So lets review where we've been so far. We've built up a picture of an expanding universe based on the key observation of the Hubble law (remote galaxies have spectra which are redshifted by an amount which depends on their distance - see lecture 11. Corroborating evidence comes from the microwave background and primordial nucleosysnthesis - both indicating that the universe in the past was hot and dense - see lecture 13. Additional evidence for an evolving universe are that the night sky is dark ( lecture 13), and that galaxy properties change as we look at large distances (remote in space means looking back in time to when galaxies were young - lecture 15). Small fluctuations in the microwave background are enough to grow by gravity into the galaxies and clusters of galaxies we see today ( lecture 15). But the uniformity of the microwave background is a big problem for a smoothly expanding universe, as at early times the horizon was rather small so different bits of the sky shouldn't be at the same temperature! (horizon problem - lecture 15)
The ultimate fate of the universe then depends on how much gravity opposes the expansion - is there enough to eventually stop the expansion, so the universe re-collapses into a big crunch (Omega bigger than 1) . Or will the universe get slowed only a bit by gravity and continue expanding for ever (Omega less than 1). The dividing live between these two is a critical universe, whose expansion is exactly balanced by gravity (Omega=1) - see lecture 14. When we try and measure how much gravity there is then we come out with Omega of about 0.3 but most of this is in dark matter (edge on spiral galaxy - rotation curves, clusters of galaxies - galaxy velocities, X-ray hot gas, gravitational lensing). Primordial nucleosynthesis restricts the amount of this which can be made of normal material (protons/neutrons/electrons) to Omega between 0.02-0.03 (although it MAY be possible to get it as high as 0.07). So while a little of the dark matter which we see can be in normal matter (white dwarfs, brown dwarfs, cold gas) MOST of it has to be something else entirely (lecture 14). But theres a problem here too - perhaps more of a philisophical issue - why is omega so close to one ? (flatness problem - lecture 15)
Both the horizon problem and the flatness problem can be solved by a period of extremely rapid inflation in the very early universe ( lecture 15) we'll do more on this in the next lecture
But how else can we get some idea as to the geometry of the universe so as to understand whether its open or closed. Well, there are two ways. 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 galaxies should be moving faster. The more gravity there is in the universe, the bigger the effect, so we can use this to measure Omega. But what what can we use other than hubble law to get distances to very remote galaxies ? we saw that for nearby ones we could use 'standard candles'. Cepheids are the best as we think the period-luminosity relation is very tight so that for a given period there is a very well defined luminosity - combine with the inverse square law and the apparent brightness and get distance. But while cepheids are bright stars, they are only 1000x brighter than the Sun, so we can only identify them in relatively nearby galaxies. We used the apparent brightness of galaxies themselves, together with the width of their lines to calibrate their absolute luminosity (tully-fisher) to go out further, but not far enough! (review ways to get distances to galaxies in lecture 11). We need something brighter which is well defined. Supernovae are bright - they can outshine all the stars in a galaxy a few weeks to a month before they fade! there are two types. Type II are massive stars, and these are not that well defined as the star mass can be anything as long as its big! But there are also binary stars where there is a white dwarf accreting from a companion. when the accreted mass pushes the white dwarf over its maximum mass which can be held up by electron degeneracy pressure then the white dwarf implodes. So its a very well defined event, and should have (and appears to have) a very well defined luminosity. Go review supernovae!
Problem is finding them - they are very rare, maybe 1 per galaxy per hundred years. So you need to look at lots of faint galaxies. But it can be done and has been done and the results are comming in. Here are some pictures of them! Firstly, they are clearly inconsistent with the amount of gravitational deceleration you'd expect if the matter density in the universe is enough to close it - Omega=1 is not allowed (watch their plot - its the other way around from the previous one ie distance is on the vertical scale and velocity is on the horizontal scale) , and again it looks a lot more like Omega=0.3. But what the data REALLY want is that the expansion is actually currently ACCELERATING ie that the velocities in the past were less than we see now - this is the curve marked by the Lambda term!!
WWHHHAAAAATTTT!! how can the universe be accelerating - is there some form of anti-gravity ?? (thats how it got reported in the press!!!). more next week !
