From a recent talk with Lawrence Krauss:

We’ve discovered the universe is flat, which most of us theorists thought we knew in advance, because it’s the only beautiful universe. But why is it flat? It’s full of not just dark matter, but this crazy stuff called dark energy, that no one understands. This was an amazing discovery in 1998 or so.

What’s happened since then is every single experiment agrees with this picture without adding insight into where it comes from. Similarly all the data is consistent with ideas from inflation and everything is consistent with the simplest predictions of that, but not in a way that you can necessarily falsify it. Everything is consistent with this dark energy that looks like a cosmological constant; which tells us nothing.

It’s a little subtle, but I’ll try and explain it.

We’ve got this weird antigravity in the universe, which is making the expansion of the universe accelerate. Now: if you plug in the equations of general relativity, the only thing that can ‘anti-gravitate’ is the energy of nothing. Now: this has been a problem in physics since I’ve been a graduate student. It was such a severe problem we never talked about it. When you apply quantum mechanics and special relativity, empty space inevitably has energy. The problem is, way too much energy. It has 120 orders of magnitude more energy than is contained in everything we see!

Now that is the worst prediction in all of physics. You might say, if that’s such a bad prediction, then how do we know empty space can have energy? The answer is, we know empty space isn’t empty, because it’s full of these virtual particles that pop in and out of existence, and we know that because if you try and calculate the energy level in a hydrogen atom, and you don’t include those virtual particles, you get a wrong answer. One of the greatest developments in physics in the 20th century was to realize that when you incorporate special relativity in quantum mechanics you have virtual particles that can pop in and out of existence, and they change the nature of a hydrogen atom, because a hydrogen atom isn’t just a proton and electron.
[Lawrence Krauss: Link]

Via Slashdot

See also:

• Physical Cosmology
• Lawrence Krauss images