Seeing objects that don't reflect light is tricky business. And black holes are as elusive as a target can be. The gravitational whirlwind of these cosmic wells draws inward with so much force that even light can't escape their grasps.
This poses a tricky problem for scientists, whose instruments typically rely on light-whether it is visible light, radio waves, X-rays or infrared-to observe objects in space.
Astronomers currently spot black holes by detecting the high-energy radiation emitted by swirling matter falling into them. Before matter passes a black hole's point of no return, called the event horizon, any radiation it emits can still escape. In a decade, however, scientists hope to spot black holes by looking at the warps in space-time created by their immense gravity.
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Seeing the invisible
Researchers have known about black holes ever since their existence was predicted by theories such as Einstein's general relativity. Knowing that they are invisible, however, scientists have devised clever ways to detect the presence of black holes indirectly.
One method has been to watch the demise of an object falling into these cosmic graves.
"If material actually falls into a black hole, it gets shredded apart and it heats up," said Roeland van der Marel, a researcher with the Space Telescope Science Institute. "As it heats up, it starts emitting radiation and this radiation we can observe. In particular, we can often see X-rays coming from black holes."
These X-rays do not penetrate the Earth's atmosphere and can only be detected with telescopes positioned in space. NASA's Constellation-X Observatory-a combination of four X-ray telescopes working together to generate 100 times the power of any X-ray satellite mission-will be able to perform such sensitive observations.
"The superior sensitivity of Con-X will give us enough information to be able to ask a fundamental question: are black holes really described by the Einstein's Theory," said Chris Reynolds, a Constellation-X science team member and a researcher at the University of Maryland.
"In essence, Einstein's theory makes specific predictions about the way the X-ray spectrum changes in time as gas orbits around the black hole," Reynolds told SPACE.com. "With Con-X, we can look for those variations in the spectrum and see if they match Einstein's predictions."
Another technique used for identifying black holes is looking at how objects neighboring one behave.
"Because a black hole is quite massive, any material, for example stars, that move close to it, will feel a lot of gravity," van der Marel said. "As a result of this, these stars will move quite fast. So another method of actually detecting the presence of black holes is looking for objects near the black hole that you can actually see that move much faster than you would naively have expected on the basis of the assumption that there would not be a black hole."
The motions of these stars are detectable by ground based telescopes like Gemini, and with space-based telescopes such as the Hubble Space Telescope.
The ripple effect
Gravitational wave detectors go about detecting black holes in a completely different way, said Doug Richstone, a Laser Interferometer Space Antenna (LISA) science team member and a researcher from University of Michigan.
Einstein's general theory of relativity predicts that concentrations of mass or energy warp the fabric of the universe. According to his theory, changes in the shape and concentration of mass or energy will cause distortions that move out like ripples on a pond.
When a black hole swallows a neutron star or another black hole, or when two black holes merge or just orbit each other very closely, they'll emit these ripples in space-time, Richstone told SPACE.com.
Scientists can detect gravity waves by how they move objects in space. For example, two probes sitting stationary in space, apart from each other, will jiggle slightly when a gravity wave rolls by. This is akin to buoys at sea displaced by waves. LISA-the first dedicated space-based gravitational wave observatory and a joint venture of NASA and ESA-will be able to measure such distortions of space-time.
"Just like you're floating on the ocean, as waves go by, you go up and down," Richstone said. "And so the trick is to measure the separation between test masses that have been isolated from all forces except gravitational forces."
Similar to LISA, a ground based observatory-with two installations situated in Hanford, Washington, and Livingston, Louisiana and called the Laser Interferometer Gravitational-wave Observatory (LIGO)-will be used for detecting cosmic gravitational waves.
LIGO works by shining a laser light to a detector. As ripples pass by, the light beam should be slightly perturbed. The different frequencies in the waves will allow astronomers to figure out the source of a disturbance. LIGO, a collaborative effort between Caltech and MIT, began its search for cosmic waves in 2002.
"It is possible that, depending on how long it will take for LISA to get to space, that LIGO will detect a gravitational wave source first. But I think it's just a question of which one will do it first," Richstone said.
LISA is scheduled to launch in 2015.
"No one has detected gravitational waves yet. If LISA flies and works properly, it will detect gravitational waves from astronomical sources," Richstone said. "If it doesn't, then Einstein's theory of general relativity is wrong."
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