So far, all the evidence we've seen has supported our sinple big bang theory. But there are some significant problems!
The cosmic microwave background is very very close to having a perfect blackbody spectrum - temperature fluctuations from place to place on the sky are tiny - one part in 100,000! And this level of fluctuation is fine to grow and be amplified by gravity into the level of structure we see around us today - galaxies and clusters of galaxies with big voids inbetween. But the speed of light is finte and the universe has a finite past from the Big Bang, so we can only see out to a finite distance, which is the light travel time since the Big Bang. The universe itself may be inifinte, but we can only see out to a horizon which is the age of the universe in light years away! If we flatten the universe out on a piece of paper, put all spatial dimensions along the horizontal axis and have time going vertically then there is a light cone which defines what we can see of the universe - only things within the lightcone can be seen as only things within the light cone are within the distance that light can travel in the time allowed. So our observable horizon is currently getting bigger. But in the past it must have been smaller and when you do this, you find that at the time the microwave background was formed, 300,000 years after the big bang, then diametrically opposite bits of sky are not close enough together to ever have exchanged photons so its very very surprising that they are at such similar temperatures. The horizon then is only about 1 degree across the sky now (the sun is about half a degree in size) This is called the horizon problem.
And what causes the microwave background flutuations in the first place ? we can have a guess that they are some form of quantum fluctuation but these would be bigger fluctuations on small size scales, whereas what we see are small fluctuations on large size scales. This is the smoothness problem.
These are both issues with the microwave background. but there are other problems too. why do we see Omega of order unity ? Both the density and critical density of the universe change rapidly with time as the universe expands, but they change in different ways. For Omega to be so close to unity now means that it must have been very very very very close to unity very soon after the big bang. This is the flatness problem, and is perhaps more of a philisophical question
A nice review of these problems is given in this cosmology tutorial (he talks you through the space-time diagrams in an expanding universe - I just cut straight to the one at the end in my lecture as you really don't want to know about the details!!)
One way to modify our standard big bang model to take account of these problems is to suppose that the early universe went through some fantastically rapid expansion - perhaps a tiny fraction of a second after the big bang ie way way before primordial nucleosynthesis (first few minutes) and the formation of the microwave background (300,000 years after the big bang). We KNOW we don't understand physics at these high energies and densities, so in some sense we can do anything (more on this next lecture). Such a hugely rapid expansion in the very early universe - a period of Inflation means that parts of the universe which were intially very close get separated. This would mean that in the past the universe was much much smaller, so things on opposite sides of our current sky DO get time to exchange photons and get into equilibrium. They DID know about each other in the past so solving the horizon problem. And the fluctuations get massively expanded in size, solving the smoothness problem. But inflation also predicts that as the universe expands so quickly then its original curvature gets very hard to see. Any initial curvature of the universe is dramatically flattened out, so our universe should have a geometry which is basically flat.
But this last point is a bit of a problem in itself. If there is only the expansion of the universe (with or without a period of inflation) which is slowed by the gravity of the universe, then the curvature of space can be directly linked to Omega, so that critical (omega=1) is flat, closed (Omega bigger than 1) is positively curved and open (omega smaller than 1) is negatively curved. So the problem is that inflation predicts that space is flat, so if all we have is gravity versus expansion then this should mean that Omega=1 BUT we measure Omega=0.3
OK, so maybe the thing to do is MEASURE the curvature of the Universe directly - going to Omega is making an assumption that all we have is expansion versus gravity. And we can measure the geometry of the universe if we have a known length scale - light bends as it travels through curved spacetime and the length we see now depends on the curvature of space. The size of the microwave background fluctuations give us a length scale - the size of the horizon at the era of the microwave background. The angular size on the sky of the fluctuations should be bigger in positively curved space, than in flat space or negatively curved space. And new data show CONCLUSIVELY that the size of the fluctuations requires that space is flat.
So good for inflation!!! but we measure Omega=0.3 but have flat space!!! so if both statements are true then there must be something else in the universe other than its expansion being slowed by its gravity! Lets look in more detail at the expansion of the universe. We'll take this further next lecture, but roughly we can see that if we can compare the expansion now, with the expansion a long way back in time, we can figure out by how much gravity has slowed down the expansion in that time. When we look at remote galaxies we are looking back in time. And since gravity should have had less time to slow the expansion then the remote galaxies should be expanding faster. But when we do this experiment (see next lecture for details) then the results show the OPPOSITE! The expansion is actually currently ACCELERATING ie that the velocities in the past were less than we see now!!! WWHHHAAAAATTTT!! how can the universe be accelerating - is there some form of anti-gravity ?? (thats how it got reported in the press!!!). Because we KNOW there is gravity in the universe which will be slowing the expansion down. so for it to be accelerating there must be something more powerful than the gravity of the whole universe pushing it out!!
So the best current bet is that big bang plus inflation can explain everything we see if Omega as measured from matter (including dark matter, whatever it is) is 0.3 but that there is also some extra term - its not just that there was some initial expansion (with inflation) but there is some ongoing component to the expansion - dark energy
