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First Significant Evidence Of Gravitational Wave Background Using Galaxy-Sized Detector

These gravitational waves can’t be observed with our usual detector.


Dr. Alfredo Carpineti


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master's in Quantum Fields and Fundamental Forces.

Senior Staff Writer & Space Correspondent

An artist impression showing a supermassive black hole binary, pulsars and waves altering space-time

Astronomers find evidence of waves from supermassive black hole mergers and maybe from the beginning of the universe itself.

Image Credit: Aurore Simonnet for the NANOGrav Collaboration

The NANOGrav collaboration, part of the International Pulsar Timing Array (IPTA), has announced the first significant evidence of the long-sought gravitational wave background, the low-frequency vibration of space-time that is expected to permeate the universe. This achievement comes from 15 years of work creating a gravitational wave detector that is as big as a galaxy.

The detector, known as a Pulsar Timing Array, works using pulsars, a special type of neutron star. A neutron star is the collapsed core of a massive star that went supernova but was not heavy enough to turn into a black hole. Some neutron stars emit pulses of radiation, acting a bit like a lighthouse, and a special class of these pulsars spin on their axes hundreds of times per second.


These millisecond pulsars are not just wonderful cosmic lighthouses – they are also precise clocks, as the interval between pulses stays stunningly regular. But there are things that affect the observed pulse timing, such as the effect of a companion star on the pulsar, the interstellar medium, and the position of the Earth. When all of that is taken into account, what is left behind is the effect of space-time being altered by gravitational waves. These waves stretch and push space-time, altering the length of the pulse by a minuscule fraction.

“Pulsar timing is the process of creating a model that predicts each and every pulse from a pulsar reaching far into the future,” Dr Thanful Cromartie, the chair of the Timing Working Group for NANOGrav, said in a press conference attended by IFLScience. “It's conducted to a precision that rivals atomic clocks, which is necessary for gravitational wave sensibility that we're trying to achieve.”

So by measuring pulsars' rotations with staggering precision, astronomers have learned about never-seen-before vibrations in space-time. They are not calling it a discovery yet, because the data has not reached the fabled 5-sigma threshold. But the chance of noise mimicking the data is 1 in 1,000, and in some tests, this is 1 in 10,000. Hence the claim that this is strong evidence, the strongest yet for the presence of the gravitational wave background.

The nanograv pulsar distribution overlayed over an artist's impression of the Milky Way
The position of the pulsars in the Milky Way with respect to the Sun (yellow star). Image Credit: NANOGrav collaboration

The frequency of these waves is much lower than what is seen in observatories on Earth, and those are already pretty sensitive. The likes of LIGO and Virgo can detect changes in space-time of a fraction of an atom. But this is going even further, and this is why the detector needs to be even bigger. There are 68 millisecond pulsars in the NANOGrav array, with each pulsar acting as the arm of the detector.


“The amplitude that we're measuring is incredibly tiny. About one part in a quadrillion,” Dr Michael Lam, co-chair of the Noise Budget working group explained. “And that is much lower than the noise that is in each individual pulsar. And so the only way that we're really able to make this measurement is by correlating the measurements between each individual pulse or arm.”

The gravitational waves that have been detected so far came from specific events. But the background whose evidence is being presented here is not from one event. It is believed to be from all the supermassive black holes in pairs located in the universe, although it could be possible that some of the waves come from the Big Bang itself or the period of cosmic inflation.

When galaxies merge, their supermassive black holes move towards each other. They will eventually end up orbiting one another and merging. We are not exactly sure how all of that happens, something called the final parsec problem, as they need to lose a lot of energy to end up merging. The gravitational wave background is believed to be made of the waves produced by supermassive black holes that have already passed that parsec threshold, and are expected to merge within 10,000 to 100,000 years. And these are massive and common events.


“To get these types of high amplitudes that we're seeing, we need fairly massive black holes, and they need to form binaries quite frequently and evolve quite efficiently,” Dr Luke Zoltan Kelley, explained.

It should be possible to distinguish between the signal from supermassive black hole binaries and what came from the birth of the universe. Key to this would be anisotropy, different areas of the sky would look different due to the former as the pair of black holes will not be distributed equally everywhere in the sky.

“This is key evidence for gravitational waves at very low frequencies,” Vanderbilt University’s Dr Stephen Taylor, who co-led the search and is the current Chair of the collaboration. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe."

The team hopes to eventually be able to do that as well as finding the location in the sky of specific binary supermassive black holes. This last goal might actually come sooner rather than later – maybe in one year or two, thanks to IPTA. The four main IPTA members are Parkes Pulsar Timing Array (PPTA), European Pulsar Timing Array (EPTA), India Pulsar Timing Array (InPTA), and NANOGrav, with groups in South Africa and China being observers, and a new group in Argentina starting to work on it.


These groups have also published their results today. Given the different sensitivity of telescopes and the number of pulsars visible around the world, data from some groups are more compelling than from others – the China one, for example, has higher confidence but on a shorter timeframe – but together they indicate that the evidence is strong and independently supported.

“The results presented today mark the beginning of a new journey into the Universe to unveil some of its unsolved mysteries,” Dr Michael Keith, Lecturer at Jodrell Bank Centre for Astrophysics at The University of Manchester and member of EPTA, said in a statement. "We are incredibly excited that after decades of work by hundreds of astronomers and physicists around the world, we are finally seeing the signature of gravitational waves from the distant Universe.”

The next step for IPTA is to combine their data and analyze it together. The IPTA catalog will have 115 pulsars and the team hopes that it will unequivocally show the existence of the gravitational wave background and even show the location of binary pairs of supermassive black holes that will be followed up with other telescopes.

To find signals left over from the Big Bang, it will take longer than just a couple of years, but if the foundation is solid more discoveries will come from IPTA. Hundreds of scientists across the world are working on disentangling all these cosmic vibrations.


The papers presenting these results are published in The Astrophysical Journal Letters.


spaceSpace and PhysicsspaceAstronomy
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  • pulsar,

  • Astronomy,

  • graviational waves,

  • pulsar timing array,

  • graviational wave background