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Gravity-wave search getting an upgrade

A $205 million upgrade will allow a laser-wielding observatory to monitor tens of thousands of galaxies for mysterious gravitational waves.
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A $205 million upgrade will allow a laser-wielding observatory to monitor tens of thousands of galaxies for mysterious gravitational waves.

Leading investigators are confident that the Advanced LIGO (Laser Interferometer Gravitational-wave Observatories) Project will be able for the first time to detect gravitational waves from neutron stars and black holes, as predicted by Einstein's theory of general relativity.

"With the limited LIGO range at time, it wasn't guaranteed detection," said Albert Lazzarini, deputy director of LIGO at the California Institute of Technology. "With Advanced LIGO, it'd be very surprising from a relativity perspective if we didn't observe anything."

Gravitational waves are ripples thought to occur in the fabric of space-time that result from interstellar collisions, explosions, or the dramatic movement of large and extremely dense objects such as neutron stars. Those ripples can then pass through the space-time that Earth occupies, causing a slight distortion which Advanced LIGO is meant to pick up on.

How it works
LIGO tries to detect gravitational waves using highly precise lasers to measure the time it takes light to travel between mirrors. Two sets of facing mirrors sit at a 90 degree angle, forming something like an "L" shape that meets at a corner. A laser beam is shot through an "L" shaped splitter at the corner, which splits the beam into two beams that strike each set of mirrors.

The laser interferometer measures how long the laser light bounces back and forth between the mirrors on the "L" legs before returning to a light detector at the "L" corner. They should theoretically return to the light detector at the same time because the mirror legs are identical distances — unless a passing gravitational wave distorts the local space-time fabric and changes the distance.

But the observatory, operational since 2002, has yet to detect the elusive, still-theoretical waves.

Scientists foresaw that advances in laser technology and mirrors would allow for even greater sensitivity when LIGO was first proposed, and so the Advanced LIGO Project became a natural upgrade for the observatory. The National Science Foundation recently approved the proposal to upgrade LIGO over the next seven years, starting with $32.75 million in 2008.

"The first several hours of observation with new instruments will equal almost the first year of observation with LIGO's current instruments," Lazzarini said. "We can probe something like several hundred galaxies out to the Virgo cluster [59 million light-years away] with LIGO, but increase that by a factor of one thousand and you go to the cosmological regime of measuring many tens of thousands of galaxies."

That thousand-fold increase in coverage comes from boosting LIGO's sensitivity 10 times over.

Larger mirrors made of better materials will reduce the background "noise" from the random motion of atoms at room temperature, and the laser power is being pumped from 10 watts to 180 watts. Advanced LIGO will also be better cushioned from any terrestrial vibrations coming through the ground, thanks to an active servo-controlled system that replaces an older, passive spring system.

"We achieved several milestones with the initial LIGO sensitivity," Lazzarini noted, pointing out that the two main LIGO facilities at Hanford, Wash., and Livingston, La., had just finished a two-year run to test the design's sensitivity. LIGO requires at least two widely separated detectors working simultaneously to rule out false signals and confirm when gravitational waves might pass through the Earth.

A global network
Advanced LIGO may eventually become part of a greater global network of gravitational wave detectors, thanks to strong international collaboration.

German and British contributors are providing the laser and mirror suspension systems respectively for the upgraded observatory, and Advanced LIGO has grown its cooperation with Europe's Virgo detector located near Pisa, Italy. Japan has also begun working towards building a gravitational wave detector.

"The gravitational wave community supports very strongly indeed the upgrade to LIGO — this upgrade was in fact planned from the very start of the LIGO project and has always been an integral part of the planned evolution of the detector performance," said Jim Hough, University of Glasgow physicist.