7)
Dark Energy
I've been seeing a lot of dark energy questions of late and thought it might prudent to add at least one more section to my review, since this is one of the most important and controversial subjects in modern cosmology.
Dark energy refers to the energy component that is driving the current
acceleration of the universe. The key relation that describes it is the equation of state; that is, the relationship between pressure and density. The simplest form of such an equation would be:
p=w\rho
where p is the pressure and \rho is the energy density. If you include a dark energy component in a general relativistic cosmological model, you'll find that, in order to cause acceleration, the "dark energy" must have w < -\frac{1}{3}.
But what is this stuff? Pretty much everything we know of and can do experiments on has a
positive pressure. Well, one possibility was pondered by Einstein (though for different reasons) back in 1917. He considered that perhaps the vacuum naturally had an energy associated with it and that, as the universe expanded, more energy would be created as space expanded. Another way of saying this is that he proposed a
cosmological constant -- a constant energy density associated with space itself. With this addition, his famous equation took the form:
R^{\mu \nu}-\frac{1}{2}Rg^{\mu \nu}-\Lambda g^{\mu \nu}=8\pi T^{\mu \nu}
where \Lambda is the cosmological constant. Because of the nature of the metric (g^{\mu \nu}), it turns out that this cosmological constant corresponds to a dark energy equation of state, w=-1.
Let's now fast forward to the end of the 20th century. In 1998, a group of astronomers observing supernovae announced that their data were inconsistent with a decelerating universe. In fact, the universe seemed to be accelerating and, in order to explain it, they needed
70% of the energy density of the universe to be made up of dark energy. This was greeted with a great deal of skepticism, partially because the methods were questionable and partially because it was physically difficult to explain. It wasn't until 2003, when the WMAP satellite announced its results from an analysis of the cosmic microwave background (CMB), that dark energy became a fixture in our cosmological models. Quite simply, the satellite made an
independent measurement of the dark energy density and came to the same conclusion that the supernovae people did -- 70% of the universe is composed of a dark energy component.
That leads to what we call the concordance model, or \Lambda CDM. This is a general relativistic model of the universe that includes ~30% matter (~90% of which is cold dark matter) and ~70% dark energy in the form of a cosmological constant. Does the dark energy have to be in the form of a cosmological constant? No, it can be in the form of a scalar field (much like the one that led to inflation), but it must have an equation of state near that of the cosmological constant because observations constrain w ~ -1 to about 30% (depending on which observations you believe). Also, it's possible that general relativity fails at large scales and our observations are simply parameterizing the breakdown of Einstein's theory.
Dark energy is one of the most puzzling aspects of modern cosmology and I think it would be naive of us to claim that we really understand what's going on here. Astronomers are working overtime to understand and quantify its effects, but we would still like a physical understanding of the mechanism that gives the vacuum energy. Is it the zero-point energy of QFTs? Is it a scalar field? Is it some exotic kind of particle? I'm happy to say that we still don't know and there is still much to be learned from our universe.
