Much of what we currently know about the initial conditions and evolution of the observable universe comes from a baby picture: the Cosmic Microwave Background (CMB) radiation. This is light that was released about 13.7 billion years ago when electrons and protons recombined to form neutral hydrogen, causing the universe to go from opaque to transparent. We are still receiving this light today, and observing it with satellites such as WMAP and Planck as well as ground-based experiments such as SPT and ACT. The CMB is extremely uniform. However, there are small anisotropies in the CMB (near one part in 10^5) which encode a snapshot of the inhomogeneities at the time the CMB was formed. A depiction of what these anisotropies look like is shown on the left. This is a Mollweide projection of the full sky, and the false colours represent the magnitude of deviations from the mean temperature of the CMB (about 2.725 K). The measured CMB also contains emission from our galaxy as well as other astrophysical sources. These are interesting in their own right, but are considered as noise when one is interested in the primordial signal [much like a conversation going on behind you during the final scene of a movie: it might be interesting for those involved, but for you it is interfering with what you want to hear!].
What do we learn from the CMB?
- The observable universe is very nearly homogeneous and isotropic.
- The observable universe has been expanding and cooling since the time the CMB was formed.
- The observable universe is very close to spatially flat.
- The observable universe is composed mostly of dark energy and dark matter.
- The large scale structure in the observable universe was seeded by nearly scale-invariant Gaussian density fluctuations.