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How do we know that the source of the CMB comes from the early universe, and we don't simply observe the rare interstellar or intergalactic dust of 3K temperature?

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The Big Bang model is the current standard model for the evolution of the universe. It is built up by using General Relativity equations and particle data and astrophysical observations. It is a successful model because there has been no unexplainable contradiction, i.e. the model developed to encompass and explain mathematically, what looked like contradictions.

That the universe is not in a steady (thermodynamic) state is an observational fact, it is expanding and recently accelerated expansion has been observed with better measurements.

The Cosmic Microwave Background is electromagnetic radiation, i.e. photons of very low frequency, not dust. There are experiments measuring and mapping the sky for this .

CMB

Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE, the most precisely measured black body spectrum in nature.[8] The error bars are too small to be seen even in an enlarged image, and it is impossible to distinguish the observed data from the theoretical curve.

The current history of the universe goes as follows:

In the Big Bang model for the formation of the universe, Inflationary Cosmology predicts that after about 10^−37 seconds the nascent universe underwent exponential growth that smoothed out nearly all inhomogeneities. The remaining inhomogeneities were caused by quantum fluctuations in the inflaton field that caused the inflation event. After 10^−6 seconds, the early universe was made up of a hot, interacting plasma of photons, electrons, and baryons. As the universe expanded, adiabatic cooling caused the energy density of the plasma to decrease until it became favorable for electrons to combine with protons, forming hydrogen atoms. This recombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old. At this point, the photons no longer interacted with the now electrically neutral atoms and began to travel freely through space, resulting in the decoupling of matter and radiation.

The color temperature of the ensemble of decoupled photons has continued to diminish ever since; now down to 2.7260±0.0013 K, it will continue to drop as the universe expands.

The extreme uniformity of the map of the sky ,

skycmb

The detailed, all-sky picture of the infant universe created from nine years of WMAP data. The image reveals 13.77 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from the our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.

Note that the color map is about 5 orders of magnitude smaller than the current black body temperature of the CMB, the all-sky is extremely uniform. It was the measurement of this uniformity that forced modeling the beginning of the universe quantum mechanically, as at the time the photons decoupled the universe could not come into thermodynamic equilibrium: due to the light cones of special relativity there were parts not possible to thermodynamically interact with other parts so as to homogenize the radiation.

We know it is radiation, we know it is uniform and has a black body spectrum, and it fits in the whole current model of the universe. Dust certainly does not.

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  • $\begingroup$ The question was why can't dust of 3K temperature explain this phenomenon. Would it have a different spectrum? $\endgroup$
    – Calmarius
    Commented Dec 21, 2015 at 16:15
  • $\begingroup$ The answer is one is not measuring dust, but electromagnetic radiation that permeates everything. Dust effects are sought on the CMB pattern, see adsabs.harvard.edu/full/1998A%26A...336...44P, dust is not uniformly distributed but is around galaxies and clusters. If it is at some level, at best it will be in thermodynamic equilibrium with the photons of the CMB and not distinguishable. It cannot generated the CMB $\endgroup$
    – anna v
    Commented Dec 21, 2015 at 16:49
  • $\begingroup$ The sentence in the second paragraph is not right. In the stady state model the universe is also expanding. $\endgroup$
    – MBN
    Commented Dec 21, 2015 at 17:23
  • $\begingroup$ please AnnaV , to avoid a quasi duplicate question : why are we sure that it is not objects getting out the Hubble horizon ? Will such farest visible radiation not show a black body curve ? it's common for any large sample $\endgroup$
    – user46925
    Commented Dec 21, 2015 at 17:42
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    $\begingroup$ +1 for "It is a successful model because there has been no unexplainable contradiction" Scientific models are models: they might be right, they might be completely wrong. The purpose of science is not to find an answer which is the "Objective Truth" with big O and bit T, but to find a model which corresponds the best to our observations. $\endgroup$
    – vsz
    Commented Dec 22, 2015 at 9:14
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Because is relative homogeneous and isotropic(it has different density variations in all directions) and therefore the photons have undergone gravitational redshift.Also, photons lifetime is of the order of lifetime the universe.

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  • $\begingroup$ variation of 10^-5 of the temperature : a typo error ? $\endgroup$
    – user46925
    Commented Dec 21, 2015 at 17:34
  • $\begingroup$ No, actually these variations were in fact the matter variations which grew beacause of gravitational instabilities to the galaxies which we see today. $\endgroup$
    – Nikey Mike
    Commented Dec 21, 2015 at 17:38
  • $\begingroup$ Here in the initial conditions they also have used 10^-3 variations corresponding to recombination era. $\endgroup$
    – Nikey Mike
    Commented Dec 21, 2015 at 17:40
  • $\begingroup$ "Also, photons lifetime is of the order of lifetime the universe." - is there any way for us to actually measure that? I don't think so... $\endgroup$ Commented Dec 22, 2015 at 3:01
  • $\begingroup$ The stability issue is discussed hereor here $\endgroup$
    – Nikey Mike
    Commented Dec 22, 2015 at 13:01
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Theory I: There is CMB corresponding to ~3 K blackbody radiation coming at us from all directions.

Theory II: There is ~3 K blackbody radiation coming to us from "rare" interstellar dust and there is no CMB.

Testable difference: There would be directions where the intensity of the radiation (photons per m^2 per steradian per second) were different. We would expect greater intensity in the plane of the galaxy (with much pollution from other light sources and attenuating objects) than perpendicular to the plane of the galaxy. There would be additional bright beacons in the direction of other galaxies. In fact, we should expect the low temperature dust beacons from other galaxies to vary in frequency (temperature) due to variations in energy density and, generally, to be hotter the farther away (further back in time, more compact) they are.

Result of observation: We do not observe this directional difference in background radiation intensity. In fact, we observe the opposite: greater intensity out of the plane of the galaxy and much scatter and loss for photons trying to traverse the plane of the galaxy. Further, we find that in the direction of other galaxies, the CMB is no more intense and does not seem to vary in temperature.

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    $\begingroup$ the CMB may be hidden by foreground objects, ie the Milky Way. Satellites results are filtered and corrected, missing photons are interpolated, mainly for the MW raw images $\endgroup$
    – user46925
    Commented Dec 21, 2015 at 23:34
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The short answer is we know that cosmic background radiation came from the early universe because it is the farthest out light we can see. And just based on chronological order and the speed of light we know what came first. It wouldn't matter what theory of the universe you believe in, we can see that background radiation came earlier than all the other light. There are a few methods they use to measure these distances. For up close things they use trigonometry and parallax techniques. Farther out they use standard candle readings of type 1-A nova which emit a certain a unique signature . For the really far stuff they use red shift and expansion comparisons.

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    $\begingroup$ But how do we know it is the 'farthest out' light we can see? All we can measure is photons hitting us - they don't have ID showing their date of birth. $\endgroup$
    – Jon Custer
    Commented Dec 21, 2015 at 16:08
  • $\begingroup$ There are a few methods they use. For up close things they use trigonometry and parallax techniques. Farther out they use standard candle readings of type 1-A nova which emit a certain a unique signature . For the really far stuff they use red shift and expansion comparisons. $\endgroup$ Commented Dec 21, 2015 at 17:02
  • $\begingroup$ Only high redshift is relevant here. Then add the black body radiation signature and the 1st fitting with the Gamow model : temperature value and constancy from all directions $\endgroup$
    – user46925
    Commented Dec 21, 2015 at 17:30
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    $\begingroup$ Which work great for "point" sources, or point like. When something is hitting from everywhere (plus/minus the structure), getting a distance is a bit trickier. Remember, the original experiments at Bell Labs were aimed at understanding atmospheric noise, with the (rapid) connection to the Big Bang being a little serendipitous. $\endgroup$
    – Jon Custer
    Commented Dec 21, 2015 at 17:30

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