Key Ideas

The Expansion of the Universe: The Hubble law describes the continuing expansion of space. On large scales, galaxies spread out in this cosmic flow, also called the Hubble flow.

• In the Hubble flow, galaxies are not actually moving through space, but the space between them is increasing as space expands.
• The redshifts that we see from distant galaxies are caused by this cosmic expansion and are called cosmological redshifts. These redshifts develop as photons travel through expanding space, getting stretched along their journey.
• The redshift of a distant galaxy is a measure of the relative size and age of the universe at the time the galaxy emitted its light.
• There is no center or edge of the known universe.

The Cosmological Principle: Cosmological theories are based on the idea that on large scales, the universe looks roughly the same at all locations and in every direction.

The Big Bang: The universe began with nearly infinite density and began its expansion in the event called the Big Bang, which can be described as the beginning of time and space.

• The observable universe extends about 14 billion light-years in every direction from Earth. We cannot see objects beyond this distance because light from these objects has not had enough time to reach us.
• During the first 10⁻⁴³ second after the Big Bang, the universe was too dense to be described by the known laws of physics.

Cosmic Background Radiation and the Evolution of the Universe: The cosmic microwave background radiation, corresponding to radiation from a blackbody at a temperature of nearly 3 K, is the greatly redshifted remnant of the hot universe as it existed about 380,000 years after the Big Bang.

• The background radiation was hotter and more intense in the past. During the first 380,000 years of the universe, radiation and matter formed an opaque plasma called the primordial fireball.
• Light from this early era gets scattered before it can carry images to us, but then there is an abrupt change. When the temperature of the radiation fell below 3000 K, protons and electrons could combine to form hydrogen atoms and the universe became transparent. We can look all the way back to this transition, which is what we see in images of the cosmic microwave background radiation.
• The universal abundance of helium is much more than stars can produce. Most helium was produced by thermonuclear reactions occurring throughout the universe during its first few minutes. It was the high temperatures required for these thermonuclear reactions that led to the prediction of the cosmic microwave background radiation, which was much hotter in the early universe.

The Geometry of the Universe: The curvature of the universe as a whole depends on how the combined average mass density ρ₀ compares to a critical density ρ_c. Due to the equivalence of mass and energy, ρ₀ also includes contributions from dark energy.

• If ρ₀ is greater than ρ_c, the density parameter Ω₀ has a value greater than 1, the universe is closed, and space is spherical (with positive curvature).
• If ρ₀ is less than ρ_c, the density parameter Ω₀ has a value less than 1, the universe is open, and space is hyperbolic (with negative curvature).
• If ρ₀ is equal to ρ_c, the density parameter Ω₀ is equal to 1 and space is flat (with zero curvature).

Cosmological Parameters and Dark Energy: Observations of temperature variations in the cosmic microwave background indicate that the universe is flat or nearly so, with a combined average mass density equal to the critical density. Observations of galaxy clusters suggest that the average density of matter in the universe is about 0.24 of the critical density. The remaining contribution to the average density is called dark energy. Therefore, about 0.76, or 76%, of the total energy content of the universe consists of some unknown entity.

• Measurements of Type Ia supernovae in distant galaxies show that the expansion of the universe is speeding up. This may be due to the presence of dark energy in the form of a cosmological constant, which provides a pressure that pushes the universe outward.

Cosmological Parameters and Primordial Sound Waves: Temperature variations in the cosmic background radiation are a record of sound waves in the early universe. Studying the character of these sound waves helps to determine that the universe is flat and other fundamental properties of the universe.