Let’s go right back to the begining, or at least try to, where did the Universe come from? How did it get here? We once thought, well just assumed really because it seemed to have always been there, that the Universe was eternal and unchanging. This is understandable as nothing seemed to have changed “up there” in all of human history. Then along came Edwin Hubble and discovered that when we look far out into the Universe, everything seems to be receeding from us, the Universe is getting bigger! But not long before that this guy-had produced a theory that was a huge leap forward in our understanding of the Universe and how it works. There was just one problem. Good old Albert assumed like everyone else that the Universe was unchanging, but his own equations showed that this could not be the case so he assumed that his equations were wrong and added the ‘Cosmological Constant’ to balance everything out and give a nice steady state universe.
The main problem with this ‘Steady State Universe’ is that it would be incredibly unstable. Gravity would be trying to pull everything in so there would have to be a repulsive force of some kind to counteract this. If something happened to make even part of the Universe shrink a little then gravity would become a tiny bit stronger and would overcome the repulsive force a little. That would make the Universe shrink a little bit more, making gravity even stronger compared to its rival and so on until the whole thing collapses in on itself. On the other hand, if it expanded even just a tiny bit then gravity would become a little bit weaker thus causing the Universe to expand some more until the whole thing expands and expands… you get the picture.
Since then more and more evidence points to a Universe that is expanding from a very small starting point, so let’s look at what that very small starting point might be and what happened just after that.
There are various ideas about how the Universe started. One idea is that there are membranes, called Branes, that are seperated by something called “The Bulk”. These (mem)Branes attract each other gravitationally until they bump into each other. This ‘bump’ is powerful enough to fill the two Branes with enough energy to create new universes and push the Branes apart. Eventually they will begin to approach each other again until they bump again and start the whole process once more.
Another idea is that the vacuum is not at its lowest possible energy level, and that a quantum fluctuation, a kind of random ‘flip’ in the fabric of spacetime, tips the vacuum over the edge and into a lower, more stable, energy level thus releasing a whole cartload of energy that sets the whole thing expanding. There are also theories of giant stars in higher dimensional space that go supernova and collapse into a black hole and that our universe is inside that black hole.
It is also possible that inflation is simply a matter of a quantum event that sets a little piece of a larger universe expanding suddenly. The logical consequence of that is that this could, and probably would, happen a lot giving rise to lots of universes, possibly an infinite number of universes each of which would probably have its own physics so we couldn’t even exist in most of them.
Whatever it was that pressed the ‘Start’ button, what happened next is that a tiny fraction of a second later, the whole thing suddenly got a serious case of ‘Inflation’. When we first discovered that the Universe was expanding it was assumed that it was a steady expansion, probably a bit faster at first, later slowing down like a bomb exploding. That sounds fine and logical but there are problems with that. The main one is what we call the ‘Horizon Problem’. Analysis of the Cosmic Microwave Background shows that it is almost exactly the same temperature everywhere. This is a problem because in order for that to happen, the equalizing of the temperature would have taken too long to have spread across the whole Universe. The Universe is simply too young for that to have happened yet. The solution seems to be that the Universe, or rather spacetime itself, expanded fantastically quickly. This spreads the energy out nice and evenly and thus explains the nice, smooth Cosmic Microwave Background.
Now don’t be afraid of this graph and all the weird numbers on it. The horizontal axis shows the age of the Universe in seconds and 10ˆ-45 means 0. and then 45 zeros followed by a one so that is a very small fraction of a second. 10ˆ5 is 1,00,000 seconds, that’s all and on the verticle side we can see the radius of the Universe at that time. What is really the whole point is to show how suddenly the Universe went from very tiny to very big. The whole inflationary period lasted only a fraction of a fraction of a moment but altered the Universe totally.
Of course this is all conjecture but all the evidence, from studies of the large scale structure of the Unniverse, to the smallest particles inside particle accelerators like the LHC keep telling us that we are at least on the right track. Maybe one day someone will find some data that contradicts the current model but until then it seems that our Universe did indeed start out unimaginably tiny, suddenly swelled up to billions of times its original size, and then settled down to expand steadily for about 11 billion years until it started accelerating again. This is sort of what we are talking about as a description of the history of the Universe up until now-
What happens in the future is not certain. Maybe the expansion will settle down again and go slower. Maybe it will stop and start collapsing again, although this seems to be unlikely, and maybe the expansion will keep accelerating until the whole thing is so big and thinly spread that the Universe dies with a thin, feeble whimper. It may even end up with the very fabric of spacetime being ripped apart in a ‘Big Rip’ that tears everything, right down to the subatomic level, apart. Watch this space…