I tried to go to the Physics meta site and was greeted with:
Not sure what is happening, or how long it has happened/will continue...
It looks like it happened to another main beta site as well:
which again came back quickly based on the comment.
Both sites are back working now though, but since it happened more than once, there may be something happening on the backend that should be investigated.
And at 9am Eastern time (10 July 2019), the main physics site also had the problem: