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Dark matter = ligh energy, Dark Energy = also light energy?

  1. Aug 3, 2014 #1
    I'm not a physicist nevertheless I wonder about these things so here goes:

    On wikipedia I read that the universe contains:
    68.3% Dark energy
    26.8% Dark matter
    4.9% Normal matter

    But I wonder why normal (light energy) is not mentioned here. Since the beginning of the universe a lot of light energy has been emitted by the stars and the big-bang itself. This light energy must have some influence on the universe as well. Now isn't it possible that this light energy is responsible for the effects artibuted to dark energy and dark matter?

    Dark energy:
    It is know that photons can excert a pressue on objects when they are absorbed or reflected by it. This could resembe the force attributed to dark energy.

    Dark matter:
    Altough gravity is tied to normal (tangible) matter, but couldn't photons also have some gravitational pull to each other and other matter however small? If this is true than maybe there is no need for dark matter also and is dark matter simply the constantly increasing amount of light energy bouncing around in the universe.

    Why photons might have gravity as well:
    If we have one mass of normal matter and another equal mass of antimatter and we let them anihilate each other in an instant (say both travel at light speed towards eachother) than first till the moment of impact both masses generate a gravitational field. Than when the two masses collide and all mass is transformed into energy than it is difficult to imagine that suddenly the gravitational field disappears. I.e. this would result into an discontinuity which may not be physical. What might happen instead is that at the instant of impact the gravitational field remains intact but than spreads out at the speed of light as the light particles travel away from the point of impact.

    Any thoughts on this? I know an idiot can ask more questions than 1000 wisemen can answer but please try ;).

    Arne Sinnema
    Last edited: Aug 3, 2014
  2. jcsd
  3. Aug 3, 2014 #2


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    google "peebles inventory"

    the famous astronomer Peebles published an inventory of all energy and energy-equivalent mass
    so it has those percentages
    AND it has the percentage for radiation too (which you didn't see in Wikipedia)

    The reason most people neglect it is because only a small fraction of a percent of the total is light (and other forms of radiation).

    Peebles cosmic energy inventory is available from "arxiv.org" free. google will tell you the arxiv link. click on that and you get the summary page with a link to the full PDF file

    Let us know if any trouble finding it.
    Last edited: Aug 3, 2014
  4. Aug 3, 2014 #3
    Oke thanks for pointing that out.

    But how about the other question, i.e. whether or not light energy also generates a gravitationa field.
    For example if we convert a mass (instanteneously at t=0) into pure energy does the initial gravitational field generated by this mass then also instanteously disapear or does this gravitational field than riple out with the speed of light. I.e. the photons generated by converting the mass into to pure energy also generate a gravitational field similar to the gravitational field generated by the mass (at least at t=0), but than as the photons move away from the initial position the gravitational field also riples out into space.
  5. Aug 4, 2014 #4


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    I believe it does.
    Last edited: Aug 4, 2014
  6. Aug 4, 2014 #5


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    At the present time, photons make up 0.005% of the energy density of our universe.

    Earlier, the photon density was much higher. The reason why it's so small right now is because radiation density falls off with expansion faster than matter density. If the universe expands so that the average distance between objects is doubled, then matter density drops to 1/8th its previous value, but radiation density drops to 1/16th its previous value.

    So early-on, radiation density determined pretty much everything about how our universe evolved. But it diluted rapidly, until matter became much more prominent. Now, matter is diluting further, while dark energy doesn't dilute at all, such that dark energy is becoming more prominent.
  7. Aug 4, 2014 #6
    Hmm okay interesting although it's not that intuitive why radiation density would fall with a factor 2 faster than normal matter. Is this because space is stretched therefore the radiation is also stretched and thus experiences a redshift therefore lowering the photon energy thus lowering it's mass?

    But if this is the case is radiation energy than not lost. I.e. does the energy conservation balance over the universe still hold?
    Last edited: Aug 4, 2014
  8. Aug 4, 2014 #7


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    Sean Carroll (cosmologist at Caltech) has a nice reader-friendly blog discussion of this issue called "Energy in not conserved".

    I think you can google it with some combination of words like "carroll energy not conserved"

    Yes, it was the first hit:

    …tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.

    It’s not that all hell has broken loose; it’s just that we’re considering a more general context than was necessary under Newtonian rules. There is still a single important equation, which is indeed often called “energy-momentum conservation.” It looks like this:
    [tex]\nabla_\mu T^{\mu\nu} = 0[/tex]The details aren’t important, but the meaning of this equation is straightforward enough: energy and momentum evolve in a precisely specified way in response to the behavior of spacetime around them. If that spacetime is standing completely still, the total energy is constant; if it’s evolving, the energy changes in a completely unambiguous way.
    Having said all that, it would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms. We all agree on the science; there are just divergent views on what words to attach to the science. In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” Which seems pretty sensible at face value.

    There’s nothing incorrect about that way of thinking about it; it’s a choice that one can make or not, as long as you’re clear on what your definitions are. I personally think it’s better to forget about the so-called “energy of the gravitational field” and just admit that energy is not conserved, for two reasons.

    First, unlike with ordinary matter fields, there is no such thing as the density of gravitational energy. The thing you would like to define as the energy associated with the curvature of spacetime is not uniquely defined at every point in space. So the best you can rigorously do is define the energy of the whole universe all at once, rather than talking about the energy of each separate piece. (You can sometimes talk approximately about the energy of different pieces, by imagining that they are isolated from the rest of the universe.) Even if you can define such a quantity, it’s much less useful than the notion of energy we have for matter fields.

    The second reason is that the entire point of this exercise is to explain what’s going on in GR to people who aren’t familiar with the mathematical details of the theory. All of the experts agree on what’s happening; this is an issue of translation, not of physics. And in my experience, saying “there’s energy in the gravitational field, but it’s negative, so it exactly cancels the energy you think is being gained in the matter fields” does not actually increase anyone’s understanding — it just quiets them down. Whereas if you say “in general relativity spacetime can give energy to matter, or absorb it from matter, so that the total energy simply isn’t conserved,” they might be surprised but I think most people do actually gain some understanding thereby.

    Energy isn’t conserved; it changes because spacetime does. See, that wasn’t so hard, was it?
    Last edited: Aug 4, 2014
  9. Aug 6, 2014 #8
    So, to recapitulate:

    Radiation density drops to 1/16 when the universe doubles.

    Normal mater density drops to 1/8, altough of course the momentum of the matter drops but this (apparently) does not have consequences for it's density. I.e. the deepest point of the gravitational well (of an object) does not become deeper or shallower when adding or substracting momentum. Altough of course the apparant shape of the gravitational well may be distorted to a stationary observer.

    This loss of momentum and radiation density is than accounted for by the gravitational field (which basically acts as some kind of spring, i.e. storing up energy).

    However from the lambda-CDM model it follows that for the universe to be flat some density is missing which is adjused for by introducing dark matter.

    For the universe to be expanding at accelerated rates, dark energy is introduced which does not dilute. Therefore I would expect that the total amount of energy insid the universe (excluding dark energy) is conserved. However due to the introduction of dark energy (which does not dilute) and since the universe is expanding at ever higher rates the total amount of energy inside the universe is increasing.

    Is this about right?
  10. Aug 6, 2014 #9


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    Some people would say that summary is about right. I wouldn't though. If you want to understand my viewpoint try this:
    google "prejudices constant"

    the first hit should be a paper available free on arxiv.org called "Why all these prejudices against a constant?

    If "prejudices constant" does not get it as first hit, then try "prejudices against constant".

    The authors' point is that it is misleading to call the Lambda constant an "energy". It occurs naturally in the Einstein equation as a curvature constant (an inverse area) and it appeared explicitly as Lambda as early as 1917. Logically it has to be included unless you have measured it and found evidence that it is zero. Like in freshman calculus you have to include a constant of integration in the answer, when you integrate, unless you know it is zero. We don't KNOW that this curvature constant corresponds to anything we would normally consider an energy. So we shouldn't use words that give the impression of something we don't know.

    There is no reason to imagine that Lambda is anything other than a naturally occurring vacuum curvature constant. A meters-2 quantity (a reciprocal area). Why artificially convert it to an energy density and bother your head about energy which doesn't behave like other energies. It is just the vacuum curvature constant. If you imagine otherwise then read the paper and see if it changes your mind.

    You can always multiply a reciprocal area (m-2) by a force (say in Newtons) and get a Nm-2 quantity, a pressure, and you can consider that as Nm/m3 = joules per cubic meter, an energy density. But that's just arbitrary algebraic manipulation.

    If you look up Einstein GR equation, or the cosmologists' version, Friedmann equation, on Wikipedia you see the iconic form of GR equation in a box at the upper right of all the articles, with a Lambda curvature constant included on the lefthand side with the rest of the curvature terms. That incapsulates my point of view. Who knows? if you read the arguments for it you might agree. It just means that one refers tothe cosmological constant in preference to "dark energy". the math is equivalent and leads to the same results.
    Last edited: Aug 6, 2014
  11. Aug 6, 2014 #10
    Also one of the thoughts I have:

    Might it be possible that black holes become instable at a certain mass (say a similar mechanism as that of atoms, i.e. atoms are stable up to iron, from that point on they again become ever more unstable till they from themselves disintegrate)

    If this might be true than the big bang might not be something special but simply is a supercritical black hole who went ballistic.
  12. Aug 6, 2014 #11


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    Actually, it does if the momentum of the matter is high enough. If the kinetic energy of the particles of matter are very large compared to their mass energy, then the loss of kinetic energy with expansion makes the energy density fall off as the fourth power of the scale factor, exactly like radiation.

    This was actually the case in the very early universe. But it hasn't been the case for a very long time: the motions of normal matter are just too small to even come close to competing with the rest mass energy.

    Yeah, sort of. Energy simply isn't a conserved quantity in curved space-time.
  13. Aug 6, 2014 #12


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    It is an old idea that black holes result in big bangs, going back to John Wheeler, I guess the 1970s or 1980s. Popularized by Lee Smolin starting in 1990s. He was at Princeton for a while and was influenced by Wheeler.

    It is not so interesting to people now, though has not been disproved. Smolin has a popular book about it (The Life of the Cosmos) and many articles. Google "cosmic natural selection" maybe even "CNS" would get it.

    More recently people seem interested in models of BH which eventually explode regardless of size.
    No "critical mass". the idea is ANY black hole, if you wait long enough will explode. And the results are visible as a type of GAMMA RAY BURST (GRB).

    Google "planck stars" if you want to learn more about that.

    Also there was a popular article in Huffington Post about this reprinted from Nature News. It quoted many prominent physicists giving their opinions about this.

    The popular piece was by Ron Cowen (science writer). You might get it if you google "Cowen explode black holes"

    Yeah, I tried googling that and got these links:

    Last edited: Aug 6, 2014
  14. Aug 7, 2014 #13
    Oke thanks for the many answers. But as usually each answer brings new questions.

    - So adding momentum to a mass gives it a higher energy therefore making the mass more dense.
    - But I also have been told that it is not possible to make from a (stationary) non black hole a black hole by
    giving it sufficient momentum.
    So therefore I would expect that it is not possibe to add momentum to an object which is at the critical density of a black hole.

    One thing that comes to mind is the following: Say we build a rocket which propels itself with an imensely powerfull laser. The weight of the rocket is however almost equal to the critical mass for a black hole. Therefore the photons emitted by the rocket will creap out of the gravity well but will have an almost infinite redshift and therefore will not be able to propel the spaceship such that it will become a black hole.

    Is this correct?
    Last edited: Aug 7, 2014
  15. Aug 7, 2014 #14


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    That's not really an accurate way to think of it. In relativity, a useful separation in energy is between mass and kinetic energy. Mass is the total energy in the internal degrees of freedom of a system, while the kinetic energy arises from the overall motion of the system.

    So if you are talking about a black hole, when you add to its motion, you're adding to its kinetic energy, but having no impact on its mass.

    If, however, you have a gas of particles, and increase the motions of the individual particles in the gas while not changing the average motion of the gas as a whole, then you're adding to the mass of the gas, because those motions are internal to the gas.

    In this way, if you have a box of gas and increase the temperature of said box, you increase the mass of the box (and its energy density).

    Does that help?
  16. Aug 7, 2014 #15
    Yes it is very helpfull

    So adding momentum to some object does add (kinetic) energy to the object but does not add to the mass of the object? Therefore it is not possible to transform a non-blackhole into a black hole simply by adding momentum to it, since the momentum does not add to its mass.

    Therefore a partical can lose or gain kinetic energy while moving through a gravitational field but it's mass (internal energy) is not affected. I.e. photons remain massless while moving through a gravitational field but their (kinetic) energy is altered.

    Also since photons are mass-less so they cannot generate a gravitational field.

    So to recapitulate again, now making a clear distinction between internal and kinetic energy:
    - When the universe doubles in size then the mass density drops to 1/8 (amount of mass is NOT affected by
    expansion of space).
    - When the universe doubles in size then the universe radiation energy density drops to 1/16.
    - When the universe doubles in size then the kinetic energy density of the matter in the universe also drops to
    - This apparent loss of kinetic energy by matter and radiation is accounted for by the
    gravitational field (which acts as some kind of spring storing up energy).
    - However from the lambda-CDM model it follows that for the universe to be flat some mass is missing which is
    adjused for by introducing dark matter (mass).
    - For the universe to be expanding at accelerated rates, dark energy (kinetic energy, NOT mass) is introduced
    which does not dilute.
    - The energy balance over the universe holds when dark energy is excluded. However including dark energy
    (and seeing it as a energy instead of a curvature of space or integration constant oid) the energy in the
    universe is not conserved since dark energy does not dilute and the universe is expanding at ever higher rates.

    Is this now (mostly) correct?
  17. Aug 7, 2014 #16


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    That part isn't true. Mass, energy, momentum, pressure, and twisting forces all contribute to the gravitational field in General Relativity. Photons may not have mass, but they do have energy and momentum.

    We didn't know about this until relativity came along because most of the time we experience the gravitational fields of non-relativistic matter. And when you're dealing with non-relativistic matter, the energy in mass is so vastly higher than the other forms of energy density that their effects are negligible.


    This isn't correct. Unfortunately, how matter loses energy with expansion is rather complicated. A basic, basic picture is this:
    1. When the temperature of the matter is high enough that the typical energy per particle is much higher than the particle's mass energy, that matter acts as if it were radiation.
    2. When the temperature of the matter is low enough that the typical energy per particle is much smaller than the particle's mass energy, that matter acts as if it had no momentum at all, for the purposes of understanding how it interacts with the expansion.
    3. In between, something rather complicated and messy happens.

    Nope. Energy just isn't conserved in curved space-time. The energy changes over time following the strict conservation of the stress-energy tensor, which forces energy to change in an expanding universe.

    I don't understand what you're saying here. Evidence for dark matter stems from a variety of sources, none of which is the assumption of spatial flatness.


    Nope. See above.
  18. Aug 7, 2014 #17
    Hmm okay well thanks for the feedback, seems things are quitte complicated (who would have though that ;-) ).
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