A percentage of the white noise on old television sets originates from the cosmic microwave background, left over from the hot dense phase of the early universe that occurred about 400,000 years after the Big Bang.
This cosmic relic is easy to detect. The reason is simple: at long wavelengths, radiation brightness scales with temperature, and room temperature is just a hundred times larger than the microwave background temperature, 2.73 degrees Kelvin above absolute zero.
Another thermal relic, the neutrino background, decoupled from cosmic matter about a second after the Big Bang. At the start of my career in astrophysics, I brainstormed with Glen Starkman on the feasibility of detecting this background and found it daunting. Indeed, 35 years later – this neutrino background was not detected yet, but only constrained indirectly based on the expansion history of the Universe.
Dark matter is even more weakly interacting than neutrinos. Therefore, its particles decoupled thermally from known forms of matter and radiation even earlier than the cosmic neutrino background. Just as with the neutrino background, it is not surprising that we have not directly detected the dark matter particles so far.
Finally, the most weakly interacting particles are gravitons, representing ripples in spacetime – also known as gravitational waves. A year ago, I published a paper with Sunny Vagnozzi on techniques to detect the cosmic graviton background, a possible thermal relic from the Planck time, 10 to the power of –43 of a second after the Big Bang.
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