If extra dimensions do exist, they must be really, really small

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Is our universe exist in more than three dimensions?

Probably not on a large scale, according to new research published July 23 in the Journal of Cosmology and astroparticle physics. The study found that over large distances in space, the universe probably works just the dimensions that we experience on Earth. The results will also help scientists better understand the enigmatic nature of the dark energy, the mysterious phenomenon behind the accelerating expansion of the universe.

In October 2017, scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) for the detection of a gravitational wave produced in the collision of two neutron stars. Dubbed GW170817, the event was also seen with traditional telescopes, allowing scientists to simultaneously study the event through gravitational waves and light waves. The dual measurements are allowing scientists to learn everything about our universe, including how many dimensions it can hold. The new results also provide additional evidence for Albert Einstein’s general theory of relativity. [8 Ways You Can See Einstein’s theory of Relativity in Real Life]

“The general theory of relativity says that the gravity should be, in three dimensions, and [the results] show that that is what we are seeing,” said Kris Pardo, lead author of the study and a doctoral student at Princeton University.

While the general theory of relativity has proven so far to be spot-on in describing our universe, there is a thing that it can’t explain very well: why our universe is the expansion. Scientists nicknamed the cause of this acceleration “dark energy”, but nobody knows what it is. Some of the theories of change of the force of gravity to explain the expansion, suggesting that the force of gravity works in a different way on a large scale. Many of these ideas predict that other dimensions exist, and these could be probed by gravitational waves.

“This whole effort on the part of the search for modified theories of gravity is, in essence, driven by the mystery of dark energy. We try to find: Is there a way we can tweak the laws of gravity to explain why the universe expansion to accelerate?” said Tessa Baker, a cosmologist at the University of Oxford in England, who was not involved in the study.

According to many of these theories, if extra dimensions exist, gravitational waves would “leak” in those dimensions, allowing the waves to weaken as they make their way through the universe. The scientists in the recent study, it was measured how far the gravitational waves and the light waves of GW170817 travelled to reach Earth, but the researchers found no evidence of the weakening that would be associated with extra dimensions.

In the wake of the GW170817 event, many studies excluded that some of the modified gravity theories, by calculating the speed of gravitational waves to determine their travel-delay. This new paper is able to dismiss a whole other set of theories, Bakertold LiveScience.

The new results exclude only large in size. As such, they do not impose any restrictions on the 10-plus dimensions predicted by the string theory is a theory in physics that suggests that everything is made of tiny vibrating strings. However, the new findings show that at scales of approximately 1 mile (1.6 km) to at least 80 million light-years away, the universe is three-dimensional. The finding further rejects even larger dimensions, but only if they have visible effects on the physics on scales smaller than 80 million light-years away.

The researchers also use the data to calculate the lifetime of the graviton, a theoretical particle that, if it exists, brings the force of gravity. That lifespan is at least 450 million years, the scientists discovered. In other words, the graviton is not lapse into lighter particles over this time. Some modified gravity theories predict such a decline, so this calculation of the graviton life can be used in the future of gravitational wave events that occur in other parts of the universe, to help in the testing of these theories.

The GW170817 collision event “was very, very local in cosmological terms — basically on our doorstep,” Baker said. But physicists would really like to see more events at larger distances (which would have occurred further back in time), because that would become known as gravity, or dark energy have changed over time, Baker added.

For now, the universe seems to be only the dimensions that we are accustomed to. But despair not, science fiction writers and-time-travel-lovers of the smaller, compact size, it can still be out there.

Originally published on Live Science.



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