Dense stellar clusters may foster black hole megamergers

Dense stellar clusters may foster black hole megamergers
A snapshot of a simulation showing a binary black hole formed in the center of a dense star cluster. Credit: Northwestern Visualization/Carl Rodriguez

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 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 .

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 . 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 .

"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.

Running stars

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.

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.

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.

Dense stellar clusters may foster black hole megamergers
A simulation showing an encounter between a binary black hole (in orange) and a single black hole (in blue) with relativistic effects. Eventually two black holes emit a burst of gravitational waves and merge, creating a new black hole (in red). Credit: Massachusetts Institute of Technology

"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."

It's all relative

When running their simulations, the researchers added a key ingredient that was missing in previous efforts to simulate globular clusters.

"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."

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.

"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."

The team decided to add Einstein's into their simulations of . 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 , creating a new generation of black holes more massive than those formed from the . What happens to those new black holes inside the cluster is a matter of spin.

"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."

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.

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 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.

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.

"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."

Explore further

Gravitational waves created by black holes in the centre of most galaxies

More information: Carl L. Rodriguez et al. Post-Newtonian Dynamics in Dense Star Clusters: Highly Eccentric, Highly Spinning, and Repeated Binary Black Hole Mergers, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.151101 ,
Journal information: Physical Review Letters

Citation: Dense stellar clusters may foster black hole megamergers (2018, April 10) retrieved 22 August 2019 from
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Apr 11, 2018
If science does not know the sequence of the formation of celestial bodies, including black holes, how can they make conclusions and give "evidence" of the characteristics and the emergence of black holes. All these are the consequences of ignorance of the structure of the universe. Science has not yet understood and found out what gravity is and how it originates, but uses Einstein's fatamorgans and deceives both itself and civilization with unnatural "evidence" based on these fatamorgans. Gravity waves do not exist, because gravity is not what Einstein claims. The two-hole holes can not collide, because the black hole does not arise in the way the doctrine interprets.

May 12, 2018
Removal of some 5-10 mistakes of Einstein's cosmic model of Riemann 4-sphere and similar rethinking of quantum theories in the new structural and energy-balance model can explain the local energy balance of 'indirect' Hulse-Taylor evidence of assumed Gravitational Waves (GW) in the inspiraling stage. The merge and ring-down phases take place according to the Mach energy principle - very little energy is left over in the form of local tidal waves for detection as GW by the present design of LIGO or LISA after their precursor and echo correlation problem is understood and corrected.

May 13, 2018
Gravitation is not what science claims. Everything on that basis that science propagates about gravity and GW is a big mistake and therefore there is no visible evidence. What is used with the LIGO experiment has nothing to do with gravity. GW does not exist because gravity has nothing to do with either the properties of matter or the possibilities of energy states and matter and energy.

May 13, 2018
Gravitation is not what science claims. Everything on that basis that science propagates about gravity and GW is a big mistake and therefore there is no visible evidence. What is used with the LIGO experiment has nothing to do with gravity. GW does not exist because gravity has nothing to do with either the properties of matter or the possibilities of energy states and matter and energy.

The same mistakes of outdated relativity and quantum theories caused the 'proven' (2011 Nobel rewarded) explanation of SN1a data in terms of Dark Energy/Matter densities in accelerated (vs. correct decelerating) expansion of the 4-radius R4. These mistakes are coming from misunderstanding the concept 'unbiased estimation'. They have confused the cosmologists since the times of Earth vs. Sun or Big Bang centered universe 400-500 years ago. Similar confusion took place in research of general matrix and tensor calculus of inverse theories, still not fully understood in physics today.

Jun 06, 2018
The writers and readers of this article should also read 'Suntola Dynamic Universe' explanation of the BH and how its captures the surrounding matter including its binary partner into a stable orbit using the DU energy balance principle without some 5-10 postulates of traditional GRT and QM theories. Before reading, open your mind by ignoring most postulates of GRT and QM of the past 100 years and pay no attention to the 1993, 2011 and 2017 Nobel 'confirmations' of cosmological truths about 'indirect GW proof', Dark Energy proof of SN1a and 'direct detection of GW'.

Jun 07, 2018
I'm glad, for the first time I see that there is someone who has realized that many of the previous theories have been misunderstood. You're right.
Today's science gives thousands of theories and "proofs" of many phenomena in the universe, yet it has not yet understood what matter is and what it forms. And worse is the situation of not knowing the cause of the emergence of gravity and magnetism, as two basic factors, without which nothing would be formed in the universe, but, of course, there must be some substance from which matter is formed.
Who logically contemplates and respects natural laws, it rejects the following illogicalities that contaminate science in its advancement: the phenomenon of BB, the spread of the universe, the existence of dark matter and dark energy, the existence of GW, the curvature of space time, Lorenz transformation, the theory of relativity and everything derived from these fatamorgana.

Jun 07, 2018

I am marveling about the conspiracies of nature to fool the mankind in several branches of science in the past history. Suntola's lifetime DU work from top-to-down energy structure had to remove most postulates of traditional relativity and quantum theories although both theories were 'proved' to provide correct predictions to observables - in the same sense as the Earth centered model with auxiliary epicycles provides nearly equal predictions to Copernicus structural or Sun centered model. So, how could Einstein and inventors of standard cosmic model repeat the same mistake? The M-M proof of constancy of speed of light C in EM bound interferometer was fooled by the nature by keeping the measured distance constant but by slowing down the ticking rate of clocks linearly with the true variable value of C. You appear working in the reverse down-to-top direction of energy frames like Prof Lehto of Suntola PFS team in rethinking particle physics. You are on right track!

Jun 08, 2018
Today's science does not know the structure of the universe and, on the basis of illogical assumptions, makes new theories that try to tackle mathematics and models. Many scientists think that mathematics is more powerful than consciousness and intuition. Therefore, the present science in this field is a kind of religion, because it believes more in formulas and models than in natural laws.
The universe must be filled with the substance from which matter is formed. But it is logical that this is a renewable process in a closed recovery cycle. The substance returns to the shape of the substance through black holes. It is a process where gravity is "in charge" to do so. But in this chain of processes there is a sequence of the formation of celestial bodies from the subatomic particles to the supernova itself, when it explodes, the chemical elements, gases, and the formation of star systems all the way to the cluster of galaxies.

Jun 08, 2018
This process of science does not know and therefore invents many false "proofs", and therefore it can never come to a final understanding of the true causes of the phenomenon.

Jun 09, 2018
I am awed about the apparent mathematical analogy of human eye-brain/memory structure to the parallel processing principle of array algebra & calculus - and about Suntola's DU presentation of the dynamic 4-sphere energy balance equations in terms of Euler complex numbers. I used the same technique in array derivation of the fast transform formulations of Fourier, discrete cosine and Karhunen-Loeve 1-D transforms and in their multi-linear expansions to higher dimensions. Like milnik and Suntola examples, the most powerful but painful process is acquiring the intuition and hands-on concrete examples of 'thought experiments' that can then be converted into 'sales demoes' of computer simulations. Otherwise it is impossible to convince 'religion based' pals to adopt your arguments until small 'story' pieces of them start making sense to next generation of scientists - if you are able to present your thoughts in some visible format like today's communications before your death.

Jun 09, 2018
Consciousness with intuition is the basis without which it can not come to any true cause. When a problem is consciously and intuitively examined, then there is no interference to prove it mathematically. Tycoon science should be changed and each state must have a way of financing the publication of scientific achievements. Current journals and academies are the greatest truth-tellers, because they are interested in how they will charge from anyone, any publication, but they are not interested in the scientific value of the document. That's why I'm waiting for that time to come and I will start publishing my intuitive findings using mathematics and geometry.

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