Theory — Galaxies, Hubble’s Law, and Cosmology
Galaxies are vast systems of stars, gas, dust, and dark matter. Studying how they are built and how they move reveals the history of the whole universe.
1. Types of galaxies
Galaxies come in three broad kinds: spirals with a disk and arms like our own; ellipticals, smooth round balls of mostly old stars; and irregulars with no clear shape. Some galaxies have blazing centres powered by supermassive black holes; the most extreme are quasars.
2. Hubble’s law
In the 1920s it was found that a galaxy’s recession speed is proportional to its distance:
v in km/s, d in megaparsecs (Mpc), H0 in km/s per Mpc
So H0 = v / d, and the more distant a galaxy, the faster it recedes.
The recession is measured from the redshift of the galaxy’s spectral lines. The relation is not because we are at a special centre; space itself is expanding, carrying the galaxies apart.
3. The age of the universe
Running the expansion backward, all galaxies were together at one moment. The time since then is roughly the inverse of the Hubble constant:
(with H0 in km/s per Mpc)
For H0 ≈ 70, the age is about 14 billion years.
4. The expanding universe
This expansion began in the hot, dense Big Bang about 13.8 billion years ago, whose afterglow we still detect as the cosmic microwave background. The universe also contains dark matter and dark energy, which together shape its expansion and fate.
Apparatus
Studying galaxies uses tools for imaging shapes and measuring redshifts. In the simulation these are modelled, but the readings match what each instrument would give.
Instructions
Work through both tabs. Calculate first by hand, then press the button to compare.
Part A — Hubble’s law and the age of the universe
- Read a galaxy’s recession velocity v (km/s) and distance d (Mpc).
- Compute the Hubble constant H0 = v / d by hand.
- Enter your value and press Check; the panel also shows the implied age ≈ 978 / H0 billion years.
Part B — Classify galaxies
- View a galaxy’s shape.
- Predict whether it is spiral, elliptical, or irregular, then press Check.
Simulation
Team Questions
Example Report
Worked example: the Hubble constant and the age of the universe
A galaxy recedes at v = 7000 km/s at a distance d = 100 Mpc.
H0 = v / d = 7000 / 100 = 70 km/s per Mpc.
The implied age is 978 / H0 = 978 / 70 ≈ 14 billion years, close to the accepted 13.8 billion years. Measuring the slope of the velocity-distance relation and inverting it is the calculate-then-compare core of the lab.