## Critical density

There is something about night time and astronomy that seems to give people an excuse to have pretty random conversations about the big questions. We shouldn't need the excuse but having one is good. Also, if you're like me, you may find yourself trying to work out something crazy such as the average density of the Sun from your own back garden without knowing how far away it is or how much mass it has. I have a thing for crazy numbers.

Recently, someone I know was preparing a talk about dark energy and was trying to think of ways to make the huge/tiny numbers involved in cosmology and particle physics easier for people to understand. This is a common problem in physics because not many people truly comprehend how much, say, 6,000,000,000,000,000,000,000,000 kg actually is (that is about the mass of the Earth in case you were wondering). For this talk the aim was to put the critical density of the Universe in context.

The critical density is the average density that everything - light, matter, dark matter, dark energy and tax returns - would have to be for the Universe to be mathematically 'flat' rather than 'open' or 'closed'. In standard models of the Universe, it turns out that this density is very small indeed. It is equivalent to about 0.000,000,000,000,000,000,000,000,000,01 grams per cubic centimetre; an incredibly tiny number! In slightly more manageable terms, it is equivalent to about 0.01 grams - about the same mass as a grain of sand - spread over the volume of the planet Earth.

Until now, I don't think I had fully realised quite how sparse the Universe really is.