Understandings: The Big Bang model; Cosmic microwave background (CMB) radiation
Applications and skills: Describing both space and time as originating with the Big Bang; Describing the characteristics of the CMB radiation; Explaining how the CMB radiation is evidence for a Hot Big Bang
Guidance: CMB radiation will be considered to be isotropic with T ≈ 2.76K; For CMB radiation a simple explanation in terms of the universe cooling down or distances (and hence wavelengths) being stretched out is all that is required
An expanding universe implies that, in the distant past, the universe was a hot dense place. For the first few hundred thousand years the young universe was opaque to electromagnetic radiation. At some point, however, the universe cooled enough for electromagnetic radiation to be transmitted through space. The photons emitted at that time travel through space forever until they are absorbed by something enroute - such as an appropriate telescope.
As the universe has been expanding for the last 13-ish billion years, the photons have been losing energy. Although they started with a temperature of around 3000 K and carried an energy of around 0.26 eV, they should now have an energy of about 0.24 mV, equivalent to a temperature of around 3K.
In 1965 engineers Penzias and Wilson first observed microwave radiation coming from all parts of the sky. The radiation has a black-body spectrum and corresponded to a 2.76 K temperature emission. The best explanation for this cosmic microwave background radiation (CMB or CMBR) is that it is the expected remnant radiation from the early universe.
Tiny variations in the CMB reveal slight differences in the density of the very early universe. Without these variations, these seeds for future galaxies and stars, then the universe would be a very different place.
A detailed look at the evidence for an ancient universe.
Oxford Physics: pages 662 - 664
Hamper HL (2014): pages 560 - 562, but with a lot of HL Cosmology mixed in.
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