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| NGC524: NGC 524 is a lenticular galaxy in the constellation Pisces. It is at a distance of about 90 million light-years away from Earth. Credit: ESA/Hubble |
Observations of supermassive black holes at the centers of galaxies point to
a likely source of dark energy—the 'missing' 70% of the universe.
The measurements from ancient and dormant galaxies show black holes growing
more than expected, aligning with a phenomenon predicted in Einstein's
theory of gravity. The result potentially means nothing new has to be added
to our picture of the universe to account for dark energy: black holes
combined with Einstein's gravity are the source.
The conclusion was reached by a team of 17 researchers in nine countries,
led by the University of Hawai'i and including Imperial College London and
STFC RAL Space physicists. The work is published in two papers in the
journals The Astrophysical Journal and The Astrophysical Journal Letters.
Study co-author Dr. Dave Clements, from the Department of Physics at
Imperial, said, "This is a really surprising result. We started off looking
at how black holes grow over time, and may have found the answer to one of
the biggest problems in cosmology."
Study co-author Dr. Chris Pearson, from STFC RAL Space, said, "If the theory
holds, then this is going to revolutionize the whole of cosmology, because
at last we've got a solution for the origin of dark energy that's been
perplexing cosmologists and theoretical physicists for more than 20 years."
Gravity versus dark energy
In the 1990s, it was discovered that the expansion of the universe is
accelerating—everything is moving away from everything else at a faster and
faster rate. This is difficult to explain—the pull of gravity between all
objects in the universe should be slowing the expansion down.
To account for this, it was proposed that a 'dark energy' was responsible
for pushing things apart more strongly than gravity. This was linked to a
concept Einstein had proposed but later discarded—a 'cosmological constant'
that opposed gravity and kept the universe from collapsing.
This concept was revived with the discovery of the accelerating expansion of
the universe, with its main component being a kind of energy included in
spacetime itself, called vacuum energy. This energy pushes the universe
further apart, accelerating the expansion.
Black holes posed a problem though—their extremely strong gravity is hard to
oppose, especially at their centers, where everything seems to break down in
a phenomenon called a 'singularity.'
The new result shows that black holes gain mass in a way consistent with
them containing vacuum energy, providing a source of dark energy and
removing the need for singularities to form at their center.
Black hole growing pains
The conclusion was made by studying nine billion years of black hole
evolution. Black holes are formed when massive stars come to the end of
their life. When found at the centers of galaxies, they are called
supermassive black holes. These contain millions to billions of times the
mass of our Sun inside them in a comparatively small space, creating
extremely strong gravity.
Black holes can increase in size by accreting matter, such as by swallowing
stars that get too close, or by merging with other black holes. To discover
whether these effects alone could account for the growth of supermassive
black holes, the team looked at data spanning nine billion years.
The researchers looked at a particular type of galaxy called giant
elliptical galaxies, which evolved early in the universe and then became
dormant. Dormant galaxies have finished forming stars, leaving little
material for the black hole at their center to accrete, meaning any further
growth cannot be explained by these normal astrophysical processes.
Comparing observations of distant galaxies (when they were young) with local
elliptical galaxies (which are old and dead) showed growth much larger than
predicted by accretion or mergers: the black holes of today are 7—20 times
larger than they were nine billion years ago.
Cosmological coupling
Further measurements with related populations of galaxies at different
points in the universe's evolution show good agreement between the size of
the universe and the mass of the black holes. These show that the measured
amount of dark energy in the universe can be accounted for by black hole
vacuum energy.
This is the first observational evidence that black holes actually contain
vacuum energy and that they are 'coupled' to the expansion of the universe,
increasing in mass as the universe expands—a phenomenon called 'cosmological
coupling.' If further observations confirm it, cosmological coupling will
redefine our understanding of what a black hole is.
Study first author Duncan Farrah, University of Hawai'i Astronomer and
former Imperial Ph.D. student, said, "We're really saying two things at
once: that there's evidence the typical black hole solutions don't work for
you on a long, long timescale, and we have the first proposed astrophysical
source for dark energy."
"What that means, though, is not that other people haven't proposed sources
for dark energy, but this is the first observational paper where we're not
adding anything new to the universe as a source for dark energy: black holes
in Einstein's theory of gravity are the dark energy."
References:
Duncan Farrah et al, A Preferential Growth Channel for Supermassive Black
Holes in Elliptical Galaxies at z ≲ 2, The Astrophysical Journal (2023).
DOI: 10.3847/1538-4357/acac2e
The Astrophysical Journal Letters (2023).
DOI: 10.3847/2041-8213/acb704.
Tags:
Space & Astrophysics