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Dense stellar clusters may foster black hole megamergers
Black holes in these environments could combine repeatedly to form objects bigger than anything a single star could produce.
Jennifer Chu
Jennifer Chu | MIT News Office
When LIGO's twin detectors first picked up faint wobbles in their respective, identical mirrors, the signal didn't just provide first direct detection of gravitational waves — it also confirmed the existence of stellar binary black holes, which gave rise to the signal in the first place.
Stellar binary black holes are formed when two black holes, created out of the remnants of massive stars, begin to orbit each other. Eventually, the black holes merge in a spectacular collision that, according to Einstein's theory of general relativity, should release a huge amount of energy in the form of gravitational waves.
Now, an international team led by MIT astrophysicist Carl Rodriguez suggests that black holes may partner up and merge multiple times, producing black holes more massive than those that form from single stars. These "second-generation mergers" should come from globular clusters — small regions of space, usually at the edges of a galaxy, that are packed with hundreds of thousands to millions of stars.
"We think these clusters formed with hundreds to thousands of black holes that rapidly sank down in the center," says Carl Rodriguez, a Pappalardo fellow in MIT's Department of Physics and the Kavli Institute for Astrophysics and Space Research. "These kinds of clusters are essentially factories for black hole binaries, where you've got so many black holes hanging out in a small region of space that two black holes could merge and produce a more massive black hole. Then that new black hole can find another companion and merge again."
If LIGO detects a binary with a black hole component whose mass is greater than around 50 solar masses, then according to the group's results, there's a good chance that object arose not from individual stars, but from a dense stellar cluster.
"If we wait long enough, then eventually LIGO will see something that could only have come from these star clusters, because it would be bigger than anything you could get from a single star," Rodriguez says.
He and his colleagues report their results in a paper appearing in Physical Review Letters.
</div> </div> <div class="caption"> <div class="text"> <p class="descr"><em> </em></p> <div class="image-caption"><em><span>Black holes may partner up and merge multiple times, producing black holes more massive than those that form from single stars.</span></em></div> <em> </em></div> </div> </div> </div> </div> <h2>Running stars</h2> <p>For the past several years, Rodriguez has investigated the behavior of black holes within globular clusters and whether their interactions differ from black holes occupying less populated regions in space. </p> <p>Globular clusters can be found in most galaxies, and their number scales with a galaxy's size. Huge, elliptical galaxies, for instance, host tens of thousands of these stellar conglomerations, while our own Milky Way holds about 200, with the closest cluster residing about 7,000 light years from Earth.</p> <p>In their new paper, Rodriguez and his colleagues report using a supercomputer called Quest, at Northwestern University, to simulate the complex, dynamical interactions within 24 stellar clusters, ranging in size from 200,000 to 2 million stars, and covering a range of different densities and metallic compositions. The simulations model the evolution of individual stars within these clusters over 12 billion years, following their interactions with other stars and, ultimately, the formation and evolution of the black holes. The simulations also model the trajectories of black holes once they form.</p> <p>"The neat thing is, because black holes are the most massive objects in these clusters, they sink to the center, where you get a high enough density of black holes to form binaries," Rodriguez says. "Binary black holes are basically like giant targets hanging out in the cluster, and as you throw other black holes or stars at them, they undergo these crazy chaotic encounters."</p> <h2>It's all relative</h2> <p>When running their simulations, the researchers added a key ingredient that was missing in previous efforts to simulate globular clusters. </p> <blockquote> <p>"What people had done in the past was to treat this as a purely Newtonian problem," Rodriguez says. "Newton's theory of gravity works in 99.9 percent of all cases. The few cases in which it doesn't work might be when you have two black holes whizzing by each other very closely, which normally doesn't happen in most galaxies."</p> </blockquote> <p>Newton's theory of relativity assumes that, if the black holes were unbound to begin with, neither one would affect the other, and they would simply pass each other by, unchanged. This line of reasoning stems from the fact that Newton failed to recognize the existence of gravitational waves — which Einstein much later predicted would arise from massive orbiting objects, such as two black holes in close proximity.</p> <blockquote> <p>"In Einstein's theory of general relativity, where I can emit gravitational waves, then when one black hole passes near another, it can actually emit a tiny pulse of gravitational waves," Rodriguez explains. "This can subtract enough energy from the system that the two black holes actually become bound, and then they will rapidly merge."</p> </blockquote> <p>The team decided to add Einstein's relativistic effects into their simulations of globular clusters. After running the simulations, they observed black holes merging with each other to create new black holes, inside the stellar clusters themselves. Without relativistic effects, Newtonian gravity predicts that most binary black holes would be kicked out of the cluster by other black holes before they could merge. But by taking relativistic effects into account, Rodriguez and his colleagues found that nearly half of the binary black holes merged inside their stellar clusters, creating a new generation of black holes more massive than those formed from the stars. What happens to those new black holes inside the cluster is a matter of spin.</p> <blockquote> <p>"If the two black holes are spinning when they merge, the black hole they create will emit gravitational waves in a single preferred direction, like a rocket, creating a new black hole that can shoot out as fast as 5,000 kilometers per second — so, insanely fast," Rodriguez says. "It only takes a kick of maybe a few tens to a hundred kilometers per second to escape one of these clusters."</p> </blockquote> <p>Because of this effect, scientists have largely figured that the product of any black hole merger would get kicked out of the cluster, since it was assumed that most black holes are rapidly spinning.</p> <p>This assumption, however, seems to contradict the measurements from LIGO, which has so far only detected binary black holes with low spins. To test the implications of this, Rodriguez dialed down the spins of the black holes in his simulations and found that in this scenario, nearly 20 percent of binary black holes from clusters had at least one black hole that was formed in a previous merger. Because they were formed from other black holes, some of these second-generation black holes can be in the range of 50 to 130 solar masses. Scientists believe black holes of this mass cannot form from a single star.</p> <p>Rodriguez says that if gravitational-wave telescopes such as LIGO detect an object with a mass within this range, there is a good chance that it came not from a single collapsing star, but from a dense stellar cluster.</p> <div class="video-callout-placeholder" data-slug="bill-nye-on-black-holes-and-wormholes" style="border: 1px solid #ccc;"> <div class="rm-shortcode" data-media_id="y3qfVcBK" data-player_id="FvQKszTI" data-rm-shortcode-id="GRGANM1563462315"> <div id="botr_y3qfVcBK_FvQKszTI_div" class="jwplayer-media" data-jwplayer-video-src="https://content.jwplatform.com/players/y3qfVcBK-FvQKszTI.js"> <img src="https://cdn.jwplayer.com/thumbs/y3qfVcBK-1920.jpg" class="jwplayer-media-preview" /> </div> <script src="https://content.jwplatform.com/players/y3qfVcBK-FvQKszTI.js"></script> </div> </div> <p>"My co-authors and I have a bet against a couple people studying binary star formation that within the first 100 LIGO detections, LIGO will detect something within this upper mass gap," Rodriguez says. "I get a nice bottle of wine if that happens to be true."</p> <p><span>Reprinted with permission of </span><a href="http://news.mit.edu/" target="_blank">MIT News</a></p>
Charles Koch Foundation
Former president of the ACLU Nadine Strossen discusses whether our society should always defend free speech rights, even for groups who would oppose such rights.
Nadine Strossen
Former ACLU president Nadine Strossen understands that protecting free speech rights isn't always a straightforward proposition.
In this video, Strossen describes the reasoning behind why the ACLU defended the free speech rights of neo-Nazis in Skokie, Illinois, 1977.
The opinions expressed in this video do not necessarily reflect the views of the Charles Koch Foundation, which encourages the expression of diverse viewpoints within a culture of civil discourse and mutual respect.
politics debate activism speech society language law
Harness the Power of Calm
Tap into the "Rest and Digest" System to Achieve Your Goals
Emma Seppälä
In the fast-paced workplaces and productivity-focused societies many of us inhabit today, it is easy to burnout.
Emma Seppälä, a Stanford researcher on human happiness, recommends tapping into the parasympathetic nervous system instead—"rest and digest"rather than "fight or flight."
Aiming for energy management rather than time management will give you the resilience you need to excel at the things that really matter in your life and career, rather than living "mostly off" by attempting to seem "always on."
Apple co-founder says we should all ditch Facebook — permanently
Steve Wozniak doesn't know if his phone is listening, but he's minimizing risks.
Derek Beres
Photo by Bryan Steffy/Getty Images
Steve Wozniak didn't hold back his feelings about the social media giant when stopped at an airport.
The Apple co-founder admitted that devices spying on his conversations is worrisome.
Wozniak deleted his Facebook account last year, recommending that "most people" should do the same.
Steve Wozniak Warns People to Get Off Facebook Over Privacy Concerns
<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="HGYB931563396188"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/2XLnnLyx7nQ?rel=0?enablejsapi=1" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span><p>The reporter never got that deep into the rabbit hole, though Wozniak sums up his privacy concerns:</p><p style="margin-left: 20px;">"I worry because you're having conversations that you think are private, you're saying words that shouldn't be listened to because you don't expect it, but there's almost no way to stop it. People think they have a level of privacy they don't." </p><p>A <a href="https://www.pewinternet.org/2014/11/12/public-privacy-perceptions/" target="_blank">2014 Pew poll</a> paints a different landscape: 91 percent of Americans either "agree" or "strongly agree" that we've lost control over our personal information to social media companies. Eighty percent of users worry how governments and businesses will use those data. </p><p>A <a href="https://www.pewinternet.org/2017/01/26/americans-and-cybersecurity/pi_01-26-cyber-00-02/" target="_blank">2017 Pew poll</a> complemented that poll perfectly: only 9 percent of Americans feel "very confident" that said companies protect their data. That's the same percentage of people that believe they have "a lot of control" over their information. Chalk up 9 percent to the clueless. </p><p>Yet we stay logged in. While a <a href="https://www.pewinternet.org/2018/03/01/social-media-use-in-2018/" target="_blank">2018 Pew poll</a> on social media discovered that 59 percent of Americans claim it would "not be difficult" to abandon these platforms, many do not. In 2017, the same agency <a href="https://www.pewinternet.org/2017/08/10/theme-3-trust-will-not-grow-but-technology-usage-will-continue-to-rise-as-a-new-normal-sets-in/" target="_blank">found</a> that even as public trust in social media companies lessens, more people are plugging into this "new normal." We know it's happening; we just refuse to give up our apps. The question remains: Do the benefits outweigh the costs? </p>
<div class="ad-tag"> <div class="ad-tag__inner"> <div class="gallery_ad" id="div-bt_gallery1"></div> </div> </div>
<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8xOTkzMDc5Ni9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTU2MzkwNjgyMH0.bH1BiCLBSrJXhgk57SwuBM6AQ50fPq_z8kshsGw94RM/img.jpg?width=1245&coordinates=0%2C59%2C0%2C15&height=700" id="a508e" class="rm-shortcode" data-rm-shortcode-id="AC07DG1563396188" data-rm-shortcode-name="rebelmouse-image" />
Photo by Andrew Matthews/PA Images via Getty Images
Recent news revealed that Amazon employees are listening in to private conversations via Alexa.
<p>Wozniak deleted his Facebook account in 2018; he is still very active <a href="https://twitter.com/stevewoz" target="_blank">on Twitter</a>. As with many tech companies, Facebook advertises its product as "free." Obviously, that's not the case. </p><p>Wozniak floats the notion of paying for privacy, though as a <a href="https://www.eurekalert.org/pub_releases/2019-01/uov-sof011819.php" target="_blank">2019 study</a>, published in <em>Nature Human Behavior</em>, states, we might well be beyond that point already. The early promise of the internet of an "interconnected world" has truly arrived, for better and mostly worse. The research team behind the study analyzed over 30 million posts by nearly 14,000 Twitter users.</p><p style="margin-left: 20px;">"With this data, they showed that information within the Twitter messages from 8 or 9 of a person's contacts make it possible to predict that person's later tweets as accurately as if they were looking directly at that person's own Twitter feed."</p><p>Blockchain solutions are touting better privacy controls on data, but we have not reached that point — nor are we sure these companies will be as benevolent as claimed once their tech scales, a <a href="https://cointelegraph.com/news/buterin-proposes-bitcoin-cash-integration-to-scale-ethereum-in-short-term" target="_blank" rel="noopener noreferrer">persistent problem</a> with these platforms. Hopes of a truly decentralized infrastructure are turning out to be more philosophical than realistic, though that doesn't mean advances in privacy protection won't arrive. It will likely just not be a romantic as promised. </p><p>Even if we're connected to others online, there are levels of control we can initiate. The simplest would be deleting Facebook (or another other platform you're concerned about). It seems to be working out for the Apple co-founder. Where this conversation leads, however, is anyone's guess. </p><p>--</p><p><em>Stay in touch with Derek on <a href="http://www.twitter.com/derekberes" target="_blank">Twitter</a> and <a href="https://www.facebook.com/DerekBeresdotcom" target="_blank">Facebook</a>.</em></p>
social media blockchain advertising big problems
Where the evidence of fake news is really hiding
When it comes to sniffing out whether a source is credible or not, even journalists can sometimes take the wrong approach.
Eli Pariser
We all think that we're competent consumers of news media, but the research shows that even journalists struggle with identifying fact from fiction.
When judging whether a piece of media is true or not, most of us focus too much on the source itself. Knowledge has a context, and it's important to look at that context when trying to validate a source.
media politics data internet journalism
New alternative to Trump's wall would create jobs, renewable energy, and increase border security
Elon Musk's Neuralink unveils device to connect your brain to a smartphone
The most culturally chauvinist people in Europe? Greeks, new research suggests.
​Scotland is generating so much wind energy, it could power all of its homes — twice over
Scientist's accidental discovery makes coral grow 40x faster
A belief in meritocracy is not only false: it's bad for you