Black holes are dark secrets, shrouded in churning space-time and scrunched into points smaller than an atom.
In recent years, astronomers have successfully penetrated some of their mysteries, but fundamental questions about what they are and how they work are still unanswered.
SPACE.com asked several black hole experts what tops their list of recent black hole discoveries and what they think the biggest questions surrounding these captivating objects are.
Several of the scientists interviewed said the single most profound discovery to emerge in their field in recent years is the intimate link that appears to exist between supermassive black holes and their host galaxies. This idea is sometimes called co-evolution.
"We have come to realize that massive black holes in galaxies are central players in the story of how entire galaxies assemble," said Christopher Reynolds, a black hole researcher at the University of Maryland. "This is a very surprising aspect of the universe that we are just starting to learn about."
"Ten years ago the idea that a supermassive black hole at the center of our galaxy might exist was just a possibility," said Andrea Ghez of UCLA. "Thanks to advances in high-resolution imaging technology, this idea has become almost a certainty."
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Supermassive black holes are thought to have masses equal to millions or even billions of Suns. Recent studies have found that black holes and galaxies can affect each other's formation and evolution.
"This started locally with the discovery of the very close correlations between the masses of black holes and the properties of their host galaxies, and then moved on to the realization that galaxy formation and star formation had to be partly controlled by the effects of black holes and accretion," said Christopher Kochanek of Ohio State University.
Black hole spin
While scientists have yet to observe a black hole directly, they have spotted X-ray radiation created by friction near the event horizon, the perimeter around a black hole beyond which nothing, not even light, can escape.
"This is an amazing achievement," Reynolds said. "Through these observations, we already have strong evidence for the significant slowing of time close to a black hole as well as the 'tornado-like' motion of space-time induced by black hole spin."
But there is still plenty of room for improvement.
"There are lots of things [about black holes] that we'd like to measure or measure better to confirm what we think," Kochanek said. "That is the biggest frustration."
For example, scientists are only beginning to accurately measure the speed at which a black hole rotates, a property called spin.
This value is crucially important for understanding black holes. Scientists can completely characterize the motions of a black hole if its mass and spin are known.
"We know of no other object that is as simple as a black hole except for an elementary particle such as an electron," said Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics (CfA).
Scientists have been measuring black hole masses for more than 30 years, but clocking the spin of one has proven much more difficult.
But last fall, a team led by McClintock and his colleague Ramesh Narayan announced they had reliably measured the speed of one black hole located 35,000 light years away.
They calculated the spin to be an astounding 950 spins per second — just below the theoretical rotation limit of 1,150 spins per second.
"We expect to determine a dozen black hole spins during the next several years," McClintock told SPACE.com.
Oldies but goodies
The questions keeping black hole researchers up at night aren't necessarily new. Some have been around since black holes were first predicted.
"The single biggest mystery is: 'What is the state of matter at the center of a black hole?'," Ghez said. "Our physical description of the universe breaks down at the center of a black hole."
For example, current black hole theory states that gravitational forces inside a black hole reach infinity. But this almost certainly can't be correct.
"Any time things go to infinity in physics, we know we haven't gotten it right," Ghez said in an e-mail interview.
The center of a black hole is known as the singularity. Theory predicts it is far smaller than an atom, yet contains all the mass and rotation of a star.
Even for scientists who study black holes for a living, this idea is still mind-boggling.
"Can you imagine your car, the Earth, the Sun, or a billion suns being crushed by gravity to a point too small to view with any microscope?" McClintock said. "How can all the forms and matter around us, and the atoms of which they are composed, disappear into a massive, spinning point?"
Many scientists suspect that fundamental black hole questions like these won't be answered until quantum mechanics and general relativity, the theories that describe how gravity works at the smallest and biggest scales of the universe, respectively, are reconciled.
"What really happens at the black hole event horizon? Or in the central singularity? Solving that would likely require, or lead to, some fundamental new physics, perhaps on par with general relativity theory itself," said George Djorgovski of Caltech.
New ground and space-based instruments expected to go online in the coming years could help answer some of these questions and, in doing so, reveal something fundamental about the universe.
"Historically, any time we have learned something new about gravity, our understanding of the universe has been changed in profound ways," Reynolds said. "There is no reason to believe that this time would be any different."
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