Red Shift, Hubble's Law & Age of Universe
Simple Explanation
Redshift is the stretching of light's wavelength (shifting toward the red end of the spectrum) observed when a light source moves away from an observer. Hubble discovered that almost all distant galaxies show redshift, and that this redshift (and therefore recession speed) increases with distance — direct evidence that the universe itself is expanding.
Why Do We Need It?
Hubble's law provides both strong evidence for the Big Bang theory and a way to estimate the age of the universe, connecting observable astronomy directly to fundamental cosmology.
Formula
Hubble's Law
v = H₀d
Distant galaxies recede from us at a speed proportional to their distance — the farther away a galaxy is, the faster it appears to be moving away, providing evidence the universe is expanding.
- v
- — Recession speed of the galaxy, in km/s
- H₀
- — Hubble's constant, ≈ 70 km/s/Mpc (kilometres per second per megaparsec)
- d
- — Distance to the galaxy, in megaparsecs (Mpc)
When to use it: Use to find a galaxy's recession speed from its distance, or to estimate distance from an observed recession speed (measured via redshift).
Worked Example
Finding a galaxy's recession speed
A galaxy is measured to be 300 Mpc away. Find its recession speed, using Hubble's constant H₀ ≈ 70 km/s/Mpc.
Why Does This Work?
If the universe's overall space itself is expanding uniformly, then galaxies farther away have more expanding space between them and us, causing them to appear to recede faster — this is exactly the linear relationship (speed proportional to distance) that Hubble observed and that gives strong support to an expanding universe originating from a single point (the Big Bang).
Real-Life Example
Estimating the age of the universe
Cosmologists estimate the universe is roughly 13.8 billion years old.
Taking the reciprocal of Hubble's constant (1/H₀, with appropriate unit conversion) gives a rough estimate of the time since the universe began expanding from a single point — a direct, simple application of Hubble's law to one of the biggest questions in cosmology.
Practice
A galaxy has a measured recession speed of 4900 km/s. Find its distance, using H₀ ≈ 70 km/s/Mpc.
MediumCommon mistake
Thinking redshift means galaxies are physically moving THROUGH space like a car driving away — cosmological redshift is primarily caused by the expansion of SPACE ITSELF between galaxies, stretching the light's wavelength as it travels, not (only) by galaxies moving through a fixed, static space.
Quick Review
- Redshift: light's wavelength stretches when its source recedes.
- Hubble's law: v = H₀d — recession speed is proportional to distance.
- The reciprocal of H₀ gives a rough estimate of the age of the universe.