The evolving Universe

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# 1.4 Inflation

Time: 10−36s to 10−32s

Temperature: rapidly changing

Energy: rapidly changing

When talking about the Universe, there is an important distinction that our discussion has, up until now, largely ignored. First, there is the entire Universe and this may be infinite in size. By implication, it makes no sense to put a value on the 'size' of the entire Universe, since infinity is larger than any number you care to think of. But there is also what we may call the observable Universe, which is that part of the Universe that it is theoretically possible for us to observe from Earth. We can calculate a value for the size of this finite region.

## Question 4

Why should there be a limit to how far we can see?

The speed of light is a cosmic speed limit — nothing can travel any faster. So, the only part of the Universe that is now observable is that fraction of it from which light has had time to reach us since the Universe began, about 14 billion years ago.

One might naturally expect that the radius of the currently observable Universe is therefore equal to the maximum distance that light can have travelled since the Universe began, as illustrated in Figure 3. Notice that, although light can travel from the edge of the sphere shown in Figure 3 to the centre within the age of the Universe, light cannot travel from one edge to the opposite edge (across the diameter of the sphere) within the age of the Universe — the Universe is simply not old enough!

Figure 3 The size of the observable Universe. Imagine that the Earth lies at the centre of the circle (really a sphere in three dimensions), with a radius equal to the distance that light can travel since the Universe began. Then light from galaxies lying within the circle has had time to reach us, but light from galaxies lying outside the circle has not had sufficient time to reach us since the Universe began. Such galaxies are simply not observable at the current time. Furthermore, light from galaxies at one side of the circle has not had time to reach galaxies on the other side of the circle.

When trying to understand the large-scale structure of the Universe that is observed today, one of he most intriguing problems is that the Universe is so uniform. The results from the Cosmic Background Explorer (COBE) satellite, launched in 1989 to measure the diffuse infrared and microwave radiation from the early universe, showed that one part of the Universe has exactly the same temperature, to an accuracy of better than one part in ten thousand, as any other part of the Universe. Furthermore, the expansion rate of the Universe in one direction is observed to be exactly the same as that in any other direction. In other words, the observable Universe today is seen to be incredibly uniform. At 10−36s after the Big Bang, when things were far closer together than they are now, the physical conditions across the Universe must therefore have been identical, to an unimaginable level of accuracy. Yet, according to conventional physics, there has not been time for these regions of space to ever 'communicate' with one another — no light signals or any other form of energy could travel from one to the other and smooth out any irregularities.

In 1981, the American physicist Alan Guth suggested that, in the early history of the Universe at times between about 10−36s and 10−32 s after the Big Bang, the Universe underwent a period of extremely rapid expansion, known as inflation. During this time, distances in the Universe expanded by an extraordinary factor — something like 1050 has been suggested although this could be a vast underestimate!

It is believed that inflation may be caused by the way in which the strong and electroweak interactions became distinct. The exact mechanism by which inflation occurred is not important here, but there are many consequences of this theory. The most important for the present discussion is that the region that was destined to expand to become the currently observable Universe, originated in an extremely tiny region of the pre-inflated Universe. This tiny region was far smaller than the distance a light signal could have travelled by that time and so any smoothing processes could have operated throughout the space that now constitutes the observable Universe. The problem of the uniformity of the microwave background and the uniform measured expansion then goes away.

Non-uniformities may still be out there, but they are far beyond the limits of the observable Universe — and always will be. Because we cannot ever hope to see beyond this barrier, we can have no knowledge whatsoever of events that occurred before inflation, since any information about such events is washed out by the rapid increase in scale. Inflation serves to hide from us any event, process or structure that was present in the Universe at the very earliest times.

If you're thinking that the inflation theory contains some pretty bizarre ideas — you're right! — but it's the most promising theory that currently exists for one of the earliest phases in the history of the Universe. We shall say no more about it here, but now pick up the story again after the Universe has completed its cosmic hiccup. The strong and electroweak interactions have now become distinct and the X bosons have therefore disappeared.

As the matter and antimatter X bosons decayed, they produced more quarks, antiquarks, leptons and antileptons — so adding to the raw materials from which the material contents of the Universe were later built. If we use X to represent a matter X boson and to represent an antimatter X boson, the types of reaction that are believed to have occurred are:

All six flavours of quark (up [u], down [d], charm [c], strange [s], top [t], bottom [b]) and all six flavours of lepton (e, μ, , e, μ, τ) were produced at this time, along with their antiparticles. Notice, however, that matter and antimatter X bosons can each decay into either matter or antimatter particles. This will be important later on in the story.

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