• The Universe is 13.8 billion years old.
• How do we know this?
• The most important ingredient is the Hubble's parameter
• Various independent methods
– Type Ia supernova explosions
– Cosmic Microwave Background (WMAP, Planck)
– Crude lower bound: age of oldest stars
• All estimates agree within the uncertainties!
2

• The Big Bang
• Very Hot Universe (< 10 -10 s)
– quantum gravity
– Inflation
– Grand Unification
– Baryon asymmetry
• Hot Universe (< 1000s)
– Formation of baryons
3
– Neutrino decoupling
• Recombination (380 000 years)
• Formation of stars (~ 1 bn years)
– e
+
/e
annihilation
– Primordial nucleosynthesis
3

Time
Temperature
Energy
0 seconds
Infinity
Infinity
• The existing physical theories break down.
• Formal application of GR: naked singularity
• New physics is needed to resolve the singularity

String theory?

Loop quantum gravity?
• What happened before the Big Bang?
 traditional answer: there was no time
 speculative ideas: Big Bounce
• Big Bang is one of the greatest challenges in physics

10
-43 s
10
32
10
28
Kelvin eV
• All forces are carried by particles (photons, gluons, W/Z)
• Gravity must be carried by gravitons. Quantum effects are small at low energies, but they dominate when field is strong.
• QG is still being developed (string theory, loop gravity, etc)
• Main applications of QG: Early Universe, black holes.
• Dynamics at this stage determines all subsequent evolution.

10
-34
10
27
10
23 s
Kelvin eV
• The strong, weak, and electromagnetic interactions are unified into one force (the Grand Unification Epoch).

10
-34
10
27
10
23 s
Kelvin eV
• The strong, weak, and electromagnetic interactions are unified into one force (the Grand Unification Epoch).
• The unification requires a doubling of the number of particles at high energies (supersymmetry).

10
-34
10
27
10
23 s
Kelvin eV
• The strong, weak, and electromagnetic interactions are unified into one force (the Grand Unification Epoch).
• The unification requires a doubling of the number of particles at high energies (supersymmetry)
• After individual forces emerged, the Universe went through a very rapid expansion (inflation).
• The entire VISIBLE Universe emerged from a TINY PART of the
Cosmos, this explains homogeneity and flatness seen in CMB.
• Inflation predicts multiverses with different physical properties.

10
-5
10
12
10
8 s
K eV
• Baryons consist of 3 quarks, mesons: quark + antiquark

10
-5
10
12
10
8 s
K eV
• Baryons consist of 3 quarks, mesons: quark + antiquark
• We see matter, but very little antimatter. The symmetry between the two was broken during baryogenesis (10
-40 s)
• Free quarks cannot exist (confinement)
• In the early Universe, quark and gluons formed a distinct state of matter called quark-gluon plasma (observed at CERN and
RHIC)
• After 10
-5 mesons.
second quarks became confined within baryons and

1 s
10
10
10
6
K eV
• Neutrinos decouple after 0.2 s (1 MeV)
– The ratio n/p is frozen, it sets stage for nucleosynthesis
– Neutrinos keep cooling off ONLY due to expansion
• Photons are produced in e
+
/e
annihilation at t < 1s

1 s
10
10
10
6
K eV
• Neutrinos decouple after 0.2 s (1 MeV)
• The ratio n/p is frozen, it sets stage for nucleosynthesis
• Neutrinos keep cooling off ONLY due to expansion
• Photons are produced in e
+
/e
annihilation at t < 1s
• Only one electron per 10 9 e
+
/e
survives
• Photons keep cooling off MOSTLY due to expansion, the ratio of photon/neutrino temperatures is set (>1)
• Remaining electrons will form atoms during recombination.

200 s
10
8
K
10
4 eV
• Stars don't explain He abundance (25% of baryonic matter)
– Nuclear reactions: He abundance is related to luminosity
– Observations: less than 0.5% of He is formed is stars
• Light elements are produced through a chain of nuclear processes involving deuterium and tritium
• Complicated equations predict the CORRECT abundances of
He (25%), D (0.01%), Li (10
-10
), etc.

380000 years
4000 K
0.3 eV
• Nuclei and electrons bind to form atoms.
• Transparent Universe (end of the “Dark Ages”)
• CMB consists of photons produced at recombination.

380000 years
4000 K
0.3 eV
• Nuclei and electrons bind to form atoms.
• Transparent Universe (end of the “Dark Ages”)
• CMB consists of photons produced at recombination.
• The most accurate info about the early Universe
• Continuous improvements (COBE, WMAP, Planck satellites)

• Cloud of hydrogen collapses due to gravity.
• High pressure ignites nuclear reactions.
• Heavy elements are produced as star burns, they are dispersed through supernova explosions.
• Galaxies and galaxy clusters are formed.
• Younger stars and planetary systems are formed
1 billion years
18 K / -255
°
C
1.5 10
-3 eV
– Solar system: 9 bn years after the Big Bang
• Life of Earth ~ 3.5 bn years ago (10.3 b.y. after the BB)

• Spacetime tells matter how to move, matter tells spacetime how to curve (J. Wheeler)
• Three forms of “matter” (energy) and their domination
– Hot matter (radiation & neutrinos) (t < 47 000 years)
– Cold matter (baryionic and dark)
– Dark energy (t > 10 bn years)
• Present breakdown: dark energy (68%), dark matter (27%), ordinary matter (5%), radiation (tiny fraction)

• Dark energy/matter/radiation fractions change with time

• Distance to the Sun is 1.5 x 10
11 m
• Distance to the nearest star ~1pc = 3 x 10
16 m = 3.3 ly
• The size of a galaxy 10
4 pc
• The size of a galaxy cluster 10
6 pc
• Universe is homogeneous & isotropic above 10 8 pc
• The size of the visible Universe is 10
10 pc
• The size of the full Universe is UNKNOWN.
• The Universe may be finite or infinite.

• The radius visible U. is 14 x 10 9 pc or 46 x 10 9 light years
• The naive radius of the entire Universe is 13.8 x 10 9 ly.
• Expansion of the Universe:
– objects move away after emitting light
– law of expansion and age give the size
• Cosmic horizon is expanding: new objects enter at the edges.
• Inflation: the Universe is at least 10 23 time larger that the observable part.

• Read the links next to powerpoint version of this talk on the course website syllabus table for Friday
• Complete written homework #4, also for Friday.
• Additional reading material will be emailed to you.