The universe is unimaginably gigantic. The observable universe is estimated to be around 93 billion light years across. It is impossible for the human mind to truly grasp just how vast the universe is, and interestingly, space is only getting bigger. One of the most fascinating facts about the cosmos is that space is expanding. How fast is space expanding and what is causing it to expand?
How Was The Expansion Of Space Discovered?
The astronomer Edwin Hubble was the first scientist to discover that the universe is expanding. Hubble set out to determine the distances to some galaxies in an effort to settle a debate in the astronomical community about whether or not the Milky Way was the only galaxy. The Hubble Telescope determined the distance to the Andromeda Galaxy, revealing that it existed far outside the boundaries of the Milky Way. Hubble had confirmed that the universe is home to many billions of galaxies, yet it was only part of his discovery. While observing distant galaxies, Hubble noticed that most galaxies are red, a phenomenon known as red-shifted. To understand why this is significant, it is crucial to understand the Doppler Effect. The Doppler Effect describes how sound waves and light waves behave depending upon the velocity of their source. When an object moves towards you, the light waves it emits are compressed and blue-shifted. When that same object moves away from you, its light waves are stretched and red-shifted. You will only notice a blue or red shift when an object is moving at extreme speeds. Since most of the galaxies Hubble could see were red-shifted, it meant that most of the galaxies in the observable universe are moving away from us at tremendous speeds. Hubble further uncovered that the speed at which a galaxy moves is proportionate to its distance. The further away a galaxy is, the faster it moves. This observation became known as Hubble’s Law.
How Fast Is Space Expanding?
Since the rate at which space expands is dependent upon distance, it’s difficult to determine a set rate for how fast space is expanding. Instead, it makes more sense to express the expansion rate relative to distance. Based on observations of multiple galaxies, astronomers estimate that the universe’s rate of expansion is 73 kilometers per second per megaparsec. A megaparsec is equal to 3.3 million light-years, meaning that for every 3.3 million light-years of distance, the expansion of space increases by 73 kilometers per second.
Interestingly, the rate at which space is expanding is accelerating. Not only is space getting bigger, but also the rate at which it’s getting bigger is also increasing. Astronomers once assumed that the rate of expansion would be gradually slowing down as the combined gravity from visible matter and dark matter would slow down the universe. However, in the 1990s, two groups of astronomers independently discovered that, rather than slowing down, space is speeding up. In order for space to accelerate, it must be saturated with a form of energy causing it to accelerate. This form of energy is called dark energy, and other than it being the cause of space accelerating, astronomers know virtually nothing about it.