Posted on 02/17/2019 8:54:27 AM PST by ETL
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.
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
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 gravitational waves emitted by binary neutron stars 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 neutron 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 space-time 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 universe 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.
Journal reference: Physical Review Letters
My pleasure.
Not. Gravity wave detection is another confirmation of General Relativity. GR prohibits any FTL energy transfers.
They propogate, as any massless particle, at c.
That makes sense. I guess. Can it travel faster than c?
A relativistic question: does a wave move a mass at c too? Can’t be. But can a mass ride the crest of a wave?
According to the theory of General Relativity gravity propagates at exactly the speed of light.
Not exactly. The hypothesis that gravity waves travel at the speed of light is used to calculate the direction of arrival, or rather the cone of arrival. The difference in the time of arrival is given by:
dt = D x cos(theta)/cThe time of arrival constrains the source to lie on the surface of a cone, whose "cone half angle" is equal to theta. If dt x c / D is greater than 1.0, then, either the signal received at the two stations is not from the same source, or gravity waves are slower than the speed of light. If dt x c = D then the source lies along the line joining the two stations, if dt = 0, perpendicular to it.theta = acos( dt x c /D)
where:
dt is the difference in time of arrival
D is the distance between the two stations
c is the speed of light
theta is the angle of arrival off the line joining the two stations.
Your point is valid. Notice that I didn’t say it proved that gravity waves travel at the speed of light, only that it indicated they travel at the speed of light.
Perhaps I should have said it was consistent with the ‘assumption’ that they travel at the speed of light.
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