How big is the universe? - Skywatching

This might sound like the ultimate unanswerable question, but thanks to astronomical knowledge accumulated over many centuries, and the power of modern astronomical instruments, we can make a guess. To do this we have three main tools.

The first is an effective means to measure how far away things are. There are stars such as Cepheid variables, and some classes of supernova explosions where we can deduce how bright they are, then measure how bright they look, and so calculate how far away they are. These techniques have enabled us to measure the distances of the most distant galaxies our telescopes can detect.  

The second tool is the expansion of the universe. As the universe expands it carries everything away with it. The further things are away, the faster objects are receding from us. Hubble and other astronomers established the relationship between distance and the rate things are moving away.  

With this information we can track the galaxies back in time and find that about 13.8 billion years ago, everything in the universe that we can see was all concentrated in one lump, which started to expand, a moment we call the Big Bang.

The third tool is the speed of light. Light moves very quickly, just under 300,000 kilometres a second. That is, 300 metres in a millionth of a second. In our everyday lives this is far too fast for us to notice, but over cosmic distances it becomes very important. In fact, one of the standard units of cosmic distance is the "light year", the distance light travels in a year, which is a tiny bit less than 10,000,000,000,000 kilometres. When we look at Sirius, that bright star shining in the southern sky these evenings, we see it as it was about 8 years ago. So, as we look further and further away, we are looking further and further back in time. If we observe something nearly 13.8 billion light years away, we are seeing it as it was 13.8 billion years ago, as it was at the beginning of the universe. 

That means, from us to those objects at the beginning of the universe lies a distance of 13.8 billion light years, and that when that light set off on its journey to us, the edge of our observable universe lay 13.8 billion light years from us.

However, the universe is expanding. Just after the Big Bang, it expanded faster than light for a little while, and then it slowed down. Since then the expansion has been gradually accelerating. That means that today, what was 13.8 billion light years away 13.8 billion years ago is now closer to 46.5 billion light years from us. The expansion also tells us something about the shape of the universe. 

When we look at the expansion, we see exactly the same relationship between expansion rate and distance no matter what direction we are looking. There are only possible answers to this. One is that we are in the exalted position of sitting right in the middle of the universe, and that everything is moving away from us. A more reasonable possibility is that our situation is no different from any other astronomer out there, and that no matter what star they orbit or galaxy they live in, they are seeing exactly the same thing. This is what ants on an expanding balloon would see. 

Every ant would see the other ants being carried away at a rate related to how far away they are. In their case they are, as far as they can see, on a two-dimensional surface expanding in a third direction. In our case, we think our universe is a three-dimensional thing expanding in a fourth dimension. This would indicate that our universe is a sort of four-dimensional balloon with a diameter of around 93 billion light years, with us wandering around on its three-dimensional "surface." 

It also means we will never find the edge of the universe, but if we travel far enough we will wind up back where we started.

• Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 
• The moon will be full on the 23rd.