Summary of Key Ideas

The Big Bang

• Astronomers believe that the universe began as an exceedingly dense cosmic singularity that expanded explosively in an event called the Big Bang. The Hubble law describes the ongoing expansion of the universe and the rate at which superclusters of galaxies move apart.
• The observable universe extends about 13.8 billion light-years in every direction from Earth to what is called the cosmic light horizon. We cannot see any objects that may exist beyond the cosmic light horizon because light from these objects has not had enough time to reach us.
• According to the theory of inflation, early in its existence, the universe expanded very rapidly for a short period, spreading matter that was originally far from our location (and hence at different temperatures and densities) throughout a volume of the universe so large that we cannot yet observe it. The observable universe today is thus a growing volume of space containing matter and radiation that was in close contact with our matter and radiation during the first instant after the Big Bang (and hence at the same temperature, pressure, and density). Inflation explains the isotropic and homogeneous appearance of the universe.

A Brief History of Spacetime, Matter, Energy, and Everything

• Four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—explain the interactions observed in the universe.
• According to current theory, all four forces were identical just after the Big Bang (although our understanding of the nature of matter and energy during this time is extremely limited). At the end of the Planck time (about 10⁻⁴³ seconds after the Big Bang), gravity became a separate force. A short time later, the strong nuclear force became a distinct force. A final separation created the electromagnetic force and the weak nuclear force.
• Observations show that the universe is flat and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe.
• In its first 30,000 years, the universe was radiation-dominated, during which time photons prevented matter from forming clumps. Then it was matter-dominated, during which time superclusters and smaller clumps of matter formed. Today it is dark-energy–dominated. Dark energy of some sort supplies a repulsive gravitational force that causes superclusters to accelerate away from each other.
• During the first 380,000 years of the universe, matter and energy formed an opaque plasma, called the primordial fireball. Cosmic microwave background radiation is the greatly redshifted remnant of the universe as it existed about 380,000 years after the Big Bang.
• About 380,000 years after the Big Bang, spacetime expansion caused the temperature of the universe to fall below 3000 K, allowing protons and electrons to combine and thereby form neutral hydrogen atoms. This period is called the era of recombination. The universe became transparent during the era of recombination, with the photons that existed back then still traveling through space today. In other words, the microwave background radiation is composed of the oldest photons in the universe.
• Clusters of galaxies and individual galaxies formed from pieces of enormous hydrogen and helium clouds, each of which became a separate supercluster of galaxies.
• All of the superclusters and some of the clusters of galaxies within each supercluster are moving away from one another.
• Supermassive black holes appear to have “seeded” the formation of most galaxies.
• During the matter-dominated era, structure formed in the universe. As the universe goes farther into the dark-energy–dominated era, the large-scale structure of superclusters of galaxies will fade away.

The Fate of the Universe

• The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe.
• The universe is accelerating outward and it will expand forever.