LIGO and Virgo: The machines that unlock the universe’s mysteries

WASHINGTON: The three machines that gave scientists their first-ever glimpse of gravitational waves resulting from a collision of neutron stars are the most advanced detectors ever built for sensing tiny vibrations in the universe.

The LIGO and Virgo detectors have previously picked up the "chirp" of black holes merging in the distant universe, sending out ripples in the fabric of space and time. The detection of these gravitational waves for the first time in 2015 confirmed Albert Einstein’s century old theory of general relativity. The two US-based underground detectors are known as the Laser Interferometer Gravitational-wave Observatory, or LIGO for short.

One is located in Hanford, Washington; the other 1,800 miles (3,000 kilometers) away in Livingston, Louisiana. Construction began in 1999, and observations were taken from 2001 to 2007. Then they underwent a major upgrade to make them 10 times more powerful.

The advanced LIGO detectors became fully operational for the first time in September 2015. On September 14, 2015, the detector in Louisiana first picked up the signal of a gravitational wave, originating 1.3 billion years ago in the southern sky.

The third underground detector is near Pisa, Italy, and is known as Virgo. Built a quarter century ago by a French-Italian partnership, the Virgo detector ended its initial round of observations in 2011 and then underwent an upgrade.

Advanced Virgo came online in April of this year, and made its first observation of gravitational waves on August 14, marking the fourth such event that scientists have observed since 2015. Virgo is less sensitive than LIGO, but having three detectors helps scientists zero in on the area of the universe where a cosmic event is happening, and measure the distance with greater accuracy.

"A smaller search area enables follow-up observations with telescopes and satellites for cosmic events that produce gravitational waves and emissions of light, such as the collision of neutron stars," said Georgia Tech professor Laura Cadonati.

These huge laser interferometers -- each about 2.5 miles long -- are buried beneath the ground to allow the most precise measurements. The L-shaped instruments track gravitational waves using the physics of laser light and space. They do not rely on light in the skies like a telescope does.