Ever since our universe exploded into being some 13.8 billion years ago, it has continued to expand. Stars and galaxies are continually moving apart — so fast that even if you were to take off from Earth at the speed of light, you'd never reach about 94 percent of galaxies, even after traveling for billions of years.
"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."
We know relatively little about this strange expanding force behind our ballooning universe, but we know that it's speeding up. Because energy is conserved in our universe, that means that if something is spreading out, there has to be some kind of energy behind it; hence, astronomers call this phenomenon "dark energy." Like dark matter, it is "dark" in the sense that its provenance is mysterious: we don't know what either is precisely, but both act as variables that help explain models of our universe.
All of this can be somewhat confusing, but essentially, many different cosmologists throughout history have come up with various theories to explain what we observe looking out into space. The universe's origin is pretty well understood: there is evidence for the events of the Big Bang theory, all the way back to the first milliseconds that the universe existed.
However, so far, it doesn't explain everything. Some things are still a mystery, but to complete the picture (and make certain math equations work), we use concepts like dark energy to make the whole thing fit. It can be quite frustrating, even for cosmologists, to have a big unknown stuck in the middle of our equations, but physicists are getting closer and closer to resolving these uncertainties.
A pair of recently published papers, one in The Astrophysical Journal and the other in The Astrophysical Journal Letters, provides some of the best evidence for what dark energy could be. The answer might lie in the core of black holes.
Astrophysicists Duncan Farrah and Kevin Croker, both at the University of Hawaiʻi at Mānoa, led this research, in association with researchers and institutions across nine countries. By scanning through existing datasets spanning 9 billion years, they found the first observational evidence for dark energy. If this new data can be supported, it would help us better understand some of the fundamental properties of the universe and could redefine our understanding of what a black hole even is.
Black holes are singularities, meaning points that effectively have no size, but huge masses and infinite density, and which are defined by the fact that nothing, not even light, can escape once they cross a certain radius around the black hole. There seems to be no upper limit to their size either, with supermassive black holes scaling as large as a billion times our own Sun. Most (but not all) black holes form from the corpses of stars that burn out, explode and collapse in on themselves. When they collide, they create massive gravitational waves of energy, which we've only recently been able to start detecting using a pair of facilities that make up the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment. That gravitational wave observatory observes huge astronomy events in the universe by observing how they warp how gravity moves through space time; typically, gravitational wave observatories only detect the biggest stellar explosions and mergers.
"When LIGO heard the first pair of black holes merge in late 2015, everything changed," Croker said in a statement. "The signal was in excellent agreement with predictions on paper, but extending those predictions to millions, or billions of years? Matching that model of black holes to our expanding universe? It wasn't at all clear how to do that."
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The first paper addresses some of the ways that black holes form and accumulate mass, by looking at elliptical galaxies at different stages of evolution. Elliptical galaxies are the most common in our universe, but are some of the oldest and most massive galaxies. They can tell us a lot about what the early universe was like. This research revealed that either observational data of black holes is more riddled with errors than previously thought, or some unknown mechanism is causing supermassive black holes to grow.
The second paper builds on this research and found that mid-sized black holes are growing at the same rate as the universe is expanding. This implies black holes are somehow contributing their own energy to the universe. Even more remarkably, this energy seems to be responsible for the accelerated expansion of the universe attributed to dark energy— and hence, could explain what dark energy is.
"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.''
Coming up with a nifty explanation for dark energy is one thing — and astronomers have been generating such theories for about a century — but this paper provides the first ever observational evidence. That is, evidence that can be measured and (importantly) tested, rather than just existing as an abstract theoretical "maybe."
"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,'' Farrah, the lead author of both papers, said in the same statement. "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.''
To back up these observations, the researchers plugged their data into numerous models and found consistency. To further confirm this, additional testing is needed, including better measurements of black holes merging and looking for signatures in the Cosmic Microwave Background, the "fossil radiation" that is left over from the Big Bang.
"This measurement, explaining why the universe is accelerating now, gives a beautiful glimpse into the real strength of Einstein's gravity," Croker said. "A chorus of tiny voices spread throughout the universe can work together to steer the entire cosmos. How cool is that?"
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