Physics: What We Do and Don’t Know


Steven Weinberg in the NYRB:

The modern era of scientific cosmology began forty-eight years ago, with the accidental discovery of the cosmic microwave background radiation. So much for the steady state cosmology—there was an early universe. This microwave radiation has been under intensive study since the mid-1960s, both from unmanned satellites in orbit and from large ground-based radio telescopes. Its present temperature is now known to be 2.725 degrees Centigrade above absolute zero. When this datum is used in calculations of the formation of the nuclei of atoms in the first three minutes after the start of the big bang, the predicted present abundance of light elements (isotopes of hydrogen, helium, and lithium) comes out pretty much in agreement with observation. (Heavier elements are known to be produced in stars.)

More important than the measurement of the precise value of the temperature is the discovery in 1977 that the temperature of the microwave radiation is not the same throughout the sky. There are small ripples in the temperature, fluctuations of about one part in a hundred thousand. This was not entirely a surprise. There would have to have been some such ripples, caused by small lumps in the matter of the early universe that are needed to serve as seeds for the later gravitational condensation of matter into galaxies.

These lumps and ripples are due to chaotic sound waves in the matter of the early universe. As long as the temperature of the universe remained higher than about 3,000 degrees, the electrons in this hot matter remained free, continually scattering radiation, so that the compression and rarefaction in the sound waves produced a corresponding variation in the intensity of radiation. We cannot directly see into this era, because the interaction of radiation and free electrons made the universe opaque, but when the universe cooled to 3,000 degrees the free electrons became locked in hydrogen atoms, and the universe became transparent. The radiation present at that time has survived, cooled by the subsequent expansion of the universe, but still bearing the imprint of the sound waves that filled the universe before it became transparent.

More here.