All about the Universe

The Hubble Deep Field
Credit: NASA

The Universe is thought to have started with what astronomers call a Big Bang. This was a huge explosion which took place almost 15 billion years ago and produced everything we see today – but this bit took time. The first stars were not born until about 400 million years after the explosion when the temperatures had cooled down a bit and the atoms had all been created. We still don’t know what caused this explosion, or what was there before – though most astronomers say that there was nothing before the Big Bang. If you could run time backwards you would see all of the galaxies and stars moving towards each other back to a single point – where the Big Bang started. This could be thought of as the centre of the Universe – but we don’t know where it is! From our position in the Universe everything looks like it’s moving away from us, but this would be the same no matter where we lived in the Universe. We sometimes say that we live in the centre of the ‘observable Universe’, because we can’t move somewhere else to see if things are different. Right now astronomers think that our position in the Universe isn’t special and that everything would look the same no matter where we lived.

We think that the Universe is still being pushed apart now by the energy from this explosion and everything is moving away from each other. We don’t think that a new big bang could ever happen. For another big bang to happen everything would need to move closer and closer together until everything in the Universe was in the same place and all merged together.

We know that the Universe is getting bigger, it’s expanding. So we think – what is it expanding into? This is an impossible question to answer. The short answer is nothing. The Universe is everything we know exists and so it can’t expand into anything – everything is within it. But – this isn’t something we can easily picture because here on Earth we are always surrounded by something.

From everything astronomers have learned over the centuries, we only know that there is one Universe – the one we live in – which contains all the stars and planets and galaxies which we can see. We still don’t understand everything about our Universe though and there are some really big questions – like where is all the material? The stuff we can see only makes up a small amount of all the things in the Universe – we know there is more stuff we can’t see. That’s because we can see the effect it has. For example we can see light moving around objects which aren’t there. There are some scientists who believe that there may be more Universes and that we are only one. Some of these Universes could be tiny. Some of them might explain the questions we still have about our own. But we haven’t seen any of these yet. We have no proof that they exist.

Between the galaxies, in the space which is expanding there is basically nothing. Most of space is empty. It is empty even of small particles like we have in the air. There is no air in space. Because there are no particles, no air, sound can’t travel in space. Space is silent. Sound waves need a material to move through, like air or water, because this doesn’t exist in space sound can’t travel. If you left the Milky Way you’d be in this empty space, for a long time. It would take 2.5 million years, even travelling as fast as the speed of light, to get to the next galaxy, called Andromeda.

Space is REALLY big. Astronomers think that the Universe is about 93 billion light years across. This means if you started on one side of the Universe it would take 93 billion years, even travelling as fast as the speed of light, to get to the other side. So space is really big, but it’s not infinite.

We don’t know how the Universe will end. There are a few different ideas. One idea is called the Big Freeze - that the Universe will get too big. This means that all of the material to make new stars will be too far away from each other and nothing new can be made. So everything already alive would slowly die and there would be nothing left.