Strong, weak and electromagnetic interaction are evidently part of a grand unified theory. These temperatures are today quite inaccessible. They were… - Sheldon Glashow

The color gauge theory postulates the existence of eight massless particles, sometimes called gluons, that are the carriers of the strong force just as the photon is the carrier of the electromagnetic force.

... If a distant galaxy is moving relative to us, its entire spectrum is Doppler-shifted in frequency. Its spectral lines are displaced relative to those of stationary light sources. Thanks to this effect, we know that distant galaxies recede from the solar system at speeds proportional to their distances from us. That's the effect that told us of the expanding universe, and of its birth, long ago, in the Big Bang.

All kinds of questions remain. Many have to do with cosmology. How did the universe originate? How did the galaxies become distributed in space like the suds in the kitchen sink, as one of my colleagues has described it? Why is the cosmological constant apparently very tiny but non-zero and has a peculiar value that leads the universe to expand more rapidly?