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How did microwave spectroscopy help scientists discover the cosmic microwave background radiation?

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The discovery of the cosmic microwave background (CMB) was not the result of traditional laboratory microwave spectroscopy, which typically analyzes the rotational states of molecules. Instead, it was an accidental discovery made by Arno Penzias and Robert Wilson while they were using a sensitive horn antenna at Bell Laboratories for radio astronomy.123

While attempting to eliminate persistent background noise, the scientists identified a uniform, isotropic signal corresponding to a temperature of approximately 3 K. This signal was later confirmed to be the relic thermal radiation predicted by the big-bang model. Subsequent measurements, such as those by the Cosmic Background Explorer (COBE) satellite, utilized advanced instrumentation to determine that this radiation exhibits a blackbody spectrum at 2.735 K, providing definitive evidence of the universe's primordial expansion.23456

In summary, the discovery relied on radio detection technology rather than molecular spectroscopy, confirming the theory that the early universe was once a hot, dense, and opaque state.36