How does the cosmic microwave background disprove a steady-state universe?

Question

BlackTom AI · Accepted Answer

The cosmic microwave background (CMB) is a nearly perfect blackbody radiation field with temperature T0 ≈ 2.725 K, filling the sky almost uniformly; this is the key observation that discriminates between cosmological models.

1) Relevant knowledge points and concepts
- Big Bang prediction: an early hot, dense phase with radiation in thermal equilibrium; as the universe expands, the radiation cools and redshifts but retains a blackbody spectrum. Temperature scales as T ∝ 1/a or T(z) = T0(1 + z).
- Steady-state model: the universe is eternal and, on large scales, unchanging in time, so it does not predict a relic, uniform thermal radiation with a precise blackbody spectrum.
- Observations: COBE, WMAP, and Planck measured a near-perfect blackbody spectrum and tiny anisotropies (ΔT/T ∼ 10^−5).

2) Step-by-step reasoning
- In the Big Bang picture, photons decouple at recombination when the temperature was roughly T_rec ≈ 3000 K.
- Those photons are redshifted by expansion to the present temperature T0 ≈ 2.725 K.
- Compute the redshift factor: (1 + z_rec) = T_rec / T0 ≈ 3000 / 2.725 ≈ 1101. This matches independent estimates of z_rec ≈ 1100 from other cosmological data.

3) Intermediate calculations
- T_rec ≈ 3000 K, T0 ≈ 2.725 K
- 3000 / 2.725 = 3000 ÷ 2.725 ≈ 1101 (shows consistency with recombination redshift)

4) Why the correct answer is correct
- The CMB’s perfect blackbody form and uniformity indicate the universe was once in thermal equilibrium: i.e., uniformly hot and dense. The precise temperature scaling and the angular-scale anisotropies predicted by an expanding Big Bang match observations quantitatively. These facts are incompatible with a steady-state scenario, which lacks a mechanism to produce such a relic blackbody background.

5) Why the other options are incorrect
- "Nebulae used to emit only microwave radiation": Nebulae are localized objects and do not produce a uniform, cosmological blackbody background; COBE measured a cosmic, not local, signal.
- "Stars were much cooler in temperature in the past": Stars do not produce an all-sky blackbody with T ≈ 2.7 K; stellar evolution doesn’t explain the uniform CMB.
- "The early universe was cold and lumpy": The CMB demonstrates the opposite—hot and extremely uniform with only small (∼10^−5) lumps (anisotropies), not a cold, lumpy cosmos.

6) Conclusion
All evidence shows the CMB indicates the early universe was uniformly hot and dense, confirming the correct answer: It shows the early universe was uniformly hot and dense.

How does the cosmic microwave background disprove a steady-state universe?Single choice

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