A surprising discovery from comparing the sizes of modern and ancient black holes could explain one of the biggest mysteries in physics.
Observations have shown that black holes are much larger than they were nine billion years ago, even compared to the size of their galaxies.
In an attempt to explain the magnitude of this anomaly, astrophysicists have found an explanation for dark energy, the mysterious force that causes the universe to fly apart in all directions at an increasing speed. Black holes may actually be the source of this mysterious dark energy.
When astronomers used the apparent brightness of supernova explosions to measure the expansion rate of the universe, they assumed it would slow down and just wanted to see how much. Instead, to their surprise, they found that it was accelerating.
However, what dark energy is remains a mystery, let alone where it comes from. Many efforts have been made to answer this question: anyone who does this is guaranteed a Nobel Prize. Now a team of astrophysicists from nine countries hopes to answer the above question with two new papers.
The scientists’ articles are based on a discovery that wasn’t really in the researchers’ theories when they started.
“This is a truly amazing result,” said Dr Dave Clements of Imperial College London in a statement. “We started by looking at how black holes grow over time and may have found the answer to one of the biggest problems in cosmology.”
The key hurdle to understanding dark energy can be boiled down to one question: if there is a force pulling the universe apart, why do we see gravity pulling objects together? The only thing we know about dark energy is that it seems to apply at least strongly, only on a very large scale, so other forces dominate closer.
So supermassive black holes are an interesting test case – they suck in material from such vast distances that we can see the transition between gravity dominance and dark energy obscured by smaller objects.
The growth of black holes is too slow and erratic to track its speed from what we see happening. However, by comparing the mass of a sample of distant black holes, as they were billions of years ago, with the mass of black holes surrounding us in nearby galaxies, we can get an idea of the average growth rate.
A team of astrophysicists report that nearby samples average 7 to 20 times the mass of those seen nine billion years ago, relative to the size of their galaxies. These are growth rates that do not match what we know about the merger and absorption rates of stars that come too close, minus evaporation due to Hawking radiation.
However, if black holes contain vacuum energy, which is associated with the expansion of the universe, they will grow faster. The authors calculated that if the supermassive black holes at the center of galaxies produce dark energy, that explains most of what we think exists. Stellar black holes in galactic disks, and possibly halos, may explain the rest.
“We’re actually saying two things at once: there’s evidence that typical black hole solutions don’t work for you in the long run, and we have the first proposed astrophysical source of dark energy,” said Dr. Duncan Farra of the University of Hawaii.
Such a major statement will undoubtedly be subject to more scrutiny than a regular article, and the authors acknowledge this possibility. They also offer tests that can confirm or disprove their theory.
Articles published in The Astrophysical Journal and The Astrophysical Journal Letters.
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