Human Universe (26 page)

But surely it makes no sense to take refuge in a vast infinity of universes to explain our existence? Absolutely correct, if that’s why the idea is introduced – it’s no better than a God-of-the-gaps explanation. If, however, there were some other reason, based on observations and theoretical understanding, that suggested an infinity of universes, then such an anthropic explanation for our perfect, human universe would be admissible. Remarkably – and that remarkably overused word is appropriate for once – this outlandish suggestion is a widely held view amongst many cosmologists.

If we look at our universe on the largest distance scales, by which I mean at distance scales far larger than the size of single galaxies, it has a number of properties that any theory of its origin has to explain. The most precise picture of the young universe we have is the photograph of the Cosmic Microwave Background Radiation (CMB) taken by the Planck satellite.

This is the afterglow of the Big Bang, a photograph of the universe as it was 380,000 years after the initial hot, dense phase when the expansion had cooled things down sufficiently for atoms to form. The most obvious feature of the CMB is that it is extremely uniform, glowing at a temperature of 2.72548 degrees above absolute zero, with small fluctuations at the level of 1 part in 100,000. Those very tiny temperature differences are represented by the colours in the photograph. This uniformity is extremely difficult to explain in the standard Big Bang model for a simple reason. Our observable universe today is 90 billion light years across. This means that if we look out to the CMB from opposite sides of the Earth, we are looking at two glowing parts of the ancient sky that are now separated by 90 billion light years. The universe, however, is only 13.8 billion years old, which means that light, the fastest thing there is, has only had time to travel 13.8 billion light years. Two ‘opposite’ parts of the CMB could therefore never have been in contact with each other in the standard Big Bang model, and there is absolutely no reason why they should be
almost
precisely the same temperature. I’ve italicised ‘almost’ in the previous sentence because, as we noted, there are very slight variations in the CMB at the level of 1 part in 100,000, and these are very important. The universe was never completely smooth and uniform everywhere, and these variations in density are encoded into the CMB as differences in temperature. The regions of slightly greater density ultimately seeded the formation of the galaxies, and so without them we wouldn’t exist. What caused these small variations in the otherwise ultra-smooth early universe?

Another fundamental property of the universe that is difficult to explain is its curvature – or lack of it – which can also be measured from the CMB. Space appears to be absolutely flat; a veritable ice rink. Recall from Chapter 1 that the shape of space is related to the density and distribution of matter and energy in the universe through Einstein’s equations. In the standard Big Bang theory, the universe doesn’t have to be flat. In fact, it requires a great deal of fine-tuning to keep it flat over 13.8 billion years of cosmic evolution. Instead, the radius of curvature is measured to be much greater than the radius of the observable universe – more than sixty orders of magnitude larger. That’s a big problem!

In the early 1980s, the need to explain these and other properties of the observable universe led a group of Russian and American physicists to propose a radical idea. The modern version, the best-known proponents of which are Alan Guth, Andrei Linde and Alexei Starobinsky, is known as the Theory of Inflation. We’ll describe a particular version of inflation below, driven by something called a scalar field, which was first described by Andrei Linde.

Spacetime existed before the Big Bang, and for at least some of that time was described by Einstein’s Theory of General Relativity and a quantum field theory like the Standard Model. The central idea in quantum theory is that anything that can happen does happen. Everything that is not explicitly ruled out by the laws of nature will happen, given enough time. One of the types of things permitted to exist in quantum field theory are scalar fields. We’ve met an example of a scalar field earlier in the chapter in the guise of the Higgs field, which we know to exist because we’ve measured it at the Large Hadron Collider. Scalar fields have the property that they can cause space to expand exponentially fast. We touched on such a scenario in Chapter 1 without being explicit about the mechanism – it is the de Sitter’s matter-less solution to Einstein’s field equations first discovered in 1917. Given General Relativity and quantum field theory, therefore, it must be the case that scalar fields will fluctuate into existence in such a way that an exponential expansion of spacetime is triggered. In this exponential phase, spacetime expands faster than the speed of light. This might sound problematic if you know some relativity, but it isn’t. The universal speed limit exists for particles moving through spacetime, but does not apply for the expansion of spacetime itself. In a tiny fraction of a second – around 10
^–35
seconds in fact – an exponential expansion of this type can inflate a piece of spacetime as tiny as the Planck length to a quite mind-boggling size: trillions of times larger than the observable universe. Any pre-existing curvature is completely washed out, leading to a flat observable universe. It’s like looking at a square centimetre-sized piece of the surface of a balloon of a light year in radius; you won’t see any curvature, no matter how hard you try.

Likewise any variations in density will be washed out, leading to the smooth and uniform appearance of the CMB. Perhaps the greatest triumph of inflationary models such as these, however, is that they don’t predict a completely uniform, homogeneous and isotropic universe. Quantum theory doesn’t allow for absolute uniformity. Empty space is never empty, but a fizzing, shifting soup of all possible quantum fields. Like the surface of a stormy ocean, waves in the fields are constantly rising and falling, and the exponential expansion can freeze these undulations into the universe. Remarkably, when calculations using the known laws of quantum theory are carried out, the sort of density fluctuations that result from such a mechanism are precisely of the form seen in the CMB. These quantum fluctuations are the seeds of the galaxies and therefore the seeds of our existence, frozen into the oldest light in the cosmos and photographed by a satellite built by the people of Earth 13.8 billion years later.

Inflation in this guise explains the observable properties of our universe, and in particular all the details of the CMB, which has been measured to high accuracy. This is why it is currently widely accepted as an essential ingredient by many cosmologists. As if this wasn’t enough to get excited about, however, there is much more.

One obvious question that arises is this: if inflation gets going, how does it stop? The answer is that inflation stops completely naturally, but with a fascinating twist that drives right to the heart of our ‘Why are we here?’ question. The scalar field driving inflation fluctuates up and down in accord with the laws of quantum theory, just like the waves on the surface of an ocean. If the energy stored in the field is high enough, inflation begins. One might expect that such a rapid expansion would dilute the energy extremely rapidly, causing inflation to stop. But scalar fields have the interesting property that their energy density can stay relatively constant as space expands. You can think of the expanding space as doing work on the field, pumping energy into it and keeping its level high. And in turn, the high level of the field’s energy continues to drive the expansion. This might sound like the ultimate free lunch, and in a sense it is, almost, although gradually the energy will become diluted and decay away. The time this takes depends on the size of the initial fluctuation in the field and the details of the field itself, but in general the higher the initial energy, the longer the field takes to fall in value as the expansion continues. An analogy often used to picture this scenario is to imagine a ball rolling down the side of a valley. The height of the ball up the valley side represents the energy density of the scalar field. When the ball is high up, the energy in the field is high, driving the inflationary expansion. As the ball rolls slowly down the valley the energy reduces and inflation turns off. At the valley floor, the ball oscillates back and forth until it comes to rest. The scalar field likewise oscillates and in so doing dumps its energy into the universe in the form of particles. In so doing it creates a hot dense soup, which we identify as the ‘Big Bang’. In other words, inflation ends naturally and the standard Big Bang follows. The decay of the scalar field that drove inflation is the cause of the Big Bang!

Let us step back for a moment and recap with broad brush-strokes, because we seem to be wandering onto Leibniz’s territory, and that’s an astonishing place for physics to have arrived at. Our claim is that there exists a quantum field that causes the universe to expand exponentially fast for some period of time, and in doing so produces all the features of the universe we observe today, including the existence of galaxies and the matter out of which they are made. This is a triumph, and is now part of cosmology textbooks. Before the Big Bang, there was inflation. Fine, our philosopher friends would say, but what happened before inflation? Here, we must leave the textbooks and become a little more speculative, but not too speculative. We are still going to be working within the domain of mainstream physics.

There is an extension of what we might term standard inflationary theory. It is known as eternal inflation. Put simply, there seems to be no reason why inflation should stop everywhere at the same time. There should always be regions of the universe where the scalar field fluctuates to such high values that the exponential expansion continues, and these regions will always come to dominate the universe, however rare they may be, because they are exponentially expanding. Where inflation stops, Big Bangs herald the beginning of more sedately expanding regions like ours. But elsewhere, there is an ever-growing exponentially expanding universe, constantly spawning an infinity of Big Bangs. This theory, known as eternal inflation, leads to an infinite, immortal multiverse, growing fractal-like without end. This is truly mind-numbing, but we must emphasise that it is an entirely natural extension of standard inflationary cosmology.

Eternal inflation opens up even more exciting possibilities. As we discussed above, one of the great mysteries in physics today is the origin of the constants of nature such as the strength of gravity, the masses of the particles and the value of dark energy. These values appear to be fine-tuned for the existence of life, and understanding where they come from is a prerequisite for understanding our existence. In eternal inflationary models, each mini-universe can have different values of these constants and different effective laws of physics. The word ‘effective’ is important. The idea is that there is some overarching framework, out of which our laws and the constants of nature are selected randomly. If this is correct, then each of the infinite number of mini-universes that branch off the fractal inflationary multiverse can have different effective laws of physics, and all possible combinations will be realised somewhere. No matter how fine-tuned our laws appear for the existence of life, it is inevitable that such mini-universes as ours will exist, and there will be an infinite number of each possible set of combinations. There is no fine-tuning problem. Given the multiverse, we are inevitable. This is reminiscent of our rejection of your own personal uniqueness whilst listening to Joy Division at the beginning of the chapter. Yes, in isolation, the odds of you existing are almost vanishingly small. But given a mechanism for producing human beings, babies are born all the time and their existence is not surprising. Here, we have a mechanism for producing universes – and with an even greater statistical sledgehammer, the mechanism doesn’t simply produce a few billion of them, it produces a potentially infinite number.

This is a quite stunning theoretical model, and I understand that it sounds like wild speculation. It isn’t, though. Inflation is probably correct in some form, in the sense that before what we call the Big Bang, there was an exponential expansion of spacetime. Scalar fields, which are known to exist, have the correct properties to drive such an expansion, although there are other theoretical models of inflation, as well. Theoretical physicists studying inflationary models have discovered that almost all of them are eternal, in the sense that they stop inflating in patches rather than all at once. This means that the potential for creating universes, in the guise of inflation, is always expanding faster than it is decaying away, and it will therefore never stop. We live in an infinite, eternal, fractal multiverse comprised of an infinite number of universes like ours, alongside an infinite number of universes with different physical laws. We exist because it is inevitable. Almost.

There is one very important caveat to this picture. Recent research suggests that eternal inflationary models may be eternal in the future, but not in the past. They never stop, but they may have to start. I can’t give you a definitive answer to this ultimate question, because nobody yet knows. I can quote from Andrei Linde’s recent review of inflationary cosmology, published in March 2014.
^fn1

‘In other words, there was a beginning for each part of the universe, and there will be an end for inflation at any particular point. But there will be no end for the evolution of the universe
as a whole
in the eternal inflation scenario, and at present we do not know whether there was a single beginning of the evolution of the universe as a whole at some moment t=0, which was traditionally associated with the Big Bang.’