The redshift of light from galaxies is one of the key pieces of evidence supporting the Big Bang theory. It indicates that the universe is expanding. The concept of redshift arises from the Doppler effect, which is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.
Here's how it works in the context of the Big Bang theory and cosmology:
1. Hubble's Law: In 1929, astronomer Edwin Hubble observed that light from distant galaxies is systematically shifted to longer (redder) wavelengths, and this redshift increases in proportion to the distance of the galaxy from us. This relationship is now known as Hubble's Law, which states that the velocity at which a galaxy is receding from us is proportional to its distance.
2. Expansion of Space: The redshift is interpreted as being due to the expansion of space itself. As space expands, it stretches the wavelength of light traveling through it, effectively reddening the light. If we go back in time, this implies that the universe was once more compact, with galaxies closer together.
3. Big Bang and Cosmic Evolution: The observation of redshift consistent with the expansion of the universe provides strong evidence for the Big Bang theory. The Big Bang theory posits that the universe began as an extremely hot, dense point approximately 13.8 billion years ago and has been expanding and cooling ever since, leading to the formation of galaxies, stars, and planets.
4. Cosmic Microwave Background Radiation: The discovery of the Cosmic Microwave Background (CMB) radiation, which is a very faint but uniform microwaves coming from all directions, also supports the redshift evidence of an expanding universe. The CMB is thought to be the remnant radiation from the very hot, dense state of the early universe, redshifted to microwave frequencies by the expansion of the universe.
5. Observational Consistency: The pattern of redshift observed across countless galaxies is consistent with predictions made by General Relativity for an expanding universe. Additionally, the observed abundances of the lightest elements (hydrogen, helium, and lithium) agree with theoretical predictions of Big Bang Nucleosynthesis, which describes the production of these elements in the early universe.
The combined observational evidence of galaxy redshifts, along with the Cosmic Microwave Background and Big Bang Nucleosynthesis, provides a robust empirical basis for the Big Bang theory as the prevailing cosmological model explaining the origin and evolution of the universe.