[Math] Are all sets in sigma-algebra measurable

elementary-set-theorymeasure-theory

In the Wikipedia article it says:

the collection of those subsets for which a given measure is defined
is necessarily a $\sigma$-algebra.

Fine, but is the opposite true? Do we know for sure that all sets of sigma algebra are measurable? If the answer is no, then is it the reason why Borel sigma algebra is so widely used in probability theory?

Best Answer

Here is one answer, which may answer the question as I understand it (but perhaps I do not understand the question correctly).

Start with a measure $\mu$ defined on a $\sigma$-algebra $\mathcal A$. Then define the outer measure $\mu^*$ associated with $\mu$, in the sense of Caratheodory. Once you have this outer measure, you can define the class $\mathcal A(\mu^*)$ of all $\mu^*$-measurable sets (still in the sense of Caratheodory).

Then, it is part of the Caratheodory extension theorem that $\mathcal A\subseteq \mathcal A(\mu^*)$. So, in this sense, the answer to the question is "Yes".

On the other hand, it is not necessarily tru that all measurable sets belong to the original $\sigma$-algebra $\mathcal A$...