New study proposes expansion of the universe directly impacts black hole growth

Date:
November 3, 2021
Source:
University of Hawaii at Manoa
Summary:
The study is the first to show that both large and small black
hole masses can result from a single pathway, wherein the black
holes gain mass from the expansion of the universe itself.



FULL STORY ==========================================================================
Over the past 6 years, gravitational wave observatories have been
detecting black hole mergers, verifying a major prediction of Albert
Einstein's theory of gravity. But there is a problem -- many of these
black holes are unexpectedly large. Now, a team of researchers from
the University of Hawai?i at M?noa, the University of Chicago, and the University of Michigan at Ann Arbor, have proposed a novel solution to
this problem: black holes grow along with the expansion of the universe.


========================================================================== Since the first observation of merging black holes by the Laser
Interferometer Gravitational-Wave Observatory (LIGO) in 2015, astronomers
have been repeatedly surprised by their large masses. Though they
emit no light, black hole mergers are observed through their emission
of gravitational waves -- ripples in the fabric of spacetime that
were predicted by Einstein's theory of general relativity. Physicists originally expected that black holes would have masses less than about
40 times that of the Sun, because merging black holes arise from massive
stars, which can't hold themselves together if they get too big.

The LIGO and Virgo observatories, however, have found many black holes
with masses greater than that of 50 suns, with some as massive as 100
suns. Numerous formation scenarios have been proposed to produce such
large black holes, but no single scenario has been able to explain
the diversity of black hole mergers observed so far, and there is no
agreement on which combination of formation scenarios is physically
viable. This new study, published in the Astrophysical Journal Letters,
is the first to show that both large and small black hole masses can
result from a single pathway, wherein the black holes gain mass from
the expansion of the universe itself.

Astronomers typically model black holes inside a universe that cannot
expand.

"It's an assumption that simplifies Einstein's equations because a
universe that doesn't grow has much less to keep track of,'' said
Kevin Croker, a professor at the UH M?noa Department of Physics
and Astronomy. "There is a trade-off though: predictions may only be
reasonable for a limited amount of time.'' Because the individual events detectable by LIGO -- Virgo only last a few seconds, when analyzing any
single event, this simplification is sensible. But these same mergers
are potentially billions of years in the making. During the time between
the formation of a pair of black holes and their eventual merger, the
universe grows profoundly. If the more subtle aspects of Einstein's
theory are carefully considered, then a startling possibility emerges:
the masses of black holes could grow in lockstep with the universe,
a phenomenon that Croker and his team call cosmological coupling.

The most well-known example of cosmologically-coupled material is light
itself, which loses energy as the universe grows. "We thought to consider
the opposite effect,'' said research co-author and UH M?noa Physics and Astronomy Professor Duncan Farrah. "What would LIGO -- Virgo observe
if black holes were cosmologically coupled and gained energy without
needing to consume other stars or gas?'' To investigate this hypothesis,
the researchers simulated the birth, life, and death of millions of pairs
of large stars. Any pairs where both stars died to form black holes were
then linked to the size of the universe, starting at the time of their
death. As the universe continued to grow, the masses of these black holes
grew as they spiraled toward each other. The result was not only more
massive black holes when they merged, but also many more mergers. When
the researchers compared the LIGO -- Virgo data to their predictions,
they agreed reasonably well. "I have to say I didn't know what to think
at first,'' said research co-author and University of Michigan Professor Gregory Tarle'. "It was a such a simple idea, I was surprised it worked
so well.'' According to the researchers, this new model is important
because it doesn't require any changes to our current understanding
of stellar formation, evolution, or death. The agreement between the
new model and our current data comes from simply acknowledging that
realistic black holes don't exist in a static universe. The researchers
were careful to stress, however, that the mystery of LIGO -- Virgo's
massive black holes is far from solved.

"Many aspects of merging black holes are not known in detail, such
as the dominant formation environments and the intricate physical
processes that persist throughout their lives,'' said research
co-author and NASA Hubble Fellow Dr. Michael Zevin. "While we used a
simulated stellar population that reflects the data we currently have,
there's a lot of wiggle room. We can see that cosmological coupling is a
useful idea, but we can't yet measure the strength of this coupling.''
Research co-author and UH M?noa Physics and Astronomy Professor Kurtis Nishimura expressed his optimism for future tests of this novel idea,
"As gravitational-wave observatories continue to improve sensitivities
over the next decade, the increased quantity and quality of data will
enable new analysis techniques. This will be measured soon enough.'' ========================================================================== Story Source: Materials provided by University_of_Hawaii_at_Manoa. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Kevin S. Croker, Michael Zevin, Duncan Farrah, Kurtis A. Nishimura,
Gregory Tarl�. Cosmologically Coupled Compact Objects:
A Single-parameter Model for LIGO-Virgo Mass and Redshift
Distributions.

The Astrophysical Journal Letters, 2021; 921 (2): L22 DOI:
10.3847/2041- 8213/ac2fad ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103200439.htm

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