Episode 705: Cosmology

The hot big bang

Cosmology is the study of the origins, history and future of the universe. The currently favoured model is the big bang.


  • Discussion: The hot big bang (10 minutes)
  • Discussion: How old is the universe? (10 minutes)
  • Student questions: The age of the universe (20 minutes)
  • Discussion: Cosmic microwave background (10 minutes)
  • Discussion: The future of the universe (10 minutes)
  • Student questions: Critical density (20 minutes)
  • Discussion: Missing mass and dark energy (10 minutes)
  • Student activity: Olbers’ paradox (20 minutes)

Discussion: The hot big bang
Imagine running a film of the universe ‘backwards’ – all matter and energy were originally in a very, very dense state. All exploded outwards = the big bang (Fred Hoyle coined this name, intending it to be derisive).

What happened before the big bang? Space-time was created at the big bang; so many cosmologists argue that the question has no physical meaning.

At the present time (2005) the overwhelming weight of evidence favours the Hot Big Bang theory. There are 3 independent pieces of evidence:

  • The observed expansion of the universe
  • The Cosmic Microwave Background (CMB) radiation
  • The cosmic relative abundance of the light elements (created by the big bang, rather than subsequently in stars or supernovae)

Discussion: How old is the universe?
Episode 704-3: Hubble’s law and the age of the universe (Word, 200 KB)

As discussed above, the Hubble constant gives us a means of estimating the age of the universe. It is ~ 14 billion years old.

Episode 705-1: The ‘age’ of the universe (Word, 102 KB)

Thus the size of the visible universe is set by how far a light beam has travelled since the big bang = c ´ age of universe = 14 ´ 109 light years or 1.3 ´ 1026 m.

Student questions: The age of the universe
Students can look at a number of estimates of the age of the universe.

Episode 705-2: Calculating the age of the universe (Word, 59 KB)

Episode 705-3 The parsec (Word, 50 KB)

Discussion: Cosmic microwave background (CMB)
Space is filled with the cosmic microwave background radiation. This allows an estimate of how much the universe has stretched since it was emitted.

Episode 705-4: The cosmic microwave background radiation (Word, 198 KB)

Discussion: The future of the universe
This is a topic that is still the focus of research and debate.

Possible futures for the universe

If there is enough mass in the universe then its gravitational attraction will eventually overcome the expansion. The universe will stop expanding and then collapse to an eventual big crunch (cf. throwing an object upwards with less than the escape velocity). This is called a closed universe.

If the quantity of mass is just right then the expansion slows to zero ‘at infinity’. This is called a flat universe.

If there is not enough mass for its gravitation effect to overcome the expansion, the universe will continue to expand forever. This is called an open universe.

A closed universe might rebound forever – a big bang eventually resulting in a big crunch which rebounds into a big bang and so on. The whole universe may be a gigantic oscillator!

The critical density is the demarcation between an open and closed universe.


Student questions: Critical density
Observation has yet to pin down the actual density with sufficient precision to decide if our universe has a density larger or smaller than the predicted critical density r0.

Episode 705-5: Critical density (Word, 36 KB)

Discussion: Missing mass and dark energy
According to the standard cosmological model, the universe consists of three categories of mass/energy. Dark matter (25%), dark energy (70%) and a smattering of normal matter (5%).

Dark matter was invented to account for the observed rotational shape of galaxies. Dark energy has been invented more recently to account for the latest red shift data. (As of 2005) at the highest red shifts the universe seems to be accelerating!

Student activity: Olbers’ paradox
To exemplify the depth of ideas that can come from thinking about a simple observation – that the sky is dark at night – students can read about Olbers’ paradox. They could report their thoughts to the class.

Episode 705-6: The sky is dark at night (Word, 46 KB)

Download this episode
Episode 705: Cosmology (Word, 455 KB)