Black Holes and the Music of the Spheres

Around-ish sixth century B.C., so the story goes, a man named Pythagoras sauntered past a blacksmith’s shop and stopped in his tracks. In the varied clang! of hammers against iron, Pythagoras heard the universe singing. He was inspired to understand music through math, and the universe through both.

The Pythagorean model of space is half-math, half-poetry. If musical notes are math, Pythagoras surmised, then it serves that math is music. Thus, he believed that planets and stars emit individual hums according to the length of their orbits.

Pythagoras was totally wrong in thinking that the earth and our neighbors are on concentric circles that jingle and jangle as we move, much like a wind chime hanging over an abyss.

Bodies in space make sound.

But he was also kind of right. Bodies in space make sound.

Or something approaching it.

At the National Center for Supercomputing Applications, Eliu Huerta Escudero is listening to the sounds of galaxies far, far away. Dr. Huerta’s team—comprised of SPIN undergraduate Bhanu Agarwal and Ph.D. student Daniel George—is using supercomputers to do the impossible.

They’re hearing black holes fall into each other in the nearby universe.

Top (from left to right): Ed, Gab, Eliu Huerta Escudero, Bhanu. Bottom (from left to right): Daniel, Justin, Haris

Using the Illinois Campus Cluster Program (ICCP), they’re teaching us how to listen, too.

The official description of Dr. Huerta’s project is that his team is extending our understanding of gravitational wave astronomy by developing new tools that detect and characterize wave sources in dense stellar environments.

“We are a leading group in the US that is developing state-of-the-art waveform models that describe the coalescence of compact binary systems involving black holes and neutron stars that retain eccentricity throughout their lifetime,” Huerta says.

Using ICCP, the team is building a waveform model and exploring how well we can measure the imprint of eccentricity in gravitational wave signals.

“Eccentricity” is the keyword here.

As the world learned earlier this year, gravitational waves are produced when compact, massive objects like black holes move toward each other very quickly (like, a significant fraction of the speed of light quickly). And just this month, researchers confirmed that black holes in sets of two (termed “binary black holes”) do exist, they merge within the lifetime of the universe, and they emit gravitational waves when they coalesce.

Many researchers anticipate that, when neutron stars or black holes formed in isolation make astrophysical systems, these systems follow a series of quasi-circular orbits when they emit gravitational waves. These waves can then be observed by detectors like the Laser Interferometer Gravitational wave Observatory (LIGO).

But what if their paths aren’t so neat?

But what if their paths aren’t so neat? If two black holes that orbit each other form in dense environments, say, in the vicinity of the supermassive black hole that lives in the center of our galaxy, then their orbits might be a bit—to use the scientific term—wonky.

Huerta’s team is creating new models to detect this kind of compact binary system in upcoming searches for gravitational waves. To really understand what kind of environments black holes form in, for instance, they need to measure the eccentricity of their orbits.

This involves a lot of math and abstraction and complicated graphs. But all this helps us answer a question as old as Pythagoras: what does the music of the spheres sound like?

The sound record

Many things can produce gravitational waves. That angry motorist shaking his fist at you on your morning commute? Waves. A starquake (an earthquake in a neutron star AND the name of your next ‘80s cover band)? Waves.

with black holes, it’s more like a death metal symphony conducted by Reavers.

When two black holes get close to each other? You guessed it, waves. It’s kind of like Pythagoras’ humming planets. Except that, with black holes, it’s more like a death metal symphony conducted by Reavers.

Remember the neat orbits of Pythagoras or the quasi-concentric orbits postulated by current researchers? Yeah, no.

“According to Einstein, matter does not move through a passive spacetime continuum. Rather, space tells matter how to move, and matter reacts back on space, telling it how to curve. Two black holes orbiting around each other warp spacetime around them. As the black holes move, they emit gravitational waves,” Huerta explains.

Waves in action

For Huerta’s team, then, the million-dollar question is: how does gravitational wave emission affect the orbits of binary black holes?

Well, instead of having the nice, quiet orbits of planets around the sun, black holes spiral into each other and ultimately merge in a blast of atomic lig--

No, because this is deep space. Many of the systems out here are dark.

The telescope for looking out, the microscope for looking in.

“We’re used to thinking about space with one sense, sight,” Huerta says. “The telescope for looking out, the microscope for looking in. But it’s dark in space. And the collision of compact objects does not always have an electromagnetic counterpart that we can follow up with detectors that collect light that comes from the Universe.”

So how do we know two black holes merged? How can we prove that objects of extraordinary mass have bent space-time?

We hear it, using technologies like LIGO.

With LIGO, we can prove that gravitational waves exist. We can also know that astronomical (literally) events like two black holes merging happened and when. And, in the coming years, we might finally be able to “see” them. Mergers involving two neutron stars, or a black hole and a neutron star, may originate electromagnetic counterparts, such as short gamma ray bursts that LIGO may be able to confirm in the future. To learn more about gravitational waves and LIGO, click here.

But, really, it’s given us a different sense entirely.

But for now, we hear. “Hearing is as close a sense as we have to understanding what LIGO does,” said Huerta with a smile. “But, really, it’s given us a different sense entirely.”

When black holes merge, it’s like we’ve reached the discordant crescendo that only LIGO can hear. The emission of gravitational waves peaks.

“Peak” might be too small of a word to describe what LIGO picks up.

“For reference, the binary black hole system detected by the LIGO detectors last September 14th 2015 had a peak gravitational wave luminosity at the time of merger that is equivalent to the energy output of all the stars in the galaxy for 500 years,” Huerta explains.

Despite being a superpower, the sixth sense LIGO gives us, in some ways, very human. Even as LIGO strains toward the heavens, it also picks up the ordinary sounds of earth.

The banal sounds of everyday living...become part of the same sound record as two dead stars

“Ocean waves, my colleague dancing in his office, someone walking down the hall outside of the lab [where LIGO is stationed], the UPS truck pulling up outside,” offers Huerta.

The banal sounds of everyday living—now, here—become part of the same sound record as two dead stars, each approximately 30 times the mass of our sun, eating each other in the black of space 1.3 billion years ago.

Limits

But even the mighty have their limits.

LIGO’s limit can be thought of as the interstellar version of Goldie Locks and The Three Bears. LIGO was designed to hear gravitational waves larger whose frequency is larger than 10Hz. We can hear stellar mass size binary black hole systems, and we know ridiculously large ones exist.

But those in the middle...remain elusive.

But those in the middle—the binary black hole systems with component masses between a hundred and a few thousand times the mass of our sun—remain elusive.

The existence of intermediate black holes has been inferred by electromagnetic observations. Unfortunately, these observations cannot provide a robust measurement of the mass of the intermediate mass black hole candidate. LIGO is in a strong position to finally unveil the existence of these objects.

“Intermediate mass black holes are very close to my heart. I have developed several models to try to detect them with LIGO. It would be just perfect if we manage to hear them coalesce in the near future,” Huerta says.

And those huge black holes—like unimaginably huge, like a million times the mass of our sun? Those are on the milliHertz spectrum. To hear them, we need to construct a wave detector in space. No more sharing the sound record with the UPS guy.

That’s next on the horizon. For now, we’ll continue to listen to the stars.

Dr. Huerta would like to thank the “kind and savvy experts” Mark Fredricksen, Campus Cluster Administrator at the National Center for Supercomputing Applications and Weddie Dion Jackson for the following: Creating a project space where we can install software that suits our particular needs; Moving to central locations several modules we have created to run our simulations; Installing/upgrading new software that is needed to use our software; Troubleshooting issues that arise when we submit jobs to the campus cluster.

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The Illinois Campus Cluster Program (ICCP) is the centralized hub of supercomputing resources at Illinois. Researchers from every field, as well as individuals, groups, and campus units, are welcome to invest in and use these resources. Researchers use the Campus Cluster for a variety of projects, including statistical modeling and data visualization. For more information, see https://campuscluster.illinois.edu/ or contact help@campuscluster.illinois.edu.