Home▸Cosmology▸Hubble's Law — Expanding Universe

🌌 Hubble's Law — Expanding Universe

Visualise Hubble's Law: galaxies recede at v = H₀ × d. Adjust the Hubble constant and watch the universe expand. See redshift, recession velocity and the Hubble sphere.

Cosmology3DEasy60 FPS

About this simulation

This visualisation animates Hubble's Law, the relation v = H₀ · d that says a galaxy's recession velocity grows in direct proportion to its distance. A central observer is surrounded by 80 galaxies that drift outward, illustrating that space itself is expanding rather than galaxies moving through it. The dashed Hubble sphere marks the distance where recession reaches the speed of light, and each galaxy is colour-coded by redshift to convey how far and how fast it is receding.

🔬 What it shows

It models uniform cosmic expansion using v = H₀d. From the chosen Hubble constant the page derives the Hubble time and Hubble radius via the identity T_H = 1/H₀ ≈ 978/H₀ Gyr (with the radius c·T_H in billions of light-years). Galaxy colour runs blue (nearby, low redshift z ≈ v/c) to red (distant, high redshift).

🎮 How to use

Drag the H₀ slider (50–100 km/s/Mpc) to change the expansion rate, or pick a preset such as Planck 2020 (67.4), SH0ES 2022 (73) or Hubble's 1929 value (~500). Toggle the Hubble Sphere and Velocity Arrows buttons to show or hide those overlays, and use Pause to freeze the animation.

💡 Did you know?

Measurements of H₀ disagree: the early-universe value from Planck (~67.4) and the local value from Cepheids and supernovae (~73) differ by more than their error bars. This unresolved gap is known as the Hubble tension and may hint at new physics.

Frequently asked questions

What is Hubble's Law?

Hubble's Law states that a galaxy's recession velocity is proportional to its distance, written v = H₀ · d, where H₀ is the Hubble constant. Edwin Hubble published the relation in 1929 after noticing that more distant galaxies showed larger redshifts. It is the central observational evidence that the universe is expanding.

Why do galaxies move away rather than through space?

In the standard cosmological picture galaxies are not flying apart through a fixed space; instead the space between them is stretching. Every observer sees all other galaxies receding, with no special centre. The simulation places you at the middle only for convenience – the same outward flow would be seen from any galaxy.

What does the Hubble constant slider change?

The slider sets H₀ in km/s/Mpc, between 50 and 100. A larger H₀ means faster expansion, a shorter Hubble time and a smaller Hubble radius, since T_H = 1/H₀ ≈ 978/H₀ gigayears. The presets correspond to real published values so you can compare the Planck and SH0ES estimates directly.

What is the Hubble sphere shown by the dashed circle?

The Hubble sphere is the distance at which the recession velocity equals the speed of light, c. Galaxies beyond it recede faster than light, which is allowed because it is space expanding rather than objects travelling through space. Such galaxies are not causally cut off forever, but their light takes ever longer to reach us.

How accurate is this model of the universe?

The v = H₀d relation and the derived Hubble time and radius are physically correct to first order for the nearby universe. The visualisation is schematic, however: it uses a looping outward animation and fixed galaxy positions rather than solving the full Friedmann equations, and it ignores acceleration from dark energy and relativistic corrections at large redshift.

Frequently Asked Questions

Does Hubble's law mean the Milky Way is at the centre of the universe?

No. Hubble's law holds from any vantage point in a uniformly expanding universe — every observer sees all other galaxies receding, just as every raisin in an expanding raisin bread sees all other raisins moving away. There is no preferred centre.

What is the Hubble tension?

The Hubble tension is a statistically significant disagreement between the Hubble constant measured from the early universe (CMB, ~67 km/s/Mpc) and from the local universe (Cepheid+supernova distance ladder, ~73 km/s/Mpc). Neither measurement is clearly wrong; the tension may indicate new physics beyond the standard cosmological model.

Can galaxies recede faster than light?

Yes. Distant galaxies in the observable universe are receding faster than light due to the expansion of space. This does not violate special relativity because it is space itself expanding, not objects moving through space. Such galaxies are not forever invisible — we can still see light emitted before the recession exceeded c.

What is the Hubble volume?

The Hubble volume is the sphere within which the recession velocity of objects is less than the speed of light (radius ≈ c/H₀ ≈ 13.8 billion light-years). Objects beyond this comoving Hubble radius are receding faster than light at the current epoch, though the boundary changes over time.

How is the Hubble constant measured?

The local method uses standard candles — Cepheid variable stars calibrate the distance ladder, then Type Ia supernovae extend it to cosmological distances, measuring redshift versus distance. The CMB method infers H₀ by fitting the power spectrum of temperature fluctuations with the ΛCDM model and extrapolating to today.

⚙ Under the hood

Vary the Hubble constant and watch galaxies recede faster with distance, mapping redshift to recession velocity and the observable universe's edge.

Hubble's lawexpanding universeHubble constantredshiftrecession velocitycosmologydark energyobservable universe

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install