Photo credit: UWM
In the wild and violent world of outer space, it is nothing unusual for black holes to collide, supernovae to collapse or for two neutron stars to coalesce. When these catastrophic events occur, vast amounts of energy are released in the form of gravitational waves. These waves ripple through the fabric of space-time at the speed of light, eventually reaching earth in a much-diminished intensity, sometimes billions of years later.
Scientists at UW-Milwaukee’s (UWM) Leonard E. Parker Center for Gravitation, Cosmology & Astrophysics are at the heart of data collection and analysis for the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO made headlines back in 2015 when it went live because in short order its supersensitive instruments managed to detect gravitational waves from pairs of coalescing black holes. Astrophysicists around the world were surprised and delighted when LIGO recorded its first “chirp” from such a coalescence.
LIGO is providing a new window on the universe, and UWM scientists are playing an active role in collecting, processing and analyzing the data the LIGO equipment is capturing. This data will likely lead to a better understanding of the universe’s mysteries, vastness and the powerful events that shape it.
Of the 1,000-plus scientists working collaboratively on LIGO around the world, 40 or so are currently affiliated with UWM or can trace their heritage to UWM. Though the UWM scientists are studying events that occurred billions of years ago in distant reaches of the universe, they work right here in Milwaukee’s backyard.
In 2016, the Leonard E. Parker Center for Gravitation, Cosmology & Astrophysics received more than $5 million in extra-mural funding from the National Science Foundation, which accounted for about one-third of the extra-mural funding received by UWM’s College of Letters and Science.
Scientists have been trying to figure out a way to measure gravitational waves for generations. Though Einstein predicted them 100 years ago as a theoretical likelihood, there was no empirical way to measure them until now.
LIGO is a realization of that dream. The LIGO interferometers, located in Hanford, Washington and Livingston, Louisiana, superimpose two or more sources of light, creating an interference pattern, which can then be measured and studied. Einstein predicted gravitational waves in the General Theory of Relativity back in 1916, but until now it was impossible to gather empirical evidence about them. Now, however, precise measurements, as small as 1/10,000 the width of a proton, are possible. LIGO’s two L-shaped interferometers, with 2.5 mile-long supersensitive antenna arms that sense invisible gravitational waves facilitate such measurements.
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Efforts to create a device to measure gravitational waves began in the 1950s. “We’ve been essentially four generations of scientists since the conception of the idea. It has taken a long time to get here and a lot of tenacity and continued focus by everybody,” says Patrick Brady, director of UWM’s Center for Gravitation, Cosmology & Astrophysics.
Scientists at the UWM Center got into the LIGO collaboration on the ground floor when UWM became a charter member of the collaboration in 1997. Jolien Creighton, Professor of Physics at the Center for Gravitation, Cosmology & Astrophysics, has been active in LIGO since the beginning.
An expert in black holes and theoretical physics, Creighton is excited about LIGO’s promise. “We can never see two black holes colliding, except through gravitational waves,” he says. “Because they’re black, they don’t emit any light. This is the most extreme environment the universe provides to study theories of general relativity. Gravitational waves give us information about the universe that we can’t get in other ways. It’s looking at a different kind of environment that is not accessible through optical or electromagnetic observations.”
Gravitational powerhouses that swallow and trap particles and light, black holes form when massive stars collapse. Sometimes, black holes collide and swallow each other, creating a massive amount of energy in the process, and an even larger black hole with even greater gravitational pull. But despite their massive size and power, until very recently, no one could observe them.
All that changed when the LIGO interferometers became operational in September 2015. Since then, LIGO has detected two separate coalescences of binary black holes. Though these coalescences occurred billions years ago, the gravitational waves engendered by these events are still cascading through the universe.
The LIGO interferometers detect the weak gravitational waves associated with the coalescence of binary black holes. A tiny little chirp that lasts only one-quarter of a second is the only evidence of those gravitational waves. Behind that short chirp, however, is a mass of data that astrophysicists can analyze to get a better picture of the dynamics of these cataclysmic events.
Brady likens it to being a bird watcher who goes out into the woods. Often, bird watchers hear birdcalls before actually spotting the birds. “By the nature of the bird call, you can tell what sort of bird it might be, a cardinal or a goldfinch,” Brady says. With LIGO, “the chirp signal that we hear—just the specific way it goes ‘whoop’—tells us about the object that is out in the universe.”
Creighton says that extraneous noise in the raw data collected by the interferometers obscures the chirping sounds related to the gravitational waves. LIGO scientists have had to create algorithms and software programs that efficiently filter out the extraneous noise so that the all-important “chirps” from the gravitational waves can be isolated.
“The signals tend to be buried, so there’s a lot of filtering of the noise to extract the exact signals,” Creighton explains. “The [raw] data kind of sounds like a vacuum cleaner. Just kind of loud noise. If you imagine that someone is chirping outside somewhere in the distance, you’re never going to hear it if you’re standing right next to your vacuum cleaner. But if you were to record the [chirping] sound and then actively try to find those individual bird chirps in that data, you can extract it.”
Creighton is optimistic about what now can be discovered about black holes, collapsing supernovae or coalescing neutron stars. Different events will produce different signals. “We want to infer what kind of thing made those waves and what its properties are,” he says. Creighton hopes to discover how those black holes were moving, what their masses were, how much they were spinning, and how far away they were when the gravitational waves were produced. He wants to learn more about the dynamics when two black holes smash into each other. As a theoretician he believes this new data may provide strong tests of the theory of general relativity and yield much information about astrophysics and the early universe.
The LIGO collaboration requires a powerful computer infrastructure to handle the massive amounts of data coming its way through the interferometers. “The LIGO problem from a practical perspective is the needle in a haystack,” says Thomas Downes, LIGO data center manager at UWM’s Center for Gravitation, Cosmology & Astrophysics. Data from the LIGO interferometers streams in 24/7, 365 days per year. “The instrument is taking data all the time. Interesting things in space happen every so often. We need to make sure that every time something interesting happens that we actually find it,” Downes says.
The Center houses close to $2 million worth of computer equipment, and its processors account for about 20% of LIGO’s total computing requirements. In 2011, the Center received a $9 million grant from the National Science Foundation to fund the LIGO Data Grid, which analyzes data from a worldwide network of gravitational wave detectors.
Keeping the computers running in optimal condition is an important way of realizing the actual value of the LIGO instrument. “There’s a lot of time, money and intellectual effort that went into building the instrument. If you don’t get the computing side right, then you make all that useless,” Downes says.
