# The Copernican principle and the multiverse

Thinking about this leaves my mind in a bit of a twist, but it is worth exploring.

I am still reading Brian Greene’s The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos: a great book (just enough maths in the footnotes to make me feel I haven’t completely lost touch yet with a clear narrative in plain English in the body).

In the book there is, understandably enough, a fair bit of discussion of the “cosmological constant” – an anti-gravitational force that is powering the universe’s expansion.

It turns out that this force is just about the right value to allow galaxies to form (if it were too high then gravity would not be able to overcome it, if it were too low then gravity might just throw everything into one lump or a black hole). Without galaxies, goes the reasoning (after Steven Weinberg), there would be no life – as galaxies allow the mixing of various elements (eg everything on the Earth that comes higher in the periodic table than iron was manufactured in a supernova, while everything that is heavier than helium surely got here in the same explosive way – we are not so much what stars are made of as opposed to being made of stars.)

But there are about $10^{124}$ different values of the cosmological constant that could have a measurable effect on our universe’s physical laws, argues Brian Greene and essentially demands that, via the Copernican Principle (that humans are not at the centre of the universe) that requires there to be approximately (in fact, rather more) that number of universes out there to show that our universe, with its physical laws (or, more accurately, its physical constants – the laws being immutable) is just another typical drop off point.

And, happily for Greene, he points out that string theory allows for about $10^{500}$ universes, so it is perfectly possible for this one, with its particular cosmological constant, to be just typical.

But, while I understand this argument and, of course, it has a beauty and is perhaps the ultimate vindication of Doctor Copernicus, it also seems to me to be flawed. There seems to me to be no need to demand these additional universes. Because we can only observe the universe we are in. Were there to be only one universe (I know that term is technically a tautology, but I hope you understand the point) and it had different physical characteristics we simply would not be around to see it.

The fact that our universe has a particular set of characteristics and we can see it seems to me to prove or demand nothing very much (ie., I am not making some argument in favour of a “grand designer” either) – other than we have “won” a physical lottery. We exist because of the physical characteristics of the universe, not the other way round, which it seems to me is quite close to what Greene demands.