Space ★☆☆ Beginner

🌌 Hubble's Law — Expanding Universe

Every galaxy recedes at a speed proportional to its distance: v = H₀ × d. Adjust the Hubble constant, observe redshift, and find the Hubble sphere where recession speed equals c.

Presets:
H₀ = 70 km/s/Mpc Hubble time ≈ 13.97 Gyr Hubble radius ≈ 13.97 Gly
v = H₀ · d  |  z ≈ v/c  |  TH = 1/H₀

Hubble's Law (1929)

Edwin Hubble found that distant galaxies recede faster than nearby ones — with recession speed directly proportional to distance. This is not galaxies moving through space, but space itself stretching. The Hubble constant H₀ sets the expansion rate.

The Hubble sphere (dashed circle) marks the distance at which recession equals c. Galaxies beyond it recede faster than light — they are not causally disconnected, but their light takes increasing time to reach us. The colour of each galaxy encodes its redshift: blue = nearby, red = distant.

About Hubble Expansion Simulator

Edwin Hubble's 1929 discovery that galaxies are receding from us at speeds proportional to their distance — v = H₀d, where H₀ is the Hubble constant — transformed our understanding of the universe. This relationship, now called Hubble's law, revealed that the universe is expanding: not galaxies flying through space, but space itself stretching, carrying galaxies apart like dots on an inflating balloon.

The Hubble constant H₀ represents the current expansion rate of the universe, with accepted values around 67–73 km/s per megaparsec. Measuring H₀ precisely has proven controversial: the "Hubble tension" is a discrepancy between values derived from the cosmic microwave background (~67 km/s/Mpc) and those from local distance ladder measurements (~73 km/s/Mpc), suggesting possible new physics.

The expansion history is described by scale factor a(t), governed by the Friedmann equations derived from general relativity. The universe began decelerating due to gravity but is now accelerating, driven by dark energy (Λ). This acceleration, discovered in 1998 through supernova observations, implies the universe will continue expanding indefinitely, with galaxies beyond the Hubble sphere eventually becoming permanently unreachable.

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.

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.