Caption: On a log-log plot, the cosmic microwave background (CMB) (which the current observable universe form of the cosmic background radiation (CBR)) plus the extragalactic background light (EBL) (in its various wavelength bands) which together constitute diffuse extragalactic background radiation (DEBRA).

DEBRA is the electromagnetic radiation (EMR) field that permeates space isotropically (or nearly so) that does NOT come directly from compact sources.

The CMB is the dominant component of DEBRA since it has the most integrated radiant flux (AKA flux) (power per unit area (perpendicular to the beam) per unit time). It has a blackbody spectrum with color temperature T=2.72548(57) K.

Note that DEBRA is NOT a blackbody spectrum overall and, other than the CMB, the components of DEBRA have NO well defined temperature???: i.e., are NOT individually blackbody spectra---or so yours truly thinks???.

Note also space is cold in the sense that the CMB is pretty cold and it is the dominant component of DEBRA.

For more information on DEBRA, see Douglas Scott, "New Physics from the Cosmic Microwave Background, Background".

Features:

1. The plot here seems to be a semi-accurate recreation of better plots. For example, see DEBRA in Douglas Scott, New Physics from the Cosmic Microwave Background, Background.

2. The horizontal axis is logarithmic wavelength in microns (μm) and logarithmic photon energy in electron-volts (eV).

   Note that photon energy E is virtually the same as frequency ν since E = hν, where Planck constant h = 4.1356 67696*10**(-15) eV-s. Planck's constant is a fundamental constant of nature.

   However, the horizontal axis CANNOT be entirely accurate since E_eV is NOT exactly equal to 1/λ_μ. The exact relationship is

         E_eV = hc/λ_μ = (1.23984 ... eV μ)/λ_μ

                 ≅ (1.239841974 eV μ)/λ_μ ≅ (1.24 eV μ)/λ_μ ≅ (1 eV μ)/λ_μ ,

   where vacuum light speed c = 2.99792458*10**8 m/s (see also NIST: CODATA: All physical constants). On a log-log plot, the difference between 1 and 1.23984 ... is NOT too important.

3. The vertical axis is logarithmic λI_λ = νI_ν (as the original caption of the plot show: see File:Extragalactic-background-power-density.jpg), where I_λ is the specific intensity per unit wavelength and I_ν is specific intensity per unit frequency (see Douglas Scott, New Physics from the Cosmic Microwave Background, Background; Wikipedia: Radiometry: Integral and spectral radiometric quantities).

   The λI_λ = νI_ν is logarithmic frequency/wavelength (log fw representation) of specific intensity. For more on this representation, see Blackbody file: log_representation.html.

4. The wavelength bands shown are the cosmic gamma-ray background (CGB), cosmic X-ray background (CXB), cosmic ultraviolet/optical background (CUVOB), cosmic infrared background (CIB), cosmic microwave background (CMB), and cosmic radio background (CRB).

5. As one can see, the CMB (with T=2.72548(57) K: see also Wikipedia: Cosmic microwave background radiation: Features) is the dominant component of DEBRA being ∼ 30 times higher in λI_λ = νI_ν flux than anything else.

   The CMB is also dominant when you integrate up the energy in the various wavelength bands. For example, you find that the CMB is ∼ 100 bigger than the cosmic infrared background (CIB) (see, e.g., Douglas Scott, New Physics from the Cosmic Microwave Background, Background).

   If the plot were non-logarithmic, nearly all one would see would be the CMB. All other components would be crushed down nearly to the horizontal axis.

6. Overall, DEBRA is NOT a blackbody spectrum.

   Besides the CMB, some of the other components may approximate blackbody spectra. But if they do at all, they probably have very different color temperatures than the CMB.

   If you extended a CMB blackbody spectrum fit to all wavelengths, it would probably fall well below the other components.???

7. Because the CMB is the overwhelmingly dominant component of DEBRA and has T=2.72548(57) K, it is fair to say in one sense that space is cold.

   Certainly, the electromagnetic radiation (EMR) incident on stars is cold because it is mostly the CMB.

   But is it because space is "cold" that heat energy flows from stars to space (i.e., from hot to cold) in the observable universe?

   NO. The hot-to-cold rule is a usual rule for most terrestrial cases, but the general rule is the 2nd law of thermodynamics (the law of increasing entropy).

   Even if the EMR in space was hotter than stars, net heat energy in the form of EMR would flow from stars to space as long as the EMR in space was very low density.

   The EMR flow from stars is spreading energy far apart, and thus increasing the disorder or entropy of the observable universe.

8. Why is the EMR density of space so low?

   This question is a form of the traditional Olbers' paradox which is "why is the night sky dark?"

   The short answer is the expansion of the universe causes the cosmological redshift which continuously redshifts EMR to lower energy.

   But where does the energy lost in the cosmological redshifting go? Most textbooks are elusive on this point---in studying physics, you learn that textbook writers frequently ignore tricky points hoping that you will NEVER notice.

   Apparently, the energy just disappears. In fact, the conservation of energy principle does NOT apply to the observable universe as a whole. General relativity (GR) gives us the general-relativity energy-momentum conservation equation (see also Car-120) as an absolute physical law to which the conservation of energy principle is just an emergent principle applying on scales much less than the whole observable universe.

9. Where does DEBRA come from?

   Well the CMB originates in the recombination era of Big Bang cosmology.

   The other components seem to all come from ordinary sources: stars, interstellar medium (ISM), accretion disks of various kinds, supernovae, supernova remnants, compact remnants, intergalactic medium, gamma-ray bursts (GRB), and so on (see, e.g., Douglas Scott, New Physics from the Cosmic Microwave Background, Background).

   The non-CMB components of DEBRA are a background radiations because:

   1. To some degree the individual sources are sufficiently remote and dense on the sky that they CANNOT be individually picked out: i.e., they are NOT resolvable.
   2. To some degree the EMR traveling through interstellar space, intracluster space, and intergalactic space is often scattered by particles (mainly free electrons in intergalactic medium (IGM)???) and cosmic dust (which is pretty much all interstellar dust: i.e., dust inside galaxies). The scattering randomizes the directions of the photons, and so the observed ones traces back to NO point source of EMR.

   Note that intergalactic space (i.e., NOT interstellar space and intracluster space) does very little scattering of the CMB and DEBRA in general because of its low density (see Wikipedia: Cosmic microwave background radiation: surface of last scattering).