# Is there an upper limit on the mass of black hole mergers we can detect?

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From the LIGO website, black hole mergers have been observed between black holes with a mass up to roughly 50 $$M_odot$$.

Are there no black holes with a mass above 100 $$M_odot$$ or is this an observational bias? Why haven't we observed any mergers between black holes with a mass in the 100 - 1000 $$M_odot$$ range?

It is quite likely there is an astrophysical upper limit to the mass of a black hole that can be produced during the core collapse of a massive star, caused by the pair instability supernova phenomenon. There isn't an observational bias against detecting more massive black holes in the range of 100 to a few hundred $$M_{odot}$$.

Details:

The frequency of the gravitational waves is twice the orbital frequency of the binary system. The general scenario is that early in their evolution, a mrging binary system will be orbiting at relatively long periods (seconds !), but as gravitational waves take energy out of the orbit, the system becomes more compact, the orbital period gets smaller and the frequency of the emitted gravitational waves goes up. This continues until the black holes get so close together that their event horizons merge.

Very roughly, we can derive (from Kepler's third law, not going into detail), using Keplerian orbits $$f_{ m max} sim left( frac{GM}{pi^2 a_{ m merge}^3} ight)^{1/2} ,$$ where $$f_{ m max}$$ is the peak frequency at merger (when the gravitational wave signal is also maximised), $$a_{ m merge}$$ is the separation of the masses at merger and $$M$$ is the total mass of both black holes.

If we let $$a_{ m merge} sim 2GM/c^2$$, the sum of the two Schwarzschild radii of the black holes, then $$f_{ m max} sim frac{c^3}{GM} left( frac{1}{8pi^2} ight)^{1/2} sim 2 imes 10^4 left(frac{M}{M_{odot}} ight)^{-1} { m Hz}$$

Now, LIGO is limited to observing frequencies above about 20 Hz. The sensitivity drops off rapidly below that because of seismic noise and other factors. If the mass of the merging black holes exceeds some critical value then the frequencies of the gravitational waves they produce never get into the sensitivity wndow of LIGO. Using the expression above, we can estimate that this happens only if the total mass exceeds $$1000 M_{odot}$$. Observing the mergers of more massive black holes would require a detector that is sensitive to lower frequencies, probably beyond the surface of the Earth (e.g. LISA).

This calculation is only good to a factor of 2 or so, but we can check it. GW150914 had a total mass of around $$65 M_{odot}$$ and merged at frequencies of about 120 Hz. Since $$f_{ m max}$$ scales as $$M^{-1}$$ this suggests 360 solar mass mergers should be just about detectable, but clearly demonstrates that LIGO could detect black holes of 100-200 solar masses. What's more, at a given distance and frequency, the signals from such mergers would be more powerful than for less massive black holes -- something like $$h propto M^{5/3}$$, which means the volume in which the mergers would be visible goes as $$M^5$$. Thus more massive black hole binaries would have to be extremely rare in order to have evaded detection.

The astrophyscal reason for an upper limit is the pehenomenon of pair instability supernovae (e.g. Farmer et al. 2019), which blows the star apart rather than leaving a black hole (or any other kind) of remnant. This likely happens for stars with initial masses of $$130+ M_{odot}$$, and means that leaving behind black holes with $$M > 50M_{odot}$$ is very difficult, with even lower mass limits for stars with a metallicity more similar to the Sun, since they lose more mass in stellar winds during their lives.

For initial masses of $$250+ M_{odot}$$ it is possible that the pair instability supernova mechanism ceases and direct collapse to a black hole becomes possible. In which case their might be a population of $$300+ M_{odot}$$ mergers that are just below LIGO's sensitivity window. New Earth-based gravitational wave detectors like the Einstein Telescope and Cosmic Explorer aim to push their low frequency response down to a few Hz and might be capable of detecting mergers in the 300-1000$$M_{odot}$$ range.

This means you could not get a merger pair between about $$100 M_{odot}$$ and $$300 M_{odot}$$ (unless they themselves were the products of a merger).

Adding to Rob Jeffries good answer - Observing stellar population and mass distribution shows a similar pattern…

• many tiny / light objects
• medium number of medium sized / weighed objects
• rather few large / massive or even super massive objects

Many of those massive / super massive black holes are active galactic cores - that rarily will have said mergers. Many Astronomers assume those had their mergers in the early phases of their galaxies - since stars that produced them lived a rather short time.

This leaves way bigger chances for mergers of rather lighter black holes or neutron stars than for the massive or super massive ones.

## Distant Black Hole Collides With a Mysterious Object

Roughly 780 million years ago and a correspondingly distant 780 million light-years away, a strange stellar object was devoured by a black hole 23 times more massive than the sun. The strange object defies categorization, being more massive than any known collapsed star and less massive than any black hole ever detected, reports Dennis Overbye for the New York Times.

This places the misfit, still 2.6 times the mass of the sun, squarely in what’s called the “mass gap,” reports Rafi Letzter for Live Science. Collapsed stars, called neutron stars, have topped out at 2.14 times the mass of the sun and their generally accepted theoretical upper limit is 2.5 solar masses, according to the Times. Black holes on the other hand don’t seem to come smaller than five solar masses.

Part of the significance of this mass gap is that neutron stars and black holes each represent possible outcomes for dying high-mass stars. The deaths of such stars entail brilliant supernovae that are punctuated in a transformation of the star’s remaining hyper-dense core into either a neutron star or a black hole, wrote Jason Daley for Smithsonian in 2019. A more massive core turns the core into a light eating black hole and a less massive core will condense into a neutron star—meaning somewhere in the mass gap there may be a tipping point, a mass beyond which a black hole is preordained and below which a neutron star forms.

“We’ve been waiting decades to solve this mystery,” Vicky Kalogera, an astrophysicist at Northwestern University and one the authors of a new paper describing the discovery, tells the Times. “We don’t know if this object is the heaviest known neutron star or the lightest known black hole, but either way it breaks a record. If it’s a neutron star, it’s an exciting neutron star. If it’s a black hole, it’s an exciting black hole.”

Astronomers discovered the confounding object on August 14, 2019, using gravitational wave detectors in Italy and the United States called the International LIGO-Virgo Collaboration, reports Pallab Ghosh for BBC News. The detectors use lasers to measure the tiny ripples in the fabric of space-time created by the collision of massive objects elsewhere in the universe. The international team’s findings were published this week in the Astrophysical Journal Letters.

Charlie Hoy, an astronomer with Cardiff University who worked on the study, tells BBC News that the discovery may call for fundamental shifts in our understanding of these phenomena. “We can't rule out any possibilities. We don't know what it is and this is why it is so exciting because it really does change our field."

Christopher Berry, a gravitational wave astronomer at Northwestern University and the University of Glasgow and co-author of the new research, tells Megham Bartels of Space.com that figuring out what tips a dying star towards becoming a neutron star will help us understand how they work. "Neutron star matter is very difficult to model," he tells Space.com. "It's nothing we can simulate here on Earth, the conditions are too extreme."

And if the mass gap turns out to be smaller than previously thought, that will require tweaking the currently accepted astrophysical models, which could have broader ramifications for our understanding of the universe, Berry tells Space.com.

The gravitational waves used to detect this interstellar oddball were theorized by Einstein but only first detected in 2016, and their use as a tool to probe the universe is still in its infancy.

## Is the Mass of Black Holes Limited?

It is believed that at the center of every galaxy lies a dormant Super-Massive Black Hole (SMBH). Some of these may be classified as Ultra-Massive Black Holes (UMBHs), black holes with a mass exceeding that of 5 billion suns. These black holes grow primarily by accreting gas from the surrounding galaxy. This growth probably begins at very high redshifts, i.e. long ago (higher redshift means earlier in time).

The new study conducted by Priyamvada Natarajan from Yale University, who is currently a fellow at the Radcliffe Institute for Advanced Study at Harvard, and Ezequiel Treister from the European Southern Observatory in Santiago, Chile, trace the accretion histories of the black holes. Using the quasar (very bright black holes) luminosity function, models of the cosmic X-ray background radiation and observational data regarding the rates of accretion in quasars at various redshifts, they suggest that SMBHs spend most of their lives in a low accretion state, and only a fraction of their life as bright quasars.

By tracing the accretion behavior of black holes the mass of the black hole can be obtained, as well as the rate of its growth. A study of the black hole’s spatial density, i.e. how many black holes are present per unit of volume, as a function of the black hole’s mass, reveals an abundance of UMBHs that is not in accordance with observations of today’s universe.

Natarajan and Treister suggest there is a self-regulation mechanism preventing the mass of a black hole from exceeding a certain value. Introducing this modification into the study of black hole spatial density, yields results that comply with observed data. These results show that while UMBHs are rare but nevertheless likely to exist, there is an upper limit on their mass.

To determine the mass limit, the scientists make use of the correlation between the properties of a black hole and those of its galaxy. In particular the strong correlation between the mass of the black hole and the velocity dispersion of its galaxy (the distribution of velocities of stars in the galaxy) is relevant to this calculation. Using various models that describe the connection between a galaxy’s velocity dispersion and the mass of the black hole lying at its center, they arrive at a limit in the area of 10 billion times the mass of the sun.

A likely place to find UMBHs is in bright and massive galaxies. The Sloan Digital Sky Survey (SDSS), a galaxy survey that began in 1998 and is still underway, may be able to detect these black holes and assist in furthering our understanding of galaxy formation and black hole assembly in the Universe.

TFOT reported on research confirming the leading theory regarding the behavior of galactic black holes, according to which the particles are accelerated by tightly-twisted magnetic fields close to the black hole. In another article TFOT covered a research that verified the blue color of quasar accretion disks. This was done by analyzing the emission spectra of the accretion disk surrounding the black hole.

Further information on the new study, scheduled for publication in Monthly Notices of the Royal Astronomical Society, can be found in the Arxiv website (PDF).

## 22 Replies to &ldquoBlack Holes Can Only Get So Big&rdquo

Well this just opens all sorts of questions concerning the Big Bang then.

AFAIK it’s hardly possible for two black holes to consome eachother, they rather attract eachother untill the bigger one repels the smaller one from the host galaxy in a renegade escape.

I wonder if Black Holes can diverge or split?

it is possible for two black holes to merge…this is one of the main locations that “we” hope to detect gravitational waves…

Hmm . . . how can a black hole radiate energy in any form if light can’t escape. How is it another part of the energy spectrum can? If nothing can escape the gravitational pull of a black hole, then why would there be an upper limit to its size or mass? Maybe in time, the universe might evolve into an infinitely large black hole. Natarajan’s study of black hole radiation interference with the consumption of matter appears to be flawed.

“I wonder if Black Holes can diverge or split?”

Maybe in a collision of two of these upper limit black holes one of them could be split in two?
That would be awesome! If we don’t see gravity waves from something like that…

What kind of mechanism could split a black hole in two?

Surely an ultra massive black hole could still get bigger in collisions with other black holes moving at high speed towards each other.

Though I can see why it would not get bigger from absorbing stars, sort of a photoevaporation effect only for black holes instead of stars.

Seems to me I read many eons ago that black holes radiated (due to zero-point energy in space and virtual particles) and that the smaller they got the greater the radiation. When they got small enough, the would explode leaving nothing.

This implies that there is greater radiation from larger black holes. Does this contradict the above? Is the above still true?

It’s called Hawking Radiation. basically, particle-anti particle pairs form spontaneously sometimes one gets sucked in and the other escape due to conservation of energy (=mass) what it carries away is taken from the black hole

This process is connected to the tidal forces near the event horizon, and therefore is actually stronger, the smaller a black hole is. That’s why those they hope to make at the LHC won’t do any harm, as they will radiate away within fractions of a second.

There is a “magical barrier”. If a black hole manages to grow beyond a certain size it will absorb more energy from the cosmic microwave background than it radiates away i.e. any black hole not actively feeding and smaller than that wil evaporate bigger than that and it will grow (*very* slowly).

Be aware that this critical mass is sinking over time due to the cooling of the CMB, so over time (*lots* of time) even big black holes might radiate away still.

If I grasp the article right Natarajan just says that black holes beyond a certain age would be so violent when swallowing something, the radiation put out in the process would clear it’s vicinity, so that it’s no longer actively feeding, thus reducing its growth to being immeasurable.

It will probably still grow (see Cosmic Microwave Background), but compared to 10^15 solar masses this won’t make much of a difference.

This is not a “hard” barrier, so they might grow further through mergers.

I’m glad we cleared that urrrp!!…. I mean up… So I can continue to feed until I’ve consumed – or merged with… other black holes… then space.. .dark matter…dark energy… time…

In this case though, I wonder if the observed limit to the growth of black holes could just be a natural part of the lifecycle of black holes and their surroundings. In other words, new physics, associated with super massive black holes, may not be needed to explain the observation.

No, this is NOT the first time someone has put an upper size limit on black holes. Amy Barger and Lennox Cowie drew the same reasoning about 3 years ago.

Duh- Conservation of energy? it can neither be created nor destroyed, therefore it cannot leave behind nothing.Given the velocity of moving black holes, the odds of it’s energy repelling an oncoming supermassive black hole are slim- THe 2 must collide and create a massave wave of some sort of energy and moswt likely massive quantities of particles. particles

The fact is there is a complete misunderstanding of the actual structure of any mass object, not just the black hole. But you will all have to wait for a little longer to read more.

The article raises two separate but related questions. One is why we have not observed ultramassive black holes more massive than about 10 billion solar masses. The second issue is whether there is a reason, a law of physics, which forbids a supermassive black hole from growing greater than this observation.

Priyamvada Natarajan answered the first one well we have not seen a more massive object. However, he misses on the second. The idea that energy radiation disrupts further growth of an ultramassive black hole is lame. That radiation results because the object grows. If it stops growing, then it stops radiating. There is now no reason for it not to start growing again.

One problem is that Natarajan just may not be thinking in the necessary time frame. He is only looking at supermassive blackholes since the beginning of the universe. He is not capable of observing any object older than the current age of the universe. Hawking Radiation will evaporate a small black hole, but a large one will continue to grow as it absorbs more energy, more mass, than it evaporates. As the universe cools, the dividing line between small and large ones gets larger. The time though is beyond any human comprehension. We are talking about maybe a trillion trillion trillion trillion times as long as the universe has existed yet before the stellar sized black holes evaporated. It would take maybe a million trillion trillion trillion times longer than that for the supermassive ones to evaporate.

Given sufficient time, there is no reason why these ultramassive black holes cannot grow. Given more time, they too will go away, but I will not be around to see it happen.

It seems reasonable. I mean if it were’nt so the universe would be littered with black holes, something must be keeping them in check. The universe could not have let these “behemoths” grow unchecked for so long it would (eventually with enough time)swallow everything.

Okay, if I’m reading this right, what Professor Natarajan is really saying is not that a black hole can grow no larger than XXX, but that the mechanics of normal growth of a black hole prevent it from getting fed once its reached XXX level of massiveness, which is not the same. What if an extreme condition arises, say a small black hole, not really affected by normal interstellar pressures, wanders by close enough to be attracted to the big guy. if it approaches at the right angle and speed can’t it still be consumed?

Another possibility is it possible that at mass XXX consumption of whatever the universe throws at the black hole is so negligible compared to its mass that the effects of so doing are not seen? Kinda like throwing a pea against the side of a skyscraper compared to a wreckers’ ball? This last one I see as a bit far-fetched, but its food for thought.

Why didn’t the universe collapse into a black hole right before the big bang?

W’r ll gnn d vntlly. t’s jst mttr f whn.

I’ve never understood how Hawking Radiation would work. I mean, don’t matter and anti-matter behave in a similar way under gravity?

That being the case, when a particle pair spontaneously appears near a black hole’s event horizon, it’s gonna be a 50-50 chance as to which one of the particles falls in.

If the matter particle fell in, the black hole would gain mass. On the other hand, if the anti-matter particle fell in, it would cancel some of the matter inside the black hole, so the black hole would lose mass.

If the two processes have an equal chance of happening, there should be no overall effect upon the mass of a black hole, because the two would cancel each other out.

Or am I missing something?

Black holes continue to enlarge until the “big crunch” which occurs after all matter in a space-time continuum is completely eradicated by blackholes through something i will call time-void phasing. During some of my early ego death experiments into the true nature of reality, i discovered much insight into this area cosmology. A fair analagy is that black holes are to physical universes as plant life is to earthlings (plants take animal CO2 waste and give lifeforms oxygen fuel) Black holes absorb matter and then “phase” as it into the next universe as energy for the next big bang.

Also bare in mind that matter is energy slowed down to a certian speed or frequency with photons being the highest frequency for physical matter. Time slows down for matter the nearer it approaches the center of the black hole which is a time void (time axis is warped so badly it no longer is connected to any of the 3 spacial axes). Eventually, the devoured matter is naturally changed by the gravity into pure energy (matter vibrating at an infinite speed/frequency) due to the particle breakdown with the law of conservation allowing it to pass out of our universe using the time void into another one (sometimes back into our own). the energy that doesnt get radiated back into ours as hawkings radiation becomes the fuel for:

1 the next universe’s big bang that occurs after our own crunches
2 a previous one that crunched before our own
3 and even recycled for the big bang responsible for our own universes creation.

Essentially this process allows an infinite number of physical universes to exist using the same finite amount of energy (pure energy mind you with infinite speed & frequency) that was created from the opposal creation of conciousness and nothingness which is what happened before the first big bang and way too complicated & abstract to explain with langauge in a timely manner on a forum. I’d imagine i’d have to use vague references to things like thought cancellation from redundant opposal awareness and chain reacting shifts in the will. It’s only completely understood while experiencing ego death (or actually dying!) since your conciousness is then existing in that same non-physical reality that the time-voids in black holes contain* but without the maecrocosmic shell limit of time’s linear sequencing effect that prevents
At least not in this universe anyway…

this coincides with my theory that galaxies are nothing more than a seed pod for future universal creation bangs…. i’m glad 2 see that im not the only person who sees this correllation… keep up the good work… chris

### Affiliations

Department Physics and Astronomy, Vanderbilt University, Nashville, TN, USA

Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA, USA

Karan Jani & Deirdre Shoemaker

Theoretical Astrophysics, California Institute of Technology, Pasadena, CA, USA

Jet Propulsion Laboratory, Pasadena, CA, USA

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

### Contributions

All authors contributed equally to the text and the primary results.

## Newly discovered black hole could have formed ‘before the first stars and galaxies’

Researchers observed the same pair of merging stars twice: once directly, and once as an 'echo' caused by the black hole.

Published: 30th March, 2021 at 09:43

A newly discovered black hole approximately 55,000 times the mass of the Sun could be an ancient relic created before the first stars and galaxies formed, scientists have said.

Such a black hole may be the seed of the supermassive black holes which exist today and could help scientists estimate the total number of these objects in the Universe, researchers said.

The discovery of the “intermediate-mass” or “Goldilocks” black hole – different to the small black holes made from stars and the supermassive giants in the core of most galaxies – is published in the journal Nature Astronomy.

Researchers estimate that there are some 46,000 intermediate-mass black holes in the vicinity of the Milky Way galaxy. The new black hole was discovered by researchers from the University of Melbourne and Monash University, through the detection of a gravitationally lensed gamma-ray burst.

The burst – a half-second flash of high-energy light emitted by a pair of merging stars – had an “echo”, caused by the intermediate-mass black hole, which bent the path of the light on its way to Earth so that astronomers saw the same flash twice.

Software developed to detect black holes from gravitational waves was adapted to show that the two flashes were images of the same object.

“This newly discovered black hole could be an ancient relic – a primordial black hole – created in the early Universe before the first stars and galaxies formed,” said study co-author, Professor Eric Thrane, from Monash University. “These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”

Fellow paper co-author Professor Rachel Webster, from the University of Melbourne, described the findings as “exciting”.

“Using this new black hole candidate, we can estimate the total number of these objects in the Universe,” she said. “We predicted that this might be possible 30 years ago, and it is exciting to have discovered a strong example.”

#### Reader Q&A: How big could a black hole get?

There is no theoretical upper limit to the mass of a black hole. However, astronomers have noted that the ultra-massive black holes (UMBHs) found in the cores of some galaxies never seem to exceed about 10 billion solar masses. This is exactly what we’d expect from the rate at which we know black holes grow, given the time that’s elapsed since the Big Bang.

Furthermore, recent studies suggest that UMBHs cannot physically grow much beyond this anyway, since they would then begin to disrupt the accretion discs that feed them, choking the source of new material.

## Astronomers say they’ve detected the most massive merger of two black holes ever discovered

Astronomers may have detected the most massive collision of two black holes ever discovered, a chaotic merger that occurred some 7 billion years ago, the signs of which have only just reached us. The cataclysmic event offered researchers a front-row seat to the birth of one of the Universe’s most elusive objects.

The distant show included two major players: one black hole roughly 66 times the mass of our Sun, and another black hole roughly 85 times the mass of our Sun. The two came close together, rapidly spinning around one another several times per second before eventually crashing together in a violent burst of energy that sent shockwaves throughout the Universe. The result of their merger? One single black hole roughly 142 times the mass of our Sun.

Such a find could be a big one for astronomers. Up until now, scientists have been able to detect and indirectly observe black holes in two different size ranges. The smaller variety are between five and 100 times the mass of our Sun. On the other end of the spectrum, there are the supermassive black holes — the kinds at the centers of galaxies that are millions and billions of times our Sun’s mass. For ages, scientists have been trying to pinpoint the black holes in between, so-called “intermediate mass black holes” that range from 100 to 1,000 times the mass of the Sun. Astronomers were certain this kind must be out there but hadn’t been able to find any direct evidence of their existence. A few potential intermediate black holes have been spotted, but are still considered candidates.

“They are really the missing link between [black holes with] tens of solar masses and millions,” Salvatore Vitale, an assistant professor at the LIGO Lab of MIT studying gravitational waves, tells The Verge. “It was always a bit baffling that people couldn’t find anything in between.”

With this discovery, detailed today in the journals Physical Review Letters and The Astrophysical Journal Letters, we may have our first detection of an intermediate mass black hole being born. The discovery could help explain why the Universe looks the way it does — with relatively bountiful scatterings of smaller black holes and a few supermassive black holes at the centers of galaxies. One theory of how supermassive black holes get so big is that smaller black holes merge over and over, consolidating until they became enormous. But if that were the case, there’d have to be intermediate black holes out there in the Universe somewhere. “That’s why astronomers have been looking for these extensively, because they would help in solving this puzzle,” Vitale says.

A plot showing GW190521 compared to the masses of other LIGO-Virgo black hole mergers Image: LIGO/Caltech/MIT/R. Hurt (IPAC)

To detect this black hole dance, scientists measured the tiny shockwaves the merger produced. When incredibly massive objects like black holes merge, they warp space and time, creating ripples in the fabric of the Universe that shoot outward at the speed of light from the event. Known as gravitational waves, these ripples are gargantuan when they’re produced, but by the time they reach our planet are incredibly faint and incredibly hard to detect.

Scientists have become pretty adept at detecting these tiny gravitational waves thanks to observatories in the US and Italy. Known as LIGO and Virgo, the observatories are specifically designed to detect these infinitesimal waves from cataclysmic mergers — by measuring how the ripples affect suspended mirrors here on Earth. Ever since LIGO made the first detection of gravitational waves in 2015, the observatories have racked up an impressive resume, detecting roughly 67 mergers of black holes, neutron stars, and black holes merging with neutron stars.

At 5.3 billion parsecs away, the detection announced today is also the farthest merger that LIGO and Virgo have ever found, with the waves taking 7 billion years to reach us. This event, called GW190521, was detected on May 21st, 2019, and it was so faint that it could have easily been missed. LIGO and Virgo only picked up four little waves from the merger in their detectors, perturbations that lasted just one-tenth of a second. Scientists working with the data used four different algorithms to find the wiggles, ultimately allowing them to pinpoint the masses of the merger and just how much energy was released. “During the process of the collision, the equivalent of seven times the mass of our Sun was destroyed and became energy leaving the system, so it’s pretty impressive in terms of energetics if you think about it,” Vitale says. “The equivalent of seven Suns was destroyed in a very small fraction of a second.”

Because of the small detection, the LIGO and Virgo astronomers are considering the possibility that they may not have actually seen a massive black hole merger, but have instead picked up waves from a collapsing star or some other weird phenomenon. However, the black hole merger is the explanation that is the simplest and makes the most sense for what they’ve observed. Astronomers estimate that mergers like this one are more rare than the smaller black hole mergers that LIGO and Virgo have seen, which would explain why it’s taken a while for the observatories to pick this kind of black hole up. “For every event like this one, there will be roughly 500 mergers of smaller black holes, so it’s very rare,” Vitale says.

But Vitale expects to see mergers like this again. Right now, LIGO and Virgo are not making observations, but the two facilities will be back online by the end of next year with some upgrades, making their instruments even more sensitive than before. “We should be able to detect more of this extremely heavy object, and then we’ll be able to say a bit more about their origins, where they come from, how rare they are, and their properties,” Vitale says. “So we’ll really able to probe the life and death of these black holes.”

## *FOUR* new black hole mergers have been found blasting out gravitational waves!

If you’re a fan of ridiculously colossal over-the-top blasts of energy screaming away in the cosmos, you’re in luck: Astronomers just announced the discovery of four new gravitational wave detections, meaning four new sets of merging black holes found out in the Universe!

This brings to 11 the total known merger events detected this way. Mind you, the first ever seen was in September 2015, so this is a brand-new field of astronomy… though it’s been decades in the making.

For background: When the first discovery was announced in February 2016, I wrote a primer with all the info you need to understand gravitational waves. In a (perhaps overly simplified) nutshell, one of the predictions of Einstein’s General Relativity is that spacetime is in some sense a thing itself, like a fabric in which everything is embedded. Any time an object is accelerated it shakes that fabric, with the force of that shake depending on the object’s mass and how much it’s accelerated. The more massive and the more rapid the acceleration, the stronger the shaking.

Artwork depicting two black holes orbiting each other. Note the spins don't align. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

In astronomy, you don’t get that combination any better than two black holes spiraling together and merging into one. They can be many times more massive than the Sun, and just before they merge they accelerate each other to very nearly the speed of light. Whipping around that rapidly they create a powerful series of ripples that expand outward: gravitational waves. They literally compress and expand space as they pass through it. They get weaker with distance, making them difficult to detect over hundreds of millions of light years, but astronomers have been trying since the 1960s to see them. The Laser Interferometer Gravitational-wave Observatory (or LIGO) was the first observatory to successfully detect them.

Even then it took a while. Over the years various upgrades to LIGO have made it more sensitive, and it was after one such upgrade that it was switched back on and almost immediately found its — and humanity’s — first solid detection of a pair of black holes merging. Since that time a second observatory called Virgo also came online, and with their combined powers more detections are being made.

These new discoveries come from observations already taken which has been reanalyzed with new computer algorithms. (Note: The paper with all this info hasn’t been peer-reviewed yet, but given the nature of the discovery I suspect not much will need to be changed… but to be clear it has not been published in a scientific journal yet.) As a test, the team of scientists was able to recover the seven previously known mergers (including the amazing event from early August 2017 when gravitational waves were detected from two merging neutron stars).

But they also found four signals that were missed before, bringing the total known binary black hole mergers to an even ten.

The eleven known mergers that have produced gravitational waves, including ten from black holes and one from neutron stars. Each one is shown as the two component masses merging (upward curved arrow) to form a new, more massive object. The objects labeled EM are comparison examples detected using light (electromagnetic waves), not gravitational waves. Credit: LIGO-Virgo / Frank Elavsky / Northwestern U.

The four new events are called GW170729, GW170809, GW170818, and GW170823 (for Gravitational Wave and the date it was detected in yymmdd format). They are all cool, but GW170729 stands out.

For one thing, the masses of the two black holes in this event are by far the highest ever seen by LIGO. They were 50.6 and 34.3 times the Sun’s mass each (with some uncertainty), making them very big stellar-mass black holes. Also, from the strength of the signal the distance to them is inferred to be a whopping 9 billion light years — two-thirds of the way across the visible Universe! That’s very exciting, because it means we can actually detect such events at cosmological distances, so far from us that the Universe has changed measurably over the time it took the signal to reach us.

This next part is what makes the hair on the back of neck stand up. When two black holes merge, a fraction of their mass is converted into energy in the form of gravitational waves. That fraction is only a few percent, but we’re talking about huge masses here. Then when you calculate the energy radiated away by using E=mc 2 , the numbers are teeth-chilling.

In the case of GW170729, the final mass of the black hole after the merger was 80.3 times that of the Sun. The math isn’t straightforward, but this means that about 4.8 times the mass of the Sun was converted directly into energy in the merger event. When you do the calculation, that means the total energy radiated was about 9 x 10 47 Joules.

Yegads. That number is terrifyingly huge. Over its entire 10 or so billion year lifetime, the Sun will emit something like 1 x 10 44 Joules of energy. That black hole merger emitted nearly ten thousand times that much energy in a fraction of a second.

That sound you hear is me running around in circles and screaming incoherently.

Yeah. That’s a lot of energy. And it’s released fast. Black holes, man. Black holes.

Artwork showing a newly-formed black hole with material swirling around it, and jets of energy and matter blasting away from its poles. Credit: NASA/CXC/M.Weiss

The team was also able to find more events that might be real, but they can’t confirm with enough confidence to say they are. Call them marginal detections, and there are 14 in total. Perhaps future analysis will be able to confirm or reject some of those as well, too.

Also of note is that an event in 2015, provisionally dubbed LVT151012 (LVT for "LIGO Virgo Trigger"), wasn’t certain enough at the time to say it really was a black hole binary merger. The new method does show it was statistically very likely to be real, so they redub it GW151012, and to be fair don’t count it as a new detection, instead including it among the seven previously known mergers.

These new detections also put some limits on the rates of these events, too, given the number of mergers they detect over a certain time and out to a certain distance. This is measured in number of events that occur per year per volume of space, and in this case specifically it’s a BIG volume: a cube over three billion light years on a side.

Doing the math, they find that black hole mergers occur somewhere between 10 – 100 times per year in that volume. Neutron star binary mergers happen somewhere between 100 and 4000 times per year in that same volume (remember, only one of these has been seen so far, so there’s a decent uncertainty to that rate). We’ve never detected a black hole eating a neutron star, which puts an upper limit to these kinds of events at 600 or so per year in that same volume — that means that if there were more than that we’d have seen one by now.

I have to say, this is very exciting. When the first black hole merger was detected we knew it was a door opening on an entirely new field of astronomy. When the neutron star merger was seen — and, for the first time, detected by the light it gave off by other telescopes — we knew we were seeing that door thrown open even more.

These new results are astronomers walking up to that doorway and taking that first step through. What will we see when we get a really good look around?

## 2. Methodology

As shown diagrammatically in Figure 1, the maximum budget for producing mergers of BBHs from stellar evolution can be calculated as

with three astrophysical constraints,

is the number of progenitor stars per unit stellar mass that will result as black holes. We utilize the piecewise initial stellar-mass function ξ(m) of Kroupa (2001) and include the numerical treatment that finds maximum star mass Mmax, given a fundamental cutoff at 1000 M (Kroupa & Weidner 2005). The choice for progenitor mass Ms is determined by the desired black hole mass MBH. We use results from the population synthesis code of Spera & Mapelli (2017) to find the mapping between the mass of progenitor star and remnant black hole (for simplicity, excluding the treatment of the pair-instability mass gap on MBH). For the massive stars that produce LIGO black holes, this mapping can vary significantly with the assumed metallicity, Z/Z, of the progenitor stars that were born at epoch . This dependence is related to the assumed BBH formation channel, and sets a relation, , between metallicity and the number of massive stars.

Figure 1. Diagram of stellar budget for merging black holes at different snapshots of time.

The fraction of progenitor stars in a tight binary is set by the parameter . While 70% of O-stars within the Milky Way are essentially binaries, this fraction is lower for the ones that are tightly bounded with orbital period of days (Sana et al. 2012). Such binaries provide a favorable chance for LIGO black holes. Therefore, unless stated otherwise, we adopt fbs = 0.1 throughout this study.

The efficiency for converting binaries of massive stars into gravitationally bound BBH sources is captured by . This free parameter solely depends on the assumed formation channel. From the asymmetric collapse, the remnant formed as black hole can get a natal kick (Hoogerwerf et al. 2001), thus decreasing the fraction ( BBH) of BBHs from two progenitor massive stars. If LIGO's BBHs are formed through dynamical capture (Rodriguez et al. 2016), then a0 could be the separation at the last encounter (provided there is no third-body interaction), and BBH will be the fraction of black holes that will find such a coalescing partner.

From the LIGO observations, we cannot infer the initial separation a0 at , the instance when the two black holes become gravitationally bound. Therefore, in this study, we adopt a delta function for a given choice of the initial BBH separation a0 (au). This simple assumption allows us to provide an upper limit on the maximum a0, regardless of an underlying astrophysical distribution for binary separation.

Assuming a circular orbit and absence of any external influence, the separation a0 sets a bound on Δtmerge (Peters 1964). As the time from the birth to collapse, , we can assume that the progenitor stars were formed at . The number of progenitor stars in the universe at this instance can be found through stellar formation rate, ψ(z) (see Equation (16) of Madau & Dickinson 2014). This sets the second constraint, , which relates a0 with the production of progenitor stars. As we are using cosmic star formation rate at an earlier epoch, , to calculate the LIGO event rate at there will be a time lag for the corresponding ψ. Furthermore, the observed redshift of LIGO detections provides a constraint, , on the maximum value of a0 such that the merger time, Δtmerge, is less than the Hubble time at that redshift.

## Primordial origins?

Another way to explain how both regular supermassive black holes and possibly stupendously large black holes formed hinges on so-called primordial black holes, Carr explained. Prior work speculated that within a second after the Big Bang, random fluctuations of density in the hot, rapidly expanding newborn universe might have concentrated pockets of matter enough for them to collapse into black holes. These primordial black holes could have served as seeds for larger black holes to form later on.

If primordial black holes do exist, they might help explain what dark matter is. Although dark matter is thought to make up most of the matter in the universe, scientists don't know what this strange stuff is made of, as researchers still have not seen it it can currently be studied only through its gravitational effects on normal matter. The nature of dark matter is currently one of the greatest mysteries in science.

"There has been a lot of interest in whether primordial black holes of modest mass could provide the dark matter," study co-author Luca Visinelli, a particle astrophysicist at the University of Amsterdam, told Space.com.

One way to detect stupendously large black holes is through gravitational lensing. According to Albert Einstein's theory of general relativity, the greater the mass of an object, the more it warps space-time around itself, and so the stronger the object's gravitational pull. Gravity can also bend light, so objects seen through powerful gravitational fields, such as those produced by black holes, are lensed. The researchers said that recent work has focused on finding gravitational lensing effects from smaller bodies, but they suggested that such research could look for stupendously large black holes as well.

Another way to detect stupendously large black holes is through the effects they would have on their environment, such as gravitationally distorting galaxies. These black holes could also generate heat, light and other radiation as they consume matter that astronomers could detect.

Aside from primordial black holes, another potential candidate for dark matter are so-called weakly interacting massive particles (WIMPs). If WIMPs exist, they would be invisible and largely intangible, but previous research suggested that if two WIMPs ever collided, they would annihilate one another and generate gamma rays, providing a way for scientists to spot them indirectly. The powerful gravitational pulls of stupendously large black holes would gather a halo of WIMPs around them, and the high-energy gamma rays that could result from WIMP annihilation might help scientists discover stupendously large black holes, Visinelli said.

All in all, "we know that black holes exist over a vast range of masses, so it's natural to ask if there is any natural upper limit," Carr said. "Some people may be skeptical about the existence of SLABs on the grounds that they would be hard to form. However, people were also skeptical about intermediate-mass and supermassive black holes until they were found. We do not know if SLABs exist, but we hope our paper will motivate discussion among the community."

The scientists detailed their findings online Aug. 18 on the preprint database arXiv and submitted the study for formal peer review.