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Learn cosmology with iCosmos

Animated lessons, short explanations and guided experiments. Everything is calculated from the cosmological model you choose, so changing a parameter changes the lesson.

Cosmic distances in motion

Six short animations, each driven by the same calculation as the calculator.

• Angular diameter distance (size on the sky): How big does a galaxy look at different redshifts?
• Luminosity distance (why distant light is so faint): Why is a distant galaxy fainter than the inverse-square law with its distance D_M would predict?
• Comoving distance (factoring out expansion): How can two galaxies move apart while their comoving distance stays fixed?
• Lookback time (the journey of light): How long has the light from a distant galaxy been travelling?
• Comoving volume (how much universe a survey covers): How much space lies within a given redshift?
• Redshift to distance (from a spectrum to a distance): What does a measured redshift tell us about distance?

Key concepts

• Cosmic units: Mpc, Gly, Gpc³ and Gyr
• Redshift and expansion
• Four distances, one universe
• Curvature comes from the energy budget
• Growth of structure

Your model against real measurements

Compare any model with published data: the Hubble tension (H₀ measurements and type Ia supernovae), BAO distances (DESI DR2) and σ₈ and S₈ (weak lensing and redshift-space distortions).

Guided experiments

Each experiment sets up the calculator, asks a question and then gives the answer with numbers from the model.

1. Why can distant galaxies look bigger?
2. Make a closed universe
3. The age problem
4. What does H₀ actually do?
5. How sensitive are distances to dark energy?
6. Radiation and the “very slightly closed” universe
7. When did the expansion start to accelerate?
8. A universe without a Big Bang

Open the iCosmos cosmology calculator

iCosmos 4.5.0 · updated 10 November 2026