After undergoing a 5-year upgrade, the world's most powerful gravitational wave detector is back online and hunting for the tiniest of tiny fluctuations in spacetime.
The Laser Interferometer Gravitational-wave Observatory (or LIGO) consists of 2 facilities (in Washington and Louisiana) designed to detect the passage of gravitational waves in local spacetime. Gravitational waves are generated by the acceleration and deceleration of huge masses in the cosmos; extreme cosmic events such as black hole collisions and supernovae are predicted to generate them. Like ripples propagating across the surface of a pond, gravitational waves ripple through spacetime, carrying energy away from these events.
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Should we have the ability to directly detect these waves, a new era of gravitational wave astronomy will be possible, where we can use gravitational wave signals to open our eyes to some of the most energetic events in the universe.
LIGO's initial observing run started in 2002 and ended in 2010, but during those first 8 years LIGO did not detect any gravitational wave signals. So, through a series of upgrades to reduce the amount of unwanted noise interfering with the facility's interferometers, Advanced LIGO is now taking a giant leap into a new regime of precision in the hunt for these elusive spacetime ripples. And on Friday, Advanced LIGO went online at a sensitivity 3-times that of its predecessor.
According to the Advanced LIGO team, the new and improved detectors should be able to detect gravitational waves "from as far away as 225 million light years." By the end of LIGO's last search, the system was only able to reach out to 65 million light-years. (For reference, Advanced LIGO can detect gravitational waves generated 10 times further away than Andromeda, the Milky Way's nearest massive galactic neighbor.) This boost in sensitivity means that Advanced LIGO can now access a volume of space 27 times that of its last observing run.
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Gravitational waves are predicted by Einstein's general relativity and astrophysicists know they are out there through indirect observations of their effects. But direct observations of gravitational waves through local space have have been maddeningly elusive. The fact that LIGO has yet to find a gravitational wave signal is a fascinating result unto itself - it means that gravitational wave signatures are weaker than predicted and we need more sensitive detectors (like Advanced LIGO) to detect them.
Although the search has been difficult so far, leading physicists behind this monumental experiment are not hiding their optimism that Advanced LIGO will detect these ripples in spacetime.
"We are there; we are in the ball park now. It's clear that this is going to be pulled off," Kip Thorne, Caltech theoretical physicist and one of the pioneers of the LIGO experiment, told BBC World Service. He added that it would be "quite surprising" if Advanced LIGO doesn't find hints of gravitational waves.
"Experimental attempts to find gravitational waves have been on going for over 50 years, and they haven't yet been found," said David Reitze, executive director of the LIGO program at Caltech, in a press release. "They're both very rare and possess signal amplitudes that are exquisitely tiny."
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But how tiny is tiny? As gravitational waves pass through local space, a minute fluctuation in distance between objects should be detectable and Advanced LIGO lasers can detect a fluctuation of one-billionth the width of an atom. But as an interferometer becomes more and more sensitive, it can start detecting unwanted signals, or "noise." This is one of the key reasons why LIGO is composed of two distant facilities situated on opposite sides of the US - should one station detect a faint signal and the other does not, the signal is likely local noise (like seismic activity or passing vehicles); if both stations detect a signal in concert, it could be a gravitational wave signal.
Now that new technology is being used to stabilize Advanced LIGO's interferometer mirrors, noise that impacted original LIGO's sensitivity has been removed, allowing the detector to "listen" for much fainter signals and, possibly, kick-start a new era of gravitational wave astronomy. It is hoped that through fine-tuning of the Advanced LIGO detectors, the observatory will be able to see 10 times further than original LIGO was capable of, making detections of massive cosmic collisions commonplace.
"Ultimately, Advanced LIGO will be able to see 10 times as far as initial LIGO and, based on theoretical predictions, should detect many binary neutron star mergers per year," added Reitze.