The Search for an Isotropic Gravitational Wave Background
World-wide radio telescope network enhances signal that may hint at ultra-low frequency gravitational waves
An international team of astronomers including a number of scientists from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has announced the results of a comprehensive search for a background of ultra-low frequency gravitational waves. These light-year-scale ripples, a consequence 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.
The results are presented online as refereed publication in the “Monthly Notices of the Royal Astronomical Society”.
The International Pulsar Timing Array (IPTA), 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). 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 their spin – combined from the independent data sets of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA), the three founding members of the IPTA.
As a collaboration of teams of astronomers, data analysts and astrophysicists around the largest European radio telescopes and multiple affiliated research institutes the European Pulsar Timing Array is part of and contributes to the International Pulsar Timing Array.
One of the main expertise of the EPTA is in data combination. “The European Pulsar Timing Array itself is an international effort and we are used to combining data from up to five different radio telescopes and even observing simultaneously. This expertise has been very useful in the creation of the current data release”, says Dr. David Champion from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, member of the EPTA collaboration and co-author of the paper. Additionally, the large number of 42 pulsars and a long baseline of up to 18 years, accounts for a relative majority of the number of observations within the release. Much of the used Bayesian methodology was developed in the EPTA to establish upper limits for the strength of the gravitational wave background (GWB) and to then understand the statistics of the emerging signal over the years.
The search for a gravitational wave background includes an extensive comparison between individual data sets from the large regional scientific collaborations and the combined data set. This search within the present release has revealed strong evidence for a 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 signal. This 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), analogous 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 collaborations over the past few years.
“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 both, the EPTA and NANOGrav collaborations, 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 the 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 the gravitational waves, depending on their separation on the sky. “The correlation of the signal across the pairs of pulsars is key to understanding the source of the signal. Gravitational waves have a very specific pattern that is difficult to account for in any other way. But we need a stronger signal to be able to detect this correlation”, explains Dr. Yanjun Guo, also from MPIfR. Intriguingly, the first indication of a GWB would be a common signal like that seen in the IPTA DR2. Whether or not this spectrally similar low-frequency signal is correlated between pulsars in accordance with the theoretical predictions for a gravitational-wave background 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 fact that the same signal is already seen in the shorter IPTA data set shows the power of the international combination and it gives great motivation to include more data from the current and new members in pulsar timing”, says Adytia Parthasarathy from MPIfR. Additionally, new data from the MeerKAT telescope in South Africa 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 GWB 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 GWB. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a GWB detection”, says Dr. Bhal Chandra Joshi, a member of the InPTA.
Recently, the EPTA produced a new dataset with 6 pulsars, extending the timespan of the observations to 24 years with more sensitive data, and analyzed it both for the search for a common signal using two independent pipelines as well as a single pulsar noise study. Work is ongoing to expand the number to at least 25; this extended EPTA dataset will be part of the next IPTA data combination.
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 next data release (IPTA DR3). Work is already ongoing on this new data release, which at a minimum will include updated data sets from the four constituent pulsar timing arrays within the IPTA. We expect the analysis for the next data release to be finished within the next few years. “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”, says Dr. Maura McLaughlin of the NANOGrav collaboration. “This is a real international team effort. The MPIfR’s group of researchers alone comprises a diverse group of junior and senior scientists from many different cultural backgrounds, all working on the same goal”, concludes Michael Kramer, Director at the MPIfR and head of its “Fundamental Physics in Radio Astronomy” research department.
The authors of the paper are J. Antoniadis, Z. Arzoumanian, S. Babak, M. Bailes, A.-S. Bak Nielsen, P. T. Baker, C. G. Bassa, B. Bécsy, A. Berthereau, M. Bonetti, A. Brazier, P. R. Brook, M. Burgay, S. Burke-Spolaor, R. N. Caballero, J. A. Casey-Clyde, A. Chalumeau, D. J. Champion, M. Charisi, S. Chatterjee, S. Chen, I. Cognard, J. M. Cordes, N. J. Cornish, F. Crawford, H. T. Cromartie, K. Crowter, S. Dai, M. E. DeCesar, P. B. Demorest, G. Desvignes, T. Dolch, B. Drachler, M. Falxa, E. C. Ferrara, W. Fiore, E. Fonseca, J. R. Gair, N. Garver-Daniels, B. Goncharov, D. C. Good, E. Graikou, L. Guillemot, Y. J. Guo, J. S. Hazboun, G. Hobbs, H. Hu, K. Islo, G. H. Janssen, R. J. Jennings, A. D. Johnson, M. L. Jones, A. R. Kaiser, D. L. Kaplan, R. Karuppusamy, M. J. Keith, L. Z. Kelley, M. Kerr, J. S. Key, M. Kramer, M. T. Lam, W. G. Lamb, T. J. W. Lazio, K. J. Lee, L. Lentati, K. Liu, J. Luo, R. S. Lynch, A. G. Lyne, D. R. Madison, R. A. Main, R. N. Manchester, A. McEwen, J. W. McKee, M. A. McLaughlin, M. B. Mickaliger, C. M. F. Mingarelli, C. Ng, D. J. Nice, S. Oslowski, A. Parthasarathy, T. T. Pennucci, B. B. P. Perera, D. Perrodin, A. Petiteau, N. S. Pol, N. K. Porayko, A. Possenti, S. M. Ransom, P. S. Ray, D. J. Reardon, C. J. Russell, A. Samajdar, L. M. Sampson, S. Sanidas, J. M. Sarkissian, K. Schmitz, L. Schult, A. Sesana, G. Shaifullah, R. M. Shannon, B. J. Shapiro-Albert, X. Siemens, J. Simon, T. L. Smith, L. Speri, R. Spiewak, I. H. Stairs, B. W. Stappers, D. R. Stinebring, J. K. Swiggum, S. R. Taylor, G. Theureau, C. Tiburzi, M. Vallisneri, E. van der Wateren, A. Vecchio, J. P. W. Verbiest, S. J. Vigeland, H. Wahl, J. B. Wang, J. Wang, L. Wang, C. A. Witt, S. Zhang, and X. J. Zhu.
MPIfR authors with first or second affiliation include John Antoniadis, Ann-Sofie Bak Nielsen, David Champion, Gregory Desvignes, Yanjun Guo, Huanchen Hu, Ramesh Karuppusamy, Michael Kramer, Kejia Lee, Kuo Liu, Robert Main, Adytia Parthasarathy, Nataliya Porayko, and Joris Verbiest.