21.7 Cosmic Redshift: Evidence for an Expanding Universe

21.7 Cosmic Redshift: Evidence for an Expanding Universe

Cosmic redshift refers to the effect in which light coming from distant galaxies across the universe is stretched toward the red end of the spectrum. This happens because the universe is expanding, causing galaxies to move away from us. As a result, the wavelength of light emitted by the galaxies increases. The greater the distance of a galaxy, the larger its redshift, i.e.; it is moving away faster. By studying cosmic redshift, scientists can measure how fast the universe is expanding. The observed change in the wavelength of light is directly related to the relative velocity between the source and the observer.

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When the observed change in wavelength $\Delta\lambda$ is positive, it indicates a redshift ($Z$). This illustrates that the celestial object is moving away from Earth. On the other hand, a negative $\Delta\lambda$, represents a blue shift, showing that the object is approaching the Earth. Astronomers study the changes in the colour of light from stars and galaxies to understand how fast and in which direction they are moving. The light from faraway galaxies appears more red than expected (redshift phenomenon). This happens because space itself is stretching as the universe expands, which also stretches the light waves and makes their wavelengths longer. The redshift pattern is observed in all directions which indicates the expansion of the universe. It is not just that galaxies are moving apart through space, rather, the space between them is increasing. These findings strongly support the Big Bang theory, which suggests that the universe originated from an extremely hot and dense state and is growing ever since. The relationship between redshift and distance enables scientists to determine the size of the universe and how it has changed over time.