Hubble's law

Hubble's law, also known as the Hubble–Lemaître law, is a fundamental observation in physical cosmology. It describes how galaxies are moving away from Earth at speeds that are proportional to their distance. In simpler terms, the farther away a galaxy is, the faster it appears to recede from us. This phenomenon provides the first observational basis for the expansion of the universe. This movement of astronomical objects due solely to the expansion of space is called the Hubble flow. It serves as a primary piece of evidence for the Big Bang model.

To understand how scientists determine these speeds, we must look at the behavior of light. Astronomers measure a galaxy's recessional velocity by looking at its redshift. Redshift is a shift in the frequency of light emitted by a galaxy. When an object moves away, the light waves it emits are stretched. This stretching changes the light toward the red end of the spectrum. By measuring this fractional shift compared to a stationary reference, scientists can calculate how fast the object is moving.

Determining the distance to these galaxies is a separate but equally vital step. Edwin Hubble used Cepheid variable stars to calculate these distances. These are specific types of stars that change in brightness. By observing them in what were then called "spiral nebulae," Hubble found they were actually located far outside the Milky Way galaxy. He combined these distance measurements with the redshift velocities measured by Vesto Slipher and Milton Humason. This allowed him to see a clear relationship between how far a galaxy is and how fast it moves.

The relationship is expressed through the Hubble constant, which is the constant of proportionality in the equation. The formula relates the speed of separation to the proper distance of a galaxy. The units for this constant are typically expressed in km/s/Mpc, which stands for kilometers per second per megaparsec. While it is called a "constant," it is actually a Hubble parameter that can vary over time. One way to state this rate is as a relative expansion rate. For example, at the current rate, an unbound structure grows by 7% every billion years.

The discovery of this expansion was a gradual process involving many researchers. In 1912, Vesto Slipher measured the first Doppler shift in a spiral nebula and found most were receding. In 1922, Alexander Friedmann used Einstein's field equations of general relativity to show the universe might expand. In 1927, Georges Lemaître independently derived the law and estimated the expansion ratio. Although Hubble's 1929 work is most famous, Lemaître had published the idea earlier in French.

This discovery fundamentally changed our view of the cosmos and corrected previous scientific errors. Before these observations, many scientists believed in a static universe that did not change size. Even Albert Einstein originally included a "cosmological constant" in his equations to force a static solution. After Hubble's work proved the universe was expanding, Einstein called his assumption a "greatest mistake." He visited Mount Wilson Observatory in 1931 to thank Hubble for providing the evidence needed for modern cosmology.

Today, Hubble's law connects to much larger questions about the nature of space and time. The reciprocal of the Hubble constant is known as the Hubble time, which is approximately 14.4 billion years. Current evidence suggests that the expansion of the universe is actually accelerating. This means that for any given galaxy, the recession velocity is increasing over time as it moves further away. The study of this expansion continues to help scientists understand the history and the ultimate fate of the entire universe.