Physics: What We Do and Don’t Know

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Robert Williams/Hubble Deep Field Team
A view of about a quarter of the ‘Hubble Deep Field,’ a tiny patch of sky about one thirtieth the diameter of the full moon, photographed in December 1995 by the Hubble Space Telescope with an exposure time of ten days, showing the ‘deepest-ever’ view of the universe. The galaxies in this picture are so far away that their light has been traveling to us for most of the history of the universe.

In the past fifty years two large branches of physical science have each made a historic transition. I recall both cosmology and elementary particle physics in the early 1960s as cacophonies of competing conjectures. By now in each case we have a widely accepted theory, known as a “standard model.”

Cosmology and elementary particle physics span a range from the largest to the smallest distances about which we have any reliable knowledge. The cosmologist looks out to a cosmic horizon, the farthest distance light could have traveled since the universe became transparent to light over ten billion years ago, while the elementary particle physicist explores distances much smaller than an atomic nucleus. Yet our standard models really work—they allow us to make numerical predictions of high precision, which turn out to agree with observation.

Up to a point the stories of cosmology and particle physics can be told separately. In the end, though, they will come together.

1.

Scientific cosmology got its start in the 1920s. It was discovered then that little clouds always visible at fixed positions among the stars are actually distant galaxies like our own Milky Way, each containing many billions of stars. Then it was found that these galaxies are all rushing away from us and from each other. For decades cosmological research consisted almost entirely of an attempt to pin down the rate of expansion of the universe, and to measure how it may be changing.

Oddly, little attention was given to an obvious conclusion: if the galaxies are rushing apart, there would have been a time in the past when they were all crunched together. From the measured expansion rate, one could conclude that this time was some billions of years ago. Calculations in the late 1940s showed that the early universe must have been very hot, or else all the hydrogen in the universe (by far the most common element now) would have combined into heavier elements. The hot matter would have radiated light, light that would have survived to the present as a faint static of microwave radiation cooled by the expansion of the universe to a present temperature of a few degrees above absolute zero.1

No search was carried on then for this leftover cosmic microwave background radiation, and the prediction was largely forgotten. For a while some theorists even speculated that the universe is in a steady state, always looking pretty…


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