Dark energy: Neutron stars will tell us if it's only an illusion

Date:
March 3, 2022
Source:
Scuola Internazionale Superiore di Studi Avanzati
Summary:
Scientists provide the first simulation of neutron star collisions
in extensions of general relativity relevant for cosmology,
offering a new approach to test gravity.



FULL STORY ==========================================================================
A huge amount of mysterious dark energy is necessary to explain
cosmological phenomena, such as the accelerated expansion of the Universe,
with 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 USA. This opens the exciting possibility of
using gravitational waves to discriminate between dark energy and 'dark gravity'," Barausse concludes.

========================================================================== Story Source: Materials provided by Scuola_Internazionale_Superiore_di_Studi_Avanzati. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Miguel Bezares, Ricard Aguilera-Miret, Lotte ter Haar, Marco
Crisostomi,
Carlos Palenzuela, Enrico Barausse. No Evidence of Kinetic
Screening in Simulations of Merging Binary Neutron Stars beyond
General Relativity.

Physical Review Letters, 2022; 128 (9) DOI: 10.1103/
PhysRevLett.128.091103 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220303112014.htm

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