The thermal cosmic microwave background radiation

A warm body radiates; you can feel the thermal radiation from a hot fire. In a closed cavity with walls that are at a fixed temperature the radiation in the cavity relaxes to a spectrum - the intensity of the radiation at each wavelength - that is uniquely determined by the temperature of the walls. The time it takes for the radiation to relax to this thermal spectrum depends on how strongly the walls absorb and emit radiation. If the walls are perfectly absorbing - black - the relaxation time is comparable to the time taken by the radiation to cross the cavity. That suggested a commonly used name: blackbody radiation is radiation that has relaxed to thermal equilibrium at a definite temperature. The thin line in Figure 2.2 shows the spectrum of blackbody radiation at temperature

above absolute zero. This is the thermal radiation - the cosmic microwave background radiation, or CMBR - that fills space.

Max Planck proposed the first successful theory for the spectrum of black-body radiation in 1900; it was also the first step into the new field of quantum

Solar Irradiance Planck Radio

Wavelength (cm)

Fig. 2.2. The spectrum of radiation that uniformly fills space, with greatest intensity at millimeter - microwave - wavelengths. In this book this is termed the CMBR. The thin line in this figure is the theoretical Planck blackbody spectrum of radiation that has relaxed to thermal equilibrium at temperature T0 = 2.725K. The thick line running over the peak shows the measurements by the NASA COBE and UBC COBRA groups. They are not distinguishable in this figure. The symbols represent other measurements at more widely spaced wavelengths. This plot was made by David Wilkinson in 1992.

Wavelength (cm)

Fig. 2.2. The spectrum of radiation that uniformly fills space, with greatest intensity at millimeter - microwave - wavelengths. In this book this is termed the CMBR. The thin line in this figure is the theoretical Planck blackbody spectrum of radiation that has relaxed to thermal equilibrium at temperature T0 = 2.725K. The thick line running over the peak shows the measurements by the NASA COBE and UBC COBRA groups. They are not distinguishable in this figure. The symbols represent other measurements at more widely spaced wavelengths. This plot was made by David Wilkinson in 1992.

physics. Richard Tolman (1931) noticed that radiation in a homogeneous universe could relax to a thermal spectrum, if there were enough matter to absorb and reemit the radiation energy often enough to cause it to relax to equilibrium. In effect, the whole universe could be the blackbody "cavity." He also showed that the expansion of a homogeneous universe would cool the radiation. Most importantly, Tolman showed that once the radiation has relaxed to thermal equilibrium the expansion of the universe preserves the characteristic blackbody spectrum, with no further need for matter to promote or maintain thermal equilibrium. The expansion of the universe causes the temperature to decrease in inverse proportion to the expansion parameter in equation (2.3), that is,

To summarize, blackbody radiation uniformly filling an expanding universe stays blackbody; only the temperature of the radiation changes as the universe expands. This is the essential signature. Since, as we now discuss, the spectrum of radiation filling our universe is close to thermal we have evidence that conditions were at one time right for relaxation to thermal equilibrium.

Figure 2.2 shows measurements of the intensity of the CMBR. It peaks at a microwave wavelength near 2 mm. The thick black line running over the peak shows measurements of the intensity at a densely sampled range of wavelengths. These measurements were made above the atmosphere, to avoid radiation from molecules in the air, independently from the National Aeronautics and Space Administration (NASA) COBE satellite (Mather et al. 1990) and from a UBC (University of British Columbia) rocket flight (Gush, Halpern and Wishnow 1990). The measurements are very close to -and not measurably different from - Planck's blackbody spectrum over a wide range of wavelengths.

The universe we see around us is close to transparent at wavelengths near the peak of this radiation. We know that because distant galaxies that are sources of radio radiation are observed at these wavelengths. This means that the universe as it is now cannot force radiation to relax to the distinctive thermal spectrum shown in Figure 2.2. And this means that the universe has to have evolved from a very different state, one that was hot and dense enough to have absorbed and reradiated the radiation, forcing it to relax to its blackbody spectrum. That is, contrary to the classical steady state cosmology, we have evidence that this cosmic microwave radiation is a fossil remnant from a time when our universe was very different.

One learns from fossils what the world used to be like. The fossil microwave background radiation is no exception: we have learned a lot from the close study of its properties. The evidence is that the thermal radiation played an important role in the history of the universe, including the thermonuclear reactions that produced light elements in the early stages of expansion and the dynamics of the growth of the mass clustering that we observe as galaxies and concentrations of galaxies. The study of both aspects, the radiation as a signature of what things were like and as a dynamical player in what happened, are recurring themes, in the recollections in Chapter 4 of research in the 1960s and in the subsequent developments described in Chapter 5 of the detailed measurements of the radiation and what the measurements have taught us. Our discussion of these themes begins with an inventory of other dynamical players: what does the universe contain in addition to the fossil thermal radiation?

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