We live in a unique astronomical epoch that allows us to study the Big Bang
The beginning of space, time and everything
Probably the most audacious idea in the whole of science is that it is possible to track the evolution of the Universe starting from a time, almost 14 billion years ago, when everything we see around us today was squeezed into a space far, far smaller than the size of a proton. It’s an astonishing claim, and in this final article in our series we will piece together the remarkable story of the birth of our Universe.
The vastness of the cosmos is hard to appreciate, but take a close look at the map of the Universe shown on the opposite page. Each tiny dot in this image is a galaxy, and each galaxy typically consists of hundreds of millions of stars. Our own Sun is one such star, residing in the Milky Way, and the Andromeda galaxy is our nearest neighbour, at a mere 2.5 million light-years away.
This galaxy map was created by astronomers from the Sloan Digital Sky Survey, and it covers about onethird of the sky: it’s just a small portion of everything that’s out there. Apart from being mind-blowing in its scale, the map is also noteworthy for the fact that the galaxies make a wispy pattern, with strands and clumps and voids. Wouldn’t it be an achievement to be able to explain how that wispy pattern came to be? Let’s put that question on hold for the moment. As we’ll find out, the observed structure of the Universe can be explained using a theory that may well be the jewel in the crown of modern cosmology.
AN EVOLVING COSMOS
The Universe wasn’t always like it is now. A little less than 14 billion years ago, the Big Bang happened (see ‘The key idea’). This is the time when all of the elementary particles that were destined to produce the stars and galaxies were first created. The Universe was simple at the time of the Big Bang (it’s the present-day Universe that is complicated and hard to understand). This is good news, because it means cosmologists can do the calculations to work out how things evolved.
At the time of the Big Bang, the Universe contained a hot, almost featureless gas of elementary particles. That word ‘almost’ is absolutely crucial here, because the density of particles in the gas was not entirely uniform: some regions had a marginally higher density than others. Knowledge of this pattern of density variations is enough for cosmologists to evolve the gas forwards in time.
At first, this gas just cooled and expanded but, gradually, gravity caused the higher density regions to draw in more matter and become increasingly dense. After a few hundred million years or so, some clouds of hydrogen became dense enough to ignite and burn via nuclear fusion – these were the first stars. Cosmologists can compute how the stuff of the Universe evolved from a hot, dense gas all the way to the formation of galaxies: the key input to their calculations is knowledge of those original variations in the density of the gas. But, of course, to know those requires some idea of what actually caused the Big Bang in the first place.
BACKGROUND NOISE
Over the past 15 years or so, cosmologists have become increasingly confident in one particular theory for the origin of
“The Universe was simple at the time of the Big Bang”
“Dark energy would cause space to accelerate in its expansion”
the Big Bang, known as ‘inflation’. Inflation is able to predict the pattern of density variations at the time of the Big Bang and this, in turn, means that we are able to compute the observed pattern of galaxies in our Universe.
As if this weren’t amazing enough, there is even more impressive proof for the theory of inflation: it can also predict the fine details of what is known as the ‘cosmic microwave background’ (CMB). Essentially, this is the cooled, faded afterglow of the Big Bang, which gives us a photograph of the Universe when it was just a few hundred thousand years old.
The CMB is light that has been travelling unimpeded across space since it started its journey 380,000 years after the Big Bang. At this point in time, light suddenly stopped interacting with the surrounding gas and headed off in largely uninterrupted straight lines. Today, we can measure this light in the form of microwaves (it actually started out as infrared light, but has been stretched on its journey across space due to the expansion of the Universe). By measuring the CMB, cosmologists are able to map the density variations in the cooling gas that existed shortly after the Universe was born (see the image on the right for the pattern of microwaves arriving at the Earth today). We can then compare these observations with theoretical computations. If we suppose that the Big Bang started out with the pattern of density variations predicted by inflation, the computations are found to give the exact CMB that we observe in reality, which is serious evidence in favour of inflation. But what exactly is inflation?
COSMIC TREACLE
The theory of inflation emerged in the early 1980s, and at first had nothing to do with the distribution of galaxies or the CMB. Instead, it began with the idea that empty space might be filled with an invisible ‘scalar field’ – a kind of cosmic treacle. This field would permeate all of space, like a still ocean, and ripples in the field would be manifested as particles.
This idea is familiar to particle physicists because one such field is the Higgs field, accompanied by the Higgs boson particle (we met this in last month’s article). Without the Higgs field, elementary particles would have zero mass and would zip around at the speed of light, so atoms would not form. Today, the Higgs boson has been observed at the Large Hadron Collider.
The existence of the Higgs field also means that there should be an energy associated with what we think of as empty space, and this energy can act as a source of ‘dark energy’. This dark energy would cause space to accelerate in its expansion. The trouble is that current theories in particle physics seem to suggest that this accelerated expansion should be far faster than astronomers observe. The fact that, today, the amount of dark energy is much less than expected is one of the biggest mysteries in particle physics.
Nevertheless, the possibility that other, similar fields might exist led cosmologists to consider that the Universe might well have undergone a period of ultra-rapid expansion sometime in its past. The remarkable thing is that this idea turns out not only to be feasible, but also offers an explanation for the origin of the Big Bang itself.
NEW BEGINNINGS
The basic idea of inflation is that empty space was once filled with an ‘inflaton field’, and that the energy stored in this field caused the space to rapidly expand. This expansion was most likely so rapid that, before too long, the Universe was a cold and
“In around 100 billion years, the Milky Way will be part of a single supercluster of galaxies”
empty place as everything rushed away from everything else. After a time, this period of rapid expansion drew to a close as the energy stored in the inflaton field steadily drained away, leaving behind a Universe filled with a cold gas of inflaton particles. These inflaton particles then decayed into other, more stable, particles and, in doing so, they generated the Big Bang, as the energy locked away in the cold gas of heavy inflaton particles got converted into a hot gas of lighter particles.
The idea of inflation was initially attractive to cosmologists because it generates a big Universe (due to that early phase of rapid expansion). However, it wasn’t too long before they realised that the theory has much more to offer: it also predicts the pattern of density variations at the time of the Big Bang. In other words, it offers a detailed description of how the matter was spread about at the time of the Big Bang, and this (as we’ve discussed) is in precise accord with the patterns seen in the observed galaxies and cosmic microwaves.
This stunning success of inflation arises as an unavoidable consequence of quantum physics. The mathematics allows us to consider going back to a time, just before the Big Bang, when the portion of space that was destined to grow into the entire visible Universe was less than a billion times smaller than a single proton. This is utterly mind-boggling but there appears to be no impediment to calculating what happened at this time.
Crucially, the inflaton field was not a perfectly still ocean – it had ripples in it. These ripples were generated as a consequence of Heisenberg’s Uncertainty Principle, a key result in the theory of quantum physics (see Part II of this series). Heisenberg’s principle tells us that nothing can ever be entirely still – even empty space is fizzing with particles that appear and disappear from nowhere (that fizzing is measured today in particle physics experiments). So, the inflaton field must have had ripples, and these were translated into corresponding ripples in the density of matter at the time of
the Big Bang. How wonderful that this extraordinary science is writ large on the sky and that we are so fortunate to be here to decode the message.
With inflation, we now have an explanation for the origin of the Big Bang and a new way of thinking about how our part of the cosmos came to be. It could even be that inflation was ongoing for a very long (possibly infinite) period of time before it halted in the region of space destined to grow into the Universe we see around us. In that case, the Big Bang was not the beginning. Rather it was just a moment in the history of the Universe.
Looking to the distant future, the fact that space is today slightly accelerating in its expansion implies that the Universe will never end. It will continue to expand forever, becoming ever more dilute and cold. In around 100 billion years, the Milky Way will be part of a single supercluster of galaxies, and all other galaxies will be racing away at such a speed that light from them could never reach the Earth. This means that astronomers of the far future would have no distant galaxies to observe – to them, the Universe would seem a much duller place than we know it to be. We are privileged to be living in an epoch when we are able to learn so much about the wonders of the cosmos and its origins.
There is a final, mind-boggling twist to the story of inflation. We have just said that inflation might have been continuing forever before the Big Bang. It is also possible that inflation did not stop everywhere in the Universe at the time of the Big Bang. It may well be that we live in a bubble of slowly- expanding space embedded in a much larger and still rapidlyexpanding space that’s undergoing inflation. There may even be other bubble universes like ours, rushing away from us at an unimaginable speed. According to some ideas in theoretical physics, it is conceivable that the laws of physics are different in each of these bubble universes. In other words, every variant on nature’s laws is played out somewhere in the vastness of this Multiverse. Now there’s a thought.
“We are privileged to be living in an epoch when we are able to learn so much about the wonders of the cosmos”