The Sachs-Wolfe Effect

The cosmic microwave background (CMB) peaks at a wavelength of 1.9 mm and frequency 160.23 GHz, if spectral radiance is defined in terms  of frequency.  If spectral radiance is defined in terms of wavelength, then the CMB peaks at wavelength 1.1 mm.  This radiation comes from all directions, and the curve of intensity as a function of wavelength very closely approximates a perfect black body having temperature 2.725 Kelvin.  Since the Big Bang 13.8 billion years ago, the universe has expanded and cooled so that today its temperature is 2.725 K.

About 380,000 years after the Big Bang, the universe had expanded and cooled enough so that for the first time it became transparent to electromagnetic radiation.  Thus when we accurately map the exact spectrum of the cosmic microwave background in different directions, we can construct a “baby picture” of the universe when it was only 380,000 years old.

Our baby picture is not smooth but has features.  At that early time, the universe had already developed into denser regions, and less dense ones. Now, it is important to note that cosmic microwave background photons that left a denser part of the universe have been gravitationally redshifted to slightly longer wavelengths (and lower frequencies) to a greater extent than elsewhere.  This is called the Sachs-Wolfe effect.


Lots of exciting cosmological information is coming out of mapping the tiny differences in the CMB spectrum as we look in different directions. I’m wondering, though, if anyone has seen temporal variations in the CMB? In other words, if you stay pointed in a particular direction and carefully measure the CMB spectrum over time, does it change or fluctuate at all (after all sources of noise have been removed)?  Even though our current understanding of cosmology might lead us to believe that the CMB would not change fast enough for us to measure, has anybody looked?

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