Initial Conditions, Inflation, and Fluctuations

A s you know, four main bodies of evidence affirm the big bang: the dark sky, Hubble expansion, element abundances, and cosmic microwave background radiation (CMBR). Parts 1-4 of this series briefly examined these topics. [1-4] Here we say a few words about the window the CMBR offers into the very early universe, including a brief but crucial epoch of " inflation. " Recall that the CMBR was first predicted in 1948 by Ralph Alpher and Robert Herman.[5] They calculated its present temperature to be about 5 K, putting the peak of its Planck spectrum (" blackbody " radiation) in the microwave band. For nucleosynthesis calculations to achieve ~0.1% agreement with observations, it was necessary to assume a billion-to-one ratio of photons to baryons. These photons need not be postulated ad hoc. When the thermal energy per photon fell below mc 2 for some particle species, after pair annihilations a matter residue was left over along with an enormous photon population, due to prior particle/antiparticle decay rate asymmetries. The radiation and matter maintained thermal equilibrium thanks to Thomson scattering between photons and charged particles. Helium synthesis occurred about 3 minutes after the big bang, and for the next 380,000 years the photon mean free path between successive Thomson scatterings remained very short. About 380,000 years after the big bang, the temperature fell to about 3000 K, cool enough to quench photoionization and allow electrons and nuclei to bind into neutral atoms. After this decoupling process, photons could propagate freely; the universe became transparent to radiation. This radiation preserved its Planck distribution, with ever-decreasing to make precision measurements of the CMBR. Over a thousand people worked on COBE. It carried three main instruments. The two that concern us here are the Far Infrared Absolute Spectrophotometer (FIRAS) and the Differential Microwave Radiometer (DMR). FIRAS was built to show the smoothness in the CMBR, and the DMR to show temperature fluctuations farther out in the decimal places. Mather led the FIRAS experiment. It measured the CMBR spectrum in wavelengths from 0.5 mm to 1 cm, which includes essentially all the active CMBR spectrum. This measurement was essential to nail down conclusively the big bang nativity of the CMBR. Before FIRAS, only fragments of the CMBR spectrum had been fit to a Planck curve. Smoot led the DMR experiment. That instrument's design featured dual receiver horns to measure the temperature difference between two points …