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LAST month, I wrote a column about the possibility that space-time is eternally inflating, and I was admittedly caught off guard by the amount of attention it got on social media. I am pleased that so many people share my interest in cosmology and specifically the first few seconds of space-time as we understand it.

At the same time, I was troubled by a supposed summary of the column that I saw in another publication which claimed I was proposing a new theory for the origins of the universe. This suggests to me that I should continue my discussion of inflation this month, including explaining my relatively minor role in doing research on it.

To start, it is worth saying again that my primary research focus is on the problem of dark matter, which we think comprises up to a quarter of the matter and energy in the universe. The cosmology and particle physics communities have been studying dark matter for about 50 years now, and while we have learned a few things about it, there is still much that we don’t know about the elusive substance.

The primary site of our ignorance is what exactly dark matter is. Is it a new particle? If so, what are its properties? Or is it comprised of primordial black holes? And is the fact that we know so little abut it just a sign that our theory of gravity is broken?

Whatever dark matter turns out to be, there is also the matter of the timeline: when did dark matter first arrive on the cosmological scene?

It is here that dark matter as a problem can be linked to the question of what happened to space-time in the early universe, and it isn’t unusual for theoretical physicists who work on ideas in response to one question to dabble in solutions to the other, and possible links between the two. In my case, around the time that I began working on dark matter, I also got involved in a bit of research about a cosmological era called reheating.

In my opinion, reheating doesn’t get enough attention from us physicists. Inflation itself is quite flashy: a short moment, in our part of the universe anyway, where space-time rapidly expands. Reheating, by contrast, is “just” the aftermath of inflation, when the universe is very cold and it needs to literally reheat and fill with the particles that form the structures we observe today, about 14 billion years later. That’s clearly not as flashy as inflation, but it is still fascinating.

“In this scenario, once inflation starts, most of the universe is always inflating, and only small parts of it may ever stop”

Our current best guess about how reheating happened is that at the end of inflation, the particle that drove the process, called the inflaton, still had some leftover energy. This energy decayed into the particles that eventually formed the atoms we see today.

It might sound as if we have it all worked out, but what I have told you is in some sense a very smart guess. We don’t have good observational evidence – yet – from this time period. And we don’t know if, for example, dark matter first started to form during this era.

For my part, I am intrigued by efforts to build models that address dark matter and reheating simultaneously, but my own research has focused on the possible outcomes for reheating if there are several different species of inflaton, which is a realistic expectation from the point of view of particle physics. There is no reason it has to be just one particle.

That is the extent of my work on inflationary models: I think about everything that happens after. This isn’t to say that inflation doesn’t continue to be an interesting moment on the cosmological timeline – and there are still many unanswered questions about it.

Though we have a sense of how it works, we are still unsure of the equations that correctly describe it. There is also disagreement about eternal inflation – the idea that inflation continues forever. In my last column, I pointed out that this process can occur eternally into the past and the future.

There are some people, like Alan Guth, the physicist who first developed the theory of inflation (and my former postdoctoral adviser), who have argued that it is eternal into the future only, meaning that there may have been an originating big bang. In this scenario, once inflation starts, most of the universe is always inflating, and potentially only small parts of it (like ours) ever stop.

Other physicists, like my master’s degree adviser Anthony Aguirre, have claimed that inflation goes in both directions. It is under Anthony’s guidance – back in 2003 – that I first learned about the subject, which means I am a bit biased about what we mean when we talk about “eternal inflation”.

But, importantly, my last column was hardly about a new proposal, and I can’t be given credit for the idea either.