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).
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 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
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.
The λI_λ = νI_ν flux---as we will call it faute de mieux---seems to be often used because it is the same whether one has I_λ or I_ν in hand. The equality λI_λ = νI_ν follows from the following derivation:
I_λdλ = I_ν(-dν) (which is just energy conservation) implies λI_λ(dλ/λ) = νI_ν(-dν/ν) and one also has λ = c/ν → dλ = -(c/ν**2)dν → (dλ/λ) = -dν/ν and together they imply λI_λ = νI_ν .In the plot, using the λI_λ = νI_ν flux prevents blueward side (short wavelength side) of the spectrum from falling very low.
Alas, there seems NO easy way do an accurate/precise integration by eye to find the total flux in the various wavelength bands (i.e., spectrum regions).
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.
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 below the other components.???
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.
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.
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:
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).