Gravity was one of the first fundamental forces studied in modern science, but fundamentally, it is not that well understood. It is a lot weaker than electromagnetism and the two nuclear forces at the level of particles and only dominates at large scales.
One potential explanation for the weakness of gravity is that it leaks into hidden extra dimensions of the universe. Testing this hypothesis is far beyond the current capabilities of our atom smashers, but scientists hoped to maybe see a glimpse of them from space. They used neutron star collisions to study the nature of gravity.
Last year, for the first time, astronomers detected the gravitational waves from such a collision and observed how much light was emitted. As reported in the Journal of Cosmology and Astroparticle Physics, this gave researchers a tool for finding the extra dimensions. They didn’t spot any.
"This is the very first time we've been able to detect sources simultaneously in both gravitational and light waves," co-author Professor Daniel Holz, from the University of Chicago, said in a statement. "This provides an entirely new and exciting probe, and we've been learning all sorts of interesting things about the universe."
The researchers looked at models with non-compact dimensions for gravity to move into while confining light in our four-dimensional spacetime. The team measured the distance of the collision using both light and gravitational waves. The two values are consistent with each other, suggesting that gravity is moving in the regular 3+1 dimensions we are accustomed to.
The idea of gravity weakening as it travels is also important to several alternative explanations for the effects that most astronomers call dark matter and dark energy. These explanations argue that general relativity as we use it doesn’t take into account the extra-dimensional leak, which is why we had to come up with the dark components of the universe. The new data suggests that the gravitational weakening simply isn't there.
"There are so many theories that until now, we didn't have concrete ways to test," co-author Maya Fishbach added. "This changes how a lot of people can do their astronomy."
There are three gravitational wave observatories in the world, two in the US called LIGO and one in Italy called Virgo. The three are currently undergoing a technical stop but will continue to collect data in a few months' time, so more detections are on the cards.
"We look forward to seeing what gravitational-wave surprises the universe might have in store for us," Holz said.