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1.9 The Radiation Backgrounds

Apart from matter in its visible form, we may look for radiation at various frequencies. In general, measurements of the electromagnetic radiation at a given frequency (or in a given range of frequencies) reveal peaks that are associated with relatively nearby discrete sources, many of which can be identified in specific directions. However, after these peaks are eliminated, there is still a residual background of radiation. This back-ground radiation could also arise from discrete sources that are located much farther away and therefore cannot be resolved, or it could arise from processes in the intergalactic spaces. Table 1.3 gives a rough estimate of the energy densities in the various wavelength ranges. It should be remembered that the measurements in X-rays, gamma-rays, and so on became possible only from the early 1960s with the advent of space astronomy.

Table 1.3. Radiation background at different levels

Wavelength, lambda
Frequency, nu Energy density
Type of radiation Energy range, E (erg cm-3)

Radio nu leq 4080 MHz leq 10-18
Microwaves lambda in 80 cm to 1 mm approx 4 x 10-13
Optical lambda in 4000 Å to 8000 Å approx 3.5 x 10-15
X-rays E in 1 to 40 keV approx 10-16
gamma-rays E geq 100 MeV leq 2 x 10-17

One thing is immediately clear from Table 1.3. Compared with the estimates of matter density, the radiation energy density is less by about three orders of magnitude. This observation is often expressed by the statement that the universe is at present ``matter-dominated''.

It is also clear from Table 1.3 that the most dominant form of radiation background is in the microwaves. The spectrum of the microwave background is very nearly that of the blackbody radiation of temperature ~ 3 K. Moreover, the extreme homogeneity of this radiation on small angular scale seems to rule out the possibility that it could have arisen from discrete sources. As we shall see in Chapter 5, the most popular interpretation of this radiation is that it is a relic of an early hot epoch when the universe was much denser than it is now. Unlike the matter distribution, this relic radiation is extremely homogeneous. This contrast further exacerbates the difficulty of understanding the origin of discrete structures against a smooth radiation background.

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