# How do we know the universe is expanding?

We have known for decades that the universe is in a process of accelerated expansion. This means that galaxies separate the ones from the others at an increasing speed. If this process continues indefinitely, a moment will arrive in which galaxies would separate the ones from the others so quickly (faster than the speed of light) that we will only be able to see the stars from our own galaxy, while the rest of the sky would complete darkness.

However, how can we be sure the universe is expanding?

#### The Doppler effect and the wavelength

The discovery for the expansion of the universe is normally attributed to the American astronomer Edwin Hubble. To understand how Hubble arrived at the conclusion that the universe is expanding, we need to know what is the wavelength.

Light propagates in the form of waves through space. Like any type of wave, they vary between a maximum point of energy, the peak, and one of minimum energy, named trough. The wavelength is the space there is between two peaks of a wave. Each type of light has a different wavelength: the more energetic, the smaller is its wavelength, and the less energetic, the bigger will its wavelength be.

As a beam of light propagates through space, its wavelength will be affected. If an object that emits light gets closer, its wavelength will get smaller (it is said that it tends to the blue, more energetic light). On the contrary, if an object emitting light gets away from us, its light will have a bigger wavelength (it is said that it tends to the red, a less energetic type of light). This effect is known as the Doppler shift, and it is the same as when we hear a police siren: when it gets closer we hear a louder sound, but when it moves away the sound weakens. In the following image, we can observe the changes in the wavelength of light emitted by moving objects:

#### Hubble’s observations

Using a telescope of Mount Wilson observatory, Hubble observed the spectra (from which we can deduce the wavelength of the observed light) of many stars and galaxies. He discovered that the light that arrived from all of them (except the ones that were found near the Milky Way) tended to the red, which means that they got away from us.

In addition, using a specific technique to be able to calculate the distance to these objects, he discovered that the majority of galaxies in the universe get away from us, and that the further away they are, the faster they do it. This process could be assimilated to some points (representing galaxies) drawn on the surface of a balloon. As the balloon inflates, the points separate the ones from the others at increasing rates. We can also observe that, the more a point is far away from another, the faster they separate. This phenomenon can be observed in the following image:

Hubble published his results in 1929. This demonstrated that the universe isn’t static (as it was thought at that time), but it was expanding, and it was doing it at an accelerated rate.

It is important to mention that this expansion process isn’t similar to a bomb that blows up (all the parts get away from the others from a common centre), but it is space itself that expands (which provokes the separation of galaxies).

#### The destiny of the universe

Nowadays, and keeping in mind the current knowledge about the universe, three theories exist that explain how could the death of the universe be. I have already talked about them in previous articles: the Big Crunch, the Big Freeze, and the Big Rip.

The two last theories seem to be the more likely to happen due to the accelerated expansion rate of the universe, which it is thought to be caused by the named dark energy, a type of exotic energy that exerts an opposite force to gravity, provoking the already known expansion of the cosmos.

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