Expansion of the universe

The expansion of the universe

For the past 5 billion years, the universe has been expanding at an increasing rate. This was first discovered in the 1920s by an astronomer called Edwin Hubble.
The other day, I read about the way Hubble managed to observe the expansion of the universe. I found it fascinating to realise that he was able to draw such a big and important conclusion with the use of simple and easy-to-understand physics.

How Hubble observed the expansion of the universe:

First step

Hubble had to prove that our galaxy isn’t the only one in the universe. At that time, people actually thought that the universe consisted only of the Milky Way (our galaxy). Hubble proved that this wasn’t the case by calculating the distance to variable stars in the universe and discovering that certain stars were more than 900,000 light years from Earth. This distance is of course way beyond our own galaxy. Hubble concluded that other galaxies had to exist.

Calculating the distance to other galaxies

Calculating the distance to stars within our galaxy is fairly simple. Because the earth orbits around the sun, by looking at a star today and then again 6 months from now, astronomers can detect a difference in the viewing angle for the star. With the use of a little bit of trigonometry, the distance from Earth to that star can be calculated. But, a problem occurs when stars are located further than 400 light years from us. They are so far away that, unlike nearby stars, they really do appear fix. In other words, a difference in the viewing angle for the stars can’t be detected.

To calculate the distance to stars in other galaxies, Hubble had to use a method known as brightness measurement. It can be explained this way:

The apparent brightness of a star is a term that describes how bright the star appears to a detector here on Earth. It depends on how much light the star radiates (its luminosity). But, the apparent brightness also depends on how far away the star is (the further, the dimmer it will look).
A simple formula can be imagined from the relationship between the apparent brightness, luminosity and distance of a star. As we already know how to calculate the distance to nearby stars, their luminosity can be worked out simply. Similarly, we could measure the distance to stars in other galaxies if we knew the luminosity of those stars.

This is exactly what Hubble figured out. Knowing that certain types of stars always have the same luminosity, he managed to find such stars in other galaxies. Hubble then used that knowledge to calculate the distance from Earth to those stars. In this way, he worked out the distance from Earth to nine galaxies.

Second step

Hubble not only calculated the distance from Earth to stars in other galaxies but he also spent his time analysing their color. He used a prism to break up the light emitted by stars into its component colors (its spectrum).

Now, you might wonder about the importance of knowing the spectra of stars. It turns out that it’s the only way for us to tell different stars apart. They are so far away that we can’t distinguish their size or shape. The light emitted by stars gives us information about their temperature and about the elements that are present in their atmosphere. That’s a lot of information!

How the spectra of stars showed the expansion of the universe

When Hubble started cataloguing the spectra of stars situated outside of our galaxy, he observed something strange. He found similar characteristic sets of missing colors as for stars in our own galaxy but, this time, they were all shifted toward the red end of the spectrum.

To understand the implications of Hubble’s discovery, it is first important to learn about a phenomenon called the Doppler effect. You might already know that light behaves like a wave (talked about in this post). The term ‘frequency’ describes the number of light waves per second and the different frequencies are what the human eye sees as different colors. The lowest frequencies appear at the red end of the spectrum and the highest frequencies appear at the blue end.

Now, imagine a star situated at a specific distance from you, emitting waves of light at a constant frequency. Suppose that the star suddenly starts moving towards you. The distance between you and that star is diminishing. This means that each light wave takes slightly less time to reach you than the previous wave. So, the number of waves you will receive each second (aka the frequency) will be higher. This corresponds to a shift toward the blue end of the star’s spectrum. Similarly, if the star suddenly moved away from you, the frequency of the waves that you would receive would be lower. This corresponds to a shift toward the red end of the star’s spectrum. The whole phenomenon is known as the Doppler effect.

So, Hubble found out that the stars and galaxies that he was observing were moving away from Earth. Even more surprising was the discovery that the size of a galaxy’s red shift wasn’t random. It was directly proportional to the galaxy’s distance from us. In other words, the further a galaxy is, the faster it is moving away from us.

Dark energy

We all know that gravity is present in the universe. So, how can the universe be expanding? Some other force must also be present in our universe. This force, named “dark energy”, has to counteract gravity and allow for the expansion of the universe. Exactly what this force is and how it operates is still poorly understood. It remains one of the greatest mysteries in science.


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