Evolution of the Universe

Question 1

State the hot big bang theory and what this means for the universe.

Answer 1

The universe started off very hot and dense and has been expanding ever since. This means that the universe has a finite age, because it started at a specific point in time.

Question 2

How can we calculate the age of the universe?

Answer 2

1/Hubble’s constant.

Question 3

Describe the evolution of the universe after the big bang.

Answer 3

• Starts infinitely hot, infinitely dense, infinitely small
• One grand unified force expands the matter
• It cools and the forces split into gravity, strong nuclear, weak nuclear and electromagnetic forces.
• Rapid period of inflation
• There is a sea of quarks, antiquarks, leptons and photons that don’t bind together because there is too much energy.
• Matter-antimatter symmetry gets broken, so there is slightly more matter
• Universe cools enough for quarks to form particles (protons and neutrons) (10^12K)
• Matter and antimatter annihilate each other, leaving a small excess of matter and loads of photons
• Protons are then cool enough to form helium nuclei (10^9K)
• Electrons combine with nuclei to form atoms (3000K)
• Recombination occurs (the universe becomes transparent since there are no free charges for photons to interact with)
• Slight fluctuation in density have allowed clumps of matter to form due to gravity (3K) (galaxies, stars, galactic clusters)

Question 4

How does density affect the shape of the universe?

Answer 4

If the density is greater than the critical density, the universe will be closed. If the density is equal to the critical density, the universe will be flat. If the density is less than the critical density, the universe will be open.

Question 5

What is the fate of a flat universe?

Answer 5

Expansion will be t=infinity, so the universe expands for ever but the rate at which it expands decreases over time.

Question 6

What is the fate of an open universe?

Answer 6

Gravity is too weak to stop expansion, so the universe will continue expanding forever.

Question 7

What is the fate of a closed universe?

Answer 7

Gravity is strong enough to stop the expansion and start the universe contracting again.

Question 8

Define critical density.

Answer 8

The density required for the universe to be flat.

Question 9

What is the density of the universe currently believed to be?

Answer 9

Close to, or equal to the critical density needed for ‘flat’ cosmology.