We may not find it on a large scale, but our universe could be made up of a range of small dimensions, according to a study published in The Journal of Cosmology and Astroparticle Physics. The work, published by researchers at Princeton University (USA), does not rule out the existence of extra small dimensions . The results also help scientists better understand the puzzling nature of dark energy, the mysterious phenomenon behind the accelerating expansion of the universe.
In October 2017, scientists used the Laser Interferometer Gravitational Wave Observatory (LIGO) to detect a gravitational wave produced in the collision of two neutron stars. Nicknamed GW170817, the event could also be observed with traditional telescopes, allowing scientists to simultaneously study the occurrence via gravitational waves and light waves. These dual measurements allow scientists to learn all sorts of things about our universe, including how many dimensions it could be. The new results also offer additional evidence for Albert Einstein’s general relativity.
“General relativity says that gravity should be working in three dimensions, and – the results – show that this is what we see,” said Kris Pardo, leader of the work.
While general relativity has so far proven to be accurate in describing our universe, there is one thing it cannot fully explain, and that is why the expansion of the universe is accelerating . Scientists dubbed the cause of this acceleration “dark energy,” but no one knows what it really is. Some theories modify gravity to explain expansion, suggesting that gravity works differently on large scales. Many of these ideas predict that there are other dimensions, and these could be investigated by the famous gravitational waves.
According to many of these theories, if additional dimensions existed, gravitational waves would “leak” into those dimensions, causing the waves to weaken as they move through the universe.
The researchers measured how far the gravitational waves and light waves from GW170817 traveled to reach Earth, however they found no indication of the weakening that was related to the extra dimensions.
There are no large extra dimensions but there are small
The new results exclude only the large dimensions. As such, they do not impose restrictions on the more than 10 dimensions predicted by string theory, a theory of physics that suggests that everything is made up of tiny vibrating strings. However, the new findings show that on scales of about 1.6 kilometers to at least 80 million light years, the universe is three-dimensional . The discovery rules out even larger dimensions, but only if they have visible effects on physics on scales less than 80 million light-years.
The researchers also used the data to calculate the lifespan of the graviton, a theoretical particle that, if it exists, transmits the force of gravity. Scientists found that their lifespan would be at least 450 million years. In other words, the graviton is not breaking down into lighter particles during this time.
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 greater distances (which would have happened further back in time), “because they would reveal whether gravity or dark energy has changed over time,” Baker added.
For now, the universe seems to have only the dimensions we are used to; But let’s not despair, it is possible that there are still smaller and more compact dimensions.
Reference: Limits on the number of spacetime dimensions from GW170817. Kris Pardo, Maya Fishbach, Daniel E. Holz and David N. Spergela. Published 23 July 2018 • © 2018 IOP Publishing Ltd and Sissa Medialab. Journal of Cosmology and Astroparticle Physics, Volume 2018, July 2018
