3.3 Observational evidence supporting the Big Bang model

Observational Evidence for the Big Bang Model

The Big Bang model rests on multiple independent lines of observational evidence. Each one points to the same conclusion: the universe began in a hot, dense state roughly 13.8 billion years ago and has been expanding ever since. The fact that these very different observations all converge on the same picture is what makes the Big Bang model so robust.

Cosmic Microwave Background Radiation

The cosmic microwave background (CMB) is the afterglow of the early universe. About 380,000 years after the Big Bang, the universe cooled enough for atoms to form, allowing photons to travel freely for the first time. Those photons have been streaming through space ever since, stretching to longer wavelengths as the universe expanded. Today they show up as microwave radiation corresponding to a temperature of about 2.7 K.

Arno Penzias and Robert Wilson discovered the CMB in 1965, initially mistaking it for instrument noise. Their detection earned a Nobel Prize because it confirmed a key prediction of the Big Bang model: if the universe started hot and dense, there should be leftover thermal radiation everywhere you look.

• The CMB is nearly isotropic (the same in every direction), which matches the Big Bang prediction of a uniform early universe.
• Tiny temperature fluctuations (on the order of 1 part in 100,000) were mapped by successive satellite missions: COBE (1989), WMAP (2001), and Planck (2009). These fluctuations correspond to slight density variations that eventually grew into galaxies and large-scale structures.
• Alternative models like the steady-state theory and plasma cosmology have no natural mechanism to produce this uniform background radiation, making the CMB one of the strongest pieces of evidence against them.

Expansion of the Universe

In the late 1920s, Edwin Hubble observed that light from distant galaxies is systematically redshifted, meaning its wavelength is stretched toward the red end of the spectrum. This redshift indicates that galaxies are moving away from us, and the farther away a galaxy is, the faster it recedes.

This pattern is exactly what you'd expect if space itself is expanding. Think of it this way: if you run the expansion backward in time, everything converges toward a single, extremely dense point. That's the core logic behind the Big Bang model.

• Redshift works through the same principle as the Doppler effect for sound. As a galaxy moves away, the light waves it emits get stretched to longer (redder) wavelengths.
• The relationship between distance and recession speed is captured by Hubble's law: $v = H_0 \times d$, where $v$ is the galaxy's recessional velocity, $d$ is its distance, and $H_0$ is the Hubble constant.
• The Tolman surface brightness test provides additional support. In an expanding universe, the surface brightness of distant galaxies dims in a specific, predictable way. Observations match the expanding-universe prediction, not the steady-state prediction.

Hubble-Lemaître Law in Cosmology

The Hubble-Lemaître law (renamed by the IAU in 2018 to credit Georges Lemaître's earlier contribution) formalizes the expansion relationship:

$v = H_0 \times d$

$H_0$ (the Hubble constant) represents the current expansion rate of the universe, measured in km/s/Mpc. Current estimates place it around 67–73 km/s/Mpc, depending on the measurement method. This range reflects an ongoing debate in cosmology known as the "Hubble tension."

The reciprocal of $H_0$ gives a rough estimate of the universe's age. With $H_0 \approx 70$ km/s/Mpc, you get an age of approximately 13.8 billion years (refined by CMB data and other constraints).

The value of $H_0$ also connects to the density parameter $\Omega$, which influences the universe's ultimate fate:

1. Open universe ($\Omega < 1$): Expansion continues forever, gradually slowing but never stopping.
2. Closed universe ($\Omega > 1$): Gravity eventually reverses the expansion, leading to contraction.
3. Flat universe ($\Omega = 1$): Expansion slows asymptotically toward zero. Current observations strongly favor a flat or very-near-flat geometry.

Galaxy Distribution and Large-Scale Structure

Galaxies are not scattered randomly through space. They organize into a vast web-like pattern that the Big Bang model successfully predicts.

• Clusters are gravitationally bound groups of hundreds to thousands of galaxies (e.g., the Virgo Cluster, the Coma Cluster).
• Superclusters are even larger groupings of clusters (e.g., the Shapley Supercluster, the Hercules Supercluster).
• Filaments are thread-like chains of galaxies stretching hundreds of millions of light-years, forming the boundaries of enormous voids, regions nearly empty of galaxies. The Boötes Void, for example, spans roughly 330 million light-years.

This structure grew from the tiny density fluctuations visible in the CMB. Slightly denser regions attracted more matter through gravitational instability, eventually forming the galaxies, clusters, and filaments we observe today.

Large galaxy surveys like the Sloan Digital Sky Survey (SDSS) and the 2dF Galaxy Redshift Survey have mapped millions of galaxy positions and redshifts, building detailed 3D maps of this cosmic web.

One striking feature is the "end of greatness": beyond scales of about 1 billion light-years, the universe's structure smooths out and appears homogeneous and isotropic. This matches the cosmological principle assumed by the Big Bang model. Steady-state and other alternative models struggle to explain both the intricate web-like structure at smaller scales and the smooth uniformity at the largest scales.