So on the one hand we have data from measuring mass that Omega=0.3, yet on the other hand we have data from measuring curvature that the universe is flat i.e. that Omega=1! And while this COULD be that we just don't understand something (more probably several interlinked things) its a bit nasty. Yet actually we CAN get a sort of consistent picture if we think again about the very early universe and inflation.
To get such rapid inflation, we need the vacuum to have energy density. In other words the 'empty' spacetime isn't empty at all but has some energy associated with it. the data from the supernovae were pretty clear that What are the building blocks of matter - what are the bits that are truely fundamenal rather than being made up of other things. We've seen protons, neutrons, electrons and neutrinos. electrons and neutrinos don't seem to be made up of anything else, but what about protons and neutrons - can we split them up further ? Take them to particle accelerators, and smash them together and look to see if there are any bits! Well, there are lots and lots of bits. To explain to zoo of particles that come out we need protons and neutrons made up of quarks. Quarks seem to be truely fundamental particles - things we can't break up any further. So we need quarks, electrons and neutrinos as the truely fundamental things. In fact we need 6 quarks (up, down, strange, charm, top, bottom) and 6 leptons (electron, electron neutrino, muon and its neutrino, tau particle and its neutrino) Heavy leptons quickly decay to electrons so are not normally seen And what do we realy mean by 'force'. At a fundamental level, a force isn't just something that happens to particles. It is a thing which is passed between two particles. So for something like the elctromagnetic force, its caused by photons being exchanged between two charged particles. It ONLY acts on electric charge - neutrons or neutrinos don't feel this force at all. So what about the rest ? We've seen the weak nuclear force in neutrino interactions - this acts only on a property called weak charge or flavor charge which quarks and leptons have, and is mediated by exchange of heavy particles called W and Z bosons. These have been found in accelerators! The strong force gets a bit more difficult. It can be described by exchanging particles called gluons - these have been indirectly observed. The strong force then acts on strong chage or colour charge which quarks and gluons have - so the force carrier in the strong interaction is itself affected by the interaction. Gravity is of course the joker in the pack - we've described it before as geometry of space-time so its a bit hard to then push it into the same framework as the other forces which are an exchange of particles. Hasn't stopped them though - the idea is that it CAN be described in this way, and that it acts on all particles by exchanging gravitons. 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. We've ended up with an awful lot of bits of matter which are 'fundamental'. quarks, leptons and force carriers (also called bosons).... And every particle has an antiparticle too. 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 - we call this 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 as the universe cooled there was a phase transition, where the electroweak split up into the electromagentic and the weak forces. We'd then need another force carrier for the electroweak force - call it the Higgs boson. But theres a problem here - ways of calculating its mass in the standard model give infinity!! Not very likely. One way to make it finite is to say that for every standard model particle there is a corresponding supersymmetric particle (or sparticle)..... These should all be very heavy, so they are trying to build very high energy accelerators to test if they exist. Can we go further - is the difference between the electroweak and strong force merely an artifact of looking at low temperatures - would they become the same thing at high enough energies ? This is what grand unified theories (GUT) are all about. And again we seem to need the supersymmetic particles, otherwise the electromagnetic, weak and strong don't seem to merge! But if they do then we'd need another force carrier for the GUT - and this might allow protons to decay on very very very long timescales - more than 10 32 years. But maybe this is enough to cause the matter--antimatter asymmetry that we need to make a universe full of matter. And how about getting gravity in there too - well it gets even harder. The main problem being that so far we've thought of gravity as curved spacetime, but the other forces as exchange of particles. A lot of these problems go away if we actually live in a world of more than 3 spatial dimensions. Wouldn't we notice ? No, not if they were very very small (rolled up!). then the fundamental particles would be vibrating bits of string (string theory). Current ones like 7 curled up dimensions, plus the 4 standard spacetime ones. Heres a picture to help you visualise this.