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Pulsars Proposed as Direct Gravity Wave Detectors

The direct detection of gravitational waves is the holy grail of modern day experimental relativity. Massive detectors both on the ground and in orbit are currently being built for this purpose. Projects such as LIGO and LISA are tremendous undertakings, costing hundreds of millions of dollars apiece. Now however, members of a small international team of physicists with no central facility and operating on a budget of only $8 million over the next decade have thrown their hats into the ring as serious contenders to be the first to directly observe gravitational waves.

When a massive object like a black hole accelerates, distortions in space-time can propagate out like a wave traveling at the speed of light. This warping of space-time manifests itself as the brief expansion and contraction of distances. On a terrestrial scale, these expansions and contractions are so tiny, they are nearly impossible to observe. However the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) expects that the tremendous distances that separate interstellar objects will amplify these otherwise tiny distortions.

The minute distortions of spacetime first predicted by Einstein in 1916 have so far eluded observation from even the most sensitive detectors. In 1993, Russell Hulse and Joe Taylor were awarded the Nobel Prize in physics after detailed observations of the binary star system PSR 1913+16 showed a loss of orbital energy at exactly the rate predicted by Einstein’s equations. However the direct observations of the associated waves have remained elusive.

NANOGrav’s novel method won’t require any costly new facilities, only the careful observation of known phenomena. They’ll scan the skies using radio telescopes to look for any slight irregularity in the uniform beat of pulsars. The radio waves emitted from a pulsar’s poles can be easily detected on Earth as they sweep by as an even rhythm of pulses, making them some of the most accurate clocks in the universe.

It’s this near perfect regularity that makes them the ideal beacon to hunt for gravitational waves. Some of the most efficient pulsars are as accurate as atomic clocks, losing only a fraction of a second over thousands of years.

Gravitational waves emitted from a source will cause a momentary expansion and contraction of the space between the observed pulsar and the Earth, This will produce a brief change in the apparent frequency of the pulsar, the signal that the NANOGrav team will be looking for.

Andrea Lommen, chair of the NANOGrav group, said that they’ve been testing their method on simulated data, “In the near future we’ll run our code on real pulsar data to see what limits we can place on existing sources,” she said.

The NANOGrav team has already been collecting information on known pulsars since August of 2007. They have identified about 30 suitable pulsars out of a total 1,500 known throughout the universe. These tend to be “recycled pulsars,” which have absorbed their stellar binary partners. They spin extremely fast and are old enough to have settled down to a smooth rotation with few glitches and little extraneous noise.

After writing the programs for collecting and analyzing the data is completed, the team hopes to move on to observations with existing powerful telescopes such as the Arecibo Telescope in Puerto Rico. As larger and more powerful radio telescopes are built, such as Allen Telescope Array and the proposed Square Kilometer Array, the NANOGrav team expects to greatly expand their catalogue of suitable pulsars for their research.

 “If we increase our sensitivity, which is what we’ll be doing in the next five to ten years, we will begin to detect super massive black hole binaries,” Lommen said, adding that this time frame puts the NANOGrav team “in the running” to be the first to directly detect gravitational waves.

“I guess you could call it competition, but it’s definitely very healthy,” Lommen said, “Our time scale is similar to that of LIGO, we both expect detection within a decade. I expect we will detect sources before LISA does.”

The ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO), which will measure the expansion and contraction along the lengths of two sets of four-kilometer long tunnels in Washington and Louisiana, should come fully online sometime in 2014. The Laser Interferometer Space Antenna (LISA), three satellites which will measure similar spatial distortions while trailing Earth’s orbit, won’t launch until 2018 at the earliest.

Right now the NANOGrav team is continuing to refine their computer program using simulated data. As algorithms are improved, finer and finer variations in pulsar timing will become apparent. The more random background noise they can filter out, the clearer signal they should be able to pick up.

“Gravitational waves will be detected within the next decade. The discovery will absolutely revolutionize astronomy and physics and I’m tremendously excited that I get to be a part of that,” Lommen said.


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