Supernova Standard-Candle Hubble Diagram

Expansion scenarios

Constant rate Decelerating Accelerating Observed supernovae

Sample

Selected supernova

Redshift z

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Predicted (constant)

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Predicted (decelerating)

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Predicted (accelerating)

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Actual observed distance

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Apparent brightness vs. constant-rate prediction

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How it works

A Type Ia supernova always detonates with almost exactly the same true peak luminosity, so it works as a "standard candle": comparing its known true brightness to how dim it actually looks from Earth gives its luminosity distance directly. Independently, the stretch of its light's wavelength (redshift z) says how much the universe has expanded since the light was emitted.

Plotting many supernovae's distance against their redshift traces the universe's expansion history. Each scenario below predicts a different curve for how distance grows with redshift:

d(z) ≈ (c/H₀) · [ z + (1−q₀)/2 · z² ]
q₀ = 0     → constant rate
q₀ = +0.5  → decelerating (gravity wins)
q₀ = −0.55 → accelerating (dark energy wins)

Decelerating predicts supernovae are closer than a constant rate — they'd look brighter.
Accelerating predicts supernovae are farther than a constant rate — they'd look dimmer.
The plotted "observed" data follows the accelerating curve with realistic scatter — exactly what the 1998 high-z supernova surveys actually found, the discovery evidence for dark energy.

Click any yellow point to inspect that supernova's redshift, its distance as predicted by each scenario, and its real plotted (observed) distance.

Constant rate Decelerating Accelerating Observed SN Ia Selected

Click a point to inspect it

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