# What does tilde denote here?

I am reading the paper https://arxiv.org/abs/1808.07032 - "Statistical mechanics of two dimensional black hole".

And here sometimes $$\widetilde{AdS}_2$$ and $$\widetilde{\text{SL}}(2, \mathbb{R})$$ are accompanied with a tilde.

Seems like this kind of notation has certain purpose, to distinguish from $$AdS_2$$ and $$\text{SL}(2, \mathbb{R})$$ in more common sense. However, I cannot infer what is meant from the paper?

Sorry, for a stupid question, but I really would like to make clear this point

If you scroll down to the appendix, that paper uses the notation from this review paper, which in turn defines $$\widetilde{\mathrm G}$$ to be the universal cover of $$\mathrm G$$.