Science

Space antennae to probe gravitational waves

A bird's eye view of Laser Interferometer Gravitational-wave Observatory (LIGO) Hanford laboratory's laser and vacuum equipment area near Hanford, Washington.  

With the second detection of gravitational waves by the LIGO detector, the space for experimental research on black holes using gravitational waves is expanding. Couple this with the success of the LISA Pathfinder experiment and you have the right mix that can boost the prospects of the evolved Laser Interferometer Space Antenna (eLISA) project.

eLISA is a spectacular plan of setting into space three spacecraft, a mother and two daughter spacecraft, which will fly in a triangular formation, trailing the earth in its orbit around the sun at a distance of over 50 million km. Each arm of the imaginary triangle, from the mother to each daughter spacecraft, will measure about a million km. Inside these spacecraft will float “freely falling” test masses – cubes with sides measuring abut 46 mm. Laser interferometers will accurately measure changes in the distance between these cubes. If they should be affected by a gravitational wave, the minute changes in this distance are measure by the interferometer.

Big space projects like this are usually planned well in advance, this mission is slated for 2034. With the success of LISA pathfinder and the detection of two sets of gravitational waves by LIGO, this may get advanced a bit.

eLISA aims to measure gravitational waves in the frequency range from 0.1mHz to about 100 mHz. To do this, it is necessary for the interferometers to have an arm length of a million kilometres and that is impossible to achieve with an earth based setup. Hence, it is necessary to have this elaborate experiment in space. This frequency range, from 0.1 mHz to 100 mHz, or even 1Hz, is expected to be sensitive to a rich set of sources of gravitational waves. These sources are such that understanding them can throw light on many problems of interest in cosmology and astrophysics and also make possible stringent tests of General Theory of Relativity. Many events, for example, mergers of binary black holes happening at all distances, black holes swallowing neutron stars, even relics from the Big Bang which took place about 13.7 billion years ago, would be studied by this space antenna and the data used to build up a more complete picture f the universe and its secrets.

However this will not displace the ground based detectors, as these would be sensitive to a different set of frequencies. “The ground based detectors receive signals in the range 10 hertz to a kilohertz. These will probe events in a different band,” said Bala Iyer, theoretical physicist who has made substantial contributions to both eLISA and the LIGO projects.

The LISA Path Finder, a space experiment mean to set the path of the eLISA, recently relayed the first set of its observations on the force between two “freely-falling” test masses in space. The results, showing a better degree of accuracy than anticipated, augur well for the performance of eLISA.

Indian teams have worked on the theoretical aspects of eLISA:in the theoretical aspects of measurement of cosmological parameters; using eLISA to set constraints on dark energy equation of state; developing techniques for time delay interferometry, and so on.

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Printable version | Oct 21, 2020 4:23:05 PM | https://www.thehindu.com/sci-tech/science/Space-antennae-to-probe-gravitational-waves/article14433813.ece

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