Asked by: Todd Andresen

So the basic answer is that we can't really measure the absolute size of the universe (much of which may even be outside our current horizon) but we can solve for the size relative to the size today.

As for black holes, their size is determined in a completely different way. By the 'size' of a black hole we typically mean the Schwarzschild radius (or 'event horizon' as people like to say). This radius is determined solely by the mass of the black hole and is given by:

We can then measure the size of the black hole by measuring it's gravitational attraction with other objects near it (like stars) -- and hence its mass.

Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley

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