Abstract
In an expanding universe, matter is diluted as the volume of the universe gets larger.
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Notes
- 1.
One degree Kelvin is equal to one degree Celsius. The Kelvin scale however starts at absolute zero (the lowest possible temperature), which is −273.15 ℃. For very high temperatures close to the big bang, there is not much difference between the two scales.
- 2.
This is not difficult to understand from the following thought experiment. Consider a black body in equilibrium with thermal radiation at some temperature T. The black body absorbs all incident radiation, and in order to maintain equilibrium, it has to emit radiation at the same rate and with the same spectrum.
- 3.
Solar radiation has comparable intensity at all wavelengths in the visible spectrum. This should be perceived as white light, and indeed the Sun looks white when viewed from outer space. However, to observers on Earth , the Sun often looks yellow. This is mostly because the blue part of the spectrum is scattered by the Earth’s atmosphere.
- 4.
This process is similar to how photons make their way from inside the Sun to the Earth. Photons that are inside the Sun (or any other star) are constantly scattered in random directions, and it can take millions of years for them to make their way to the surface of the Sun. Once they get there, they are no longer jostled about, and stream freely towards us, arriving within a mere 8Â min.
- 5.
The observed value of the CMB temperature (3 K) is close to the theoretical prediction (5–10 K). The discrepancy between the two was mostly due to the uncertainty in the average matter density that was used in the calculations.
- 6.
The satellite was named after David Wilkinson who played a major role in CMB research. (Remember, Wilkinson was one of the young fellows that Robert Dicke recruited to build a CMB detector in the1960s).
- 7.
The Greek letter gamma, \(\gamma\), is often used to denote photons.
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Perlov, D., Vilenkin, A. (2017). The Hot Big Bang. In: Cosmology for the Curious. Springer, Cham. https://doi.org/10.1007/978-3-319-57040-2_11
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DOI: https://doi.org/10.1007/978-3-319-57040-2_11
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