Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Inflation and Dark Energy

  1. May 11, 2014 #1
    I read an article that says that inflation at the early stages of the Big Bang explains why the universe is as big as it is today suggesting that if the universe had expanded at the rate it's expanding today it would be no bigger than a full stop.

    To me this doesn't make sense because I thought the universe was expanding at an ever increasing rate due to dark energy but the above seems to state that the universe expanded far more rapidly at early stages than it does now. Wouldn't this mean there was a lot more dark energy back then and if so what has happened to it?
  2. jcsd
  3. May 11, 2014 #2


    User Avatar
    Gold Member

    "I read an article that ..." is not considered a valid source on this forum.

    Inflation in the early stages is NOT known to be the same thing as dark energy and the inflation is not a proven fact, although it explains more facts that any other theory so far.

    Expansion is a different beast. This is all discussed in the link in my signature.

    Also, I suspect that you were reading something about the OBSERVABLE universe, not the universe.
  4. May 11, 2014 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    He's not providing a source, he's asking a question.
  5. May 11, 2014 #4
    No, this is incorrect, without considering inflation. lets start with time of CMB. Then consider the volume of the observable universe then. Now keeping the energy density of the cosmological constant (aka dark energy). the total amount of dark energy is lower. As the universe expands there is a larger volume as the energy density stays constant. So today the total energy contribution of the cosmological constant today is higher than in the past. However the energy density per meter3 is still the same.

    Now as for the expansion being faster than today is a consequence of distance mathematics. A simple way to think of it is if you had a VERY fast rate of expansion say 100% for ease of calculations. When you measure the distance between two points at 1 meter. the rate of expansion would go as follows

    4+100%=8 etc

    see how you now have an exponential expansion? or accelerating expansion (kind of a misnomer)
    Last edited: May 12, 2014
  6. May 12, 2014 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    This is an interesting question, however it is not thought that dark energy and inflation are linked in any particular way, other than both causing an accelerated expansion of the universe. The inflationary era is driven by a scalar field (or a fluid with negative pressure) that causes the universe to accelerate in its expansion to solve the problems of the big bang. However, once inflation is over, the scalar field responsible (the inflaton) decays into the particles of the standard model that fill our universe today. Dark energy only becomes dominant in the recent epoch of the universe. We know a lot less about what is responsible for the dark energy (whether it's some weird fluid or a modification to gravity). Since the energy scales are very different, it is not expected that they are linked.
  7. May 12, 2014 #6
    EDIT: I meant to reply to the OP. Fixed.

    To add to (and to reiterate) the above, just because expansion is accelerating today and that the expansion rate was larger in the past does not follow that there was more dark energy. The reason is that the expansion hasn't always been accelerating. After inflation ended the expansion was decelerating because the dominant contents of the universe were matter and radiation whose gravity resisted the expansion. However, as the universe expanded the matter was diluted making its relative contribution to how the universe behaves smaller than that of dark energy whose effect is to accelerate the expansion. Thus at some point (something like 5 billion years ago) the expansion changed from decelerating to accelerating.

    However, you are right in a way. During inflation there was even more dark energy than today. Dark energy is just a placeholder for something that causes accelerated expansion. Inflation is a period of accelerated expansion and so there must have been a type of dark energy driving it. The important part though is that the dark energy that was driving inflation is not the same as the dark energy which is driving today's accelerated expansion though it could have been very similar. The reason that it can't be the same is that we know that there was a period in the history of the universe, between inflation and today's accelerated expansion, which was dominated by matter. That matter must have come from somewhere and pretty much the only place it could have come from is from the dark energy that drove inflation.

    Why a misnomer?
    Last edited: May 12, 2014
  8. May 12, 2014 #7
    the amount of expansion you measure depends on the separation distance of the measurement. The amount of expansion is determined by the cosmological constant, and the effects of mass.(mass/gravity-contracts, cosmological constant-expands) If you measure the rate using two points, not gravitationally influenced by each other. With the cosmological constant at 67.3 km/s/Mpc. Which is constant. Then the separation distance between the two objects would be accelerating. However the rate of expansion is still 67.3 km/s.Mpc

    In other words if you determine the rate of expansion as per the units of expansion in km/s/Mpc, then the rate of expansion is constant at 67.3 km. If you define the rate of expansion as the separation between two objects, then the separation distance would be accelerating. See the misnomer? in one case the rate is the relation between two objects and expansion, in the other case the rate is the amount of distance change per second per Mpc.

    When were using the term "rate" one needs to clarify the rate relation being used. the rate between two objects separation distance per second, or the rate of expansion in the units of the cosmological constant. km/s/Mpc
    Last edited: May 12, 2014
  9. May 12, 2014 #8


    User Avatar
    Science Advisor

    That's the Hubble parameter that you quote -- it is not constant. Only in a universe with only cosmological constant is H also constant.
  10. May 12, 2014 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    It is pretty confusing to say that "During inflation there was even more dark energy than today." During inflation the universe was dominated by a scalar field which drove the accelerated expansion of the universe. At the present time, the universe is dominated by "dark energy" which might be a scalar field but could also be a placeholder for something else we don't yet understand. Though the two eras are qualitatively similar, they are not driven by the same quantity.
  11. May 12, 2014 #10
    I'm afraid I don't follow. I presume by "cosmological constant" you mean the Hubble constant (which isn't really a constant, now that's a misnomer). How is that relevant to 'accelerated expansion' being a misnomer. [itex]\ddot a>0[/itex], hence expansion really is accelerating, just like it says on the tin.

    Do we know that inflation was driven by a scalar field? Right until the BICEP announcement the Starobinsky [itex]R^2[/itex] inflation sat right at the center of the Planck constraints. Admittedly, if BICEP is correct it's disfavored but I'm not sure that all modified gravity models etc. are disfavoured. Either way it's premature, in my opinion, to say that we know.

    I can see where you are coming from, but personally don't see much of a problem in saying that what drove inflation was a type of dark energy. I did stress that it's not the same thing as what is causing accelerating expansion now. The bottom line is that you had energy which was constant and the energy density was much higher than it is now.
  12. May 12, 2014 #11
    :redface :eek:ops yeah your both right I am mis-thinking that D'oh. I feel embarassed....
    Last edited: May 12, 2014
  13. May 12, 2014 #12

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I agree with cristo. Well not wrong, I think it is non-standard terminology, and thus possibly confusing, to refer to whatever drove inflation as "dark energy".

    One possible difference: a cosmological constant has not been ruled out as dark energy, but a cosmological constant cannot be the the cause of inflation.
  14. May 12, 2014 #13
    to add to that we don't don't know which inflationary model is correct. There are 60+ inflationary models. One model I can readily think of that's still a good fit to obvervation AFIAK. is the Higg's inflation. In this model the Higg's Boson is responsible not the inflaton. The beauty behind this one is you don't need to add an exotic particle (The inflaton). Which isn't part of the standard model.

    Last edited: May 12, 2014
  15. May 12, 2014 #14
    Higgs inflation is somewhat similar to [itex]R^2[/itex]. It has been a very good fit to data right up until BICEP but is disfavoured if there are large gravitational waves. I think Shaposhnikov and company have since proposed another scenario where inflation would happen in a shallow false vacuum or plateau in the Higgs potential which exists for very specific values of SM parameters and which should be compatible with BICEP. I haven't followed that very closely though.

    I'm thinking of this paper:

    Of course if it hadn't been for BICEP Higgs inflation would have been very attractive because you don' need new suff (though you do need non-minimal coupling to gravity) and it produced exactly the right spectral index. But I think there was debate over some fundamental aspects of it, like violation of unitarity. But again, I'm not too cought up onthe details.
  16. May 12, 2014 #15
    good paper, I personally think it to early to rule out the Higg's inflation. There is simply too much we don't understand about the Higg's itself. We don't have a large enough data set on the Higg's bosons masses yet, for one thing. I've been reading lately numerous Higg's papers on its thermodynamics. The equations of state itself changes from paper to paper lol. Even between two CERN papers
  17. May 26, 2014 #16
    The standard model of cosmology suggests there have been two inflationary phases. The first at t<10^-32 sec, was rapid, and must have stopped or else we wouldn't be here. The second, more recent, is slow. In between, the inflationary expansion was supposed to have reverted to a big bang-type expansion, decelerating because of its mass density. However, as the story goes, the recent inflation is coming out from under the big bang expansion (since a redshift of ~2), so that about 72.8% of the expansion is due to inflation, the rest is due to big bang expansion. To me, this sounds crazy. Why not have an inflation that gradually reduces from rapid at first to a progressively slower rate in time. Early inflation is needed to be rapid just to keep massive black holes from forming. But I question the legitimacy of dark energy; it seems completely unfounded; only conservation of energy suggests that some positive energy was inserted. But why dark energy and not matter? In fact, my doctoral dissertation was investigating clouds of hydrogen in low-redshift galaxy voids (2002 ApJ 574, 599; two more in 2003). We found that cloud line density was an order of magnitude greater than predicted by numerical simulations with the standard model. Subsequent work shows that clouds were concentrated near void centers, and recent work finds galaxies forming in void centers (see e.g. Kreckel et al.).
  18. May 26, 2014 #17


    User Avatar
    Science Advisor

    This has been looked into: it's called quintessential inflation (primordial inflation + late-time acceleration due to a quintessence field). These models work but are generally considered fine-tuned and I believe the production of matter has problems in these models (there is no reheating because there is no inflaton decay; my understanding is that particle creation results from the changing gravitational field a la Parker et al.)
    Because matter doesn't cause accelerated expansion...are you questioning whether the universe is actually accelerated, or do you accept this and are instead disputing that dark energy is responsible? There are other sources of evidence for dark energy beyond the expansion history obtained from supernovae, the CMB for one.
    Last edited: May 27, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook