Key Ideas and Terms

15-1 The darkness of the night sky tells us about the nature of the universe

• Cosmology is the science concerned with the structure and evolution of the universe.
• Olbers’s paradox is that the night sky is actually dark, although there are countless stars in the sky in every direction.

15-2 Our observations show us that the universe is expanding

• Galactic redshift, z, is wavelength difference (Δλ), divided by the unshifted wavelength (λ₀).
• Hubble found that the more distant a galaxy, the greater its redshift and the more rapidly it is receding from us.
• The Hubble law states that the greater the cosmological redshift of a distant galaxy, the greater is its distance.
• The Hubble constant, about 73 km s⁻¹ Mpc⁻¹, is the rate at which the universe is expanding. It helps estimate the age of the universe.
• Every point in the universe appears to be at the center of the expansion, and thus it follows that our universe has no center at all.
• The cosmological principle assumes that the universe is homogeneous and isotropic.

15-3 The expanding universe emerged from a cataclysmic event called the Big Bang

• The beginning of the expanding universe is named the Big Bang.
• Calculating how long it has taken galaxies to get to their present position yields an age of the universe of nearly 14 billion years.

15-4 The microwave radiation that fills all space is compelling evidence of a hot Big Bang

• There is far more helium in the universe than could have been created by hydrogen fusion in stars alone.
• Observed low-temperature radiation that fills all of space from the Big Bang is called the cosmic background radiation.
• Recent satellite missions have confirmed the lumpiness of the cosmic microwave background, which helps explain the current structure of the universe.

15-5 The universe was a rapidly expanding, hot, opaque plasma during its first 300,000 years and has slowly cooled

• Radiation and matter are both important constituents of the universe.
• Charged electrons and protons existed as a plasma until 380,000 years after the Big Bang, when electrons recombined to form atoms in the era of recombination.
• Inflation during a short inflationary epoch accounts for the relative isotropy of the microwave background.
• The four fundamental forces—gravitation, electromagnetism, and the strong and weak forces—explain the interactions of everything in the universe and are joined in grand unified theories (or GUTs) and the theory of everything (or TOE).

15-6 The shape of the universe indicates its matter and energy content

• Possible geometric shapes of the universe are flat, closed, and open.
• The existence of some type of dark energy is needed to reconcile the measured geometry of space with the total amount of matter in the universe.

15-7 Observations of distant supernovae indicate that we live in an accelerating universe

• The universe is expanding in such a way suggesting we live in a dark-energy-dominated universe.