Closing in on Gravitational Waves
For as long as man has looked at the sky, we have relied on electromagnetism to probe our universe, gradually expanding from visible light into other areas of the spectrum: infrared, microwave, x-ray, gamma ray, and so forth. But certain things aren’t detectable using conventional methods: black holes, for instance, or cold dark matter. Now gravity provides an exciting new tool with which to probe the universe.
Move a large mass very suddenly — or have two massive objects suddenly collide, or a supernova explode — and you would create ripples in spacetime, much like tossing a stone in a still pond.
WATCH VIDEO: Did you know there’s a black hole in the center of our galaxy?
These waves were first predicted by Einstein’s general theory of relativity in 1916. At the time, we didn’t have the technology to detect them, since they are very weak and fade very quickly, although scientists found indirect evidence of their existence in observations of a binary pulsar — work that won the Nobel Prize in Physics in 1993. But now we have the technology, and these gravitational waves should be detectable with very sensitive instruments — if scientists had a good idea where to look.
Working with his thesis advisor, an undergraduate astrophysics major at University of California, Santa Cruz named Luke Zoltan Kelley has completed a study that should tell astrophysicists where such mergers are mostly likely to occur.
The details were just published in the December 10 issue of Astrophysical Journal Letters.
One of the best possible sources for gravitational waves would be a merger between compact binary neutron stars. A neutron star is the remnant of a supernova explosion, packing a large amount of mass in such a small space that it is incredibly dense: imagine all of the sun’s mass crammed into a sphere just a few miles wide. Two such stars rotating around each other would form a binary system, getting closer and closer until they merged in a violent explosion.
That collision would produce ripples of gravitational waves that should be detectable by a a gravitational wave detector — namely, the upgraded Laser Interferometer Gravitational Observatory (LIGO), which has been searching space for gravity waves since it opened in 2002, and is due for an upgrade to further increase its sensitivity. LIGO — pictured below — is a joint project between scientists at MIT, Caltech, and many other colleges and universities.
How do you make a giant interferometer that can pick up tiny ripples in spacetime? You take one very large mirror and hang it to form an arm, then hang two more mirrors perpendicular to it to form an L-shape. Viewed from above, the two arms form an L shape.
Then you send laser light through a beam splitter to divide the beam between the two arms, and let the light bounce back and forth a few times before returning to the beam splitter. LIGO actually has three such detectors, two in Richland, Washington and one near Baton Rouge, Louisiana, since LlGO needs to operate at least two detectors at the same time as a control, so they don’t get false positives.
How will the detectors know if a ripple of gravity passes through the Earth? Per Caltech’s official LIGO Website:
If the two arms have identical lengths, then interference between the light beams returning to the beam splitter will direct all of the light back toward the laser. But if there is any difference between the lengths of the two arms, some light will travel to where it can be recorded by a photodetector. The space-time ripples cause the distance measured by a light beam to change as the gravitational wave passes by, and the amount of light falling on the photodetector to vary. The photodetector then produces a signal defining how the light falling on it changes over time. The laser interferometer is like a microphone that converts gravitational waves into electrical signals.
But space is very large, and if LIGO scientists could pinpoint their search, they might detect those ripples sooner. Also, should they detect them, they will need to match that data with telescope observations of the merger event to confirm.
This is where Kelley’s recent work can help: his findings suggest that the galaxy catalogs currently proposed as a means of following up on potential gravity wave detections need to account for some unusual behavior if they’re to be useful for confirmation. And the scientists might want to look beyond the nearest galaxies.
See, compact binary systems — pairs of neutron stars, black holes, or one of each — don’t just spiral around each other, they are also speeding through space.
“By the time the two objects merge, they are likely to be located far away from the galaxy where they were born,” Kelley said in a press release announcing his results. That’s because they get a sort of recoil “kick” from slight asymmetries in their parent supernova explosions — in the case of a compact binary system, the maximum velocity would reach 200 kilometers per second. That means when the two stars finally get around to merging, they could be far away from the galaxy of their birth — good news, as it happens, for optical confirmation.
Kelly and his advisory, Enrico Ramirex-Ruiz, ran multiple computer simulations to investigate how different kick velocities would affect where compact binaries were likely to merge. After running their model for a simulated 13.8 billion years (the current age of our universe), he found that variations in the kick velocity definitely affects how such systems are distributed.
Also, he concluded that a large survey telescope would be more likely to observe such a merger if it looked for such events away from a galaxy’s bright light. This in turn would give LIGO scientists an extra clue as to where to search their data for a gravity wave signal.
Kelley and his UCSC colleagues are now trying to determine what such an optical signal might look like. It’s quite impressive work for someone who hasn’t even finished college yet. The USCS release reveals that as Kelley completes his senior thesis, he is also teaching an astrophysics class “and deciding where he will go to graduate school.” I’m guessing he’ll have a lot of options.
Leading image: Visualization of the production of gravitational waves after a black hole merger (MPI for Gravitational Physics/W.Benger-ZIB)