The Cosmic Microwave Background radiation consists of photons that were released from their constant interactions with matter at a time coincident to another important event. Which one of the following events coincided with the formation of the CMB?

Question

BlackTom AI · Accepted Answer

To address the question, I will evaluate each historical epoch in relation to the origin of the Cosmic Microwave Background (CMB).

Option a: The end of the era of nucleosynthesis. While Big Bang nucleosynthesis occurred early and set the primordial abundances of light elements, the photons that became the CMB were not released at this time. Nucleosynthesis ended long before the decoupling of matter and radiation, so this choice does not match the event associated with CMB formation.

Option b: The era of galaxy and star formation. The formation of galaxies and stars happens much later in cosmic history, long after the CMB was already released. The CMB corresponds to a much earlier epoch, when the universe was still largely uniform and photons began traveling freely, not to a period dominated by structure formation.

Option c: The end of the era of matter-antimatter annihilation. The matter-antimatter annihilation era ended earlier and its end does not coincide with the decoupling of photons from matter. The CMB originates from a later process when photons stopped scattering off free electrons, not from annihilation events.

Option d: The time during which most of the ionized matter in the Universe became neutral. This is the recombination epoch, when electrons combined with protons to form neutral hydrogen and the universe became transparent to photons. Photons released around this time are what we observe as the CMB. This directly matches the defining moment of CMB formation.

Option e: The era of inflation. Inflation refers to a very early exponential expansion that occurred before the hot, dense state of the Big Bang became radiation-dominated. The CMB originates after inflation, not during it, so this option does not describe the event coincident with CMB formation.

Overall, each incorrect option either points to an earlier or later epoch or to a process not directly tied to the decoupling of photons from matter. The correct description is the recombination period when neutral atoms formed and the photon mean free path increased dramatically, producing the CMB.

ASTR 350 ONL FA25: Big Bang,Black Holes,Universe (Liu, X)

The Cosmic Microwave Background radiation consists of photons that were released from their constant interactions with matter at a time coincident to another important event. Which one of the following events coincided with the formation of the CMB?

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