Which of the following is NOT a characteristic observed in the cosmic microwave background?

The cosmic microwave background radiation (CMB) is a relic from the early universe, specifically from the time when atoms first formed and the universe became transparent to radiation. The characteristics of the CMB help us understand the early conditions of the universe.

The CMB is known for its overall uniformity, which indicates that the universe was once in a hot, dense state and has since expanded. It also exhibits slight variations in temperature, known as anisotropies, which provide crucial information about the structures in the universe and its evolution. Additionally, the CMB reflects conditions present shortly after the Big Bang, serving as a snapshot of the universe when it was just about 380,000 years old.

However, the CMB does not contain prominent spectral lines of helium as would be expected in the spectrum of a gas. Instead, it is a nearly perfect black body spectrum at about 2.7 K, and any spectral lines would be so faint that they do not significantly contribute to our observations of the CMB. Therefore, the absence of prominent spectral lines of helium distinguishes it from other types of astronomical data and is why this is the correct answer.