I am unaware of any new advancements in physics that suggest that time travel to the past is possible. And other than time dilation, the same goes for time travel forwards. Also, there is absolutely no evidence that multiverses exist let alone whether we can travel to them.
These things aren't possible simply because everything we have observed tells us that this does not happen. We cannot explain why it isn't possible, only tell you that it is not the way we have observed the universe to work. Anyone claiming that either of these things is possible would need to provide evidence for this exact phenomena.