Filtering Gravity

Most of the time, checking the arxiv's new publications is kind of boring. It doesn't happen very often that someone comes up with a really new idea. Therefore I want to draw your attention to a recent paper that I found really interesting:

Degravitation of the Cosmological Constant and Graviton Width
Authors: Gia Dvali, Stefan Hofmann, Justin Khoury
hep-th/0703027

Abstract: We study the possibility of decoupling gravity from the vacuum energy. This is effectively equivalent to promoting Newton's constant to a high-pass filter that degravitates sources of characteristic wavelength larger than a certain macroscopic (super) horizon scale L. We study the underlying physics and the consistency of this phenomenon. In particular, the absence of ghosts, already at the linear level, implies that in any such theory the graviton should either have a mass 1/L, or be a resonance of similar width. This has profound physical implications for the degravitation idea.

If I understand it correctly, the idea is to look at the cosmological constant problem from a different point of view. The question that troubles many physicists is why the observed value of the cosmological constant is what it is, especially why it is not 120 orders of magnitude larger. In their paper, the authors point out that the question what we measure for the cosmological constant does depend on it's coupling to gravity. They propose a scenario of modified gravity in which the coupling strength of a source depends (loosely speaking) on it's typical size of structures. The more homogeneous the distribution, the less it couples to gravity. (More technically, one takes the Fourier transform of the density, and the coupling depends on the wavelength. Large wavelengths couple weaker.)

The cosmological constant is perfectly homogeneous, and therefore would couple only very weakly, or maybe not at all. In this work, they do not explicitly address the issue why it is non-zero or has the observed value, but I think this idea has a large potential.

The specific scenario that they investigated is one with a massive graviton. Because of the graviton acquiring a mass, additional polarizations arise. This concrete model is one example for the more general class of gravitational high-pass filters that they have proposed. The challenging question is of course whether this approach can be utilized to compute the value of the cosmological constant. What I find specifically intriguing is that in this case, the gravitational sector might be the cause for the observed value of the cosmological constant, and it would be essentially independent on the expected value from the matter fields.

Here is an example to underline the importance of filters. Last week, a disaster happened:

From: System Administration
To: alllusers@itp.****.de
Subject: [allusers] Spam Surge

Last night at around midnight, a major increase in spam has caused an overload of the spam filter daemon. As an unfortunate result, the daemon gave up completely and let through all the spam until recently.

I received about 120 spam emails per hour. Luckily, I am essentially decoupled from this particular server.

See also: The Gravity Defyer

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TAGS: PHYSICS