90 New Cosmic Collisions Revealed by Gravitational Waves

The Universe Just Flinched 90 Times

In 2017, we heard the universe ring like a bell. Two dead stars, each more massive than our sun but crushed into the size of a city, spiraled into each other. The collision sent ripples through spacetime itself. LIGO and Virgo caught those ripples. It was the first time humans ever detected gravitational waves from a neutron star merger. It was a miracle of engineering, a triumph of physics.

Now, six years later, the same collaboration has released GWTC-3, the third Gravitational-Wave Transient Catalog (Abbott et al., 2023). It contains 90 new probable gravitational-wave candidates. That is not 90 detections across the entire history of the field. That is 90 new ones, added to the pile, from just the second part of the third observing run.

The universe is not quiet. It is screaming.

What Did They Actually Find?

The authors analyzed data from LIGO and Virgo between November 2019 and March 2020. That is five months. In those five months, the detectors registered 90 new events (Abbott et al., 2023). To put that in perspective: the first detection happened in 2015. The second in 2017. Now we are getting dozens per year.

The catalog includes three types of collisions:

• ▸Binary black holes: Two black holes merging. These dominate the catalog.
• ▸Binary neutron stars: Two neutron stars merging. Rarer, but more informative.
• ▸Black hole neutron star binaries: One of each. The rarest and most puzzling.

The authors found that the mass distribution of these objects is not random. There are gaps. There are clusters. There are surprises.

Why Black Holes Refuse to Be Simple

Black holes are supposed to be simple. General relativity says they can be described by just three numbers: mass, spin, and charge. That is it. No hair, no complexity, no personality.

But the new catalog suggests black holes have something like a personality after all. The authors found that black holes in binary systems tend to have masses that cluster around certain values (Abbott et al., 2023). There is a pileup around 8 to 10 solar masses. There is another around 30 to 40 solar masses. And there is a gap between 10 and 30 solar masses where almost nothing appears.

One possibility is that black holes form in two distinct ways. The lower mass ones might come from the collapse of massive stars. The higher mass ones might form through repeated mergers. But that is speculation. The data shows the pattern. The explanation is still open.

The Neutron Star Problem

Neutron stars are the densest objects in the universe that are not black holes. A teaspoon of neutron star material would weigh about a billion tons. They are cosmic extremes.

The catalog includes several binary neutron star mergers (Abbott et al., 2023). These are precious. Each one tells us about the behavior of matter at densities far beyond anything we can create in a laboratory. The authors measured the masses of the neutron stars involved. They found that the maximum mass a neutron star can reach before collapsing into a black hole is around 2.2 solar masses (Abbott et al., 2023). That number is critical. It constrains the equation of state of nuclear matter. It tells us how stiff or squishy neutron stars are.

But there is a tension. Some theoretical models predict a higher maximum mass. Others predict a lower one. The data from GWTC-3 does not settle the debate. It sharpens it.

The Black Hole Neutron Star Mystery

The rarest events in the catalog are black hole neutron star mergers. The authors found several candidates (Abbott et al., 2023). These are collisions where a black hole swallows a neutron star. The neutron star gets shredded. Some of its material gets flung into space. That material produces light, X-rays, and gamma rays.

The problem is that the authors did not detect any electromagnetic counterparts to these events (Abbott et al., 2023). No light. No X-rays. No gamma rays. Just the gravitational wave signal.

Why? One possibility is that the neutron stars were too small. They got swallowed whole before they could be shredded. Another possibility is that the black holes were too massive. They swallowed everything before any material could escape. The data does not tell us which explanation is correct. It just tells us that something is missing.

How Do You Detect a Ripple in Spacetime?

The methodology behind GWTC-3 is worth understanding because it reveals how fragile these detections really are.

LIGO and Virgo are giant L-shaped interferometers. A laser beam is split, sent down two perpendicular arms, reflected off mirrors, and recombined. If a gravitational wave passes through, it stretches one arm and compresses the other. The laser beams get out of sync. That tiny mismatch is the signal.

The problem is noise. Trucks driving on nearby roads cause vibrations. Seismic waves from earthquakes cause noise. Even quantum fluctuations in the laser itself cause noise. The authors had to filter out all of that. They used a technique called matched filtering: they compared the incoming data against a library of predicted gravitational wave waveforms (Abbott et al., 2023). If a match was found, and if the match was strong enough, it was flagged as a candidate.

But not every candidate is real. The authors set a false alarm rate threshold. They accepted only candidates with a false alarm rate of less than one per century (Abbott et al., 2023). That is conservative. That is rigorous. That is why the catalog is credible.

What the Catalog Does Not Tell Us

The catalog is a list of events. It is not a complete explanation of those events. There are several things the data does not tell us.

First, the catalog does not tell us the exact origin of every black hole. Some could be primordial, formed in the early universe. Others could be stellar mass, formed from collapsing stars. The data shows masses and spins, but not birth certificates.

Second, the catalog does not tell us the rate of mergers across cosmic time. The authors estimated rates, but those estimates have wide error bars (Abbott et al., 2023). The true rate could be twice as high or half as low.

Third, the catalog does not tell us whether black holes have hair. General relativity says no, but some alternative theories of gravity say yes. The gravitational wave signals from these events were consistent with general relativity. But the tests are not yet sensitive enough to rule out small deviations.

Fourth, the catalog does not tell us what happens inside a neutron star. The equation of state remains uncertain. The data constrains it, but does not determine it.

These are not failures. They are invitations. Every open question is a direction for the next experiment.

The Future Is Loud

The authors of GWTC-3 note that the sensitivity of LIGO and Virgo is still improving (Abbott et al., 2023). The next observing run, O4, is expected to start in 2023 or 2024. It will be more sensitive. It will detect more events. The rate could double or triple.

And then there is LISA, a space-based gravitational wave observatory planned for the 2030s. LISA will detect mergers of supermassive black holes. It will see events that LIGO cannot. The combination of ground based and space based detectors will give us a complete picture of the gravitational wave universe.

We are in the early days of gravitational wave astronomy. The first detection was in 2015. The first catalog had 11 events. The second had 39. The third has 90. The fourth will have hundreds. The fifth will have thousands.

The universe is not quiet. It is a symphony. We are just learning to hear it.

What This Actually Means

• ▸The mass gap in black holes is real and unexplained. If you are a theorist, this is your problem to solve. If you are an observer, this is your target to refine. The gap between 10 and 30 solar masses is not an artifact. It is a clue.

• ▸Neutron stars cannot be heavier than about 2.2 solar masses. This is a hard constraint. Any theory of nuclear matter that predicts a higher maximum mass is wrong. Any theory that predicts a lower one is incomplete.

• ▸Black hole neutron star mergers do not always produce light. If you are planning an electromagnetic follow up campaign, adjust your expectations. You might see nothing. That nothing is still information.

• ▸The rate of mergers is higher than we thought. The universe is more violent than we assumed. Black holes and neutron stars are colliding more often than previous estimates suggested. This changes our models of galaxy evolution and star formation.

• ▸Gravitational wave astronomy is now a routine science. It is no longer a single heroic detection. It is a catalog. It is a data set. It is a field with its own methods, its own puzzles, and its own future. The era of discovery has begun.