An amazing chain of events was unleashed by the big bang, culminating some 13 billion years later in molecules, life, planets, and everything we see around us. Running the expansion of our universe in reverse, back to the big bang, we can be confident there was a time when it was so hot that the universe was just a soup of the elementary particles. Researchers are beginning to speculate about even earlier times when particles did not even exist and our universe was a quantum mechanical soup of strange forms of energy in a bizarre world of fluctuating geometry and unknown symmetries and even an unknown number of spatial dimensions.
The journey to the universe we know today is depicted in Figure 1.1.1. It began at the end of inflation, when vacuum energy and quantum fuzziness became a slightly lumpy soup of quarks, leptons, and other elementary particles. Ten microseconds later quarks formed into neutrons and protons. Minutes later the cooling fireball cooked the familiar lighter elements of deuterium, helium, helium-3, and lithium (the rest of the periodic table of chemical elements was to be produced in stars a few billion years later). Atoms, with their electrons bound to nuclei, came into existence only a half million years or so later. The cosmic microwave background is a messenger from that era when atoms were formed. Along the way, dark matter particles and neutrinos escaped annihilation because of the weakness of their interactions, and for that reason they are still here today.
The slight lumpiness of the dark matter— a legacy of the quantum fuzziness that characterized inflation—triggered the beginning of the formation of the structure that we see today. Starting some 30,000 years after the beginning, the action of gravity slowly, but relentlessly, amplified the primeval lumpiness in the dark matter. This amplification culminated in the formation of the first stars when the universe was 30 million years old, the first galaxies when the universe was a few hundred million years old, and the first clusters of galaxies when the universe was a few billion years old. As the dark matter clumped, the ordinary matter followed, clumping because of the larger gravitational pull of the more massive dark matter. Ordinary matter would get the final word, as its atomic interactions would eventually allow it to sink deeper and form objects made primarily of atoms—stars and planets—leaving dark matter to dominate the scene in galaxies and larger objects.
This gulf of time between the decoupling of matter and radiation and the formation of the first stars is aptly referred to as the “dark ages.” Mountain-top observatories on Earth and the Hubble Space Telescope reveal evidence of the