The Age of the Universe

Astronomers estimate how long the Universe has been expanding since the Big Bang. Think of rewinding a video of galaxies moving apart. If we know how fast they are separating now, we can estimate how long ago they were all much closer together.

Key ideas

• Redshift: Light from distant galaxies is stretched to longer (redder) wavelengths. This shows they are moving away.
• Hubble’s law: The speed a galaxy moves away is proportional to its distance: $v = H_0 d$. Here, $H_0$ is the Hubble constant.
• If expansion has been roughly steady, the time since everything was together is about the inverse of $H_0$.

Estimating the age

Time is distance divided by speed. Using Hubble’s law for any galaxy, $\text{age} \approx d / v = 1 / H_0$. This gives a simple estimate for the age of the Universe.

Typical measured values of $H_0$ give an age of about 14 billion years.

Block equations:

$$v = H_0 d$$

$$\text{Age of Universe} \approx \frac{d}{v} = \frac{1}{H_0}$$

What this means

Because $1/H_0$ gives a finite time, it suggests all matter in the Universe was once much closer together. The expansion is like dots on an inflating balloon: every dot moves away from every other dot as the surface stretches. Other evidence, such as the cosmic microwave background radiation, also supports this Big Bang picture.

Common misconceptions

• It was not an explosion into empty space; space itself is expanding.
• There is no single “centre” of the Universe that everything moves away from.
• $1/H_0$ is an estimate. The expansion rate has changed over time, so precise ages use detailed models.