We saw last lecture that the evidence that the universe in the past really was a lot hotter and denser than it is today - the cosmic microwave background, 3000K, 300,000 years after the big bang, even more impressively, primordial nucleosynthesis, 1 billion K, 3 mins after the big bang. Protons and neutrons created from photon collisions in the early universe (matter and energy are equivalent - Einstein's famous E=mc2) can survive after about 0.001 seconds after the big bang - the photons no longer have the energy to destroy neutrons and protons as soon as they are formed. But these particles are moving very fast (are very hot) so we can have nuclear fusion reactions (and the fact that there are neutrons and protons mean they go a fair bit faster than if we had mainly protons as there isn't the repulsive electrostatic These fusion reactions can only happen for a short time as the universe is expanding and cooling so rapidly. So some (but not all!!) hydrogen undergoes fusion reactions. Thus there is some deuterium, tritium and helium which is NOT produced in stars, but in the very early universe. If we look at very old stars which were formed before there was much chemical enrichment from supernovae then we see these primordial abundances.
so it seems that we really can do this extrapolation back in time, to a hot dense early universe. so what about a forwards extrapolation, what about the ultimate fate of the universe ? Will the Universe keep on expanding forever ? What can stop it ? Gravity! The gravitational force between all the matter must be slowing the expansion down. Can it slow it down enough to stop the expansion, and bring all the galaxies back together again ? Obviously this depends on the balance is between how fast the Universe is currently expanding and how much mass (i.e. gravity) there is in the Universe to pull it back. The average density of matter which forms the borderline between these two outcomes is called the critical density - its TINY, corresponding to 5-6 protons (or hydrogen nuclei) per cubic metre! The ratio between the actual density of matter and this critical density is called Omega. For Omega < 1 then the Universe will expand forever, the stars will all eventually die out - a dark and cold death of everything. For Omega > 1 then there is enough matter to eventually halt the expansion, and recollapse the Universe into a big crunch. Perhaps this could be a cyclical process, seeding a new big bang ? More on ultimate fate of the Universe .
So, what is the density of matter in the Universe ? We know more or less how massive stars are, and we know more or less how luminous they are, so we can look at the apparent brightness of a galaxy, get its distance using any of the ways described in lecture 11, so get the luminosity of the stars and so get the mass of stars. Do this for all the galaxies we can see within a certain distance, and then work out the density! And its WAY WAY too small to close the Universe, giving something like Omega ~ 0.01!!
But what we can see isn't all of what there is! When we estimated the mass of our Galaxy we did it two ways, first by looking at the numbers of stars, and secondly by looking DIRECTLY at the amount of gravity by looking at the motion of stars orbiting around the center - with a velocity and a distance we can use our understanding of gravity to get the mass within the orbit. Stars are OK to do this with, but gas is better as the gas extends out a fair bit further than the stars and we want now to look at the mass of as much of a galaxy as possible. Dust is always a problem, so we want to go to long wavelengths. And we want a spectral line which is very strong so we can easily track it. Hydrogen has that line in the radio (nice long wavelength) at 21cm which is ideal for this. Get an edge-on spiral and map the rotation velocity of the 21cm line at all distances from the center. As we go towards the edge of a galaxy the starlight falls off (definition of the 'edge' of a galaxy!), so we expect that this is all the mass, so the rotation curve should fall away too. It doesn't!!! There must be large amounts of DARK mass in a halo around the galaxy in order to match the observed rotation curves. A nice site on including a java animation of rotation curves. Masses measured in this way show 10-100x more mass than that which we can see in stars! So the mass of the Universe is dominated by DARK MATTER. But even including this DARK MATTER we get Omega ~0.3 i.e. not enough to close the Universe
But what is this dark matter ? Maybe its failed stars, or dead stars (white dwarfs, neutron stars, black holes), or black holes left over from the big bang, or.... But there is a problem. Primordial nucleosynthesis in the big bang puts a fairly stringent constraint on how much of this can be in the form of protons/neutrons (collectively known as baryons) - and its something like Omega~0.06. So, a small faction of this dark matter can be normal stuff, but the majority of it has to be something else entirely!! More about dark matter. Nasty isn't it - most of the mass in the Universe is in the form of something which we don't yet understand!