Lecture 18: The Big Bang

Question 1

Big Bang Model

Answer 1

• Simplest Model is just an adiabatic expansion of hot gas
• temperature and density as function of time
• good model should have verifiable predictions
• Two predictions have been verified, the existence of the cosmic microwave background, and the expected helium abundance in the universe

Question 2

Temperature of universe has cooled over time

Answer 2

• Blackbody spectrum has become redder
• light was trapped when the universe was hot and dense
• after cooling a few thousand degrees, the universe became transparent and emitted visible photons
• Cooled to where peak is now radio waves

Question 3

Cosmic Microwave Background

Answer 3

• universe cooled to 3000 K and free electrons became bound to atoms of H and He, which emit photons
• Universe became transparent without free electrons to scatter photons
• Happened at 380,000 years after the big bang, at a redshift of z=1000
  First mapped over the whole sky by COBE, 1989-1993
• slightly hotter in one direction of the sky than the other due to Earth’s relative motion, doppler shift
• very smooth and uniform sky
• Blackbody temperature of CMB very close to the predicted 3ºK, 2.728ºK

Question 4

Structure of CMB

Answer 4

• brighter and darker or redder and bluer regions show temperature changes
• How much power is there on a particular angular size scale: multipoles

Question 5

What is the universe made of

Answer 5

• 68.3% dark energy
• 26.8% dark matter
• 4.1% free hydrogen and helium
• only 0.5% stars
• 0.3% neutrinos
• 0.03% heavy elements

Question 6

History of universe

Answer 6

• Planck Era (10^-43 sec), so early that physics fails
• GUT Era (up to 10^-38 sec), physics forces created, inflation
• Electroweak Era (up to 10^-10 sec) forces become different
• Particle Era (up to a millisecond), final protons and neutrons form
• Era of nucleosynthesis (up to 3 min) fusion, mostly to He
• Era of nuclei (up to 380 million years) neutral atoms form and CMB forms
• Era of Atoms (up to a billion years) cosmic dark ages, gravity begins to build structures
• Era of Galaxies (up to present day) galaxies assemble

Question 7

Cosmic Helium Abundance

Answer 7

• In the era of nucleosynthesis, protons and neutrons equal in number when greater than 100 billion degrees
• proton-to-proton neutron reactions no longer occur but neutrons still decay into protons
• less than this protons begin to outnumber neutrons, and products of fusion reactions no longer break apart
• helium, deuterium and lithium remain stable
• At this time (3 min after big bang) the model predicts 7-to-1 proton to neutron ratio

Question 8

Problems with big bang model

Answer 8

• where does structure come from
• why isn’t it filled with antimatter
• why is the large-scale universe so smooth
• why is the density of matter almost exactly the critical density?, why so flat?
• Inflation is the idea that there was enough energy released when the strong force froze out of the GUT force for the universe to expand by 10 to the 30 times in less than 10 to the negative 36 seconds

Question 9

Why is Density of Matter almost critical?

Answer 9

• if matter were 10% more dense, universe would have collapsed already
• if matter were 10% more dense, galaxies would never of formed
• imbalance of gravitational pull and kinetic energy of expansion creates a curvature of spacetime
• when they balance, spacetime is “flat”
• the effect of rapid inflation is to flatten spacetime
• inflation imposed the balance of these energies
• a universe that starts out flat will stay perfectly flat, but a universe that starts out not perfectly flat will deviate quite quickly (hours)

Question 10

Large Scale Structure of the Universe

Answer 10

• Can locate many galaxies by various distance indicators
• We use large surveys of many galaxies to map out the distribution of the galaxies
• distribution of nearby galaxies is clumpy
• long strings of galaxies and galaxy clusters surrounding voids
• universe if foamy, but expect that at some very large scale the universe is uniform

Question 11

Galaxy Survey Strategies

Answer 11

• Find galaxies and measure their redshifts to get distances
• pencil beam surveys cover small area in the sky but go deep
• slice surveys cover areas of the sky but do not go as deep

Question 12

Modelling Galaxy Formation

Answer 12

• unable to see back to the time when galaxies first formed
• must rely on theoretical models to describe how galaxies formed
• assumptions were that the universe was filled uniformly with H and He for the first million years after the big bang
• some regions were slightly denser than others
• started after about a billion years
• Gravity brings mass together
• collapsing gas became protogalactic clouds
• clumping and collapse aided by presence of cold dark matter

Question 13

What determines galaxy type?

Answer 13

Galaxy Interactions
- when two spiral galaxies collide
- tidal forces randomize the orbits of stars
- gas either falls to the centre to form stars or it is stripped out of the galaxies
- the disk is removed and the resulting galaxy becomes an elliptical

Question 14

Models of Galaxy Formation

Answer 14

Hierarchical
- Lots of small galaxies form, then merge to make bigger ones
- Problem is that simulations leave far too many small ones around
Monolithic
- Large clouds of inter-galactic gas collapse to form large galaxies
- Problem is that it is difficult to make such large clouds of gas in such a short time
- Other things to consider are how long ago were merger events and how massive were merging galaxies