Is dark energy just an illusion?

We know how much dark energy there is because it affects the expansion of the universe. Other than that, it’s a complete mystery. It turns out that about 68% of the universe is dark energy.

What if dark energy was just an illusion and general relativity itself had to be modified?

A new SISSA study offers a new approach to answering this question. Through computational and mathematical effort, they created a simulation of binary neutron star mergers in theories beyond general relativity that reproduce dark energy-like behavior on cosmological scales.

The simulation allowed scientists to compare Einstein’s theory and modified versions. Once sufficiently accurate data is obtained, scientists will be able to solve the mystery of dark energy.

Enrico Barausse, an astrophysicist at SISSA, raises the question of dark energy.

According to him, “the existence of dark energy could be just an illusion; the accelerated expansion of the Universe could be caused by some as yet unknown modifications of general relativity, a kind of ‘dark gravity'”.

When neutron stars collide, they stretch the space-time around them and produce strong gravitational waves. The merger of neutron stars offers a scenario that allows scientists to test the aforementioned hypothesis.

According to Barausse, “we can use the data acquired during these events to study how gravity works and test Einstein’s theory in a new window.”

In this study, scientists collaborating with physicists at the Universitat de les Illes Balears in Palma de Mallorca produced the first simulation of binary neutron star mergers in modified gravity theories relevant to cosmology.

Miguel Bezares, first author of the article, states: “This type of simulation is extremely difficult due to the highly non-linear nature of the problem. It requires an enormous computational effort – months of execution on supercomputers – which was also possible thanks to the agreement between SISSA and the CINECA consortium, as well as the novel mathematical formulations we developed. This represented a major hurdle for many years until our first simulation.”

“Surprisingly, we found that the ‘dark gravity’ hypothesis is as good as general relativity at explaining the data acquired by the LIGO and Virgo interferometers during past binary neutron star collisions. In fact, the differences between the two Theories in these systems are quite subtle, yet they could be detectable by next-generation gravitational interferometers such as the Einstein telescope in Europe and the Cosmic Explorer in the US This opens up the exciting possibility of using gravitational waves to discriminate between dark energy and “dark gravity”.

Sources, credits and references:

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