Universe: expansion speed may be slowing down – 04/04/2024 – Science

The Universe continues to expand, but this process may be slowing down, according to the first results of an international astronomical survey.

The first signals were identified by the Dark Energy Spectroscopy Instrument (Desi), installed on a telescope at the American Kitt Peak observatory.

The instrument is equipped with 5,000 thin robotic optical fibers. Each of them observes a galaxy for 20 minutes, which allows them to later calculate, with a spectrograph, its distance and, therefore, the age of the Universe when it emitted its light.

“We measure the position of galaxies in space and also in time, since the further away they are, the further we go back in time, towards an increasingly younger Universe”, explains Arnaud de Mattia, from the Atomic Energy Commission (CEA) from France, who co-leads the cosmological data interpretation group.

After one year, Desi, a collaborative project of 70 international institutions led by Berkeley Laboratory in the United States, had already mapped six million light sources, galaxies and quasars, which represent the last 11 billion years of the history of the Universe.

This Thursday (4), a conference in Switzerland and another in the USA announced the first results, ahead of a series of scientific articles in the Journal of Cosmology and Astroparticle Physics.

Desi’s main objective is to help understand the nature of dark energy, a theoretical element supposedly responsible for accelerating the expansion of the Universe.

Dark energy would be the force that increases the distance between galaxy clusters, as if the space that separates them never stops expanding.

Dark energy

In the standard cosmological model, the observable Universe is composed of 5% baryonic — or common — matter, 25% cold hypothetical dark matter and 70% dark energy.

For more than a century, scientists have known that the Universe began to expand after the Big Bang, 13.8 billion years ago. But recently astronomers discovered that this expansion accelerated significantly about 6 billion years ago.

While baryonic and dark matter are slowing their rise, dark energy is accelerating. And clearly the latter has an advantage, according to the model called Lambda-CDM, with Lambda being the cosmological constant related to dark energy.

“So far we see that the data matches our best model of the Universe, but we also see some potentially interesting differences that could indicate that this dark energy has evolved over time,” said Michael Levi, director of the international Desi project, quoted in a statement. from the US Department of Energy’s Berkeley Laboratory.

In other words, “the Desi data seems to show that the cosmological constant Lambda would not really be a constant”, since dark energy would have a dynamic behavior depending on the periods considered, said Arnaud de Mattia.

This may suggest that, after accelerating 6 billion years after the Big Bang, this speed may have “slowed down in recent times”, according to CEA physicist Christophe Yeche.

The scenario of dark energy oscillation over time still needs to be confirmed with more data from Desi and other instruments. But, if this slowdown is confirmed, we would have to rethink the idea of ​​the Universe based on the erratic behavior of the dark energy constant.

For example, replacing the cosmological constant with a force field related to a particle, yet to be identified. Or even modifying the equations of relativity in general, “so that they behave slightly differently on the scale of large structures”, according to Mattia.

It has not yet been necessary to make these changes, since, according to the researcher, the history of science shows cases “in which we saw deviations of this type that were resolved over time”.

After all, more than a hundred years after its formulation, the theory of general relativity still works perfectly.