Question

I'm watching "Horizon's How big is the Universe?". They said the observable universe is 46bn light years to the edge (they showed a sphere with a radius from the centre as they said it) so I assume they'd mean from the centre to any part of the edge. But if the universe is 13.9bn light years old and nothing goes faster than light... How can it be that big? Surely the max diameter is approx 28bn? Thanks!