General relativity and quantum mechanics work incredibly well, but they fail spectacularly when they are put together. For this reason, physicists have been looking for a new theory of quantum gravity that goes beyond them.
An overlapping theory is needed with some urgency in areas like black holes and the Big Bang, where the extreme conditions require the use of both theories. In one particular area, though, cosmologists from the University of Portsmouth and University of Tehran seem to have made some breakthrough.
In a paper, published in Physical Review Letters, the scientists looked at the period of cosmic inflation, the sudden expansion of the universe that went from microscopic and subject to quantum fluctuations to macroscopic.
Those fluctuations were tiny differences in density, but once blown up to cosmic proportions, allowed for an uneven distribution of matter.
“Quantum fluctuations during inflation are thought to be the origin of all structure in the universe," lead author Dr Vincent Vennin, from the University of Portsmouth, said in a statement. "Structures we see today such as galaxies, stars, planets and people can be traced back to these primordial fluctuations."
It is not easy to work out what these fluctuations are like. There are many different scenarios for inflations and, while many have been ruled out over the years, we don’t have the correct one yet. The British-Iranian team looked at what these fluctuations should look like from a probabilistic point of view.
The approach is a bridge between relativity and quantum mechanics, and while it doesn’t go into the dirt and grit of a theory of quantum gravity, it does allow them to produce a new approach to the quantum effects on cosmological fluctuations.
“Physicists do not yet know how to combine theories of gravity and the quantum world," said Vennin. "Yet both play a crucial role in the very early universe where the expansion of space is driven by gravity and cosmological structures that arise from quantum fluctuations.
"We haven’t solved quantum gravity but we’ve learnt a little more about how it would work.”
General relativity and quantum mechanics are two of the highest intellectual achievements of humanity, describing the large and heavy as well as the small and light, respectively. Their unification is the biggest challenge in physics and the only way to truly understand the universe.
