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Did the Grus star get expelled from the Milky way because it was accelerated by the Sagittarius A black hole?
On Wikipedia there is an article that says a star may have been expelled from the Milky Way because it interacted with Sagittarius A.
Sagittarius A* from Wikipedia:
In July 2019, astronomers reported finding a star, S5-HVS1, traveling 1,755 km/s (3.93 million mph) or 0.006 c. The star is in the Grus (or Crane) constellation in the southern sky, and about 29,000 light-years from Earth, and may have been propelled out of the Milky Way galaxy after interacting with Sagittarius A*.[58][59]

Your title and the body of the question don't match. Please decide. You answer the title question yourself and should read the references linked in wikipedia.

It often helps to follow the references in wikipedia. It is supposed to link to the primary sources - in science often research papers like here. They often are freely-accessible on arxiv or sometimes elsewhere. Regardless, it's always worth to take a look and read those. Having read those, your questions would profit - and you might be able to answer some of your questions even yourself, and quicker. And you learn and train a valuable skill: literature research and evaluation - and you gain the knowledge from those papers.informed.

Additionally those primary references give you a good insight in how the knowledge is actually obtained, and you will understand much of the background on the fly while reading those papers as they often describe the methodogy and thus help to judge the confidence of the results and the constraints to knowledge.

@planetmaker: As it happens, Wikipedia actually discourages relying on primary sources, because it's too easy for it to turn into original research and personal interpretation. That said, the primary sources are often linked at one remove through the (preferred) secondary sources.

The linked reference 59 in the wikipedia article you quote (Koposov et al (2019)) closes with the question

The fact that S5-HVS1 was ejected with a velocity almost twice
that of all other known HVS potentially originating from the GC
poses two questions: were all the known HVS produced by the same
mechanism and has the HVS velocity spectrum been constant in
time?

Thus at that time there seem to be no other known hypervelocity stars leaving the galaxy due to interaction with Sgr A*.
Searching through ADS with the keywords 'hypervelocity', 'milkyway' and 'ejected' gives me a paper from Evans (2022) which tries to come up with a likelyhood of finding these kind of stars. They state

Constraints from Gaia EDR3 and S5-HVS1 become slightly more vertical in κ − η space as we now expect more (fewer) HVSs for κ < −1.3 (κ > −1.3) when compared to a single power-law, but otherwise remain unchanged. For
our fiducial model, this prescription would predict 14.2 +5.8
−5.0 total HVSs in Gaia DR4.

That leaves me to believe that they also don't know any further confirmed HVS accelerated by Sgr A* to velocities greater than the galactic escape velocity - but a careful analysis of Gaia DR4 data might reveal a few candidates.

I didn't know about the ADS astrophysics database .

From the references you quote a maximum of  1 in 100 interactions per year with Saggitarius A result in a hypervelocity star.  And the true figure could be 1000 times lower .Even so, over millions of years there should be more HVS outside the milky way What is holding them back ? Collisions with other stars?

lifetime of the stars and our observational capabilities due to their large distance and possibly hidden behind molecular clouds. The milkyway has estimated 100...200 billion stars, with Gaia we know ~ 1...2 billion, thus approx. 1%.


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Are stars attracted to one another by their magnetic fields?
What effect do the magnetic fields of stars have on one another:do they tend to repel or attract one another?

They behave just as other magnetic fields do - whether the force is attractive or repulsive would depend on the relative orientation of the fields. But magnetic forces are negligible compared with other (gravitational) forces in terms of accelerating the stars as a whole.
A model for the large scale magnetic field of a star could be that of a magnetic dipole (like a short bar magnet). If so, then the magnetic field strength falls off as the inverse cube of distance. The magnetic potential energy associated with the interaction of two magnetic fields is proportional to the product of the magnetic field due to one of the stars and the magnetic dipole moment of the other, so also falls as the third power of their separation, and the apparent force between them (whether attractive or repulsive, depending on how the stars' magnetic fields are oriented with respect to each other) is the (negative) gradient of this potential energy and so falls as the fourth power of their separation.
This means that magnetic forces rapidly become totally negligible compared with gravitational forces that only fall as the inverse square of separation.
As a rough order of magnitude calculation - imagine two sun-like stars separated by a distance $r$. Each will have a magnetic dipole moment equivalent to a field of about $10^{-4}$ T at the solar photosphere; in SI units $p \simeq 10^{29}$ T m$^3$. The mutual magnetic potential energy between the two stars will be of order
$$ U \simeq \left(\frac{\mu_0}{4\pi}\right) \left(\frac{2 p_1 p_2}{r^3}\right) = \frac{\mu_0 p^2}{2\pi r^3}$$
and the force between them (see here for example) is
$$ F_{\rm mag} \simeq \pm \frac{3\mu_0 p^2}{2\pi r^4}\ ,$$
where the sign is positive of negative depending on whether the magnetic dipoles are aligned or anti-aligned (the magnitude of the force would be less if it were somewhere in between).
The ratio of gravitational forces to magnetic forces is therefore
$$ \frac{F_{\rm grav}}{F_{\rm mag}} \sim \frac{2\pi G M^2}{3\mu_0 p^2} r^2 \, ,$$
where $M$ is the mass of each star.
Putting in the numbers for a pair of one solar-mass stars separated by say $r = 10R_\odot$ then $F_{\rm grav}/F_{\rm mag} \sim 10^{18}$.
That is not to say that the interaction of the magnetic fields of a star and their surroundings are always negligible. If material is ionised, then the Lorentz forces on that material due to a stellar magnetic field can be dominant over gravitational forces - e.g. magnetically, funnelled accretion. The two magnetic fields of stars in a binary system (or the magnetic field of a star and a close-in exoplanet) may also interact, releasing some fraction of the stored magnetic potential energy in the form of flares or heating of plasma, although direct evidence for this happening is sparse.
Edit:
It's also interesting to consider "more magnetic" stars - some white dwarfs have magnetic fields at their surface of about 10 T (about $10^5$ times the solar dipole field) and neutron stars can have surface fields of up to $\sim 10^{9}$ T. However, these stars are also much smaller by factors of $10^2$ and $10^5$ in radius and and since the magnetic dipole moment scales as surface field multiplied by volume, their magnetic dipole moments are not that much higher than the Sun's. They can get much closer to each other of course, but not close enough that magnetic forces ever get close to those of gravity.

while "it's too small to care about" might be a popular answer, suppose an explicit (neglecting forces other than those associated with the stars' magnetic fields) were added. I wonder if interstellar material provides a degree of shielding at large distances, or if it's pure -4 all the way until the observable universe effects set it?

Ugh, another magnet-mania musing. “F*ing magnets, how do they dork?”
The hype about magnets and magnetism is … hype. The Sun’s overall (static) magnetic field is irrelevant at, let’s say, Mercury (hard to draw a line in the sand on such a question). Solar effects are primarily from storms such as CMEs and the larger (X-class) flares, which are not the same thing as the general IMF. As already stated, the force then falls by the inverse fourth power, which makes it really tiny, really quickly.
The solar wind, with or without help/hindrance from photolysis (ionospheres), is far more relevant. But the person on the street- the nonscientist, scientism-swallower- doesn’t grasp the ionosphere and plasma physics, so we never hear that bit from YouTube and its regurgitators.
Of course, it’s hard to generalize about the universe… the _entire universe _ . One could always kluge together some bizarro example involving, say, a Giant in a globular cluster, doing a close pass by … another giant, while flaring. I don’t need to indulge bizarro kludges.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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How does a scalar field linked to dark matter affect the fine structure constant?
Olive & Pospelov (2001) "[hep-ph/0110377] Evolution of the Fine Structure Constant Driven by Dark Matter and the Cosmological Constant" https://arxiv.org/abs/hep-ph/0110377 propose that the fine structure constant is affected by a scalar field that can change the permeability of space. Dark matter is also said to be linked to the scalar field.
It is my understanding that permeability is a measure of magnetic field strength in a region of space. Does the scalar field change the permeability because it has a magnetic potential at each point in space, and how can dark matter which isn't electromagnetic affect the scalar field?
Abstract:

Bekenstein’s model of a scalar field, φ, that affects the electromagnetic permeability (usually identified with “changing α”) predicts tiny variations of the effective fine structure constant up to very high red-shifts, |α(z = 3.5)/α0 − 1| < 10−10, when the constraints from Eötvös-Dicke-Braginsky types of experiments are imposed. We generalize this model by allowing additional couplings of φ to both a dark matter candidate and to the cosmological constant. We show that in a supersymmetric generalization of Bekenstein’s model, the coupling to the LSP, which is assumed to contribute significantly to the dark matter density, can be up to six orders of magnitude stronger than the coupling to the baryon energy density.
This allows one to evade the present limits on the non-universality of the gravitational attraction due to φ-exchange and at the same time accommodate the effective shift in α at the level of α(z = 3.5)/α0 − 1 ∼10−5, reported recently from observations of quasar absorption spectra.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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When two galaxies with dark matter halos merge do they provide evidence for fermionic or bosonic dark matter?
If dark matter is made from fermions these should collide and cause dark matter to become denser in some places than others Bosons wont collide in this way so there should a different effect .So is there any observational evidence to suggest density changes of one kind or another?

"If dark matter is made from fermions these should collide", please outline why you think this is. Dark matter is generally assumed to have little/no self-interaction.

@ProfRob Fermions  cannot occupy the same space at the same time but Bosons can

However, I now understand what you are asking about.

There is no law of physics that prevents fermions getting arbitrarily close to each other. The Pauli Exclusion Principle says that no two fermions can occupy the same quantum state. Basically, this means that two fermions can be in the same place if they have different spins (though this can only be up or down) or occupy different energy states.
What this means, is that closely packed fermions experience degeneracy pressure - the fermions have an average kinetic energy even at zero temperature. This degeneracy pressure would be there even if the fermions were non-interacting.
One way to work out whether this is an important effect is to compare the "phase space" occupied by each particle with the cube of Planck's constant. Essentially there will be a significant pressure if
$$(\Delta x\ \Delta p)^3 \leq h^3\ , $$
where $\Delta x$ is a typical separation of the fermions and $\Delta p$ is a typical spread in their linear momenta, which depends on their mass and spread of speeds. Once the phase space occupied by each fermion gets down to this level then their wavefunctions overlap and fermion degeneracy becomes an issue.
We can play some back-of-the envelope games: Let's assume that the density of dark matter at the solar radius is $\sim 0.01 M_\odot$ per pc$^{3}$ and that the dark matter is WIMPS with a rest-mass energy of 100 GeV. These are reasonable numbers - although the density of dark matter in the halo of the Galaxy would be an order of magnitude or two lower than this. The velocity dispersion in the dark matter halo is let's say about 300 km/s.
Turning these numbers into self-consistent SI units, we have a WIMP mass $m_w \sim 1.8 \times 10^{-25}$ kg and a WIMP number density of $n_w \sim 4000$ m$^{-3}$. Thus $(\Delta x_w)^3 = n_w^{-1}$ and $\Delta p_w \sim 3\times 10^5 m_w$, which leads to
$$ \left(\frac{\Delta x_w\ \Delta p_w}{h}\right)^3 \sim 10^{38}\ . $$
i.e. The phase space occupied by 1 WIMP is $10^{38}$ times bigger than a value that would generate significant degeneracy effects.

How is one wimp mass and velocity  a spread of its  momenta?  I don't see how separation distance  multiplied by momentum spread cubed is less than Planck constant cubed given your figures

Using "Viktor T. Toth - Hawking radiation calculator" https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator. wimps of 10^-25 kg would be the peak emission of a  black hole with a starting temperature of about 3 K  and a mass 10^12 kg.

It isn't, it's $10^{38}$  times bigger?? That's why the effect you are proposing doesn't happen.

Okay yes I got it the wrong way around.


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Do black holes emit dark matter as Hawking radiation?
When a star made from protons neutrons and electrons becomes a black hole that black hole can emit protons neutrons and electrons as Hawking radiation.
Dark matter is unusual matter: is there any evidence for unusual particles being radiated by black holes ( that may have swallowed dark matter) ?
If not perhaps black holes can't be made from dark matter because it doesn't have a particle antiparticle pairing that would enable the emission of  hawking radiation.

note that having a distinct anti particle is not a requirement.  Photons are part (perhaps a dominant part) of Hawking radiation (in theory) and the photon is its own anti-particle.  However there is no evidence of radiation of any kind being emitted.  Hawking radiation remains a theoretical thing.

Dark matter must be able to fall into black holes or else it is repelled by them and it is not because then stars would repel it too being made from the same material as black holes

The matter that falls into a black hole is irrelevant.  This is part of the "no hair" theorem.  A black hole made of normal matter, a black hole made of dark matter, a black hole made entirely of light, or a black hole made of a mixture: all would have exactly the same properties of mass, charge, rotation, and nothing else.

Black holes are expected to be able to radiate every particle. There is no connection to what kinds of particles fell into the black hole.
And indeed, evaporating black holes in the early universe have been considered as a production mechanism for dark matter (example).
The caveat is that a particle is only efficiently produced if its mass is lower than the black hole's temperature. Stellar remnant black holes today have Hawking temperatures around $10^{-12}$ eV. Most dark matter candidates are not that light, although some are.

The lower limit for dark matter mass in the link for light DM  gives a de Broglie wavelength of about one sun earth distance 10^11 metres.


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Which wavelength is most likely to be emitted over the lifetime of a black hole by hawking radiation and which is most likely at any given time?
What equations tells us the most likely wavelength emitted for a given black hole mass at any instant in time and over the whole life of the black hole?

See Viktor Toth's Hawking radiation calculator

Great link. I used it to find that a WIMP particle of mass 10^-25 kg could be the peak emission of a black hole of mass 10^12 kg which would have a starting temperature of 3 K.

@PM2Ring Nice... A 1E11 kg black hole would provide around 36GW of power and only require a steady influx of new matter to maintain its size.

@Michael Sure, but the peak wavelength is 415 MeV gamma rays, which isn't a very convenient form of energy, and you need to fight that gamma ray flux to feed 396 micrograms / second of matter to an event horizon that's smaller than a proton.

A black hole is black... and is a nearly perfect black-body radiator.  It emits across the whole range of wavelengths, but the peak wavelength is given by Wien's law
$$\lambda_* = 2.898×10^{-3} / T$$
Where temperature is in Kelvin.
The effective temperature of a black hole is
$$T = \frac{\hbar c^3}{8 \pi G M k_\mathrm{B}}$$
In which $\hbar$, $c$, $G$ and $k_\mathrm{B}=1.38×10^{-23}$ are Planck's constant, the speed of light, the gravitational constant, and boltzmann's constant, and $M$ is the mass of the black hole.
From these you can find that a stellar mass black hole has a temperature well below the temperature of the cosmic microwave background, so any stellar mass black hole will be gaining more mass (from the cosmic background even if nothing else) than it is losing in Hawking radiation.

In the early universe perhaps small black holes gave out more energy than they received from the CMB.

If black holes emitted dark matter would  the most common dark matter particles obey Weins law too.

Stellar mass black holes aren't mentioned in the Q. Whether they absorb more than they emit does not change what they emit.

No, that's just some background context.

Looks like$$T_{\text{1 solar mass}}\approx 6.17 \cdot {10}^{-8} \mathrm{K}$$.


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How does superfluid dark matter keep stars orbiting at high speeds in galaxies?
According to A paper on Ultra-Light Dark Matter, Superfluid dark matter is a kind of Bose-Einstein condensate. But how does it help stars keep their high or  velocities in galaxies. Can somebody explain this .Section 4.2.2  Superfluid Dynamics is the relevant section, I think

Information necessary to understand your question belong into the question, not into the comments.

Are you wondering why the DM-baryon interaction term (equation 195) does not slow stars?

@Sten Yes why doesn't it slow stars?

The theory is wrong, for multiple reasons.
Why the theory is wrong:
We need a complete revision of what a Bose-Einstein condensate actually is.
A Bose-Einstein condensate is basically a dilute gas of atoms which are cooled down to near absolute zero. As the temperature approaches absolute zero, the atoms coalesce into one (relatively) giant "superatom". Basically, thousands of atoms coalesce and behave like one single atom, that is they represent a single matter-wave function.
Dark matter, in contrast defies all of the properties of a true BEC. Modern theories suggest that dark matter's particles (if they do exist), the axions, do not interact with even themselves. BEC, on the other hand, have literal particles interact with each other to form a single matter-wave function. Dark matter also cannot behave like a superfluid. We already have made Superfluid helium on earth, and we know that despite zero viscosity, superfluids are still fluids and obey Bernoulli's principle. Meaning that despite zero viscosity, even superfluids exert viscous drag on an object passing through it. Meaning that if the universe was filled with superfluid dark matter, according to the theory, the Universe would have actually collapsed into a black hole or its original state, a infinitesimally small singularity! Because stull being fluids, superfluids would exert viscous drag on the atoms of matter and thus lose speed and collapse back into a black hole.
I am afraid that the theory of a BEC or superfluid dark matter is much more inaccurate than the theory that the universe is a simulation and black holes are glitches in space-time, which is also inaccurate.
Tl;dr: I am afraid that the theory is wrong.
So, in short that theory does not explain how dark matter can keep stars and galaxies moving at high velocity.

Why the downvote? If there is anything wrong in this answer, please let me know, and I will correct it.

Nothing wrong I downvoted accidentally while writing a response .Your link to evidence for non self  interacting dark matter was was interesting.

Axions are a useful DM candidate because they can explain the Strong CP problem   "Strong CP problem - Wikipedia" https://en.m.wikipedia.org/wiki/Strong_CP_problem and this https://www.newscientist.com/article/2369958-weird-dark-matter-waves-seem-to-warp-the-light-from-distant-galaxies/

While I don't know much about the model, here are some issues I'm seeing with the answer: (1) While superfluidity requires self-interactions, Bose-Einstein condensation does not. (2) Dark matter self-interactions are not ruled out anyway, only constrained to certain degrees in many regimes. (3) Axions are only one of many dark matter candidates. (4) Viscous drag isn't relevant to expansion, as that doesn't involve shearing motion. (5) Your link doesn't support the claim that superfluids exert viscous drag.

I should point out that the article you link (for which the associated research article is 1708.04245) concerns self-interacting dark matter, which has a very different phenomenology from superfluid dark matter. Also, it doesn't rule out self-interacting dark matter; it only counters previous claims that the system they are studying shows evidence for self-interacting dark matter.


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Did liquid helium help the solar system to form by cooling the nebula that collapsed to form it?
Was the gas cloud that collapsed to form the solar system cooled by liquid helium at some point in time? I ask because the Cosmic microwave background had a temperature around 3K when the solar system formed and helium is a liquid at around this temperature and if the pressure is 1 atmosphere and it absorbs heat well

Nothing is liquid out there in space. The temperature of the radiation of the cosmic microwave background is not the temperature of the gas or dust in the disc.

4.6 billion years ago corresponds to a redshift of about 0.45, so the CMB would have been $T_0(1+z) \simeq 4$ K.

No, because the pressure was never close to 1 atmosphere and the nebula that condensed into the solar system was hotter than 3 K.
The surface density of the protoplanetary disk was maximally about 4.3 kg per square centimeter... but the thickness was about 0.2 AU, giving a gas density of $10^{-7}$ kg per cubic meter. Not much pressure.
The temperature of the outer disk is believed to have been 50-150 K, too warm for liquid helium.

The temperature and pressure in an undisturbed nebula may have been unsuitable but suppose a supernova  caused a low temperature cavity into which compressed gas was ejected.Then the conditions may have been suitable  There are exoplanets that have been found surrounded by helium "Helium Detected in Atmosphere of Exoplanet for First Time | Astronomy | Sci-News.com" https://www.sci.news/astronomy/wasp-107b-helium-05969.html. The helium extends 16000 km  and more above the planet

@DaveTheWave - Supernovas produce high temperature cavities.

Rapidly expanding gases cause cooling by other mechanisms. https://en.wikipedia.org/wiki/Boomerang_Nebula     Is 1K temperature. Here is why. "We Finally Know why the Boomerang Nebula is Colder than Space Itself - Universe Today" universetoday.com/135920/

https://www.universetoday.com/135920/finally-know-boomerang-nebula-colder-space/.   Link for previous comment


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How did people in Mali know that there were two stars appearing as one in Sirius before telescopes were invented?
Did they invent a telescope and keep it quiet?
Given that glass has been around for 4000 years (made in mesopotamia in 2000 BC) was it technically possible to polish it and make convex shapes from it a long time ago?

An European explorer made contact with the after the Sirius B was discovered and told them about it. The story was slowly inserted in their folklore. Most Dogon today confirm that there was no mythology associated with that, though. So, nothing anachronistic at all.

@PierrePaquette-- would be a good answer if you have sources?

seeing as you probably weren't there (in Mali before telescopes were invented), we are going to need a source for this claim.

This is more suitable for SE.Sceptic. It is worth noting that Temple was rather selective with his reporting of their beliefs making it sound like a scientifically accurate description of the Sirius system while the original stories apparently were incompatible with observations.

Sirius B is apparent mag 8.4, so it cannot be seen without a telescope.  There is no planetary nebula, so it did not become a white dwarf recently.


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Why did it take so long to invent telescopes given glass was used 4000 years ago in Mesopotamia?
Is a telescope difficult to make? Does glass have to be polished and shaped very precisely? Or is a device using two or more lenses to magnify things just not obvious?

It might seem obvious to us, but people were making spectacles for centuries before the telescope was invented. From https://www.space.com/21950-who-invented-the-telescope.html "One story goes that he got the idea for his design after observing two children in his shop holding up two lenses that made a distant weather vane appear close". And then it took a year or so before a telescope was used (by Galileo) to view celestial objects.

As a very minor addition to James K’s excellent answer, keep in mind that to make a refracting telescope (the one that uses lenses), you need perfect optical-quality glass, plus you need to perfectly polish not one but two surfaces for each lens. Mirrors, on the other hand, don’t require perfect-quality glass, and only one surface needs to be polished. But there, too, progress was slow, and optical-quality mirrors were late in coming.

You can check History of Science and Mathematics Stack if you are more interested.

Ancient glass was opaque or at most translucent.  It was also often full of bubbles.  Glass was not even suitable for windows until the first century AD, and even then it was as a means of letting some light in, it wasn't useful for looking through.
Lenses were made in Mesopotamia, but using quartz crystal instead of glass, and were costly. Nero is said to have had glasses made from emerald.  Only an emperor could afford such luxury.
Transparent glass and the development of techniques to make flat planes of glass date from about 1000 AD and was developed in Venice.  This is just an example of slow evolution of technology.  Making transparent glass requires a pure source of silica and soda ash (or potash, but that comes with other issues), chemistry to supply various oxides that improve transparency, more advanced metallurgy, better furnaces and so on. Transparent glass is said to be the discovery of Angelo Barovier in the middle of the fifteenth century.
Glass had developed to a quality that made is a suitable substitute for quartz in lens making by the 16th century, but it was still secret knowledge that few held. The process of grinding and polishing glass to a suitable degree takes time, and needs the development of fine abrasives (or you just end up with lens that is so scratched to be opaque again).
It was only after the printing press had been developed that the theory of lens-making became widely known, and the first telescopes were designed. Even then, the idea of using a telescope at night wasn't immediate.  The people making lenses weren't the same as the people who tracked the planets.
Lenses for telescopes need to be an order of magnitude better in quality than lenses for correcting long-sightedness. The glass does have to be shaped and polished very precisely. The technology and skills required for this were not fully developed and shared before 1600.
sources:
Ancient glass
lenses
telescope
Angelo Barovier

Anyone doing photography with a real camera would understand the issues of quality. You can see specks of dust on your lens as a giant blob.

When you say it was secret knowledge in the 13th century, does that mean that it was knowledge that only a few experts had so most people didn't know it, or that the relevant group of experts was actively keeping it a secret and not sharing outside their group?

@terdon Guilds had a strong tendency to keep knowledge secret in the era before patents, and in some cases was main reason they existed. Especially easy in this case where there are 'magic' ingredients where the workers know the mix white powders into the sand sand but not know what those materials are or most importantly, how they were assessed for quality. Similarly preparing the materials for grinding glass can be kept a guild 'mystery' but the results used (and consumed) by outsiders.

@GremlinWranger yes, I know, that's precisely why I am asking if this was something actively kept secret or simply very technical information that the average person wouldn't know.

@terdon not 100% certain in this case, but I'd bet that it was actively kept secret.  This would not be difficult in a time when there was no printing press, and knowledge tended to be hoarded and passed only from master to student. But even with the knowledge, the skill and materials required would have prevented many copies.

@terdon Not exactly lens making but kind of adjacent and it illustrates the lengths to which guilds would go to protect their trade secrets: https://barbaraathanassiadis.com/history/venetian-glassmakers-in-paris

@biziclop as I told GremlinWranger, I know this. Which is precisely why I was curious and asked whether this was a case of a guild guarding its secrets, or if it was simply that most people don't know technical details. That is a neat example though, thanks for the link!

@terdon I'm not entirely sure but I know Venice did have a guild of eyeglass-makers, relying on their ability to manufacture high quality glass. It also seems that eyeglasses were sold widely in Europe by the 14th century, and more affordably than mirrors, so I suspect they didn't have a monopoly. Maybe the reason few people made them was simply the industries that support it didn't exist yet. Sure, you need good glass but you also need optical pitch, various abrasives (these are typically suspensions, so they're not trivial to make) and of course technique that can't be learnt from books.


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Has the missing dark matter in some galaxies been explained yet?
There are galaxies that should have dark matter to explain their existence but they do not.
"Mystery of Galaxy's Missing Dark Matter Deepens | NASA" https://www.nasa.gov/feature/goddard/2021/mystery-of-the-galaxys-missing-dark-matter-deepens
"This weird galaxy is missing all of its dark matter and scientists can't explain it | Space" https://www.space.com/dark-matter-missing-from-galaxy

https://astronomy.stackexchange.com/questions/53314/have-we-discovered-a-galaxy-lacking-in-dark-matter Does this answer your question?

https://www.nature.com/articles/s41586-022-04665-6 shows that DF2 is part of a chain formed by two galaxies passing through each other.  The stars and dark matter passed through each other unimpeded.  A chain of dwarf galaxies between them formed from gas clouds (sans dark matter) that rammed into each other.  This is called a bullet interaction.


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Are vacuum fluctuations that produce dark matter correlated to fluctuations that produce baryonic matter?
When cosmic inflation began vacuum energy fluctuations created matter . But were the fluctuations that created dark matter dependent on the fluctuations that created matter ? Is the ratio of dark matter to baryonic matter random for galaxies for which this has been assessed?

Yes. This question was only settled in the last decade or so, but initial variations in the baryon density and in the dark matter density seem to agree at the percent level. See for example "Baryons still trace dark matter: probing CMB lensing maps for hidden isocurvature".


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If the energy density during inflation were lower, how would the CMB look in comparison to the observed CMB today?
I am assuming that a big bang followed by inflation is still theoretically possible with a different starting density. Would the CMB look the same but just colder?

By "less mass" Do you mean "if the density of the universe was less than the critical density".  Observations suggest that the density is equal to the critical density (to within the accuracy of the observations) If the density is less then the universe is open and expands forever.

@James K  I just meant that if the  inflation potential energy created  photons quarks leptons etc  of a total mass less than the total mass of those particles when the present universe started out, would the CMB have a physically different shape on a plot or  would it just look colder? I think this is ultimately about how  does the inflaton develop when it starts with with less potential energy.

Then again, most (70%?) of the mass/energy budget of the universe is dark energy, and apparently not "particles" at all, and only a small amount of the mass is "photons, quarks, leptons"  Most of the mass-in-the-form-of-matter is "etc"  (ie dark matter)

Dark energy doesn't affect the  inflation period when particles were being created. The amount of DE grew  with time

If the mass is finite which it could be what would theory say about a n inflaton with smaller potential energy?  There must be some sort of answer to this?

Should probably ask this question on  physics  stack exchange.

To confirm: you are asking about the implications of varying the inflaton energy density? (Mentioning the mass of the universe is confusing here.)

@Sten Yes  the energy density of the entire universe as inflation began

We don't know the energy density during inflation. This should immediately tell you that it doesn't (directly) impact any of the observables that we are able to measure.
Even though the inflation energy density sets the "initial temperature" of the Universe, the known processes that give rise to today's Universe, like nucleosynthesis and recombination, are connected to the temperature of the Universe and not the time. That is, they occur at a set temperature, irrespective of what the initial temperature was. For example, the temperature at which the cosmic microwave background (CMB) last scattered is set by atomic physics: it's the temperature at which neutral hydrogen atoms become thermodynamically favored over free protons and electrons. By the way, this temperature doesn't even affect the overall CMB temperature today; it only affects the temperature variations.
There are also unknown processes responsible for

the abundance of an asymmetric component of ordinary matter (more matter than antimatter);
the abundance of dark matter;
the initial variations in the density of the Universe;
the abundance of dark energy.

The outcomes of these processes affect the temperature variations in the CMB. It's possible that these outcomes depend on the inflation energy density (e.g. if both the matter and the radiation were decay products of the inflaton field), but any claim to this effect would depend on the particular speculative model of baryogenesis, dark matter, inflation, or dark energy that you adopt.

I think the temperature just depends on the photon/baryon ratio and $\Omega_bh^2$ right?

@ProfRob i.e. the photon density today, yes. But I'm aiming at the more general question of what the CMB looks like (not just the monopole), which depends on many more cosmological parameters.


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If phosphine gas is to be proof of life on a planet, shouldn't there be other gases too?
If microbes on Venus produce phosphine, wouldn't there also be other gases produced by microbes since earth has many kinds of bacteria and the fast reproduction rate of bacteria make genetic mutations likely?

I think you're confusing the idea of something being a possible indicator of life with something being proof of life.  Two completely different standards.

It's a hypothesis. There's no proof that PH3 is originated from a biological source. The paper you mentioned explained that clearly. Again, there is no proof that there is any microbial entities on Venus.

You can check previous discussions if you are interested: https://astronomy.stackexchange.com/questions/38914/phosphine-on-venus, https://astronomy.stackexchange.com/questions/39394/is-extra-planetary-phosphine-actually-a-new-discovery, https://astronomy.stackexchange.com/questions/38947/what-other-hints-of-possible-life-in-venusian-atmosphere-have-we-dismissed, https://astronomy.stackexchange.com/questions/38888/what-are-the-prospects-for-follow-up-observations-of-phosphine-on-venus/39163

That article is arrant garbage. You may as well try and say that there are tigers on Jupiter, or there are hedgehogs living on Mars. Anyways, as pointed out by @NilayGhosh in the comments, phosphine can be produced by inorganic sources as well.

I'd love to upvote a question similar to this but the "is to be proof" is pulled from thin air. The premise of your question is that it is considered scientific proof and that's not a thing, so -1 for a faulty premise. Certainly your linked article in no way says "microbes on Venus produce phosphine"!

Sure, if it is something reactive like oxygen or fluorine
Basically, if you find anything in the atmosphere that is way too reactive to exist for a long period of time, like oxygen, fluorine or chlorine, then there are only two is only one logical explanation:

Some moronic alien dumped the gas onto a planet(pun intended)
The planet has life on it.

In fact, we don't even need an hypothetical for this. This example already lies on Earth.
Earth was basically a slightly upscaled version of Venus 3 point something billion years ago. Its atmosphere comprised of hydrogen, methane, ammonia and water vapor. And the only lifeforms were either methanogens or sulfur bacteria. There were photosynthesizers, but those were mainly purple sulfur bacteria, releasing stinky fart gas hydrogen sulfide, a foul-smelling gas, instead of oxygen.
Then things changed. Some bacteria (most likely stinky sulfur photosynthesizer) got a mutation from UV rays or something like that, and the next thing we know, the bacteria becomes green and now uses carbon dioxide and water and sunlight to produce glucose and oxygen and what not. We know this happened as we have banded-iron formations on Earth, which happened when oxygen attacked the dissolved iron present in the ocean into rust (hydrated ferric oxide). Rust, having a lower solubility than water, precipitated out of the water and settled out to the bottom of the ocean, where it got crushed into hard red rocky formations that we call banded-iron formation.
In the early days, oxygen was consumed up by the reducing atmosphere of baby Earth. However, as the cyanobacteria (as they are called that) started multiplying exponentially, they started producing more oxygen faster than it could be consumed by the reducing atmosphere. Some of them even started fixing nitrogen from the atmosphere (maybe ammonia) and turning them into nitrates.
The result: Earth's atmosphere turned from a reducing one, to that of a oxidising one. This killed off the methanogen and other pre-cyanobacteria lifeforms, like the sulphur bacteria. Now, methanogens still exist, but only in oxygen-deprived areas like cow bowels, animal dung, swamps, wetlands. And today, you can only find purple sulphur bacteria in stagnant water bodies, hot springs or microbial mats.
Similarly, although we wouldn't likely identify hydrogen sulfide as a lifeform(as hydrogen sulfide is also formed inorganically as well), we would very likely find out if there is life on a planet, if there was a reactive gas that shouldn't exist on that planet, like oxygen or fluorine. The reason we know this can happen: It already has happened.

Not all oxygen implies life. Oxygen in Ganymede's atmosphere is generally deemed to result from ultraviolet radiation acting on the water ice at the moon's surface.

Yeah, but I said, Oxygen in "unusually high" quantities. Ganymede has a tenuous atmosphere, which means that oxygen can easily, if briefly, form due to ultraviolet light

Your claim that there is only one logical explanation has no useful evidence to support it, as we only have a sample size of 1 planet with life and none with significant amount of reactive gases. We have loads of theories, but all based on observation.


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Finding Position angle and ellipticity to Deproject galaxy image using IRAF
I am trying to obtain position angle and ellipticity for a galaxy image by fitting elliptic isophotes using IRAF routine stsdas.isophote.ellipse package. After fitting the isophotes, I have a table which contains the variation of postion angle and ellipticity with major axis of elliptical isophotes. I am not understanding how to know the Position angle and ellipticity for my galaxy from the table.


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Could there be bacteria in the 6000 km water jet of Saturn's moon discovered by the James Webb telescope?
James Webb telescope discovers gargantuan geyser on Saturn's moon, blasting water hundreds of miles into space.
If there is life inside the moon Enceladus perhaps now it is airborne. Could water carry bacteria 6000 km high ?

Many things are conceivable but that doesn’t imply they occur necessarily.

@JonCuster your comment feels unhelpful, it's not like the OP does not know that conceivability does not necessarily imply occurrence. Your not helping the post, but instead suggesting the OP said something that they did not say. I've changed "would" to "could" perhaps that's better than casting shade on the question itself. Since the height that water-borne organisms could reach is of great importance to future astrobiology missions, I think the question is important and preventing others from posting answers is unproductive. voting to leave open!

Different but related: How can "Geysers" on Europa reach heights of 100km? and Is there an established distinction between a geyser and a cryovolcano in solar system bodies? and Enceladus; why use the words "geysers", "jets", and "plumes" interchangeably?

Enceladus has what is known as a tenuous atmosphere composed mostly of water vapor, along with trace amounts of other gases such as carbon dioxide, methane, and nitrogen. This atmosphere originates from the geysers present near Enceladus' south pole, which expel water vapor and icy particles into space.  The density is not enough to hold up a bacteria for any length of time (0.001 Pascals of pressure), even with the small gravity of this moon. But the answer is yes, bacteria (if they exist there) could be ejected at high speeds and pressures by the geysers, and they could make it all the way to Saturn's E-ring.  Of course, it is unlikely they would stay alive for very long outside of their subsurface ocean.

But the equivalent of bacterial spores might survive for quite some time, and there's dehydration-resistant yeast and tardigrades-type scenarios as well. Related in Biology SE: Is it known how some heat-resistance Bacillus spores repair their DNA after having been heated to 420 °C? (but not much higher) and Will proper autoclave treatment really "inactivate all resistant bacterial spores"? If not, how widespread are autoclave-resistant bacterial spores?


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How does the number of CMB photons vary with time?
When the CMB temperature is calculated does it take into account photons absorbed by gas clouds and dust over billions of years?

If you receive CMB photons then those photons haven't been absorbed.
The CMB spectrum is modified by absorption and emission (mainly) along the line of sight. Particularly due to dust in our own Galaxy and even in our own Solar System. These are small perturbations and are taken into account when analysing the CMB.
Only a tiny fraction of CMB photons can have been absorbed. There are about a billion photons for every proton in the universe and that ratio is thought to be very similar now to what it was when the CMB was produced. Even if every atom in the universe could somehow absorb a CMB photon (they can't and most protons are not in atoms) this could only remove one billionth of the photons.

To be clear, CMB photons can be absorbed, but emission or re-emission along the line of sight is not, by definition, a CMB photon.  But, technically, what you wrote is correct in that the spectrum one measures when looking for the CMB photons is modified by emission along the line of sight.


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When dark energy became significant 6 billion years after the Big Bang was there anything else of significance going on in the universe?
I read that a lot of supernovas appeared at this time but were other major events or phenomenon occurring? I ask this in case there could be some physical process helping dark energy to strengthen or perhaps being helped by dark energy 6 billion years after the Big Bang

The dark energy density doesn't strengthen. In the simplest model it stays exactly constant. What changes is the density of normal and dark matter, both of which decrease as the universe expands.
5 billion years ago, corresponding to a redshift of about 0.5, is just the time at which the energy densities of matter and dark energy were similar

When their energy densities were similar did that mean space was expanding at a constant rate.


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Can the cosmological constant be coupled to redshift of radiation
Can space-time be more embedded with the things inside it than we actually think?Can the redshift of radiation be a property of spacetime and that dark energy exists only to conserve the overall energy of the universe?

The energy density of the radiation goes as $a^{-4}$, but the energy density of the dark energy is constant.  So, they do not track each other.  However, there seems to be a conspiracy that made the universe flat, ie $\Omega = \Omega_m +  \Omega_{rel} + \Omega_{\Lambda} = 1$


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If black holes contain dark energy would the expansion of the universe be uniform?
Because black holes are different sizes presumably they would contain different quantities of dark energy and so some galaxies would accelerate faster in some directions than others.
Dark energy from supermassive black holes? Physicists spar over radical idea

In that model, black holes are assumed to spread out uniformly throughout the universe after their formation. This is typical behavior for dynamical dark energy models, but it's particularly necessary here to evade established observational constraints on the abundance of massive black holes within galaxies. In Appendix B of their ApJ Letter, where the authors demonstrate their model's compatibility with these limits, they say that they "have assumed a uniformly dispersed population in the computation of Figure 3". It is also clear from the referenced figure that this assumption has indeed been made.
(Whether the implied departure of stellar remnants from galaxies is itself consistent with observations is a separate matter that, to my knowledge, has not been addressed.)

Stellar-remnant black holes do remain in galaxies -- that's where we've seen them! (Both via accretion disks from companion stars and from microlensing.)

And, no, the model doesn't assume that black holes "spread out uniformly throughout the universe".

@PeterErwin I agree that stellar remnant black holes stay in galaxies (and that's one reason why the black holes as dark energy idea makes no sense)

@PeterErwin But the people proposing the idea do assume that black holes spread out uniformly. See section 4.1 and appendix B of https://arxiv.org/abs/2302.07878

@Sten How would black holes spread out  uniformly  if they expand space at different rates? This is like  energy being asymmetrical ly distributed as shown by the CMB fluctuations

@DaveTheWave The suggested mechanism is that there is a pressure-like effect ("anisotropic stress" in the linked article) that drives the black hole distribution toward uniformity. This doesn't have to be the case, but if it is not, then the model is already ruled out at the 4-5 order of magnitude level by (at least) microlensing.

If dark energy is vacuum fluctuations then as a black hole increases its mass it increases its entropy and gains information  Isn't this problematic and similar to the information loss paradox.  Something must be losing entropy / information to balance the situation and if that something is the space around the black hole then that space will lose vacuum energy to the black hole and will shrink not expand !

@Sten There’s nothing in the article (either the Science news piece or the ApJ Letter) about BHs being “driven toward uniformity”.

@Sten Looking at some of the earlier articles, the idea seems to be that if you have a population of dark-energy-filled black holes formed at $z \sim 10-20$ from Pop III stars, and if these objects have extremely high spins, then you can supposedly get a large scale repulsive effect that might produce “uniformity”, before most galaxy formation. They note that the observed spins of present-day stellar-mass black holes are much too low to qualify.

E.g., Croker et al. 2020

@PeterErwin Regardless of earlier work, when assessing compatibility with microlensing/wide binary/ultrafaint dwarf constraints in the recent Letter (that I linked), they assume the black holes are uniformly distributed. Notice how in figure 3, the black holes are taken to contribute $\sim 10^{-5}$ the local density of the Milky Way halo. If the black holes remained clustered, they would contribute the majority of the halo's density, violating the observational limits horrendously.

@PeterErwin The relevant text in Appendix B is, "Consistent with the discussion in §4.1, we have assumed a uniformly dispersed population in the computation of Figure 3"

I think the paper is just wrong. I wrote this answer about it. Their assumption that the mass of the quasi-black-hole objects depends on the scale factor is inconsistent with general relativity.

@benrg Totally agree that the theoretical basis for the model is in error.

@Sten Hmm.... I think I agree with you now: they do assume a uniform density of BHs, even though that requires implausibly high spins and contradicts the observed distribution of the supermassive BHs they're basing the paper's argument on...

@benrg I think your argument in that physics.stackexchange answer is wrong; I just added a comment there about that. (I do agree that the paper is wrong, but for more observational reasons.)


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If astronomers didn't know about redshift could we still determine the universe is expanding?
What other evidence is there for the expansion of the universe?
Would the equations of physics look wrong without it?

Why would redshift not be known? The (varied) reasons have many knock-on implications.

This is rather like asking "if scientists didn't know about 'falling' could we still determine a theory of gravity"

Yeah , if redshift isn't known, then the astronomers don't know how to measure the speed of light, or it's infinite and astronomers are disintegrated by Olber's paradox.

The density rises as you look far away.  Distance would come from other methods.  Cluster distances and densities could do it.  Eventually, enough galaxies will have an SN1a measurement.

I would think that electrons protons etc from supernovae explosions would have less energy than predicted by theory That would be a good indicator of  Hubble expansion.

Given the technological and knowledge requirements to measure distances and velocities in the universe of the other methods, it does not sound reasonable to assume that spectroscopy is unavailable - especially as imaging and taking spectra are some of the most direct methods to obtain data.


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Negative Rotation Speed (vsini)
I've repeatedly seen the rotation speed of a star given as a negative. Does this mean that the star started spinning in the opposite direction at some point?

Your link doesn't work for me.


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As redshift increases are there more large galaxies from mergers because there was less dark energy in the past?
Dark energy moves galaxies apart.Is this reflected in an increasing number of large galaxies as redshift increases and dark energy decreases?

The merger rate was indeed larger in the past, i.e. at higher redshifts. However, this is not due the smaller density of dark energy (since this only started dominating relatively recently, and only does so on scales larger than gravitationally bound structures), but simply because the Universe hadn't expanded so much, so galaxies were closer together.
Moreover, once galaxies have merged, they don't "unmerge". Hence there cannot be more larger galaxies at higher redshifts (in principle you could have more small galaxies forming at late time, thereby reducing the fraction of massive galaxies, but that doesn't happen; in fact smaller galaxies tend to form at earlier times, in the so-called "hierarchical structure formation").
Minor vs. major merging
Merging can be divided into minor and major merging, depending on the ratio between the merging galaxies' mass. Major merging involves galaxies of comparable masses and can be quite violent, whereas minor merging are of more "accretion-like" nature. The following figure (from O'Leary et al. 2021) shows how the rate of major mergers (here defined as a mass ratio of $>0.25$) increases for various halo masses with redshift, as predicted by the empirical galaxy formation model EMERGE:

For convenience, I annotated the lookback time in gigayears. Dark energy only started dominating at redshift $z\simeq0.3$ (marked by "Λ ~ M"), some 3½ Gyr ago.
Similar trends are seen for minor mergers, e.g. in this plot (from Rodriguez-Gomez et al. 2015) showing the merger rates in the cosmological simulation Illustris:

The dark blue line in the latter plot shows major merging, whereas "greener" colors show successively minor-er mergers. Black lines show predictions from a fitting function from Genel et al. (2010).
Dry vs. wet merging
Another way to divide merging is according to whether the two merging galaxies are gas-rich or gas-poor, called "wet" and "dry" merging, respectively. Whereas wet merging tends to trigger star formation and quasar activity, dry merging is typically less dramatic. Merging can also be "mixed", involving a gas-rich and a gas-poor galaxy, which may shut down star formation in the gas-rich one. The following plot (from Lin et al. 2008) shows how the fraction of wet mergers was larger in the past, at the expense of dry/mixed mergers (the data points are observations and the lines are a semi-analytical model from Khochfar & Burkert 2003):

This evolution reflects how galaxies in general were more gas-rich in the past, whereas at late times the fraction of red, gas-poor galaxies has increased.

Why was the fraction of wet mergers higher in the past compared to dry mergers Is this because wet galaxies had stronger gravitational attraction for one another.

@DaveTheWave No, it's mostly because galaxies in general were more gas-rich in the past. With time, the galaxies use their gas to form more and more stars, and while massive stars quickly return their gas to the interstellar medium, the bulk of the stars formed are small and hence long-lived. Some galaxies also lose their gas, either by intense starbursts or quasar activity blowing it out of the galaxy, or by being ripped out of the galaxies during merger events.

$\Lambda$ did not start to dominate on the scale of galaxy mergers at z=3.  It never dominates in a region with above closure density.

@eshaya No that's true. I can see that my answer gives that impression, so I removed that part. Thanks!

Downvote though?

@eshaya Though to be clear, dark energy still has an impact where it does not dominate, and it is indeed one influence on the merger rate today.

@Sten  The influence that dark energy has on merger rates is through weak coupling from its effects on very large scales where densities are low.  It is making large voids slightly bigger, so more matter in walls of voids and clusters, but walls are more stretched.   Its affect on whether 2 galaxies in a group will merge is negligible.

@eshaya Rough estimate: collapse (merging) happens around a factor of 2 in time later than turnaround (farthest separation of the two galaxies). For objects merging today, that would put turnaround around redshift 0.8, when dark energy was 40% as dense as matter. At turnaround, the system's average matter density is ~6 times the mean, so dark energy would contribute around 40%/6=7% as much energy density as the system's matter. Its repulsion would be around 14% of the matter's attraction (with the factor of 2 arising from the pressure contribution). Is a 14% effect negligible?

To be clear, dark energy isn't preventing mergers. It's delaying them, which reduces the merger rate.

Galaxy mergers today are not happening from turnaround, generally.  Field galaxies are rare and at very very low densities.  Galaxies move around in a group potential and eventually bump into another galaxy.  Perhaps you are looking at halo statistics from N-body calculations, which means you are looking at groups merging with one another or more precisely matter falling into halos/groups.  Hardly any of this is expected in the next Hubble Time.

@eshaya I actually didn't interpret the question as "could dark energy rip galaxies apart", but more along the lines of "could dark energy move them so far apart today that the merger rate is smaller". But I see that it could also be interpreted as the former.

I deleted my comment.  I see now that many think a region of space collapses and the material in it mergers to form a galaxy.  This is oversimplified, but it leads to the question, now that dark energy dominates will it truncate collapsing regions?  And, why wouldn’t galaxies forming by collapse therefore be smaller now?

@eshaya Yes, I'm thinking more about halo mergers. For objects within already virialized structures, dark energy would be a percent-level contribution today.


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Can dark matter accumulate at Lagrange points?
Interplanetary dust can accumulate at Lagrange points .
"Kordylewski cloud - Wikipedia" https://en.m.wikipedia.org/wiki/Kordylewski_cloud
But can dark matter accumulate at Lagrange points and is there evidence for this for example between pairs of  galaxies or between the earth and moon?

If dark matter is dissipationless and only interacts gravitationally, then no it can't. How could it be brought to approximate rest$^1$ with respect to a Lagrangian point, its inertia would mean it just carried on with whatever kinetic energy it had?
If dark matter is weakly interacting then it might be possible for it to accumulate in deep gravitational wells formed by ultra-dense matter - i.e. in white dwarfs or neutron stars. But such ultra-dense matter does not exist at Lagrangian points in the Solar System.
$^1$ Something "at Lagrangian points" (the wording of the question) is necessarily at rest on average with respect to those Lagrangian points. They are actually effectively in orbit around the more massive body. There is no mechanism to decelerate and capture dark matter into an orbit with a speed of $\sim 30$ km/s (e.g. for an orbit near Earth) when dark matter is travelling past the Sun with typical speeds of hundreds of km/s.
Even should one argue that the dark matter has a distribution of speeds and some of it will be travelling at close to zero velocity with respect to the Sun, that matter will be accelerated towards the Sun as it approaches and would stil not be captured at the Lagrangian points.

The smoother than expected profile of dark matter suggests to me that it does dissipate https://phys.org/news/2016-12-dark-smoother.html

Huh? It's less clumpy than expected. @DaveTheWave

@DaveTheWave Also, that article concerns scales much larger than what would be affected by dissipation in the dark sector.

temporary, reversible (-1) because I think you are oversimplifying the problem in order to write an easy answer. Associated with the regions of Lagrange points are unstable orbits that one can enter, then spend some substantial time in the general vicinity of a Lagrange point and the exit. That mechanism could result in a higher concentration without require inter-particle collisions or other lossy interactions. Accumulation ≠
trapped, much less "at rest" as you stipulate.

Accumulate means rate of particles entering a volume  > rate of particles out that volume @uhoh

No, that's what currently accumulating means. Having accumulated means there's an excess density, but it could certainly be steady state. And it does not require "trapped" or "at rest" Do all explanataions for a ""Kordylewski cloud" require dust-dust collisions for its formation? (I don't know, but it's a good question)

Objects near Lagrange points aren't at rest. They maintain a constant relative position to the Sun and Earth, but that system is itself rotating.

@eyeballfrog No, in the parlance of the restricted three body problem of which we speak (the only scenario where the L-points can be defined) we work in a rotating frame of reference. In this case the five libration points (or Lagrange points) are indeed at rest because we let the pseudopotential take the place of centrifugal effects.

@ProfRob This is all so interesting! I'd never heard of these clouds before now, so I've just asked Are dust-dust collisions necessary to explain Kordylewski_clouds at Earth-Moon L4/5? Aren't the cross-sections, number density timescales too small?


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Can WIMPS be stable for a long time given that they are 60 times heavier than a proton and other particles heavier than protons decay rapidly?
WIMPS are dark matter candidates  But how can they be stable for billions of years ?

Depends on what they actually are, their physics, and their possible decay paths.

Just because they could decay doesn't mean they do it quickly.

@ProfRob - indeed, Bismuth-209, decidedly more than 60 times heavier than a proton, has a half-life (alpha decay) estimated at 4.6E19 years, significantly more than a paltry billion years.


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How to get target_id column from GAIA DR3 basic search?
When using the GAIA search through the website, the target_id column seems to pop up without any displayed column specified and disappears just as mysteriously. Does anybody know how to trigger that column to display when downloading data?
Thank you


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What are these dark artifacts in this New Horizons image?

Are these photographic artifacts of some kind and if so how would they occur?
They are almost the same exact shape and size.
It is a photo of the Kuiper Belt from the New Horizon space probe taken directly from NASA web site.
If it is not a common photographic event then what could it be?

They’re present only next to bright stars, so I would suppose it has something to do with imaging captor “bleed” of the light to neighbouring pixels…

Looks like maybe the CCD is being read-out raster wise and the pixels following saturated pixels can't be read correctly.  Looks like some of the less-saturated pixels have the same effect, but shorter.  Also probably a lot of data compression going on with images from New Horizons.

Weaver et al. 2020 discusses New Horizons LORRI instrument performance in detail.
The introduction says:

The LORRI CCD incorporates anti-blooming technology
to eliminate bleeding of the electrons along columns
when bright targets saturate
(i.e., when the signal in a pixel exceeds
the full-well capacity of ∼80,000 electrons).

Otherwise the tails of bright stars would be white and blocky.
Figure 5, another deep image of a star field, explains similar artifacts:

The brightest stars are saturated and have black tails due to amplifier undershoot.

Jenkins et al. 2010, section 2, addressed a similar issue for the Kepler mission:

The analog electronics chain exhibits memory,
necessitating the application of a digital filter to remove this effect,
called Local Detector Electronics (LDE) undershoot.


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Compute rotation angle of fits image
I have a fits image and the header has the keys PC1_1, PC1_2, PC2_1, PC2_2. I want to compute the angle by which my image is rotated so that North will be up. I use the formula below:
angle = np.degrees(np.arctan2(PC1_2, PC1_1)) .
The answer I get is 156.0 deg however when I load the image on DS9 I get the following direction.

Does this mean that the angle is measure from the negative x-axis and so I should use angle_true = 180 - angle or am I using a wrong formula?

It depends on how you count and orient your pixel coordinate system. Often its origin is in the top left.


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What would happen if Venus and Earth collided?
The first thing we would need to consider is that Earth has already been hit by a protoplanet half its size 4.5 billion years ago, meaning the Earth may have a chance of staying intact after the collision.
The next thing we would need to think about is the Moon. The impact would have a gigantic force and may disrupt the Moon's orbit, which would lead to two possibilities: either it would collide with the Earth-Venus body, or it would get ejected into space, without any planet to orbit. It may even start orbiting the Sun. The Earth could get completely destroyed, but again, it went through that before.
I also am wondering if it might disrupt any of the planets in any way, and if Earth's orbit might be changed. Thanks for answering the question!

there would be an "Earth-shattering Kaboom!"

nice question for Randall Munroe, of https://www.xkcd.com

That protoplanet collsion is the one that created the Moon, right?

Yes, Theia did crash into the Earth 4.5 billion years ago, and 20% of Theia's mass became the Moon. Please let me know if I'm mistaken. Thanks, and I hope this helps!

Qualifying the Earth after impact with Theia as "intact" seems like a far fetch. More like "molten".

@GenericPerson your question is totally unanswerable.  It's like asking "what happens if two cars collide".  When two cars collide on a freeway going the same direction, on a highway head-on, in a parking lot, in your garage etc, the situation is totally different.  You need to explain the type of collision you have in mind.

"Intact" is an elastic term ;-)

@Peter-ReinstateMonica no, kinetic energy is converted to heat, so even if it's a 2-body final state, if they are "molten" then it's not an elastic collision.

This is the kind of thing that computer simulations are done for, but I am not aware of any that were specifically for a Venus-sized body hitting the earth.
The specific details would depend very much on the velocity and angle of the impact. But, in general, as the two planets approached, tidal forces would deform both planets in the hours before collision, causing significant heating. The collision would liquify and splatter both planets, but also probably slow down their relative velocities so that most of the matter of both planets would form a spinning cloud of giant droplets and later coalesce. The moon would be caught up in the cloud and might end up combined with one composite planet. Or you could end up with a planet plus a moon, with some of the current moon in the planet and some of both planets in the new moon.
Be aware that this is  not the sort of thing that can "just happen". The orbits of Venus and earth are stable on long time scales. But simulations have been done showing that there could (with very low probability) be collisions between inner planets, billions of years from now if instabilities slowly built up in their orbits. But they ran numerous simulations and only had a few near misses far in the simulated future.
This article shows simulations of collisions that are more unequal, and more similar to the moon-creating collision. But the fluidic, gravity-controlled nature would be the same.

The outcome of a Venus - Earth collision depends on the impact speed. At a relatively low speed, the matter is likely to condense to form a new planet. But if the speed is high enough the planets will be totally disrupted and the matter will disperse. We can calculate this speed from the gravitational binding energy. For a spherical body of uniform density, this is
$$U = -\frac{3GM^2}{5R}$$
where $G$ is the gravitational constant, $M$ is the mass of the body, and $R$ is its radius.
Real planets don't have uniform density. They have dense cores, and their actual binding energy is somewhat higher, but the above formula gives a reasonable rough approximation.
Notice that the GBE (gravitational binding energy) is negative. If two planets collide with kinetic energy equal to the negative of the sum of their GBE they will become totally unbound. So we just need to solve for $v$ in
$$-\frac12( M_{Venus}+M_{Earth})v^2 = U_{Venus}+U_{Earth}$$
This results in $v\approx8.384\,\rm km/s$, which I calculated using this Python script.
By way of comparison, the mean orbital speed of Earth is $29.78\,\rm km/s$ and of Venus is $35.02\,\rm km/s$. A smooth Hohmann transfer ellipse from the current Venus orbit would meet Earth at a relative speed of $2.495\,\rm km/s$, well below the disruption speed. However, if Venus were on a Solar System escape trajectory and it collided with Earth, the relative speed could easily be higher than the Earth's orbital speed.

As Mark Foskey mentions, a collision between Venus and Earth is quite unlikely. However, there is a chance that interaction between Jupiter and Mercury will disrupt the inner Solar System before the Sun becomes a red giant.
From Wikipedia:

Mercury–Jupiter 1:1 perihelion-precession resonance
The planet Mercury is especially susceptible to Jupiter's influence because of a small celestial coincidence: Mercury's perihelion, the point where it gets closest to the Sun, precesses at a rate of about 1.5 degrees every 1,000 years, and Jupiter's perihelion precesses only a little slower.
At one point, the two may fall into sync, at which time Jupiter's constant gravitational tugs could accumulate and pull Mercury off course with 1–2% probability, 3–4 billion years into the future. This could eject it from the Solar System altogether or send it on a collision course with Venus, the Sun, or Earth.

If inner system disruption doesn't occur, Mercury and Venus will likely get swallowed when the Sun expands. Earth may survive the Sun's early red giant phase, but eventually it too will probably succumb.
It's difficult to make solid predictions about such far future events. We don't know exactly how the Solar System will respond as the Sun sheds mass. By the time the Sun becomes a white dwarf it will have roughly 50% of the mass it had when it was a new main sequence star.

+1. I think this answer is better than mine because it's more comprehensive and detailed, and covers the high-velocity possibility that I ignored. (Not that I'm not happy with mine. I just think I got a first-mover advantage.)


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What would the collapse of a white dwarf to a neutron star look it
It’s hypothesised that some white dwarves, when raised in mass above the Chandrasekhar limit, will instead of producing a type 1A supernova, will instead collapse into a Neutron star.
What would this look like? How bright would it be? How strong would the Neutrino flux be?
Could you end up with a neutron star lighter then is lower possible limit for neutron stars produced by type II supernovae.

see https://astronomy.stackexchange.com/questions/14740/why-does-the-chandrasekhar-limit-affect-white-dwarfs-differently


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Is there a minimum size for a moon?
Is there a minimum size for a moon? Is there a lower bound below which it's simply considered as being a rock? Or could you can every piece of rock and Ice in Saturns rings as an individual moon.


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How to compute object counts in narrowband imaging?
How do we go about computing the object counts from an emission nebula in H alpha?
Do need to search the object in Vizier, get the reported FLux for the object in a catalog?..
Do I use synphot-astropy somehow?
The general formula I use for computing the counts from an extended object in typical BVRI filters from a known BVRI magnitude, is:
Targetelectrons = (Flux/PhotonEnergy) * QE * FilterBandwidth * EffectiveTelescopeAperture * imagescale^2

I suppose a similar formula will work for narrowband too, but there is no narrowband magnitude so that I can compute the Flux, hence my lack of understanding how this is done.
Update:
Could this be a valid approach for Flux?
# Retrieve Object H-alpha Flux from VizieR in erg/s/cm²/Å else do a conversion for the whatever flux units you find
h_alpha_flux = your_h_alpha_flux * units.erg / (u.s * u.cm**2 * u.Angstrom)

# Define H-alpha Wavelength
h_alpha_wavelength = 656.3 * u.nm

# Create filter for H-alpha
Hydrogen_filter = SpectralElement(Box1D, amplitude=1, x_0=6563, width=30)

# Create Source Spectrum for H-alpha
spectrum = SourceSpectrum(ConstFlux1D, wave=h_alpha_wavelength, flux=h_alpha_flux)

# Convolve Source Spectrum with Hydrogen_filter
observed_spectrum = Observation(spectrum, Hydrogen_filter)

# Integrate Over Wavelength to Calculate Photon Count Rate
integrated_flux = observed_spectrum.integrate(flux_unit=units.FLAM)


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How can I better align stamps from Hubble images?
I am trying to do difference imaging with Hubble ACS/WFC images and have been using TweakReg to align images and cutout stamps. The problem is that residuals haven't been great and I'm looking for ways to improve alignments. X and Y RMS stands at around 0.4 pix each. Is there a python module I can use to further align the cutout stamps?


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What is a "Gentle Giant" Black Hole?
I have seen articles about the Milky Way's "gentle giant" black hole. For example:

https://nypost.com/2022/05/12/image-of-gentle-giant-black-hole-at-milky-ways-center/
https://www.reuters.com/lifestyle/science/black-hole-scientists-announce-groundbreaking-milky-way-galaxy-discovery-2022-05-12/

Both of these articles say the black hole is not eating very much matter, which is the only clue I have from the articles.
A similar phrase, "gentle singularity," is used in the movie Interstellar. There has been some discussion about that, in which the consensus seems to be that the size of a supermassive black hole reduces the tidal forces at the event horizon, such that a falling entity would not be spaghettified. (I acknowledge that a movie quote is far from authoritative.)
So I have two hints at a definition of "gentle giant" black hole:

A black hole that consumes little matter.
A supermassive black hole whose tidal forces will not cause spaghettification.

Is either of these correct? If not, is there an authoritative definition of a "gentle giant" black hole?

It sounds very much like newspaper science popularization language, and probably means 1 in these cases, but all supermassive black holes do fall in category 2 too.

Black hole expert Kip Thorne was a consultant for Interstellar, but they had to "bend" the physics a little for storytelling purposes. See https://physics.stackexchange.com/q/148567/123208

There's no "authoritative" definition, this is just the common meaning of the word "gentle" used in a popular science and science fiction context.
A black hole might be gentle because the tidal forces at its horizon are gentle (supermassive black holes have gentle tides at their event horizon, but rapidly more extreme tides beyond the event horizon (and once you cross, you can't come back). Black holes without a hot accretion disc won't be emitting large amounts of harsh radiation, again relatively "gentle".
I can find one mention of "gentle black hole" in the literature: The quantum gravitational black hole is neither black nor white, which contrasts the stable (and hence "gentle") Schwarzschild state with a dynamic model of quantum gravitational collapse.
So in conclusion, no, there is no authoritative definition of "gentle" when applied to a black hole.


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Why is the Milky Way today about the same diameter as the visible universe was when recombination happened 378000 years after the Big Bang?
Has the Milky Way undergone an expansion of space time independent of the universe as a whole or did the gas that created the Milky Way heat up and expand? Whatever the answer, why the current diameter ?

Why is my car about the same length as an elephant?

They aren't the same size. The part of the Universe that is observable today was ~86 million lightyears in diameter at recombination. The observable Universe at that time was much smaller, but still ~1.7 million lightyears, more than a factor of 10 larger than the Milky Way.

The visible Milky Way is about 100,000 light years in diameter (Goodwin et al. 1998) - so no coincidence there. Of course, this is a bit of a "how long is a piece of string problem" - the "edge" of the Milky Way is not sharp and some stars can be found, certainly as far as 300,00 light years from the centre (Kafle et al. 2014).
If you include the dark matter halo, then it is possibly ten times bigger again, so I can't see any numerical coincidence.
Also note that the Milky Way is just one galaxy. Other galaxies can be bigger or smaller, even in the local group of galaxies. Thus, any quoted similarity between the size of the Milky Way and the light travel time at the epoch of recombination must just be a coincidence.
Gravitationally bound objects like the Milky Way do not expand with the universe.

The Baryonic portion of the Milky way (i.e not including dark matter) is 100,000 light years in diameter. The epoch of recombination, and the present galactic evolution's "finetuneness" is rather a coincidence. Big bang expanded rapidly due to dark energy i.e cosmological constant and inertia whereas the Milky way galaxy evolved slowly through mergers, formation of nebulae, primordial interstellar medium accumulating to form galaxies, so it is a mere coincidence. The present diameter is due to the amount of baryonic mass distribution (it has no galactic nucleus) it has swallowed, and that depends the rotational velocity which in turn concludes the diameter, other than things like the inertial speed of the gas when it started to accrete to form a proto galaxy.
Milky way has always been a condensed region ever since it started accretion, the cosmological constant was weaker at that time, so the Hubble velocity did not play a role in the galactic evolution. Nor did gas heat up, and  cause it to expand, as it solely relies on mergers, and especially the tidally interactions by the local group which was more closer at that time, had affected the arm rotational speed a little bit, and it would not heat up, rather it would cool down. However in few galaxies the quasars may actually play a role to expand but this is not the case for the Milky Way.
Thanks, hope it helps you!


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Is dark matter made from the particles that had charge but no mass in the early part of the Big Bang
It is assumed that massless charged  particles acquired mass after the early phase of the Big Bang. But could most charged particles be massless now as dark matter. They wouldn't interact with photons or electrons electromagnetically because electrons have mass and charge and photons are charge less massless Neither are charged massless

Dark matter particles (whatever they might be - axions, wimps something else...) must have mass, but no charge.  So the can't be particle with charge but no mass.

I would dispute the claim that dark matter particles have mass. The cold dark matter solution has problems associated with it such as too few dwarf galaxies. The hot model also has problems but doesn't as yet consider charged massless particles. If charged massless particles are like gluons (massless with colour charge) they could self interact and hold each other together.

But a key property of dark matter is that it has negligible self-interaction, apart from gravitation, which is due to mass. Haloes of such particles give the necessary mass distribution required to account for galaxy rotation curves. Charged massless particles would give a very different distribution.

What would be the point/need for massless "dark matter"?

see https://physics.stackexchange.com/questions/7905/massless-charged-particles

Charged, massless particles would be a form of "hot" dark matter. i.e. A form of energy density that was relativistic at the epoch of structure formation and of course, still relativistic now, since massless particles travel at the speed of light. That is not what is needed to explain structure formation in the universe or the dynamical properties of galaxies and galaxy clusters now, which need "cold" (non-relativistic) dark matter.
However, a more fundamental problem appears to be that massless charged particles should not exist. If they did then electrons and positrons would quickly decay into their massless counterparts whilst conserving charge. Massless charged particles should also be rather easy to produce in particle accelerators and none have been observed. Indeed it should even be possible for "pair production" just to take place with everyday (visible) photons.

(+1) Nice answer!

@ProfRob The charged massless particles would not have interacted with the Higgs field in the early universe for some reason .That reason may also be why electrons and protons now can't decay into them and why visible photons can't form charged pairs  today .I should think that failure to interact with the Highs means a particle is carrying a quantum number (property) that is incompatible with the Highs field theory

@DaveTheWave  I gave an answer based on physics that is known. If you postulate physics from another universe then anything is possible. The reason I have given is essentially why muons are not common, everyday particles - all particles will decay to their least massive manifestation that does not violate conservation laws.

Minor point: "hot dark matter" traditionally refers to stuff that is nonrelativistic today (hence "matter"), but transitioned relatively recently so that it has a long free-streaming length and low phase-space density.

No, because if "dark matter" had no mass, it wouldn't be dark matter anymore.
Keep in mind dark matter needs to exert gravitational force, and massless particles do not exert gravitational force (at least not in the same way massive particles do).
If dark matter actually did have no mass, it would become "dark radiation", which behaves in a fundamentally different way (see Wikipedia's technical definition section for dark matter). It's possible that a small amount of dark radiation exists, but not a lot of it, and certainly not anywhere near the amount of dark matter in the universe (source).

I don't agree that massless charged particles would behave like radiation . The charge makes them different to light  and the masslessness different to baryons so they  may not move at the speed of light or slower They must be faster than light! Dark matter is more complicated than we think.


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How do stars 1 billion times the volume of our sun form and can they become black holes?

UY Scuti is a star 1 billion times the volume of our sun.
A billion solar mass black hole formed 750 million years after the Big Bang.

Could it have formed from a star like UY Scuti?
Rare black hole 1 billion times the mass of the sun could upend our understanding of galaxy formation

are we talking about stars or black holes? The most massive star known to date has a mass of 226 solar masses (226 $M_\odot$). A star with a mass of 1 million solar masses would be something phenomenal https://en.wikipedia.org/wiki/List_of_most_massive_stars Overall, an interesting question!

"What is the biggest star in the universe? | Space" https://www.space.com/41290-biggest-star.html

Your question starts off talking about volume, then switches to mass. Which do you mean?

@Mark Morgan Lloyd I wrongly assumed mass was proportional to volume.

@DaveTheWave The volume of a neutron star of 1 billion solar masses isn't going to be the same as that of a supergiant ("red-hot fog"). Even if we believe that we understand all the relevant physical mechanisms, it's important to remember that these are subservient to observation. So it's entirely possible that a star of unexpectedly-large mass or diameter could be detected in a sky survey, hence the importance- I feel- of careful phrasing in this sort of question.

It is theoretically possible to have stars on the order of a million solar masses, but they are very short lived. https://en.wikipedia.org/wiki/Quasi-star

UY Scuti is a hypergiant star. The name refers to its size, not its mass. It may well have a billion(ish) times the volume of the Sun, but it is only about ten times the mass of the Sun.
A supergiant/hypergiant is an evolutionary phase that at least some massive (10-100 solar mass) stars go through, shortly before the ends of their lives. There is no special formation mechanism - the envelope inflates and becomes very tenuous as a result of the physics of what is going on inside the star - a more extreme version of the giant phase that even the Sun will eventually go through.
Supermassive black holes do not form directly from stars. There was no billion solar mass stellar progenitor. The formation mechanism is somewhat mysterious but likely involves the accretion of lots of gas or the merger of lots of black holes.

@Anders Sandberg
UY  Scuti (30 solar masses)  is expected to have high metallicity because it is in a high metallicity part of the Milky Way   It is curious that population 3 stars of similar mass to UY Scuti are said to have  low metal content . It seems to me that population 3 stars  may not have really existed.

@DaveTheWave Where did you find that 30 solar masses figure? Everything I've read says seven to ten solar masses.

Also bear in mind that to have one billion times the volume of the Sun, it "only" has 1000 times the radius (actually maybe more like 1700, that "one billion" is just the order of magnitude). 1000 solar radii is approximately the distance from the Sun to Jupiter. So, once you believe the Sun will inflate to approximately the Earth's orbit, then it's probably not such a surprise that other stars inflate a few multiples further.

@DaveTheWave: The reason population III stars are said to have low metal content, is that prior to population III stars it seems there was nothing to produce heavier elements. So I don't think it's really curious: population III stars were formed from H and He, they started producing heavier elements, therefore later stars (including UY Scuti) can have higher metallicity. Of course this is conjecture, but a difference in composition between early and late stars isn't a reason to doubt the existence of early stars: it's predicted by the theory.

@Steve Jessop Why would stars with different metallicity be of similar mass?  High metal stars expand and cool and can attract more gas than low metal stars which emit more heat and drive gas away .

@DaveTheWave: sorry, I don't know the details of the proposed formation of population III stars. Just from a Google search, https://www.aanda.org/articles/aa/full_html/2015/09/aa26617-15/aa26617-15.html talks about formation of large low-metallicity stars, and I think even without talking about population III it's not controversial that high-mass low-metallicity stars do exist. Quoting that introduction, "low-metallicity massive stars can also be found in the local Universe: some of the nearby dwarf galaxies form massive stars at a high rate (Tolstoy et al. 2009; Weisz et al. 2014)"

So, if you're going to say that pop III cannot have existed because large low-metallicity stars cannot exist, then you need some other way to explain what appear to be observations of large low-metallicity stars.

You are confusing volume with mass. While UY Scuti is indeed five billion times the size of the Sun in terms of volume, it is only seven to ten times the size of the Sun in terms of mass. That most likely is not massive enough to form a black hole at the time of its death. It will instead form a neutron star. That neutron star might combine with another to form a black hole, which might in turn combine with other stellar mass black holes to form intermediate mass black holes.
The formation of supermassive black holes is still a bit of a scientific mystery. That they exist is not a mystery. That they exist is a scientific fact.

You are right 7 to 10 solar masses I mixed up a volume with mass.

Small radiating black holes could have caused reionizamtion instead of population 3 stars.

Supermassive black holes are believed to form through several processes, like massive early (population III) stars massing up to a thousand or more solar masses, accretion in very dense environments, and mergers with other black holes. Early supermassive black holes are trickier to explain, but there may have been overdense regions of the early universe where dark matter halos got inside its own Schwarzschild radius and collapsed, or metal-free gas clouds that collapsed. But it is an active research area with many unknowns right now.

The formation of stars like UY Scuti and black holes with a billion solar masses involves different processes and timescales, and they are not directly related in the way one might initially think.
Formation of Massive Stars like UY Scuti
UY Scuti is a red supergiant star, and while it is indeed about a billion times the volume of our Sun, it is not a billion times the mass. Its mass is estimated to be around 7-10 times that of the Sun. Stars like UY Scuti form from massive molecular clouds in a process that involves the gravitational collapse of gas and dust. The mass of the star is determined by the initial conditions in the molecular cloud, including its density and temperature.
Formation of Billion Solar Mass Black Holes
Black holes that are a billion times the mass of the Sun are generally thought to form in the centers of galaxies and are the result of multiple processes, including the collapse of massive stars into smaller black holes, which then merge and accrete mass over time. The formation of such massive black holes so soon after the Big Bang is still a subject of research and debate among astrophysicists. One leading theory is that they form through the direct collapse of large gas clouds, bypassing the star formation stage entirely.
Could a Star Like UY Scuti Turn Into a Billion Solar Mass Black Hole?
The short answer is no. When UY Scuti exhausts its nuclear fuel, it will likely undergo a supernova explosion and may form a black hole. However, the resulting black hole would be much smaller, on the order of 10 solar masses, not a billion. The process of forming a billion-solar-mass black hole involves not just the end stages of one star's life but likely the complex interplay of many stars and other black holes, as well as the accretion of a large amount of matter over a long period.
Implications for Our Understanding of Galaxy Formation
The discovery of black holes with a billion solar masses so soon after the Big Bang does challenge our understanding of galaxy formation and black hole growth. It suggests that there are mechanisms for black hole formation and growth that are faster than those currently understood, which has implications for theories of cosmic structure formation.

@Heylts Tony I should think that black holes form quickly because of magnetic fields bringing them or the stars or gas that form them closer together . "impact of magnetic fields on cosmological galaxy mergers – II. Modified angular momentum transport and feedback | Monthly Notices of the Royal Astronomical Society | Oxford Academic" https://academic.oup.com/mnras/article-abstract/526/1/224/7263282?redirectedFrom=fulltext

HeyItsTony ... adding two spaces to the end of a line, followed immediately by an 'enter' will force a phrase/sentence to the next line. When you originally posted your answer, your header lines were commingled with the paragraphs... fixed and italicized.