By Jonathan Amos
BBC News Online science staff in Denver
The hunt is well and truly joined. Scientists report here that one of the greatest observatories ever constructed works as expected and is now ready to go for goal.
The Ligo facility, built at a cost of nearly $300m, is trying to detect gravitational waves, the ripples created in the fabric of space-time that occur every time a star explodes or black holes collide.
If it succeeds, scientists will not only confirm cherished theories, they will also have a new window on the Universe that will enable them to probe the nature of the cosmos right back to the beginning of time itself.
Researchers connected with the project came to the annual meeting of the American Association for the Advancement of Science in Denver with good news.
They said initial experiments showed Ligo could achieve the extraordinary sensitivities required to pick up the waves.
The installations' tunnels are in an L-shape
"We've been through a very complicated commissioning phase but we have now started to do what we can call science," said Professor Barry Barrish, the director of Ligo.
"We've improved the sensitivity and over the next year or two we will slowly move to the design performance."
The researchers also reported they would be putting in a request this week for $150m of further funding to boost the power of the facility.
Ligo (Laser Interferometer Gravitational Wave Observatory) is a huge engineering undertaking.
It consists of two widely separated installations - one in Washington State, the other in Louisiana - in which lasers are bounced off mirrors at the end of two four-kilometre-long vacuum tunnels.
The returning light beams are recombined in detectors. If a gravitational wave passes through the beams, the light will be ever so slightly disturbed.
The UK's Dr Harry Ward, an international partner on Ligo, explained: "The effect of a gravitational wave - if you imagine it propagating towards you - would be to lengthen space between your head and feet, and contract your width. It distorts you in that way.
"So the way we try to detect gravitational waves is to lay out, L-shape, a set of mirrors. The gravitational waves would shorten one arm between two mirrors while simultaneously lengthening the other arm between two mirrors."
Building started in 1996 and finished in 2000
Facilities located at Hanford WA and Livingston LA
Include the most sensitive interferometers ever built
But the weak nature of gravity means these disturbances are unimaginably small, even when they originate from some of the Universe's most violent events, such as the collapse of supermassive stars.
The scientists at Ligo have to measure deviations in their light beams equivalent to one one-thousandth of the width of a proton, one of the particles that make up all atoms.
One of the major tasks for engineers has been to insulate the installations from vibrations - from passing lorries and earthquakes - that might swamp the real data.
Having the two installations so far apart helps scientists see past the extraneous "noise" in the system.
Joy of discovery
After initial adjustments to instrumentation, researchers felt confident to start their first 60-day "science run" on Ligo this weekend.
They do not expect to find gravitational waves immediately. Ligo's sensitivity is still some way short of the level needed to detect an event with confidence.
But when the eureka moment arrives, we will cross the threshold into a new era of science.
Professor Kip Thorne, one of the lead researchers on the project, told the BBC: "It'll be wonderful. We'll have the joy of discovering things about the dark side of the Universe that cannot be discovered in any other way.
"We'll have the joy of testing theories about black holes that cannot be tested any other way. We'll be seeing the Universe and the fundamental laws of space-time in a manner that we have never seen before."