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THROUGHOUT all of human history, people have looked up at the night sky and wondered about the universe and how it came to be. But in one respect, we鈥檙e very different from our ancestors: we more or less understand what we鈥檙e looking at.

Take an image from the Hubble Ultra Deep Field, for example. We know the blotches of light on it are not stars, but entire galaxies similar to own Milky Way. And because it takes time for light to travel through space, we鈥檙e not seeing what these galaxies look like today, but rather what they were like over 13 billion years ago, a few hundred million years after the big bang.

A little over a century ago, scientists didn鈥檛 have the faintest understanding of our universe鈥檚 distant past, and they certainly knew nothing about its origin. We didn鈥檛 have the tools even to conceptualise questions about how the universe might change or evolve. All of that changed with Albert Einstein. With his general theory of relativity, he how showed how space isn鈥檛 static and unchanging. It can be curved: it can warp and deform: it can expand and contract.

In 1929, Edwin Hubble observed that the universe is in fact changing. Every galaxy is receding from us: every two points in space are getting farther apart from each other as time advances. The universe is expanding.

If the universe were smaller in the past, and we know how much matter and energy it contains, we can deduce that its matter and energy density must once have been higher. Billions of years ago, it must have been in a denser, hotter state, and expanded into the cooler world we see today. That is the basic premise of the big bang theory.

According to this picture, wind back to some 380,000 years after the big bang, and we reach a point when the universe had first cooled enough for atoms to form. It suddenly became transparent to radiation, dumping an enormous amount of light into the cosmos that鈥檚 been propagating throughout space ever since. Today we see this light as the cosmic microwave background, a sea of radiation cooled to 2.7 degrees above absolute zero. Its existence gives us confidence that we understand the universe and its evolution from this point right up to the present day.

Going back even further, to the first seconds and minutes after the big bang, we encounter a time when the universe was about a billion degrees, 100 times as hot as the sun鈥檚 core, and functioning as a giant nuclear fusion reactor. We can predict how much deuterium, tritium, helium, lithium and beryllium we think should have been made in this era 鈥� and again, the predictions agree with what we observe today.

Going back even further, we can鈥檛 make direct observations, but we can recreate the conditions of the early universe using particle accelerators such as the Large Hadron Collider near Geneva, Switzerland. The LHC accelerates protons to about 99.999997 per cent of the speed of light and collides them head on at a rate of about 600 million times every second, exploiting Einstein鈥檚 famous equation E = mc² to convert as much of the energy released as we possibly can into mass. This allows us to create a variety of exotic forms of matter that are very rare in our universe now, but were extremely common in the incredibly hot first trillionth of a second after the big bang.

鈥淵ou might think we know a lot about the universe鈥檚 first fraction of a second That just isn鈥檛 true鈥�

From all I鈥檓 saying here, you might now be under the impression that we know a lot, and with a great deal of confidence, about the universe鈥檚 first small fraction of a second. But sadly, that just isn鈥檛 true. I鈥檓 going now to talk about four puzzles or problems that cosmologists have discovered or revealed over the last few decades, which all point to something missing in our understanding. To solve them, I increasingly think we鈥檙e going to have to radically rethink what we think we know about the universe鈥檚 very early history.

The first puzzle has to do with the simple fact that atoms exist. Everything we know from particle accelerators and other such experiments tells us that, for every kind of matter that exists in the universe, there exists a kind of equal and opposite mirror version, antimatter. When you create more matter, you create an equal amount of antimatter: when you destroy one of them, you destroy the other along with it. So whatever created the universe鈥檚 matter should have created an equal amount of antimatter. As the universe expanded and cooled in its first fraction of a second, we calculate that matter and antimatter should have destroyed each other almost entirely. There should be no atoms, no molecules, no stars, no galaxies, no planets and no life.

Our second puzzle has to do with matter, too, but not the kind that consists of atoms. It is 鈥渄ark鈥� matter that doesn鈥檛 appreciably reflect, radiate or absorb light. Since the 1970s, astronomers have been measuring how fast stars in other galaxies are moving in their orbits. They鈥檝e consistently found that stars in galactic outskirts are moving too fast for the amount of visible material the galaxies contain. Nearly all galaxies seem to contain a small amount of visible material, compactly located in the centre of a larger 鈥渉alo鈥� of dark matter.

Making some assumptions about how dark matter must work, we can create computer models to find out how it would have impacted the universe鈥檚 evolution. When we do that, we find near-perfect agreement with the distribution of galaxies and clusters of galaxies we see in the universe today. With dark matter, we can explain it: without dark matter, we can鈥檛.

If you鈥檇 asked me what the dark matter consists of maybe 10 or 15 years ago, I would have given you a very confident-sounding answer about how it was probably made up of 鈥淲IMPs鈥�, short for weakly interacting massive particles. We thought we knew how to detect these particles, so we built impressive, super-sensitive detectors for them deep underground in places like the Gran Sasso laboratory in Italy 鈥� but we still haven鈥檛 observed any dark matter particles.

Our third puzzle has to do with how fast our universe has been expanding over time. General relativity gives you basically three possibilities. You can have a universe that expands for a while, reaches a maximum size and then starts to contract: you can have a universe that expands forever, but with its expansion slowing down: or you can live on the boundary between those two cases, and have a universe that gets bigger for a while and then approaches a maximum size.

For decades, cosmologists set out to try to measure which of these three cases describes our universe. And the answer turned out to be: none of the above. Instead of slowing down, in the past few billion years our universe鈥檚 expansion rate has been getting faster. The universe is accelerating.

Within the context of Einstein鈥檚 theory, the only way to explain this behaviour is to posit that space itself contains a fixed density of 鈥�dark energy鈥�. Unlike matter and other forms of radiation, dark energy doesn鈥檛 get diluted as space expands, so it plays an increasingly important role, ultimately driving the universe to speed up its expansion rate.

The fourth and final puzzle has to do with the extremely early universe, maybe something like 10-32 seconds after the big bang. If you take the big bang theory as it was envisioned in the 1960s and 1970s, it is very hard to explain why our universe is so uniform, and also what we call geometrically flat 鈥� basically, it follows the rules of agoconventional Euclidean geometry. There鈥檚 no reason why either of these things should be.

鈥淲e鈥檙e going to have to radically rethink the universe鈥檚 early history鈥�

In the 1980s, physicists began to posit an explanation: cosmic inflation. In its very early stages, our universe expanded in explosive fashion, growing exponentially by a factor of something like 1075 in volume over a very, very brief period of time, smoothing itself out as it did so. The best thing is that inflation made some very specific predictions about patterns of light we would observe in the cosmic microwave background 鈥� and we have observed them.

One thing that I find really compelling 鈥� or exciting, anyway 鈥� about inflation, is it takes even a very tiny amount of space, and it rapidly turns it into a multitude of universes: a multiverse. Quantum physics says that different patches of that space will stop inflating at different times, essentially creating something like our universe. But this doesn鈥檛 happen just once: in fact it happens without limit. Inflation seems to lead inevitably to the conclusion that there should be an infinite or nearly infinite number of universes in existence, some maybe a lot like ours, some very different.

That gives us a lot to ponder about the possible varieties of existence we might find throughout the multiverse. But all these puzzles give us also a lot of reason to doubt that we understand the whole story of the first fraction of a second of the universe.

It brings to mind a question I鈥檓 fond of asking my colleagues: what would it have been like to be a physicist in 1904? The reason I pick 1904 is because it鈥檚 when physicists seem to have had the most confidence that they really understood the universe. Newtonian physics had reigned supreme for over two centuries. It had been applied to problem after problem, and it just kept working. There was every reason to think that Newtonian physics could just be applied to anything 鈥� heat, electricity, magnetism 鈥� if we just thought long and hard enough about it.

But back then there were also a few loose ends: a few problems and puzzles that hadn鈥檛 been resolved. One was the way light seemed always to travel at the same unvarying speed, no matter what frame of reference you were in. Another was how Mercury鈥檚 orbit didn鈥檛 fit what was predicted by Newtonian physics. People had posited that maybe there was an additional planet out there called Vulcan that had perturbed Mercury鈥檚 orbit, but no one had ever been able to see it.

A third puzzle was that no one knew how the sun got its energy. Geologists had already shown us that Earth and the sun were billions of years old. Even if the mass of the sun were made of some fuel like gasoline or coal, it should have run out of energy a long ago. And then a fourth puzzle: Newtonian physics couldn鈥檛 explain the inner workings of atoms. It predicted they were unstable, and couldn鈥檛 say why particular atoms gave off special patterns of light, what we call spectral lines.

The resolution to these problems was not an incremental change of Newtonian physics. It came with a revolution ushered in by Albert Einstein and people who followed his work. In 1905, Einstein introduced his theory of relativity that explained the uniform speed of light, and would eventually explain the Mercury鈥檚 orbit. When you combined relativity with what we learned about quantum physics, we could explain the nuclear fusion that powers the sun, and begin to understand the inner workings of the atom. The revolution that came forth in 1905 tore down Newtonian physics and left something else, something that would have been unimaginable in 1904, in its place.

Right now as a cosmologist, I wonder if 2020 is the 1904 of cosmology. I hope so, because that means in 2021, or some other short time down the road, we are going to have a revolution that will be very exciting to live through. Of course, I could be wrong. But that鈥檚 what I鈥檓 hoping for, that鈥檚 what I鈥檓 excited about 鈥� and all these Puzzles make me think that it鈥檚 atleast a little more likely.

From beginning鈥� to end
Cosmologist Katie Mack will be exploring our theories about the universe鈥檚 ultimate fate in an online event on 13 August.
Details on all events at