Key Takeaways

Plain English Takeaway

Scientists used precise measurements from many pulsars to look for ripples in space called gravitational waves. They found strong hints of these waves, but more work is needed to be sure what causes them.

Study Aim

The main goal of this study is to search for an isotropic stochastic gravitational wave background (GWB)—a random, persistent signal from many sources—at very low (nanohertz) frequencies. The researchers use the second data release from the European Pulsar Timing Array (EPTA), along with data from the Indian Pulsar Timing Array (InPTA), to look for the unique pattern of timing changes in pulsars that would signal the presence of a GWB. They aim to determine whether the observed signals are consistent with gravitational waves or could be explained by other sources of noise. Simply put: The study wants to find out if tiny, regular changes in pulsar signals are caused by gravitational waves passing through space.

Study Design

The research analyzes timing data from 25 millisecond pulsars observed over up to 24.7 years by the EPTA, with some data sets also including 3.5 years of InPTA observations for 10 pulsars. The team examines four data sets: the full EPTA data, a 10.3-year subset using only modern equipment, and both of these combined with InPTA data. They use advanced statistical methods, including Bayesian and frequentist analyses, to search for a common signal across pulsars and to distinguish gravitational wave signals from other noise sources. Multiple software pipelines and noise models are used to check the reliability of the results. Simply put: The researchers carefully studied years of pulsar data, using different methods and tools, to look for signs of gravitational waves.

Findings

The study reports evidence for a gravitational wave background in the most recent 10.3-year EPTA data, with a strong statistical significance (Bayes factor of 60 and a false alarm probability of about 0.1%). The full 24.7-year data set shows only marginal evidence (Bayes factor of 4, false alarm probability of 4%). Adding InPTA data improves noise modeling but does not change the main results. The detected signal matches the expected pattern for gravitational waves, but the exact shape of the signal's spectrum is uncertain and differs between data sets. If the spectral index is fixed to the value expected from supermassive black hole binaries, the amplitude is consistent across data sets. The study recommends further investigation to clarify the source and properties of the signal, and expects future data from international collaborations to improve the results. Simply put: The team found strong hints of gravitational waves in recent pulsar data, but more research is needed to fully understand and confirm the signal.

Abstract

We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). A robust GWB detection is conditioned upon resolving the Hellings-Downs angular pattern in the pairwise cross-correlation of the pulsar timing residuals. Additionally, the GWB is expected to yield the same (common) spectrum of temporal correlations across pulsars, which is used as a null hypothesis in the GWB search. Such a common-spectrum process has already been observed in pulsar timing data. We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of 4%. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of 60 and a false alarm probability of about 0.1% (≳3 σ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The latest EPTA data from new generation observing systems show non-negligible evidence for the GWB. At the same time, the inferred spectrum is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of (2.5 ± 0.7) × 10 −15 at a reference frequency of 1 yr −1 . Further investigation of these issues is required for reliable astrophysical interpretations of this signal. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.

Referenced In

StarTalk Show Notes

22 days ago

StarTalk

Season 17, Episode 33: Gravitational Waves from Colliding Neutron Stars

Hey StarTalkians! Episode 33 of Season 17 was another Cosmic Queries edition, with Neil and Negin working through a grab-bag of questions covering everything from LIGO to lycanthropy. One interesting question asked about visible sources of gravitational waves:

Cosmic Queries – LIGO, Light, & Lycanthropy - StarTalk Radio

(from 34:00)

Neil addressed this question very well, but after the excitement of the famous first gravitational wave observation, this “visible” result got comparatively little attention.

The GW170817 Observation: Seeing the Source

Neil’s answer is based on the GW170817 observation in August 2017. The LIGO and Virgo gravitational wave detectors picked up a signal consistent with two in-spiralling neutron stars.

This is kind of fitting, because the first indirect evidence of gravitational waves came from a binary neutron star system.

Neutron stars aren’t particularly massive – around 1.4-times the mass of the sun – but they are incredibly compact, crammed into a radius of just 10 km or so. If the Earth was shrunk until it was as dense a neutron star, it would end up just 305 meters in diameter.

Observations of two neutron stars orbiting closely showed a decrease in orbital energy, which physicists assumed was a sign of gravitational wave emission.

The GW170817 observation corroborates this. Two US-based LIGO detectors made the observation, and the Virgo detector in Italy helped localize the source. Such binary neutron star mergers also create gamma-ray bursts, and right around that time, NASA’s Fermi space telescope spotted a matching burst.

It’s like hearing a siren from inside your apartment. You know there’s an emergency somewhere, but you don’t know the exact source until you follow the sound and locate the flashing light.

How Pulsars Can Become the Detector

While checking into Neil’s comment, something else incredibly cool came up. Other researchers used pulsar signals to detect gravitational waves in a different way. Pulsars are neutron stars that give off regular flashes, like a lighthouse whipping around and periodically pointing at you. They’re like cosmic clocks.

Tracking 25 pulsars, researchers looked for slight variations in the timing of the pulses, a sign of disturbance by a gravitational wave. Like a spider waiting for vibrations along its web, they were able to detect a gravitational wave background. These were probably created by binary black hole systems.

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Created: Jun 3, 2026