Experiment #53 · Scientific experiment

Hubble's Redshift Law

The universe is expanding

Edwin Hubble (with Vesto Slipher's spectral data) · 1929 · Cosmology

First published: E. Hubble, "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae", *PNAS* 15 (1929): 168–173.

Distant galaxies recede with velocities proportional to their distance. The universe is not static; it is expanding.

Using Cepheid variable stars to measure distances to nearby galaxies, and Vesto Slipher's earlier spectroscopic data on their redshifts, Hubble in 1929 plotted recession velocity against distance and found a linear relationship — galaxies are receding with velocities proportional to their distance from us, *v = H₀d*. The result implied a non-static universe, in tension with Einstein's 1917 cosmological model and with the steady-state intuitions of most astronomers. Extrapolated backward, it implied that the universe had a finite age (~14 billion years on modern values) and arose from a hot, dense initial state. The work is the empirical anchor of modern cosmology; Lemaître's 1927 derivation of the expansion from general relativity and the discovery of the CMB in 1964 complete the case for the Big Bang. Modern values: H₀ ≈ 67–73 km/s/Mpc.

Formulation

For galaxies at distance d, measure spectroscopic redshift z; compute recession velocity v ≈ cz (for z ≪ 1). Plot v vs d. Observed: linear relation v = H₀d. Modern interpretation (general relativity): expansion of space itself, characterised by the scale factor a(t).

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Dimensions Engaged

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Space

A fundamental finding about Space · Ontological Status: space itself is expanding (not just objects moving through static space). The metric of spacetime is dynamical.

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Time

Forces Time · Extent to be finite: extrapolating backward yields a singular origin ~14 billion years ago. The universe has a beginning.

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Matter

Bears on Matter · Conservation at cosmological scale: matter density decreases as the universe expands; the CMB-determined photon-to-baryon ratio and primordial nucleosynthesis follow from the expansion history.

Responses — How Schools Engage

Affirms / takes the bait 3

Naturalism

A model of empirical cosmology: quantitative observations decisively favouring a non-static universe. The Big Bang model is built on this foundation.

Realism

The expansion is real; the universe had a finite beginning. Steady-state cosmologies are empirically untenable.

Catholic/Thomistic

A cosmology with a temporal beginning is congenial to creation *ex nihilo*. Lemaître himself was a Catholic priest; Pius XII's 1951 endorsement set the tone for later theological reception.

Reframes the question 3

Eternalism

The block universe accommodates a finite past: the beginning is a boundary of the manifold, not a mystery requiring temporal "creation." The expansion is a structural feature of the geometry.

Presentism

A challenge: light from distant galaxies brings evidence of states that no longer exist. Presentists must distinguish what is real (only the present) from what is evidentially accessible (records of the past).

Multiverse Theory

Hubble expansion plus inflationary cosmology generically predicts eternal inflation and a multiverse of bubble universes — though the inference from Hubble alone is much weaker than from the CMB.