Expansion of the Universe and Redshift

The expansion of the universe refers to the increase in the distance between any two distant parts of the universe over time. It is an intrinsic expansion of space itself, rather than galaxies moving “through” pre-existing space. This phenomenon is a cornerstone of the Big Bang Theory and is supported by three primary pillars of observational evidence: redshift, the Hubble Constant, and the Cosmic Microwave Background radiation.

The Doppler Effect and Redshift

The Doppler Effect is the change in frequency of a wave in relation to an observer moving relative to the source of the wave. In astronomy, this principle is applied to light waves to determine the motion of celestial bodies.

• Redshift (Moving Away): When a light source moves away from an observer, the light waves are stretched, shifting them toward the longer, redder wavelengths of the electromagnetic spectrum.
• Cosmological Redshift: Unlike a standard Doppler shift caused by local motion, cosmological redshift is caused by the stretching of space itself while the light is in transit.
• Blueshift (Moving Toward): If an object moves toward the observer, the waves are compressed, shifting them toward shorter, bluer wavelengths. Most distant galaxies exhibit redshift, proving they are receding from Earth.

Hubble’s Law and the Hubble Constant

In 1929, Edwin Hubble observed that the redshift of galaxies is directly proportional to their distance from Earth. This relationship is formalized as Hubble’s Law.

• The Formula: v = H₀ ·d
  • v is the recession velocity of the galaxy.
  • d is the distance to the galaxy.
  • H₀ is the Hubble Constant.
• Significance of H₀: The Hubble Constant represents the current rate of expansion. It is measured in kilometers per second per megaparsec (km/s/Mpc).
• The Hubble Tension: There is a current discrepancy in physics known as the “Hubble Tension.” Measurements of the local universe (using Cepheid variables and Supernovae) yield a value of approximately 73 km/s/Mpc, while measurements of the early universe (using CMB data) yield approximately 67 km/s/Mpc. Resolving this is a major goal of modern cosmology.

Cosmic Microwave Background (CMB) Radiation

The CMB is the oldest light in the universe, dating back to approximately 380,000 years after the Big Bang. Before this time, the universe was a hot, dense plasma that trapped light.

• Recombination Era: As the universe expanded and cooled, protons and electrons combined to form neutral hydrogen atoms. This allowed photons to travel freely for the first time.
• Characteristics: The CMB is a nearly uniform “glow” in the microwave part of the spectrum. It has a temperature of approximately 2.725 Kelvin.
• Expansion Proof: The CMB photons have been redshifted from high-energy gamma rays/visible light to low-energy microwaves over billions of years due to the expansion of space.

Comparative Summary of Expansion Evidence

Expansion Milestones and Facts

• Age of the Universe: By inverting the Hubble Constant (1/H₀), scientists estimate the universe is approximately 13.8 billion years old.
• The Horizon Problem: The CMB is remarkably uniform in temperature across the sky. This led to the theory of Inflation, a period of extremely rapid expansion in the first fractions of a second after the Big Bang.
• Observable Universe: Because space can expand faster than the speed of light (a concept that does not violate Special Relativity since no “information” is traveling through space faster than c), there is a limit to how much of the universe we can ever see.
• The “Balloon” Analogy: A common way to visualize expansion is to imagine galaxies as dots on the surface of an inflating balloon. As the balloon grows, every dot moves away from every other dot, and those further apart move away faster.