Black holes may explain a mysterious form of energy that makes up most of the universe, according to astronomers. The existence of “dark energy” has been inferred from observations of stars and galaxies, but no one has been able to explain what it is, or where it comes from.
The things, or matter, that make up the known world around us are only 5% of everything in the universe. Another 27% is dark matter, a shadowy counterpart to ordinary matter that does not emit, reflect or absorb light. However, the majority of the cosmos – around 68% – is dark energy.
The new evidence that black holes can be the source of dark energy is described in a scientific article published in The Astrophysical Journal Letters. The study was carried out by 17 astronomers in nine countries and was led by the University of Hawaii. The collaboration included researchers in the UK, based at STFC RAL Space, The Open University and Imperial College London.
By searching through data spanning nine billion years of cosmic history, astronomers have uncovered the first evidence of “cosmological coupling,” which would mean that the growth of black holes over time is linked to the expansion of the universe itself.
The idea that black holes may contain something called vacuum energy (a manifestation of dark energy) is not particularly new and was actually discussed theoretically as far back as the 1960s. But this latest work assumes that this energy (and therefore the mass of the black holes) will increase with time as the universe expands as a result of cosmological coupling.
The team calculated how much of the dark energy in the universe could be attributed to this process. They found that black holes could potentially explain the total amount of dark energy we measure in the universe today. The result could solve one of the most fundamental problems in modern cosmology.
Our universe began in a Big Bang about 13.7 billion years ago. The energy from this explosion of space and time caused the universe to expand rapidly, with all the galaxies flying away from each other. However, we expect this expansion to gradually decrease due to the effect of gravity on all things in the cosmos.
This is the version of the universe we thought we lived in until the late 1990s, when the Hubble Space Telescope discovered something strange. Observations of distant exploding stars showed that in the past the universe expanded more slowly than it is today.
So the expansion of the universe has not slowed down due to gravity, as everyone thought, but has instead accelerated. This was very unexpected and astronomers struggled to explain it.
To account for this, it was proposed that a “dark energy” was responsible for pushing things apart more strongly than gravity pulled things together. The concept of dark energy was very similar to a mathematical construct Einstein had proposed but later discarded—a “cosmological constant” that counteracted gravity and prevented the universe from collapsing.
But what is dark energy? The solution, it seems, may lie in another cosmic mystery: black holes. Black holes are often born when massive stars explode and die at the end of their lives. The gravity and pressure of these violent explosions compress enormous amounts of material into a small space. For example, a star roughly the same mass as our Sun would be squeezed into a region of only a few tens of kilometers.
A black hole’s gravitational pull is so strong that not even light can escape it – everything is sucked in. At the center of the black hole is a place called a singularity, where matter is crushed to a point of infinite density. The problem is that singularities are a mathematical construct that should not exist.
The black holes at the center of galaxies are much more intense than those born when stars die violently. These galactic “supermassive” black holes can weigh millions to billions of times the mass of the Sun.
All black holes increase in size by accreting matter, by swallowing stars that come too close, or by merging with other black holes. So we expect them to get bigger as the universe ages.
In the latest paper, the team looked at supermassive black holes at the center of galaxies and found that these black holes gain mass over billions of years.
The team compared observations of elliptical galaxies, which lack star formation, in the past and today. These dead galaxies have used up all their fuel, so any increase in black hole mass during this time cannot be attributed to the normal processes by which black holes grow by accumulating matter.
Instead, the team proposed that these black holes actually contain vacuum energy and that they are “coupled” to the expansion of the universe, so that they increase in mass as the universe expands.
This model nicely provides a possible origin for the dark energy in the universe. It also circumvents the mathematical problems that affect some studies of black holes, because it avoids the need for a singularity at the center.
The team also calculated how much of the dark energy in the universe could be attributed to this coupling process. They concluded that it would be possible for black holes to provide the necessary amount of vacuum energy to account for all the dark energy that we measure in the universe today.
This would not only explain the origin of dark energy in the universe, but would also make us radically rethink our understanding of black holes and their role in the cosmos.
Much more work needs to be done to test and confirm this idea, both from observations of the sky and from theory. But we may finally see a new way to solve the dark energy problem.
Chris Pearson, Astronomy Group Lead, Space Operations Division at RAL Space, and Visiting Fellow, The Open University and Dave Clements, Reader in Astrophysics, Imperial College London
This article is republished from The Conversation under a Creative Commons license. Read the original article.