How big is the universe?

Have you ever wondered just how big the entire Universe could be? Before we attempt to answer this I should give you a few definitions and put some things into context.

Measuring the Universe

Firstly, a measure of distance used by astronomers is called a Light Year – LY. This is the distance travelled by light in one year in the vacuum of space. In miles that is 5.9 trillion and with all those zeros it is easier to write 1 LY.

To put this into perspective, our Sun is only 0.000016025 light years away from earth. Remarkably close!

The next star to the Sun is Proxima Centauri and this is 4.37 light years away.

We exist in a Galaxy of hundreds of billions of stars and this is known as the Milky Way. How far is it to the next spiral Galaxy? It is 2.5 million Light Years away.


The Universe- IT’S BIG!

The Universe is made up of galaxies spread out through the vastness of the cosmos. Is the Universe – simply all that we can see? No, it certainly goes beyond that. But how big?

The first challenge in answering ‘how big’ is that the universe is expanding. The second is that light takes time to travel across space. Therefore it is difficult to have a single meaning of ‘distance’. Imagine you are swimming in a pool from one end to the next and the pool is expanding as you do so. The distance at the beginning is different to that when you finish. If at the start it is ¼ mile long and it normally takes you 10 minutes to do that distance what if when you arrive it has taken 15 minutes and you swam at the same speed, how long is the pool?

So astronomers focus on what is called redshift- the term can be understood literally – the wavelength of the light is stretched, so the light is seen as ‘shifted’ towards the red part of the spectrum. This can be observed as a result of the universe expanding. The greater the redshift the longer the light from the galaxy has taken to reach us. The amount of redshift is denoted by a factor called ‘Z’.

If we take an example of a galaxy which has a redshift of 7.5 – this equates to the light having taken 13 billion light years to reach us. So does that make the galaxy 13 billion light years away? Unfortunately it is not so simple and the answer is ‘no’ we cannot say it is that far away.

13 billion years ago the universe was much smaller than it is now. Therefore something with a redshift of 7.5 means that when the light left the galaxy, 13 billion years ago, the distance was less than 4 billion light years. As the universe has been expanding all this time then how far away is that galaxy now? The answer is almost 30 billion light years away.

So how far away is that galaxy? 4 billion LY? 13 billion LY? 29 billion LY?

We can say it is 29 billion LY away and this is what we mean when we say how far is something in the universe now- it is known as the comoving distance.

What is the most distant thing we can observe and how far away is it? This turns out to be radiation from the Big Bang, known as the Cosmic Microwave Background (CMB) radiation. This has a redshift of 1000. The comoving distance of the CMB is 46 billion light years. Therefore a radius of 46 billion LY’s must give a diameter of at least 2X46 billion light years or 92 billion light years.

So we can say that the universe is at least 92 billion light years across. But we do not know the true size as there is nothing we can measure beyond the CMB radiation.

Maybe the universe expanded at a faster rate than the speed of light in the first few seconds of its existence, allowing it to be even larger than we could imagine. Maybe it is infinite? Maybe there are multiple universes intwined and crossing dimensions? Maybe we will never know?

Hope you weren’t expecting a number! But you can say, it’s big!