According to astronomers, black holes could explain a mysterious form of energy that makes up most of the universe. 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 came from.
The stuff or matter that makes up the familiar world around us is only 5% of everything in the universe. Another 27% is dark matter, a shadowy counterpart of ordinary matter that does not emit, reflect, or absorb light. However, the majority of the cosmos – about 68% – is made up of dark energy.
New evidence that black holes may be the source of dark energy is detailed in a scientific article published in The Astrophysical Journal Letters. The study is the work of 17 astronomers in nine countries and was led by the University of Hawaii. The collaboration included researchers in the UK based at the STFC RAL Space, the Open University and Imperial College London.
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 related to the expansion of the Universe itself.
The idea that black holes might contain something called vacuum energy (a manifestation of dark energy) is not particularly new, having been discussed theoretically as far back as the 1960s. However, this latest work posits that this energy (and thus the mass of the black holes) would increase over time as the universe expands due to 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 possibly 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 was created with a big bang about 13.7 billion years ago. The energy of this explosion of space and time caused the universe to expand rapidly, with all galaxies flying away from each other. However, we expect that this expansion would gradually slow down due to the effects 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 the universe used to expand at a slower rate than it does today.
So the expansion of the universe is not slowing down due to gravity, as everyone thought, but is accelerating. This was highly unexpected and astronomers have struggled to explain.
To explain this, it has been suggested that a “dark energy” is responsible for pushing things apart more than gravity pulls things together. The concept of dark energy was very similar to a mathematical construct that Einstein proposed but later discarded – a “cosmological constant” that counteracted gravity and kept 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 formed when massive stars explode and die at the end of their lives. The gravity and pressure in these violent explosions compress massive amounts of material into a small space. For example, a star roughly the same mass as our sun would be squashed into a space of just tens of kilometers.
The gravitational pull of a black hole is so strong that not even light can escape – everything is sucked in. At the center of the black hole is a location called a singularity, where matter is compressed into a point of infinite density. The problem is that singularities are a mathematical construct that shouldn’t exist.
The black holes found at the centers of galaxies are much more powerful than those formed when stars die violently. These galactic “supermassive” black holes can weigh millions to billions of times the mass of our Sun.
All black holes grow by accumulating matter, swallowing stars that come too close, or merging with other black holes. So we expect them to get bigger as the universe gets older.
In the latest publication, the team studied supermassive black holes at the centers of galaxies and found that these black holes increase in mass over billions of years.
The team compared past and present observations of elliptical galaxies lacking star formation. These dead galaxies have used up all their fuel, so an increase in their black hole mass during this time cannot be attributed to the normal processes by which black holes grow through accumulation of 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 their mass increases as the universe expands.
This model cleanly provides a possible origin for the dark energy in the universe. It also circumvents the mathematical problems that plague some black hole studies, as 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 of the dark energy we measure in the Universe today.
Not only would this explain the origin of dark energy in the universe, but it would also lead us to 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 through observations of the sky and through theory. But maybe we’ll 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, associate professor of astrophysics, Imperial College London
This article was republished by The Conversation under a Creative Commons license. Read the original article.