Gravitational waves will settle cosmic conundrum

February 14, 2019, Simons Foundation
When neutron stars collide, they emit light and gravitational waves, as seen in this artist's illustration. By comparing the timing of the two emissions from many different neutron star mergers, researchers can measure how fast the universe is expanding. Credit: R. Hurt/Caltech-JPL

Measurements of gravitational waves from approximately 50 binary neutron stars over the next decade will definitively resolve an intense debate about how quickly our universe is expanding, according to findings from an international team that includes University College London (UCL) and Flatiron Institute cosmologists.

The cosmos has been expanding for 13.8 billion years. Its present rate of expansion, known as "the Hubble constant," gives the time elapsed since the Big Bang.

However, the two best methods used to measure the Hubble constant have conflicting results, which suggests that our understanding of the structure and history of the universe—the "standard cosmological model"—may be incorrect.

The study, published today in Physical Review Letters, shows how new independent data from emitted by binary neutron called "standard sirens" will break the deadlock between the conflicting measurements once and for all.

"We've calculated that by observing 50 binary neutron stars over the next decade, we will have sufficient gravitational wave data to independently determine the best measurement of the Hubble constant," said lead author Dr. Stephen Feeney of the Center for Computational Astrophysics at the Flatiron Institute in New York City. "We should be able to detect enough mergers to answer this question within five to 10 years."

The Hubble constant, the product of work by Edwin Hubble and Georges Lemaître in the 1920s, is one of the most important numbers in cosmology. The constant "is essential for estimating the curvature of space and the age of the universe, as well as exploring its fate," said study co-author UCL Professor of Physics & Astronomy Hiranya Peiris.

"We can measure the Hubble constant by using two methods—one observing Cepheid stars and supernovae in the local universe, and a second using measurements of cosmic background radiation from the early universe—but these methods don't give the same values, which means our standard cosmological model might be flawed."

Feeney, Peiris and colleagues developed a universally applicable technique that calculates how gravitational wave data will resolve the issue.

Gravitational waves are emitted when binary neutron stars spiral toward each other before colliding in a bright flash of light that can be detected by telescopes. UCL researchers were involved in detecting the first light from a gravitational wave event in August 2017.

Binary star events are rare, but they are invaluable in providing another route to track how the universe is expanding. The gravitational waves they emit cause ripples in that can be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo experiments, giving a precise measurement of the system's distance from Earth.

By additionally detecting the light from the accompanying explosion, astronomers can determine the system's velocity, and hence calculate the Hubble constant using Hubble's law.

For this study, the researchers modelled how many such observations would be needed to resolve the issue of measuring the Hubble constant accurately.

"This in turn will lead to the most accurate picture of how the is expanding and help us improve the standard cosmological model," concluded Professor Peiris.

Explore further: Could gravitational waves reveal how fast our universe is expanding?

More information: Stephen M. Feeney, Hiranya V. Peiris, Andrew R. Williamson, Samaya M. Nissanke, Daniel J. Mortlock, Justin Alsing and Dan Scolnic, 'Prospects for resolving the Hubble constant tension with standard sirens' will be published in Physical Review Letters on Thursday 14th February 2019.

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8 comments

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torbjorn_b_g_larsson
4.3 / 5 (6) Feb 14, 2019
"We show that, with existing data, the inverse distance ladder formed from BOSS baryon acoustic oscillation measurements and the Pantheon SN sample yields an H0 posterior near-identical to the Planck CMB measurement. The observed local distance ladder value is a very unlikely draw from the resulting PPD.

Turning to the future, we find that a sample of ∼ 50 binary neutron star "standard sirens" (detectable within the next decade) will be able to adjudicate between the local and CMB estimates." [ https://arxiv.org...3404.pdf ]

Not too bad, but I had hoped for 5 years at the most. One factor they did not factor in which may shorten the decade is that the coming Indian detector may start observe in 2024 [ https://en.wikipe...rvations ].
valeriy_polulyakh
1 / 5 (1) Feb 14, 2019
If we believe that our World has started sometimes ago we are still in the position to decide which hypothesis, Lemaître's or Gamow's was closer to reality. There is an opinion that the problems in the standard cosmology could be solved by adjusting of details. Our suggestion is that we have to go back to the conceptions and use the observations accumulated since.
https://www.acade...osmology
https://www.acade...he_World
joel in oakland
3 / 5 (1) Feb 15, 2019
That's great! Let's move on.
Da Schneib
2.3 / 5 (3) Feb 15, 2019
To what?
antialias_physorg
5 / 5 (2) Feb 15, 2019
Not too bad, but I had hoped for 5 years at the most.

Depending on upgrade schedule some of the LIGO-type detectors may be down for months/years at a time. Remember that it takes 40 days alone to create the vacuum inside the arms.

That said the analysis itself is going to take a while, too. The code for this hasn't really been written yet.

Reg Mundy
2.6 / 5 (5) Feb 16, 2019
How close to Earth does the Hubble Constant apply? A hundred light years? One light year? A kilometer? A millimetre? How's about a Planck Length? Everything is expanding, hence the effect we call Gravity. See "About Time:.." for how the Hubble Constant causes time.
granville583762
5 / 5 (4) Feb 20, 2019
Observing Earth

At this Hubble constant
Reg Mundy> How close to Earth does the Hubble Constant apply? A hundred light years? One light year? A kilometer? A millimetre? How's about a Planck Length? Everything is expanding, hence the effect we call Gravity. See "About Time:.." for how the Hubble Constant causes time.

At 13.8billion Lys, the Hubble constant applies to Earth
so
by definition
as
we sit on Earth, this Hubble constant
Applies, at a kilometre, a millimetre, a Planck Length
Reg Mundy
5 / 5 (1) Feb 20, 2019
Observing Earth

At this Hubble constant
Reg Mundy> How close to Earth does the Hubble Constant apply? A hundred light years? One light year? A kilometer? A millimetre? How's about a Planck Length? Everything is expanding, hence the effect we call Gravity. See "About Time:.." for how the Hubble Constant causes time.

At 13.8billion Lys, the Hubble constant applies to Earth
so
by definition
as
we sit on Earth, this Hubble constant
Applies, at a kilometre, a millimetre, a Planck Length

So, everything is expanding then?
I've been saying this for upteen years, and pointing out that this expansion causes the effect we call gravity, not to mention the passage of time.

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