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Which force drives the expansion of the universe?

  1. Jun 14, 2004 #1
    Which force drives the expansion of the universe?
    The mental model is a explosion and subsecuent expansion product of the
    presure of matter and energy, but is this right? The most probable model of
    the universe do not have limits, so matter and energy cannot push over
    boundaries of the universe because there is no boundaries. From the very
    begining the universe was limitless. I am not talking about the moment of
    creation. Imagine a universe the size of an orange with the quantity of
    matter and energy of ours. How it may evolve? May it crash instantly into a
    singularity? Why?
    By the way, did somebody calculate the energy necessary to spread apart all
    the matter and energy of this universe from the plank time to present time?
    (something like the potential energy of all the universe)
    To many questions for now....
    Thanks for your answers
  2. jcsd
  3. Jun 16, 2004 #2
    What drives the expansion...
    I can tell you that there's a thing called dark energy culprit of the acceleration of the universe. Dark energy has not still an identity: it can be vacuum energy, quintessence, Chaplygin gas,...
    But you will be mislead if you think that without dark energy would not be expansion. In fact, the cosmological paradigm until the discovery of dark energy was the called Einstein-De Sitter universe, that was expanding but decelerating. So, you add dark energy to the Einstein-De sitter universe and you get the actual model.
    But the question is: What was driving the expansion in the Einstein-de sitter universe and consequently also contributes to the expansion in the actual model?
    I have my own ideas, but prefer not to say it, cause are very radical. Perhaps it would be useful to postulate the existence of some field, independent of dark energy, also contributing to expansion?
    Last edited: Jun 16, 2004
  4. Jun 16, 2004 #3
    Ah, the universe is expanding is it?
    Of course it is! The expansion of the universe is all the galaxies moving away from each other, no wait, they are standing still, but the space is expanding making them look like they are moving, no wait it's both.

    Ah, we know this because we know that distance is proportional to redshift, and we have absolutely no doubt that this redshift is caused exclusively by the Doppler effect, no wait, it's the expanding space making the light lose energy, no wait, it's both.

    I am so glad that we are so certain, it makes me feel good inside.

  5. Jun 16, 2004 #4


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    Pietro please think about a historical example

    it does not take a force to push the planets along their orbit paths around the sun

    kepler did not realize this and he thought there must be angels pushing behind every planet, like farmers pushing a wagon

    it does not take any force to "drive" the expansion
    once it has started it will just continue expanding

    it must take something (not exactly a force but an energy density) to slow the expansion down!

    this is what the Friedmann equation says, it says the energy density in the universe will cause expansion to slow down (except in the very special case of dark energy)

    leaving out dark energy (ie. the pressure term) it says:
    [tex]\frac{a''}{a} = -\frac{4\pi G}{3} \rho[/tex]

    a is the scalefactor showing size and a' is the rate of expansion and a'' is the acceleration, so the lefthandside is a measure of acceleration
    and rho is the energy density so it makes the acceleration negative. Deceleration. the expansion slows
    but only very gradually because the average density of energy in space is very very sparse

    putting dark energy (ie. the pressure term) back in it says:
    [tex]\frac{a''}{a} = -\frac{4\pi G}{3}( \rho + 3p)[/tex]

    dark energy is something with negative pressure so that it can make the righthand side of that equation positive (a minus of a minus)

    maybe the Friedmann equations do not explain things but they
    are currently the prevailing model people have

    what do the Friedmannequations explain, nothing except they tell you what needs to be explained

    what needs to be explained is not a force "driving" the expansion of space, space does not need a force to push it to expand, it does not work so, it likes to expand and will keep expanding unless you slow it down, by rho, the energy density----this is what the equation says

    the mystery is what special kind of energy can have a very very gentle negative pressure in space and cause this slowing down NOT to happen

    (like Newton's equations for the planets going around the sun----they dont explain everything but at least they have the angels pushing in the right direction-----he understood that the planets liked to go along their path and did not need a force pushing them, what they need is a force to bend their path)
    Last edited: Jun 16, 2004
  6. Jun 17, 2004 #5
    Lambda or Cosmological Constant. When Einstein introduced this factor into his Gravitational Equations, it was to produce a stability against Dynamical interactions, that he thought made the Universe very un-stable.

    Recently there are models that define this as an Energy/Space density product, which turns into a Repulsive Field Force.

    The Space that is between Galaxies has a ratio of Density different of that of Space between Atoms.

    Example I have seen the Equations that show the Vacuum Density causes Light to be variable according to Vacuum Density. Now the interesting thing is that Photon Density and Energy Density are two different aspects of the Vacuum Density, or a specific Electro-Magnetic-Vacuum Density.

    Just as the speed of light is gauged by the Vacuum Density it travels through(producing a constant speed in Labs), the density of E-M-Vacuum is altered by the speed and density of Photons.

    As one pulls light out of a chamber, it gets Dark, the Energy contained is allways expansive (this is what a vacuum is), but when one send a photon into the Vacuumated chamber, you are introducing a certain (hv) energy into a certain Energy Density, caused by the lack of photons.

    In E=MC2 energy and light are exchanged at a certain speed, but if you treat the Photon as a form of 'Energy Density' value, the smallest energy density value being low-energy, or infra-red for instance, then the corresponding smallest Mass-Energy Density is..well lets not say anything for now.

    Redshift can be a measure of the Density of Space and the density of Mass-Energy of Light, and as a consequence, the drop in the speed of Light transversing or Relative to other Energy Density Regions, produces a change in the Energy Value of E=MC2, resulting in where there is Darkness, there is Repulsive Expansive Energies.

    In simplistic terms without the equational values, the 'Darker' the Energy Density, the more Negative the Gravity, the less speed a photon can achieve.

    This is according to my work, but it is an extension to Einsteins Field Equations, and very relevant to Sakharov's 'induced' gravitational work, I will be posting the full extensions very soon.
    Last edited: Jun 17, 2004
  7. Jun 17, 2004 #6

    I look forward to it. :smile:
  8. Jun 17, 2004 #7


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    The expansion of space is driven by the dominant component of the energy density at each cosmological epoch.

    It is assumed that today this component is the dark energy. The dark energy has a positive energy density but it exerts a negative pressure (against the positive energy density), which leads to an acceleration of the expansion of space (see marcus’ post).

    But this was not always the case. Before recombination (the formation of neutral hidrogen from the radiation-matter plasma which filled the universe until 400.000 years after the big-bang) the dominant component was radiation.

    After recombination (more or less) matter became the dominant component until 'today' (IIRC it is assumend that at z ~ 0.5, the dark energy is becoming the dominant component). Both components, radiation and matter, lead the expansion of space to decelerate.

    The radiation dominated epoch can be traced back until the end of inflation, a period at the very early universe. During inflation the dominant component was again some sort of energy with a positive energy density but exerting a negative pressure, which lead to an accelerated expansion of space.

    Last edited: Jun 17, 2004
  9. Jun 18, 2004 #8
    Sol, you have to look at this paper!


    If one reads it, (pay attention to the equations on page 8, and paragraph thereafter:Quote-Although the size of the Universe increases in each successive cycle, its size with respect to Planck units remains the same, unless of course we consider a model in which the field [tex]phi[/tex] and radiation may exchange energy.

    The words[unless we consider] seem pretty new to me :smile:
  10. Jun 20, 2004 #9
    There is an obvious flurry of activity in Scientific circles at this moment, and clarity is evolving, so for the benefit of PF users, look at this paper by no-other than S.Hsu and A.Zee!


    Only three pages long, but has some very relevent and highly speculative insights!
  11. Jun 22, 2004 #10
    *ahem* GRAVITY*ahem*
  12. Jun 22, 2004 #11
    I know there is all this dark energy - negative pressure malarky going on... but what if there some longer range (on extragalatic scales) force at work? In essence a dissociative force which scales to gravity as gravity scales to electromagnetism?
  13. Jul 23, 2004 #12


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    Applying Occam's razor, the simple answer appears to be gravity. lamda represents the distance at which gravity ceases to be attractive and becomes repulsive. Fortunately, this crank theory [everyone is entitled to at least one crank theory and I claim this one] is easily disproven. Compare the distance indicators between relatively [to earth] massive regions of the universe to less massive regions and check for discrepancies in recessional velocities. Only cosmologically remote bodies need apply...
    Last edited: Jul 23, 2004
  14. Jul 24, 2004 #13
    All systems will evolve to the lowest possible state of energy or the highest state of disorder and will tend to stay there unless energy is supplyed or order is restored thru the expenditure of energy.
    --------"After all is said and done, Gravity Rules."------------- :approve:
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