The Great Unsolved Problems in Physics: Cosmic Inflation

| September 27, 2014 | 0 Comments

How did the universe begin? It seems that every school of thought and religion has its own answer. The favourite answer of Christians is, of course, “God did it”. Buddhism offers, in my mind, a more elegant explanation — there was no beginning, nor there will be an end. Meanwhile, the favourite answer of most physicists is the Big Bang.

The idea of the Big Bang can be summarized as follows: the universe started from a single point which contained all the matter/energy. This idea essentially comes from the observation that the universe is expanding and is also backed up by such evidence as the cosmic microwave background. Despite the far-reaching predictions and vast observational evidence, however, the theory of the Big Bang is not flawless.

The picture of the universe according to the standard cosmology

The main problems of the so called standard cosmology are all about the universe as we see it today. The cosmological principle, in simple words, states that the universe on the large scale is isotropic and homogeneous, which implies that it looks the same from place to place and in each direction. This principle is at the roots of standard cosmology and is firmly confirmed by observational evidence. However, the standard model of cosmology simply cannot explain how the universe “communicated” with all its parts in order to account for the isotropy and homogeneity. This is the so called horizon problem. The nature of it lies in the fact that the universe, which, of course, is vast and rapidly expanding has a large number of causally disconnected parts due to the finite speed of light. So, to put it simply, there wasn’t enough time from the beginning of the universe for different parts of it to “communicate” and set the mean temperature, energy density and other quantities, which are roughly equal everywhere.

Another problem originates from our modern understanding of particle physics. It is known in standard cosmology, that the further we go back in time, the denser and hotter the universe becomes. If we go back early enough, the universe is hot and energetic enough to create hypothetical particles known as magnetic monopoles. However, as we all know, we don’t see monopoles anywhere around us, which indicates that either our understanding of particle physics is wrong or that we need to revise our ideas about the early universe cosmology.

Another important observation in modern cosmology is that our universe is very close to being perfectly flat. This, as you might have guessed by now, is again not what you expect given the framework of standard cosmology. To put it simply, the solution to the equations of standard cosmology indicate that a flat universe is unstable, unless the initial conditions at the moment of the Big Bang are very specific.

Finally, it’s difficult to explain how the structure of the universe turned out to be as it is observed today. To put it simply, it’s hard to explain how the currently observed distribution of galaxies and galaxy clusters was formed.

So given all these drawbacks, isn’t the Big Bang a bit of a failure? Most scientists realized that even despite the vast observational evidence, standard cosmology cannot be a complete theory. So two paths emerged — either the theory gets patched up or it get’s overthrown. So far it seems that the first route will be the one taken by modern physics.

So the (almost) all healing cure for the Big Bang theory is the so called cosmic inflation. Inflation is a period of exponential expansion in the very early stages of the universe, which, according to most modern cosmologists, get’s rid of most of the problems in the standard cosmology. The idea goes as follows: sometime between 10−33 and 10−32 seconds after the Big Bang for around 10-36 the universe expanded exponentially. This exponential expansion is the key to solving the previously discussed problems. The expansion is caused by a hypothetical scalar field called inflaton. Invoking our understanding of general relativity and quantum field theory we find that the inflaton offers an elegant solution to the cosmic structure. The wonderful thing about the field is that it is affected by random quantum fluctuations, which, during the exponential expansion, grow significantly and create the first seeds of the structure of the universe. Fluctuations in the field mean that different parts of the universe expanded differently, leaving behind different values of the energy density at different patches in the cosmos. This, in turn, at much later stages through the process of gravitational collapse formed the first galaxy clusters.

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Andrei Lined and Alan Guth — the fathers of the cosmic inflation theory with their wizard hats on

 Another important aspect is that the quantum fluctuations got stretched to scales much larger than the observable universe (a part of the whole universe that can interact causally). These fluctuations basically made the universe homogeneous and isotropic on the large scale. That is exactly why nowadays, when we look at the skies and see the newly uncovered regions of the sky, the universe looks exactly the same. So this essentially gets rid of the horizon and the structure problems.

Another wonder of inflation is that the solution of the equations shows that the flat universe case doesn’t depend on the initial conditions, given that inflation continued for long enough. So, to put it simply, it doesn’t really matter how the universe looked at the very first moments — if the inflation continued for a required period of time, the universe will still turn out to be flat. Finally, during the period of rapid expansion, the energy density drops, essentially cooling down the universe, which explains why there aren’t any magnetic monopoles around (or why there are so few that we can’t see them in the observable universe).

So it seems that the relatively simple idea of inflation gets rid of most of the problems in the standard cosmology framework. However, a very important question remains — what is the nature of the inflaton field. Currently the answer is that we don’t know. The problem lies in the fact that there are many theoretical models of inflation, which draw the same picture of the universe.  To express this mathematically, there are many forms of the inflation potential, which lead to the same predictions. With the advance of space and earth-based instruments, observations with an ever-increasing accuracy are made, which will hopefully rule out the unsatisfactory models of inflation. So, given enough time, it is likely that we will more or less have a good idea of the form of the inflationary potential.

The universe with a period of cosmic inflation (scitechdaily.com)

Then we have to ask another big question. What is the mechanism of the inflaton field? That is, how can we explain the existence of such field through particle physics, or any other framework? The answer, at this point, as you might guess, is that we don’t have a clue. The main approaches are deriving the field from particle physics, embracing the weirdness of string theory, or modifying general relativity. One of the original ideas was that the famous Higgs field could act as the inflaton field. This prediction, however, has lost enthusiasm among many cosmologists, due to unfavourable experimental results.

It is also often believed that inflation could be a natural consequence of spontaneous symmetry breaking dynamics of some unified theory particle theory, say, a grand unified theory or string theory. The problem with these approaches is that, as of today, a simple theoretical framework, for both string theory and the grand unified theory, which would be testable and agree with observations haven’t been achieved yet. Finally, some believe that inflation could be explained through the modifications of our theories of gravity. Most likely, however, quantum mechanics and general relativity would have to be unified firstly (quantum gravity) in order to fully explain inflation.

So to wrap it up, it seems that the theory of cosmic inflation is indeed a cure to the major problems of the Big Bang theory. However, the fundamental nature of the inflaton field can only be understand when we form a firmer understanding of particle physics and gravity. Top put it simply, the theoretical understanding of inflation is intimately linked to our knowledge of the grand unified theory, string theory or quantum gravity. And that, of course, means that we have to be patient.

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