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Why the universe is expanding faster and faster

  1. Apr 18, 2007 #1
    it seems the big bang can't explain it
  2. jcsd
  3. Apr 18, 2007 #2


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    Your thorough explanation of modern cosmology is astounding :rolleyes: Have a quick read of these forums, or indeed anything written about modern cosmology then come back when you actually have a question or and interesting problem to pose in a little more detail.
  4. Apr 19, 2007 #3


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    We do have possible explanations for why the universe's expansion is accelerating - try a google search for "dark energy" or "cosmological constant".

    The area remains one of active research - currently, an accelerating expansion appears to be the "best fit" to observation. This acceleration is not a problem to explain theoretically via a "dark energy" mechanism, but since this is the only observation that supports the existence of dark energy (AFAIK, anyway), some caution is advised. Ideally we'd like to have multiple experimental confirmations.
  5. Apr 19, 2007 #4
    i really know little about this problem and i am searching about dark energy.

    thank you for your guide,my dear pervect.
  6. Oct 9, 2007 #5
    i could rant for hours on end about this. but i'll put it into a couple lines.

    the matter in the universe in not dense enough to at least slow down the universe temporarily. hence, it simply follows the laws of physics, and gets bigger. there's nothing stopping it.
  7. Oct 10, 2007 #6
    thank you,keinve.it's really more conciser,but seems havn't explained the acceleration.
  8. Oct 10, 2007 #7

    George Jones

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    Maybe on cosmological scales gravity is repulsive. Maybe the vacuum has energy. Maybe some other field causes the acceleration.

    More detail.
  9. Oct 10, 2007 #8
    As I have read several articles recently about dark matter. Instead of attracting, the dark matter repels. And darkmatter occupy about 85% of all matter, so it accelarate the expanding of the universe.
  10. Oct 11, 2007 #9
    thanks Regards and pixel01.
  11. Oct 11, 2007 #10


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    Sorry, but what articles where these? I think you're confused and thinking of dark energy.
  12. Oct 11, 2007 #11
    Those articles are on space.com about a month or so ago. Even scientists are confused dark matter and dark energy, the two things are linked and influence each other. Anyway, I think the explanation is quite good for the acceleration of the universe expansion.
  13. Oct 12, 2007 #12
    keinve: I'm not a cosmologist, but does that mean that as the universe becomes less dense the force of gravity decreases causing acceleration of the expansion from the original point of highest density to increase?
  14. Nov 4, 2007 #13
    as far as i know... this expansion of the universe is the result of the big bang explosion.at the begining , the the explosion occurred & the universe started to xpand from the zero state(ie.infinitely large density n infinitely small volume).till now the universe is xpanding & now the xpansion rate is growing higher and higher...coz..the repulsion force between the elements (ie. stars, planets..etc..) is growing higher. and i should mention here that this repulsion force between them is directly proportional to the distances between them.and now if u r asking the question..."mention the time till when the the universe... will expand..".then i will answer u...."as the universe is xpanding...the density is decreasing...n goinh almost near to 1..when it will reach 1 ,the universe will collapse to a point with an infinite density & after that moment another big bang will occur and a new universe will be created. this process is eternal.".....

    and at last of all....i want to tell u that.... these all theories (given by different scientists) may be proved wrong in futute...coz...no one can say that....this or that theory is completely right,,,, they may change in future.
  15. Nov 4, 2007 #14


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    This thread seems to be full of confusion.

    A low matter density does not imply an acceleration, but only that the expansion speed decreases more slowly with time.

    Dark matter must be attractive in order to explain what it is suppost to explain (dynamics on galactic, cluster, and large scale structure scales.)
    The connection between dark matter and dark energy is just something some scientists are hoping for, and is in any way much more complicated then what seems to be the case from the post by pixel01. At the moment I havn't heard of any promising attempts of explaining the cosmic acceleration with the help of a dark matter model.

    I think you need to be more clear on what you are trying to say. All I see is a lot of words and incoherent claims (which even one bye one not seem to be correct).
  16. Nov 4, 2007 #15


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    enricfemi, an empty universe will expand at a constant rate - the Mine model.

    Add matter evenly throughout the universe and the mutual attraction between the particles will affect the curvature of space and the expansion rate and slow it down, the universe will decelerate in its expansion.

    Add positive pressure into the universe (a hot gas or radiation field such as the CMB) and counter-intuitively under GR gravitational theory the universe's deceleration will increase.

    Add an exotic form of pressure which has a negative pressure (tension) and this effect is reversed, the universe's deceleration would decrease. If the negative pressure were high enough the universe would eventually accelerate in its expansion. For acceleration to happen the total pressure would have to be less than minus one third (in geometric units) of the total density. p < [itex] - \frac{1}{3}\rho c^2[/itex].

    A cosmological constant, [itex]\Lambda[/itex], acts like a negative pressure with p < [itex] - \rho c^2[/itex].

    Distant SNe Ia are fainter than previously expected, which, if they are standard candles, is interpreted as them being further away than expected because the universe's expansion has accelerated.

    Therefore it seems this mysterious negative pressure dominates the universe and it has been given the title Dark Energy.

    I hope this helps.

    Last edited: Nov 4, 2007
  17. Nov 4, 2007 #16

    Chris Hillman

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    Dark Energy versus Dark Matter

    Hi, pixel101; ditto Cristo: I think you have confused dark energy and dark matter.

    Depending on what you mean by "influence each other", this could be wrong.

    space.com sometimes has some decent articles, but it also has some very bad ones. As always when surfing the web, be careful to distinguish between authoritative and nonauthoritative sources of information, factor in the PR machinery used by universities to promote their image (often at the expense of truth or at least of balance), and so on.
  18. Nov 5, 2007 #17
    I would like to make a remark about making predictions about the consequence of a cosmological constant on the evolution of the universe, as based on the Einstein field equations. If [itex]\Lambda[/itex] is related to quantum fluctuations (as is often assumed), how can we then rely on predictions using the Einstein field equations when we don't have a satisfactory theory for quantum gravity ? Why should the field equations give the correct answer for an entity for which it was never conceived ?

    Rudi Van Nieuwenhove
  19. Nov 5, 2007 #18


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    The Cosmological Constant ([itex]\Lambda[/itex]) is a possible component of Einstein's field equation. It serves the function of an integration constant in that its presence does not violate the conservation properties of the Einsteinian tensor with respect to covariant differentiation. It need have no counterpart in quantum physics, it is simply part of how gravity on its own might behave.

    [itex]\Lambda[/itex] represents a repulsive force that becomes significant only at large ranges, whereas the normal Newtonian force becomes increasingly significant at shorter ranges. If weak enough [itex]\Lambda[/itex] would be undetectable in the solar system yet dominant at cosmological ranges.

    Cosmic acceleration may be evidence of [itex]\Lambda[/itex] having a non-null value.

    The zero point energy field, detected only as the weakest of forces, the Casimir force, is something different; although it should behave gravitationally identically to [itex]\Lambda[/itex] and is often confused with it.

    Theoretically ZPE is of huge energy density and if it affected gravitational fields it would totally dominate over all else, being OOM 10120 times larger than [itex]\Lambda[/itex].

    As you indicate the resolution of this enigma awaits a full quantum gravity theory.

    Last edited: Nov 5, 2007
  20. Nov 9, 2007 #19
    What about quintessence ?

    I've been reading about quintessence which has the property that it has a spatial and temporal dependence. As you explained before, the cosmological constant is a possible component of Einstein's field equation but it is really a constant, independent of space and time. What I don't understand however is how this can be reconciled with quintessence. Why is there suddenly a new component which is not a real constant and yet compatible with Einstein's field equation? How does this mathematically makes sence?
  21. Nov 9, 2007 #20


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    A good question!

    One problem with [itex]\Lambda[/itex] is that its energy density is roughly equal to that of matter in the present epoch. Why should this be so as [itex]\Lambda[/itex] is a constant, as you say, and matter-energy density has decreased by a huge factor ~~1080 since the Inflation era?

    If we look at the Einstein Field Equation:

    [tex]G_{\mu \nu} = 8\pi G T_{\mu \nu}[/tex]

    [itex]\Lambda[/itex] enters in on the LHS of the equation as part of the [itex]G_{\mu \nu}[/itex] description of the nature of space-time curvature, whereas quintessence, ZPE, etc. etc. would enter in on the RHS of the equation, if they exist.

    The differences between these various speculative possibilities is their equation of state, that is, how their energy relates to their pressure.

    Cosmic acceleration demands that overall p < -[itex]\frac{1}{3} \rho[/itex]
    and if DE is given the equation of state p = - 1[itex]\rho[/itex], then that fits the data pretty well.

    If you move [itex]\Lambda[/itex] across to the RHS of the equation then it also has this equation of state, which also happens to be the equation of state of the false vacuum, ZPE.

    Hence either [itex]\Lambda[/itex] or false vacuum is the leading candidate, but that suggestion then has to explain why its density is so small.

    As an alternative you can make up whatever equation of state you want for a hypothetical 'quintessence' to make it fit the data, but until you have identified it in the laboratory its just 'pixie dust' IMHO!

    Last edited: Nov 9, 2007
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