The American astronomer Edwin Hubble (November 20, 1889 – September 28, 1953) was the first scientists to prove that the universe is expanding. He showed that the other galaxies are moving away from the Milky Way at a speed directly proportionate to their distance from it, a theory referred to as Hubble’s Law today.
Hubble is also known for his system for classifying galaxies. The Hubble telescope was named after him, as well as the constant term in his equation linking the recession velocity of galaxies and their distance, known as Hubble’s constant.
Our modern perception of the universe dates back to 1924, when this prominent American astronomer showed that our galaxy is not the only one. In fact, his discovery revealed the existence of many more cosmic galaxies separated by vast space. To prove this, Hubble had to determine the distance to these other galaxies, which are so far away that they appear static to us. Thus, Hubble had to use indirect methods to measure the distances. He noticed that certain stars always had the same luminosity when they were close enough to be measured; he used this as a “standard candle” to determine distances to stars in other galaxies. In this way, Hubble measured the distances to nine different galaxies. Today, we know that our galaxy is just one in hundred billion galaxies that can be observed with modern telescopes, and each of these galaxies comprises hundred billion stars.
The Universe Is Expanding
We live in a slow-rotating galaxy that’s about one hundred thousand light years in diameter. The stars located in the galaxy’s spiral poles rotate around its axis once in hundred million years. Our sun is a sheer, average-sized, yellow star close to the inner edge of one of the spiral poles. A long time has passed since the time of Aristotle and Ptolemy, when it was believed that the Earth was the center of the universe!
In the years following the discovery of other galaxies, Hubble spends his time categorizing their distances and observing their spectrum. At the time, it was widely believed that galaxies move randomly in the universe. Thus, it was expected that the number of redshift galaxies will be the same to the number of blueshift ones. So, the discovery that most galaxies are redshifted, meaning they are moving away from us, came as a complete shock. What was even more astonishing was Hubble’s 1929 discovery that even the size of the redshift of galaxies is not arbitral, but proportional to the distance of the galaxy from the Earth. In other words, the further the galaxy, the faster it distances from the Earth. This means that the universe is not static as it was previously believed, but, that it’s expanding, and the distance between galaxies increases over time.
This discovery is considered one of the greatest scientific discoveries of the 20th century. From today’s viewpoint, it’s quite surprising why no one before realized that if the universe was static, it would eventually start shrinking due to gravity. On the other hand, the idea that the universe is expanding at a rate that defies gravity, it could go on infinitely.
This can be compared to launching a rocket from the Earth – if the velocity is low, gravity will eventually stop the rocket, and it’ll start falling down. But, if the velocity is higher than the critical point (approximately 12 km/s), gravity can’t pull the rocket down, meaning it’ll continue distancing from the Earth infinitely.
This could have been predicted by Newton’s theory of gravity in the 18th century, even at the end of the 17th century. However, the belief that the universe is static persevered until the beginning of the 20th century.