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When we look at our image reflected off a mirror at a distance of 1 meter, we see the way we looked 6 nano-seconds ago, the light travel time to the mirror and back. If the mirror is spaced 1019 cm = 3pc away, we will see the way we looked twenty one years ago. Light propagates at a finite speed, and so by observing distant regions, we are able to see how the Universe looked like in the past, a light travel time ago. The statistical homogeneity of the Universe on large scales guarantees that what we see far away is a fair statistical representation of the conditions that were present in in our region of the Universe a long time ago.

Figure 1

Figure 1. Cosmology is like archeology. The deeper one looks, the older is the layer that one is revealing, owing to the finite propagation speed of light.

This fortunate situation makes cosmology an empirical science. We do not need to guess how the Universe evolved. Using telescopes we can simply see the way it appeared at earlier cosmic times. Since a greater distance means a fainter flux from a source of a fixed luminosity, the observation of the earliest sources of light requires the development of sensitive instruments and poses challenges to observers.

We can in principle image the Universe only if it is transparent. Earlier than 0.4 million years after the big bang, the cosmic plasma was ionized and the Universe was opaque to Thomson scattering by the dense gas of free electrons that filled it. Thus, telescopes cannot be used to image the infant Universe at earlier times (or redshifts > 103). The earliest possible image of the Universe was recorded by COBE and WMAP (see Fig. 2).

Figure 2

Figure 2. Images of the Universe shortly after it became transparent, taken by the COBE and WMAP satellites (see for details). The slight density inhomogeneties in the otherwise uniform Universe, imprinted hot and cold brightness map of the cosmic microwave background. The existence of these anisotropies was predicted three decades before the technology for taking this image became available in a number of theoretical papers, including [<355, <308, <297, <338, <282].

Figure 3

Figure 3. The optical depth of the Universe to electron scattering (upper panel) and the ionization fraction (lower panel) as a function of redshift before reionization. Observatories of electromagnetic radiation cannot image the opaque Universe beyond a redshift of z ~ 1100.

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