Hubble's law
Galaxies recede faster the farther they are from Earth.
Hubble's law, officially the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. It is considered the first observational basis for the expansion of the universe and one of the pieces of evidence most often cited in support of the Big Bang model.
- field
- Physical cosmology
- known_for
- Hubble's law (Hubble–Lemaître law), the linear relationship between galaxy recessional velocity and distance
- key_contributors
- Edwin Hubble, Georges Lemaître, Alexander Friedmann, Vesto Slipher, Henrietta Swan Leavitt
Lore & Background
The discovery of Hubble's law is attributed to work published by Edwin Hubble in 1929, but the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. The Friedmann equations showed the universe might be expanding, and presented the expansion speed if that were the case. In 1927, Georges Lemaître concluded that the universe might be expanding by noting the proportionality of the recessional velocity of distant bodies to their respective distances. He estimated a value for this ratio, which—after Hubble confirmed cosmic expansion and determined a more precise value for it two years later—became known as the Hubble constant.
Reader's Guide
Hubble's law is foundational to modern cosmology, providing the first observational evidence that the universe is expanding. The law is described by the equation v = H0D, where H0 is the Hubble constant, currently valued at about 70 km/s/Mpc. The reciprocal of H0 is the Hubble time (14.4 billion years). The law's discovery resolved the Shapley–Curtis debate over the size of the universe and led Albert Einstein to abandon his cosmological constant, calling it his 'greatest mistake.' The Hubble constant remains a subject of active measurement and debate, with different methods yielding slightly different values. The law also underpins the Big Bang model and the concept of the Hubble flow, the motion of astronomical objects due solely to cosmic expansion.
Did You Know?
- Hubble's law is also known as the Hubble–Lemaître law.
- The Hubble constant is currently about 70 km/s/Mpc, meaning a galaxy 1 megaparsec away recedes at 70 km/s.
- The reciprocal of the Hubble constant is the Hubble time, approximately 14.4 billion years.
Frequently Asked Questions
What is Hubble's law?
Hubble's law (also called the Hubble–Lemaître law) states that galaxies drift away from Earth at speeds that scale linearly with their distance. Put simply, the farther a galaxy sits, the faster it appears to recede.
Who figured out Hubble's law?
Edwin Hubble is most commonly credited with the discovery, but the result built on the work of Georges Lemaître, Alexander Friedmann, Vesto Slipher, and Henrietta Swan Leavitt. Slipher's redshift measurements and Leavitt's period-luminosity relation were the key observational pieces that made the velocity-distance link possible.
Why does Hubble's law matter?
It supplied the first direct observational evidence that the universe is expanding, becoming a cornerstone of the Big Bang model. Without that linear velocity-distance relationship, the case for cosmic expansion would lack its most frequently cited pillar.
Is Hubble's law the same as the Hubble–Lemaître law?
Yes, the two names refer to the identical relationship between a galaxy's recessional speed and its distance. The combined label acknowledges Georges Lemaître's earlier theoretical groundwork alongside Hubble's observational confirmation.
What field does Hubble's law belong to?
It sits squarely within physical cosmology, the branch of astronomy concerned with the universe's large-scale structure and evolution. Its linear velocity-distance finding remains one of the most often-cited pieces of evidence supporting the Big Bang framework.
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