WHITE HOLES AND WORMHOLES
According to a list of frequently asked questions answered by Ted Bunn at a website of the University of Berkeley, white holes and wormholes can be concieved of and are mathematically possible, so it is possible that they could exist. I learned the following from the previously mentioned site. The equations of general relativity are symmetric in time. If one assumes that time moves backwards instead of forwards, on can get an equally valid solution that is exactly opposite of a black hole, called a white hole. This means that if time is reversed, a white hole will only spit things out, whereas a black hole will only take things in. Although this can be conceived of as possible, it is almost certain that these things do not exist, since it is impossible to produce. Producing a white hole is as impossible as destroying a black hole, because of time.
A wormhole exists when a black hole is connected to a corresponding white hole. It is theorized that the white hole may be in a different universe, which is defined as being a space-time that is completely disconnected from our region except by a wormhole. In theory, a wormhole could lead to anywhere- perhaps to another universe or perhaps in the past. However, it is almost certain that wormholes do not exist. Black holes that form from the collapse of ordinary matter do not form wormholes, and scientists do not think it is possible for any other type of black hole to exist. Also, if a wormhole formed, it would probably not be stable, and would collapse if anything tried to move through it. Even if it were stable, if one tried to pass through it one would be “fried” by the concentrated light waves of the stars and the cosmic microwave background.