麻豆传媒

THERE may not have been a beginning to the thing we understand as 鈥渢he universe鈥�. Before I explain what I mean, I should say: of course, this isn鈥檛 the story I expected to tell audiences when I was a child who wanted to be just like Stephen Hawking.

I was certain, in fact, that the job I was signing up for was the one where we figured out exactly what happened in the very beginning, to a level of detail that humanity has never before known. As a grown-up scientist, I have had the wonderful opportunity to investigate that era and to discover again and again that the universe is more bizarre than we previously imagined.

The old story from 1980 or so goes that in the beginning of space and time, space-time exploded out of nothing and then rapidly expanded. The expansion was so fast that it grew faster than the speed of light, because it turns out that the only thing that can violate the universal speed limit is space-time itself. This era, known as inflation, was first simultaneously hypothesised by Alan Guth, Alexei Starobinsky, Andrei Linde and a team comprised of Martin Einhorn and Katsuhiko Sato. They were all motivated by a desire to try to explain phenomena that astronomers had observed.

Before we even try to imagine what inflation means, we have to grapple with the expansion of space-time and what space-time even is.

The idea that space and time aren鈥檛 completely separate entities is a relatively new one. Their merger is a theoretical necessity induced by Albert Einstein鈥檚 relativity, which tells us that when two of us are moving with respect to each other, your space and my time can mix: time and space aren鈥檛 as independent as they feel on everyday, human scales.

The expansion of space-time itself can also be hard to fully grasp because it is so different from everyday life. Readers may have previously heard an analogy that space-time expansion is akin to galaxies racing away from each other like fast cars driving in opposite directions. The reality is more fantastical than that.

Imagine a not-yet inflated balloon covered in little dots. As you blow up the balloon (hopefully not with precious helium), the distance between the dots grows. This is what the expansion of space-time is like. Galaxies aren鈥檛 racing apart but rather space-time is growing between them.

鈥淭o understand cosmic inflation鈥檚 exponential expansion, watch how mould spreads on a loaf of bread鈥�

This expansion isn鈥檛 anything to worry about, because it is only happening on very large scales, not on the scale of our solar system, where gravity is playing its part to keep things together.

When our space-time was less than a second old, this expansion accelerated faster than the speed of light for a very brief moment. Imagine a percentage of a second with 40 zeros after the decimal. That is how long cosmic inflation occurred for. This expansion was exponential.

To get a sense of what this means, buy a loaf of bread and wait until it shows signs of mould. Once there is a little, it will become a lot very quickly. This is because the microorganisms ruining your bread are reproducing at an exponential rate.

One of the challenges that inflation theory faces is that while it fits all of our cosmological data almost perfectly, we haven鈥檛 been able to work out the details. We still don鈥檛 have an exact equation to describe the energy that governs inflation.

What we have learned, however, is that many reasonable candidates for this energy equation implicate space-time in a fantastic trick: it may be eternal. New bubbles of space-time may pop up and grow continuously, with no beginning and no end.

Not everyone loves this idea. In fact, one of the early architects of inflation, Paul Steinhardt, has since become one of the fiercest critics of inflation 鈥� and especially eternal inflation 鈥� and a source of many editorials challenging its status in mainstream cosmology. But so far, no one has offered an alternative idea for why the contents of space-time look the way they do that matches the data as gracefully as inflation does.

In the meantime, the search for the right energy equation continues and there are those of us who are thinking beyond inflation to the energy that is left over after the process is done.

In a series of papers, I and my colleagues have begun to unravel how the energy from inflation can plant the seeds of all the matter we see in space-time: galaxies, planets and us. I struggle with the idea that inflation could be eternal and the whole of space-time may not have had a beginning.

That aspect of inflation theory may not be testable, which makes many people ask whether it is still science. I think it is, and I try to remember that the universe wasn鈥檛 designed to be easy for me to understand.