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© Joe McNally/GettyThe hunt continues
For the first time, detectors on Earth have put a meaningful limit on the strength of gravitational waves - the ripples in space-time - created during the first instants of the universe's existence.

According to Einstein's general relativity, gravitational waves should have been emitted during inflation, when the universe expanded exponentially moments after the big bang. "[Gravitational waves] can tell us how the laws of physics operated at that time," says Vuk Mandic of the University of Minnesota, Minneapolis. "This is very valuable because we cannot reproduce these high-energy conditions in the lab."

The latest measurement, made jointly by the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and VIRGO, its European counterpart, was sensitive to gravitational waves at frequencies around 100 hertz. But they found nothing. The null result, however, puts an upper limit on the energy density of gravitational waves in the infant universe, the most convincing yet. The results improve upon the limits set by the theory of big bang nucleosynthesis, based on the observed abundances of light elements such as hydrogen and helium (Nature, DOI: link).

"This is a milestone," says Mandic, who is a principal investigator with the LIGO and VIRGO collaboration. "It's one indication that gravitational wave cosmology is coming of age."

Harald Pfeiffer of the Canadian Institute for Theoretical Astrophysics in Toronto, agrees. "It's very gratifying to see the work of the past 10 years paying off," he says.

The recently launched Planck satellite is also searching for primordial gravitational waves by looking for their imprint on the cosmic microwave background, the radiation left over from the big bang (New Scientist, 11 May 2009).

The hope is that the detectors will eventually find gravitational waves themselves. Any they do find will likely be from a nearby source, such as pairs of neutron stars in tight orbits or black holes that are spiralling in towards each other. LIGO probably has the edge over the other detectors. Last month the experiment entered a new, enhanced phase by doubling its sensitivity. Advanced LIGO, which commences in 2014, will be a tenfold improvement.