Speed of Gravitational Waves is very near the speed of light

Proof of gravitational waves came on Sept. 14, 2015, when two giant, L-shaped, 2-mile-long laser instruments, one set up in a swamp in Louisiana and the other in Hanford, Washington, detected a tiny ripple in space, a “chirp” that reached Earth from the gigantic collision of two black holes a billion years ago.

The 2017 Nobel prize in physics was awarded to Rainer Weiss of MIT, 85, Kip Thorne, 77, and Barry Barish, 81, both of Caltech.

“I view this more as a thing that recognizes the work of about 1,000 people, a really dedicated effort,” Weiss said.

Over four decades, the American public has bet about a billion dollars on the LIGO project, which stands for Laser Interferometer Gravitational-Wave Observatory. Five LIGO observatories were set up around the world, based on faith that Einstein was right.

Neil Cornish, Montana State University astrophysicist is working with teams of scientists on other ways to detect gravitational waves, like the Pulsar Timing Array and space-based detectors, to go “hunting monster black holes.”

“This is just the beginning of gravitational wave astronomy,” Cornish said. “It’s like when Galileo first turned his telescope to the heavens.

Bounding the speed of gravity around the speed of light has many significant implications for fundamental physics and cosmology. One of the biggest implications is that the tight bounds provide a more precise test of general relativity and rule out proposed alternatives to general relativity.

“Many alternative theories of gravity, including some that have been invoked to explain the accelerated expansion of the Universe, predict that the speed of gravity is different from the speed of light,” Cornish said. “Several of those theories have now been ruled out, thereby restricting the ways in which Einstein’s theory can sensibly be modified, and making dark energy a more likely explanation for the accelerated expansion.”

The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO Scientific and Virgo Collaborations we constrain the gravitational waves propagation speed c gw to the 90% credible interval 55% to 142% of the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order 20 detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.

Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light very tightly.

The reason for the huge leap in precision is that the neutron star event did not emit only gravitational waves, but also electromagnetic radiation in the form of gamma rays. The simultaneous emission of both gravitational waves and light from the same source allowed the scientists to set bounds on the speed of gravity that is many orders of magnitude more stringent that what could be set using gravitational wave signals alone.

The scientists measured an arrival delay of just a few seconds between signals that traveled a distance of more than one hundred million light years. Such a small delay across this distance is considered virtually nothing.

Physics Review Letters – Bounding the Speed of Gravity with Gravitational Wave Observations (Neil Cornish, Diego Blas, and Germano Nardini)

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is 5.0 X 10^ -8. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of (+  1.74 0.05 s ) between GRB 170817A and GW170817 to:

(i) constrain the difference between the speed of gravity and the speed of light to be between 3X 10^-15 and 7 X 10 ^ -16 times the speed of light,

(ii) place new bounds on the violation of Lorentz invariance,
(iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.