Follow humbly wherever and to whatever abyss Nature leads, or you shall learn nothing. -T.H. Huxley
We’ve spent a little bit of time talking about dark energy, including what we think of it, how we first discovered it, and how we knew that there wasn’t just something out there blocking the light. It seems to be the latest abyss that Nature is leading us, so we needed to look beyond the type Ia supernova data and see what else the Universe was telling us.
So what do we do? First off, we can try to measure how much matter is in the Universe independent of anything else. How do we do this? We use the most accurate method available, of course. This means taking giant surveys of galaxies and clusters of galaxies, combined with a knowledge of gravity.
Then you take this actual clustering data and you compare it with simulations of Universes with different matter compositions. You take a Universe with 10% matter, then you take another one with 20%, 30%, 40%, etc., and see which one matches the Universe you actually have in front of you.
From clustering data, we can tell that the Universe has somewhere between 25 and 30% of its energy in the form of normal matter. Independent of any supernova data, we learn that most of the energy in the Universe is not normal matter.
So what’s the rest of it? We need the cosmic microwave background to tell us that.
These tiny little fluctuations tell us a tremendous amount about what’s in our Universe. Moreover, they tell us whether space in the Universe is curved positively like a sphere, flat like a sheet of paper, or curved negatively like a saddle.
These three different curvature cases would lead to the hot and cold spots looking different from one another, and the differences are striking. BOOMERANG was able to tell these cases apart.
Only the middle case — a flat Universe — holds up to the data. In fact, the limits are that if the Universe is curved, the amount of curvature is less than 2% of the total energy density. So we have not only supernovae, but clusters of galaxies and the cosmic microwave background too, all pointing towards the same Universe. One where it’s spatially flat, full of about 25-30% matter, and where the remaining 70-75% is some mysterious form of energy. Seriously, all these different data sets point towards the same conclusion:
The Universe is mostly full of dark energy, which would need to exist even without the supernova data! It’s a very unusual thing for all of these different sources of data to come in all at once, like they have over the past decade, and all support the same conclusion.
But this is what we’ve got, and it’s supported from every angle. So take Huxley’s advice, and follow Nature into the abyss of dark energy, or — the horror — you shall learn nothing.