How many Borel sigma algebra are there on the real number set?

Question

I’m learning measure theory and a question came up and struck me. I know that Borel sigma algebra on the real number set is generated by a topology set of open intervals. My question is that is it required that topology must contains all of the open intervals in the real number set? Or any set of some open intervals can be used to generate the Borel sigma algebra on real number set? In other words, is THE Borel sigma algebra on real set unique?

Answer 1

First you learn what an open interval is, in say high school or possibly sooner. You define these as sets of the form $(a,b)$ where $a<b$. Then in a topology course you define what a $topology$ is.... namely a collection of objects with certain properties and we call these objects "open sets". It just happens that the way we define the Borel Sigma Algebra is that it is the topology that is generated by the open intervals (where "open" here refers, first and foremost, to your naive high school definition of open). Obviously, though, because you have defined your basis as these open intervals, it necessarily follows that they are open sets.... in THIS topology. You could have just as easily defined closed intervals as open sets, what sort of topology would this generate? Check for yourself. There is more than one way to generate the Borel sigma algebra but yes there really is only one.