Testing the Hot Big Bang: What the Evidence Can and Cannot Prove
A student group is preparing a debate on the origin of the universe. They compile the following evidence. Twelve nearby galaxies show a linear relationship between distance and redshift, with a best-fit slope of H₀ = 71 km s⁻¹ Mpc⁻¹. The cosmic microwave background has a blackbody spectrum at 2.725 K, is isotropic to about one part in 10⁵, and shows faint temperature ripples. The primordial abundance of helium is about 25% by mass, with hydrogen about 75% and only trace lithium. The oldest globular clusters are dated at 12.5–13.0 billion years. The opposing team argues for a steady-state universe with continuous creation, in which redshift is caused by light losing energy over vast distances ("tired light") rather than by expansion. The group must judge which evidence is strongest and what would count as a fair test.
1. Explain how the redshift–distance relation and the CMB each support a hot, expanding universe, and calculate the implied expansion age from H₀.
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Redshift–distance: a straight-line relation between distance and recession velocity is exactly what uniform expansion produces - every observer sees all distant galaxies receding with speed proportional to distance (Hubble's law). A tired-light explanation must instead produce the same linear law without expansion, and it struggles to explain why the relation is so tight across widely different galaxy types.
CMB: a blackbody at 2.725 K filling all space is naturally explained as radiation from a hot, dense early universe, stretched and cooled by expansion. Its near-perfect isotropy shows the same temperature everywhere, as expected if all regions shared a common hot origin. The tiny ripples are the seeds of structure.
Age calculation:
- H₀ = 71 km s⁻¹ Mpc⁻¹. Convert: 1 Mpc = 3.09 × 10¹⁹ km, so H₀ = 71 / (3.09 × 10¹⁹) s⁻¹ ≈ 2.30 × 10⁻¹⁸ s⁻¹.
- 1/H₀ = 4.35 × 10¹⁷ s ≈ 13.8 billion years.
This is consistent with globular cluster ages of 12.5–13.0 billion years, and crucially the clusters are younger than 13.8 billion years. That consistency is a strong check: a universe cannot contain objects older than itself. The calculation is an upper bound because expansion has not been constant.
2. Evaluate which single observation is hardest for the steady-state/tired-light model to explain, and justify your choice.
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The CMB blackbody spectrum plus its isotropy is the hardest to explain. A tired-light model has no hot, dense phase, so it needs an unidentified process that produces a perfect blackbody at 2.725 K in every direction, with no preferred source and no spectral distortion. No known tired-light mechanism does this; scattering or absorption processes tend to distort a blackbody and produce blurring of distant images, which is not observed.
The redshift–distance relation alone is a weaker discriminator: a tired-light model can be tuned to fit it. Primordial helium abundance is also strong evidence, because 25% He requires a hot, dense phase with a specific neutron-to-proton ratio; a steady-state model must invoke an earlier generation of stars to make it, but then the predicted abundance and the trace lithium are hard to match simultaneously.
The CMB is therefore the strongest single test because it independently fixes a temperature, a spectrum and an isotropy level, all at once. The judgement is about explanatory power: the Big Bang predicts these numbers, whereas the alternative must add unexplained mechanisms for each.
3. Describe one observation that would falsify the hot Big Bang, and explain why it would be decisive.
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A decisive falsification would be a stable object or structure older than 13.8 billion years - for example, a well-dated star or galaxy at 16 billion years, with its distance and age independently confirmed. The hot Big Bang with H₀ = 71 km s⁻¹ Mpc⁻¹ implies a finite age; a genuinely older object would contradict a core prediction and could not be explained by adjusting details.
Other valid answers: a CMB with strong spectral distortions or a preferred direction that no expansion model can reproduce; primordial helium abundance far from the predicted ~25%; or a redshift–distance relation that is clearly non-linear in a way that expansion cannot fit.
The key is that the observation must be decisive, not merely awkward. A single anomalous galaxy can be explained by measurement error or peculiar motion, so it would not falsify the model. A confirmed age conflict, or a CMB spectrum that is not a blackbody, would require either abandoning the model or adding an entirely new physical mechanism - which is what makes it a fair test.