What will be the biggest change in our view of the sky that happens before the last stars burn out?

Question

BlackTom AI · Accepted Answer

Because cosmic expansion is accelerating due to dark energy, light from galaxies that are not gravitationally bound to us will be stretched (redshifted) and their flux will fall until they effectively disappear from our sky. Relevant concepts: cosmic expansion and redshift, the cosmological event horizon in a Λ-dominated universe, gravitational binding of the Local Group, and the long-term stellar-evolution timeline (stelliferous era).

Step-by-step reasoning:
1. Expansion and redshift: As space expands, wavelengths of photons are stretched by a factor (1 + z). Each photon's energy decreases by 1/(1 + z), and the rate at which photons arrive is reduced by another factor 1/(1 + z). Thus observed energy flux from a distant source is suppressed roughly by ~1/(1 + z)^2 (plus the usual inverse-square geometric dimming via the luminosity distance). As z grows without bound for receding objects beyond the cosmological horizon, the received flux tends to zero.
2. Event horizon and binding: Because dark energy causes accelerated expansion, there is a cosmological event horizon: light emitted beyond a certain comoving distance will never reach us. Galaxies not gravitationally bound to the Local Group (a few Mpc scale) will recede beyond detectability and eventually beyond the horizon.
3. Timescale relative to last stars: Before the very last stars burn out (trillions to quadrillions of years in the far future), accelerated expansion will already have carried distant galaxies so far and redshifted their light so strongly that they become undetectable; only the merged remnants of the Local Group will remain visible.

Why the correct answer is correct: "All galaxies beyond our local group will be undetectable" follows directly from accelerated expansion and the cosmological event horizon making external galaxies redshifted and their flux negligible.

Why the other options are incorrect:
- Blue stars will not become more plentiful; stellar populations become dominated by long-lived red dwarfs, so the sky reddens.
- The CMB will fade and redshift to lower temperature and intensity, not brighten.
- Dark matter does not become “plentiful enough” to cause overall collapse; dark energy-driven expansion prevents global collapse and dark matter density declines with expansion.

Therefore the final answer is: All galaxies beyond our local group will be undetectable.

What will be the biggest change in our view of the sky that happens before the last stars burn out?Single choice

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