Simply put: Why gravitational waves hold out promise of a window into dark matter and energy

Last week’s ‘vision’ was historic, but it isn’t very clear yet. More precise instrumentation is needed, says Amitabh Sinha.

4 min readFeb 16, 2016 12:50 AM IST First published on: Feb 16, 2016 at 12:50 AM IST

Existence of gravitational waves had been predicted by Albert Einstein exactly 100 years ago in his paper on the General Theory of Relativity. (Source: Illustration by Subrata Dhar)

First there was light and sound, the physical senses. Then came the discovery of x-rays and a variety of other similar radiations that make up the electromagnetic spectrum, visible light being a part of it. Human beings’ perception of, and knowledge about, faraway objects, or those too small to be seen, depends on the detection of the electromagnetic waves either emitted or reflected by them.

These electromagnetic waves very often carry information that is characteristic of the objects they are emitted by. Different kinds of detectors have been constructed over the years to trap these electromagnetic waves and read the information they are carrying. Almost the entire body of knowledge of the physical universe is built from the information captured through the electromagnetic waves.

As we know now, about 95 per cent of the universe is dark matter or dark energy. Both of these have only been postulated in theory, and neither has been directly ‘observed’. That is because these are ‘transparent’ to electromagnetic waves. As such, they cannot be directly detected by any existing detector.

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Black holes, those unimaginably dense areas in the universe from which even light, or other electromagnetic waves, are not able to escape, had also not been ‘observed’ because of this reason. Until September 14, 2015, that is, when scientists found the first direct evidence of the existence of black holes through the detection of gravitational waves generated by a pair of fast-spiralling blackholes 1.3 billion years ago.

It is this ability to offer a completely new ‘vision’ to human beings — to ‘see’ previously invisible objects in the universe — that has made the discovery of gravitational waves one of the most important ever, on a par with the discovery of electromagnetic waves about 150 years ago.

Like electromagnetic waves, gravitational waves too are thought to be characteristic of the event that generated them. That is why scientists could tell that the one captured at the LIGO observatories in the United States on September 14 last year were the result of two black holes coalescing into one.

To be sure, this new ‘vision’ is not very clear as of now. The gravitational waves are extremely weak signals, and it took extremely complicated instrumentation to catch a faint glimpse of an event that produced a mind-boggling amount of energy. Gravitational waves should be routinely produced by a variety of other events much nearer to the earth as well, but would require the precision of instrumentation in the observatories to go up by several orders of magnitude.

For this reason, and several others, the detection of gravitational waves only offers the possibility of a window — and does not, so to speak, open the window itself — to observe dark matter and dark energy. There are large crumbs of dark matter floating around, and it is possible that they too might be coalescing together like the black holes that were captured. But it would require evolved forms of current observatories to record these.

But the detection of gravitational waves is almost certain to trigger the construction of more LIGO-like observatories to detect and study them. Several are already in the planning or construction stages, including one in India. Europe already has a functional one in Italy, called VIRGO, and is in the process of setting one up in space, named eLISA, or Evolved Laser Interferometer Space Antenna.

India has also proposed to set up a LIGO-like gravitational wave observatory. The proposal has been awaiting government approval since 2012. In the mean time, scientists have zeroed in on two potential sites, one in Rajasthan, and the other in Maharashtra. The design and sensitivity of the observatory would be almost identical to that of LIGO. If timelines are followed from now on, it would take eight years for the project to start doing the science runs from the date it gets the government’s approval.

Another observatory, with a different design, is being planned in Japan as well. It is proposed to be smaller, and would be underground.