Science Gazette

Dark energy: Neutron stars will inform us whether it’s a mirage

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With Einstein’s hypothesis, a massive quantity of enigmatic dark energy is required to explain cosmic phenomena such as the rapid expansion of the Universe. But what if dark energy is an illusion, and general relativity must be altered? A recent SISSA research published in Physical Review Letters takes a fresh look at the topic. Scientists constructed the first simulation yet of merging binary neutron stars in theories beyond general relativity that mimic a dark-energy-like behavior on cosmic scales, thanks to massive computational and mathematical effort. This allows for a comparison between Einstein’s theory and updated versions of it, and with enough exact data, it may be possible to answer the enigma of dark energy.

General relativity has proved quite effective in characterizing gravity on a range of regimes for nearly 100 years now, passing all experimental tests on Earth and the solar system. However, in order to explain cosmological data such as the observed rapid expansion of the Universe, we must incorporate dark components like as dark matter and dark energy, which are currently unknown.

Enrico Barausse, astrophysicist at SISSA (Scuola Internazionale Superiore di Studi Avanzati) and main investigator of the ERC initiative GRAMS (GRavity from Astrophysical to Microscopic Scales), wonders whether dark energy exists or if it is a breakdown in our understanding of gravity. “The presence of dark energy may be an illusion,” he adds, adding that “the rapid expansion of the Universe may be driven by some yet undiscovered modifications of general relativity, a kind of ‘dark gravity.'”

The merging of neutron stars provides a unique opportunity to test this concept since the gravity surrounding them is pushed to its limit. “Neutron stars are the densest stars that exist, often with a radius of barely 10 kilometers but a mass of one to two times that of our Sun,” adds the expert. “This causes gravity and the spacetime surrounding them to be very strong, allowing for an abundance of gravitational waves to be produced when two of them clash. In a separate window, we may utilize the data collected during such episodes to explore the workings of gravity and put Einstein’s theory to the test.”

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SISSA scientists worked with physicists from Universitat de les Illes Balears in Palma de Mallorca to create the first simulation of merging binary neutron stars under theories of modified gravity important to cosmology in this research, which was published in Physical Review Letters: “”Because of the very non-linear nature of the issue, this form of simulation is incredibly difficult,” explains Miguel Bezares, the paper’s lead author. It requires a massive computational effort – months of running on supercomputers – which was made feasible by an agreement between SISSA and the CINECA group, as well as unique mathematical formulations devised by us. For many years, they were substantial hurdles until our first simulation.”

Researchers can now compare general relativity with modified gravity thanks to these simulations. “Surprisingly, we discovered that the ‘dark gravity’ explanation is just as good as general relativity in explaining evidence from prior binary neutron star collisions as the LIGO and Virgo interferometers. The differences between the two theories in these systems are indeed small, but they may be observable by next-generation gravitational interferometers like the Einstein telescope in Europe and Cosmic Explorer in the United States. This brings up the intriguing prospect of employing gravitational waves to distinguish between dark energy and ‘dark gravity.’ “Barausse draws a close.

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