The following is an extract from our Lost in Space-Time newsletter. Each month, we dive into fascinating ideas from around the universe. You can sign up for Lost in Space-Time here.

Cosmic inflation is a problem. During the first tiny fraction of a second of the universe, it is generally believed that the universe expanded by a factor of around 10³⁰. And then, as quickly as it began, this exponential growth just stopped. The idea was first proposed because if you rewind the evolution of large-scale cosmic structures, galaxies and stars as we see them, you come to the conclusion that everything everywhere began in a big bang. Inflation solved several problems with big bang cosmology at once, but in certain corners it remains somewhat controversial. And for researchers working to unite the laws that govern the very large with those that govern the very small – perhaps the biggest problem in modern cosmology – it’s the biggest stumbling block of all.

First, the good news. Aside from explaining how the universe got from very small to much bigger, inflation explains what I’ll call the structure problem – that is, how anything bigger than a planet formed at all. Before inflation, the universe would have been largely homogeneous, with only the tiniest of variations due to quantum effects. Inflation would have blown those variations up and introduced new ones, eventually making them significant enough to start matter clumping together by gravity and then forming galaxies and stars and everything else that we see in the universe now. Without inflation, we’d have no stars, let alone structures as immense as galaxies and superclusters.

Perhaps counterintuitively, inflation also explains why everything across the universe looks about the same. This is called the horizon problem: if we look as far as we can in two opposite directions, the views are extraordinarily similar. But two regions of space on opposite ends of the observable universe are much too far from one another to have interacted in any meaningful way, even just through light, so without inflation there’s no reason they should be anything alike. With inflation added to the model of the big bang, though, we can say that all regions of space were once close enough together to interact and come to equilibrium before rushing outward. The structure problem and the horizon problem are two sides of the same coin; inflation explains both why the universe is chunky and why it’s smooth. There are a couple of other empty spaces in the big bang hypothesis that inflation fills, as well. Put simply, it is extraordinarily good at explaining why the cosmos looks the way it does today.

Nevertheless, it has its detractors, and not without reason. For one thing, we don’t really know why inflation would happen. In order to kick off inflation, the universe would need to have had extraordinarily specific initial conditions, which leads to what cosmologists call the fine-tuning problem: we can’t explain why the universe would have those initial conditions, so it starts to look like we’re tweaking the numbers to match our theories, rather than simply finding the theory that naturally fits best. Researchers disagree about whether inflationary theories evoke the fine-tuning problem, but they nearly all agree that fine-tuning is bad science. The spectre of it in any theory is enough to make all sorts of physicists nervous.

So a mechanism to start inflation is tough to come up with, and so is one to end inflation once it’s begun, for similar reasons. There are, of course, many different models of inflation, each with evidence in its pro and con columns. Things only get more complicated when you start to consider the other mysteries in cosmology alongside it.

The biggest of those is the relationship between general relativity, which is the physics of the extremely large (mediated by gravity), and quantum mechanics, which is the physics of the extremely tiny. Those two should meet and mesh somewhere in the middle, allowing them to be combined into a theory of quantum gravity, but they don’t.

The inflationary epoch, and the minuscule moments of time preceding it, are one place where the extremely small and the extremely large are linked, where everything is so dense that the usually weak force of gravity becomes extraordinarily powerful, and everything is so small that it’s rife with quantum effects. For quantum gravity, that makes the inflationary epoch the perfect place to call home.

A successful model of quantum gravity must, then, not only account for both the effects of relativity and quantum mechanics in the current universe, but also how and why inflation would start and end. The more you think about the problem, the knottier it becomes.

Solving inflation

One potential solution comes from loop quantum gravity, which posits that the universe’s beginning and end are roughly symmetrical – it’s a big bounce scenario, where the universe inflates, and later on it deflates only to rebound back out again. Another comes in the form of infinite inflation: if there are some areas where the universe goes on inflating exponentially forever, you don’t have to worry about how to end inflation. But you do have to worry about creating an infinite fractal multiverse in which the various inflating areas become their own universes, so far from our own that we could never access them, which is a pretty significant worry that largely killed support for that particular model. An infinite multiverse is a hard sell for many physicists, because in such a reality, everything that could possibly happen will happen somewhere, so any remotely plausible predictions made within the infinite multiverse would be effectively impossible to test.

As simpler scenarios have been ruled out, increasingly complex ones have arisen. There’s hybrid inflation, which introduces at least two new fields (not like fields of grass or fields of study, but like electromagnetic fields) carrying the energy required to start, slow and end inflation. There’s brane inflation, which comes from string theory and is so complicated it may be impossible to explain briefly, but essentially it proposes that our cosmos exists on a membrane between different dimensions, which could explain away any confounding issues with inflation.

There’s also an idea called quadratic gravity, which involves modifying a model of gravity so that it still works at the extraordinarily high energy densities where general relativity tends to break down. When you add quantum corrections to those equations, out pops inflation, and the behaviour that we attribute to relativity arises on its own as the universe grows. In short, it satisfies the rules of quantum mechanics and matches with the tenets of general relativity – two extremely well-tested pillars of physics. That’s a great start.

The main problem with quantum quadratic gravity is that it suggests we ought to see massive “ghost particles”, but so far we haven’t found any in experiments. However, a recent paper that proposes a new take on this strange idea suggests a reason for that. The findings indicate that as the universe grew exponentially during inflation, gravity became stronger, which led to “the containment of ghosts”, as the researchers put it.

Inflation explained and ghosts busted, it’s a promising idea. The other upside for quantum quadratic gravity is that it should come with a side order of ripples in space-time created in the early universe, and as weak as those gravitational waves would be, the next generation of detectors could be able to spot them.

Personally, I suspect that inflation will remain somewhat controversial for decades to come. The measurements we need to prove how it happened are almost unspeakably precise – in addition to the extremely weak gravitational waves, we would need to do extremely precise measurements of the cosmic microwave background (CMB), the radiation imprinted across the cosmos from the dawn of time. Then there’s the difficulty of making sure we’ve interpreted the measurements correctly once we have them. We’ve misread the CMB before – supposed gravitational wave signatures created by cosmic inflation, once hailed as the discovery of the century, turned out to be simply galactic dust. This one nearly infinitesimally small moment in cosmic history has the power to break physics as we know it – but it also has the power to unite the two best theories we have to describe the universe we live in.