
The Question
(Submitted March 20, 2007)
The acceptance of Big Bang theory assumes the universe is ultimately finite
and therefore measurable. But given the physical and temporal constraints
that govern all our activities, the observable and measurable (empiric)
universe may surely only represent a fraction of what exists across space/time.
How do we know where the universe begins and ends? And therefore  how can the
concept of an infinite universe (or even multiverse constructs) be ruled out?
The Answer
You are right that the observable universe may be a fraction of the universe.
The total energy and total volume of the universe may be infinite (we do not
know), but the energy density (energy per unit volume) of the universe is
finite, and this is a more important quantity for our understanding. Various
observations, such as the observed expansion of the universe, the nature of the
cosmic microwave background (CMB), etc. show that the universe started with the
Big Bang (at the beginning, the whole space and time was confined to a point,
and then it started expanding).You may see,
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html
http://en.wikipedia.org/wiki/Big_Bang
The multiverse theory is not ruled out, but it is still in the conceptual and
mathematical stage.
Hope this helps,
Sudip & Koji
for "Ask an Astrophysicist".
