The Little Book of Cosmology by Lyman Page is a compact and delightful summary of recent human progress in understanding the Universe. Much of the story is written in the Cosmic Microwave Background (CMB), the leftover glow in the sky from the era when everything was incredibly hot and dense – blackbody radiation now redshifted by expansion to a chilly 2.725 K, barely above Absolute Zero.

But the CMB isn't quite uniform, and therein lies great knowledge. The Little Book explains how the tiny ~0.003% temperature fluctuations between patches of sky in different directions are important clues about the birth of the cosmos. Bottom line: astronomical observations of the past few decades have led to powerful models of the Universe that are testable and, so far, seem about right. The world, writ large, depends on only half a dozen key parameters:

• Atoms – the ~5% fraction of the net cosmic density that is ordinary material (like what we're made of)
• Dark Matter – another ~25%, invisible stuff (maybe new fundamental particles) yet to be directly observed but necessary to hold galaxies together as well as to explain ripples in the CMB in various directions
• Dark Energy – the "Cosmological Constant", the remaining ~70%, responsible for driving the accelerating expansion of the Universe
• Optical Depth – the fraction (5%-8% or so) of CMB photons that have been scattered on their way to us, due to reionized gas created by early stars and galaxies
• Amplitude of Primordial Density Fluctuations – how massive the local "seeds" were in the earliest moments of the Universe, regions that grew into galaxies, clusters of galaxies, and the vast voids between
• Angular Scale of Primordial Density Fluctuations – the relative numbers of "seeds" of various sizes

So strange, wonderful, and knowable – this marvelous Universe within which we live!

_{(cf Cherished Beliefs (2000-04-19), Universal Knowns (2002-06-13), ...) - ^z - 2023-03-31}