HW 8 - due 11 April

R+D: 27-1, 27-4, 27-6, 27-13, 27-16

Problems: 27-1, 27-7

Current Event

Radiation and Matter Dominance

Now we live in a matter dominated Universe

As Universe expands, both densities of radiation and matter decrease

But radiation decreases faster as cosmological redshift reduces the effective energy of photons

The early Universe was dominated by radiation

Fig. 27.1

Cosmic History

Fig. 27.4

Major Epochs in History of Universe

Table 27.1

Pair Production (Where Matter Comes From)

Pair production= 2 high energy photons create a particle and its anti-particle (see (c)).

High threshold temperatures; 10⁹ K for electrons, 10¹³ K for protons (2000 times more massive than electrons)

Below the threshold temp, particles "freeze out"

What does high temp mean for the photons?

Pair annihilation= when a particle and its anti-particle annihilate and create photons

Thermal Equilibrium

Thermal equilibrium

Particles are created from radiation (photons) at the same rate at which particles annihilate and create radiation

Matter Universe

When Universe expands and cools, the temperature falls until protons and then electrons cannot be produced by pair production

All matter in the Universe was made by pair production in this early phase of the Universe

But thermal equilibrium implies that equal amounts of matter and anti-matter were produced:

Why do we have a matter Universe? Slight asymmetry left more matter than anti-matter

From this matter, all stars and galaxies were produced

3 Fundamental Forces

Gravity, weak, strong, and electromagnetic (electricity and magnetism)

Weak: the force that moderates certain nuclear reactions and radioactive decay

Strong: the force that binds the particles together in the nucleus, does not affect electrons or neutrinos

Range of forces: strong 10^-15m, weak 10^-17m, gravity and EM are inverse square laws

Strong force is 137 times stronger than EM, 100,000 times stronger than weak, and 10³⁹ times stronger than gravity

GUTs

GUT - Grand Unified Theory

Unified means that all the forces can be explained as one complete force that manifests different aspects: EM, weak, and strong

In 1960s, the electromagnetic and weak forces were shown to be the same- electroweak force

GUTs (many different models) unify the the electroweak and strong nuclear forces

GUTs occur only at high temperatures >10²⁸ K

Creation of Particles

Note the times involved!

Hadron epoch- all protons and neutrons were in thermal equilibrium with radiation

Hadron: particles that interact through the strong force

Protons and neutrons are themselves made up of 3 quarks

Leptons- light particles; electrons, neutrinos, etc.

All particle reactions are mediated by particles: bosons for weak force, gluons for strong force (Fig. 27.10)

Observational Problems

Flatness:

space-time appears flat, which implies Ð₀=1

Why are we so close to critical density (Fig. 27.9). Small departure from critical density causes large departures later on

Horizon:

Microwave background is the same everywhere

so various regions must have started out with same density and temperature

Epoch of Inflation

Fig. 27-11

Some regions of Universe, entered an unstable state called a "false vacuum"

Caused by quantum fluctuation

Empty space created an enormous pressure

Universe expanded rapidly in size by 10⁵⁰ times

Inflation took only 10^-32 s to happen

Flatness of Space

Fig. 27-13

Space Time would appear flat if the Universe were very large

If Universe is flat, then Ð₀=1

Horizon Problem

Fig. 27-12

Universe is homogeneous

All parts of the Universe must have had time to communicate and become homogenous early on

Matter and radiation cannot move faster than c, but there is no such limit on space-time

Dark Matter

Dark matter interacts only weakly with radiation and normal matter

Natural tendency of dark matter to clump was not hindered by radiation background in early Universe

Dark matter stars clumping at redshift 6000, normal matter cannot clump until redshift 1500

Hot dark matter- lightweight particles (like neutrinos), cannot form small scale structures (like galaxies and clusters)

Cold Dark Matter- massive particles, possibly formed in GUT epoch, can form small scale structures easily

Probably some mix of hot and cold dark matter is needed

Structure Formation

Initially dark matter and normal matter are mixed evenly

Dark matter starts to clump (time=1000 yrs)

Dark matter forms large structures into which normal matter flowed, ultimately forming galaxies that we see today

Fig. 27.14

Summary

Characteristics of Universe immediately after its birth

How matter emerged from primeval fireball

Describe the epochs of the evolution of our Universe

How and when simplest nuclei and atoms formed

Summarize flatness and horizon problems, why inflation might be a solution

Explain formation of large-scale structure in cosmos and observational evidence for various models