3.3 Observational evidence supporting the Big Bang model

The Big Bang model is supported by compelling observational evidence. From the cosmic microwave background radiation to the expansion of the universe, these findings paint a picture of a universe that began in a hot, dense state and has been expanding ever since.

Hubble's law, galaxy distribution, and large-scale structures further bolster the Big Bang theory. These observations provide crucial insights into the universe's age, expansion rate, and overall structure, challenging alternative cosmological models and reinforcing our understanding of cosmic evolution.

Observational Evidence for the Big Bang Model

Cosmic microwave background radiation

• Discovered by Arno Penzias and Robert Wilson in 1965 as a uniform background radiation filling the entire universe with microwave wavelengths corresponding to a temperature of about 2.7 K
• Remnant heat from the early stages of the universe after the Big Bang that cooled as it expanded, shifting the radiation to longer wavelengths (microwaves)
• Provides strong support for the hot, dense early state of the universe predicted by the Big Bang theory
• Nearly isotropic and homogeneous, consistent with predictions of the Big Bang model (COBE, WMAP, Planck satellites)
• Existence cannot be explained by alternative cosmological models (steady-state, plasma cosmology)

Expansion of the universe

• Galaxies moving away from each other, with more distant galaxies receding faster (Hubble's observations)
• Redshift of light from distant galaxies indicates their recessional velocity (Doppler effect)
• Hubble's law: relationship between a galaxy's distance ($d$) and its recessional velocity ($v$) expressed as $v = H_0 \times d$, where $H_0$ is the Hubble constant
• Natural consequence of the Big Bang model, extrapolating the expansion backwards in time implies a single origin point
• Alternative models, such as the steady-state theory, cannot explain the observed expansion (Tolman surface brightness test)

Hubble-Lemaître law in cosmology

• Mathematical description of the relationship between a galaxy's distance and its recessional velocity, expressed as $v = H_0 \times d$
• $H_0$ represents the current expansion rate of the universe, measured in km/s/Mpc (estimated around 70 km/s/Mpc)
• Reciprocal of $H_0$ provides an estimate of the age of the universe (about 13.8 billion years)
• Value of $H_0$ influences predictions about the ultimate fate of the universe:
  1. Continued expansion (open universe, $\Omega < 1$)
  2. Contraction (closed universe, $\Omega > 1$)
  3. "Flat" universe ($\Omega = 1$)

Galaxy distribution and structures

• Galaxies not randomly distributed but form clusters (Virgo, Coma), superclusters (Shapley, Hercules), and filaments
• Voids: Large regions of space containing few or no galaxies (Boötes void)
• Galaxy surveys and redshift surveys map the 3D distribution of galaxies using their positions and redshifts (2dF Galaxy Redshift Survey, Sloan Digital Sky Survey)
• Observed large-scale structure consistent with predictions of the Big Bang theory and the growth of structure through gravitational instability
• "End of greatness": Beyond about 1 billion light-years, the universe appears homogeneous and isotropic, as predicted by the Big Bang model
• Alternative models (steady-state theory) struggle to explain the observed large-scale structure and the "end of greatness"