You might call it the universe’s ultimate Clash of the Titans.
As if the idea of monster black holes lurking in the heart of a galaxies isn’t ominous enough, imagine two of them crashing together like a pair of Sumo wrestlers.
This is an inevitable outcome when galaxies collide. But such a death match has never been directly detected, at least not yet.
Astronomers are eagerly looking forward to the day when gravitational wave detectors are sensitive enough to pick up the fabric of space-time ringing from such a smashup. This would allow theorists to precisely test general relativity under extreme conditions where strong gravity is at work.
These events that happened long ago and far away are a prime target for space based gravitational wave detectors, like the long-planned Laser Interferometer Space Antenna (LISA) — a joint mission between NASA and the European Space Agency.
According to theory, a black hole merger will first look like a sinusoidal wave on LISA’s detectors. It will wiggle, increase in frequency and then flat-line after the black holes coalesce. The gravitational waves will tell scientists about mass, spin and orbital properties of the merger.
The beauty of gravitational wave astronomy, when it finally comes of age, is that it can look back into time to the birth of galaxies and it covers whole range of galaxy evolution.
Until then astronomers are hoping that upcoming mammoth all-sky optical surveys, conducted with Pan-STARRS (Panoramic Survey Telescope & Rapid Response System) and the LSST (Large Synoptic Survey Telescope) might serendipitously pickup the flicker of a collision. These will be sensitive to galaxies probably no farther away than 7 billion light-years. More importantly how would astrophysicists recognize a collision?
Astrophysicists are relying on computer modeling of black hole collisions to predict just what a merger would look like. It would be a brief burst of light from the heart of an interacting galaxy.
When a pair of galaxies collide and merge — an event taking a billion years to elapse — their black holes should merge too. Spiraling toward the center of the newly forming merged galaxy, the black hole pair will feel each other’s gravity at a distance of about 30 light-years, depending on their masses.
The orbits continue to shrink as gravitational waves carry away energy from the system. In some simulations the black holes recoil from the center.
But to be optically visible, the merger will need a lot of gas trapped in a giant space-time mixer accompanying the collision.
And the mergers are elusive. The brightening from coalescence would could be as short as one to two hours. During that period the glow from the central region of gas surrounding the black hole rises dramatically. The light would flicker out as the newly formed daughter black hole forms and gulps up the gas.
“These are robust feature that all light curves show in our models,” says Tamara Bogdanovic of the University of Maryland, “seeing such a glow would be a smoking gun for a black hole merger.”
So stay tuned. Within a decade we should have actual observations of black holes going “bump in the night.”