Detecting waves

On February 11 this year, with much fanfare, the Laser Interferometer Gravitational Wave Observatory (LIGO) announced the first direct detection of gravitational waves, almost exactly a century after Einstein first predicted their existence. LIGO found the signal, which was the expected and predicted signature from the merger of 2 black holes. This was a symmetric wiggly line that gradually increased in height and then dropped in agreement with the theoretical solutions. The merger occurred a billion years ago, leading to the formation of a 60 solar-mass black hole.

Gravitational waves go through everything in their path and are not affected by the medium. This implies that the information carried by gravitational wave is precisely the same as when the astronomical system sent it out. This kind of signal, which is like a perfect, faithful messenger, is rare in astronomy. Light and other electromagnetic radiation are often distorted and diminished by the various media they pass through. For instance, we cannot see light from certain regions of our own galaxy because of the dust that is in the way. However, the intensity of gravitational waves is very weak and they cannot be generated in the laboratory.

It’s only when we have binary systems of compact objects like neutron stars and black holes revolving close to each other (with short periods) that the gravitational wave emission becomes significant. It was expected that when 2 neutron stars (stars with radii
Detectors like LIGO can be affected even if this kind of merger takes place in the Virgo cluster of galaxies, 50 million light years away. We have had strong indirect evidence for gravitational waves from binary neutron stars (the binary pulsar) for the past few decades. Their orbital periods shorten as they approach each other, in accordance with the theoretical formula for gravitational wave emission from such a system. When such objects merge, the signal would be much stronger. LIGO did not detect any signals from merging neutron stars.

Black holes, on the other hand, being more massive objects, emit more gravitational radiation when they merge, and this is the signal which LIGO saw, or rather heard, as the dominant frequency is several hundred hertz, and when the electrical signal is converted into sound, it is well within our hearing range!

Inference
The conclusion was that 2 massive black holes merged a billion light years away to form a single 60 solar-mass black hole. LIGO would ultimately have 5 detectors in different continents and countries (including one in India and Japan). Simultaneous detection would confirm that it is truly a celestial signal.

However, LIGO detectors can pick up only limited frequencies. Each arm of LIGO is 4 km long (laser beams bounced between mirrors can detect changes in path length to one part in 10^22, when a gravitational wave passes through.) But picking up frequencies, which are the richest in gravitational waves (like less than a millihertz) caused by mergers of supermassive black holes found in most galactic centres requires distances of hundreds of thousands of kilometres of arm length rather than just 4 km like that in LIGO. This is achieved more easily in spaceborne detectors.

Space-based gravitational wave detectors can avoid fluctuations in the earth’s gravitational field, which can obscure signals. Moreover LIGO’s sensitivity is limited by several noise sources (thermal noise from mirror suspensions), seismic activity at low frequencies etc. As one expects a rich spectrum of gravitational waves at lower frequencies (unlike the kilohertz one associated with LIGO) of a few millihertz to a fraction of a millihertz, spaceborne detectors have been suggested.

The Laser Interferometer Space Antenna (LISA) mission was suggested by NASA and later European Space Agency (ESA) some years ago. LISA will consist of 3 spacecrafts positioned about 5 million km apart and flying in an equilateral triangle formation around the sun.

The system would trail about 20° behind the earth orbit. Laser beams bounced between the spacecraft will form 3 separate interferometers. It was first scheduled for 2015, but NASA later withdrew it. The ESA is however going ahead. In the European E LISA, the spacecraft separated at a distance of 2 million km from each other. The project expected to be launched in 2034. The distances between the spacecrafts would be disturbed by passing gravitational waves and this can be measured by methods such as Doppler ranging. It can detect the merger of supermassive black holes billions of light years away.

Meanwhile, the Chinese have proposed their own space-based detector called Taiji (which means supreme ultimate), which is more ambitious than E LISA. In the proposed Taiji scheme (or C LISA), the 3 spacecrafts are separated by 3 million km giving access to different frequencies. The proposed launch of the Chinese mission is in the year 2033, 1 year ahead of LISA. Another project, the Tianqin project, twice cheaper than the Taiji, proposes to put the 3 spacecrafts much closer, to specifically detect the gravitational waves emitted by the white dwarfs binary HM Cancri.

In the meantime, the non-profit, Simons Foundation (USA) would fund a new observatory to search for signs of stretching in the very early stages of the expanding universe. It would hunt for the so-called Big Bang gravitational waves. This hunt for primordial gravitational waves would get 40 million dollars from the Simons foundation. The primordial gravitational waves is predicted by inflation models (of the early universe), which suggest that the universe ballooned exponentially in a fraction of a second to a large volume.

Curling patterns
Such a rapid wrenching expansion would give rise to gravitational wave ripples in space-time, that would have been imprinted in the Cosmic Microwave Background Radiation (CMBR). This would be in the form of so-called ‘B modes’, a curling pattern in the orientation of the CMBR. There is a good chance of getting a definite answer as to whether such an inflation did occur, and in what way. About 2 years ago, there was a false alarm with the announcement of primordial gravitational waves by the Background Imaging of Cosmic Extragalactic Polarisation (BICEP) experiment at the South Pole.

They claimed that they saw strong evidence of ‘B modes’, but this later turned out to be from contaminated dust in our galaxy. If the Simon observatory does discover the ‘B modes’, it would be a so-called smoking gun proof for inflation, but still would not convince everyone. Primordial gravitational waves have not been seen so far, so sceptics feel that this already rules out the most plausible versions of the inflation model. The Simons project will take off in a couple of years. The upgraded BICEP (at the South Pole) is underway.

In short, these future detectors are likely to open up the whole new area of gravitational wave astronomy, which has already seen the thrill of 2 black holes colliding and merging. Mergers of neutron stars, faraway supermassive black holes colliding and the gravitational waves generated in the fraction of a second before the Big Bang are all waiting to be discovered.