We saw in lecture 11 that the galaxies had spectra which are generally redshifted, implying that they are receeding from us with a velocity proportional to their distance (the Hubble law). This is a nice way to find distance, but what does it mean ?
The redshifts are often talked of in terms of a doppler shift, to give a recession velocity. If everything is receeding from everything else then the universe must be expanding. There is a subtlty here as the redshifts in an expanding universe are not really the same as a doppll but it not quite the same as the standard . Einstein's general theory of relativity tells us that space (or rather space-time) is the same thing as a gravitational field. So space-time can't exist apart from the matter and energy that creates the gravitational field. So we can't talk about the universe expanding into empty space, because space apart from the universe doesn't exist! So what we are seeing is not a universe expanding through space, but space itself expanding. Blowing up a balloon is an analogy which is often used for the expansion of the universe See this nice diagram
So if its expanding, then at some time in the past the universe must have been much smaller than it is now. So it must also have been much denser (same mass in a smaller space). As things expand they cool, so the early Universe must have been much hotter. If we extend this line of thought backwards to its logical conclusion then the entire universe (space and time and mass/energy) was once compressed to unimaginable densities and temperatures, expanding explosively outwards. This is the 'big bang' - the creation of the entire universe.
But whats the evidence! In general we trust a theory if several (the more the better) independant lines of evidence come up with the same story. Where is the corroborating evidence for this ? The wilder the story, the more evidence we'll need to justify it, and this is pretty wild.
If the Universe was originally incredibly hot and dense, then it would radiate like a blackbody, with photons and electrons interacting. Electrons couldn't recombine with nuclei to form atoms because another photon would come along and knock it out of the atom. But as the universe cools, these blackbody photons would cool along with it. Eventually the Universe would get to a temnperature where photons no longer had enough energy to immediately knock any electron out of an atom - this is the era of recombination. The blackbody photons then don't interact with the electrons any more. So the blackbody cools with the expansion of the universe. If you take the expansion from the Hubble law, then this predicts that this cooling blackbody radiation left over from the hot early universe should be at temperatures of only a few Kelvin i.e. that this peaks in the microwave region of the spectrum
.This cosmic microwave background radiation was found experimentally at much the same time as it was predicted theortically!
. But what else is there in the Universe ? We've seen that there are normal galaxies, shining by the light of stars, and active galaxies, shining both by the light from stars and by the huge luminosities that can be produced by accreting material onto a central supermassive black hole. Just as we used the observed distribution of stars to understand something about our galaxy, we're now going to use the distribution of galaxies to try to understand more about the Universe.We see that galaxies are often clumped together into groups or clusters of galaxies - we might expect this as gravity draws them together. About 75% of galaxies are in clusters. Our own Milky Way is part of a fairly small cluster called the local group (2 big spirals - the Milky Way and the Andromeda Galaxy) and a smattering of smaller galaxies). The nearest moderately sized cluster is the Virgo cluster (hundreds of galaxies), while Coma is the nearest big (or rich) cluster. Clusters themselves clump together into superclusters, with voids separating them. The galaxy distribution is not uniform - and we wouldn't expect it to be. Galaxies have mass, so they have gravity, so they attract each other.
But hang on, how does this tie in with the observations that galaxies are mostly redshifted, implying that they are moving apart ? In general the velocities from Hubble law are so large that these other motions in clusters make only a very small difference to the total redshift.