Cosmology

The Origin, Evolution, and Destiny of the Universe

Questions

• What is the flatness problem

• What is the horizon problem?

• What is the solution of the flatness and horizon problems?

• What is the long term future of the universe?

• Why does the universe exist?

Time since t = 0 Temperature Description

< 10-43 s > 1032 K All forces unified

~ 10-34s 1027 K Inflation begins, strong force separates

~ 10-11 s ~ 1015 K Weak and electromagnetic forces separate, neutrons and protons created

~ 180 s ~ 109 K Nucleosynthesis

~ 300,000 years 3000 K Recombination, origin of the CBR

~ 109 years 20 K Stars and galaxies form, re-ionization occurs

~ 10-2 s 1011 K Electrons and positrons created.

Dark Age

Where did all the protons, neutrons, and electrons come from?

• When the temperature of the universe was about 1015 K, lots of photons had energies high enough for the reactions + → p + p- and + → n + n' to occur.

• As the expansion and cooling continued, the number of these high-energy photons decreased.

• Of course, the reverse reactions also occurred: p + p- → + and n + n' → + , so the net result might have been a universe without protons and neutrons.

• However, some asymmetry in the laws of elementary particle physics resulted in the survival of the protons and neutrons that we find in the universe today. Antimatter exists only under extreme circumstances for very short times.

• At a temperature of about 1011 K, the reaction + → e- + e+ and its inverse began to occur and left a residue of electrons.

General relativity the universe must either expand or collapse.

Time (t/tH)

Scal

e Fa

ctor

c closed

c flat

c open T

he P

rese

nt (

t = 1

3.7

Gyr

)

0r t R t r

R t "scale factor"

The distance between any two points in the universe depends on time.

r0 = the present distance between the two points.

• The mass density includes the mass equivalent of energy given by E = mc2.

• The universe could have negative curvature (like the curvature of a saddle), zero curvature (like a flat surface), or positive curvature (like the surface of a sphere).

• There is a critical density c such that(a) If < c , the universe is open (negative curvature)(b) If = c , it is flat (c) If > c , it is closed (positive curvature).

• The present value of c 9.47 × 10-27 g/cm3

• Density of baryonic matter ≈ 4.17 × 10-28 g/cm3.

General relativity the average mass-energy density of the universe determines its geometry.

Even though the density of baryonic matter is only about 0.04 times the critical density, the universe is actually flat, which implies that = c. , which implies that most of the universe’s mass-energy in not baryonic.

The Horizon and Flatness Problems

• The flatness problem:Why is the density of the universe exactly equal to the critical density?

• The horizon problem:How can two regions of the cosmic background radiation have the same temperature even though there could have been no causal relationship between them at the time of recombination?

3410 s445 10 s 1110 s

Unification and Spontaneous Symmetry Breaking - Inflation

Inflation

Accelerated Expansion of the Universe

Time (t/tH)

Scal

e Fa

ctor

WMAP

Flat

The cause of this acceleration of the universe’s expansion is called “dark energy”. We don’t yet know what it is (cosmological constant, vacuum energy, something else?).

Wilkinson Microwave Anisotropy Probe (WMAP)

Regions colored red and yellow are warmer than those colored green or blue.

Black dots: WMAP data.

Red solid line: theoretical model 73% dark energy,27% matter

T0 = 2.725 K

dark energy source = cosmological constant,

Age = 13.7 billion years

Density and Age of the Universe

• Visible mass of galaxies

• Dark matter in galaxy clusters

• Deuterium and lithium abundance in the early universe

• WIMPS (Weakly Interacting Massive Particles)

• MACHOS (Massive Compact Halo Objects)

• Density of baryonic mass = 4%.

• Density of dark matter = 23%.

• Density of dark energy = 73% of total mass-energy.

• Time elapsed since beginning = 13.7 billion years.