• The sea of galaxies and a big question

    From jacob navia@21:1/5 to All on Thu Feb 25 00:19:37 2021
    I have posted several times here about a "sea" of galaxies just beyond
    the limit of our telescopes.

    Two new obsevations start confirming the existence of this "sea".




    ... It boggles the mind that over 90 percent of the galaxies in the
    universe have yet to be studied. Who knows what interesting properties
    we will find when we discover these galaxies with future generations of telescopes?

    The second obsevation is:



    New Horizons Spacecraft Answers Question: How Dark Is Space?

    The New horizons spacecraft, enjoying a sky 10 times darker than Hubble
    has measured the background light of the Universe, i.e. the light from
    space beyond ours and all other known galaxies.

    I quote from the press release:

    "... there may be many more faint, distant galaxies than theories
    suggest. This would mean that the smooth distribution of galaxy sizes
    measured to date rises steeply just beyond the faintest systems we can
    see -- just as there are many more pebbles on a beach than rocks..."

    Reference paper is:
    arXiv:2011.03052v2 [astro-ph.GA] 9 Nov 2020

    Now to my BIG question :-)

    What is the difference between the cosmic background measured by the New Horizons spacecraft and the CMB?

    I know, one is in the micro-wave and the other is in visible. OK.

    But couldn't the CMB be the same thing, i.e. the scattered, red-shifted
    light of all the billions of galaxies that are just beyond the reach of
    our telescopes?

    If we assume that the Universe goes on forever, within a given solid
    angle we should find more and more galaxies until all the field of view
    is blocked by them. Since the red-shift decreases the energy of the
    emitted light, the more distant galaxies just disappear completely,
    red-shifted out of existence (for us). So there would have to be a
    cutoff somewhere. And as we approach that limit, the galaxies lose their individuality and become just a continuum of red-shifted light: the CMB

    Why I am wrong?

    Thanks in advance.


    [[Mod. note -- It seems to me that there are three difficulties with
    trying to explain the CMB as the scattered and red-shifted light of many
    faint galaxies:
    1. Individual galaxy spectra are non-black-body -- they have absorbtion
    features from gas & dust within the galaxy, and from the stellar spectra
    of the stars comprising the galaxies. If we add a bunch of such
    non-black-body spectra, each with its own different (individual)
    redshift, we'll get *some* composite spectrum. But it's highly
    unlikely that all the non-black-body spectral features would cancel
    out to 99.999% everywhere in the composite spectrum to yield a pure
    single-temperature black-body spectrum, which is what the CMB looks
    like (to about 1 part in 100,000).
    2. That last 1-part-in-100,000 of the CMB varies over the sky in a very
    specific way ("multipole decomposition"), and I can think of no reason
    the superposition of many redshifted galaxy spectra would reproduce
    that variation.
    3. To red-shift a galaxy spectrum (color temperature of maybe 5000 K)
    to a temperature 2.73 K would take a redshift of about 2500, corresponding
    to a time of only about 5 million years after the big bang. That's
    nowhere near enough time to form all these galaxies.
    -- jt]]

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