The increasing observations of supermassive black holes at the centers of galaxies indicate a probable source of dark energy, the main component of the Universe. Measurements show that black holes grow more than expected, in line with a phenomenon predicted in Einstein‘s theory of gravity. The surprising result means that we don’t have to overturn our understanding of the cosmos to explain the presence of dark energy. Black holes, in combination with Einstein’s gravity, are the main source.
The Introduction of Dark Energy
The conclusion was reached by a team of 17 researchers from nine countries, led by the University of Hawaii and including physicists from Imperial College London and STFC RAL Space. Work is published in two papers in The Astrophysical Journal (ref.) and The Astrophysical Journal Letters (rif.). Study co-author Dr. Dave Clements from Imperial College said, “This is a really surprising result. We started by observing how black holes grow over time and we may have found the answer to one of the biggest problems in cosmology”.
Co-author Dr. Chris Pearson from STFC RAL Space said, “If the theory is valid, then this will revolutionize the entire cosmology because we finally have a solution to the origin of dark energy that has baffled cosmologists and theoretical physicists for over 20 years”. In the 1990s, it was discovered that the expansion of the Universe is accelerating. Everything is moving away from everything else at an increasingly fast pace. A concept that is difficult to imagine because the gravitational attraction between objects in the Universe should slow down the expansion.
To explain this apparent paradox, dark energy was introduced as being responsible for this acceleration stronger than gravity. The principle is connected to an idea that Einstein had proposed but later discarded. The famous cosmological constant that opposes gravity and prevents the Universe from collapsing. However, black holes posed a big problem. Their extremely strong gravity is difficult to counteract, especially at their center, where everything seems to break down into a phenomenon called singularity.
The evolutionary analysis of black holes
The result of the studies was obtained by studying the evolution of black holes. These are formed when massive stars reach the end of their lives. When they are instead located at the center of galaxies, they are called supermassive black holes and have masses from millions to billions of times that of the Sun in a very small space. Black holes can increase their size by swallowing matter, such as stars that get too close or by merging with other black holes.
To discover whether the growth of supermassive black holes is determined solely by these phenomena, the team examined data covering nine billion years. The researchers examined a particular type of galaxy: giant ellipticals. These evolved at the beginning of the Universe and then became dormant. They are so-called because they have stopped star formation, leaving little material available for the growth of the black hole at their center.
This means that any growth cannot be explained by normal astrophysical processes. But despite these logical deductions, the comparison between distant galaxies with local elliptical galaxies has shown a much larger growth than expected from accretion or mergers.
The conclusions of the studies
Further measurements, with populations of galaxies related to different points in the evolution of the Universe, show good agreement between the size of the cosmos and the mass of black holes. The measured amount of dark energy can be explained by the vacuum energy of the black hole. This is the first evidence that black holes actually contain vacuum energy and are coupled to the expansion of the cosmos.
Their mass increases as the Universe expands, a phenomenon called cosmological coupling. If further observations confirm this, 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, said, “We are asserting that there is evidence that typical black hole solutions don’t work on a long timescale, and we have the first astrophysical evidence for dark energy”.
“However, it’s not that other researchers haven’t proposed sources of dark energy. But this is the first observational paper where we’re not adding anything new to justify the source of dark energy. Black holes, in Einstein’s theory of gravity, are the dark energy”.