@crypto (crypto) posted a new topic, subject "space - that final frontier". It read:

   questions
    
      1. If you believe in the big bang as the origin of all things out there in space, then everything should be moving away from everything else. If this is so, how is the Andromeda Galaxy heading toward the Milky Way?
    
      2. I found a projection of the collision between the Milky Way and Andromeda on line somewhere (sorry, can't remember where, but i'll keep looking), and the prediction is that not one star will collide with another star. Simply put, there would be no physical contact between any two stars of the two colliding galaxies.
    
    
    
    
    a) how can this be?  
    b) assuming this is true, how can they know that? I understand the vastness of space, but there would be between one and two billion stars or so heading toward each other. How could they pass without at least one star coming in contact with another?
    
    c) doesn't the contradiction of the collision of two galaxies (and there are pictures of more of them found by Hubble) negate the big bang theory?

@Asympotically_fat (Asympotically fat) replied

    Stink Fish Pot:
    
    > questions
    > 
    >   1. If you believe in the big bang as the origin of all things out there in space, then everything should be moving away from everything else. If this is so, how is the Andromeda Galaxy heading toward the Milky Way?
    > 
    >   2. I found a projection of the collision between the Milky Way and Andromeda on line somewhere (sorry, can't remember where, but i'll keep looking), and the prediction is that not one star will collide with another star. Simply put, there would be no physical contact between any two stars of the two colliding galaxies.
    > 
    > 
    
    > 
    > a) how can this be?  
    >  b) assuming this is true, how can they know that? I understand the vastness of space, but there would be between one and two billion stars or so heading toward each other. How could they pass without at least one star coming in contact with another?
    > 
    > c) doesn't the contradiction of the collision of two galaxies (and there are pictures of more of them found by Hubble) negate the big bang theory?
    
    No,  
    By Hubble's law (providing I've got my units correct) Andromeda has a recession velocity* of approximately 60 km/s from the Milky Way. This may seem like quite a lot, but it's only 0.0002c and, as in the case of Andromeda and Milky Way, is easily small enough to be compensated by the random motion of the Milky Way and Andromeda relative to the cosmological background frame. So Andormeda is moving towards us at about 100 km/s instead of away from us at 60 km/s  
    *Or more properly a cosmological comoving observer in the Andromeda galaxyhas a recession velocity of 60 km/s to a cosmological comoving obsever in the Milky Way and that's not even considering what effects local inhomogenity may have.

So @automagic (automagic) replied

    The vastness of space is beyond human comprehension. The stars are so far apart that the possibility of one star colliding with another is so small you can call it zero.
    
    The Milky Way is 100,000 light years across and Andromeda is not much bigger. Assuming each galaxy has 100 billion stars with the average size of the star being the size of our sun(1,000,000 miles across) and the average distance between the stars is 4 light years (distance to Alpha Centurai) then the ratio of the average distance between the stars and the average size of a star becomes about 23,000,000.
    
    Imagine you are walking blindfolded in an extremely large field full of people but the average distance between the people is 4,300 miles (23,000,000 feet) than what are the chances that any one is going to collide with anyone?

@Der_Trihs (Der Trihs) replied

    Stink Fish Pot:
    
    >   1. If you believe in the big bang as the origin of all things out there in space, then everything should be moving away from everything else. If this is so, how is the Andromeda Galaxy heading toward the Milky Way?
    > 
    
    
    Because local gravity and velocity are overcoming the general expansion of the universe among those particular objects. In other words, for much the same reason Earth doesn't explode from the expansion of the universe.
    
    Stink Fish Pot:
    
    > c) doesn't the contradiction of the collision of two galaxies (and there are pictures of more of them found by Hubble) negate the big bang theory?
    
    No. If during an explosion two chunks of shrapnel happen to collide with each other that doesn't negate the fact that the explosion is in fact expanding.

To which @Candyman74 (Candyman74) replied

    No. Simple common sense can tell you otherwise. No need to be an advanced physicist or mathematician.
    
    Expansion of space is at rate X. Gravitational attraction between objects is at rate Y.
    
    Where objects are close enough that Y is larger than X, net motion will be towards each other.
    
    Where objects are far enough apart that X is larger than Y, net motion will be apart.
    
    Most objects in the universe are far apart, so they recede from each other. Andromeda is really close, so it moves towards us.
    
    Simples. You should be able to reason this out yourself, as it requires no special knowledge.

@Candyman74 (Candyman74) replied

    As an addendum to my above post - actually, I guess it does require one piece of knowledge as well as one piece of statistical probability:
    
      1. the knowledge required is that gravity decreases by distance. I guess if you don't know that, you wouldn't reason out the conclusion. But then you should be bewildered by why the moon isn't landing on your head. Assuming you're not bewildered by the latter, one has to assume you know the former.
    
      2. the likelihood that you've stumbled on a a bit of logic which disproves everything physicists believe is very, very slim. I guess it's possible for geniuses to exist and disprove decades of solid science with a transient thought they had in the bath, but the odds of that being you are very unlikely. But we can all hope!

To which @crypto (crypto) replied

    These are my own pants:
    
    > No,
    
    There is a lot in my OP that you could say No to.  But allow me to take a stab at answering your posting of “No” and see if I at least got the correct item. Please see my answer to **Der Trihs** , below.
    
    automagic:
    
    > The vastness of space is beyond human comprehension. The stars are so far apart that the possibility of one star colliding with another is so small you can call it zero.
    > 
    > The Milky Way is 100,000 light years across and Andromeda is not much bigger. Assuming each galaxy has 100 billion stars with the average size of the star being the size of our sun(1,000,000 miles across) and the average distance between the stars is 4 light years (distance to Alpha Centurai) then the ratio of the average distance between the stars and the average size of a star becomes about 23,000,000.
    > 
    > Imagine you are walking blindfolded in an extremely large field full of people but the average distance between the people is 4,300 miles (23,000,000 feet) than what are the chances that any one is going to collide with anyone?
    
    Yes, I do get this, but I think you did a fabulous job of painting a mental picture that is pretty easy to see, so I think it's worth quoting in its entirety. I think what I have a difficult time wrapping my head around is that with all of these stars passing through each other, I find it very hard top believe that with the inevitable gravitational pull that the two galaxies will have on each other, no two stars (one from each galaxy) will occupy the same space at the same moment of time. Is this just a wag on the part of the cosmologists, or did they actually do the math to prove that statistically, at least, the odds fall outside the 2 degrees of freedom required to make it a reasonable assumption?
    
    Der_Trihs:
    
    > Because local gravity and velocity are overcoming the general expansion of the universe among those particular objects. In other words, for much the same reason Earth doesn't explode from the expansion of the universe.
    > 
    > No. If during an explosion two chunks of shrapnel happen to collide with each other that doesn't negate the fact that the explosion is in fact expanding.
    
    Are you saying that over time, Andromeda's celestial “speed” has slowed with respect to its neighbor (MWG)? So, it is still moving away from us, but instead of 65 MPH it's now at 35 MPH, permitting MWG to “gain” on AG? (yes, I know the 65 and 35 MPH are farcical units of measurements. I used them for simplicity of reference). In other words, the AG and the MWG aren't nearing each other… the MWG is overtaking the AG?

@automagic (automagic) replied

    Stink Fish Pot:
    
    > Yes, I do get this, but I think you did a fabulous job of painting a mental picture that is pretty easy to see, so I think it's worth quoting in its entirety. I think what I have a difficult time wrapping my head around is that with all of these stars passing through each other, I find it very hard top believe that with the inevitable gravitational pull that the two galaxies will have on each other, no two stars (one from each galaxy) will occupy the same space at the same moment of time. Is this just a wag on the part of the cosmologists, or did they actually do the math to prove that statistically, at least, the odds fall outside the 2 degrees of freedom required to make it a reasonable assumption?
    
    I did leave out a key piece of information. The strength of gravity weakens exponentially as the distance from the source of gravity is larger. The sources of gravity in this case are the stars. Since the average distance is so large between them and their relative motions are so great (120 km/s*) then gravity simply does not have the strength to pull individual stars together, but that does not mean there is no effect. If there was no effect then the stars would just pass right through each other. The effect is that the trajectories or paths of individual stars are thrown off but because of the distances collisions will be extremely unlikely.
    
      * http://en.wikipedia.org/wiki/Andromeda–Milky_Way_collision#Stellar_Collisions
    
    
    
    I do not know the equations on how to figure all this out but in my limited knowledge of astrophysics? I believe this is the right answer.

@Candyman74 (Candyman74) replied

    Stink Fish Pot:
    
    > Are you saying that over time, Andromeda's celestial “speed” has slowed with respect to its neighbor (MWG)? So, it is still moving away from us, but instead of 65 MPH it's now at 35 MPH, permitting MWG to “gain” on AG? (yes, I know the 65 and 35 MPH are farcical units of measurements. I used them for simplicity of reference). In other words, the AG and the MWG aren't nearing each other… the MWG is overtaking the AG?
    
    In the hope that you won't refer to me by some bizzare name related to pants (what the hell was that about?):
    
    Expansion of the universe is currently 50mph (and accelerating).
    
    Gravitational attraction is 100mph.
    
    So the galaxies move towards each ruben88@example.org.
    
    The further apart objects are, the lower the gravitational force. So an object much further away than that galaxy would be using the figures of Expansion 50mph and gravitational attraction 5mph, resulting in them moving away from each other at net 45mph.
    
    Farcical numbers, of course.
    
    Add in the complication that the 50mph expansion is accelerating (so it becomes 51mph, 52mph, 60mph, 100mph, 200mph, 500mph… endlessly increasing) while the gravitational force stays the same. So eventually - in time - the expansion will override the stronger gravitational forces resulting in a very sparse and empty universe.

To which @Bearflag70 (Bearflag70) replied

    Candyman74:
    
    > Expansion of space is at rate X. Gravitational attraction between objects is at rate Y.
    > 
    > Where objects are close enough that Y is larger than X, net motion will be towards each other.
    
    Andromeda is like a plane on a treadmill.

After which, @what_do_I_type_here (what do I type here) added

    Candyman74:
    
    > In the hope that you won't refer to me by some bizzare name related to pants (what the hell was that about?):  
    >  .
    
    I…I think I'm about to get whooshed but he was talking to a poster named **These are not my pants**
    
    I just got whooshed didn't I.

To which @The_Hamster_King (The Hamster King) replied

    The entire Local Group of galaxies is gravitationally bound together. They're close enough to each other, and massive enough, that their gravitation overcomes the metric expansion of space.
    
    However, other clusters of galaxies that are farther away than the Local Group are receding from us. The farther away they are, the faster they are receding.

Then @Qwakkeddup (Qwakkeddup) said

    No cosmological scientist here, but I think it is just a big WAG.  
    There are two extremely large bodies moving together, with lots ‘n' lots of Shtuff in them.  
    It may well be that no two stars will collide head on, there are asteroids, dust, small dwarf planets, lots of Shtuff. In my mind that means there will be collisions.
    
    Point one, the scientific machine use to say that there is no chance of anything big hitting the earth, nowadays they Know of several large impacts, and have witnessed Jupiter's bashing by the Shoemaker–Levy comet. Now they only contest it is very unlikely to happen in Man's reign on Earth.
    
    There will likely not be anyone around to witness the galaxies colliding and even if there were, what could anyone do about it. If it were going to happen inside of 100 or 1000 years, they would be speculating, taking pictures, measurements, and bets with outrageous odds.
    
    Chaos will do as it wants, not as the scientist or any other learned man would tell it.

@cmyk (cmyk) replied

    Qwakkeddup:
    
    > Point one, the scientific machine use to say that there is no chance of anything big hitting the earth<schnip>
    
    Cite?

To which @Triskadecamus (Triskadecamus) replied

    I don't think anyone has come up with a scientifically promulgated theory that stellar collisions are impossible during galactic collisions. But the observed interactions of galaxies that appear to be colliding, or have collided seem to support the view that such events are as rare as the statistical likelihood predicts.
    
    When a group of objects with diameters of thousands or even millions of kilometers, separated by distances measured in light years pass through each other at speeds several orders of magnitude higher than the gravitational acceleration possible between them over the duration of the probable galactic transit there is a highly reliable probability that collision events between stars will be extraordinarily rare. Probably rare enough to be considered unlikely for even one to occur. That doesn't mean it cannot happen. I might get nominated by the Iowa caucus as the Republican candidate for the presidency, twenty or thirty times during this century. No one expects it, though.
    
    A billion years later, though, lots of new stars will be formed by the colliding wave fronts of clouds of gas flung about by the gravity of the two galaxies. And it seems that nearly undetectable regions with large concentrations of dark matter will be flung about as well. Or maybe not.
    
    It's a zoo out there.
    
    Tris

@Candyman74 (Candyman74) replied

    what_do_I_type_here:
    
    > I…I think I'm about to get whooshed but he was talking to a poster named **These are not my pants**
    > 
    > I just got whooshed didn't I.
    
    No - it was my fault. I was on my phone, and somehow didn't see any of the other posts. I can see them now, and they make my post make no sense whatsoever!

To which @njtt (njtt) replied

    Candyman74:
    
    > No. Simple common sense can tell you otherwise. No need to be an advanced physicist or mathematician.
    > 
    > Expansion of space is at rate X. Gravitational attraction between objects is at rate Y.
    > 
    > Where objects are close enough that Y is larger than X, net motion will be towards each other.
    > 
    > Where objects are far enough apart that X is larger than Y, net motion will be apart.
    > 
    > Most objects in the universe are far apart, so they recede from each other. Andromeda is really close, so it moves towards us.
    > 
    > Simples. You should be able to reason this out yourself, as it requires no special knowledge.
    
    I believe that, in general, whether galaxies are moving together or apart has much more to do with the random momentums (momenta?) that they have acquired over their history than with any specific gravitational attraction between the two objects in question. An astrophysicist friend once described the motions of galaxies to me as [closely analogous to] the random motions of molecules in a gas.
    
    I suppose that gravity must have played a large role in (and may perhaps have been entirely responsible for) how those momenta were acquired in the first place (largely soon after the big bang, when everything was much closer together, and so interacting gravitationally much more stringly), but, with the universe now as spread out as it is, I think the effects of current local gravitational interactions between galaxies are usually trivial compared to the effects of pre-existing momentum.

Then @Candyman74 (Candyman74) said

    njtt:
    
    > I believe that, in general, whether galaxies are moving together or apart has much more to do with the random momentums (momenta?) that they have acquired over their history than with any specific gravitational attraction between the two objects in question. An astrophysicist friend once described the motions of galaxies to me as [closely analogous to] the random motions of molecules in a gas.
    > 
    > I suppose that gravity must have played a large role in (and may perhaps have been entirely responsible for) how those momenta were acquired in the first place (largely soon after the big bang, when everything was much closer together, and so interacting gravitationally much more stringly), but, with the universe now as spread out as it is, I think the effects of current local gravitational interactions between galaxies are usually trivial compared to the effects of pre-existing momentum.
    
    I don't have a cite handy, but I'm 99.999999% sure that the general view is that the galaxies in our local cluster will all eventually combine due to gravitational attraction.

Then @ZenBeam (ZenBeam) said

    The Wikipedia pages on _Galaxy groups and clusters_ and on _Superclusters_ are worth reading.
    
    From the intro in the first link:
    
    > Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. […] Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.
    
    and from the second:
    
    > Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.
    
    Andromeda is in the Local Group, and it and the Milky Way are gravitationally bound.

After which, @Grey (Grey) added

    Lets say the rough spacing of stars is 1 in a sphere with a radius of 4 light years.
    
    Volume - 3.546784408363E+48 m3  
    Volume of a star with a radius 4 times that of our sun - 1.409223939637E+27 m3  
    V of Sun/V of Space Sun resides in = 3.97324387 × 10-22
    
    So yeah, there's a fair bit of space in space.

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