How do galaxies form? The European Pulsar Timing Array marks an important step forward
The research collaboration EPTA (the “European Pulsar Timing Array”) reports on the result of a 24-year observation campaign with the five largest European radio telescopes. The campaign has led to a possible signal for the long-sought gravitational-wave background (GWB), which is expected to be orbiting supermassive black holes in the centers of galaxies at a short distance. The cooperation brings together teams of astronomers at the institutes of the major European radio telescopes, as well as research groups specializing in data analysis and modeling of gravitational wave signals. Among them are astrophysicists from the research group of Professor Dr. Joris Verbiest from the Faculty of Physics at Bielefeld University. Although a clear proof has not yet been achieved, it is an important step to detect gravitational waves at very low frequencies in the nanohertz range for the first time. The results are published online in the journal “Monthly Notices of the Royal Astronomical Society”.
This result was made possible by a data set collected over a long period of 24 years with the five major European radio telescopes. These include the 100 m radio telescope of the Max Planck Institute for Radio Astronomy near Effelsberg in Bonn, the 76 m Lovell telescope in Cheshire/Great Britain, the Nançay telescope for decimeter radio waves in France, the 64 m radio telescope at Pranu Sanguni (Sardinia/Italy) and the 16 antennas of the Westerbork Synthesis radio telescope in the Netherlands. In the observation mode of the Large European Array for Pulsars (LEAP), these five telescopes are connected to each other in such a way that they represent a virtual fully movable 200 m radio telescope, which significantly improves the sensitivity of the EPTA to gravitational waves.
The researchers observe the rays emitted by the magnetic poles of the rotating pulsars as pulses as they pass through the line of sight, similar to the light of a distant lighthouse. Pulsar Timing Arrays (PTAs) are networks of very stable rotating pulsars that are used as detectors for gravitational waves on a galactic scale. They are particularly sensitive to very low-frequency gravitational waves in the billionth of a hertz or nanohertz range. This expands the observation window for gravitational waves from the high frequencies of hundreds of hertz currently observed by ground-based observatories (LIGO, Virgo, KAGRA). While their detectors study short-term collisions of stellar black holes and neutron stars, the Pulsar Timing Arrays can be used to study gravitational waves as emitted by systems of rotating and slowly approaching supermassive black holes in the centers of galaxies. The addition of the gravitational waves released by a cosmic population of these binary systems forms the gravitational wave background.
“We can measure small changes in the arrival times of pulsar radio signals on Earth caused by the deformation of space-time due to a passing gravitational wave of very low frequency. In practice, these deformations in space-time show up as sources of very low-frequency noise in the series of observed arrival times of the pulses, a noise that is detected jointly by all pulsars of a pulsar timing array,” explains Dr. Jun Wang, who recently completed his doctorate on this topic at Bielefeld University.
However, the amplitude of this noise is incredibly tiny (estimated to be between ten and a few hundred billionths of a second), and in principle, many other effects could transmit a corresponding noise to every single pulsar in the pulsar timing array. To validate the results, several independent evaluation programs with different statistical frameworks were then used to exclude alternative noise sources and to search for the gravitational-wave background. Importantly, two independent procedures were used throughout the analysis to ensure mutual consistency.
The analysis with both methods in the context of the EPTA observations resulted in a clear can data signal for a gravitational wave background.
Einstein’s General Theory of Relativity predicts a very specific relationship between the space-time deformations experienced by the radio signals from pulsars located in different cardinal directions. The scientists refer to this as the spatial correlation of the signal or the so-called hellings-downs curve. Their detection can identify the observed noise as clearly caused by a gravitational-wave background. Dr. Siyuan Chen, researcher at the Laboratoire de Physique et de Chimie de l’Environnement et de l’Espace in Orleans, one of the two lead authors of the study, notes: “At the moment, the statistical uncertainties in our measurements do not yet allow us to identify the presence of the spatial correlation expected for the gravitational-wave background signal. For further confirmation, we need to include an even larger amount of pulsar data in the analysis, but the current results are already very encouraging.”
Prof. Dr. Joris Verbiest, group leader at Bielefeld University and one of the leading members of the European Pulsar Timing Array Consortium, summarizes: “It is really satisfying to finally see the first indications of a signal that underpin the expectation that we will soon open up a new part of the gravitational wave spectrum that will allow us to detail the formation history of galaxies over the course of cosmic time. study.”
Contact:
Prof. Dr. Joris Verbiest Bielefeld University Phone: 0521 106 3184 E-Mail: verbiest@physik.uni-bielefeld.de The authors of the original publication include S. Chen, R. N. Caballero, Y. J. Guo, A. Chalumeau, K. Liu, G. Shaifullah, K. J. Lee, S. Babak, G. Desvignes, A. Parthasarathy, H. Hu, E. van der Wateren, J. Antoniadis, A.-S.
Bak Nielsen, C. G. Bassa, A. Berthereau, M. Burgay, D. J. Champion, I. Cognard, M. Falxa, R. D. Ferdman, P.C.C. Freire, J. R. Gair, E. Graikou, L. Guil-lemot, J. Jang, G. H. Janssen, R. Karuppusamy, M. J.Keith, M. Kramer, X. J. Liu, A. G. Lyne, R. A. Main, J. W. McKee, M.B. Mickaliger, B.B. P. Perera, D. Perrodin, A. Petiteau, N. K. Porayko, A. Possenti, A. Samajdar, S. A. Sanidas, A. Sesana, L. Speri, B.W. Stappers, G. Theureau, C. Tiburzi, A. Vecchio, J. P. W. Verbiest, J. Wang, L. Wang and H. Xu.
Among them are the following authors from Bielefeld University: Ann-Sofie Bak Nielsen, Joris Verbiest, Jun Wang.
Original publication: S. Chen et al: Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: Inferences in the stochastic gravitational-wave background search, 2021, Monthly Notices of the Royal Astronomical Society ( https://doi.org/10.1093/mnras/stab2833 or https://academic.oup.com/mnras/article/508/4/4970/6410749 )
More information: European Pulsar Timing Array (EPTA) http://www.epta.eu.org/ International Pulsar Timing Array (IPTA) http://www.ipta4gw.org/
This is an traslation of the news report by the Bielefeld University. Orignal in German
