Which of the statements is true about the cosmic microwave background?

Question

BlackTom AI · Accepted Answer

To tackle this question, I will evaluate each statement about the cosmic microwave background (CMB) in turn.

Option a: 'All of the options.' This is a summary claim that if every other statement is true, then this one must be true as well. We need to verify each of the other options before accepting this.

Option b: 'It appears to be cosmic: It is everywhere in our sky, coming from every direction in space.' This is correct. The CMB is observed uniformly across the entire sky, coming from all directions, which is a hallmark of its cosmological origin.

Option c: 'It is a blackbody: Its spectrum is that of a blackbody source with a single temperature of 2.725 degrees above absolute zero everywhere.' This is also correct in the standard description of the CMB. Its spectrum is that of a near-perfect blackbody with a current temperature of about 2.725 K (approximately 2.725 degrees Kelvin above absolute zero) that is effectively the same across the sky to a high precision.

Option d: 'It is the best blackbody known: Unlike the spectra of stars, the CMB's blackbody spectrum is not contaminated by absorption lines.' This statement is true in spirit. The CMB exhibits a nearly perfect blackbody spectrum with no spectral absorption lines like those seen in stellar spectra, making it an excellent example of a blackbody spectrum in nature.

Option e: 'It is uniform: It looks remarkably the same in every direction.' This is largely true on large scales. The CMB is extremely uniform, with tiny anisotropies at the level of about one part in 100,000, but overall it appears highly uniform in different directions.

Putting it together, each of options b, c, d, and e conveys a correct aspect of the CMB, and therefore option a ('All of the options.') is true as it correctly encompasses the rest. It's worth noting that the language in option c is conventional (temperature in kelvin rather than degrees, though the meaning is clear), and option d emphasizes the absence of absorption lines in its spectrum, distinguishing it from many stellar spectra.

Which of the statements is true about the cosmic microwave background?Single choice

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