“Nixed!” –Neutron Star Mashup Spikes Theories That Gravitational Waves Travel Faster Than Speed of Light (VIDEO)

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On October 16, physicists announced that the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, and VIRGO,  the European Gravitational Observatory, detected gravitational waves from a neutron star merger that emitted high-energy light shortly after merging (view the sequential videos below). The Fermi space telescope spotted that light coming from the same region of the sky 1.7 seconds after the gravitational wave detection that proved for the first time that gravitational waves, the ripples in spacetime set off when massive bodies collide, travel at the speed of light to within a tenth of a trillionth of a percent.

Within 24 hours, five papers were posted at arXiv.org nixing hundreds of expanding universe theories that predicted gravitational waves should travel faster than light — an impossibility without changes to Einstein’s laws of gravity reports sciencenews. These theories “are very, very dead,” said the coauthor of one of the papers, cosmologist Miguel Zumalacárregui of the Nordic Institute for Theoretical Physics, or NORDITA, in Stockholm, Sweden. “We need to go back to our blackboards and start thinking of other alternatives.”



The most popular explanation for the acceleration of the expansion of the universe is that spacetime is filled with x-factor dubbed dark energy. “You can think of it like a mysterious fluid that pushes everything apart and counteracts gravity,” says cosmologist Jeremy Sakstein of the University of Pennsylvania, coauthor of another new paper.

Radical new research is attempting to characterize the properties of a fifth force, dark energy, that disrupts the predictions general relativity makes outside our own galaxy, on cosmic-length scales. University of Pennsylvania astrophysicist Bhuvnesh Jain and his colleague Sakstein, says the nature of gravity is the question of a lifetime. As scientists have been able to see farther and deeper into the universe, the laws of gravity have been revealed to be under the influence of this unexplained force.

Two branches of theories have sprung up, each trying to fill its gaps in a different way. One branch — dark energy — suggests that the vacuum of space has an energy associated with it and that energy causes the observed acceleration. The other falls under the umbrella of “scalar-tensor” gravity theories, which effectively posits a fifth force (beyond gravity, electromagnetism and the strong and weak nuclear forces) that alters gravity on cosmologically large scales.

“These two possibilities are both radical in their own way,” Jain said. “One is saying that general relativity is correct, but we have this strange new form of energy. The other is saying we don't have a new form of energy, but gravity is not described by general relativity everywhere.”

In the simplest version of this theory, the density of this dark energy has not changed over the history of the universe, so physicists call it a cosmological constant. This doesn’t require any changes to gravity because gravity has been well-tested inside the solar system. The cosmological constant idea matches observations of the wider universe, but it has some theoretical difficulties. Dark energy is about 120 orders of magnitude weaker than theorists calculate it should be, a finding that makes scientists squirm.

Also, different methods for measuring the rate of expansion come up with slightly different numbers: Measurements based on exploding stars suggest that distant galaxies are speeding away from each other at 73 kilometers per second for each megaparsec (about 3.3 million light-years) of space between them. But observations based on the cosmic microwave background, ancient light that encodes information about the conditions of the early universe, found that the expansion rate is 67 km/s per megaparsec. The disagreement suggests that either one of the measurements is wrong, or the theory behind dark energy needs revision.

So instead of invoking a the dark energy x factor to counteract gravity, theorists tried to explain the expanding universe by weakening gravity itself. “It’s quite hard to build a theory that accelerates the universe and also doesn’t mess up the solar system,” says cosmologist Tessa Baker of the University of Oxford, coauthor of still another paper. “From my perspective, the exciting thing about this paper is that it improves tests of gravity in a regime that is not well-tested at the moment,” Tessa Baker, post doctoral researcher at the University of Oxford told Gizmodo. She pointed out that there is a lot of clutter and potential factors in the center of the galaxy that can cloud a measurement there, but “it seems like they’ve done a comprehensive and careful job of accounting for these.”

“This field of modified gravity theories is a zoo,” says Baker says. Some suggest that gravity leaks out into extra dimensions of space and time, while others account for the universe’s speedy spreading by adding a different mysterious entity — some unknown particle perhaps — that saps gravity’s as the universe evolves.

But the new entity would have another crucial effect: It could slow the speed of light waves, similar to the way light travels more slowly through water than through air. That means that the best alternatives to dark energy required gravitational waves to travel faster than light — which they don’t.

Justin Khoury, a theoretical physicist at the University of Pennsylvania who has worked on several of the alternative gravity theories but was not involved in the new papers, says “The fact that we’re learning something about dark energy because of this measurement is incredibly exciting.”

The discovery gravitational waves and light waves at the same time offers a third, independent way to measure how fast the universe is expanding suggesting that the rate lies frustratingly right between the two clashing measurements scientists already had, at 70 km/s per megaparsec. Once LIGO and other observatories have seen 10 or 20 more neutron star collisions, they should be able to state which measurement is correct and whether dark energy needs an update, Zumalacárregui says. “Gravitational waves may kill these models, but eventually they have the potential to tell us if this discrepancy is for real,” he says. “That’s something that is in itself very beautiful.”

The Daily Galaxy via dailygalaxy and  sciencenews.com

Image credit: LIGO/Aurore Simonnet/Sonoma State University







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