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Tidal stresses during inflation

  1. Sep 21, 2006 #1
    Over in the Relativity forum, in the thread "Metrics and Forces", I have been brought (in fact dragged kicking and screaming, by Pervect) to the following view: In the FRW universe with matter, as gravity slows expansion, compressive tidal stresses develop in any entity that is bound by non-gravitational interactions. These stresses are due to accelerations of H ^ 2 per unit separation of the bound entity's elements, where H is the Hubble constant in units of (sec ^ -1).

    In a universe with lambda = 0 and where H is about 70 Km/sec/Mpc, or about 10 ^ -19 /sec, these accelerations (and the corresponding compressive stresses they generate in, say, solids bound by electromagnetic interactions) would be entirely imperceptible.

    In the inflating early universe, where expansion is exponential, these accelerations are bigger. Much bigger. If one estimates H roughly, from the postulated inflation of the universe by a factor of about 10 ^ 43 in 10 ^ -34 sec,(see Liddle, Intoduction to Modern Cosmology, p. 106), H turns out to be about 10 ^ 77 /sec. The acceleration per unit separation of any bound elements is then very large, about 10 ^ 154 m/sec ^2. The accelerations are due to the speeding up of expansion and would cause dilation pull-apart stresses in any bound entity.

    There may not be any bound entities in an inflating universe, which I understand is postulated to be a universe where all the forces of nature are the same and where all the force carriers are massless and travel at c. I don't know whether in such a situation the FRW model is even thought to apply.

    But does anyone know if such extreme internal accelerations are thought to have any consequences for the inflationary scenario? Say to disrupt even Higgs particles? Or to prevent them forming?
    Last edited: Sep 21, 2006
  2. jcsd
  3. Sep 21, 2006 #2
    Ok. First off ill admit im at a loss as to most of what youre saying, so ill have to guess (this is because im dumb not becuase your post sucked :P)

    you are asking if the tidal forces, or gravitational forces, of an exponentially expanding universe would rip things appart and prevent your higgs particle? (if this is wrong, ignore the rest :P)

    whilst its a good point, its always good to point out when discussing inflation that it is space time expanding. This means that there is no actual relativistic velocity between two points, even though they are 'moving apart'. This, as far as i know, means a.) the universe can expand faster than the speed of light b.) time dilation and other affects dont count

    from this i would assume that the normal tidal forces experienced in such a motion also wouldnt apply

    not sure tho
  4. Sep 21, 2006 #3
    Yes, this is more or less what I assumed before starting my thread in the relativity forum. But after quite a struggle (while I kicked and screamed), I was in the end persuaded that there are real (compressive/dilational) tidal forces that develop as expansion (slows down/speeds up). Trust the general relativity experts!
  5. Sep 21, 2006 #4


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    Just a quick note: the tidal forces due to the gravity of a uniform distribution of matter are compressive (in GR). The tidal forces due to the cosmological constant in a De-sitter space-time are of the opposite sort, i.e. they'd cause tension.

    Out of a long thread, the post that most clearly (IMO) computes the value of the tidal forces is

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