There are additional constraints from local

There are additional constraints from local - There are...

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There are additional constraints from local (or indeed laboratory) observations. Most prominent is the cosmic microwave background . This is the radiation field at the epoch of (re)combination, when the universe first became transparent enough for radiation to propagat freely over cosmologically interesting distances. At the high densities then, the radiation field was fully themalized (blackbody). Since then the observed temperature has dropped due to redshifting (a redshifted blackbody is another lower-temperature blackbody). See Peebles, chapter 7, for some complications. Recombination was somewhere around z ~ 10 4 set by the temperature at which hydrogen ionizes and the observed CMB temperature (which we know from COBE to be 2.785 K, Mather et al 1990 APJLett 354, L37 and Smoot et al 1991 ApJLett 371, L1). The CMB is highly isotropic (except for a dipole term thought to reflect our motion with respect to the large-scale velocity defined by the CMB emission surface) and embarrassingly smooth. Its spectrum is as perfect a blackbody as can be measured: from Fig. 4 of
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There are additional constraints from local - There are...

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