Transcription
It's not every day that I see an explanation for the beginning of the universe. Today I have one that was published in PRL, which is the top journal in physics, and it did make quite a few headlines. This new idea elegantly does away with the problem of quantum gravity, and it also tells us what happened before time began. Let's have a look.
The biggest open problem in the foundations of physics is that Einstein's theory of gravity, general relativity, doesn't cooperate with quantum mechanics. It's a problem because gravity is caused by matter, and we know that the matter has quantum properties. So, gravity must react to these quantum properties of the matter somehow. We just don't know how to make these two theories work together. We need a theory of quantum gravity.
Such a theory of quantum gravity is necessary to understand what happens in places where gravity becomes very strong, because then the quantum fluctuations of space and time are big. This plays a role most importantly inside of black holes and at the big bang, the beginning of the universe.
The issue with quantum gravity is not that we don't know how to convert Einstein's theory into a quantum theory. Einstein's general relativity can be converted into a quantum theory, like the other interactions: electric and magnetic, strong and weak nuclear force. It's just that the quantum version of gravity that one obtains this way breaks down exactly when one needs it, inside of black holes and at the big bang.
The authors of the new paper found a way to remedy this breakdown. You see, the reason that quantizing gravity is so difficult is ultimately easy to understand, is because the strength of gravity is given by Newton's constant, and that constant is dimensionful. It's got units. The strength of the electromagnetic interaction, on the other hand, is given by alpha, which is approximately 1/137, and that one doesn't have units. It's similar for the nuclear forces. To make quantum gravity work out, one would need the strength of gravity to be dimensionless too.
And there is a version of gravity which has this property. It's called quadratic gravity, because the action is quadratic in the curvature, and then the coupling constant has no units. Don't worry if you don't know what this means. The relevant point is that this quadratic gravity does have a neat quantum version. It just isn't compatible with our observations.
Then again, this isn't necessarily a problem. It could be that in the cases that we observe gravity, we have Einstein's normal gravity, and then at high energies and strong curvature, it goes over into quadratic gravity, which resolves the problem with quantizing it. Then everything would work out. This is the idea in the new paper.
The authors, a group from Perimeter Institute, looked at how the origin of the universe would work out with the transition to quadratic gravity. They find that the universe originally does not have time at all. It's four-dimensional space. But because of quantum fluctuations, there is a border in this space where one of the dimensions turns into time. So this is where our universe properly began.
After this happens, the quantum properties drive a rapid expansion of the universe. This is very similar to what's usually called inflation. This process eventually runs out of steam and stops. Then the energy from the expansion converts into matter and radiation, including dark matter.
So this idea solves several problems at once. It explains how the universe started. It explains how our universe came to be that big from what should be a tiny quantum fluctuation. It explains why the universe stopped growing that rapidly and where matter came from. And it does all that without running into the problem with quantum gravity. They even make some predictions for how this idea could become measurable in the future from the gravitational waves that be produced in the early universe.
Sounds too good to be true. And yes, there is a caveat: is that this particular way that they connect Einstein's gravity in the low energy, low curvature range that we observe to the quadratic gravity at high energy and strong curvature is well motivated, but not strictly speaking derived. This means that basically they have some artistic freedom in designing just how the transition happens. So there is somewhat of a gap in the logic, but maybe it's a gap that can be fixed with further work. I give this paper a two out of 10 on the meter.
There are a lot of theories for the origin of the universe, but few are as minimalistic as that. The idea that quadratic gravity can solve the problem with quantum gravity is not entirely new. It's also behind an approach called asymptotically safe gravity that goes back to Steven Weinberg. But this is the first time that I've seen this idea apply to the big bang. In summary, they solved quantum gravity, explained the big bang, and avoided all known problems. The only remaining issue is whether it's actually true.
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