In the nineteenth century, astronomers discovered that studying the light emitted from a star, a study known as spectrum analysis, revealed what elements stars are made of.
Changes in a star’s spectrum indicate the movement of the star, and analysis could determine the star’s speed and direction.
It was revealed that stars only appear fixed in the sky because of their great distance from Earth. They are actually moving at tremendous speeds.
At the same time that Edwin Hubble was fixing the galaxies’ location, the American astronomer Vesto Melvin Slipher was using spectrum analysis to determine their velocities.
He measured 41 galaxies and found their speed was as staggering as their distance. Their average speed was 375 miles per second, and all of them were moving away from Earth.
Hubble took the experiment one step further. He measured the distance to 24 of the galaxies Slipher had measured and noticed something strange.
The farther away the galaxy, the faster it was moving, and the galaxies were moving away from each other at a rate constant to the distance between them. He saw only one explanation: the universe was expanding.
Over the years, Hubble confirmed his findings by measuring 150 galactic velocities.
The finding became known as Hubble’s law. He also realized that with such distances involved and the time it takes light to travel, observers on Earth are also looking far back in time.
For example, light from the Sun takes about eight minutes to reach Earth, so we see the Sun as it appeared only eight minutes before.
However, light from the farthest visible stars, 12 billion light-years away, has taken 12 billion years to reach Earth.
We see these stars as they appeared 12 billion years ago, or around the time of the birth of the universe.
Many years before Hubble’s findings, the physicist Albert Einstein had theorized that the universe is expanding.