Cosmology seeks to explain the universe’s origin, evolution, and ultimate fate. The prevailing scientific explanation is the Big Bang theory, which posits that the cosmos began in an extremely hot, dense state and has been growing ever since. A theory describing all of space and time requires powerful, repeatable observations to support its claims. The most compelling observational data that underpins this model comes from analyzing the light of distant celestial objects, which provides a concrete measurement of the cosmos’s dynamic nature.
Understanding the Redshift Phenomenon
Redshift is a phenomenon where light waves emitted by an object are stretched as they travel through space. The electromagnetic spectrum ranges from high-energy gamma rays to low-energy radio waves. Visible light spans from the short-wavelength violet end to the long-wavelength red end. When a light-emitting object moves away from an observer, its waves are elongated, causing them to shift toward the red, or longer, end of the spectrum.
This behavior is analogous to the Doppler effect observed with sound waves, such as the changing pitch of a siren. When a sound source approaches, waves are compressed (higher pitch); when it recedes, waves are stretched (lower pitch). Light behaves similarly: a source moving toward an observer experiences a “blueshift” (waves compressed toward violet), while an object moving away causes a redshift (measurable lengthening of the wavelength).
The degree of shift is directly proportional to the speed at which the light source moves away. Scientists use a spectrometer to analyze light from stars and galaxies. They compare the unique pattern of absorption lines—dark lines corresponding to elements absorbing specific wavelengths—to those measured in a laboratory on Earth. If the pattern of lines is displaced toward the red end, it confirms that the source is receding. The redshift observed in deep space is accurately described as a cosmological redshift, distinct from motion through space.
The Discovery of Cosmic Expansion
Applying redshift measurements to distant celestial bodies led directly to the realization that the universe is not static. American astronomer Edwin Hubble pioneered this work in the 1920s by meticulously measuring the light from “extragalactic nebulae.” He used observations of variable stars, specifically Cepheid variables, to accurately determine the distance to these faint smudges of light. By the end of the decade, he correlated the measured distance of these galaxies with their corresponding redshift.
Hubble’s foundational finding was that nearly all distant galaxies exhibited redshift, confirming they were moving away from the Milky Way. He discovered a linear relationship between a galaxy’s distance and its recessional velocity, formalized as Hubble’s Law. This law states that the farther away a galaxy is, the faster it appears to be moving away from us. This consistent, uniform recession of distant galaxies in every direction means the entire structure of space is expanding. The observation does not suggest that our galaxy is at a unique center, but rather that space itself is stretching everywhere at once.
Redshift as Evidence for the Big Bang Model
The observation of cosmic expansion, confirmed by widespread galactic redshift, provides a powerful framework for the Big Bang model. If all galaxies are currently moving away from each other, reversing the timeline necessitates they must have been closer together in the past. Tracing this expansion backward in time, using the measured rate of recession, leads inevitably to a moment when all matter and energy were compressed into an extremely hot, dense state, a singularity. This initial state and subsequent rapid expansion is the core premise of the Big Bang theory.
The cosmological redshift acts as a direct, measurable scale factor for the universe’s growth. As light travels across the vast expanse of space, the expansion of the universe literally stretches the light’s wavelength. The more distant an object is, the longer its light has been traveling, and the more the intervening space has expanded, resulting in a greater observed redshift. This quantifiable stretching allows cosmologists to determine the age and expansion history of the universe with precision.
This evidence strongly supported the Big Bang theory over rival concepts, such as the Steady State model. The Steady State model proposed an eternally unchanging universe that simply created new matter as it expanded. However, the clear observation that the cosmos was once smaller and denser, shown by light from highly-redshifted galaxies, cannot be reconciled with a static model. Redshift provides a quantifiable signature of the universe’s dynamic evolution from an initial, hot, concentrated state.