© Q D Wang et al / UMass Amherst / CXC / NASAThe centre of the Milky Way, as seen from NASA's Chandra X-ray Observatory. New evidence strongly suggests that there is a black hole lurking in there.
Falling into a black hole is aone-way trip - once matter or light crosses a threshold called the event horizon, it can never escape. While astronomers have identified many dark, dense objects they strongly suspect are black holes, it is difficult to prove that they possess event horizons, the defining feature of such objects. Among the proposed alternatives are dense balls of exotic matter called boson stars, which don't have event horizons.
Now Avery Broderick of the Canadian Institute of Theoretical Astrophysics and his colleagues have analysed previous infrared and radio observations of the galactic centre and put forward the strongest evidence yet that an object at our galaxy's centre does indeed have an event horizon.
The team reasoned that if the object were not a black hole, it should glow in the infrared. This is because the kinetic energy of matter hitting the object would be converted into heat. Given the rate that matter appears to be falling onto the central object, it should have a temperature of at least a few hundred Kelvin, they calculate. The resulting infrared glow would be 250 times as bright as the actual glow coming from the region containing the massive object and its disc of matter, when previously measured during quieter moments when the disc is not flaring up.
They conclude, therefore, that incoming matter really is disappearing behind a black hole's event horizon. "Too much energy is falling into the object for us to not see the emission from the surface," says Broderick.
Mitchell Begelman of the University of Colorado in Boulder says the new results put the tightest limits yet on any surface radiation from a candidate black hole, but falls short of proving the existence of an event horizon, because it is not a direct measurement. The argument depends on how quickly matter is falling onto the central object, something no one knows for sure, although the team's estimates look reasonable, he concludes.
Even assuming the team's arguments are ironclad, Broderick admits there is still one viable alternative to a black hole. A wormhole connecting one region of the universe to another could collect matter and energy at the galaxy's centre and spit it out somewhere we would not be able to see it, he says.
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