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Gravitational waves: A new type of astronomy

Gravitational waves provide key to some of the universe's greatest mysteries

Big Bang

The first time physicists announced that the Laser Interferometer Gravitational-wave Observatory (LIGO) had detected gravitational waves, on September 14, 2015, it was breaking news. The discovery coincided with the 100-year anniversary of Einstein's theory of General Relativity, which predicted the existence of gravitational waves.

Last week's announcement that LIGO had detected a second round of gravitational waves proved that the first signal was not a fluke. The tremendous effort from thousands of physicists, engineers and computer scientists to design, build, and maintain LIGO, and analyse its results, have paid off. The detector is alive and working well – uncovering signals from the most violent events happening in our universe.

But in the near-distant future, the detection of gravitational waves will not make headlines anymore. Sightings will be more common as LIGO is upgraded to become even more sensitive. The range of signals it can "hear" in the universe will be expanded – from the big loud crashes from supermassive black holes colliding to tiny whispers from supernovae bursts.

"By the end of the decade we'll probably be able to detect at least one gravitational wave event per month," Ken Strain, professor of physics at University of Glasgow involved in upgrading LIGO, told The Register.

Initially, LIGO failed to find any gravitational waves. The detectors were not sensitive enough to catch them passing through earth, and over $600 million was spent to upgrade LIGO to Advanced LIGO. The upgrade has paid off, and scientists were excited and relieved to finally announce that they had managed to catch the first glimpse of a gravitational wave in action.

How Advanced LIGO works

The detector is made up of a laser, an interferometer and mirrors. The laser is shone into an interferometer that splits the laser light into two directions. At either end is a mirror, which reflects the light beams so that they recombine and travel to the detector – which measures how much one signal is lagging behind the other.

Initially, the mirrors at LIGO were suspended from steel wires. To eliminate the risk of other sources such as sound waves disturbing the mirror pendulums, all the air has to be sucked out to create an ultra-high vacuum. However, at room temperature there are still thermal motions in the atoms of the steel wires that jog the mirrors and disturb the light, explained Professor Strain, who was involved in improving the pendulum mirrors for Advanced LIGO.

Strain is part of a larger team at the University of Glasgow, who were awarded £8 million by the Science and Technology Facilities Council (one of Europe's largest multidisciplinary research organisations), along with other Scottish universities, to design and build improvements to the LIGO detector. LIGO is a big collaborative project and other countries contributed to upgrade other components in the detector.

The new suspension system hangs four 40kg mirrors – made out of ultrapure fused silica glass and worth half a million dollars each – with threads of super-strong glass fibres only 0.4mm thick. The glass fibres hold the mirrors more steady, as they are less susceptible to thermal vibrations, allowing scientists to detect the tiniest wobbles caused by passing gravitational waves.

Over long distances, the amplitude of the gravitational waves dwindles and by the time they reach earth, the signal is tiny. The second pair of colliding black holes produces less energy than the first, and the passing gravitational waves only moved the mirrors by 0.7 of a thousandth of a femtometre (10^-15m) – smaller than a proton.

The detector will only get more sensitive, Strain says. "The laser power is only working to a third of its capability." In three years time, the power will be cranked up to 200W, allowing LIGO to detect gravitational waves from greater distances.

A new era for astronomy

LIGO ushers in a new kind of astronomy. Although the energy released from both pairs of colliding black holes was greater than all the energy radiated by every star in the observable universe, it would have remained invisible if it wasn't for LIGO.

Gravitational wave astronomy allows scientists to observe the universe in a new way. More exotic objects that were hidden in the dark depths of the universe will now be brought to light, providing new answers to questions that were impossible before. How do stars collapse into black holes? What is happening inside a black hole? What makes supernovae explode?

"LIGO has shown that gravitational waves exist and makes it feasible to scale up the size of objects we want to find," says Dr Francesco Shankar, a lecturer at the School of Physics and Astronomy at the University of Southampton in England.

"We are interested in the real monsters – the supermassive black holes that are millions or even billions of solar masses at the centre of galaxies."

LIGO has provided the first step toward building the technology required for the Evolved Laser Interferometer Space Antenna (eLISA) spacecraft – the first gravitational wave observatory in space. The current LISA pathfinder is whizzing around in space, testing the technologies needed for eLISA to be launched in 2034.

Shankar hopes that eLISA will uncover the secrets to how black holes and galaxies evolve. Studying this might even allow scientists to get a better idea of how dark matter behaves. It is thought that all galaxies evolve at the center of dark matter holes.

"If we can trace back the origins and distances of supermassive black holes, we might get more information on how the universe is expanding. It will be a new way to understand dark matter and dark energy," says Shankar.

A whole network of gravitational wave detectors

There are currently three gravitational wave detectors: two in America and one in Italy. Two more are on the way. On the day that the discovery of gravitational waves was announced, Narendra Modi, Prime Minister of India, tweeted that he was proud of India's participation in LIGO and promised India would play an even bigger part in the future, as it would be building its own gravitational-wave detector.

Japan is also joining in and is currently in the process of building the Kamioka Gravitational Wave Detector (KAGRA), which is expected to be ready in 2018. KAGRA will be even more sensitive than Advanced LIGO, as it stifles any thermal vibrations by chilling the mirrors down to cryogenic temperatures. It should be sensitive enough to detect binary stars merging and the brightest supernovae explosions.

Having five detectors in place also means that the location of the source behind the gravitational waves can be pinpointed more accurately.

Scientists have signed a memorandum to ensure that all data is shared and achievements will be credited to all scientists. Collaboration is essential in science – with more accurate detectors in place and a growing team of international scientists joining the LIGO Scientific Collaboration, the new branch of astronomy promises to be exciting and fruitful. ®

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