From University of Birmingham (UK) : “International collaboration offers new evidence of a gravitational wave background”

From The University of Birmingham (UK)

12 Jan 2022

Beck Lockwood
Press Office
University of Birmingham
+44 (0)781 3343348.

The results of a comprehensive search for a background of ultra-low frequency gravitational waves has been announced by an international team of astronomers including scientists from the Institute for Gravitational Wave Astronomy at the University of Birmingham.

Artist impression of the IPTA experiment – An array of pulsars around the Earth embedded in a gravitational wave background from supermassive black hole binaries. The signals from the pulsars measured with a network of global radio telescopes are affected by the gravitational waves and allow for the study of the origin of the background. The distances have been reduced for visual purposes, notably the supermassive black holes are much further away in reality. (Credits: C. Knox)

These light-year-scale ripples, a consequence of Albert Einstein’s Theory of General Relativity, permeate all of spacetime and could originate from mergers of the most massive black holes in the Universe or from events occurring soon after the formation of the Universe in the Big Bang. Scientists have been searching for definitive evidence of these signals for several decades.

The International Pulsar Timing Array (IPTA) [EPTA European Pulsar Timing Array; NANOGRAV-The North American Nanohertz Observatory for Gravitational Waves(US);PPTA Parkes Pulsar Timing Array], joining the work of several astrophysics collaborations from around the world, recently completed its search for gravitational waves in their most recent official data release, known as Data Release 2 (DR2), published in MNRAS.

The river of stars in the southern sky. ESA/GAIA (Gaia DR2 skymap).

ESA GAIA Release 2 map.

This data set consists of precision timing data from 65 millisecond pulsars – stellar remnants which spin hundreds of times per second, sweeping narrow beams of radio waves that appear as pulses due to the spinning – obtained by combining the independent data sets from the IPTA’s three founding members: EPTA European Pulsar Timing Array, NANOGRAV-The North American Nanohertz Observatory for Gravitational Waves(US), and PPTA Parkes Pulsar Timing Array.

These combined data reveal strong evidence for an ultra-low frequency signal detected by many of the pulsars in the combined data. The characteristics of this common-among-pulsars signal are in broad agreement with those expected from a gravitational wave “background”.

The gravitational wave background is formed by many different overlapping gravitational-wave signals emitted from the cosmic population of supermassive binary black holes (i.e. two supermassive black holes orbiting each other and eventually merging) – similar to background noise from the many overlapping voices in a crowded hall.

This result further strengthens the gradual emergence of similar signals that have been found in the individual data sets of the participating pulsar timing collaborations over the past few years.

Professor Alberto Vecchio, Director of the Institute for Gravitational Wave Astronomy at the University of Birmingham, and member of the EPTA, says: “The detection of gravitational waves from a population of massive black hole binaries or from another cosmic source will give us unprecedented insights into how galaxy form and grow, or cosmological processes taking place in the infant universe. A major international effort of the scale of IPTA is needed to reach this goal, and the next few years could bring us a golden age for these explorations of the universe.”

“This is a very exciting signal! Although we do not have definitive evidence yet, we may be beginning to detect a background of gravitational waves,” says Dr Siyuan Chen, a member of the EPTA and NANOGrav, and the leader of the IPTA DR2 search and publication.

Dr Boris Goncharov from the PPTA cautions on the possible interpretations of such common signals: “We are also looking into what else this signal could be. For example, perhaps it could result from noise that is present in individual pulsars’ data that may have been improperly modeled in our analyses.”

To identify the gravitational-wave background as the origin of this ultra-low frequency signal the IPTA must also detect spatial correlations between pulsars. This means that each pair of pulsars must respond in a very particular way to gravitational waves, depending on their separation on the sky.

These signature correlations between pulsar pairs are the “smoking gun” for a gravitational-wave background detection. Without them, it is difficult to prove that some other process is not responsible for the signal. Intriguingly, the first indication of a gravitational wave background would be a common signal like that seen in the IPTA DR2. Whether or not this spectrally similar ultra-low frequency signal is correlated between pulsars in accordance with the theoretical predictions will be resolved with further data collection, expanded arrays of monitored pulsars, and continued searches of the resulting longer and larger data sets.

Consistent signals like the one recovered with the IPTA analysis have also been published in individual data sets more recent than those used in the IPTA DR2, from each of the three founding collaborations. The IPTA DR2 analysis demonstrates the power of the international combination giving strong evidence for a gravitational wave background compared to the marginal or absent evidences from the constituent data sets. Additionally, new data from the MeerKAT telescope and from the Indian Pulsar Timing Array (InPTA), the newest member of the IPTA, will further expand future data sets.

“The first hint of a gravitational wave background would be a signal like that seen in the IPTA DR2. Then, with more data, the signal will become more significant and will show spatial correlations, at which point we will know it is a gravitational wave background. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a gravitational wave background detection,” says Dr Bhal Chandra Joshi, a member of the InPTA.

Given the latest published results from the individual groups who now all can clearly recover the common signal, the IPTA is optimistic for what can be achieved once these are combined into the IPTA Data Release 3. Work is already ongoing on this new data release, which at a minimum will include updated data sets from the four constituent PTAs of the IPTA. The analysis of the DR3 data set is expected to finish within the next few years.

Gaia EDR3 StarTrails 600

Dr Maura McLaughlin of the NANOGrav collaboration says, “If the signal we are currently seeing is the first hint of a gravitational wave background, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future.”

See the full article here .


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University of Birmingham (UK) has been challenging and developing great minds for more than a century. Characterised by a tradition of innovation, research at the University has broken new ground, pushed forward the boundaries of knowledge and made an impact on people’s lives. We continue this tradition today and have ambitions for a future that will embed our work and recognition of the Birmingham name on the international stage.

The University of Birmingham is a public research university located in Edgbaston, Birmingham, United Kingdom. It received its royal charter in 1900 as a successor to Queen’s College, Birmingham (founded in 1825 as the Birmingham School of Medicine and Surgery), and Mason Science College (established in 1875 by Sir Josiah Mason), making it the first English civic or ‘red brick’ university to receive its own royal charter. It is a founding member of both the Russell Group (UK) of British research universities and the international network of research universities, Universitas 21.

The student population includes 23,155 undergraduate and 12,605 postgraduate students, which is the 7th largest in the UK (out of 169). The annual income of the institution for 2019–20 was £737.3 million of which £140.4 million was from research grants and contracts, with an expenditure of £667.4 million.

The university is home to the Barber Institute of Fine Arts, housing works by Van Gogh, Picasso and Monet; the Shakespeare Institute; the Cadbury Research Library, home to the Mingana Collection of Middle Eastern manuscripts; the Lapworth Museum of Geology; and the 100-metre Joseph Chamberlain Memorial Clock Tower, which is a prominent landmark visible from many parts of the city. Academics and alumni of the university include former British Prime Ministers Neville Chamberlain and Stanley Baldwin, the British composer Sir Edward Elgar and eleven Nobel laureates.

Scientific discoveries and inventions

The university has been involved in many scientific breakthroughs and inventions. From 1925 until 1948, Sir Norman Haworth was Professor and Director of the Department of Chemistry. He was appointed Dean of the Faculty of Science and acted as Vice-Principal from 1947 until 1948. His research focused predominantly on carbohydrate chemistry in which he confirmed a number of structures of optically active sugars. By 1928, he had deduced and confirmed the structures of maltose, cellobiose, lactose, gentiobiose, melibiose, gentianose, raffinose, as well as the glucoside ring tautomeric structure of aldose sugars. His research helped to define the basic features of the starch, cellulose, glycogen, inulin and xylan molecules. He also contributed towards solving the problems with bacterial polysaccharides. He was a recipient of the Nobel Prize in Chemistry in 1937.

The cavity magnetron was developed in the Department of Physics by Sir John Randall, Harry Boot and James Sayers. This was vital to the Allied victory in World War II. In 1940, the Frisch–Peierls memorandum, a document which demonstrated that the atomic bomb was more than simply theoretically possible, was written in the Physics Department by Sir Rudolf Peierls and Otto Frisch. The university also hosted early work on gaseous diffusion in the Chemistry department when it was located in the Hills building.

Physicist Sir Mark Oliphant made a proposal for the construction of a proton-synchrotron in 1943, however he made no assertion that the machine would work. In 1945, phase stability was discovered; consequently, the proposal was revived, and construction of a machine that could surpass proton energies of 1 GeV began at the university. However, because of lack of funds, the machine did not start until 1953. The DOE’s Brookhaven National Laboratory (US) managed to beat them; they started their Cosmotron in 1952, and had it entirely working in 1953, before the University of Birmingham.

In 1947, Sir Peter Medawar was appointed Mason Professor of Zoology at the university. His work involved investigating the phenomenon of tolerance and transplantation immunity. He collaborated with Rupert E. Billingham and they did research on problems of pigmentation and skin grafting in cattle. They used skin grafting to differentiate between monozygotic and dizygotic twins in cattle. Taking the earlier research of R. D. Owen into consideration, they concluded that actively acquired tolerance of homografts could be artificially reproduced. For this research, Medawar was elected a Fellow of the Royal Society. He left Birmingham in 1951 and joined the faculty at University College London (UK), where he continued his research on transplantation immunity. He was a recipient of the Nobel Prize in Physiology or Medicine in 1960.