Curvity and Black Holes
If enough mass was packed into a small enough area, gravity within would eventually go negative. In other words, as more and more matter was added to a star, gravity would increase to a point but after that, any additional matter would make gravity go negative and the star would explode. In Curvity, the universe has a safety valve because instead of black holes, there exist supernovas.
Black holes are important to Curvity because it is one way that Curvity can be falsified. If black holes really exist then Curvity is false.
The reason black holes are thought to exist is because Relativity predicts that they do and there is lots of observational evidence to support it. One example is the M87 giant elliptical galaxy from the Messier catalog. When the red and blue shift of light leaving the center of the disk is measured, they get speeds of 550km/s which suggests a huge gravitational field which is larger than can be accounted for by the visible stars. The other feature of the M87 galaxy is that it has a 6500 light year long jet of high-speed electrons being ejected from the debris as it falls into the center, where a black hole is presumed to exist.
Another candidate for a black hole is the V404 CYGNI, which is modeled as a binary system consisting of a star circling a black hole. From spectra, they could determine that the star is circling the black hole with at an astonishing speed of 210km/s which results in a complete revolution every 6.74 days. More importantly, when the mass of the object the star is rotating around is measured, it works out to 6.26 solar masses, 6.26 times the weight of our own sun, which substantially exceeds the maximum size for a neutron star.
However, recent studies have shown that stars are orbiting a lot closer to black holes then they should be if the black holes existed, for example, even the black hole at the center of our galaxy has some stars orbiting very close.
"Two years ago, astronomers found the first cluster of young stars 0.7 light years from the black hole. How they got there is unclear, because the black hole's gravity ought to tear apart the clouds of gas and dust from which new stars form." "The latest group of five young stars ... is moving in convoy even closer to the galactic centre - just 0.26 light years away . The stars appear to be only about 10 million years old and don't seem to be held together by a smaller black hole. "What's amazing is that this little group of stars can survive in this hostile environment," says Lu. "You would think that the stars would be quickly torn apart."
Maybe those young stars can exist at the center because there is no high intensity gravity from a black hole but the calm push of negative gravity from surrounding galaxies.
The very evidence for black holes, the speed at which the stars orbit, could also be caused by the cumulative negative gravity from surrounding galaxies or stars. There are other more fundamental problems with black holes, for example:
"One such problem arises from the idea that once matter crosses a black hole's event horizon - the point beyond which not even light can escape - it will be destroyed by the space-time "singularity" at the centre of the black hole. Because information about the matter is lost forever, this conflicts with the laws of quantum mechanics, which states that information can never disappear from the universe.
Another problem is that light from an object falling into a black hole is stretched so dramatically by the immense gravity there that observers outside will see time freeze: the object will appear to sit at the event horizon for ever. This freezing of time also violates quantum mechanics.
"People have been vaguely uncomfortable about these problems for a while, but they figured they'd get solved someday," says Chapline. "But that hasn't happened and I'm sure when historians look back, they'll wonder why people didn't question these contradictions."
If Curvity is going to do away with black holes, it must be able to explain the large amounts of energy, mostly in the form of x-rays, that stream away from black holes. One possibility is that as material falls into the gravity well, it is compressed and goes past into the "negative portion of the curve" at which point gravity between the atoms goes negative and the material explodes. The x-rays that are released after a supernova are likely the same thing.
Curvity makes two points about black holes, first they do not exist and secondly, what astronomers currently take for black holes in galaxy centers is actually the "null balance" point, explained in the quote below, formed by surrounding galaxies or other very heavy masses.
We must, therefore, seek a spot which is remote from any of the existing galaxies, and approximately equidistant from the nearer ones. Even in this remote area of space we will find countless numbers of particles of matter, anti units of charge; electrons, protons or simple atoms, which have achieved escape velocity from some star, or which have been formed in space by random approach and capture. In short, we have all of the building blocks of nature, present in an exceedingly tenuous and diffuse state.
Since each of the particles of matter has mass, each has a force of attraction existing between it and ever other particle of matter in the area.
If we accept the concept of the non linearity of natural law as previously outlined in this text, we find that each of these particles is also being repelled slightly by the surrounding galaxies or galactic clusters.
These forces are almost inconceivably small, yet the net result of their action is to create a tendency upon the part of each randomly moving particle to move ever closer to the center of the area of attraction, which is also approximately but not exactly the center or 'null balance' point of the repulsion of the surrounding galaxies.
We can understand this if we realize that the center or null point of the force of repulsion is determined only by the distribution and the distance of the surrounding galaxies, while the center of the force of attraction is determined by the distribution of matter within the area of condensation. Since the center of 'push' is not at the same point as the center of 'pull', there is a tendency toward the creation of an angular velocity. That is: the particles, instead of falling directly toward the center, will tend to spiral inward. Eventually this rotational motion will become general throughout the mass. 
Yet another prediction by Curvity is that there should exist galaxies which have a "black hole", i.e. a "null balance" that isn't at the center of the galaxy. Because the "null balance" point created by the surrounding galaxies doesn't line up with gravitation attraction of the particles that form the galaxy, the "black hole" created can exist outside the center of a galaxy. Again, evidence of such off-center "black hole" exists at the center of M82:
Recently, Chandra may have discovered an important link between stellar-mass black holes and the supermassive type: a 500 solar mass black hole in the nearby irregular galaxy M82. But it is puzzling, too, because the black hole is not centered in M82's nucleus. 
and from BBC news:
"No-one was sure that such black holes existed, especially outside the centres of galaxies," said Professor Martin Ward, of Leicester University, UK, one of the scientists to report the discovery.
"Finding it is like finding the missing bit of a puzzle."
The new type of black hole was located about 600 light years from the centre of the M82 galaxy using Nasa's Chandra X-ray observatory. It is part of a binary system in which the hole and a very much smaller object move around each other.
Another interesting point is that black holes seem to exist everywhere in a variety of sizes, which could possibly be explained by the differences in surrounding galaxies.
Einstein himself never believed they [black holes] could exist. He was convinced that nature had a way, not yet discovered by physicists, to protect us from what he considered an absurd implication of his theory. Today, though, it would be hard to find a physicist or an astronomer who doesn't believe in black holes.
- "Mysterious ring of stars guards Andromeda's heart", David L. Chandler, Newscientist.com, September 2005
- "Stars spotted on the edge of a massive black hole", Jessica Lu, Astrophysical Journal Letters, vol 625, pg 51
- Astrophysical Journal Letters, vol 625, p 51
- "Three cosmic enigmas, one audacious answer", Zeeya Merali, New Scientist, issue 2542, March 9th, 2006, page 8.
- "Atoms, Galaxies and Understanding", Daniel W. Fry, 1960, Understanding Publishing Co., pg 95 - 97
- "A Galactic Center of Mystery", Nasa, February 2002
- "Middleweight black hole discovered", BBC news, September 2000
- "Black Hole Boldly Goes Where No Black Hole Has Gone Before", ESA Chandra Press Release, January 2007, Tom Maccarone.
- "Black Holes Spin?", Robert Kunzig, Discover Magazine, July 2002