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Ratio of radiaton energy to mass energy?

  1. Jun 23, 2007 #1
    A typical star like the Sun is emitting radiation energy.
    Over a duration, say the star emitted radiation energy, E.
    This would have required the mass, m=E/(c^2), to be converted to radiation.

    For the Universe, is the ratio of radiaton energy to mass energy increasing with time?
  2. jcsd
  3. Jun 23, 2007 #2
    Stars are converting a fraction of nuclear potential energy to radiation, but as the universe expands radiation gets red-shifted, so I'd suspect the ratio of radiation energy to "matter energy" is decreasing. (Wasn't the early universe thought to be radiation dominated?)
  4. Jun 23, 2007 #3


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    I believe you are right Frog,
    the ratio of matter to radiation increases as the scalefactor a
    because the radiation density falls off as a-4
    and the density of matter falls off as a-3
    at present the radiation from the stars would be changing the ratio very much
    so the expansion effect dominates

    there is a published matter/energy inventory for the universe that lists all kinds that the authors could think of
    I don't remember but you could find it on arxiv by searching with keywords like "cosmology" and "inventory" and "energy" I think

    the amount of energy density in the form of light----over and above the CMB----was a small percentage of the total IIRC
    Last edited: Jun 23, 2007
  5. Jun 24, 2007 #4
    So, due to universe expansion and the radiation being red-shifted, has this radiation lost energy, since a red photon has less energy than a blue photon?
    With regard to energy conservation, to what form of energy would this lose in energy have being converted to?

    Would it be gravitational energy, in a similar way that a photon trying to escape a gravitational field is red-shift?
  6. Jun 24, 2007 #5
    You can check this link :

    http://colloquesetconferences.u-strasbg.fr/video.asp?idvideo=4934 [Broken]

    This is a video presentation + slides from Andrei Linde on the origin and fate of the Universe. You will find very interesting slides and there is a whole part on energy conservation laws in an expanding universe (from slide 9...).

    I personally love the way this guy talks about things, despite its complexity, its abstract character, he makes things so concrete and easier to understand
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  7. Jun 24, 2007 #6


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    Nasher, I have not yet looked at the link offered by Chrisina which may teach me something new and cause me to change my understanding of what happens.

    But up til now at least I always thought that in Gen Rel there is no global energy conservation law.

    AFAIK the loss of energy by the CMB is a massive violation of this (non-existent :smile:) "law". In the course of its long history the CMB has lost about 999/1000 of its original energy.
  8. Jun 24, 2007 #7
    I don't agree with the whole presentation from Andrei Linde (not the end part when he starts talking of the landscape...).

    But I do find the ideas on where did the energy of the universe come from very apealing :

    ie, that the total matter/radiation energy is equal to the opposite of the gravitational potential energy in the form of a cosmological constant (vaccuum energy), so that the sum has always been zero.
  9. Jun 24, 2007 #8


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    I have seen this idea explained several times but always with some hand-waving. Alan Guth has expressed a similar notion famously by saying that "the universe may be the ultimate Free Lunch"

    he was talking about the massive Conservation Violation that happens during inflation

    you have a uniform scalar "inflaton" field that represents X number of joules per cubic kilometer-----this is imagined to cause a rapid accelerating expansion like today's "dark energy" but much faster-----now you have many more cubic kilometers and approximately the same density of energy of X joules in each----so you have much much more energy.

    Where did all that additional energy "come" from? All the explanations I have seen, that try to show it "came" from somewhere seem forced to me.

    And then this huge amount of energy created by expanding the volume occupied by the scalar field DECAYS into more usual material forms of energy and we have "reheating" that produces quarks electrons photons etc which eventually condense to form galaxies and us.

    So in a sense the "inflaton" field created us, along with a huge amount of energy that nobody paid for. So that is very interesting.

    What Guth (and perhaps Linde) are trying to say, I think, is that it is like a bucket being lowered into a well, where the pulley-rope turns an electric generator which powers a matter-creating machine using the electric energy to create particles, which then go to fill the bucket. So that matter fills the bucket and gives it weight, so that it can even better produce energy as it is lowered down the well, and produce more matter.

    In the end the bucket is full of created matter and it sits at the bottom of this very deep well. So the matter that was created is a "free lunch" that is balanced to zero by the fact that it sits at the bottom of a deep gravity well. And so negative gravity-potential balances out positive mass-energy and the sum is zero.

    I have been busy and I still have not gone to watch the Linde talk. Perhaps what i am saying is wrong or does not fit with what he said.
    But anyway, right or wrong, I am doubtful and skeptical about this.

    Basically I think that there are problems even DEFINING global energy in a curved spacetime context and also problems defining time globally (except in simplified solutions). So I do not understand that one should EXPECT there to be a global energy conservation law. The mathematical proof of energy conservation depends on assumptions that do not pertain to the universe, so it is nice when it happens but I do not insist on nature obeying it in all situations (nobody has to agree with me :smile: )
    Last edited: Jun 24, 2007
  10. Jun 25, 2007 #9


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    Expansion, in and of itself, does not violate energy conservation. The energy 'lost' due to redshifting is recovered via time dilation.
  11. Jun 25, 2007 #10
    Marcus, "curved spacetime" : what if the curvature had always been zero.
    I don't think that the idea that the Hamiltonian of the universe, when taking into account EVERYTHING, has always been zero, is necessarily a wrong assumption...
  12. Jun 25, 2007 #11


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    Hi Chrisina,
    not sure what you mean. spatial curvature equals zero means space is flat but if expanding then spacetime is still curved.

    BTW I listened to LINDE'S TALK at the University of Paris.
    It was immensely charming and conducive to the imagination.
    I did not find myself convinced.

    I remember being delighted in the 1970s by the cover art on the Scientific American which Linde and his son produced (for their article) showing his vision of eternal or fractal inflation. It was my favorite SciAm article for a while.

    the sense of beauty remains, after more than 30 years, but I have found nothing rigorous to hold on to, with Linde, only brilliant imaginings.
  13. Jun 25, 2007 #12
    yes I meant spatial curvature k=0 in the Friedmann equations.

    In this case, the first equation transforms suggestivly in the following form:

    1/2 a' + V(a) = 0 where V(a) = - (Mo/a + Lambda a^2/6) (taking the radiation energy away which is valid for most of the history, and with c=G=1)

    Does that remind you something ?
  14. Jun 25, 2007 #13
    sorry, I meant,

    1/2 a'^2 + V(a) = 0
  15. Jun 25, 2007 #14
    btw Marcus, congratulations for your 10000th post...
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