Hořava-Lifshitz Gravity.

Now that we discussed loop quantum gravity I wanted to point your attention to another quantum gravity theory that has been generating a lot of attention: Hořava-Lifshitz gravity.

About one year ago, Petr Hořava proposed a new theory of gravity with the following attributes:

1. It is 4 dimensional just like the space we observe is. (4th dimension being time).
2. It is a "small" theory in that, like loop quantum gravity, it only addresses gravity and doesn't aim to unify all physics.  (Though it may be embeddable in a larger theory like string theory.)
3. Here is the kicker part 1: It is anisotropic in space and time on small scales meaning the traditional "equivalence" of space and time breaks down. 
4. Here is the kicker part 2: Because of #3 above, Lorentz invariance only holds on large scales.  On small scales it breaks down.
5. It is re-normalizable by virtue of #3 and #4 above.

Basically, what I took from the paper is this:

Hořava wanted to work on a "small" gravity theory that didn't have to solve all the worlds problems at once, just quantum gravity in 4 dimensions.  He noticed that the main problem with quantum gravity is that it blows up at high energies. He understood a concept from condensed matter where there is a scaling between space and time producing an anisotropy that can be parametrized by a variable z.  He applies this scaling to spacetime.  If z = 3 Lorentz invariance becomes broken at small scales but quantum gravity doesn't blow up!  Moreover, at large sclaes, like we observe, Lorentz invariance and the equivalence of space and time is recovered making it a viable quantum gravity theory!

So, with that we have a new theory of quantum gravity.  Hořava has this to say:

It is difficult to imagine how Lorentz symmetry can survive as a fundamental symmetry in a framework in which the space itself is viewed as an emergent property of the theory. In string theory, quantum mechanics appears to be more fundamental than the symmetries of special of general relativity. As a result, we adopt the perspective that Lorentz symmetry should appear as an emergent symmetry at long distances, but can be fundamentally absent at high energies.

Does Hořava-Lifshitz gravity have issues?  Of course, but to it's defense its only been around a year.  The issues, however, are too technical for me to understand without really digging deep.  The interested reader with consult here and here for papers describing problems with the theory.

If nothing else, it's a fun theory to think about.

The Loop Quantum Big Bounce

Theoretical physicist Martin Bojowald of Pennsylvania State University in University Park has a cosmology model, based on Loop Quantum Gravity, that claims to to be able to peer back before the big bang. What does his analysis find? It finds that our universe formed from another universe that collapsed down to a point then bounced back. This previous universe may have been physically different then ours in various ways, but after the bounce it became the universe we know and love. Also, he predicts the relics of the previous universe are probably too faint to be detected.

Not everyone is satisfied with the result. I for one a skeptical of loop quantum gravity since I don't understand it (yet) and the vast majority of theorists think other methods are better. Personally I really like the String Landscape model, which is being covered extensively at TASI 2007. (The theme is "The String Universe") Plus, the blog is named after the idea that the universe is formed from some sort of eternal inflation process so the bias has to come out.

Others like Caltech Cosmologist Sean Carroll have already blogged against it. A quote from him: "Someday we’ll understand how the Big Bang singularity is resolved in quantum gravity. But the real world is going to be more complicated (and more interesting) than these simple models."

Or as the Scientific American states:

Physicist Donald Marolf of the University of California, Santa Barbara, says the finding would be strengthened if it turned up in other models of quantum gravity, such as string theory. "No one has good control over this physics in any approach to quantum gravity," he says, "and it is important to explore a broad range of models and ideas."

Here is an article by New Scientist.

Hopefully in the future we will be able to test such theories.

Quantum-gravity phenomenology, Lorentz symmetry, and the SME

I read an interesting paper from the arxiv today: Quantum-gravity phenomenology, Lorentz symmetry, and the SME by Ralf Lehnert.

In the paper, Lehnert explains that in order to have a quantum theory of gravity me must understand physics at the plank length. He propses one practical way for doing that would be to search for symmetry violations. If we could find direct evidence of a symmetry violation, such as Lorentz Invariance, we would know something about the plank length.

He gives various examples. My favorite is if we could find a particle described by a vector field which has a direction in the ground state we would have a violation since there should be no preferred direction in Lorentz Invariant physics. There are other violations he discusses as well.

So is we could find some "sacred" symmetry breaking it would be wonderful for quantum gravity theories for we could use the violations to probe the plank length and adopt quantum gravity thoeries where such violations are possible and discard the rest.