The Big Bang Theory (BBT) is the prevailing scientific model describing the origin and evolution of the observable universe. This cosmological framework posits that the universe originated from an extremely hot, dense state and has been continuously expanding and cooling over the last 13.8 billion years. The model describes the evolution of the cosmos immediately following that initial state, not the moment of creation itself. Scientists support the BBT based on a collection of three independent and specific observational lines of evidence that align precisely with the model’s predictions.
Observing Universal Expansion
The first foundational evidence supporting a non-static universe came from observations of distant galaxies. In the 1920s, astronomer Edwin Hubble analyzed the light emitted by these cosmic structures, utilizing the Doppler effect to determine their movement relative to Earth.
When an object moves away from an observer, its light waves are stretched, shifting their wavelength toward the red end of the spectrum—a process called cosmological redshift. Conversely, light from objects moving toward us shifts to shorter, bluer wavelengths. Hubble’s analysis revealed that almost every galaxy exhibited a significant redshift, indicating they are all moving away from us.
Hubble correlated a galaxy’s distance with the degree of its redshift. He found a proportionality: the farther away a galaxy is, the faster it is receding, a relationship now codified as Hubble’s Law. This finding is interpreted as evidence that space itself is uniformly expanding, carrying all galaxies along with it, rather than Earth being at a special center.
The expansion is similar to points on the surface of an inflating balloon, where every point moves away from every other point. Tracing this motion backward in time requires that all matter must have been compacted into a much smaller volume in the past. This observation of universal expansion provided confirmation that the universe is dynamic and evolving, contradicting the previously held idea of a static cosmos.
The Cosmic Microwave Background
The second line of evidence is the existence of the Cosmic Microwave Background (CMB) radiation. The BBT predicts the universe was once an extremely hot, dense, opaque plasma where photons constantly scattered off free electrons and protons. This state persisted until the universe cooled sufficiently, about 380,000 years after the Big Bang, when the temperature dropped to 3,000 Kelvin.
At this temperature, electrons and protons combined to form the first stable, neutral atoms, primarily hydrogen. This event, known as recombination or decoupling, allowed photons to travel freely through space as the plasma fog cleared. These photons, released at the “surface of last scattering,” represent the thermal “afterglow” from the early hot universe.
As the universe expanded, the wavelengths of these photons were stretched dramatically, shifting them from visible light into the microwave region of the spectrum. The CMB was accidentally discovered in 1964 by Arno Penzias and Robert Wilson, who detected a persistent, uniform background noise. This noise corresponded to a temperature of 2.725 Kelvin, exactly matching the BBT prediction for the relic radiation today.
Initial measurements showed the CMB was remarkably uniform across the sky, conforming to a perfect blackbody spectrum. Since the universe contains structure (galaxies and clusters), the BBT predicted the CMB should exhibit minute temperature fluctuations, known as anisotropies. These anisotropies represent the initial density variations that served as the seeds for later structure. Satellite missions like COBE and WMAP successfully mapped these tiny variations, finding temperature differences of only about one part in 100,000.
Predicted Abundance of Light Elements
The third major pillar of evidence is the prediction of the cosmic abundance of the lightest chemical elements. The BBT includes a specific period known as Big Bang Nucleosynthesis (BBN), which occurred between one second and twenty minutes after the initial expansion began. During this brief window, the universe had cooled enough for nuclear fusion to occur and be sustained.
The BBN model predicts the universe’s initial composition would be almost entirely hydrogen and helium, plus trace amounts of other light isotopes. Specifically, the model predicts that approximately 75% of baryonic matter should be Hydrogen and about 25% should be Helium-4 by mass. It also predicts the formation of small quantities of Deuterium and minute amounts of Lithium-7.
These predicted ratios depend on the density of protons and neutrons present at that time. Observations of the oldest, most pristine regions of the universe, where stellar fusion has not altered the chemical composition, consistently confirm these predictions. For instance, the observed cosmic abundance of Helium-4 is consistently found to be around 25%, a figure that cannot be explained by stellar processes alone.
The isotope Deuterium is easily destroyed inside stars, meaning nearly all Deuterium observed must be primordial, formed during BBN. The measured amounts of primordial Deuterium and Lithium-7 match the BBN model’s quantitative predictions. This agreement between theoretical calculations and astronomical observations solidifies the BBT as an accurate description of the universe’s evolution.