Dark Energy… And Zombie Stars!

31 Replies to “Dark Energy… And Zombie Stars!”

1. As we dig through the ditches and burn through the witches…

   Click Here.

   1. quoting from a comment in your link:
      24 people are not wise in the way of science, weigh the same as a duck, didn’t get better from? being turned into a newt, whose fathers smelled of elderberries, do not know the average airspeed velocity of an unladen swallow, fell over the rail half-way across the bridge of? death, got retarded from hearing “Ni, Payng and Nuuuuuwong” and has no way of knowing how to always look on the bright side of life!

2. As an armchair cosmologist I really appreciated Mr Howell’s insights –
   “Space itself has some energy associated with it,” said Howell. “That’s what the results seem to indicate, that dark energy is distributed everywhere in space. It looks like it’s a property of the vacuum, but we’re not completely sure. We’re trying to figure out how sure are we of that – ”

   I am starting to understand why many scientists favor Dark Energy as a placeholder or hypotheses… I also find refreshing Mr Howell’s candor and honesty in simple easy to understand language.

   Nice article Ms Plotner – I also enjoyed Mr Steve Nerlich’s insights in some recent articles here about DE that if indeed there is DE, it should not be distributed evenly or else it would violate three well tested laws of thermodynamics – to some extent I am mentally cut and pasting – if some more knowledgeable here wish to comment I would be interested.

   Thanks again UT 🙂

   1. Dark energy is distributed quite evenly in space, and in time. This still obeys the laws of thermodynamics.

      LC

      1. If I’m not mistaken, the part that may violate thermodynamics is how all the new space that is being created as the universe expands all has this energy, seemingly created out of nothing.

         But I know that I’m an ignorant layman so i won’t speculate on it coming from other universes or branes crashing together.

      2. The FLRW equation for the scale parameter a = a(t)in flat space is

         (a’/a)^2 = 8?G?/3

         where a’/a = H, the Hubble parameter and a’ = da/dt. There is an equation of state for the mass-energy in the spacetime

         d(?a^3)/dt + pda^3/dt = 0

         which is most relevant for the case of radiation and matter dominated universe with p = ?/2 and ? a^3 = constant for radiation. The general equation of state is p = w?. The equation of state p = -? corresponds to a case where the total energy is zero and the first law of thermodynamics is dF = dE – pdV = 0 means the energy that is increased in a unit volume of the universe under expansion is compensated for by a negative pressure which removes work from the system. Further pdV = d(NkT), and for a constant thermal energy for the vacuum and Nk = S this is the entropy of the universe. This is due to a negative pressure which “absorbs work” from the universe. As a result the expansion of the universe appears to generate more dark energy, say with the expansion of any volume of space with a constant vacuum energy, but the negative pressure acts as “negative work” to absorb this apparent creation of energy.

         Conservation of energy in general relativity is a funny issue. A conservation law is equivalent to some symmetry, and in the case of energy conservation it requires the existence of some symmetry or isometry which preserves a timelike vector along a time direction. This vector is called a Killing vector. The Killing vector is an eigenvector of the Weyl curvature, which gets into deep territory in general relativity. However, as a rule if a metric comonent has a time dependency there is no Killing vector which is preserved along a spacetime path. The metric for the de Sitter spacetime is

         ds^2 = dt^2 – exp(t sqrt{?/3})(dr^2 + r^2d?^2)

         which means there is no obvious time dependency. However, for the above equation of state with w = -1 this imposes a conservation of energy. This leads to the prospect the universe has a net zero mass-energy.

         LC

      3. No need for such a long post. Not very effective. You just need one sentence to say that energy creation is zero. Maybe one equation. You have to work with what they said. It’s a creation of spacetime and constant density of Dark energy that seems to come out of nothing. Am I right?

      4. You are entitled to your opinion but I for one greatly appreciate Lawrence’s posts. Though I struggle with the maths, I also learn a lot from them and they add a great deal of science to the popularised basic stories.

      5. Dr. Crowell writes in a thought provoking manner, it is an honor to have him here on the UT forums.
         Invariably when you cross-reference his postulations his articulations are meticulously fastidious.
         A mathematical posit is hereby proposed whereby a graph with an algebraic formulae be incorporated acceding proportional benevolence thus eliciting future fan “likes” in his favor.

      6. I love the maths part of LC, even if I do not understand everything when he repeats enough I will. And his explanation is very clear compared to what you have to read from text books.

3. A well placed Rob Zombie lyric at the end there from the song Dragula. 😀

4. Sorry Tammy but this article seems a tad confused:

   “It’s called a Type Ia supernovae and it shines with the luminosity of a billion suns. For all intents and purposes, once they explode they’re dead… But it ain’t so. They might have a core of ash, but they come back to life by sucking matter from a companion star.”

   I thought that a Type Ia fully disrupted the progenitor white dwarf so there would be nothing left onto which to accrete? Perhaps you are suggesting this might not always be the case and what is left could go through another cyle of accretion and explosion.

   “Howell presented a hypothesis to understand this new class of objects. “One idea is that two white dwarfs could have merged together; the binary system could be two white dwarf stars,” he said. ‘Then, over time, they spiral into each other and merge. When they merge, they blow up. This may be one way to explain what is going on.’ ”

   Possible but those two mechanisms are quite different so what is the link between “zombie stars” and Howell’s reiteration of the old merger idea?

   http://www.universetoday.com/56461/merging-white-dwarfs-set-off-supernovae/

   1. I found myself pondering this as well. I had thought that SNIa’s blew themselves into space with nothing left. The explosion of a white dwarf that passes the Chandrashekhar is a sort of giant 1.3 solar mass hydrogen bomb core. It was my understanding that nothing remained such as a core, but the whole thing blew itself into space. This all seems to be putting additional conditions on these SNIa’s, which could mean there is now a range of different physical objects we now think of as SNIs’s.

      LC

      1. It’s the article. Compare it with this version:

         http://www.sciencedaily.com/releases/2011/06/110630131836.htm

         “[Howell] calls Type Ia supernovae “zombie” stars because they’re dead, with a core of ash, but they come back to life by sucking matter from a companion star. Over the past 50 years, astrophysicists have discovered that Type Ia supernovae are part of binary systems — two stars orbiting each other. The one that explodes is a white dwarf star. “That’s what our sun will be at the end of its life,” he said. “It will have the mass of the sun crammed into the size of the Earth.””

         It makes it clearer that he was using the term to refer to the progenitor white dwarf, not a leftover after the supernova.

      2. This is then the standard model for SNIa’s. LC

   2. Don’t blame Tammy; she was just reporting on what Howell said in the original press release.

5. Dark energy is a phenomena hypothesised in response to the observational data that the universe is expanding with a uniform acceleration.

   OK. But couldn’t we just say that the universe is expanding with a uniform acceleration and leave it there?

   I can’t see what we gain by saying that there must be some utterly mysterious form of energy that appears out of nowhere, breaks the laws of thermodynamics, expands the universe and then disappears without trace after having done so. What is the point of flagging this place-marker concept as ‘energy’?

6. Without there being some additional effect, the expansion should slow, and that is what happened for the first 6 billion years or so, but since then the rate has been increasing. The acceleration was initially negative but is now positive so can’t be described as “uniform”.

7. Albert Einstein introduced the cosmological constant to help justify his theory of relativity, but it only applied to a static state. It didn’t take long before Edwin Hubble corrected him and Einstein later referred to his failure to predict the expansion of the universe as the “biggest blunder” of his life. But it wasn’t.

   Neatly related as befits a short article. Now I’m an impatient student of history, and general relativity isn’t a subject I’ve studied, but FWIW I think the expansion of the compressed history goes something like this:

   Earlier observation and theory, including Hubble’s own observations, had all suggested a static universe. (If threatening paradoxes like why the sky is dark, instead of hot and bright as star surfaces, in a sufficiently large static universe is placed on hold.)

   Einstein saw that he could introduce his cosmological constant (cc). It applied generally, but by finetuning its value he could explain the finetuned appeareance of a static universe.

   However Hubble’s later observations failed a static universe. Also there was the revelation that not only is the cc finetuned but the model using them have an unstable steady state. They tend to either collapse or expand from slight disturbances out of uniformity, say by galaxy clustering.

   Einstein then rejected the whole idea of a cc he didn’t see any relevance too. And ironically that may have been his “biggest blunder”!

   One reason Einstein reacted like that was, it is easy to believe, because he saw his cc as a constituent on the space-time side of the general relativity balance of “space-time curvature ~ matter-energy density”. If there were no space-time effects to see, what use was it for his theory?

   Today we see it, prompted by standard cosmology relevance as an energy factor, on the other side. Then it has relevance for understanding the local and global energy of the vacuum. And the matter of finetuning can more generally be seen as the value of a bias, which is a much more common situation. (Well, if you stomach the idea that it is a bias of a universe, potentially a multiverse parameter.)

   @ Wesley Jackson, Steve Nerlich:

   I think it is arguable to term dark energy a placeholder or place-marker. Let me expand on this, while remembering that obviously my first language isn’t english so I may be mistaken:

   A placeholder, the stronger of the two terms, is most often something that is used to stabilize a construction temporarily. Say a box used instead of content while stacking shelves tightly.

   This can be used in models where you want something more detailed to go in a later stage. Say, putting in the assumed amount of heat flow in a model with gases without bothering about if the mechanism is radiation or convection.

   So it is quite apt to talk about placeholders, if we are talking about a general situation with many possible models.

   That is not the case here. We have a tested and true theory, standard cosmology, with no obvious contenders. The parameter of dark energy is equivalent to the observation of accelerated expansion within the theory. Thus it is no longer an ad hoc placeholder.

   [And of course there are now direct observations of dark energy through such effects like the late-time integrated Sachs–Wolfe effect.]

   Here is the other side of this arguable topic: these terms can be, and are sometimes, used to devalue perfectly fine theories, or their perfectly observable constituents, like they are one among several possible hypothetical models. (Models that can be used as gedanken experiments, sand boxes for theorists, et cetera.) IMHO that is not fair to these theories, nor is it good for theory study in general.

   1. http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric#Name_and_history

      “The main results of the FLRW model were first derived by the Soviet mathematician Alexander Friedmann in 1922 and 1924. Although his work was published in the prestigious physics journal Zeitschrift für Physik, it remained relatively unnoticed by his contemporaries. Friedmann was in direct communication with Albert Einstein, who, on behalf of Zeitschrift für Physik, acted as the scientific referee of Friedmann’s work. Eventually Einstein acknowledged the correctness of Friedmann’s calculations, but failed to appreciate the physical significance of Friedmann’s predictions.”

      Friedmann had presented Einstein with the solution before Hubble’s observations, but he was so intent on modelling the static universe he missed the implications. Had he realised the consequence, he could have predicted the expansion before Hubble’s publication of his measurement of expansion, that was the blunder to which he later referred. As you say, it is ironic that it resulted in the CC which accidentally ‘predicted’ dark energy.

   2. http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric#Name_and_history

      “The main results of the FLRW model were first derived by the Soviet mathematician Alexander Friedmann in 1922 and 1924. Although his work was published in the prestigious physics journal Zeitschrift für Physik, it remained relatively unnoticed by his contemporaries. Friedmann was in direct communication with Albert Einstein, who, on behalf of Zeitschrift für Physik, acted as the scientific referee of Friedmann’s work. Eventually Einstein acknowledged the correctness of Friedmann’s calculations, but failed to appreciate the physical significance of Friedmann’s predictions.”

      Friedmann had presented Einstein with the solution before Hubble’s observations, but he was so intent on modelling the static universe he missed the implications. Had he realised the consequence, he could have predicted the expansion before Hubble’s publication of his measurement of expansion, that was the blunder to which he later referred. As you say, it is ironic that it resulted in the CC which accidentally ‘predicted’ dark energy.

   3. Thanks – but again, I have no issue with the observational evidence that the universe is expanding in an accelerating manner – which the late time SW effect aptly demonstrates.

      I agree it is extraordinary that space-time is appearing out of nowhere and I have no explanation for this – I just don’t see how we can ascribe the cause of it to invisible energy.

      I fail to see how the SW effect can be interpreted as a direct observation of dark energy, but maybe it’s just me.

   4. Einstein introduced the cosmological constant as a way of reversing the collapse of matter in the entire universe. If you have a lot of galaxies arrayed through space with no relative motion to speak of their mutual gravity will result in their collapse. Einstein then used a bit that Kahler had mathematically explored, which is a Ricci tensor that is proportional to the metric itself. So Einstein introduced the ?g_{ab} into his field equations to reverse this collapse. Hubble found not long afterwards that the universe was expanding and so Einstein threw out his term.

      Friedmann wrote a solution to the Einstein field equations which is similar to the dynamics of a projectile. If the energy of the projectile is too little, kinetic energy less than potential, it falls back. If it is just at the escape velocity, total energy = 0, it will in an infinite time period reach zero velocity at “infinity.” If the kinetic energy is larger than the potential the projectile escapes to “infinity” with some terminal velocity > 0. These in a cosmological setting correspond to the closed space, an open flat space and the hyperbolic saddle shaped space. Friedmann’s model was not regarded widely, in part because Friedmann died not long afterwards of tuberculosis. His model was picked up in the forthcoming decade by Robertson,Walker and Lemaitre who worked out more of the physical consequences or observable aspects of this spacetime.

      At around the same time the Dutch mathematician Willem de Sitter worked the Einstein field equations in 5 dimensions and examined the curved 4 dim spacetimes that could exist as embedded spacetimes. These are the de Sitter and anti-de Sitter spacetimes. The de Sitter spacetime is a form of the Friedmann-Lemaitre-Robertson-Walker spacetime for a constant energy density assigned to the space. This constant energy is what we call dark energy and is a cosmological term intrinsic to the spacetime metric.

      These developments were conducted in the early 1920s, on the heels of Schwarzschild’s solution for a static gravity field. These early developments in general relativity were eclipsed by the much bigger interest in quantum mechanics that developed in the mid to late 1920s. The next interruption came with world war II, where Oppenheimer and Snyder worked out much of the physics of black holes in 1939, and three years later Oppy was up to his behind in matters of fission, uranium gun barrels, plutonium implosion and fat-man at Los Alamos. Broad interest in general relativity only resurged about 10 years after WWII.

      LC

      1. Thanks, that makes sense. I reread my comment, and found that while I wasn’t explicit the text may be read as implying that Einstein thought the blunder was the cc itself. That was probably what I was thinking when writing it.

         So again thanks, for providing the history details!

   5. I agree it is extraordinary that space-time is appearing out of nowhere and I have no explanation for this – I just don’t see how we can ascribe the cause of it to invisible energy.

      I fail to see how the SW effect can be interpreted as a direct observation of dark energy, but maybe it’s just me.

      First, let me note that no one says that expansion of space-time is caused by an apparently visible* “invisible” energy. Expansion is ongoing, it is the acceleration that is of concern here. Those are entirely different things. (Say, when warming water, heat is responsible for increased temperature, increased heat flow for accelerated temperature increase.)

      Essentially, without dark energy expansion would still be free-wheeling. Inflation is what once caused free-wheeling expansion to happen.

      Also we are not considering appearance of initial spacetime, that is a matter of inflation before the free-wheeling expansion of space-time.

      Second and more importantly, now we are asked to leap between two different theories.

      That the acceleration can be interpreted as dark energy/negative pressure is built into standard cosmology. So that shouldn’t be surprising, it is what the theory says.

      What is the causation here is a matter for more theory. Considering standard cosmology it could be that:

      1) Dark energy is caused by an accelerated expansion. I dunno about the physics of that, but it is permitted.

      2) The accelerated expansion is caused by dark energy. This is what the hypotheses of vacuum energy comes out of, and the observation that dark energy can be interpreted as a cosmological constant allows for this.

      3) The acceleration/energy observation is caused by a constraint of zero energy. This is what some of the results discussed in another thread can be used for.**

      4) A combination of standard cosmology parameters causes (a constraint on) DE. Again, I dunno about the physics of that, but it is permitted.

      – Any other physics outside of standard cosmology than 3) – 4) causes DE.

      Third and not important at all, I can’t resist leading off into more physics =D :

      I don’t think it is correct to criticize a perfectly valid theory for what theories outside of it predict. That DE exists shouldn’t be a matter of argument, if one accepts standard cosmology theory. What DE is, should be questioned.

      You know, this is easily turned on its head. If there is no zero point energy of fields where did it go? What causes the finetuned canceling of vacuum energy? These are the questions that arise if we find that DE isn’t tied to the vacuum but something else entirely.

      Considering that, I find it encouraging that there is an available hypothesis that solves all of this. While raising other questions, naturally. Why is the cc so low? Is there any other viable theory than inflationary multiverse theory that predicts this?

      —————–
      * By visible I mean “observable”, and by observable I mean “observable according to theory”.

      ** Personally I think the constraint is somehow a result of general relativity/the equivalence principle as per some of those results, and the cc is specifically “the cause” of dark energy.

8. I think you have to read lcrowell’s comment just above. Dark energy isn’t only consistent with thermodynamics, it seems to be needed to preserve it. (Preserve local zero energy density but also global zero energy of FLRW universes.)

   Didn’t we all have this conversation a few weeks back? How there is no mystic energy appearing from ‘nowhere’.

   Or conversely how can anyone accept that spacetime expands out of ‘nowhere’ but balk that some of its constituents does!?

   To add to that – now we are leaving physics to go into the world of argumentation “do and don’t” – and take on another characterization that strikes me as unfair here: inflation admits multiverse solutions where universes appears, expands briefly but because their cc is so much larger than ours as quickly contracts and disappears without trace.

   Is inflation invalid because such universes may exist? Is the point of flagging inflation as expansion, necessary for standard cosmology et cetera, invalid? I may be mistaken, but I think the argument since it is no longer a question of “how does it work” but “what is the point” amounts to the fallacy of “argument from incredulity”!?

   As for the specific point here, I just answered that in another comment, so I refer to that on the general question.

   1. Dark energy with the equation of state w = -1 imposes a conservation of the de-Sitter vacuum free energy. The cosmological constant written according to a source in a stress-energy tensor is

      ?g_{??}= 8?G/c^4(?g_{00} – pU_iU_j)

      where the energy density ? is a time-time component of the tensor and the pressure p is a space-space component. The condition p = -? gives conservation of total energy, or the free energy dF = d? – pdV is zero. In effect the negative pressure performs “negative work,” and work = energy, so this absorbs the dark energy which appears to keep pouring forth from the expansion of the universe.

      Indeed this subject keeps coming up, and a couple of months ago I saved writings on this so that I can quickly reuse things to write up responses on this. I found about every 2 weeks I was spending up to an hour writing a variant on what I previously had written. Because new people keep showing up and because some people do not quite get the idea the first time this subject is going to keep appearing. At least I don’t have to get into Anti-de Sitter conformal field correspondence, and how this whole gemish is a boundary effect on this.

      LC

      1. Are we going to have to start talking about “Dark Pressure” too now 😉

      2. A dark pressure, sure enough. Of course one has to see there is a big problem with all of this. Dark energy is a quantum vacuum effect, but where the quantum eigen-states are unknown. Various theoretical proposals have been made, but so far things do not work very well. The problem is this is often approached with some large number of quantum modes, or a huge ensemble of degrees of freedom, which is problematic. I tend to think the vacuum states of the de Sitter spacetime is due to a fermion condensate, with type II string realizations, where the condensate state is similar to superconductivity. In that case all the degrees of freedom of the superconducting carriers (Cooper paired electrons etc) fall into the same quantum state, or a few number of states, and so the degrees of freedom for the system is far less.

         The situation has some reminiscences with the aether theories of the late 19th and early 20th centuries. A large number of degrees of freedom (or a continuum of such degrees) were assigned to a type of fluid. The solution to how electromagnetic radiation propagated in space consistent with dynamics was simply due to 6 parameters in the special relativistic boosts and rotations. We are facing a similar problem these days.

         LC

9. It would seem the container has mass…how surprising.

10. I read an extract about dark energy:
    ‘Dark energy’ is not energy, it has energy which is a subtle but important difference. Energy is a property of dark energy, expressed in joules, calories, BTU’s etc.. Similarly, matter is not mass. Again, mass is a measurable value, a property of matter.

    Specifically, one property that Dark Energy does not have in its ‘at rest’ or inertial, non accelerating state is ‘mass’. However, in any type of acceleration mode it does have mass and, as a consequence, will exhibit matter like attributes. If this seems difficult to envisage, the photon, or any electromagnetic wave, is another example of something which if it were ever ‘at rest’ would appear to have no mass yet when it moves it has momentum.

    The statement *Gravity makes an apple fall to the ground* is obviously true but it is similar to saying *Fire makes us warm*. In the case of fire, it is electromagnetic radiation that makes us warm, an energised form of dark energy. Matter, the mass of which is subject to the famous equation e=mc2, is another manifestation of dark energy and, you will need to take this on trust for a moment, but gravity is caused by a third form, as detailed in the table below.

    It helped me, maybe the extract could help others?

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