Colliding Black Holes and the Dawn of Gravitational Astronomy
BY IAN O'NEILL
New simulations of the most energetic collisions in the universe are helping astrophysicists understand how gravitational waves are generated, possibly giving us an exciting glimpse into the future of gravitational astronomy.
Black hole mergers are thought to be the most energetic events the universe has seen since the Big Bang, nearly 14 billion years ago. These events occur when two (or more) spinning black holes become trapped in their mutual gravitational wells, orbit and then collide, merging as one. The energy generated in these merging events are thought to create a very specific signature of gravitational wave emissions.
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According to Einstein’s theory of general relativity, gravitational waves should be created when massive objects accelerate through space. However, they have not been directly observed. Indirectly, we can see their impact when white dwarf binaries, for example, orbit one another — over time, as their orbits shrink, energy is lost. This energy must be carried away from the system by gravitational waves.
Although we have a pretty good idea about their properties, gravitational waves are notoriously difficult to detect directly, but should they become detectable in the future, a new era of gravitational astronomy may be possible. And black hole mergers could be the key to making this happen.
“An accelerating charge, like an electron, produces electromagnetic radiation, including visible light waves,” Michael Kesden, of the University of Texas at Dallas, said in a press release. “Similarly, any time you have an accelerating mass, you can produce gravitational waves.”
Kesden is the lead author of new research into black hole mergers published in the journal Physical Review Letters.
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“Using gravitational waves as an observational tool, you could learn about the characteristics of the black holes that were emitting those waves billions of years ago, information such as their masses and mass ratios, and the way they formed,” added co-author Davide Gerosa, of the University of Cambridge, UK. “That’s important data for more fully understanding the evolution and nature of the universe.”
Currently, there are several projects underway that are attempting to detect gravitational waves. Perhaps the most famous detector is the Laser Interferometer Gravitational-Wave Observatory (LIGO) situated at two locations in the US — in Louisiana and Washington. LIGO is set up to detect the passage of gravitational waves through our local volume of space.
Using precision lasers along two 4 kilometer-long tunnels in “L” shaped structures, the very slight perturbations of spacetime should be detectable as gravitational waves pass through our planet. Although LIGO has yet to detect a positive gravitational wave signal, it is currently undergoing upgrades that will boost its sensitivity. “Advanced LIGO” is scheduled to go online later this year. Europe is also building its own detector called VIRGO and the LISA Pathfinder Mission is planned to set up a gravitational wave detector in space.
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“The equations that we solved will help predict the characteristics of the gravitational waves that LIGO would expect to see from binary black hole mergers,” said co-author Ulrich Sperhake, also of the University of Cambridge. “We’re looking forward to comparing our solutions to the data that LIGO collects.”
The researchers have specifically focused on modelling the spin and precession of binary black holes as they orbit one another.
“Like a spinning top, black hole binaries change their direction of rotation over time, a phenomenon known as procession,” said Sperhake. “The behavior of these black hole spins is a key part of understanding their evolution.”
“With these solutions, we can create computer simulations that follow black hole evolution over billions of years,” said Kesden. “A simulation that previously would have taken years can now be done in seconds. But it’s not just faster. There are things that we can learn from these simulations that we just couldn’t learn any other way.”
The researchers hope that, with the help of their computer models, new details behind black hole mergers may be revealed. In doing so, the specific gravitational wave signal may be characterized so that when detectors such as Advanced LIGO register their first signals, we may quickly untangle what is generating the emission.
Like optical astronomy revolutionized our naked eye view on the universe and X-ray astronomy highlighted some of the most energetic phenomena in the cosmos, gravitational wave astronomy could give us a view of a previously invisible realm — a realm of massive interactions and collisions that characterize the very evolution of our universe.
Source:University of Cambridge