Key takeaways

  • For the first time, astronomers have observed a burst of light caused by the collision of two black holes, 7.5 billion light-years away.
  • The collision happened in the hot, swirling matter called the accretion disk, which surrounds a larger supermassive black hole.
  • LIGO and Virgo detected gravitational waves from the collision, confirming the event along with the observed light.
  • The light flare was created when the newly formed black hole sped through the accretion disk, causing a visible reaction in the gas.
  • This finding helps scientists understand black hole collisions better and opens the door to more discoveries using both gravitational waves and light observations.

Astronomers have discovered a burst of light caused by the collision of two black holes for the first time.

The objects collided and fused 7.5 billion light-years away, in a vortex of hot, swirling matter that surrounded a larger, supermassive black hole.

This whirlpool is known as the accretion disk, and it orbits a black hole’s event horizon, which is the point at which gravity is so strong that even light cannot escape.

That is why scientists have never observed two black holes collide. In the absence of light, they can only detect such mergers using gravitational waves, which are disturbances in spacetime caused by huge object collisions.

graph of black hole accretion disc
(ESO, ESA/Hubble, M. Kornmesser; Business Insider)

Albert Einstein predicted the phenomena, but he didn’t believe gravitational waves could ever be discovered. They appeared too feeble to pick up on Earth amidst the cacophony and tremors.

For 100 years, it looked that Einstein was correct. However, in 2015, a pair of instruments in Washington and Louisiana caught their first gravitational waves: signals from the merger of two black holes approximately 1.3 billion light-years apart.

The finding established a new discipline of astronomy and earned researchers the Nobel Prize in Physics for their contributions to the experiment, known as the Laser Interferometer Gravitational-Wave Observatory (LIGO).

For the first time, scientists have connected a black-hole collision recorded by LIGO to an outburst of light, which was previously thought to be impossible since black holes don’t emit any lights.

The researchers believe that when the two black holes merged, the power of the collision propelled the newly formed black hole careening through the gas of the accretion ring surrounding the larger black hole.

“It is the reaction of the gas to this speeding bullet that creates a bright flare, visible with telescopes,” Barry McKernan, an astronomer on the California Institute of Technology team that captured the light, stated in a press statement.

The researchers published their findings in the journal Physical Review Letters on Thursday. They expect to see another flare from the same black hole in a few years, when it will reenter the supermassive black hole’s accretion disk.

“Looking for flares like this is critical since it greatly aids astrophysics and cosmology research. If we can repeat this and detect light from the mergers of other black holes, we will be able to pinpoint the locations of these black holes and understand more about their origins,” research co-author Mansi Kasliwal, an assistant professor of astronomy at Caltech, said in a statement.

A’spectacular’ flare coincided with gravitational waves

Both LIGO, which comprises of two gravitational-wave detectors in the United States, and its Italian counterpart, Virgo, detected anomalies in space and time.

Just a few days later, telescopes at the Palomar Observatory near San Diego detected a bright flash of light coming from the same location in the universe.

When the Caltech researchers later reviewed archive footage of that region of the sky, they noticed the flare-up. Over the course of a month, the light gradually disappeared. The timeline and location coincided with LIGO’s observations.

“This supermassive black hole was burbling along for years before this more abrupt flare,” said Matthew Graham, a Caltech astronomy professor and the study’s principal author, in a statement.

“In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities.”

However, the researchers ruled out the notion that the light was caused by normal explosions in the supermassive black hole’s accretion disk. That’s because the disk was largely peaceful for the 15 years preceding the latest flare-up.

“Supermassive black holes, like this one, emit flares all the time.” They are not quiet objects, but the timing, size, and location of this flare were outstanding,” Kasliwal explained.

simulation of neutron star merger
This supercomputer simulation shows one of the most violent events in the universe: a pair of neutron stars colliding, merging and forming a black hole. (NASA Goddard)

How LIGO Detects Colliding Black Holes

Both the LIGO and Virgo experiments have two arms that are 2.5 miles (4 kilometers) long each.

The detector fires a laser beam and splits it in two. One of the split beams is sent down a 2.5-mile-long tube, while the other goes down an identical, perpendicular one.

The beams bounce off mirrors and then converge at the beam splitter. When everything is still, the light waves return at the same length and line up in such a way that they cancel each other out on the detector.

When a gravitational wave strikes Earth, it temporarily alters spacetime, making one tube longer and the other shorter. This cyclic stretching-and-squeezing distortion lasts until the wave passes. When this happens, the two light waves do not converge at equal lengths and so do not cancel each other out. As a result, the detector records a few flashes of light.

Measuring fluctuations in brightness enables scientists to detect and observe gravitational waves passing across Earth.

That’s how the observatories discovered the merger of two neutron stars in October 2017 and what they believe was a black hole engulfing a neutron star in August 2019. More than 30 observatories have discovered possible gravitational waves.

A new gravitational wave observatory will help additional telescopes detect severe cosmic collisions.
Scientists anticipate additional findings like this one in the coming years, as a new gravitational-wave observatory called the Kamioka Gravitational-wave Detector (KAGRA) goes online.

Scientists anticipate that KAGRA, LIGO, and Virgo will help them narrow down the location of huge collisions three times more accurately. That would make it much easier for telescopes to confirm the collisions that cause gravitational waves, as well as detect the light they emanate.

Vicky Kalogera, an astrophysicist at Northwestern University and LIGO, previously told Business Insider that the new global network may detect 100 collisions per year.

As the worldwide gravitational-wave network detects more and more collisions with greater precision, scientists will be able to learn more about the physics of these gigantic mergers.

This article was first published by Business Insider.

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