[Figure: Supernovae Cosmology Project]
The most puzzling experimental result of the last decade that has inspired hundreds, if not thousands of papers, is the matter composition of our universe. The measurements indicate that the usual matter we are made of is only a small fraction, 4%, of all the matter content of the universe. According to present data, 23% is dark matter, whereas the remaining 73% is dark energy. The matter densities are usually normalized to the so-called critical density ρcrit = 8 π G/3 H2, upon which one obtains the more convenient dimensionless parameters ΩM for the fraction of matter (us + dark), and ΩΛ for the dark energy part.
The plot above shows best fit confidence regions in the ΩΛ versus ΩM plane. It combines data from supernovae redshift, galaxy clustering, and analysis of the Cosmic Microwave Background. The shaded upper left corner indicates a region where there would be no Big Bang (scale factor doesn't go to zero). The diagonal line is a flat universe, and divides the areas of closed and open models. The slightly upward bended line divides region with and without recollapse (derivative of the scale factor can have a zero). If ΩΛ = 0, then a closed universe recollapses eventually. Roughly speaking, more matter requires more dark energy to have continuing expansion, so the line dividing continuing expansion from recollapse bends upwards. (Sean Carroll explains you how to compute these boundaries here.)
The shaded lower right corner would imply the the universe was younger than the oldest observed stars. The supernovae results and the CMB data constrain combinations of both parameters, such that the best fit regions lie on diagonal ellipses, whereas the cluster data is dominantly sensitive to the amount of matter, though the ratio of X-ray gas mass to total mass depends on depends on both parameters, so it yields some weak constraints also on ΩΛ (Allen, Schmidt and Fabian, Mon. Not. Roy. Astron. Soc. 334 (2002) L11 arxiv:astro-ph/0205007).
The present data is compatible with a flat universe. Though one has to be somewhat careful what this analysis actually shows. It shows that the ΛCDM model - a flat cosmology with the above mentioned fractions of dark matter and dark energy - is a parametrization of observed effects that is the best fit to the presently available data. But so far we have no experimental knowledge about the microscopic nature of the unknown constituents of the universe.
Actually, writing this series of posts about today's data and how it is compatible with our theories I am impressed how much the homo sapiens sapiens has learnd about the world around him, ;-).
This post is part of our 2007 advent calendar A Plottl A Day.