Cosmic Sirens: The Dawn of Gravitational Astronomy

Tuesday 15th October 2013

Our speaker was Professor Sathyaprakash from Cardiff University who began his talk "Cosmic Sirens: The Dawn of Gravitational Astronomy" by commenting that until recently astronomers had only used electromagnetic radiation to study the Universe.

The actual science of astronomy began using wavelengths of electromagnetic radiation that more commonly would be referred to as light. But light that is visible to the human eye is only a tiny part of the electromagnetic spectrum. As the field of observational astronomy has advanced, telescopes have been designed that can capture other wavelengths of light so that most of the spectrum, from radio to gamma rays can be used in research.

However, there is a fundamental limit to how far back in space (and time) that these instruments can observe. Up until roughly 380,000 years after the Universe came into being it was so hot that photons of light could not travel freely. They could only traverse tiny distances before being swallowed up by the dense plasma of particles that pervaded space.

There is another type of wave that can flow unimpeded through the early Universe. These are gravitational waves which are ripples in the actual fabric of space-time itself and were predicted by Einstein in his Theory of General Relativity. Just as a stone thrown into a still pond sends circular waves out from its point of impact, a moving celestial object will emit gravitational waves moving outwards at the speed of light.

Although gravitational waves have not yet been directly observed, two physicists won the Nobel Prize for Physics in 1993 for discovering a new type of twin star system known as a binary pulsar. This "Hulse-Taylor binary pulsar" consists of two incredibly compact stars known as neutron stars that are in a tight orbit around each other. Whilst studying the system they noticed that their orbital period was steadily decreasing by 10 microseconds a year. From this they could conclude that the neutron stars were losing energy as they emitted gravitational radiation in the form of gravitational waves and were gradually spiralling inwards.

To try and directly observe gravitational waves scientists and engineers have built a number of instruments across the world. Currently, there are about half a dozen in operation with others undergoing upgrades and some still on the drawing board. Most of them are designed to detect the tiny fluctuations that occur when a gravitational wave distorts the paths of a pair of laser beams set at right angles to one another.

Prof Sathyaprakash is hopeful that within the next decade, as new gravitational wave observatories are built and others upgraded, we will see the first direct observations of gravitational waves. These ripples in cosmic space-time are most likely to come from the merger of black holes or neutron stars, or perhaps the rarer event of a supernova explosion.