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The 2019 Nobel Prize in physics honored humankind's fascination with the starry sky and our progress in understanding the nature of the universe. Except for sex, the stars might be our longest-standing fascination. It arguably originated over a million years ago as the human brain rapidly increased in size prior to the evolution of Homo sapiens.

In these pages on Nov. 5, I discussed the award of half of that prize for leadership in discovering of the many exoplanets orbiting other stars in our Milky Way galaxy. Today, I describe the work of Princeton University astrophysicist James Peebles, who won the other half. Peebles laid the foundations of modern-day cosmology, the study of the large-scale structure, origin, evolution and fate of the universe.

Early in his career, in 1965, Peebles predicted the big bang origin of the universe should have left an afterglow, a prediction soon verified by the discovery of the cosmic microwave background, or CMB. The idea of the big bang originated with Edwin Hubble's observation in 1929 that the other galaxies are moving away from us in all directions. But do not suppose that this means our galaxy is at the center of the universe, because the same observation would be made in every other galaxy. The two trillion galaxies in the "observable universe" are all moving away from each other: The universe is expanding.

If one extrapolates this expansion backward in time, one concludes all the galaxies were together at some time in the past. Today's observations and theories peg that time at about 13.8 billion years. Peebles' theoretical predictions, and their subsequent verification by observation of the CMB cemented the big bang as the prevailing cosmological model.

Peebles and colleagues developed the concepts underlying our present understanding of the large-scale universe. He predicted the big bang's afterglow would today have a temperature of just a few degrees absolute, now measured at 2.73 Kelvins, or minus 455 degrees Fahrenheit. This radiation fills every nook and cranny in the universe. Stars and planets are small but important deviations from this norm. Without the large temperature differences between stars and the CMB, radiation would not stream from the stars and structures such as living organisms would be impossible.

Working from present knowledge of the CMB and the current expansion rate of the universe, Peebles' group predicted the early universe was made of 73 percent hydrogen and 27 percent helium, a figure later confirmed. He proposed that high temperatures implied the early universe consisted of bare hydrogen and helium nuclei, mixed with "free" (unattached to the nuclei) electrons. After 380,000 years, this cauldron of electrically-charged matter had expanded and cooled sufficiently to form ordinary hydrogen and helium atoms (nuclei with electrons orbiting them).

This "recombination" of nuclei and electrons to form the earliest atoms was a big deal. Prior to that time, photons (particles of radiation) could not travel through the highly charged, dense mix of material, because these photons were constantly interacting with that material. After recombination, the photons interacted only minimally with the electrically neutral atoms, so radiation could flow freely throughout the universe.

The same photons are still flowing. While moving, they are stretched to longer wavelengths by the expansion of the universe; this expansion literally stretches space itself, hence stretching everything in space, including photons. Having never interacted with anything these 13.8 billion years, these cosmic photons are nearly the most primitive structures in the universe. Some CMB photons fall on our planet, where they are easily detectable as a component of the visual interference seen on old pre-digital television sets.

These photons, released by the newborn universe, continue to provide us with our most detailed cosmological knowledge. Cosmologists such as Peebles predicted that the violent sloshing of nuclei and electrons prior to recombination would have stretched some regions and compressed others, and that these less dense (hence cooler) and more dense (hence warmer) regions would be detectable in the cosmic microwave background. By 1989, NASA launched its Cosmic Background Explorer satellite carrying a microwave detector that could tune in on details of these photons. The resulting gold mine of information revolutionized our view of the universe to include the dark matter comprising 27 percent of the universe, the dark energy comprising 68 percent, and much more.

It's been a while since early humans were inspired by the starry heavens. I am proud to belong to a species capable of such wonder and able to puzzle out some of the fantastic history of how it all came to be.

Commentary on 01/07/2020

Print Headline: Exploring the starry sky

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