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Astrophysicists Listening as Black Holes Collide

gravitywavesCataclysmic events, like the merger of two massively dense black holes, result in vibrations or ripples in the universe that eventually reach Earth. That is how Embry-Riddle Aeronautical University astrophysicists describe gravitational waves that stretch and compress the fabric of space.

“The metaphor that we like to use is that gravitational waves let us hear the universe,” said ERAU physicist Dr. Brennan Hughey. “Most astronomy is done through photons in one way or another – everything from radio to optical to gamma rays is an example of photons at different energies, at different wave lengths. Gravitational waves are a completely different messenger that allows us to listen to the universe rather than look at the universe.”

To receive this messenger, a team of ERAU scientists has been deeply involved in the LIGO (Laser Interferometer Gravitational-Wave Observatory) project from the design and construction of interferometers to analyzing the data. In September, Advanced LIGO detectors in Livingston, Louisiana and Hanford, Washington successfully “heard” the waves produced during the final fraction of a second when two black holes collided and become one massive spinning black hole a billion light years away.

“Gravitational waves are a natural consequence of general relativity, where space and time meet,” said Hughey. “Einstein published the first paper on them a year after discovering general relativity in 1915. So, basically, it’s taken a century for this prediction of gravitational waves that came from Einstein’s theory of general relativity to actually be something that we’re able to detect out in space.”

The black holes that LIGO sensed each had a mass 30 times the density of the sun. Because the universe is so vast and the waves are so weak – about a billionth of an atom – Hughey says it takes a major event like this to be picked up. When heard through an audio file, the merger sounds like a water droplet.

“The thing that’s maybe a little surprising about this observation is that the black holes are as heavy as they are. There’s a whole range of masses of black holes, but this is what we talk about as being stellar mass black holes, which are black holes on the scale of stars and these are on the upper end of what we’d expect to see there.”

As Hughey explains, black holes do not emit light, so they cannot be seen, but LIGO interferometers will help scientists understand the lifecycles of black holes and where they are in the universe.

“Black holes are merging all the time in our universe. The universe is a very big place, so it’s not happening in our own galaxy, but if you look out to millions, billions of light years, there are a lot of these events happening. So we expect that by the time we get our LIGO instruments to design sensitivity, which is going to be a region of about a factor of three farther into space than we’re able to look at right now, we should be able to pick up tens to hundreds of these events in a year.”

Up until September, LIGO had been a physics experiment. But Hughey is hoping this technology will open a new field of study, gravitational wave astronomy. He says LIGO will also help scientists learn more about the of collision of neutron stars, the core collapse of supernovae and possibly the existence of cosmic super strings, which are theorized to be topological defects in the universe formed billions of years ago.

“We do think space is a pretty noise place. Simulations suggest the universe may sound like a jungle at night where you’re going to get some sounds like frogs or crickets out in the darkness and this would be the sounds of the universe with these binary coalescences. We’re not there yet, but that’s where we want to get in another decade or so and I think that will be pretty amazing to be able to listen to the universe at night the same way that you listen to crickets chirping outdoors.”

From the ERAU LIGO Lab, Hughey has been analyzing the gravitational wave signal. Currently he is working to remove background noise to make the sounds even clearer as he searches for transient waves.

Dr. Michele Zanolin is the principal investigator of the Embry-Riddle LIGO group. He also coordinates the supernova subgroup of LIGO, which focuses on detecting gravitational waves from core collapse supernovae and extracting astrophysical information from them.

Dr. Andri Gretarsson helped design and characterize the mirrors that form the heart of the LIGO detectors.

The Embry-Riddle astrophysics group also includes doctorate student Marek Szczepanczyk, plus undergraduate students Kiranjyot Gill, Marina Koepke, James Pratt and Sophia Schwalbe. It is the only LIGO team in Arizona and has been funded by the National Science Foundation since 2006.

Hughey says the next Advanced LIGO “observation” run could happen as early as this summer.

By Bonnie Stevens






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