Confirmation of a “background hum” of low-frequency gravitational waves in the cosmos is the culmination of 25 years of work by astronomers. It has been hailed as “the start of something big”, opening up a new frontier of research on black holes; the most extreme objects in the universe.
Details of compelling evidence for the gravitational waves was published on Thursday by an international collaboration of European astronomers, under the European Pulsar Timing Array (Epta), including scientists from Trinity College Dublin, together with colleagues in India and Japan.
The results have emerged from observations using six of the world’s most sensitive radio telescopes, which detect and amplify radio waves from space, turning them into signals that astronomers use to enhance understanding of the universe.
In a series of papers in Astronomy & Astrophysics and on ArXiv.org, they show the data are consistent with a “background hum” of low-frequency gravitational waves. The researchers believe these ripples in space-time, predicted by Einstein’s general theory of relativity, are produced by pairs of “supermassive black holes” with masses billions of times that of the sun.
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Einstein’s first great leap was the realisation that space and time were different dimensions of the same underlying thing – space-time. We are all moving through space-time.
His second great insight was that gravity and space-time interact (gravity curves space-time) such that travel times through objects with strong gravitational fields are slowed down. This can be seen as light passes by any massive object, eg with “pulsars”, which are signals from extinguished stars, we see this as light passes by Jupiter and other planets on its way to Earth. In fact in this way scientists can weigh the planets.
A black hole is a region of space-time where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts a sufficiently compact mass can deform space-time to form a black hole.
As these black holes orbit each other they lose energy through gravitational wave emission causing them to “in-spiral and ultimately merge”, explained one of the researchers Prof Evan Keane, of Trinity College Dublin’s school of physics, who is also head of the Irish LOFAR telescope – a low frequency radio telescope based at Birr Castle, Co Offaly. He described their results as “the start of something big”.
The European Pulsar Timing Array (Epta) is a collaboration of scientists from more than 10 institutions, which brings together astronomers and theoretical physicists to use observations of ultra-regular pulses from pulsars to construct a galaxy-sized gravitational wave detector.
“Pulsars are basically super clocks in space. By monitoring the ‘ticks’ from these clocks, which are spread throughout our galaxy, we can see the impact of passing gravitational waves in making the pulsar signals arrive earlier or later,” Prof Keane said.
He added: “As we collect more data, and as more telescopes join in and we refine our analysis techniques, we can expect an ever more precise view of what the universe looks like in gravitational waves. I can’t wait to add more layers to the painting,” Prof Keane added
The Epta announcement is co-ordinated with similar publications from other teams located across the world; in Australia, China and North America, which provide supporting evidence for their findings.
The analysis is in line with what astrophysicists expect, though Prof Alberto Vecchio from the University of Birmingham in the UK noted: “The gold standard in physics to claim the detection of a new phenomenon is that the result of the experiment has a probability of occurring by chance less than one time in a million.”
The Epta results as well as evidence from other international collaborations do not yet meet this criterion. However, combining all the worldwide datasets, under the International Pulsar Timing Array, should allow the astronomers to obtain irreproachable proof and achieve further understanding of the history of the universe using gravitational waves.
The aim is to expand the current datasets by exploiting an array consisting of more than 100 pulsars, observed with 13 radio telescopes, and agglomerating more than 10,000 observations for each pulsar.
“We are opening a new window in the gravitational wave universe,” said Dr Caterina Tiburzi, a researcher at NAF, Osservatorio Astronomico di Cagliari in Sardinia.
