I went to an excellent colloquium yesterday at the SUSY 2005 conference here in Durham, just before the singing and dancing I mentioned in a previous post. It was by Carlos Frenk, of the Institute for Computational Cosmology, also based here (in fact, both it and the IPPP I mentioned earlier are in the same building, the Ogden Centre for Fundamental Physics.) The title of the talk was "The Emergence of a Cosmological Paradigm", and he was talking about what readers of Sean's previous writing might know is called the "Lambda-CDM" model of the universe. What is that? Well, the title shows a refreshing honesty and emphasizes two remarkable things about our universe about which we know very little, in some sense. We don't know what the vast majority of our universe is made of, but the unknown components are currently believed to be an unknown form of matter, and an unknown form of energy. We can't directly see either of these forms and so they are called, helpfully, "Dark Matter" and "Dark Energy". While we do not know exactly what these missing forms are, we've been clever enough to give them names after the sort of behaviour that they have that characterizes what they do to make us believe they are there.
(I say "we", but I mean this to represent something like "humankind", or something equally non-specific. I'm not personally a cutting-edge cosmologist naming anything. I should be doing the string theory beat, but our cutting-edge cosmologist who usually takes this as his beat is busy....Doing the string theory beat. Excellent.)
How bad is the situation? Well, the rough numbers are as follows: First, know that we have just enough matter and energy in the universe to make it almost perfectly flat. Not positively curved, like ball's surface, and not negatively curved, like a pringle chip or saddle. Flat, like a table top. (Those were two dimensional surfaces, and I'm really talking about the geometry of the three dimensions we live in, but the analogy is accurate.) Next, perhaps surprisingly, only 30% of the matter-energy budget of the universe is matter. But only about 5% of it actually glows so that we can see it, in the form of stars, etc. Most (all but a little bit) of the rest of that 25% is now expected to be "Cold Dark Matter". This means that it is not "hot", which means moving at near relativistic speeds. It is "cold", made of some massive particle moving at sub-relativistic speeds. The properties of this particle, if you work out what it needs to be to fit the observations, turn out not to be possessed by any particle we have yet found, but it is of great excitement to "phenomenologists" (like those people dancing and playing and singing) that certain theories for what happens after we go beyond physics described the Standard Model of Particle Physics, which people hope will begin at the upcoming experiment called the LHC, do contain the right particles. The most popular of these are "SUperSYmmetric" theories, or "SUSY" theories. (Which first found their natural home in string theory, by the way.) (And this explains the title of SUSY 2005 conference mentioned in several of my recent posts. In case you wondered. The world seems to be split into particles for matter, and particles for forces: Supersymmetry unites these two types, essentially.) Finally, about 70% of the matter-energy budget of the universe is in a form that is not matter. It is energy. Its main claims to fame is that there is a lot of it, it is "dark", and it is everywhere, and it likes to expand. So it pushes outwards to create more space. Really. It is accelerating the expansion of our universe, which includes space itself, and we can see it happening by looking closely at certain types of star. The "Lambda" comes about because people (including stringers like me) in the field use that Greek letter in their notebooks to denote something called a "cosmological constant". It is a term in Einstein's equations governing gravity which tells you about the energy density contained in spacetime itself. If it is positive, you get this repulsive pushing-out behavior we talked about. No-one has proven that the dark energy is a positive cosmological constant, but it is behaving quite a bit like one, and so it is simpler to use Lambda in the title. (Try out "Something not quite like a positive cosmological constant but it's an awful lot like it-CDM Model". Just does not trip off the tongue like "Lambda-CDM".) There's a lot of background and cross-checked data and observation going on behind those facts I put out there in the last paragraph, but if this is not to get bogged down, I've to skip them. Let me refer you to writing on this here and here. So why am I telling you this? Well, there is a crucial reason why we know that it is cold dark matter and not hot dark matter that we're looking for. Basically, the stuff has to move slowly enough to help give rise to that which we see all around us today. What's that? Lumpiness. This is not a personal attack on you or the people around you, but instead refers to the fact that matter is clumped into stars, galaxies and clusters of galaxies, with large voids in between. How did that come about? The simplest thing would have been to have a nice smooth porridgey distribution of matter. We'd either all be smoothed out structureless porridge-creatures, or maybe not evolved (or whatever) at all. As it is, we're lumpy, and we're here to ask why. The clumping came because gravity, which ultimately amplified small deviations from smoothness into large ones, got a foothold due to some mechanism which produced these "inhomogeneities" as we like to call them. They formed at some very early stage, as we know because we can effectively take pictures of these lumps when they and the 13+ billion year old universe were very young. Just a few hundred thousand years old. In fact, the nice backdrop forming our blog's banner is a piece of one of those photos taken by the WMAP satellite, looking at the "light" that was emitted that long ago. The CDM has to have the right properties to be the seeds of clumping and to give just the right sort of clumping. You don't want stuff to clump too fast, or too slowly. The right sorts of processes which switch on and off at various stages in our universe must happen at just the right rate, or you'll get something completely different at the end, even if you have the right ingredients. Ever tried to make a cake? Or bread? Or almost any cooking for that matter. Even if the ingredients are right, you have to do things at the right stages, and at the right rates, to get the final product the way mama used to make it. Same thing here. The "hot" dark matter just moves too fast, and does not get the clumping right at all. To show that this assertion is correct -to test out the CDM idea- why not just bake the cake? But with the science budget what it is these days, not even the USA can pay for us to make them in the lab, so you do the next best thing: Computer Simulations. So this is what Carlos was talking about. There are wonderful sets of simulations which he talked about, called the "Millennium Simulation" (ahem...remember an earlier post?). The bottom line is that these guys simply took really big computers and put in as much as we know about the basic equations of the universe, and then play with sprinkling in different amounts of cold dark matter and simulate the evolution of the universe tracking 10 billion individual particles in the simulation. They let the computers run until they get to the equivalent time of where we are now in the universe, and then then stop and look inside the computer. They take out the universe they've made and compare it to our universe. For that, you need to do an accurate survey of where all the stars and galaxies are in a large piece of the universe so that you can get good data on the distributions of the lumps, and other structures. The survey his team compared to was the "2DF" survey. So what did they see? How well did their universes do? Well, in short, they look an awful lot like our universe when you have (the right amount of) cold dark matter sprinkled in. ("An awful lot" is not yet an established scientific term. Heck, it is not established English either. However, there are very specific tests (comparison of the power spectra of the size distributions of the structures) which work very well.) This is great stuff, and confirms several other pieces of work by other teams. (See the references in their paper I'll give the link to below.) Here's the really fun part you can do right from here. You can look at all the wonderful slides he showed by looking at the video of this talk when it comes up on the SUSY 2005 site, but even better you can download some of the high quality movies he showed right now! These are movies of flying around inside these newly created universes and seeing all of the wonderful organic-looking structures which form due to the clustering seeded by the CDM. You can see some of the hotspots that form at the intersections of some of these filamentary tendrils, which will be the birthplaces of stars. It is all rather beautiful. Have a look at this all here! (There are some wonderful stills too.) Cheers, -cvj P.S. It seems that the other cutting-edge cosmologist who sometimes talks about this stuff was typing a post on Dark Energy at the same time I was! Have a look!
