The Big Bang theory is the widely accepted model explaining the universe’s origin and evolution over nearly 13.8 billion years. This model describes the universe originating from an extremely hot, dense state and continuously expanding and cooling ever since. The most compelling observational data supporting this narrative comes from the Doppler effect. This principle governs the behavior of all waves and provides the framework for interpreting the light received from distant galaxies. The shifting of this light offers a direct measurement of cosmic movement, establishing the expansion of the universe.
Understanding the Doppler Effect
The Doppler effect describes the change in wave frequency perceived by an observer when the source of the waves is in motion relative to that observer. It is a fundamental principle of physics that applies to any type of wave, including sound, water, and light. A common experience occurs when a vehicle, such as an ambulance or a train, passes by an observer at high speed.
As the source approaches, the sound waves it emits become compressed in front of the moving object. This compression results in a higher wave frequency, which an observer perceives as a higher pitch. Conversely, as the sound source moves away, the waves are stretched out, leading to a lower frequency and a lower pitch. This change is solely due to the relative motion between the source and the listener.
The degree of compression or stretching is directly related to the speed of the source relative to the observer. Faster relative motion produces a more pronounced change in the perceived frequency and wavelength. If the source and observer are stationary, no shift in frequency occurs. This relationship between motion and wave characteristics is the foundation for understanding how astronomers measure the movement of cosmic objects.
Applying Doppler to Light: Redshift and Blueshift
This principle of frequency and wavelength alteration applies to electromagnetic waves, or light, traveling through space. Visible light is only a small part of the electromagnetic spectrum, and different wavelengths correspond to different colors. Shorter wavelengths are perceived as blue or violet, while longer wavelengths appear red.
When a light-emitting object moves toward an observer, the light waves are compressed, shortening their wavelength. This shift toward the shorter, higher-frequency end of the visible spectrum is called a blueshift.
Conversely, when a light source is moving away, the light waves are stretched out. This stretching increases the wavelength, causing the light to shift toward the longer, red end of the spectrum. This is known as a redshift. The magnitude of this spectral shift can be precisely measured using spectroscopy, which allows scientists to determine the velocity of the light source relative to Earth.
Hubble’s Observation
The application of the Doppler effect to light became significant in the 1920s through the work of astronomer Edwin Hubble. Using the 100-inch Hooker telescope, Hubble observed and measured the light spectra of distant nebulae, which he demonstrated were galaxies far outside our own Milky Way. He sought to compare the measured redshift of these galaxies with their distance from Earth.
To determine the distance to these remote galaxies, Hubble relied on specific types of variable stars, such as Cepheid variables. Their intrinsic brightness is related to the period of their light variation. By comparing their known intrinsic brightness with their apparent brightness, he could calculate their distance. Simultaneously, he measured the spectral lines of the light from these galaxies, which consistently displayed a shift toward the red end of the spectrum.
The presence of this pervasive redshift indicated that almost all distant galaxies were moving away from the Milky Way. Hubble’s plot of galactic distance versus recessional velocity revealed a linear relationship. He found that the farther away a galaxy was, the greater its redshift, and the faster it appeared to be receding from us. This relationship, formalized as Hubble’s Law, demonstrated a uniform, large-scale expansion of the cosmos.
The Expanding Universe and Big Bang Confirmation
The ubiquitous redshift observed by Hubble implies that the space itself is expanding, carrying the galaxies along with it. This concept is often illustrated with the analogy of a loaf of raisin bread dough rising in an oven.
As the dough (representing space) expands, the raisins (representing galaxies) move farther apart. No single raisin is at the center of the expansion. An observer on any raisin would see all other raisins moving away from them, with the more distant ones receding faster. This illustrates that the cosmological redshift is a stretching of light waves as they travel through the expanding fabric of space, not merely a Doppler shift caused by motion through space.
The continuous expansion of the universe, evidenced by the pervasive redshift, provides the most direct support for the Big Bang theory. If the universe is currently expanding, tracing this motion backward in time must lead to a point where all matter and energy were concentrated together. Projecting the expansion back yields a state of infinite density and temperature—the singularity—which represents the initial condition of the universe. Thus, the spectral shift of light from distant galaxies serves as the primary observational evidence for the dynamic, evolving nature of the cosmos.