Evidence that black holes are a source of dark energy

15 Feb. () –

Observations of supermassive black holes at the center of galaxies point to a possible source of dark energy, the ‘absent’ 70% of the Universe.

Measurements made in old and inactive galaxies show that black holes grow larger than expected, which is consistent with a phenomenon predicted in Einstein’s theory of gravity. The result could mean that nothing new needs to be added to our picture of the Universe to explain dark energy: black holes combined with Einstein’s gravity are the source.

This conclusion has been reached by a team of 17 researchers from nine countries, led by the University of Hawaii and made up of physicists from the STFC RAL Space and Imperial College London. The work is published in two articles in The Astrophysical Journal and The Astrophysical Journal Letters.

Study co-author Dr. Chris Pearson of STFC RAL Space said it’s a statement: “If the theory holds true, this is going to revolutionize all of cosmology, because we finally have a solution for the origin of dark energy, which has stumped cosmologists and theoretical physicists for more than 20 years.”

Study co-author Dr. Dave Clements, from the Department of Physics at Imperial, said: “This is a truly amazing result. We started by studying how black holes grow over time, and we may have found the answer to one of the biggest problems of cosmology”.

GRAVITY VERSUS DARK ENERGY

In the 1990s it was discovered that the expansion of the Universe is accelerating: everything is moving away from everything at an ever faster rate. This is difficult to explain, since the pull of gravity between all objects in the Universe should slow down the expansion.

To explain it, it was proposed that a “dark energy” was responsible for pulling things apart with more force than gravity. This was linked to a concept that Einstein had proposed but later discarded: a “cosmological constant” that opposed gravity and prevented the Universe from collapsing.

This concept was revived with the discovery of the accelerated expansion of the Universe, whose main component is a type of energy included in spacetime itself, called vacuum energy. This energy pushes the Universe further apart, accelerating the expansion.

However, black holes posed a problem: their very strong gravity is difficult to counteract, especially in their center, where everything seems to break apart in a phenomenon called a “singularity”.

The new result shows that black holes gain mass in a way consistent with their containing vacuum energy, providing a source of dark energy and eliminating the need for singularities to form at their centers.

To reach this conclusion, nine billion years of evolution of black holes have been studied. Black holes form when massive stars reach the end of their lives. When they are at the center of galaxies, they are called supermassive black holes. They contain within them millions or billions of times the mass of our Sun in a comparatively small space, which creates extremely strong gravity.

Black holes can grow larger by accumulating matter, for example by swallowing stars that come too close, or by merging with other black holes. To find out if these effects alone can explain the growth of supermassive black holes, the team analyzed data spanning nine billion years.

The researchers looked at a particular type of galaxies called giant elliptical galaxies, which evolved early in the Universe and then went dormant. Inactive galaxies have finished forming stars, leaving little material for the black hole at their center to accumulate. which means that any subsequent growth cannot be explained by these normal astrophysical processes.

Comparison of observations of distant galaxies (when they were young) with local elliptical galaxies (which are old and dead) showed much larger growth than predicted by accretion or mergers: Today’s black holes are between 7 and 20 times larger than they were nine billion years ago.

Other measurements with related populations of galaxies at different points in the evolution of the Universe show a good agreement between the size of the Universe and the mass of black holes. This shows that the amount of dark energy measured in the Universe can be explained by the vacuum energy of black holes.

This is the first observational proof that black holes actually contain vacuum energy and that they are “coupled” to the expansion of the Universe, increasing in mass as it expands, a phenomenon called “cosmological coupling.” If other observations confirm it, Cosmological coupling will redefine our understanding of what a black hole is.

The study’s first author, Duncan Farrah, an astronomer at the University of Hawaii and former Imperial PhD student, said: “We’re actually saying two things at once: that there is evidence that typical black hole solutions don’t work on a very, very long time scale, and that we have the first proposed astrophysical source for dark energy.”

What this means is not that other people have not proposed sources of dark energy, but that this is the first observational work in which we are not adding anything new to the Universe as a source of dark energy: Black holes in Einstein’s theory of gravity are dark energy.”