02 September 2009 by Stephen Battersby
DARK energy – the mysterious stuff that is causing the universe’s expansion to accelerate – could be illuminated by another dark enigma: the black hole.
Supermassive black holes are thought to exist at the core of most galaxies. When two galaxies collide and merge, these black holes will go into orbit around each other, spiral inwards and eventually collide. According to Einstein’s theory of general relativity, two orbiting black holes will generate powerful gravitational waves, ripples in the fabric of space-time. NASA and the European Space Agency (ESA) are planning a joint mission called LISA to detect these waves.
The pattern of gravitational waves from colliding black holes can tell us how far they are from us. Analysing light or X-rays from the merged galaxies allows us to determine the red shift – a measure of how much the universe has expanded since the waves were emitted. By studying the distances and red shifts of black-hole pairs at different eras of cosmic time, it is possible to calculate the expansion history of the universe. That should reveal whether the strength of dark energy has changed or remained constant – a strong clue as to which of several theories of dark energy is correct.
The distances and red shifts of orbiting black holes give us the universe’s expansion history
It was a study involving the distances and red shifts of supernovae that led to the discovery of dark energy in the first place. Using colliding black holes to do the same job was suggested some years ago, but early calculations indicated that this would yield no more information than what had already been gleaned from supernovae.
Now Adamantios Stavridis of Washington University in St Louis, Missouri, and colleagues have found that they can do better by exploiting the rotation of the black holes themselves. Pairs of spinning black holes will wobble slightly, and this modulates the gravitational waves they emit. The extra information in the modulated signal should allow us to more accurately determine the distance to the orbiting black holes. “It gives quite a drastic improvement,” says Stavridis….
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