Talk:Dark energy

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Talk Archive 1

[edit] clarification

I made some edits to the article. I made what I think is an important clarification in the "Nature of phenomena" (whatever that means) section: the SN Ia data tell us the expansion of the universe is accelerating. The CMB tells us the universe is flat. Various things tell us that baryons + dark matter = 30% of the critical density. However, all three are required to suggest that dark matter makes up the extra 70%. --Joke137

I assume, you want to say: However, all three are required to suggest that dark energy makes up the extra 70%.

And put in the data on element abundances, as merging in the Big Bang nucleosynthesis constraints gives even smaller error bounds on the cosmological parameters.

And, BTW: What's your opinion on having separate articles on Dark energy, Quintessence (physics), abd Cosmological constant? Seems some overlap to me.

Pjacobi 00:49, 2005 Feb 7 (UTC)

Well, I think there is plenty of overlap and they could probably be merged. I'm new, and I'm not really sure what the standard is re: when it's better to have one article, and when it's better to have three. --Joke137 06:09, 7 Feb 2005 (UTC)

For what its worth, I vote for keeping the articles seperate. They clearly are related issues, and the quintessence / cosmological constant articles could use some polish, but they also seem to me to be sufficiently distinct ideas as to warrant elaboration on their own pages. Dragons flight 13:14, Feb 7, 2005 (UTC)

[edit] Rewrite

I decided to take a stab at reorganizing this page, rewriting much of it and adding content, per User:Pjacobi's suggestion below. I think a lot of work probably still needs to be done, but it will ultimately result in a better article. I have taken a first stab, drawing on material in the articles quintessence (physics), cosmological constant and accelerating universe, all of which I edited extensively over the past weeks. I think these articles, along with ultimate fate of the universe and Equation of State (Cosmology) should be retained, but should perhaps be more technical, with this sort of an umbrella page for this set of ideas. Please let me know what you think. P.S. Incidentally, if someone wants to move the old talk to an archive page, please go ahead. I don't know how, so I just nuked it. P.P.S. I added some, hopefully simple, equations in the cosmological constant section. Do you think the argument is comprehensible to the reader with little specialized knowedge? --Joke137 00:55, 4 Mar 2005 (UTC)

I think you did a very good job with rewriting this article. There are a few places where I thought your presentation was ambiguous or imprecise, and I may try and do something about that, but overall a remarkably good effort. Dragons flight 01:27, Mar 4, 2005 (UTC)

Thanks a lot. It is something I had been thinking about doing for a while. I've tried to touch it up a little bit, and will probably come back to it again in a few days once it has had some time to settle in my mind. Please edit away where you've found ambiguities. --03:07, 4 Mar 2005 (UTC)

I redirected the accelerating universe page to this one, since it now contains a more complete discussion of SN Ia etc. --Joke137 15:50, 10 Mar 2005 (UTC)

[edit] Split articles

Adding to the thread above: These articles must be seperated, I think! The Accelerating universe is an observational fact (with low proabaility of being incorrect), but the rest of the articles are theories to explain that fact! They can not be mixed, just like the observation of spectral lines can not be mixed up with Bohr's theory of the atom. Awolf002 15:59, 24 Mar 2005 (UTC)

Right now, there is a huge pressure against raising concerns about whether redshifts are entirely due to cosmological expansion (with small contributions from proper motion), or whether some redshifts might be intrinsic.

The question is whether the drop in luminosity is the result of simple distance, or if it is the result of the metrics of an expanding universe. -- Orionix 08:20, 1 Apr 2005 (UTC)

What I advocate is to re-instate the article Accelerating universe in its own right, not having it as a redirect page! Observations should be kept separate from theories, at least as long as the latter are in a realistic competition. Your concerns (which I do not share) would also be better framed in such an article, instead of being in here. Awolf002 18:40, 1 Apr 2005 (UTC)

I think it would be OK to have an accelerating universe page with a detailed technical discussion of the SNIa results, including figures. But I don't think it would be valuable unless it is much more extensive than that in the dark energy article. The problem is that our belief in dark energy comes from complementary sources: big bang nucleosynthesis, the cosmic microwave background, SNIa, large-scale structure of the cosmos etc. Dark energy is the unifying concept between these, the idea that 70% of the cosmic energy budget is unaccounted for, and has to be made up of something that doesn't cluster or evolve much with time, which GR therefore predicts will cause acceleration. If it were only the direct observation of an accelerating universe -- the supernova Ia measurements -- then we wouldn't believe in dark energy with nearly the confidence we have now. Systematic errors, such as a failure in the standardizability of SNIa populations or attenuation in the observed luminosity of supernovae by the intergalactic medium, would be much more worrisome. But the fact is that we have a number of indirect, independent checks, and together they make up something more general than the accelerating universe, called dark energy.

As for keeping observations and theory seperate, I think that is misguided. While it is important to make it clear that we don't have an agreed-upon theory for dark energy, and that the Lambda-CDM model is merely a very good phenomenological model, it is still important to keep the proposed explanations in the same article as the observations, particularly since the cosmological constant, whatever its flaws, perfectly accounts for every observation by adding only one parameter to cosmology. --Joke137 19:37, 1 Apr 2005 (UTC)

I hear what you're saying. The Lamda-CDM model is important and unique in the sense of explaining current observations with minimal changes. It should have its proper place. But I also think it would be worth while to have the Accelerating universe be an article focussing on the SNIa data and have some well written detail on its meaning and history, since this was the first hint of a discrepency from earlier models and had to survive some strong challenges before it was accepted. If I get the time, I will try my hand in this. Awolf002 15:47, 4 Apr 2005 (UTC)

[edit] Other explanations for an accelerating universe

1. Cosmic inflation

2. Leaking gravity (into other dimensions)

3. A failure of general relativity. Spacetime curvature is fictitious and dark matter does not exist.

4. Incorrect interpretations. Hubble's law is not correct and redshift is not a reliable distance indicator.

The 3rd and 4rd options seem the most promising candidates to me in the future. -- Orionix 13:36, 31 Mar 2005 (UTC)

The Lambda-CDM model is an excitingly successfull theory. Unless you can cite an alternate theory which passes the test of the scientific method, is not yet falsified but falsifiable, there aren't alternatives worthy mentioning in the article. Private speculations and opinions are better discussed on USENET. --Pjacobi 14:31, 2005 Mar 31 (UTC)

The PSR J0737-3039 is a good varification to general relativity but the fact that dark matter and gravitational radiation have never been directly detected is suspect to me. Just because general relativity makes correct predictions doesn't mean that gravitational waves really exist. These could just be geometrical entities. --Orionix 14:53, 31 Mar 2005 (UTC)

Gravitational waves almost certainly do exist, as was shown by measurements of binary pulsars. And they are "just" geometrical entities, as is all of general relativity. For 1, cosmic inflation and dark energy are hard to reconcile, because of the very different energy scales on which they occur. The recent attempt by Kolb to connect them seems to have been quickly refuted.

But 2 and 3 are good possibilities. Number 2 is another kind of dark energy, but coming from higher dimensional geometry. It is hard to precisely define the difference between number 3 and dark energy. They used to call a cosmological constant a failure of general relativity, but now write it on the other side of Einstein's equation and call it a new form of energy.

Hubble's law is well established, and it would be very surprising if 4 were the case. It could be the case, although it seems very unlikely, that there are systematic problems with supernovae, but that would not be a failure of Hubble's law, which is a robust prediction of any geometrical theory of an expanding universe. --Joke137 19:59, 1 Apr 2005 (UTC)

[edit] Negative Pressure

I've been working on a paper about dark matter and dark energy for a writing class I'm taking. I found a Scientific American article from January 2001 titled "The Quintessential Cosmos", and based on that article it appears that the discussion in this article about negative pressure could use a bit of work. It's probably correct in the case of the cosmological constant but details seem to be lacking. I'll probably edit it in a while (couple weeks) when I'm not so busy and sick of writing, but the key point is that when the ratio of pressure to energy density drops below -1/3, gravity becomes repulsive. --DonJuan 16:30, 2005 April 21 (UTC)

What, specifically, are the problems with the discussion of negative pressure? I wrote that, as well as the more technical, detailed discussion in Equation of state (cosmology). I was trying to keep the article from getting technical, but in the case of the cosmological constant I couldn't resist putting a short, simple argument about the thermodynamics of a box of cosmological constant in. --Joke137 19:38, 21 Apr 2005 (UTC)

[edit] Correa Link

Let me start out by saying that I have no interest in spending my time debating whether dark energy is real. That is not the purpose of wikipedia. It suffices to say that it is a firmly established theory in cosmology.

I have moved to Correra article User:SamuelR to non-standard cosmology. I hope this is a suitable compromise. The article itself is pseudoscience, that displays a fundamental misunderstanding of dark energy and general relativity. Here are some points:

by more recent so-called computations, this rate of expansion is accelerating, and one obtains all the ingredients for a modern scientific religion - a metaphysics of physics [...] but please remark further that, despite thousands of papers published on the subject, there is literally no experimental evidence for any of them. That is incorrect. Read the evidence for dark energy section. Type Ia supernovae, big bang nucleosynthesis and galaxy clustering are ample evidence.
Indeed, the idea that the universe had a beginning is nothing more than an interpretation, and at that, one that is not legitimized by the First Law of Conservation of Energy. The conservation of energy is well established in general relativity, and satisfied by dark energy and the big bang model in particular. Since it is impossible to define the energy of the universe in Einstein's theory, it means that the stress-energy tensor is covariantly conserved.
There could never be Dark Energy without mass. And there could never be massless energy. Not, at least, according to Albert. Anyway, this is a minor detail, since the Dark Energy that our particle physicists talk about is only 'massless' for laughs - it was 'massless' in a distant past, but is supermassive today. Nonsense. Mass has energy, sure, but energy doesn't need to be massive in the traditional bricks-and-rocks sense. Light is a perfectly good example.
One first assumes a beginning for the universe and postulates, by dint of sheer interpretation, that the universe expands. Backwards. General relativity and observations clearly suggest the universe is expanding.

After that, I got sick of reading it. –Joke137 22:58, 23 May 2005 (UTC)

-- Response from SamuelR --

  Let me start out by saying that I have no interest in spending my time 
  debating whether dark energy is real. That is not the purpose of 
  wikipedia. It suffices to say that it is a firmly established theory 
  in cosmology.

  I have moved to Correra article User:SamuelR 
 <http://en.wikipedia.org/wiki/User:SamuelR> to non-standard cosmology 
 <http://en.wikipedia.org/wiki/Non-standard_cosmology>. I hope this is 
  a suitable compromise.

The name is Correa, not Correra.

  The article itself is pseudoscience, that displays a fundamental 
  misunderstanding of dark energy and general relativity.

Rather, the conventional theory of Dark Energy - along with Relativity - are pseudoscience. Just because they are universally accepted does not make them any less 'pseudoscience'. Many religions claim universal acceptance but that does not make their dogmas true. Only fanatic followers claim such nonsense.

It is truly remarkable that in a "community encyclopedia", a publication one of whose very purposes is to provide a vehicle for presenting our store of knowledge and understanding through open discourse, you would set yourself up as a watchdog of the "purity" of science-as-religious-dogma, as posited and promoted by the officiating "scientifc" institutions.

  Here are some points:

   * by more recent so-called computations, this rate of expansion is
     accelerating, and one obtains all the ingredients for a modern
      scientific religion - a metaphysics of physics [...] but please
     remark further that, despite thousands of papers published on
      the subject, there is literally no experimental evidence for any
      of them. That is incorrect. Read the evidence for dark energy
      section. Type Ia supernovae
     <http://en.wikipedia.org/wiki/Supernova>, 
    big bang nucleosynthesis
      <http://en.wikipedia.org/wiki/Big_bang_nucleosynthesis> and
       galaxy clustering
     <http://en.wikipedia.org/w/index.php?title=Galaxy_clustering&action=edit>
       are ample evidence.

All three lines of evidence are interpretive. Even for Relativity there is no incontrovertible evidence. Other hypotheses can explain the same findings without recourse to the dogmas of Relativity.

    * Indeed, the idea that the universe had a beginning is nothing
      more than an interpretation, and at that, one that is not
      legitimized by the First Law of Conservation of Energy. The
      conservation of energy is well established in general
      relativity, and satisfied by dark energy and the big bang model
      in particular. Since it is impossible to define the energy of
      the universe in Einstein's theory, it means that the
      stress-energy tensor
      <http://en.wikipedia.org/wiki/Stress-energy_tensor> is
      covariantly conserved.

Einstein created formalisms that permitted the universe to run down while energy would be conserved. The question is whether the universe is or is not running down. The answer that it is running down is not a fact but an interpretation. Citing a mathematical model for an interpretation does not make it into a fact. A more consistent view of the universe, and moreover one that does not need to abandon simultaneity, is to conclude that the universe is neither running down nor had a beginning.

Since you cannot prove your contentions other than with suppression and interpretation, you are defending a faith (the 'Einsteinian faith'), not science.

    * There could never be Dark Energy without mass. And there could
      never be massless energy. Not, at least, according to Albert.
      Anyway, this is a minor detail, since the Dark Energy that our
      particle physicists talk about is only 'massless' for laughs -
      it was 'massless' in a distant past, but is supermassive today.
      Nonsense. Mass has energy, sure, but energy doesn't need to be
      massive in the traditional bricks-and-rocks sense. Light
      <http://en.wikipedia.org/wiki/Light> is a perfectly good example.

Light has never been demonstrated to have or not to have mass. It is an issue still open to contention. The notion that Dark energy was massless at the origins and became supermassive in contemporary time is built right into the Higgs model and the so-called unification of weak and electromagentic interactions. If you do not know this, that you are not even very cognizant of the interpretations that are foundational for your faith in relativity. So, you are repressive, a-scientific, and also ignorant.

Understandably, you fear that others might read the link and make their minds up by themselves. Your faith only has to lose and nothing to gain.

    * One first assumes a beginning for the universe and postulates,
      by dint of sheer interpretation, that the universe expands.
      Backwards. General relativity and observations clearly suggest
      the universe is expanding.

A suggestion is not a fact.

   After that, I got sick of reading it. -Joke137 
   <http://en.wikipedia.org/wiki/User:Joke137> 22:58, 23 May 2005 (UTC)

You appropriately named yourself what you are - a joke. And since Dirac and others are wrong about 137, the entire cosmos has put still another joke on you and your fellow interpreters. It's 138, my dear Joke!

It is funny how a text that makes me laugh makes you sick. Maybe science is not for you.

Sam

An encyclopedia has to report the universally accepted view. New theories must battle for their acceptance or even superiority in academic discussion, not in Wikipedia articles and not on Wikipedia discussion pages. --Pjacobi 17:26, 2005 May 24 (UTC)

User:SamuelR, As I wrote on your talk page, I would appreciate it if you read no personal attacks and no original research. You will not win many friends on here if you suggest that I'm a Nazi, that I'm ignorant, that I'm gutless or that I'm a joke. You say

You have misunderstood the purpose of Wikipedia: it is not meant as a community for open discourse. I am merely following official Wikipedia policy. If the policy were any different, I would likely choose not to contribute. In case you don't care to refer to the latter page, let me quote:

The phrase "original research" in this context refers to untested theories; data, statements, concepts and ideas that have not been published in a reputable publication [...] Reputable publications include peer-reviewed journals, books published by a known academic publishing house or university press, and divisions of a general publisher which have a good reputation for scholarly publications.

Finally, let me suggest that science is all interpretation and suggestion. You imply that it is possible to prove scientific theories, particularly ones so far outside the realm of everyday experience as cosmology and relativity, by some other means. I doubt that. I expect you could learn something about this on the philosophy of science page. –Joke137 17:44, 24 May 2005 (UTC)

[edit] No evidence for dark energy

I think that it was correctly stated in the header that dark energy is an hypothetical form of energy. Then, why does the first section have the title Evidence for dark energy? I would rather give it the title The dark energy puzzle or something that would not imply a so strong indication that dark energy at such actually must exist. It may turn out that the true explanation of the mystery won't be appropriately described by the term energy. Also, in the same section, instead of saying that The type Ia supernovae provide the most direct evidence for dark energy, I propose something like The dark energy hypothesis is able to explain the type Ia supernovae observations.. --Philipum 12:46, 27 May 2005 (UTC)

Ahem, in scientific discourse, provide evidence and is able to explain, is nearly equivalent, with some portion of Occam's Razor added. As the Lambda-CDM model is beautifully d'accord with Occam's Razor, I don't see the need to change the formulation. --Pjacobi 13:45, 2005 May 27 (UTC)

Nov 16 2005

This is a good point, but I think it's just a matter of semantics. A prosecutor can gather "evidence" to show that a suspect has done murder even if the suspect is innocent. So there is evidence for dark energy (DE), i.e. there are strange observations that DE can explain, but that does not prove its existence.

Meanwhile, I've edited the "Evidence for Dark Energy" section - I've deleted a few things. For instance, Big Bang Nucleosynthesis (BBN) does not provide DE evidence - BBN was shown to be successful long before the notion of DE, and DE is not thought to affect BBN very much. BBN does not predict the total matter content of the Universe, only the baryon density. It goes a long way as evidence for dark matter (combined with other observations), but not so much dark energy.

Measuring the Hubble constant does not tell you anything about the acceleration of the Universe, only its current expansion rate. Also, the age of the Universe is a derived quantity - it is not really evidence for anything. Though DE affects our estimate of the age, it doesn't go the other way - there are no precision measurements of the age that allow us to measure the DE.

I think the current list of references should be enough if someone wants to verify all this. May I also suggest the textbook "Modern Cosmology" by Dodelson, 2005.

CAS

[edit] newbie request

ok.. instead of having:

((supernova|type 1a supernovae))

shouldn't it be:

type 1a ((supernova|supernovae))

[edit] MOND: Modified Newtonian Dynamics

Moti Milgrom's idea of Modified Newtonian Dynamics should be included in the text. Please see the FAQ and MOND Website. helohe 22:16, 29 July 2005 (UTC)

MOND has nothing to do with dark energy. See dark matter. –Joke137 22:19, 29 July 2005 (UTC)

Woops, I didnt read that Topic correctly. helohe 00:15, 30 July 2005 (UTC)

[edit] Medium of the universe/aether

These edits seem to be original research, and so I removed them. Please, provide references to published articles, before you add this. Awolf002 12:59, 20 August 2005 (UTC)

[edit] a form of energy?

In cosmology, dark energy is a hypothetical form of energy...

In what sense is it a form of energy? Is this a good use of the term energy? --ExtraBold 19:44, 30 September 2005 (UTC)

Well, it's "stuff". And it's not matter. And it has an energy density, certainly, so it "is a form of energy" in the same sense that matter is also a form of energy. Past that, if you can think of a better name/description I'll pass it on to Michael Turner (cosmologist) the next time I'm in Chicago. -- SCZenz 19:49, 30 September 2005 (UTC)

[edit] 120 or 123

10^-120 is the often quoted density. The discrepancy is due to whether you used the Planck mass with G=1 or the reduced Planck mass, with 8πG=1: the difference is 1/(8π)². The number universally quoted by cosmologists and particle theorists is -120, which is, I think, the number we ought to use here. –Joke137 18:35, 2 October 2005 (UTC)

I've always assumed that with an exponent that large, nobody cared about "rounding off" a factor of 1000. I still that might be a contributing factor in what cosmologists cite, but I don't really care either. Go for it. -- SCZenz 19:01, 2 October 2005 (UTC)

[edit] Einstein's constant

As far as I know, Einstein did NOT propose the cosmological constant to obtain a static universe. The Einsteinian universe is indeed static but that was not what he was seeking (and stable is a characteristic that Einstein's universe does not have). He wanted a closed universe as only a closed universe was in his eyes physical. He also hoped to be able to explain mass generation through the cosmological constant until de Broglie showed that it is possible to have a curved universe with cosmologial constant and no mass (de Broglie universe). Then, Einstein saw, his constant had no need and said, it was a mistake to propose it.

That Einstein wanted a static universe is a widely spred declaration. Nevertheless, it is not completely right as can be read in Einstein's original article. So, I am thinking to make some changes to that part in the article.

N.M.B.R.-KN.141.70.111.178 09:03, 31 October 2005 (UTC)

I've read several reputable sources that say he wanted a static universe. Now, if you have primary sources that say the opposite, by all means go ahead—but please cite them. -- SCZenz 10:01, 31 October 2005 (UTC)

[edit] Accelerating universe

I was looking for info about the acceleration of the universe, and was redirected to this article, but there does not seem to be any info here about the acceleration of the universe. Is there any way to get back the accelerating universe article.

GoldenBoar 16:34, 3 January 2006 (UTC)

The information has been folded into this article (under "implications for the fate of the universe") and ultimate fate of the universe. --Christopher Thomas 00:39, 8 January 2006 (UTC)

[edit] The receding speed of the farthest galaxies

can you please explain or verify? are galaxies farther then the Hubel horizon receeding from us in a speed greater than the speed of light?

Yes. Space is allowed to expand faster than light (this is how most proposed FTL drives nowadays work). This is discussed at observable universe, and also briefly at de Sitter universe. --Christopher Thomas 00:24, 9 January 2006 (UTC)

Thank you very much Thomas.

this is very interesting. I have read the following type of explanations before and some thing seems to be missing.

"However, the Special Relativity speed limit only applies to motion through space; space may expand faster than c. This occurs when space expands behind a photon in transit, and the photon may travel distance which is now greater than the Hubble distance, the distance beyond which objects recede at faster than the speed of light, or the traditionally and incorrectly defined edge of the observable universe[1]."

"the universe expansion behind the photon" - doesnt help much. (help me anyway)

How can one distinguish between:

1. An object moving away from you through space

2. An object moving away from you due space expansion

the first is limitted by the speed of light and the second is not

My name is Miki Ganor, I live in Tel Aviv

[edit] Changing "negative pressure"

"dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure." -- Changing "pressure" to "repulsive effect". We wouldn't normally speak of "the pressure of gravity", "the pressure of magnetism", "the pressure of the nuclear force". Also, since we currently have no very good idea what causes the effect that we label as "Dark energy", I think it's a toss-up at this point whether this force/pressure/effect is "positive" or "negative". Presumably in a few years we'll know better. (Here [1] for a definition of "repulsion".) -- Writtenonsand 13:40, 17 January 2006 (UTC)

Actually, physicists routinely talk about pressure when dealing with cosmological models--it just means force per area, after all. Whether it's less clear than another term in this case might be a different question. -- SCZenz 14:32, 17 January 2006 (UTC)

I switched the wording back, largely because the new wording made the first two sentences seem almost redundant, but also because it was less precise. "Negative pressure" is not a statement about cause. Anyway, I welcome more discussion, of course. -- SCZenz 14:38, 17 January 2006 (UTC)

(1) I don't see why my version was less precise.

(2) Is "force per area" actually the relevant concept when discussing dark energy? (again, we don't usually discuss "force per area" when discussing gravity, magnetism, etc. (Or perhaps this is a cosmological convention?)

(3) My quibble on "negative pressure" is twofold:

(3a) In lay uses of the word "pressure", "negative pressure" indicates an attractive effect (if I put a ping-pong ball in a tube and create a negative pressure on one side, the ball will move toward the negative pressure). As I understand it, dark energy is a "repulsive effect" and would be more clearly understood by lay people as a "positive pressure".

(3b) Since we don't know what's causing this effect, we don't really know whether it's a "negative" or a "positive" effect. (A "push" or a "pull".) (Insofar as these terms have any meaning when discussing physics: but if they don't, then why use "negative" or "positive" at all?).

(4) The article Negative pressure begins with "Negative pressure is a term used to describe a pressure less than that of a surrounding fluid (such as the air)." This would appear to be seriously misleading with regard to dark energy (fluid??). (However, I may be mistaken about this.)

- As I assume is obvious, I'm viewing this from a lay position, but we have to assume that many readers of this article will be as well. -- Writtenonsand 16:06, 17 January 2006 (UTC)

Valid points. It is a cosmological convention, but I guess that's not explained anywhere at all. My only quibble with your edit is that saying a "repulsive effect" acts like a force does seem a bit redundant; but with that in mind, you should fix it up however seems best to you. -- SCZenz 16:12, 17 January 2006 (UTC)

Yes, the term "negative pressure" can only be understood in the context of the Equation of state (cosmology) article. To help the lay person, maybe this word should not be used that early. Also, when it is, it should be in clear connection with the equation of state. Would that help? Awolf002 16:17, 17 January 2006 (UTC)

I think Awolf002 nailed it. Early in the article, call it "repulsive force". Then when you get to a place further into the article, where you have some room to maneuver, where you have room for several sentances, then write soomething along te line of "...repulsive force, which corresponds to a negative pressure in the cosmological equation of state".

Hmmm ... now I'm actually confused too. I thought that even in cosmology, that positive pressure correspnded to general expansion/replusion, or am I tripping over the signature of the metric? i.e. in the FLRW metric, one has energy-pressure = (rho, -p_x, -p_y, -p_z) so for positive rho (positive energy density), the acceleration is negative (energy is atractive); while for positive pressure p, the acceleration is positive ... i.e. positive pressures are repulsive in cosmology. Disclaimer: I am not a cosmologist in any way shape or form, and never really had to think about the sign in this metric. Writtenonsand is right: even if one thinks one knows something about cosmology, the sign here is confusing. linas 23:05, 18 January 2006 (UTC)

Hm, I somehow missed the boat on this discussion. Pressure is defined so that radiation has positive pressure. If you have a box filled with radiation, there is a force on the walls of the box, and if the box expands the radiation does p dV work on the box. There is nothing mysterious about pressure in cosmology! Now, if you have negative pressure, expanding the box does work on the contents of the box. So it's all topsy-turvy for the universe, which wants to speed up its expansion when it sees a negative pressure (although, it would be happy speeding up its contraction, too, if that were its current thing). This is all determined by the acceleration equation

$\frac{\ddot{a}}{a}=-\frac{1}{6}(\rho+3p),$

whence negative pressure makes the universe want to accelerate. I'm a little uncomfortable with "repulsive force." It's not a force, it's an utterly homogeneous potential energy density behaving exactly how you would expect it to from simple thermodynamic arguments. –Joke 05:10, 2 February 2006 (UTC)

(Speaking as a layperson here) Isn't it a force? It makes matter move. It makes matter accelerate. Anything that has this effect is a "force", right? I dunno -- I had originally advocated changing "force" to "effect" in the article. From Force: "In physics, a force is an external cause responsible for any change of a physical system." Okay, so "external" is problematical in this context. "Force in its most primitive definition can be thought of as that which when acting alone causes an object to accelerate." That fits Dark energy, right? --- Again, a large percentage of the people arriving at this article are going to be laypeople. IMHO, we need to start with some sort of (probably bastardized, but physicists are used to that) lay-compehensible definition/explanation, and then proceed to the nitty-gritty (which will be incomprehensible to 95%+ of the public and probably to a majority of the people reading this article.) Everyone's comments on this are exceedingly welcome! -- Writtenonsand 14:55, 2 February 2006 (UTC)

It doesn't really make matter accelerate, it makes the expansion of space itself accelerate, and the matter just happens to be in it. For example, the solar system, because it is gravitationally bound, is not accelerating. But I see your point – to the lay reader, it sounds an awful lot like a force. I really don't know what the best balance between technically correct and easily intelligible is. –Joke 14:59, 2 February 2006 (UTC)

(After edit conflict) You describe accurately, what a lay person already thinks about when reading the word "force." And that's why we should avoid using it, here. The description of the cosmos with GR describes the "growth" of space with time, not the acceleration of objects inside of it. That's why only after you understand the "equation of state" of the Universe as a description of the effects "allowed" by GR theory then you can identify dark energy as "negative pressure" for a possible explanation for the acceleration of the "growth" of space. Again, for the sake of lay readers, we should IMO describe the expected effect of dark energy in the introduction, without confusing them with "pressure" or "force" concepts. Awolf002 15:07, 2 February 2006 (UTC)

That sounds reasonable. Although how do you explain simply that the expansion of space is accelerating (as opposed to decelerating if the universe were filled with ordinary matter)? –Joke 15:12, 2 February 2006 (UTC)

I'm mulling over this for a while now, and it is tricky. I think we need to describe that the "expansion" of the universe can be influenceb by the content of it, and that dark energy somehow is expected to have the opposite effect of normal matter. But I still trip up when trying to spell this out. Awolf002 15:17, 2 February 2006 (UTC)

Joke: personally, I find your comments here perfectly comprehensible ("it makes the expansion of space itself accelerate, and the matter just happens to be in it"). On the other hand, I know that there are a lot of people who have trouble with the whole "expanding Universe" concept" (viz "But when we get to the edge, what's beyond that?"). And on the other other hand, as everyone here is noticing, anything nore sophisticated than that is going to be really tough to explain. --- Potentially idiotic question: It's common to explain gravity to laypeople with a "rubber sheet" diagram showing a gravity well. Is there any possibility of using some sort of diagram of this sort to illustrate the concept of Dark Energy? -- Writtenonsand 00:42, 3 February 2006 (UTC)

Here is proposal for a new lead. Please comment:

In cosmology, dark energy is a hypothetical form of energy which permeates all of space and has the effect of accelerating the expansion of space-time in the universe. According to the theory of relativity, space-time evolves under the influence of the content of space, including normal matter and energy. Until recently, it was thought that this influence should have a net effect of deceleration, when the discovery that the universe is expanding at an accelerating rate challenged that assumption. Adding a large amount of dark energy to the equation of state for the universe, providing “negative pressure,” is currently the most popular method for explaining this acceleration, and also helps to account for a significant portion of the missing mass in the universe.

Awolf002 17:50, 7 February 2006 (UTC)

Comment on proposal for a new lead by User:Awolf002. I think this is quite good and seems comprehensible to the lay reader. We might want to tweak the grammar a bit. I'm also concerned that some people get excited whenever they see the word "evolves" -- it's possible that we might want to change that to avoid needless arguments and vandalism. -- Writtenonsand 23:58, 7 February 2006 (UTC)

[edit] dark energy and acceleration of the universe

OK. So the universe is "missing" approximately 70% of the expected dark energy. The universe is also accelerating at rates faster than in the past. Then could it be that, the reason for the acceleration of the universe is that it is being PULLED by the dark energy; where 70% of the missing dark energy actually lies outside of the known universe? Is this a logical hypothesis?

John

I'm afraid that doesn't work; let me try to clarify. The universe's energy density is known, but it's 70% unaccounted for, and that extra unknown 70% is called dark energy. One of its effects is to accelerate the universe. However, this cannot be due to it pulling from "outside" because the universe is uniform at large scales. Furthermore, the limits of the "known universe" are simply the furthest distance light has travelled since the big bang; since gravitation travels at the same speed as light, there can't be anything "outside" pulling. Does that help? -- SCZenz 18:20, 26 January 2006 (UTC)

[edit] Contraction of local space

What is the evidence of the universe expanding as opposed to the local region of space contracting. Also, I have noticed things like microwaving of food taking less time, which may have something to do with a change in the (cosmological?) relationship between the food, microwaves, and time. Also, the [[Lagrangian point]s occur to me in this context. A lagrange point in the center of mass of two objects makes sense, but what about the other 4?Hackwrench 18:32, 3 February 2006 (UTC)

There is a dissertation famous for its title, "Big Bang leftovers in the microwave". –Joke 18:53, 3 February 2006 (UTC)

[edit] Universe Expansion Accelerating?

How about the theory that the Universe is expanding with increasing speed? I found some references on the net, but not in wikipedia.

http://astro.isi.edu/notes/accelerate.html http://www.space.com/scienceastronomy/solarsystem/accelerating.html http://www.nsf.gov/od/lpa/nsf50/nsfoutreach/htm/n50_z2/pages_z3/01_pg.htm

Huh? Did you read this article? Awolf002 14:35, 10 February 2006 (UTC)

[edit] A possible anwser to the "Unsolved problem in physics"

Could be what indicates this web the answer to the "Unsolved problem in physics" marcked in the article? The page seems interesting, but I don't know if the creator is a genius or crazy. Llull 19:19, 21 February 2006 (UTC)

I think we will have to wait for some peer-reviewed publications, otherwise any inclusion of this would violate the no original research policy. Awolf002 19:44, 21 February 2006 (UTC)

I concur. Website belongs to individual who is likely to be a crank. Can't know for sure until seeing his unpublished works (coming out this year). - JustinWick 21:51, 21 February 2006 (UTC)

[edit] Why is the expansion of the universe accelerating?

Could it be that the rate of elapse of time has been slowing over the last 5 billion years?

The expansion appears to be accelerating, which would mean galaxies that are 13 billion years away don't appear red-shifted enough compared to galaxies that are within 5 billion years away. If the elapse of time had slowed down, the red-shift of the most distant galaxies would appear less^*1 than it would otherwise (and those galaxies would therefore appear to be receding, and therefore the universe expanding, less fast than they would otherwise).

Then consider the galaxies that appear to rotate too fast for their supposed mass. If the elapse of time had slowed down, the rotations would look faster now than they would have done at the time the light set off towards us. The galaxies might tell you how much time had slowed down by.

Then you could correlate the history of the rate of elapse of time with the supernova data to see if it resulted in the decelerating universe that everyone had expected before it was discovered that the expansion of the universe is accelerating.

Why should the rate of elapse of time be slowing down? Well, I saw some famous scientist on the telly a few years ago who had a theory that the speed of light had varied over the history of the universe. Instead, his speed of light could have been constant, and it could have been his speed of time that was changing.

^*1 One might think that if time had since slowed down, the most distant galaxies would look as though they were receding faster, not slower. But we can't see their motion; all we can see is the red shift. Red shift means lower frequency light. If time were slower now, that frequency would look higher to us. That would look like less Doppler effect and therefore a lower receding speed.

--Vibritannia 18:41, 25 March 2006 (UTC)

The rate of time does change based on the frame of reference you are in. This is simply a consequence of relativity. transformations in space and time are interconnected. however this shouldnt have any affect on the data presented in Type Ia Sne (or any other physical data). If you are saying that two objects in the "same" frame of references have different "elapsed" time then thats a different story. Im not sure how that would work based on any currently accepted theory. Keep in mind that the evidence of Dark Energy wasnt based on merely one sets of observation but on many observations that culminated with Sne Ia. if the rate of elapsed time is different based on location than relativity as we know it is wrong and it would also throw off alot of the particle physics too. Of course how we define an elapsed time is tricky and is really a discussion that belongs to entropy and arrow of time discussions. --Blckavnger 16:42, 17 November 2006 (UTC)

I think people forget to consider that there may be matter outside our hubble sphere, cause whats inside our hubble sphere to accelerate outwards toward that matter. Danorux 03:56, 13 December 2006 (UTC)

SCZenz covered that below. --Falcorian ^(talk) 04:35, 13 December 2006 (UTC)

[edit] Global Dimming?

Here's a very different possible explanation... the basics are that very distant things (recently measured) are far dimmer than they aught to be, thus further. What about global dimming?--Smkolins 20:47, 20 April 2006 (UTC)

There isn't any proposed mechanism for the standard candles being dimmer. They're chosen precisely because they always have (roughly) the same brightness, due to the physics of how Type Ia supernovae work. Changing this requires proposing that the laws of the universe have changed over that time period. If there's intervening matter causing dimming, 1) it should be nonuniform, and 2) it should leave absorption lines in the spectrum of the supernovae. Observations to date instead indicate that the light is relatively unhindered, giving a distance range within the error bounds stated in the original publications. Given this, and the other circumstantial evidence for the presence of dark energy, it seems to be the most reasonable explanation available given the present state of observations. --Christopher Thomas 20:56, 20 April 2006 (UTC)

[edit] Lay man's view......

As a newcomer to this topic and openly admitting my ignorance of detailed cosmology, I have a one comment and a couple of questions.

The discovery that the expansion of the universe is apparently accelerating must have come as a major shock to professional cosmologists, and the concoction of an entirely new and poorly understood form of energy to explain the observation seems a little knee-jerky at best. Clearly our understanding of the nature of space (the distance/area/volume that exists between isolated quantities of matter) is very poor even compared with our limited understanding of matter itself.

Is the dark energy seen as fundamental property of space, enabling it to inflate/unfold (or some other expanding description) with time or is it seen as a separate entity inhabiting space and exerting a negative pressure on it? With its alleged ultra-high uniformity, the former seems more reasonable.

Is the dark energy concentrated or otherwise manipulated by strong gravity sources as they distort the space around them? Would the effects of dark energy be much stronger in the immediate localities of black holes? —The preceding unsigned comment was added by PeterKM (talk • contribs) on 21:23, 21 April 2006.

The simplest model of dark energy is simply the cosmological constant, a term in Einstein's equations which was historically assumed to be zero. This would make it a property of space. The existence of the cosmological constant isn't new - it falls naturally out of the equations governing General Relativity. Most scientists just hadn't expected it to have a nonzero value.

More complex models of dark energy do sometimes allow for the amount of pressure to vary with time and/or space. Quintessence is one of these models. You'd have to ask a physicist familiar with quintessence about what happens to it in the vicinity of black holes.

The introduction of dark energy in either form solves some of the problems with models of the early formation of the universe, too (mass density needed to cause matter in the early universe to clump into the large-scale structures we see today; not all of this was solved by dark matter). So, given that we're pretty sure the acceleration is happening, and that there are relatively simple ways of adding a negative pressure, and that doing this not only explains accelerating expansion but solves other problems, it seems to be the best thing to do given our current observations and understanding of the universe.--Christopher Thomas 04:29, 22 April 2006 (UTC)

Even though the majority of the universe, based on the most accepted models in the community, is dark energy, the cosmological constant is tiny. First, if it were of substantial value, it would have been verified during the early years of general relativity calculations. Second, if you plug the number of approximately 75% energy density attributed to dark energy, you will get a small cosmological constant. Since general relativity has yet to find any problems experimentally, this should suffice for now. Basically, the effects from a cosmological constant shouldnt effect anything on scales less than cosmologic.

Based on early papers, the detection of the accelerated expansion (expansion of the universe was already known before Sne Ia) was definately shocking. Scientists were already trying to find a source for dark matter and then they have this new thing to worry about. Of course there are many interesting problems in cosmology beyond this like the horizon, early inflation period. Prior to data from Sne Ia, i think many people expected a currently matter dominated universe that would eventually collapse; now it seems like it will expand forever.

To say dark energy is a little knee jerky is a little harsh. Its not really invented, its just the name given to this new element of the universe needed to explain the data. However, the actual source of dark energy is quite perplexing. We (scientists) are not sure even of its dynamical behaviour (if its not a constant) let alone a mechanism to describe. Just another mystery for cosmologists to look into.

Of course it should be said that the data from the Sne Ia experiments could be interpretated wrong. People have a good idea of the mechanism of these types of supernova but looking at the data its clear we arent for sure. the data from the CMB also agrees with Sne Ia so that gives us more confidence to trust this current accelerated expansion. Of course if general relativity is invalid at cosmological distances then its all pretty much wrong --Blckavnger 22:22, 15 November 2006 (UTC)

[edit] Finite & Philosophy

We were discussing the cosmological argument for the existence of God in my philosophy course; and the teacher started talking about negative gravity or dark energy; and how the universe is speeding up and expanding, etc. I wasn't sure what he meant; but it sounded like he was theorizing that if the universe was indeed infinite, than the universe would be a dark cold place right now; so since it isn't, it would prove that it is a finite universe. He was attempting to argue against one of the 4 options brought up in the Cosmological argument (he tried to disprove 3 of the possible options; which would prove that option 4 is the only one. Option 4 [last one] states that the universe had to have a beginning (creator) or a finite first cause). The option he tried to eliminate out of the 4, with this argument/evidence, is that the universe is possibly infinite, needing no finite cause. But what I don't understand is why he had to argue this in the first place; wasn't the traditional view of the big-bang thoery already implying that the universe is finite (it blows up, shrinks due to gravity; then shrinks again, etc.). So the traditional view would already imply the universe is finite; why would he rely on this relatively new finding to cross out that option? 24.23.51.27 22:06, 28 June 2006 (UTC)

It doesn't make much sense to me, either. The current view is that there is no evidence to suggest that the universe has finite spatial extent, so it could be spatially infinite (if it were, we would never be able to tell). On the flip side of the coin, there is no evidence to suggest that the universe is more than 14 billion years old: nobody knows what, if anything, came before the big bang, and there appears to be little immediate prospect of finding out. –Joke 22:31, 28 June 2006 (UTC)

[edit] Expansion "accelerating"

The article says several times that the expansion is accelerating, but in terms of the Hubble factor this is not really true, since the Hubble factor is still thought to be decreasing. --Michael C. Price ^talk 08:47, 11 August 2006 (UTC)

Corrected, I hope, with the aid of cosmic acceleration. --Michael C. Price ^talk 16:47, 11 August 2006 (UTC)

[edit] Occam's Razor?

Considering the various issues which are not yet satisfactorily explained by GTR and QM, how likely is it that a paradigm shift and new set of theories will eventually be developed to explain everything satisfactorily? This might be worth a paragraph of discussion either in this article or a related one. Strikes me as a bit embarrassing to be unable to explain why 70% of the universe appears to be missing. --Scott McNay 04:59, 31 August 2006 (UTC)

What kind of discussion do you have in mind, and what sources would you use? -- SCZenz 05:07, 31 August 2006 (UTC)

[edit] Please use some judgement on external links

I've removed

http://news.bbc.co.uk/2/hi/science/nature/6156110.stm News story: More evidence for dark energy being the cosmological constant

O.K. NASA did a press conference and now several sites and papers will pick up something about that.

But:

Generally speaking, our own article should be better and more comprehensive than a news story, even from the better sources, like BBC
Specifically regarding the new insights: Let's wait for the scientific papers, and reactions by other scientists. Then bring our article up to date and link to these papers as references.

Pjacobi 14:44, 17 November 2006 (UTC)

Yes, perhaps our article should be more comprehensive, but in the mean time..... --Michael C. Price ^talk 14:47, 17 November 2006 (UTC)

Please point out only item of (valid) information which is in the BBC take but is missing here. --Pjacobi 14:56, 17 November 2006 (UTC)

If there are none then your 2nd point was irrelevant. Links to news stories are still useful for background, verifiability and links to more information, other related stories. --Michael C. Price ^talk 15:17, 17 November 2006 (UTC)

Here's the Hubble newsitem and another link newstory on the same discovery. The scientific paper will be by Adam Riess et al, published on Astrophysical Journal on February 10, 2007. _└ ^VodkaJazz / _talk ^┐ 01:31, 19 November 2006 (UTC)

New Hubble Space Telescope Discoveries of Type Ia Supernovae at z > 1: Narrowing Constraints on the Early Behavior of Dark Energy Riess et al. Haven't read it yet, but I do believe that's it. --Falcorian ^(talk) 05:52, 20 November 2006 (UTC)

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