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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,

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".

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