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Accelerating universe

  1. May 7, 2007 #1
    This site: http://curious.astro.cornell.edu/question.php?number=575 [Broken] , states that "Astronomers now have strong evidence that we live in an "accelerating universe" how is that possible, where does the repulsion force come from?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. May 7, 2007 #2


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    It can be misleading to try to think in terms of a mechanical analogy (since space is not a material----more like a web of distance-relationships).

    One can reply to such a question, staying in the context of mechanical analogy, and say there is an unseen "dark" energy which amplifies the expansion, making the expansion accelerate. One can say that in a manner of speaking this "dark energy" exerts a "repulsion force".

    But I worry that it just temporarily satisfies the questioner and will lead eventually to deeper confusion.

    Maybe it is safer just to say that since 1998 we have had observational data indicating that the expansion (which has been known since 1929) is accelerating, but beyond that not a great deal is known.

    Maybe it is safer to say that we don't really know yet why this acceleration, there are only speculative guesses. But the effort going into fitting the LCDM model to the data, and measuring the parameters of this acceleration is truly impressive.

    Lambda is the main numerical parameter that governs this acceleration, and it has given its name to the prevailing consensus model called LambdaCDM.
    I admire the work of the people fitting LCDM to the data, but I think they are not telling us why or how it works yet----they are just fitting their model with exquisite precision to the data of several different kinds coming from a range of wonderful instruments (some in orbit and some on the ground).

    Eventually from more accurate measurement will come constraints on what causes it-----measurement will narrow down the range of acceptable ideas
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  4. May 8, 2007 #3


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    Dark energy is one option, along with scalar fields, a cosmological constant, and quintessence. The differerences between these models are sometimes subtle, sometimes esoteric.
  5. May 8, 2007 #4
    A quick google search on dark energy and th effect of a cosmological constant for instance didn't leave me with an answer to the difference between the two. Isn't 'dark energy' just mathematically the same thing as a positive cosmological constant, but forms a physical interpretation of the latter?
  6. May 8, 2007 #5


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    dark energy is a generic term for a host of theories (some which do not actually refer to a genuine energy) that are similar, but not in general identical to a cosmological constant. This statement:

    I find a little confusing, since Dark Energy is the generic term for a class of theories which include scalar fields, cosmological constant and quintessence.
  7. May 8, 2007 #6
    Where does dark energy come from? I thought it came from the big bang
  8. May 8, 2007 #7


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    :smile: how would it sound to ask this:

    "Where do unicorns come from? I thought they came from the big bang."

    The question is assuming something which has not yet been demonstrated, namely the existence of unicorns.
    We don't know there is any substance or radiation corresponding to "dark energy". so it is premature to be asking where it comes from.

    IF it is a type of energy then according to the preponderance of astronomical data it appears to have a constant density of about 0.6 joules per cubic kilometer. Recent observations appear to confirm that this is constant throughout space and time as far as we can tell.
    that would mean that ALMOST NONE of what now exists was present within a second or two of the BB. Because space has expanded, what there was then would have been almost nothing compared with what we now have.

    because the volume of what is now observable universe would have been very small----and since there is a constant energy density of so and so much per liter, or cubic meter, there would have been only a very small amount of dark energy.

    so even if it is eventually shown to exist as some form of energy, it would still not be true that this energy appeared at the time of the big bang.

    It would necessarily be being constantly created along with the constant expansion of space---every new cubic kilometer of space that appears would bring with it a new 0.6 joule quantity of dark energy

    (because at all times there are exactly 0.6 joules of it per cubic kilometer, as far as astronomers can tell------or else there is some non-energy explanation for the acceleration)
    Last edited: May 8, 2007
  9. May 8, 2007 #8
    So if that's not it, what IS propelling the universe in its acceleration
  10. May 8, 2007 #9


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    I don't think marcus is saying that dark energy is not the solution (correct me if I'm interpreting you wrong, marcus), just that we do not know the solution! As crosson says, there are other models. IMHO, it's too early to answer your question, as this is an ongoing research field.
  11. May 8, 2007 #10
    I am new here so forgive me if I seem ignorant, but we have absolutely no idea of what the universe looks like "right now". For all we know, we could be in a "Big Crunch" and all the galaxies could be converging.

    The most recent example we know of about what the universe is actually doing is Andromeda correct? ...and that galaxy is heading right for us (or us to it)

    I for one, find it a stretch to assume based on redshifts that our entire universe came from a singularity. That is quite an extreme jump in logic, to say the least.
  12. May 8, 2007 #11


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    The logic is a little more complicated than that.

    First of all, redshifts do give us convicing evidence that distant objects are moving away from us, especially when combined with other evidence such as the light curves from supernova.

    Methods of trying to account for the redshift such as tired light have been tried and discarded. There is no way that a "tired light" model can explain the fact that distant supernovae appear to happen in "slow motion" compared too nearby supernovae.

    Tired light also has other problems, such as a prediction of a general "smearing" of momentum, this is discussed on for instance Ned Wright's cosmology page (along with the supernovae results).

    So, if everything is moving away from us now, it is hardly a stretch of logic to say that in the past, everything must have been closer together than it is now.

    Taking the logic all the way back to a singularity requires a little more than this. It basically follows from attempting to model the universe within the context of GR, which is our most successful theory of gravity.

    Current cosmology comes in large part from General Relativity. People are continuing to test this theory, though so far GR has held up very well in all the predictions its made. (GPB is an example of the sort of current tests that are being done - so far, GR appears to have passed).

    Thus, our best candidate for understanding the universe is an initial period of extremely rapid inflation, followed by a slower period of more gradual expansion.

    Furthermore, this theory makes important predictions that have been tested, the existence of the cosmic microwave background radiation. If the universe was initially very small, it must have also been very dense. The CMB is predicted to come from light that was emitted when the universe first became transparant. The expansion of the universe has redshifted this light into the microwave spectrum.

    Any successful theory is going to have to explain the observed redshifts, avoid the known problems with tired light, and also explain the existence of the cosmic microwave background. Current big bang theory explains all of these observations.
  13. May 9, 2007 #12
    So is gravity responsible for the "Big Crunch?" That makes sense but that would imply a center point. Would that be the same as the big bang reversed?
  14. May 9, 2007 #13
    I wasn't trying to imply that it was "crunching", I was just trying to illustrate the point that we are looking at where things were and where they were going billions of years in the past and have no way of really knowing where these things are today or "right now" except to assume they stayed on the same course and had no interactions to change their trajectory or whatever.

    So since that over time the universal expansion has been accelerating, do we observe the distant galaxies as much closer together universally? They should be given that logic.
  15. May 9, 2007 #14


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    I agree with the excellent discussion Pervect just gave. Additional points:
    A 'big crunch' is not predicted to happen.
    The 'big bang' expanding universe model does NOT imply a centerpoint.

    (that idea is one of several popular misconceptions based on mistaken notions of BB, there was a Scientific American article about that, it's online. Google the author Charles Lineweaver and the title "popular misconceptins of the big bang"---bug me if I don't have the title right :smile:)

    If a 'big crunch' were in the cards, it would NOT imply a centerpoint.
    (you need to flush the obsession with a "centerpoint" out of your head)

    If a 'big crunch' were in the cards, then you could certainly say "gravity is responsible". The model of gravity that people use is not the Newtonian force model but the Gen. Rel. geometric model. It works better.
    Our idea of how the universe looked in the past is based on a MODEL that we test in the present.
    Our idea of what the universe looks like at present is also based on a model (the same model) that we have tested many times in the present era (since 1919 or so, when Eddington's group first performed an observational check).

    All our conceptions about the universe are inferred using (more or less trustworthy) models provided by the laws of physics.

    there is no absolute logical certainty: 5000 years ago a pixie could have made the earth and put dinosaur bones in it and set all the photons, with just the right redshifts, on their way to us so it would LOOK like there are receding galaxies----it could be all a practical joke.

    But the pixie model is not very interesting, and if you adopt the model cosmologists use based on Gen. Rel.---which has been tested a lot and works to amazing precision---then you have to admit that we are very far from having "absolutely no idea" about the past present future of the cosmos.

    We don't have absolute logical certainty, but we have some pretty clear ideas based on tested physical law. (And hopefully the models will continue to be tested and will be improved, so that the ideas get adjusted to fit the data better)
    Last edited: May 9, 2007
  16. May 9, 2007 #15
    Still, the universe "expands" from one essential point. BoomBoom is right, there is a theory that the universe is "contracting", in a way, to anothere big bang.
  17. May 9, 2007 #16


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    Could you explain your reasoning...how you come to that conclusion?

    It sounds to me like you are saying that there exists a "center point" in space and that the expansion is outwards from that point.

    Or are you talking about a hypothetical point that is thought to have existed in the past---some 13 or 14 billion years ago?

    What do you mean by a "center point" and how do you arrive at the conclusion that one exists?
  18. May 9, 2007 #17


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    How about citing a source when you make statements like this, so we know what theory you are talking about? Note that we try to limit discussions here on PF to published peer-reviewed theories.

    A quick google finds, for instance http://www.newscientist.com/article.ns?id=dn2759

    Since new scientist is unfortunately sometimes a bit speculative, we can further track Linde's name down to papers published in reputable journals such as


    Of course I have no idea if that's what you had in mind.

    So, how does this fit in with my argument? I mentioned that we assumed that GR was the correct theory of gravity in making our predictions of what happened at the big bang. I'm not sure if I mentioned this explicityly, but I'll say that as you go back further and further in time, the predictions of cosmology probably become less and less reliable. But let's go back to the Linde scenario of a collapse.

    This Linde scenario assumes a rather more speculative theory of gravity than GR, which is our standard theory of gravity. So if GR is wrong, and this more speculative theory of gravity is correct, then it is possible that we will have a big crunch. But going with the evidence, one has to pick GR as being the better experimentally supported theory of gravity at the current time.

    I hope that this illustrates a bit better what we know, and what we don't know. Science, in general, does not offer any "absolute answers".

    Meanwhile, we continue to test our theories (such as the very important theory of gravity, GR) both in laboratory and in cosmological contexts, to try and confirm or deny its predictions.
    Last edited: May 9, 2007
  19. May 21, 2007 #18
    Is it possible the expansion and acceleration of the universe is a collapse to the point of a big bang and the point of the big bang is over there, past the farthest of what we can observe as expansion and acceleration. Please be kind.
  20. May 21, 2007 #19


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    that reminds me of how a 'slinky' toy walks down stairs.

    do you know those toys? cylindrical coil springs
    you put the spring on stair 10
    and flip the top over onto stair 9 and the upper portion continually expands and flows to the lower portion until finally it is all on stair 9

    (which you could call the point of a big bang, because it starts over)

    then of its own accord it then flops over and starts flowing from stair 9 down to stair 8

    so one portion is always expanding and being sucked in to a part that is contracting, and it keeps on going as long as there are steps in the stairway

    well you said to be kind. I don't want to be either kind or unkind, I just want to react honestly to the idea. I think it is a nice idea....visually. I can see it.
    It is an analogy (not a model that I would know how to write down mathematically as a model of the universe)

    the bottleneck is always getting from a verbal or visual idea to a mathematical model, isn't it:smile:
  21. May 22, 2007 #20
    Thank you Marcus and that was a kind answer.

    That slinky does seem to apply nicely. Practically I would think the universe is a galaxy of galaxies, slowly rotating and expanding and we can only see a small portion. I sure would love to be around when the answer is found but that will probably be a long time coming.

    Again, thank you.

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