Gravitational Waves

Gravitational Waves[1]

The Laser Interferometer Gravitational-Wave Observatory, or LIGO, launched the era of gravitational wave astronomy in February 2016 with the announcement of a collision between two black holes observed in September 2015.

The scientific collaboration that operates the two LIGO detectors netted a second merger between slightly smaller black holes on December 26, 2015. (A third “trigger” showed up in LIGO data on October 12, 2015, but ultimately did not meet the stringent statistical significance standard that physicists generally insist on.)

Instead, scientists focused on sharpening theoretical estimates of how often various events occur. In particular, they are eager to see collisions involving neutron stars, which lack sufficient mass to collapse all the way to a black hole. Neutron star collisions are thought to be plentiful, but would emit weaker gravitational waves than do mergers of more massive black holes, so the volume of space the LIGO detectors can scan for such events is smaller.

LIGO scientists are also looking for signals from individual pulsars—rapidly rotating neutron stars that are observed on earth as pulses of radio waves. A bump on a pulsar’s surface should produce gravitational waves, but so far, no waves with the right shape have been picked up. This absence puts a limit on the size of any irregularities and on the emission power of gravitational waves from nearby pulsars such as the Crab and Vela pulsars, said Michael Landry, head of the Hanford LIGO observatory, and could soon start putting limits on more distant ones.

A few hints of possible excitement to come: LIGO data taken through the end of January, 2017 produced two short signals that were unusual enough to exceed the experiment’s “false alarm” threshold—signals with shapes and strengths expected to show up once a month or less by chance alone. Both LIGO collaboration members and astronomers at conventional telescopes are investigating the data to determine whether they represent real events.

LIGO is not the only means by which scientists are searching for gravitational waves. Some scientists are using powerful radio telescopes to track signals emanating from dozens of extremely fast-rotating pulsars. A specific pattern of correlations between tiny hiccups in the arrival times of these pulses would be a signature of long-wavelength gravitational waves expected from mergers of distant supermassive black holes.

[1] See Gabriel Popkin, “Gravitational Waves: Hints, Allegations, and Things Left Unsaid,” in APSNEWS (36, 3, March 2017, p. 1)

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