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Stupid question

  1. May 31, 2007 #1
    This question concerns Omega(matter+dark+lambda).
    We take into account protons(baryons), electrons(leptons), neutrinos, pions, etc etc when trying to determine Omega from the CMB. but what about photons? I mean there are A LOT more photons than baryons. All those photons carry energy, shouldn't that be taken into account somehow when measuring Omega? are they? perhaps the lambda portion atleast?
    i think i have some fundamental misunderstading here.

  2. jcsd
  3. May 31, 2007 #2


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    Yes, we do need to take into account photons. However, at the current epoch of the universe, they don't contribute very much to Omega, because the universe is matter dominated.

    Even as early as recombination at z=1100, the effect of photons is small enough that it is usually ignored in cosmological calculators (for instance Ned Wright's distance calculator doesn't include radiation terms).

    However, if you go back far enough in time, photons (or more generally, radiation) are the dominant factor in omega. This is in the so-called "radiation dominated era".

    Google for "radiation dominated era", for example http://www.cambridge.org/resources/0521546230/575_p267-268.pdf

    I recall Space Tiger saying that in terms of energy density, the CMB is currently the dominant form of energy in the universe. It's not very large, but it's everywhere, while light from suns (for instance) is only found inside galaxies and is not present in intergalactic space.


    I believe ST has given references for this statement in another post, but I couldn't find it
  4. May 31, 2007 #3
    Yes. thank you very much.
    which leads to my next question inspired by your statement that
    "However, at the current epoch of the universe, they don't contribute very much to Omega, because the universe is matter dominated."

    It is the sum of all Omega's(matter/darkmatter/lambda) that cosmologists are trying to determine is equal to, greater than or less than one. But Omega is the ratio of density now over the critical density where the critical density is a function of Hubble's "constant" which itself is a function of time.
    density critical ~ H(t)^2.
    So my question is assuming the Omega matter = .3 and omega lamba =.7 today so the sum = 1(approximately), that does not mean that Omega matter = .3 and omega lamba =.7, say, 5 billion years ago? correct?
    but the sum did equal 1 (approximately)? is that right? if so, why would the sum be the same but the ratios different?
    i guess maybe I would understand this if i understood the graph on page 51 of this paper better?


    thank you in advance for your help.
  5. May 31, 2007 #4
    Isn't that contradictory? Do you mean that on large scales CMB is the dominant form of radiation?
    Last edited: May 31, 2007
  6. May 31, 2007 #5
    Isn't the dominant form of energy ( density) dark energy, the energy associated with empty space?
  7. Jun 1, 2007 #6


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    Yes this is correct. Different forms of energy evolve differently as the Universe expands. Matter energy density for instance goes as the inverse cube of the scale factor, or equivalently, with the inverse of volume. So you double the volume you halve the energy density, makes sense.

    For photons, they get the inverse cube for the same reasons as matter, but they also lose energy as the Universe expands, going as the inverse of the scale factor this means that as the universe expands radiation density drops faster than matter. Hence in the past radiation was dominant but the energy density in radiation dropped faster than matter, leading to a matter dominated era.

    The cosmological constant is different again. The energy density actually stays constant (hence the name) so as the Universe expands further and the matter density drops eventually Lambda becomes dominant.
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