The size of the Universe

I’m quoted today in article in New Scientist, “Universe Weighs in Surprisingly Light”. I spoke to the author, Zeeya Merali, last week about a recent article by Hans Fahr and Martin Heyl, in which they define a “radius of the universe” and therefore the amount of mass inside that radius. Because their calculation is done in the context of Einstein’s theory of General Relativity, we have to be very careful when we define any coordinate system — and that’s just what a definition of “radius” is.

Unfortunately, as far as I can tell, their definition seems to be purely an artifact of the way they’ve decided to write down their coordinates. This is in contrast to a more standard definition — such as the so-called “particle horizon“, which is the distance a photon could travel from the big bang until some particular time. But their definition has the interesting property that it is proportional to (and just a little smaller than) the radius of a black hole that has the same density as the average density of the Universe at a particular time.

A subscription is required to get the whole article, but I suppose I can reproduce my own words:

Stating the obvious: “Our universe is supposed to be infinite and expanding.”

A meagre attempt at cross-disciplinary profundity: “What’s interesting is that they have managed to reproduce this physical correspondence with black holes just by starting from an essentially philosophical question: what does it even mean to ask about the mass of the universe?”

And back to the obvious (or is it profound?): “This is just one way to define the mass of the universe…. There’s room for different definitions — and they may not give the same answer.”