Next-gen gravitational wave detectors could spot the first black holes

A few hundred million years after the Big Bang, the first stars ignited—literally the “let there be light” moment for the universe. Now known to astronomers as Population III, or Pop III, stars, these giants were very different from the stars we know today. They formed from pristine hydrogen and helium, with almost no “metal” (i.e., other elements) holding them back. They were also huge, growing to tens to hundreds of times larger than the sun. And they died young, in many cases collapsing into the universe’s earliest black holes.

Some of those black holes even partnered up, eventually colliding with one another and creating gravitational waves that, if we have instruments sensitive enough, we could potentially detect. A new study posted to the arXiv preprint server led by astrophysicist N.V. Krishnendu of the University of Birmingham and colleagues shows just how much we can learn about them with the new suite of gravitational-wave detectors about to come online.

Current gravitational-wave observatories, such as LIGO in the U.S., Virgo in Italy and KAGRA in Japan, have proven themselves very capable. They’ve detected around 400 events, with the first one occurring only about 11 years ago. However, they’re limited by distance—which also means time when talking about space. Because of their high-frequency limitations, they can see back only about 8 billion years. Still impressive, but not enough to capture any data on Pop III stars.

Enter the newcomers. Two projects are set to take over the next round of gravitational-wave astronomy. Cosmic Explorer (CE), based in the U.S., is a planned L-shaped facility similar to LIGO, with arms stretching up to 40 km (25 miles). The Einstein Telescope (ET), on the other hand, is a planned underground facility on Sardinia, built in a 10-km (6-mile) triangle.

While these new observatories are planned to be very capable, trying to sort out what a black hole from near the beginning of the universe would look like is hard. A gravitational wave traveling from around 14 billion years ago would be stretched by a factor of 19 by the time it reached Earth because of the expansion of the universe itself. This not only causes black holes to appear more massive, but also lowers the frequency of their signal. For example, a pair of 30-solar-mass black holes merging back then would look to our detectors like a gigantic black hole binary weighing 1,100 solar masses.

This isn’t the first paper to point that out, nor is it the first to attempt to figure out how well these new detectors would do at catching these ancient mergers. Previous efforts often relied on a mathematical model called Fisher matrices to estimate whether the new telescopes could determine the properties of a very early black hole merger. The new paper, on the other hand, modeled a realistic set of Pop III stars and used Bayesian supercomputer simulations to test how well a combination of the ET and CE could determine the properties of mergers occurring more than 13.54 billion years ago.

It all comes down to frequency. The researchers modeled two detectors—one that could measure vibrations at 10 Hz and a more ambitious one that could get down to 5 Hz. While a 5 Hz difference might sound small, for gravitational waves it’s massive. A detector capable of 10 Hz could detect only the final seconds of an “inspiral”—the death spiral in which the two black holes end up essentially touching and vibrating together—followed by the “ringdown.”

A 5 Hz detector, on the other hand, could catch part of the inward spiral early enough to trace several full orbital cycles before they rammed into each other—a massive difference in understanding the properties of these mergers and allowing astronomers to make sure they don’t confuse these distorted primordial beacons for more mundane, nearby black holes.

Another upside to these simulations is that the detectors appear able to measure the physical mass of these ancient black holes to within around 12% of their actual mass. That feature will be extremely helpful, as physicists are currently locked in a debate about why supermassive black holes existed so early in cosmic history. Cataloging their exact masses will help scientists understand how they grew so big or whether their measurements were biased in some way.

A third feature of the simulation is that the new set of detectors should be able to pinpoint the location of these merging binaries to within around 60 square degrees—at least in 60% of cases. Combining that localization with information from giant radio telescopes like the Square Kilometer Array (SKA) will allow scientists to link black holes with other cosmological features, such as neutral hydrogen flowing across the universe.

Admittedly, all of this potential science relies on two as-yet-unbuilt detectors in very different parts of the world. Each represents a set of engineering challenges that have yet to be overcome. But if they do, we as a species will have our first clear window into the dying breaths of the stars that gave us first light.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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