String Theory: Problems with Wormholes
The problem with using wormholes to travel in space or time is that they are inherently unstable. When a particle enters a wormhole, it creates fluctuations that cause the structure to collapse in upon itself. There are theories that a wormhole could be held open by some form of negative energy, which represents a case where the energy density (energy per volume) of space is actually negative.
Under these theories, if a sufficient quantity of negative energy could be employed, it might continue to hold the wormhole open while objects pass through it. This would be an absolute necessity for any of the previously discussed theories that allow a wormhole to become a time portal, but scientists lack a real understanding of how to get enough negative energy together, and most think it’s an impossible task.
In some models, it may be possible to relate dark energy and negative energy (both exhibit a form of repulsive gravity, even though dark energy is a positive energy), but these models are highly contrived. The good news (if you see possible time travel as good news) is that our universe appears to have dark energy in abundance, although the problem is that it looks like it’s evenly distributed throughout the universe.
Trying to find any way to store negative energy and use it to sustain a wormhole’s stability is far beyond current technology (if it’s even possible at all). String theory can provide potential sources of negative energy, but even in these cases, there’s no guarantee stable wormholes can occur.