What exciting new discoveries await astronomers in the field of gravitational wave astronomy?

Aerial view of LIGO Hansford
An aerial view of LIGO Hansford.

A new-and-improved gravitational wave observatory is open for business. After a five-year-long upgrade, the Advanced Laser Interferometer Gravitational-wave Observatory (ALIGO) resumed its search for gravitational waves on September 18, 2015. In this first run, its sensitivity is already three times that of the initial LIGO project’s best run, and eventually it will be running at ten times the sensitivity.

When any massive body accelerates through spacetime, it acts like a paddle moving through water, generating ripples known as gravitational waves. The more massive the body and the faster it moves, the larger the ripple. But even though they’re expected from the theory of general relativity and indirectly detected in pulsar observations, gravitational waves have eluded direct detection so far. ALIGO might fix that predicament.

“It is incredibly exciting — and satisfying — to see the planning, designing, building, testing, installing, and commissioning of Advanced LIGO come together so successfully. Kudos to the whole team!” David Shoemaker, director of the MIT LIGO Laboratory, said in a press release.

The original LIGO was built to detect gravitational waves from mergers of neutron stars and stellar-mass black holes. It didn’t succeed, but it never had the capability to detect anything beyond the most powerful sources. Now, with their improved sensitivity, the new ALIGO detectors could spot inspiraling binary systems 3.5 times farther away than the original LIGO project — that’s as far as 225 million light-years away. And since ALIGO can now detect gravitational waves with shorter wavelengths, the instrument could even detect supernovae exploding outside our galaxy.

Advancing to ALIGO

LIGO optical path
This diagram shows the L-shaped arms of the LIGO detector. The beam splitter sends a laser beam traveling down both arms at once, bouncing off mirrors at the ends of the arms, and traveling back to create an interference pattern that the photodetector picks up.

LIGO is a unique observatory consisting of two L-shaped detectors, one based in Livingston, Louisiana, and another on the Hanford Nuclear Reservation Site along the Columbia River in Washington State. A vacuum fills each detector, and laser beams travel up and down through this vacuum, bouncing off mirrors before returning to interfere with each other. A photodetector records the interference pattern.

A passing gravitational wave would warp spacetime, altering the length of one or both arms according to the theory of general relativity, affecting the resulting interference pattern. Keep in mind, the sort of changes ALIGO is looking for are tiny, less than 1/1000th of the width of a proton. Though the system is isolated and various mechanisms help amplify the signal, it’s still incredibly difficult to pick the signal out amidst the background noise of low-level seismic activity or human-generated rumbling.

Since gravitational waves travel at the speed of light, we wouldn’t even have time to blink before a signal passed us by. But two detectors, placed 1,865 miles apart, help researchers triangulate the signal and pinpoint its source after the fact.

Over the last five years, the LIGO team has lengthened the L-shaped arms of the detectors, replaced the laser with a more powerful one, improved the isolation from seismic activity, and made other improvements. Not only is ALIGO’s sensitivity greater, but it also has access to a slightly greater range of gravitational wave frequencies, so it could detect gravitational waves not just from inspiraling neutron stars and black holes, but also from faraway supernovae.

ALIGO’s first observing run will last three months. Then engineers will fine-tune the instruments before conducting additional runs, eventually getting ALIGO to ten times the sensitivity of the original LIGO project.

And should a detection occur, 75 observatories around the world have agreed to follow up immediately afterward, observing everything from radio waves to X-rays to determine the event’s origin.

Will ALIGO Succeed?

Advanced LIGO Control Room
Advanced LIGO control room (Hanford site) during the start of the first operational run.
K. Burtnyk

Astronomers have been attempting to detect gravitational waves directly since the 1960s, even before indirect evidence from pulsars supported their existence. But LIGO, a $620 million project, detected nothing in its years of operation between 2002 and 2010.

To some extent, this result was expected — detection was considered plausible for LIGO, but not likely. And to some extent, this result was useful. For example, by not detecting the primordial gravitational waves that would have been generated in the Big Bang, LIGO ruled out some more exotic versions of the theory of cosmic inflation, which describes how the universe expanded shortly after the Big Bang.

Moreover, LIGO showed that detection could be done. Scientists periodically inserted artificial signals, known as blind injections, into the data, and they were able to recover these artificial signals. If a real, loud-enough signal had passed by, LIGO would have heard it. Now, with ALIGO, the probability of detection moves from plausible to likely.

While the first direct detection of gravitational waves would surely garner scientific accolades, a non-detection from ALIGO could prove just as fruitful — not seeing anything this time around would send theorists back to the drawing board.

The Future of Gravitational Wave Astronomy

Sensitivity and Frequency of LIGO, ALIGO, and eLISA
This plot compares the sensitivity and frequency range (i.e., type of gravitational-wave-producing object) for LIGO, ALIGO, and eLISA. LIGO could plausibly have detected the most powerful gravitational waves signals, but ALIGO's better sensitivity gives it a better chance. Click for a bigger version or go to the Gravitational Wave Sensitivity Curve Plotter by Christopher Moore, Robert Cole, and Christopher Berry to plot your own version.

Future systems will push the bounds of gravitational wave astronomy even further. The Laser Interferometer Space Antenna (LISA) Pathfinder is scheduled to launch on December 2nd. Though this mission won’t detect gravitational waves themselves, it’s designed to test key technologies for a full mission known as eLISA (short for “evolved” LISA), which might launch sometime in the 2030s.

LIGO India will also utilize technology from a planned-for, but never-built, third LIGO array stored at the Hanford facility. Engineers expect LIGO-India to be operational in the early 2020s, and its observations will further the existing LIGO capabilities.

Looking to hunt for gravitational waves yourself? You can enlist your computer’s idle time to run Einstein@Home. Similar to the famous SETI@Home, this new project sifts through data from LIGO, Arecibo, and the Parkes Multibeam Survey to search for radio pulsars and continuous gravitational wave sources. Though Einstein@Home users haven’t detected gravitational waves yet, they have discovered 51 new pulsars as of February. And who knows what the future holds in store?


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Peter Wilson

October 14, 2015 at 10:50 pm

Like searching for harmful effects of marijuana, a search for a desired result is contrary to the scientific method, but at some point, the negative results would seem to call for it.

Start with the pulsars suspected of emitting gravitational waves. I know: their frequency is too low. But that frequency is precisely defined, as well as their direction, power and distance; orientation is known approximately. Even with the obvious potential for bias, verifying the detection method by using a suspected source, whose parameters are already established, would seem newsworthy.

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October 23, 2015 at 7:28 am

The search for a desired result makes the use of the scientific method far more useful and efficient than it would otherwise be. The necessary insistence upon duplication of results and peer review is an acknowledgement of human bias.
Desire is not only the cause of suffering, but also the impetus for attainment of greater knowledge and insight.

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