Here's why:
* Redshift: When light is emitted from an object moving away from an observer, its wavelength gets stretched, shifting towards the red end of the electromagnetic spectrum. This is called redshift.
* Hubble's Observation: Hubble noticed that the light from distant galaxies was redshifted, and the farther the galaxy was, the greater the redshift. This indicated that these galaxies were moving away from Earth.
* Expanding Universe: Hubble interpreted this redshift as evidence for an expanding universe. He proposed that the universe was not static, but rather expanding outwards, causing the galaxies to move away from each other.
This discovery was revolutionary and formed the basis of the Big Bang theory, which states that the universe began in a hot, dense state and has been expanding ever since.
