Lecture 13

The Early Universe
Ch. 17
Big Bang Theory
Big Bang Theory
Early Universe exceedingly hot and dense
Energy of photons high enough
to create matter and vice versa
Big Bang Theory
Big Bang theory explains how the early state
of the Universe cooled and produced the
Universe that we observe today
Big Bang Theory
•
•
•
•
Early Universe was so hot that all four forces were merged
As Universe cooled, four fundamental forces became unique
GUT - combination of strong force and electroweak force
Supersymmetry - combination of GUT force and gravity
Planck Era
• t<
• T > 10 K
• Current physics unable to understand
-43
10 s
32
times before the Planck era
• Need to unite physics of small scale
(quantum mechanics) with physics
of large scales (general relativity)
GUT Era
•
• 10 K > T > 10 K
• Supersymmetry force splits into GUT
10-43s < t < 10-38s
29
32
force and gravity
Electroweak Era
•
•
•
•
10-38s < t < 10-10s
15
10 K
>T >
29
10 K
GUT force separates into strong and
electroweak forces
•
Huge release of energy → inflation
Intense radiation produces matter and
antimatter, which annihilate to photons
b
a
l
•
•
•
•
•
!
d
e
t
s
e
t
Particle Era
10-10s < t < 0.001s
1015K > T > 1012K
Electroweak force separates into weak
force and electromagnetism
•
All 4 fundamental forces are distinct
Temperature too low for continued
conversion between particles and photons
Quarks are produced, which turn into
(anti-)protons and (anti-)neutrons
•
billion+1 protons for every billion antiproton
!
d
e
t
s
e
t
Era
of
b
a
l
Nucleosynthesis
• 0.001s < t < 5min
• 10 K > T > 10 K
• Fusion begins
• Temperature still so high that He
12
9
nuclei can break apart
• Expansion reduces density so that
fusion stops despite T>109K
• 75% H and 25% He, D, Li
b
a
l
!
d
e
t
s
e
t
Era of Nuclei
• 5min < t < 380,000yrs
• 10 K > T > 3000K
• Electrons and nuclei are separate
9
since photons still hot enough to
ionize any atoms that might form
• Photons cannot travel far since they
are scattered by electrons
b
a
l
!
d
e
t
s
e
t
Era of Atoms
• 380,000yrs < t <
• T < 3000K
• Temperature drops low enough for
9
~10 yrs
atoms to form
• With electrons bound to atoms,
photons can now travel large distances
•
• Cosmic Microwave Background
Dark Ages
Era of Galaxies
•
• Universe filled with nearly
t > ~109yrs
homogeneous distribution of matter
and dark matter after Era of Atoms
• Regions of slight overdensity collapse
to form first stars and galaxies
• Gravity is the strongest influence in
the Universe
Evidence for Big Bang
Big Bang Theory makes testable predictions
1. Leftover light from the end of Era of Nuclei
and beginning of Era of Atoms
2. Amount of Helium produced during the
Era of Nucleosynthesis should match
observed values
Cosmic Microwave
Background
Cosmic Microwave Background
(CMB)
•
Theorized to form at the end of Era of Nuclei
and beginning of Era of Atoms
•
•
•
Age of Universe - 380,000yrs
Black body radiation
•
Temperature ~ 3000K
Universe expanded by a factor of 1000 since
then - CMB should have cooled as much
•
Observed CMB temperature = 2.73K (~3K)
CMB Spectrum
COBE
WMAP
PLANCK
CMB fluctuations
Seeds of structure formation
Added support for WIMP dark matter
• without DM, fluctuations not large enough
to form galaxies within few billion years
Helium Abundance
• Observed He makes up 25-28% of matter
•
Can only have created 10% of He in stars
•
Big Bang Theory & CMB temperature tell us
that 25% of matter was He by the end of the
Era of Nucleosynthesis!
• Need most of He produced before stars
Helium Abundance
• Need Deuterium (D) to form He
• Amount of D determined by the relative
number of neutrons and protons
• Neutrons quickly decay into protons
• 7 protons for 1 neutron for nucleosynthesis
Helium Abundance
Assuming all neutrons go into forming He
Get 1 He atom for every 12 H atoms
In terms of mass,
4/16th = 25% is He; 12/16th = 75% is H
Element Abundances
Not all D is fused into He
Abundance of D determines
density of ordinary matter
If density was higher,
more D goes into He,
less D is left
Ordinary matter density is 4% of critical (total) density
Lingering Questions
• Where did the large scale structure come from?
• Why is the Universe so uniform?
• Why is the density of the Universe so close to
the critical density?
Inflation
Inflation
• Expansion of the universe by a factor of
30
10
in
-36
10
seconds!
• Energy for expansion comes from
separation of strong force from GUT force
Inflation and Structure
• Huge expansion would make tiny (quantum)
fluctuations in density into large ripples
•
atom-sized fluctuations
solar-system sized ripples
• Leads to density enhancements that give rise
to structure
Inflation and Uniformity
•
Expect uniformity if different parts of the
universe are in “causal contact”
•
•
light can travel travel between different parts of the universe
within the age of the universe
Parts of universe initially in causal contact
moved out of causal contact due to inflation
Shape of the Universe
Three possible shapes depending
on density of universe
Density of our universe is
(DE, DM, normal matter)
very close to critical density
Universe is flat!
Inflation and Flatness
Inflation and CMB
Support for Inflation
Predictions using inflation theories
match CMB data very well
Olbers’ Paradox
Why is the night sky dark?