Colossal collisions in space send shockwaves through the universe: Gravitational waves break records

ASTROPHYSICS Gravitational waves shake the very fabric of the universe when black holes collide. Today we measure these events and use them to understand the cosmos. Now University of Copenhagen researchers and their international collaborators have published a treasure trove of new gravitational..
Illustration of spiraling towards a black hole collision
More than a billion years ago, two black holes collided each 30 times heavier than the Sun. This year, the ripples reached Earth. Illustration: Simonnet (SSU/EdEon)/LVC/URI.

For millions upon millions of years, they orbited each other, bound by gravity. The distance between them grew shorter until, in a fraction of a second, they collided with velocity nearing the speed of light.

When the most compact and heavy objects in existence—black holes—merge, the forces involved are so immense that they ripple through the universe, sending waves that distort space and time.

"These are waves in spacetime itself—like ripples in water—that travel at the speed of light. They don’t move through space; they are waves of space: a rhythmic stretching and compression of the very structure of the universe," explains Jose Maria Ezquiaga from the Niels Bohr Institute, who leads NBI’s LIGO-Virgo-KAGRA group and contributed to the new scientific publications and observations.

Exactly ten years ago, Einstein’s prediction of gravitational waves was confirmed with the first measurement. Now, an almost identical black hole collision has provided gravitational wave researchers with a stronger and clearer signal than ever before: The measurement reveals two black holes, each having around 30 times the mass of the Sun.

"The properties of this merger are a type we know well from previous measurements. What makes this discovery truly exceptional is the very strong signal. It opens entirely new possibilities for testing our fundamental understanding of gravity and the nature of black holes," says Jose Maria Ezquiaga.

Already, the observation has confirmed—at more than 99% certainty—a longstanding theory by the renowned physicist Stephen Hawking, which states that a black hole formed from merging black holes must have a larger area than the combined area of its progenitors. Because gravitational waves fade quickly after a merger, it has previously been difficult to confirm the theory through observations. But the strength and clarity of the new record signal has made it possible.

The spacetime disturbances caused by gravitational waves are extremely small. To detect them, researchers must measure changes that are 700 trillion times smaller than the thickness of a human hair.

The reason the signal from GW250114 - the name given to the collision - was so strong is largely due to the advancements in measurement equipment made by the LVK collaboration. This development continues and promises a bright future for the field.

Facts: How do the measurements work?

When researchers measure an event that has emitted gravitational waves, it happens by the observatories in the USA, Europe, and Asia detecting the same shift in the time it takes highly precise lasers to traverse a distance. Gravitational waves stretch space and time leave a signal by passing through the detectors and making the sets of lasers go out of phase – so their waves are no longer in sync.

By comparing data from the three stations, the location in space and the distance can be calculated.

While orbiting each other, black holes disturb spacetime with powerful gravitational waves. In this part of the measurement, researchers have ample data to work with and good opportunities to assess the properties of the binary e.g. the mass and area. After the merger, the signal starts to fade. This is the so-called ringdown phase, when the new black hole vibrates like a bell that has been struck.

It is due to the unusually strong and clear signal that the mass and area in the subsequent phase could also be calculated, thereby confirming Hawking’s theory.

The strongest measurement vs. the biggest clash

The record-breaking measurement is part of a massive package of gravitational wave observations now released by the research coalition. Not only does it double the number of gravitational wave measurements available for study, but thanks to several remarkable observations, it marks a significant leap forward for the field.

About LIGO-Virgo-KAGRA

The LIGO-Virgo-KAGRA collaboration includes more than 1,000 researchers from three gravitational wave observatories in the USA, Italy, and Japan.

The LVK collaboration will, in the coming years, improve their detectors and build a new one in India—LIGO India—which will make it easier to locate gravitational wave signals.

In the longer term, the Cosmic Explorer is planned with 40 km long arms, and Europe’s Einstein Telescope with over 10 km long underground facilities. These will be able to detect the earliest black hole mergers and provide insight into the origin of the universe.

GW250114 is not the only record among the group’s new observations. They also include a measurement of the merger of two black holes weighing approximately 100 and 140 solar masses, forming a black hole of at least 225 solar masses: the largest black hole merger ever observed.

Until now, researchers have seen that binary systems—where black holes orbit each other—typically have masses up to 50 solar masses. Beyond this, the number of observations drops significantly.

GW231123, the name of this collision, breaks that pattern. The resulting black hole may weigh up to 260 solar masses, placing it outside the normal classification of “stellar” black holes formed from stars, and instead into the range known as intermediate-mass black holes—between 150 and 100,000 solar masses. This size is extremely rare and puzzling to scientists.

"The observation challenges our understanding of how black holes form. Black holes of such large mass shouldn’t arise through ordinary stellar collapse. One possibility is that the two black holes in this system were themselves formed by previous mergers of smaller black holes, but in truth, we don’t know. It’s also possible that the signal was distorted as it traveled through the universe," says Jose Maria Ezquiaga.

In addition to their extreme masses, both black holes rotate at unusually high speeds, making the observation even more remarkable. The downside, however, is that this signal is very short and weak compared to GW250114, which makes analysis and interpretations much harder.

A research field driving innovation

Einstein predicted gravitational waves nearly 100 years ago as an inevitable aspect of his famous theory of relativity. Ten years ago, the theory became empirical reality when the first gravitational waves were measured. Now, the decade is marked by a wealth of new observations, including these two spectacular ones. Both studies are milestones that demonstrate we can not only hear the universe’s deepest resonances—we can begin to decode its structure.

Facts: Innovations from gravitational wave research

LIGO’s technological advancements since the 1980s have led to several groundbreaking innovations:

Laser stabilization: A new method for stabilizing lasers, called the Pound–Drever–Hall technique. It is now widely used, including in atomic clocks and quantum computers.

Mirror coatings: LIGO has developed advanced mirror coatings that almost perfectly reflect laser light, which is crucial for precise measurements.

Quantum squeezing: Using so-called "quantum squeezing," LIGO has improved sensitivity and reduced noise, which is otherwise limited by quantum mechanical laws.

Artificial intelligence: New AI methods developed in the research field can remove unwanted noise and further enhance signal processing.

As often happens in physics, the fundamental research driving these discoveries can also lead to new technologies. Already, the extremely sensitive instruments used to measure gravitational waves have led to new types of lasers and optical systems in quantum computers and atomic clocks, and AI techniques are used to reduce noise and much more.

The development is expected to continue. Future waves of observations are anticipated to eventually include all gravitational wave signals from black hole collisions that the universe has to offer. This will be achieved by expanding the LVK collaboration with a new observatory in India and through planned new instruments.

"We expect this research field to be crucial for our fundamental understanding of the universe. We’ve only just reached the end of the beginning," says Jose Ezquiaga.

The research article on the observation GW231123 has not yet passed peer review but is expected to do so very soon. The article on the first mentioned observation, GW250114, can be read in the journal Physical Review Letters.

For more information about the observations read LVK's press release here

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