A supernova would predictably dim over time whereas a white hole may not.
Not sure about a quasar, but the B.A. seems confident that they are different enough. (Or perhaps Occam’s Razor keeps the observation as a quasar.)
An exploding galaxy? Is there such a beast? I don’t see how.
http://www.leaderu.com/offices/billcraig/docs/kalam_davis.html
In short, Roger Penrose says that white holes cannot exist because they violate the Weyl Curvature Hypothesis. He also states this in his book “The Emperor’s New Mind”
Einstein’s theory of general relativity predicts that there is a singularity at the center of a black hole, but Einstein’s theory is incomplete. It does not include quantum mechanics, which makes a big difference at small distances (like a distance of zero, the length of a singularity). Modern physics, using string theory & quantum theories of gravity, predicts that the mass inside a black hole will collect in a small nugget with each dimension measuring the Planck length, 10^-33 cm. Modern physicists also theorize that the universe began as a Planck-sized nugget.
That’s not the interesting part. The interesting part is that black holes have entropy proportional to the area of the event horizon (the event horizon is the surface at the distance that is the closest you can get without being sucked in). This is called the Bekenstein bound, and you can read about it in Lee Smolin’s “Three Roads to Quantum Gravity” or other recent physics books. Entropy is a measure of information of a system because it represents the number of possible states that the system can be in. Black holes have to have entropy for the laws of thermodynamics to hold, so that when an object with entropy falls into a black hole, that entropy is not lost. Instead, the black hole grows and its entropy increases. The black hole can also give off radiation. Stephen Hawking figured this out so it’s called Hawking radiation. This returns the entropy & information from the black hole to the rest of the universe. How can this happen? Because of quantum mechanics. Particles can “borrow” energy to escape from the black hole as long as they pay it back later. This happens very slowly, though, so don’t expect to see a black hole evaporate any time soon. A black hole with the mass of the sun would take 10^57 years to evaporate.
White holes do not exist, but black holes aren’t completely black because they’re emitting Hawking radiation. This means they also have to have a temperature. When the black hole gets smaller, it gives off radiation faster. No one knows what happens once it gets to be teeny-tiny (Planck sized) because general relativity and quantum mechanics don’t mix so well. This this probably hasn’t happened in our universe because stuff tends to fall into black holes faster than it can spew it out.
By the way, when two black holes come together, they should just combine and form one bigger black hole.
<hijack> The Great Philosopher… welcome back. 
</hijack>
I answered this in an earlier thread, so I’ll just reprint what I said then.
Short answer: No.
“White” holes, as conceptualized in 1965 by the Soviet scientist Igor Novikov and the Israeli physicist Yuval Ne’eman, would be a time-reversed version of a black hole. As you recall, a black hole tends to suck up all the matter and energy around it, forever trapping it behind its event horizon, of which nothing can escape. A white hole, on the other hand, would continuously pour out energy and matter from its “antihorizon”, of which nothing can enter. The prevailing theory for the time was black holes and white holes were linked by what are called Kerr tunnels. If, say, you were to enter a black hole in one quadrant of the universe, and managed to survive the trip through the Kerr tunnel, you would exit via the white hole in some other part of the universe. Just imagine the travel opportunities!
Unfotunately, it was too good to last. A physicist named Douglas Eardley analyzed Novikov and Ne’eman’s models, and published his “White Holes Are Shy” Theorem in 1974. Eardley proved that white holes are inherently unstable. The matter and energy a white hole ejects would build up outside its antihorizon, unable to enter. After a short time, the white hole would be surrounded by a dense shield of energy, which Eardley called a “blue sheet”. This sheet causes the space-time region around the antihorizon to become sharply distorted. Eventually, the space becomes so warped that the former antihorizon becomes a true horizon, and from that point on prevents any more material from leaving the hole. Hence, the former white hole is now a black hole.
This conversion process happens fairly rapidly: A white hole of ten solar masses turns into a black hole in less than a thousandth of a second. A white hole of a million masses would take under two minutes.
Conclusion: If there ever was a white hole, it would have turned into a black hole long ago.
When two black holes stray too close they begin to emit ripples of spacetime curvature called gravitational waves. These waves push the holes closer together while causing them to spiral inward faster and faster. As they move closer and closer they radiate much of their mass away, half of it converted into gravitational waves while the rest is spent increasing the holes’ orbital speed. When the holes are traveling near the speed of light their horizons touch, temporarily forming a dumbell-shaped object. More ripples are emitted, and the shape coalesces into a single hole.