Black holes may be growing as the universe expands

The black holes in the universe are bigger than astrophysicists expected. Now a new study suggests why: Every black hole may be growing as the universe expands.

The new hypothesis, called “cosmological coupling,” holds that as the universe expands outward after the Big Bang, all objects with mass also grow with it. And black holes, like some of the most massive objects out there, grow the most.

This hypothesis stems from gravitational waves in spacetime that occur when two massive black holes get locked in orbit, spiral inward, and collide. Since 2015, scientists at the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo interferometer, which are designed to detect these gravitational waves, have observed many of these black hole mergers.

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But the waves hold a mystery. Based on the estimated size distribution of stars in the universe, black holes should have masses less than about 40 times the mass of the sun. But data taken from these gravitational waves shows that many black holes have more than 50 solar masses and some are close to 100 solar masses.

A common explanation for this mismatch is that black holes grow over time by filling up with gas, dust, stars, and even other black holes. But because black holes often form after giant stellar explosions called supernovae, many black holes emerge in regions of space without any of this material. Astronomers have suggested alternative explanations, but they all propose invisible changes in scientists’ current understanding of the life cycles of stars. And no one can explain the astonishing diversity of sizes of merged black holes that gravitational wave observatories have detected.

The new paper, published Nov. 3 in The Astrophysical Journal Letters, proposes an explanation for the large and small masses of merging black holes: The ballooning masses of black holes are not the result of anything they are eating, but which in a way they are. tied to the expansion of the universe itself.

This would mean that all the black holes in the universe, including merged black holes detected in gravitational wave experiments, wandering black holes on the outskirts of our galaxy, and even huge supermassive black holes at the centers of most galaxies, are growing over time. .

To investigate their hypothesis, the researchers chose to model two merging blacks in a growing universe, rather than the static universes that other research teams build in order to simplify complex equations (derived from the theory of general relativity of Einstein) that provide the basis for black. hole fusion models.

It takes just seconds for two spiral black holes to merge, so assuming a static universe in that short amount of time, as previous work has done, seems sensible. But the researchers disagree, they say that if scientists assume a static universe in their models, they could be ruling out potential changes in the two black holes over the billions of years they existed before reaching the collision point.

“It’s an assumption that simplifies Einstein’s equations, because a universe that doesn’t grow has much less to follow,” said study first author Kevin S. Croker, a professor in the Department of Physics and Astronomy at the University of Hawaii at Mānoa. in the statement. “However, there is a trade-off: predictions can only be reasonable for a limited period of time.”

By simulating millions of pairs of stars, from birth to death, the researchers were able to study those that died to form pairs of black holes and link how much they grew in proportion to the expansion of the universe. After comparing some predictions made by the model universe that had grown up with data from LIGO-Virgo, the researchers were surprised to see that they matched well.

“I have to say, I didn’t know what to think at first,” co-author Gregory Tarlé, a professor of physics at the University of Michigan, said in a statement. “It was such a simple idea, I was surprised that it worked so well.”

The hypothesis may seem outlandish, but the cosmological coupling exists elsewhere in astrophysics. The most famous example of this is probably the “redshift”, in which receding objects have their light stretched out at longer (and therefore redder) wavelengths.

This means that as the universe expands and the stars move away from each other, like dots drawn on an inflated balloon, the light particles, or photons, emitted by the stars become redder over time and lose energy as do it. The energy of light is said to be cosmologically coupled with the expansion of the universe.

If the researchers are correct, it means that everything that has mass is getting bigger: suns, neutron stars, planets and even humans. Of course, this coupling would be much weaker for us than for black holes.

“The cosmological coupling applies to other objects and materials in the universe, but the force of the coupling is so weak that its effects cannot be seen,” Croker told Live Science. “For the types of black holes that we have hypothesized, the coupling may be a million times greater than what you would expect from the Sun’s core. And even for these types of black holes, you may have to wait hundreds of millions years to double your mass. “

That might be an interesting idea for now, but as gravitational wave detectors become more sensitive over time, more and more data will be available to test the hypothesis, Croker said.

“The planned updates to LIGO-Virgo, plus the data they will collect over the next decade, will describe many more black hole mergers,” Croker said. “The more data that is collected, the more powerfully we can test our hypothesis. Space-based gravity wave experiments, such as LISA [the Laser Interferometer Space Antenna], it can allow us to see the mass gain directly in individual systems. “

Originally posted on Live Science.

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