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Simulated merger of a neutron star binary. Credit: Miguel Bezares—GRAMS group SISSA |
A huge amount of mysterious dark energy is necessary to explain cosmological
phenomena, such as the accelerated expansion of the Universe, using
Einstein's theory. But what if dark energy was just an illusion and general
relativity itself had to be modified? A new SISSA study, published in
Physical Review Letters, offers a new approach to answer this question.
Thanks to huge computational and mathematical effort, scientists produced
the first simulation ever of merging binary neutron stars in theories beyond
general relativity that reproduce a dark-energy like behavior on
cosmological scales. This allows the comparison of Einstein's theory and
modified versions of it, and, with sufficiently accurate data, may solve the
dark energy mystery.
For about 100 years now, general relativity has been very successful at
describing gravity on a variety of regimes, passing all experimental tests
on Earth and the solar system. However, to explain cosmological observations
such as the observed accelerated expansion of the Universe, we need to
introduce dark components, such as dark matter and dark energy, which still
remain a mystery.
Enrico Barausse, astrophysicist at SISSA (Scuola Internazionale Superiore di
Studi Avanzati) and principal investigator of the ERC grant GRAMS (GRavity
from Astrophysical to Microscopic Scales) questions whether dark energy is
real or, instead, it may be interpreted as a breakdown of our understanding
of gravity. "The existence of dark energy could be just an illusion," he
says, "the accelerated expansion of the Universe might be caused by some yet
unknown modifications of general relativity, a sort of 'dark gravity'."
The merger of neutron stars offers a unique situation to test this
hypothesis because gravity around them is pushed to the extreme. "Neutron
stars are the densest stars that exist, typically only 10 kilometers in
radius, but with a mass between one or two times the mass of our Sun,"
explains the scientist. "This makes gravity and the spacetime around them
extreme, allowing for abundant production of gravitational waves when two of
them collide. We can use the data acquired during such events to study the
workings of gravity and test Einstein's theory in a new window."
In this study, published in Physical Review Letters, SISSA scientists in
collaboration with physicists from Universitat de les Illes Balears in Palma
de Mallorca, produced the first simulation of merging binary neutron stars
in theories of modified gravity relevant for cosmology: "This type of
simulations is extremely challenging," clarifies Miguel Bezares, first
author of the paper, "because of the highly non-linear nature of the
problem. It requires a huge computational effort—months of run in
supercomputers—that was made possible also by the agreement between SISSA
and CINECA consortium as well as novel mathematical formulations that we
developed. These represented major roadblocks for many years till our first
simulation."
Thanks to these simulations, researchers are finally able to compare general
relativity and modified gravity. "Surprisingly, we found that the 'dark
gravity' hypothesis is equally good as general relativity at explaining the
data acquired by the LIGO and Virgo interferometers during past binary
neutron star collisions. Indeed, the differences between the two theories in
these systems are quite subtle, but they may be detectable by
next-generation gravitational interferometers, such as the Einstein
telescope in Europe and Cosmic Explorer in U.S. This opens the exciting
possibility of using gravitational waves to discriminate between dark energy
and 'dark gravity'," Barausse concludes.
Reference:
Miguel Bezares et al, No Evidence of Kinetic Screening in Simulations of
Merging Binary Neutron Stars beyond General Relativity, Physical Review
Letters (2022).
DOI: 10.1103/PhysRevLett.128.091103
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