Black holes today are much bigger than they were nine billion years ago, even compared to the size of their galaxies, observations have found. In an effort to explain the scale of this anomaly, astrophysicists have hit on an explanation for dark energy, the mysterious force causing the universe to fly apart at ever increasing rates. Black holes might actually be the source.
When astronomers used the apparent brightness of supernova explosions to measure the rate of expansion of the universe they expected to find it slowing down, and just wanted to see how much. Instead, to their astonishment, they found it was accelerating. A tiny proportion of the physics community remain unconvinced, but the data has been confirmed so many times there is overwhelming agreement the acceleration is happening and something called dark energy is responsible.
What dark energy is, however, remains a mystery, let alone where it comes from. Plenty of efforts have been made to answer this – there’s a guaranteed Nobel Prize for anyone who does so to general satisfaction – but so far none have gained widespread traction. A team spread across nine nations hope to change that with two new papers.
Like the original identification of dark energy, their papers came from a discovery that was not really on the researchers’ radar when they began. “This is a really surprising result,” said Dr Dave Clements of Imperial College London, in a statement. “We started off looking at how black holes grow over time, and may have found the answer to one of the biggest problems in cosmology.”
A key obstacle to understanding dark energy can be summed up in one question: If there is a force pushing the universe apart, why do we see gravity attracting objects together? The one thing we know about dark energy is that it seems to only apply, at least strongly, at very large scales, so other forces dominate closer in.
Supermassive black holes therefore provide an interesting test case – they suck in material from such enormous distances that we might see the transition between the domination of gravity and dark energy in a way concealed by lesser objects.
Black hole growth is too slow and erratic to track its rate from what we can see occurring. However, by comparing the mass of a sample of distant black holes as they were billions of years ago with those around us in nearby galaxies, we can get a sense of average growth rates.
The team report in one paper that the nearby samples average seven to 20 times the mass of those seen nine billion years ago, relative to the size of their galaxies. That’s a growth rate that doesn’t fit with what we know about merger rates and consumption of stars that get too close, minus evaporation through Hawking Radiation.
However, if black holes contain vacuum energy that is linked to the expansion of the universe, they would grow faster. Indeed, the authors calculate, if supermassive black holes at the center of galaxies are producing dark energy it would account for most of what we think exists. Stellar black holes in galactic disks, and possibly halos, might account for the rest.
“We're really saying two things at once: that there's evidence the typical black hole solutions don't work for you on a long, long timescale, and we have the first proposed astrophysical source for dark energy,” Dr Duncan Farrah of the University of Hawai'i said. “What that means, though, is not that other people haven't proposed sources for dark energy, but this is the first observational paper where we're not adding anything new to the universe as a source for dark energy: black holes in Einstein's theory of gravity are the dark energy.''
Such a big claim will undoubtedly face more scrutiny than the typical paper, and the authors acknowledge the possibility: “Selection and measurement biases are both underestimated.” They also suggest some future tests that could support or reject their theory. However, they consider a number of possible explanations for the unexpected growth they find to be unlikely, leaving the possibility of black holes as dark energy factories as the strongest contender.
Crucial to the finding is the long-standing observation that, in the nearby universe, “More massive black holes tend to reside in more massive galaxies.” The black hole masses in this study were compared with the mass of the visible stars in the galaxy, ruling out the possibility they had grown through more mergers than expected, or that the team were somehow studying undersized galaxies from billions of years ago.