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When black holes collide: Puzzling out the strange light of dead stars by Wladimir Lyra

(Originally published in the Las Cruces Sun News on September 13, 2020 – link)

A long time ago in a galaxy far, far away, two black holes collided. The Sun did not yet exist. The energy of the event, radiating away at the speed of light, traveled for two billion years when remote chunks of rock and metal clumped together to form the Earth. Five billion years later the pulse reached our planet, where thinking beings had evolved and were ready with their instruments to try and decode it. Astronomers weighted one of the black holes to be 85 times the mass of the Sun; the other 65. The pair, weighing 150 solar masses together, merged to produce a black hole of 142 solar masses.

What happened to the remaining 8 solar masses, you might ask.

Lost. Converted into pure energy according to Einstein’s celebrated E=mc^2.

That’s enough energy to outshine all the stars in the universe. Yet, it’s a flash no human eye can see, for this radiation is no light at all. What we see as light are electromagnetic waves. This event produced gravitational waves.

Gravitational waves are emitted by any object in orbital motion, shrinking its orbit. The Earth emits them orbiting the Sun, but the energy lost is minuscule: it would take 1500 times the Earth’s age to lower its orbit by a mere inch. Close black hole pairs are another matter altogether: there the orbital decay can be seen in real time.

Black holes are among the most alluring objects in the Universe. They are so dense that not even light, faster than anything else, can escape from their relentless gravity. A black hole is a one-way road. Information from the universe can enter it, but none can leave it. All we know about them are properties felt from the outside, like mass.

They were predicted in Einstein’s theory of general relativity over a century ago, but their properties were so bizarre that they were first dismissed as a mathematical pathology, not a physical reality that could exist in nature. Yet, in the 1930s, evidence for them appeared. It was understood that stars are in a balance between the inward pull of gravity and the outward pressure produced by nuclear fusion. But what happens to a star once the nuclear fuel is gone? It is as if the floor disappeared from under your feet: the outer layers of the star start to fall toward the center. The inescapable conclusion is that the star will implode under its own weight.

For some stars the contraction will eventually halt because two objects cannot occupy the same place in space: as subatomic particles are squeezed together, a repulsion arises between them. Yet, above about two solar masses, even this repulsion is no match for gravity and there is no force known to physics that can stop the inevitable. The star will crush itself into a point of infinite density and, finally, rest in peace.

We have been learning a lot more about black holes since 2016, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) finally detected the gravitational waves from a collision between a pair of them. Since then, a dozen such events have been recorded. The one mentioned here is particularly interesting because the pair was too massive, 85 and 65 solar masses, to be the direct remnants of dead stars. What exactly formed them remains a mystery, but one of the main ideas (which this columnist, full disclosure, is working on) is that these hefty black holes came from earlier mergers of smaller black holes.

In the centers of galaxies, clusters of stellar-mass black holes exist, so repeated mergers could work their way up to build more massive black holes such as the ones observed. We do not know in which galaxy exactly this black hole merger happened, but about a month after the event a flare of light was seen in a galaxy near the estimated location of the merger. Was this a shock related to the merger? Perhaps. Astronomers need to keep observing and refining the models to make sense of the data and construct a theory.

Following LIGO, the Laser Interferometer Space Antenna (LISA) led by the European Space Agency, will be launched in 2034. In addition to massive black holes, LISA will perhaps be able to detect gravitational waves from the early phase of the Big Bang, the beginning of time itself. The detection of gravitational waves is, in a way, similar to the invention of the telescope: a new window to the universe has been opened. We can only begin to imagine the fascinating surprises we will find out about the cosmos.

Harrison Cook from NMSU and Mordecai-Mark Mac Low from the American Museum of National History in New York City contributed to this article.

The image is an Artist’s impression of two black holes colliding Credit: Mark Myers, OzGrav.