Friday, March 23, 2007

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

    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
    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



  1. Filtering Gravity --- interesting.

    "The supersymmetric particles is an idea that people have been developing for decades, and it actually emerged from string theory: this notion that there should be more particles than the ones that we know about, and the reason we don't yet see them is because they're so much heavier(?) and you need a more powerful machine to conjure them up."
    Brian Greene does theoretical physics ...

    Nothing like conjuring up heavy particles with a powerful machine.

    And if you wondered where all the old computers, any lost data and information and filtered spam goes to, take a look at the
    Information blackhole

    But I've always wondered where the information on the blackboard goes when you wipe the chalk off ...
    could anyone restore or 'conjure' from the dust the information that was earlier written on the b/board and composed by the chalk dust?

  2. Ich verstehe hier nur Bahnhof..

    Wishing you all a nice weekend ;-)


  3. Just to clarify information

    I understand that if we compress information into a zero 0
    then zero contains 'all' the information we have compressed.

    Unless we have photographs we have no 'idea' what Hiroshima or Nagasaki actually looked like 62 years ago or San Francisco before the great fire (earthquake).

    If we had no photographs we would have no idea what the Twin Towers would have looked like.
    We have no 'accurate' idea of what Jerusalem looked like before the Temple was destroyed ...
    and whilst we can attempt to decipher dead languages and hieroglyphs ... we have no 'clear' idea what they sounded like.

    You yourself used the analogy of looking at a car crash and trying to determine what the 'victim' had for dinner the night before ... and that is still easier to determine or decypher ... than what 'they' were thinking or doing the night before.

    Ooops I seem to have veered off gravity at a Tangent.

  4. Hi Klaus,

    :-) Interessant! Ich hab mich schon immer gewundert, wo der Spruch herkommt.

    Bezüglich dem erwähnten Paper, die Idee ist im wesentlichen dass Gravitation einen Hochpass-Filter hat und das 'Gewicht' einer Masseverteilung davon abhängt wie homogen sie im Universum verteilt ist. Die Details vom Paper hab ich auch nicht alle kapiert (um ganz ehrlich zu sein hab ich auch nur die erste Hälfte gelesen).

    Viele Grüsse :-)


  5. Hi @ All,

    Danke Sabine :-)

    Highpass and lowpass are commonly known in low voltage electronics, ie. amplifiers and antenna circuitry. What "passes" refers to frequency, measured in Hz.

    If I understand you right we are in this case talking about "frequencies" refering to statistics.

    In electrotechnics we are dealing with the term cutoff frequency. Would that also apply to the "gravity filter"?

    Perhaps one of you guys could add to the wiki-articles about high pass filters.



  6. -oh by the way..!

    what if "our normal world" is placed on the cut-off side of the high pass?

    could this be the reason why we observe the huge disproportion (20:1 at least) between dark (DM)and normal matter.

    I mean.. maybe there aint so much DM after all,, the filter simply favours the observation of DM thereby the picture is decieving, some parts blown up, out of their true proportions.

    Sunday will be great! (I should know, being 15 hrs ahead of you)


  7. Hi Klaus,

    you know what, I thought yesterday understanding 'Bahnhof' is totally outdated. The next time I sit in a seminar, feeling lost and stupid, and someone asks me I'll tell them 'I understand only airport'.

    Anyway 'what if "our normal world" is placed on the cut-off side of the high pass?'

    If I understand correctly, the filter doesn't distinguish what kind of source is coupled (baryonic, dark matter, dark energy), but only what size its structures are. You can't make dark matter 'heavier' than baryonic stuff if both clumps on comparable sizes.




COMMENTS ON THIS BLOG ARE PERMANENTLY CLOSED. You can join the discussion on Patreon.

Note: Only a member of this blog may post a comment.