Lecture 34

Question 1

What does the big bang theory explain and what does it not explain?

Answer 1

It explains how the universe has evolved but not how the initial conditions came to be

Question 2

What are some important ideas regarding the big bang?

Answer 2

• at earlier times the energy (temperature) was much higher
• as soon as something can happen, it does happen
• as soon as something can’t happen, it stops happening
• physics is at the extreme

Question 3

What are the three main pieces of observational evidence for the big bang theory?

Answer 3

1. galaxies are moving away from each other- the universe is expanding
2. cosmic microwave background- the universe was once hot and uniform
3. abundances of atomic elements- H and He are common whereas heavier elements are rarer

Question 4

What were the conditions in the early universe?

Answer 4

The early universe was very dense and very hot; everything is a plasma

Question 5

What occurred within the plasma?

Answer 5

…

Question 6

What are some key features of the cosmic microwave background (CMB) spectrum?

Answer 6

• it is a nearly perfect black-body spectrum
• the temperature is 2.725 Kelvin
• the radiation is from the edge wall around 400,000 years after the big bang

Question 7

What does the CMB show?

Answer 7

The emission is uniform with very small temperature fluctuations

Question 8

What does the CMB mean?

Answer 8

• it is the light from recombination of the universe- the surface of last scattering
• as the earliest, hottest universe was opaque the first universe we can at the CMB

Question 9

What are some features of the region of the early universe that we can see?

Answer 9

• 1/1000th the scale of the current universe
• t= 400,000 years after the big bang
• 10^9 times denser

Question 10

What is speculated to have occurred at t<10^-43s?

Answer 10

The Planck Era- quantum gravity and extremely high temperatures of 10^32K

Question 11

What is speculated to have occurred at t~10^-34s?

Answer 11

Inflation- the strong and electro-weak force decouple and the universe grows by ~10^40 times

Question 12

What is speculated to have occurred at t<0.001s?

Answer 12

The particle era- the incredibly high temperatures causes photons to produce exotic particle pairs including quarks and antiquarks

Question 13

What is speculated to have occurred at t~0.001s (1)?

Answer 13

The formation of nuclei- T<10^12K and pair production stops, matter-antimatter annihilates but with a slight imbalance of matter left over

Question 14

What is speculated to have occurred at t~0.001s (2)?

Answer 14

The era of nucleosynthesis- protons and neutrons form nuclei in the first moments and it ends with H, He and other light elemental nuclei. It lasted around 3 minutes and the starting conditions had an equilibrium between protons and neutrons

Question 15

What occurred due to neutrons being unstable outside the nucleus?

Answer 15

They can spontaneously transform into protons and at t=1s and protons dominate 6:1
- at this point in time the temperature is still too high for heavy nuclei and gamma rays blast and dissociate them as soon as the form

Question 16

What occurred at around 2-3 minutes during the era of nucleosynthesis?

Answer 16

• the ratio of protons:neutrons became 7:1
• the temperature is still hot enough for H to He fusion but not too hot so that He nuclei are destroyed by gamma rays
• the quantity of helium atoms is limited by the number of available neutrons

Question 17

What would have occurred if the universe cooled faster?

Answer 17

There would be more helium toms as faster cooling leads to less neutron decay and therefore, more He formation

Question 18

How does the abundance of helium match the big band theory?

Answer 18

The ratio of H to He in stars means that big bang nucleosynthesis is consistent with measured abundances

Question 19

Why were no heavy elements produced in the big bang?

Answer 19

Density and pressure quickly decreased with time. After the helium was produced, there was no time for He to form C

Question 20

Why is our universe isotropic and homogenous?

Answer 20

It is isotropic as it is the same in all directions (seen in the CMB) and homogeneous because all places are alike (galaxies are similar in all places)

Question 21

What does the CMB show about the curvature of the universe?

Answer 21

It shows that it is flat as light rays from opposite sides of a hot spot do not bend at all and the spot appears to be its true size