Einstein-Rosen bridge

This geometry would contain:

• A black hole.

• A white hole, which is a black hole running backward in time. It is essentially, the opposite of a black hole. It cannot be entered from the outside, however, matter and radiation can escape from it. However, these violate the 2nd law of thermodynamics.

• Two universes connected at their horizons by a wormhole. This is the Einstein-Rosen Bridge.

However, we do not observe these in nature, because, when stars collapse, they do not form a wormhole, they form a black hole. Even if one were found, it would be unstable and it would fly apart.

The solution actually has two parts:

• the black hole part in one universe where matter flows in...

• the white hole part in another universe where matter flows out...

The connection between our universe and this “mirror universe” is known as an Einstein-Rosen bridge. It is named after Albert Einstein and Nathan Rosen. The Einstein-Rosen bridge connects two different universes. However, Einstein believed that anyone would entered the bridge would be crushed and that there could be no communication between these two universes.

As mentioned previously, not only would there have to be a black hole with an event horizon, however, there would also necessitate the existence of a white hole (cannot be entered from the outside, however, matter and light can move out from it) interior. However, white holes violate the 2nd law of thermodynamics. Hypothetically, however, the presence of these two interior regions, would give way to two different exterior regions. These two exterior regions would be the different universes. 

To sum it up: the idea of the wormhole came from Albert Einstein and his collaborator Nathan Rosen (and will be later popularized by John Archibald Wheeler). What Einstein and Rosen speculated was that the interior of a black hole, could connect to some very far away point in spacetime. The wormhole would be a shortcut through spacetime, connecting two black holes that are billions of light years away from each other, at their horizons.