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The LIGO and Virgo laser interferometer gravitational-wave observatories in the U.S. and Italy have begun a third yearlong observation run with the hope of yielding new astronomical observations.
The O3 run, as it is being called, will likely add to the milestones achieved in the first two runs. These include the detection of gravitational waves from 10 binary black-hole mergers and from the collision of a pair of ultradense neutron stars. The latter detection, coordinated with observations from more traditional optical, x-ray, and gamma-ray telescopes in an example of “multimessenger astronomy,” resulted in new scientific information.
This current run comes just three years after the first detection of gravitational waves (GW). The LIGO/Virgo Scientific Collaboration (LSC) will make data about possible GW detections publicly available in near real time.
LIGO team member Alena Ananyeva at the LIGO Livingston Observatory, installing “baffles” to control stray light. (Photo courtesy of LIGO/Caltech/MIT/Matt Heintze)
LSC scientists are confident that the LIGO and Virgo observatories will log observations at an even faster clip in O3, as a result of technical improvements implemented since the end of the last observation run, O2, in August 2017.
The improvements include a doubling of the power of the facilities’ lasers, which in these observatories are amplified in two Fabry-Pérot cavities 3 to 4 km long that form the arms of a gigantic, L-shaped Michelson interferometers located in Livingston, La., and Hanford, Wash. Also installed in the upgrade were scattered-light suppressors, or “baffles,” designed to control stray light within the huge interferometers.
In addition to laser power, other recent upgrades at the U.S. facilities have centered on efforts to boost sensitivity by locating and eliminating noise sources in a range of subsystems.
At LIGO, this has included the significant engineering challenge of swapping out a number of the 40-kg mirrors, or test masses, suspended at either end of the laser interferometer arms. As a passing gravitational wave ripples through space-time, tiny movements in these hefty mirrors result in infinitesimal changes of the interferometer arms’ lengths, which are read as picowatt-scale power fluctuations at the dark port of the interferometer. The new, better-performing versions of the mirrors include improved coatings to diminish thermal noise.
At Virgo, meanwhile, the steel wires suspending the main mirrors have been replaced with fused silica versions that quiet down vibrational noise and extend the facility’s ability to pick up low- and medium-frequency GWs. During O3, both LIGO and Virgo will use a trick of quantum mechanics, the injection of a “squeezed” state of light at the photodetector to narrow down the uncertainties in photon arrival times attributable to fluctuations in the quantum vacuum.
These and other technical improvements were partly developed and matured at yet another facility, GEO 600, a smaller GW observatory in Europe that has served as a vital testbed for technologies to sharpen the observing power of the larger sites. GEO 600 is also participating in the O3 run.
The recent sensitivity upgrades will enable the global GW network to sample a much-expanded slice of the cosmos for evidence of high-energy astronomical events. In the O3 run, for example, LIGO’s sensitivity in the wake of the recent upgrades should enable it to sniff out binary neutron-star mergers to a distance of 550 million light-years — more than 190 million light-years farther than in O2.
That, coupled with an eightfold expansion of the volume of space now visible to Virgo, could increase the rate of detection of binary black-hole collisions to anywhere from a few events per month to a few per week, and binary neutron-star mergers to between one per year and one per month. There’s also the possibility of picking up more exotic, previously inaccessible events, such as the merger of a black hole and a neutron star.
Among the notable changes, the public will have near-immediate access to this harvest of discoveries through new software developed by LSC scientists. The software will be “able to send open public alerts within five minutes” after a GW detection, said Sarah Antier, a postdoctoral research associate at the Université Paris Diderot in France.
That will allow rapid public access to parameters such as type of signal, sky position, and estimated distance for a given GW event. Those parameters, in turn, will let both professional and amateur astronomers looking at various slices of the electromagnetic spectrum quickly train their instruments on the right patch of sky to follow up on the GW observation.
The ability to locate GW sources quickly and precisely could receive yet another boost late in the yearlong O3 period, with KARGA’s long-awaited debut. KAGRA, an underground 3-km laser interferometer GW observatory whose design includes suspended sapphire test masses cryogenically cooled to 20 K, has been under construction in Japan since 2010. But its development has been plagued by ongoing difficulty in banishing vibrational noise attributable to cryocooling equipment and even to water infiltration in the underground facility.
In January this year, the KAGRA team finally reported a successful 10-day test of the interferometer at cryogenic temperatures. With that significant milestone behind it, the team is optimistic that the facility will be able to make its first scientific observations in late 2019.READ MORE