<i>Astronomers call this boundary the Kuiper cliff, because the density of space rocks drops off so steeply. What caused it? The only answer seems to be a 10th planet. We're not talking about Quaoar or Sedna: this is a massive object, as big as Earth or Mars, that has swept the area clean of debris.</i><p>They've got this kind of backwards. They describe this like something basically pushed parts of the Kuiper belt inward to form the steep gradient. We're pretty sure Neptune and Pluto pushed things out to form the Kuiper belt. As well Mike Brown's work that found Xena, Santa, Easter Bunny etc. would have picked up on a Mars sized object past the Kuiper Belt but close enough to shepard things.<p><i>Maybe we can't work out what dark matter is because it doesn't actually exist. That's certainly the way Rubin would like it to turn out. "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances," she says. "That's more appealing than a universe filled with a new kind of sub-nuclear particle."</i><p>Actually modified Newtonian Dynamics or MOND equations are WAAAAY harder to swallow than dark matter.<p>Dark Matter: The majority of particles don't interact significantly with photons<p>MOND: Particles shooting through space either spontaneously replicate or behave non geometrically.
Gravity falling off at 1/R^2 is a consequence of geometry, the surface area of a sphere. MOND proposes 1/R^2 + R/N (for some very large but arbitrary number N) which makes NO physical sense.
Meanwhile gravitational lensing observations continue to provide evidence for "invisible" matter.<p><i>If the observations are correct, the only vaguely reasonable explanation is that a constant of physics called the fine structure constant, or alpha, had a different value at the time the light passed through the clouds.<p>But that's heresy. Alpha is an extremely important constant that determines how light interacts with matter - and it shouldn't be able to change. Its value depends on, among other things, the charge on the electron, the speed of light and Planck's constant. Could one of these really have changed?</i><p>Actually any sophomore or junior in college is exposed to the concept of evolution in the fine structure constant, while it isn't taught as definitive, it's hardly heresy.<p>AND it goes on, none of these (at least the physics based ones) really have much debate in the scientific world.