A black hole (Credit: NASA/CXC/M.Weiss)
At the core of a black hole is believed to be an infinite density called a singularity. Black holes are believed to have a tremendous gravitational attraction, but no observable surface features in themselves (although we can see their nearby effects). A black hole is believed to be a very simple object that can be completely described by only a few numbers, one of which is its mass.
But yesterday physicist Laura Mersini-Houghton published a scientific paper (not yet peer-reviewed) claiming to show that black holes don't actually exist. According to her calculations, when the most massive type of star is undergoing gravitational collapse, it shrinks to a very small size, but then quantum mechanical effects start to dominate, preventing the star from collapsing to become an infinitely dense singularity.
I gather that by claiming that black holes don't really exist, the idea is not that black holes are a complete illusion, but that they are simply things that don't really have an infinitely dense singularity, as astrophysicists assumed. We know from astronomical observations that the cores of many galaxies have objects that are something like black holes, regardless of whether they have an infinitely dense singularity.
I am a bit skeptical about this “black holes don't exist” claim, partially because Mersini-Houghton is the same scientist who made some previous cosmological claims that I found to be unwarranted. But let's ignore that, and assume that Mersini-Houghton may be right about this matter. What are the implications if it turns out that black holes don't really exist?
I can think of two big implications. The first implication is that the nonexistence of black holes would completely destroy Lee Smolin's theory of cosmological natural selection. That is the theory that attempts to account for fine-tuning in our universe by imagining that our universe is the product of a “cosmic natural selection” process that supposedly occurs because black holes collapse to become new universes.
The theory of cosmological natural selection is truly a “flight of fancy” which is 99% wild speculation and 1% fact. We have no reason for believing that a black hole collapse would form another universe. As the physicist Leonard Susskind has pointed out, the theory of cosmological natural selection violates a central finding about black holes, which is that information cannot be transferred from a black hole. As Susskind put it in his “Final Letter” in the Smolin/Susskind debate (The Universe page 198):
No information about the parent can survive the infinitely violent singularity at the center of a black hole. If such a thing as a baby universe makes any sense at all, the baby will have no special resemblance to the mother. Given that, the idea of an evolutionary history that led by natural selection to our universe makes no sense.
As mentioned here, the theory of cosmological natural selection also has been stymied by observations that contradict its predictions. If it is true that black holes do not even exist, that would be the final nail in the coffin of the theory of cosmological natural selection (because black holes are a crucial pillar of the theory).
If no black holes exist, it could also have implications relating to the Big Bang. Contrary to what a few people have suggested, if black holes are ruled out, it would not endanger the Big Bang theory. The Big Bang theory was introduced long before the idea of black holes came into prominence, and the Big Bang theory has no dependence on the existence of black holes formed from stars. The central reason for believing in the Big Bang is the fact that the universe is expanding; "run the film backward" on such an expansion, and you are forced to begin with something like the Big Bang.
The only relation to black holes and the Big Bang theory is that the event described by the Big Bang theory is rather like a black hole collapse in reverse (although vastly larger). Scientists say that the Big Bang was an expansion of the universe from an infinitely dense singularity. So it was a little like the collapse of a star into an infinitely dense singularity (a black hole), except the opposite, and involving incomparably more matter.
This similarity doesn't really do much of anything to make the Big Bang seem less astonishing. But at least it gives scientists some thread of similarity they can use to compare the Big Bang to a natural event. It's a very, very thin thread of similarity, because saying that the Big Bang is like a black hole collapse in reverse is kind of like saying, “No big deal,” after seeing a three-egg omelet jump back into three egg shells, on the basis that it's just the breaking of the eggs in reverse.
But what if there are no black holes? Then even this slim thread of similarity to a natural event is eliminated, and we are left with a Big Bang that would be absolutely unlike any event in nature, going forward or going in reverse. That would make the Big Bang seem all the more miraculous.