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Why is the Universe expanding at an accelerated rate?

  1. Jul 13, 2003 #1
    I may be out of touch, but I don't think I've ever read it for sure so what are your theories on this? I mean the big bang theory may provide the inertia to expand but why at an accelerated rate? What force is being added if any to accelerate this expansion and where does it come from?
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  3. Jul 13, 2003 #2


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    what experts mean by the expansion of the universe is
    the expansion of space itself as modeled by a very simple
    pair of equation (Friedmann eqs.) which govern the evolution of
    a scale factor a(t).

    the universe expanding means the time deriv da/dt is positive

    this also written as a(t) with a dot over the letter----or as at---just alternative ways to write the time-deriviative or time-rate-of-change.

    accelerating expansion means the second deriv---as att----is positive.

    You say WHY. Two kinds of response (1) how do we know that it is? Because of Type 1A supernova data gathered in 1998. It was unexpectedly found in this data.

    (2) what is the CAUSE of the observed expansion? In a mathematical science the cause is found in some term in a differential equation. Or more generally in some mathematical MODEL. In this case the model is of the simplest imaginable kind, a differential equation

    att/a = (-4pi/3) (rho + 3p)

    rho is the average energy density in space----assumed to be positive---and p is pressure (essentially zero for ordinary matter and negligible for light at the present time).
    The cause of the observed acceleration----the fact that the LHS is positive----is that the RHS is positive. This happens because
    the term (rho + 3p) is negative.

    Since energy density rho is positive, how can that term be negative? A form of energy is postulated which has pressure p = -rho. (einstein had already thought it up in 1916 but it was a sleeper for 80 years or so until people saw the supernova data and realized they needed it.)

    I dont see any other way to say why besides explaining how the term (rho + 3p) = (energy density + 3 times pressure) can be negative. And the only way to explain this must involve an energy with negative pressure. But hey no problem a constant density VACUUM energy automatically has just that amount of negative pressure----the negative of its density whatever that density happens to be.

    So to solve and explain it all we just need to postulate a constant density vacuum energy-----that happens to be 73% of the total universe-wide average. Then accounts balance and all is well.
    Last edited: Jul 13, 2003
  4. Jul 14, 2003 #3
    There are other conclusions to be reached by the 1a supernova data - the constants may have been different billions of years ago - and if the older supernova events involved less mass energy, they would appear to be dimmer - ergo, the observed dimming may be the result of less energy rather than greater distances.
  5. Jul 14, 2003 #4
    Gravity is inversely proportional to the distance squared. So as the universe expands, the gravitational effect is getting weaker, therefore the force that is causing the expansion increases.

    Does that explain why the rate of expansion of the universe increases?
  6. Jul 14, 2003 #5
    Thanks Marcus although I don't understand what is exactly this negative vacuum density that should account for it, I see the how the math works out nicely, but I don't understand what this term means and how it works.
    I'm also inclined to the same concept as hyperreality, that it has to do with gravity more likely, but I would differ that the universe is accelerating in expansion because gravity is accelerating in attraction, or on the relatively small scale things are compressing but on the larger scale things are decompressing.
    One reasonable expanation for it might go that if you drop a baseball to the Earth it had a bunch of potential energy but as it falls it gives off that energy into kinetic energy to aggitate the air and vibrate through the soil in sound and heat, but when a solar system is forming and everything is comming together things are moving from point a to point b and decreasing in potential energy in the same way under gravity but where does that kinetic energy of motion go to if in space there is no air to disrupt and sometimes no ground to collide with if something is moving in space by a force of attraction toward another thing maybe this energy of motion is transferred to the expansion of things around those 2 things.
    Another view might be that if you increase the velocity of an object it's energy is increased or it takes and equal amount of energy to slow it down, so if gravity is pulling masses of charge together it is adding energy of motion to things that has to be nullified or come or go somewhere to balance out the equations.
    Another thing is it could be that the gravity of the other big bang universes is pulling our universe outward from all directions, or that because the universe is expanding gravity is attracting.
  7. Jul 16, 2003 #6
    Forgive my ignorance, and I have no idea about the math's, I've still got problems with 1 and 1 equaling 2
    But is what marcus said about the universe expanding into a negative vacuum ? like a dead universe Or something then ?
  8. Jul 16, 2003 #7
    Why is the Universe expanding at a decreasing rate?


    The rate of expansion of the universe is decreasing ! Long time ago (far ago), the rate of expansion was greather than it is actually.

    Therefore, "Why is the Universe expanding at an accelerated rate?" shall be "Why is the Universe expanding at a decreasing rate?"
  9. Jul 17, 2003 #8
    The rate of expansion of the universe is speeding up, or atleast that is what is suggested by the supernova data. What's causing this is some sort of anti-gravity and is called dark energy (which I believe is what marcus was refering to). What I find intersting about this is that around 60% of our universe is composed of this and we know nearly nothing about it. There was a Nova on this awhile back, maybe they will show it again.
  10. Jul 18, 2003 #9


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    Re: Why is the Universe expanding at a decreasing rate?

    Imagine, you have suggested an excellent article and supplied us with a link, but your post here is not consistent with the article

    The article is "The Quest for the Cosmological Parameters" by Plionis

    It is especially useful because it presents a "consensus" model or a "concordance" (as Plionis calls it) picture of the evolution of the universe. Many cosmologists have come to agree with the main features of this picture.

    The general features can be seen in Figure 1 on page 10 of the article by Plionis which you yourself have suggested and this clearly shows accelerating expansion for the past 5 billion years.
    The is a natural consequence of the consensus value of the cosmological constant Lambda, which Plionis uses for his calculations.

    Also you can see in the Figure 1 that this period of accelerating expansion was preceded by a period of DECELERATING expansion. But the regime of decelerating expansion ended already some 5 or 6 billion years ago.

    The decelerating expansion lasted (in Plionis "preferred Lambda" model) only during the first 60 percent of the history of the universe. This can be seen from the curve, which is convex for the first 60 percent and then has a point of inflection and is subsequently curving upwards. At the inflection point the curve changes from convex to concave.
  11. Jul 21, 2003 #10
    Re: Re: Why is the Universe expanding at a decreasing rate?

    Bonjour Marcus,

    I got a look to that figure. It's represening R/R0 versus t/t0. Instead, IMHO, we shall see t/tH for Hubble time. Right?

    Also, may be I misinterpret Hubble constant. When I talked about the decelerating rate, I was talking about the observed decreasing recession speed at "this" moment, not about H(t).

    If I understand correctly, at this moment, the expansion rate (Hubble's speed) is greather in the past (far away in the space) than it is in a near present (not so far away in the space).

    At "this" moment, if I look into the sky and observe a galaxy. Its recession speed would be equivalent to the expansion rate at "that" moment (in the past). No? Its recession speed would be greather than a nearer galaxy. Right? That's what I called decreasing expansion rate "at this moment".

    Am I wrong?
  12. Jul 21, 2003 #11


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    Re: Re: Re: Why is the Universe expanding at a decreasing rate?

    Bonjour Imagine,

    you have something left to discover about the Hubble constant which I suspect will surprise and delight you (as it did me when I first recognized it)

    The Hubble law actually refers to recession velocity which we cannot see, because it is the velocity of recession at the present moment (!) which we can only infer.

    Ned Wright goes to some length to explain this in his cosmology tutorial (which I was so pleased to discover has now been translated into both French and Italian)

    At various times several posters at PF have said things that cause me to suspect that they have not yet comprehended the fact that Hubble law refers to both distance and velocity of recession inferred to be occurring at the present moment.

  13. Jul 21, 2003 #12


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    it is always a pleasure to hear from you and to have the opportunity to converse.
    I have decided to construct a very elementary example just to get some concrete numbers out in the open. this should be the sort of thing that you yourself could construct to explain the Hubble parameter to a young person with only simple arithmetic.
    Probably no one here at PF needs such a simple example but I will sketch it out anyway.

    You know the hubble time is around 13.7 or 13.8 billion years
    and H0 is the reciprocal.
    For easy calculation I will pretend that it is 14 billion years.
    So at the present moment the Hubble parameter is 1/(14 GY).

    this means that at this very moment a stationary galaxy which is 1 billion LY from us must be receding with speed 1/14 of the speed of light

    And a stationary galaxy which is 2 billion LY from us must be receding at 2/14 of the speed of light.

    We cannot see them or measure the distance and recession at the present moment, but we believe that this is the reality.

    this is quite a curious law because it relates things which we cannot directly know at the present moment! It may even seem to be in bad taste! A kind of intellectual solecism or faux pas on the part of Hubble and the other astronomers.

    But so it is, that is what the law means. And in fact the situation is not really so bad if one looks only at galaxies with small redshift, as Hubble did.

    If the galaxy is only 100 million LY away at present, so that the present velocity is 1/140 of the speed of light, then its distance was not very different from that when it sent out the light that we see. So that the ancient distance and the ancient speed (which the law is NOT officially about) are nevertheless rather close to the 2003 distance and 2003 speed (which the law IS relating).

    So for a galaxy that today is only 100 million LY distant from us, in the past 100 million years it can hardly have moved even by one percent! Also its recession speed can only have changed proportionately by very little. So there seems to be no harm in applying the law in a naive way to uncorrected observational data.

    The fact that this works so well for nearby galaxies has led people to speak and act carelessly. In fact the distance the law refers to is not the "light travel time" distance and the recession speed is not the speed "back then".

    But for nearby galaxies it works sufficiently well to use the wrong distance and the wrong speed because they APPROXIMATE the present day distance and speed of recession.

    I hope this is not too long-winded---suis un peu bavard, je sais.
  14. Jul 22, 2003 #13
    Bonjour Marcus,

    I just came back from a hard disk crash. I forgot to read Ned Wright. It's schedule for today.

    I will have to understand the Hubble constant measurement and metric context before really replying to your bavardage :wink:

    Have a nice day.
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