D4 Big Bang: Microwave Radiometry of the Early Universe

Topic

Central to this model is the spectrum of the cosmic microwave background itself, measured with ever-increasing precision by successive space missions.

A pioneering satellite carried three distinct instruments: one measured the full spectrum of this cosmic background with unprecedented precision using a cryogenic interferometer, confirming it to be a nearly perfect blackbody with extremely small deviations—a result that later earned a Nobel Prize; another instrument discovered temperature anisotropies at large angular scales.

A subsequent mission substantially improved the angular resolution with which this same spectrum is measured and determined its power spectrum down to much finer scales, enabling the age of the universe to be calculated with one percent precision.

The underlying physics is straightforward: a hot Big Bang necessarily produces a pure blackbody spectrum, so any deviation from that spectrum would signal the presence of new energetic processes in the early universe.

The most remarkable feature of the first instrument is its differential design: it compares sky radiation directly against an internal reference blackbody—temperature-controlled to within millionths of a degree—thereby eliminating any signal originating from the instrument itself and revealing only the pure imprint of the cosmic background. Detecting temperature variations at the largest possible angular scales further demonstrated that the seeds of large-scale structures were already imprinted as early as that epoch. Thus, from the cosmic microwave background spectrum with which this document begins to the imprinted structural seeds with which it concludes—taking in the cryogenic interferometer and the improved angular resolution of the later mission along the way—Big Bang radiometry shows how a single spectrum, measured with increasing precision, ultimately reveals both the pure thermal nature of the early universe and the origin of all the structure that followed.

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