What is Hubble's Law?

Hubble's Law states that the speed at which a galaxy moves away is proportional to its distance from us.

Hubble's Law is a fundamental concept in cosmology, named after the American astronomer Edwin Hubble, who discovered it in 1929. According to this law, galaxies are receding from us at speeds that increase with their distance. This relationship can be expressed with the formula: \( v = H_0 \times d \), where \( v \) is the velocity at which a galaxy is moving away, \( H_0 \) is the Hubble constant, and \( d \) is the distance to the galaxy.

The Hubble constant (\( H_0 \)) is a crucial value in cosmology, representing the rate of expansion of the universe. Its units are typically kilometres per second per megaparsec (km/s/Mpc). A megaparsec is a unit of distance used in astronomy, equivalent to about 3.26 million light-years. The exact value of the Hubble constant is still a topic of research and debate, but it is generally accepted to be around 70 km/s/Mpc.

Hubble's discovery was groundbreaking because it provided strong evidence for the Big Bang theory, which posits that the universe began from an extremely hot and dense state and has been expanding ever since. Before Hubble's observations, many scientists believed the universe was static and unchanging. His work showed that the universe is dynamic and constantly evolving.

To measure the speed at which galaxies are moving away, astronomers use the redshift of light. When a galaxy moves away from us, the light it emits shifts towards the red end of the spectrum. This phenomenon, known as redshift, allows scientists to determine the velocity of a galaxy's recession. By measuring the redshift and the distance to various galaxies, Hubble was able to establish the proportional relationship that forms the basis of his law.

Understanding Hubble's Law helps us grasp the vastness of the universe and its continuous expansion, providing a window into the past and the future of cosmic evolution.