We saw in the last lecture that when we look at the universe around us we measure Omega(matter)=0.3 - predominately in dark matter NOT normal stuff made of protons/neutrons. But we also saw that we could use the size on the sky of the tiny fluctuations in the cosmic microwave background to measure the overall geometry of the UNiverse. and its FLAT. But if there is ONLY an initial expansion (with or without inflation) which is progressively slowed down by its own gravity, then a flat universe MUST have Omega(mass)=1. So, the assumotion is WRONG - there isn't only an initial expansion (with or without inflation) which is progressively slowed down by its own gravity - there is an acceleration term in there as well, kind of like strapping a rocket to something which is being thrown up in the air - its distance is then NOT determined just by how fast its initially thrown up and the strength of gravity pulling it down, it also depends on how the rocket fires. Our best guess for this 'rocket', this acceleration term (also called the cosmological constant, or dark energy, and given the symbol Lambda), is that it comes with 'empty' space. The more the universe expands, the more empty space we have, so the more important this term becomes. The overall curvature is set by the SUM of the lambda and omega terms - we think we live in a universe with omega=0.3 and lambda=0.7 so space is flat, but the evolution of the universe is no longer simply determined by the balance between the initial expansion and gravity, this accelerating term takes over and the universe expands faster and faster.....
But if thats the fate of the universe, how about its beginning ? 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. Oh, and every particle has an antiparticle. and there are also forces between them. We know of 4 different types of force, electromagnetic (ties electrons to protons to form atoms), weak nuclear force (the only way neutrinos interact), strong nuclear force (that binds atomic nuclei together, the velcro force in fusion) and gravity
Together, these particles and forces make up 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. 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 ? We don't know!
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, a brief history of teh universe and a tour of the beginning of the universe.
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.'