We saw in the last lecture that the supernovae distances to remote galaxies gave Omega < 1 quite convincingly. Omega=0.3 looked much better than Omega=1. But there is also hits of something else in there too - I said in passing that it looked like there was an acceleration term! Now, that data isn't yet dramatically convincing, but there are other hints. We can get the curvature of the Universe directly 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 of the fluctuations should be bigger in positively curved space, than in flat space or negatively curved space. And new data seem to show that the size of the fluctuations requires that space is flat. So isn't this inconsistent ? Many lines of argument about mass give us Omega=0.3, and ONE experiment on curvature gives Omega=1. So why don't we wait for this one experiment to be confirmed (or not) before we get too worried (the MAP space mission in 2001)? Its back to inflation again - we liked inflation as a way out of the horizon and flatness problems and inflation predicts flat spacetime. And there is one way which we can have Omega=0.3 in mass and flat curvature and thats if there is something else which curves space as well as mass. How about if spacetime itself has some energy.... an energy density associated with the vacuum of empty spacetime. This is called a cosmological constant, written as the capital greek letter Lambda. Then the curvature is given by Omega+Lambda and if Lambda=0.7 then we can have flat spacetime, and Omega(mass)=0.3. But this energy density of the vacuum is strange stuff. When you work out its properties then its total energy (add up energy density and pressure but its pressure is NEGATIVE) is actually negative so it curves space the other way round to gravity. In fact it breaks the nice relations we had before about the curvature of the universe and the energy density in it - with a big enough cosmological constant you can have Omega(mass)=2 and an open universe!
Uggggghhhh! but why might the universe be doing this ? Well actually it'd be the same as inflation but scaled down. Inflation is basically just a big cosmological constant causing rapid expansion so if it was big at some point in the past, it could be nonzero now! We just don't know because we don't know enough physics
How far back can we push our knowledge of physics ? Lets go back to the early universe, before the cosmic microwave background and before primordial nucleosynthesis. At earlier times the Universe would have been hotter still. The protons would smash together, but the Universe was so hot that the blackbody photons would break the nuclei apart as soon as they were formed. Protons are not fundamental particles, they can be split apart into bits called quarks. The 'standard model' of particle physics has 6 quarks and 6 leptons (electron, electron neutrino, and two other sorts with their neutrinos) as truely fundamental particales that can't be split apart. And forces: at fundamental level, a force isn't just something that happens to particles. It is a thing which is passed between two particles. So we also have the force carriers: photons (electromagnetic), W and Z bosons (weak force), gluons (strong force) and gravitons (gravity! though its very hard to get our ideas of curved spacetime into the standard exchange of particles format!!). Oh, and every particles has an antiparticle too...
So this is the standard model - it provides a very good description of phenomena observed by experiments BUT it cannot explain why some particles exist as they do, and it cannot PREDICT things like particle masses and the strengths of the forces. But are we really free to choose all these fundamental constants ? Each one gives us a rather different universe (gravity stronger/weaker, strong force stronger/weaker) and we've seen that actually we need these constants to be rather finely balanced to get a universe which can produce life (the anthropic principle). It looks more like we're missing something. And there are so many bits! Is it really so complicated ? Perhaps there are some simplifications which would come in at higher energies. Ice and water look very different, but if we heated them up they would BOTH turn into steam. They are the same thing, but there is a phase transition at low temperatures - termed symmetry breaking. We now can see that this is the case for the electromagnetic and weak nuclear forces - at high enough energies they turn into the same thing - called the electroweak force. So perhaps at very very high energies there was only one force....and then as the universe cools we get phase transitions, and things which are really the same start to look different. Theres a nice review of particles and forces at the particle adventure site.
So lets go back as far as we can. At even earlier times, when the Universe was less than a millisecond old and its temperature was 1012K then the Universe would be so hot that the blackbody photons crashing into each other would make particles and their antiparticles (remember E=mc2 - matter and energy are the same thing!). Protons must then slightly have outnumbered antiprotons (by only 1 in a billion!) in order to have the Universe made of protons as we see it today. Why ? Plainly there are some observable constraints here!
Back even further and the Universe getts hotter and hotter. At 10-10 of a second after the big bang then the temperature was more than 1015 K. At this point the electromagnetic force and weak nuclear force (the one that the neutrino interacts by) become one and the same thing. We have experimental evidence backing up these theories from particle accelerators - the theories predicted some new particles (W and Z bosons), and these have been seen!
Back even further, and we are into temperatures/energies which are higher than we can currently attain in particle accelerators. Theories have been developed which unify the electroweak and strong nuclear force into a single force at very high energies - maybe something like 10-35 seconds after the big bang (these are called grand unified theories or GUT's). Its in these theories that we have to look for the slight imbalance between matter and antimatter that could result in the fact that we and the rest of the Universe is made of matter. And there are GUT theories can do this, which is interesting and makes them potentially believable even if we can't yet test them directly in particle accelerators. And they might also make some odd, heavy particles which make up the dark matter we talked about in lecture 14.
And most physisics believe that if you go to high enough energies then all the forces (ie GUT+gravity) unify into a single force. But that is on timescales of 10-43 seconds (called the Planck time) after the big bang. We don't yet have even a theory of how to unify gravity with the other forces, so we can't get anywhere closer to the origin of the big bang than this planck time (though its pretty close!). Here is a nice overview of beginning of the universe, and another tour of the beginning of the universe, and some stuff about unification of forces.
So, the fact that we are made of matter gets us into the GUT era which isn't well understood. So we can put inflation here. An accessible list of articles on cosmology is kept at the astronomy cafe For the truely brave (or foolhardy!) here is a link to frequently asked questions in cosmology.
So, here we are, at the very end of our current knowledge. Lemaitre described the big bang model lyrically in one popular account: `The evolution of the world could be compared to a display of fireworks just ended - some few red wisps, ashes, and smoke. Standing on a well-cooled cinder we see the slow fading of the suns and we try to recall the vanished brilliance of the origin of the worlds.'
