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Another dark energy question.

  1. Mar 17, 2009 #1
    In the late 1920's it was observed that galaxies in virtually all directions were moving apart from us, and/or we from them, at a rate that increases with distance. Implicit within this observation was the fact that there is a mechanism that would have caused this increasing rate of separation. There would have been no reason to suppose that this mechanism was not still operating. Why then was surprise expressed when observation showed that it is indeed still operating and galaxies are still continuing to separate at an increasing rate?
  2. jcsd
  3. Mar 17, 2009 #2


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    The original expansion, as observed in the 1920's, was supposed to have arisen from the big bang and was presumed to be slowing down. The speedup observed about 10 years ago was a complete surprise.
    Last edited: Mar 18, 2009
  4. Mar 18, 2009 #3
    peter - you have asked a number of good questions lately - keep it up. at the time of the work you reference, the balloon analogy was developed to explain the observational results, and remains an effective analogy today. the remaining issue was a question regarding whether there was sufficient mass contained within the universe to slow down the expansion over time to a point where expansion stopped, or stopped and slowly reversed toward collapse, or continued to expand forever at a slowly decreasing rate.
  5. Mar 19, 2009 #4
    Thank you Mr. jnorman. I will. With regard to your above post, take a look at the deepest view ever, (Hubble ultra deep field view), disregard the technical stuff, and their interpretations and see what you make of it. Particularly the exponential rate of acceleration over the last few hundreds of light years. And remember, there has never been sight of any galaxy composed of the large hot blue stars that are thought to have preceded the typical yellow starred galaxies of today.
    hubblesite.org is the address.
    Last edited: Mar 19, 2009
  6. Apr 1, 2009 #5
  7. Apr 1, 2009 #6


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    This is not correct as the surprise is due to the suggestion of a second mechanism.

    The original expansion mechanism was taken to be inertial. So a one time push and then the universe freely rolled on (with gravity then acting as a brake to cause the universe to coast to a halt at t=infinity).

    Dark energy would be a second mechanism, some extra source of acceleration. Which would be a surprise.

    Alternatively, the original inertial big bang story was too simple and some third story may emerge in which both the strongly inertial and the weakly accelerative components are accounted for by a single mechanism.

    I favour this latter outcome. But the original mechanism (in the way it was originally envisaged at least, as the pure inertia of a one-time bang producing an entropically isolated system) could not still be operating.
  8. Apr 2, 2009 #7


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    There seems to be a confusion of ideas in this thread, I will attempt to make things a little clearer.

    There is no doubt or question that inertia exists in the Universe. The same laws of physics that require you to apply the brakes on your car to slow down also requires that any two galaxies will relative motion away from each other will continue in that state of motion. There is no mystery behind why the Universe continues to expand unless otherwise acted on, it is just everyday Newtonian mechanics.

    Now, in the current model we have for expansion history of the Universe, we have two main things that can change the relative velocity of galaxies. The gravitational effect of the matter in the Universe continually acts to reduce this velocity, while the effect of dark energy has the opposite effect, that of increasing it.

    Both of the effect of gravity to slow the expansion rate and dark energy increasing it have always been acting since the expansion started at some very early time. Now, early on the Universe was very dense, so the gravitational deceleration was very strong and for the first part of the history of the Universe the rate of expansion was continually slowing because of this. The fact that the expansion continued was simply inertia, a body in motion continues in its state of motion unless otherwise acted on, but the slowing of the expansion rate was due to the effects of gravity.

    Now, as the Universe expands it gets less dense, so the gravitational slowing of expansion weakens. On the other hand the property of dark energy is that is remains roughly (possibly exactly, we aren't sure) the same density at all times, even as the Universe expands. This mean early on the high matter density swamped the repulsion due to DE and the expansion slowed. At some point though the matter density got low enough such that the repulsion due to DE became larger than the attraction of gravity and the expansion rate started accelerating. Again though, this affects the second time derivative of expansion (the acceleration) this should not be confused as something 'required' to continue expansion, this is still simply inertia, in the absence of matter or dark energy expansion would simply continue, but it takes a force to apply an acceleration, causing a change in velocity (i.e. Newtons second law, F=ma).

    Another key point to realise though is that we don't 'see' acceleration or decceleration of the Universe directly. We don't see the redshift of a particular galaxy changing such that we can infer whether its recession from us is getting faster or slower. Instead what we see is a 'snapshot' of the redshift of a series of galaxies at different distances from us. From this we can reconstruct the expansion history of the Universe, but the process to do this is a little complex and requires a model (we use a particular solution of General Relativity) to interpret. If this model is wrong (gravity might be different on large scales to that expected from GR for instance) then in fact the Universe may not actually be accelerating at all. The evidence for 'acceleration' is not independent of the model.

    Anyway I digress, the main point I wanted to make is that you cannot compare 'inertia' and 'dark energy' as two comparable mechanism that can 'continue the expansion'. Clearly inertia is always present and expansion would continue unchanged if there were no forces. The gravitational effects of matter and dark energy are examples of accelerations that can and do change this expansion rate.

    I hope that helps to clear things up!
  9. Apr 2, 2009 #8
    "inertia" of real matter has little to do with the acceleration of space
    Real matter is only 4%
    dark matter is 23%
    dark energy is 73%
    2% error
    Real matter and dark matter will decelerate space
    Dark energy density approaches a constant relatively quickly in Tamara Davis's computer simulation her 2004 phd at university of new south wales
    A recent NASA Ames paper points out that the measured dark energy density is precisely the inverse area of our far future cosmic horizon in Davis's simulation.
    This suggests retrocausality according to that paper.
    Last edited: Apr 2, 2009
  10. Apr 2, 2009 #9
    I should have added that in that paper the dark energy we measure today is advanced wheeler feynman radiation from the future horizon that is also a hologram in lenny susskind's sense.
  11. Apr 2, 2009 #10


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    Sorry but you're going to have to define what you mean by 'space' before you can give it quantities such as acceleration, velocity etc. The equivalence principle explicitly refutes the idea that 'space' can have such qualities, so you can't be talking about general relativity. Note that the theory that underpins modern cosmology does not invoke the acceleration of space or any other such concept. There are pop-sci explanations that invoke such phrases, but these should not be confused with physics.

    These are the values today, as I explained in my post, in the past these numbers were different, you only have to go back a few billions years and you find instead that matter (dark + baryonic [what you called 'real']) add up to almost 100%. When that was the case, the expansion decelerated, that is to say the mass in the universe (such as galaxies) slowed the rate at which it moved away from any other chosen bit of mass (such as another galaxy).

    I think you are confusing the physical density of dark energy (which is close to constant or possibly exactly constant at all times by definition) and the dimensionless energy density [tex]
    \Omega[/tex] which is the relative amount of dark energy compared to other energy terms. In the early universe this was small, now it is aroud 0.7 (i.e. your comment that dark energy density is about 70% today)/

    I really have no idea what you are saying here, but Davis's 2004 paper was simply an unusually clear exposition of standard general relativity and cosmology. It certainly doesn't imply retro-causality?? You are going to need to supply that reference and explain your proposition much more clearly.
  12. Apr 2, 2009 #11
    The scale factor a(t) in the FW metric
    See rocky kolb's lectures Slac 2005 online
    For the equation.
    True tamara does not discuss retrocausality
    That's the NASA Ames paper. Tamara's fig 1.1 shows the future event horizon
    So does hawking & ellis fig 18
    Tamara's fig 5.5 shows the evolution of the area of the future horizon
    She did not know that the reciprocal area she computes when plugged into Einstein's field equation gives precisely the observed dark energy density.
    That's the NASA paper. The numbers using the past particle horizon do not work.
    Therefore you must have Wheeler Feynman retrocausality argues the NASA paper to be published in iop proceedings if dice 2008 workshop in Italy sept 2008
    One of the coauthors cl is a top NASA scientist aide to USAF general pete Worden ret space command now head of NASA Ames
    Creon is also winner of Feynman prize in nanotechnology.
    Last edited: Apr 2, 2009
  13. Apr 2, 2009 #12
    That's fw metric not few metric
    Equation is also in tamara davis's thesis
  14. Apr 2, 2009 #13
    Typo that's fig 5.5 for area of future horizon
    Am on iPhone small keyboard
  15. Apr 2, 2009 #14
    0902 0032
    Archive physics section
    Google Creon Levit emergence of gravity
    I cannot copy & paste on iPhone
  16. Apr 2, 2009 #15
    I am now on laptop not iphone so I can copy & paste
    the NASA paper is

    Rocky Kolb's lectures are at
    image 5 has the relevant equations for the expansion of 3D space according to Einstein
    Rocky uses a(t) instead of R(t)
    see also Tamara Davis
    on Wheeler-Feynman see
    http://www.npl.washington.edu/npl/int_rep/dtime/node2.html [Broken]
    on retro-causality see
    on holography see references in 0902.0032
    Last edited by a moderator: May 4, 2017
  17. Apr 2, 2009 #16


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    FlashStarwalker - the notion of retrocausality from a future event horizon is very appealing as a general mechanism.

    Can you set aside your own enthusiam enough to give a sketch of its current status within theory circles? Who is for it, who against it - the major lines of argument either way?

    For a start, there seem two quite different final fates depending on whether the universe is cruising to a halt, of forever gently accelerating away (big crunch and big rip cosmologies might well be ruled out by retro-causal considerations now?).

    And does it connect yet with Lineweaver-Davies story about the blackbox photons (the radiation of event horizons) that will be all that is left at the Universe's heat death?

    It seems a very significant new idea. So the ins and outs would be interesting to discuss.
  18. Apr 2, 2009 #17
    Flash to Physics
    show details 8:32 PM (0 minutes ago)


    On Thu, Apr 2, 2009 at 6:07 PM, Physics Forums <gregbernhardt@gmail.com> wrote:
    Dear FlashStarwalker,

    apeiron has just replied to a thread you have subscribed to entitled - Another dark energy question. - in the Cosmology forum of Physics Forums.

    This thread is located at:

    Here is the message that has just been posted:
    FlashStarwalker - the notion of retrocausality from a future event horizon is very appealing as a general mechanism.

    "Can you set aside your own enthusiam enough to give a sketch of its current status within theory circles? Who is for it, who against it - the major lines of argument either way?"

    James Woodward a professor of physics at Cal State in LA, Michael Ibison at Earth Tech Austin as well as Levit and Sarfatti are only ones active in the cosmological field today. Possibly John Cramer at U. Washington. AAAS had a retrocausality workshop June 2006 published by Dan Sheehan chairman of physics dept USD. Feynman & Hoyle first proposed it in 1940 and it was taken on by Fred Hoyle and J Narlikar. Bernard Carr, a former assistant to Hawking has written about it recently. He is at Queen Mary College, London. The point is if you try to explain dark energy density (not Omega DE) from the past horizon you get the wrong answer - it's way too big. You only get the right answer using the future horizon. Ergo ...

    "For a start, there seem two quite different final fates depending on whether the universe is cruising to a halt, of forever gently accelerating away (big crunch and big rip cosmologies might well be ruled out by retro-causal considerations now?)."

    exactly - it's ruled out say Levit & Sarfatti

    "And does it connect yet with Lineweaver-Davies story about the blackbox photons (the radiation of event horizons) that will be all that is left at the Universe's heat death?"

    i am not familiar with that - there is no heat death with dark energy. the accelerating universe is never in thermal equilibrium. that's why there is a future horizon - light takes infinite amount of proper time of the observer to travel a finite distance - that finite distance defines the location of the future horizon of the observer at any moment in her world line - see Davis Fig 1.1

    "It seems a very significant new idea. So the ins and outs would be interesting to discuss."

    understatement ;-)


    "There may also be other replies, but you will not receive any more notifications until you visit the forum again.

    All the best,
    Physics Forums"

    Last edited: Apr 2, 2009
  19. Apr 4, 2009 #18
    The paper http://arxiv.org/abs/0902.0032 reflects " the notion of retrocausality from a future event horizon is very appealing as a general mechanism."....

    As soon as I read flashstarwalker's initial posts, an event horizon came to mind.....
    but overall the math in the above article is beyond my capability... I have to admit the paper is either one of the most advanced I have read or the most obscure..maybe both...most of the mathematical statements in section 1-3 were beyond me....

    when a paper explains something as basic as "divergence" as (flow) and seems to leave all rather complex conclusions until the end...well, call me confused and perhaps even suspicious. If there are direct links between the conclusions in section 4 (expressed for example as "...virtual "bosons" ...manifest as dark energy") and all the "hipper dipper" in earlier sections I missed it. And how did "virtual fermion-antifermion pairs" emerge from any of the earlier sections of development??. It seems like the conclusions are unrelated to the earlier discussion....but that may reflect my lack of understanding...

    So I second flashstarwalker's request for an explanation for the major lines of argument....so I can better understand the intermediate steps the authors take....
    As another example the title of section 3 includes "Calabi-Yau from torsion".. but Calabai-Yau is never mentioned until the last sentence of that section....How are Calabi Yau shapes related to torsion fields??
  20. Apr 4, 2009 #19
    Re posts #6 and #7. Dark energy aside, this is exactly how I view the universe, the only difference is that I used the term "ballistic" to describe the movement, ie. an initial shove and then only gravity to to alter things, as opposed to "inertial". Within these two posts is all the information necessary to understand why the slowing of expansion is expressed by an increased rate of separation. I'm constantly being told that slowing will be evidenced by our being able to see a slowing of the rate of separation, but this is to put us in a position apart from the universe, rather than being within it. If the universe is expanding outward, (and it is), from a single point, as stated in the "einstein.online" website that is constantly referred to on these pages, then, as you say, density will decrease, but not uniformly. The outer regions will become less dense before the inner universe. This will shift the centre of gravity inward. This in turn will apply greater restraint to the inner regions, slowing them at a higher rate than the outer regions. The more that this self perpetuating process continues, the greater the rate at which galaxies will separate. With the inner galaxies slowing at a greater rate than we are, they appear to be accelerating away. We are slowing at a greater rate than the outer galaxies and hence they appear to be accelerating away from us. The universe has been slowing at an increasing rate since it's birth. The "greater with distance" redshift confirms this view, as do the "Hubble wars" with different rates being seen with different angles of view.
    I fully expect the full might of the PF. contributors to denounce this interpretation of the universal evidence. Which begs the question, "why were you not told that expansion is due to expanding space?"
  21. Apr 4, 2009 #20


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    I think it has been repeatedly stated that this is expansion is from every point, not a single point.
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