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Cosmology IV: The Early Universe
Lecture 30
Lec 30: The Early Universe 2
Announcement
Prelim #3 on Wednesday, Nov. 14 In class: 11:15am - 12:05pm (Uris Auditorium)
Will emphasize lectures 22-31 (Galactic Center to Habitability of Worlds)
Closed notes and closed book
Lec 30: The Early Universe 3
Lecture Topics
Problems with the Big Bang
The Early Universe Theory of everything
Inflation
Pair-production
Nucleosynthesis
Pillars of the Big Bang
The Multiverse
The Anthropic Principle
What grade does Cosmology get?
Lec 30: The Early Universe 4
Problems with Big Bang
The Big Bang described thus far is very
successful in may aspects.
However, there are two major problems that
need to be addressed
The Horizon problem
Why is the CMB so uniform?
The Flatness problem
Why are we so close to Wk = 0 (a flat universe)?
Lec 30: The Early Universe 5
What is a Horizon?
Our horizon (in a Cosmological sense) is the
maximum distance we can see out to in the Universe.
More generally, for any point in the Universe, the
horizon is the maximum distance from which light
could have reached that point, within the age of the
Universe.
Nothing outside your horizon can have any effect on
you, because it has never been in causal contact.
Lec 30: The Early Universe 6
The Horizon Problem
Looking at one part of the sky and looking in the
opposite direction radio telescopes see the same
CMB temperature to 1 part in 100,000
Suppose the Universe is 14 billion years old, then the
two directions are separated by 28 billion lightyears
Earth
28 billion lightyears
Lec 30: The Early Universe 7
The Horizon Problem
Looking at one part of the sky and looking in the
opposite direction radio telescopes see the same
CMB temperature to 1 part in 100,000
Suppose the Universe is 14 billion years old, then the
two directions are separated by 28 billion lightyears
Thus they should not be “causally connected”
That is, they don’t know about each other
The two regions should not have the same temperature
In the past the situation is even worse.
100,000 years after the Big Bang the separation would be 10
million lightyears
Lec 30: The Early Universe 8
The Flatness problem
Measurements of the curvature of the Universe indicate it is almost exactly flat.
However, both the average density and critical density change with time.
In the past, right after the BIG BANG if the average density was slightly larger or smaller we would have a very obviously closed or open Universe.
At the beginning of the Big Bang the density would have to be very close to the critical value (1 part in 1060!).
Lec 30: The Early Universe 9
Epochs of the Universe
From the Big Bang until now, the universe can be viewed as proceeding through different “epochs” (time periods).
Distinguishing characteristics of epochs Each succeeding epoch is cooler and less dense.
Different “forces” and/or “particles” may dominate!
Lec 30: The Early Universe 11
Epochs
10-50 10-30 10-10 1 1010 10-30 10-10 1 1010
Radius (cm)
GUT GUT = E-M, Weak, & Strong forces unified
Planck All four forces unified (Quantum Gravity) 10-45
10-35
10-25
10-15
10-5
105
1015
1032
1027
1022
1017
1012
107
102
Present t
(se
c)
T (
K)
Lec 30: The Early Universe 12
Theory of Everything
Unite gravity with the other fundamental forces. Merging of gravity with quantum
mechanics and other forces.
We don’t have a theory yet but the most promising ones involves “string theory” and “higher dimensions”
String Theory suggests there are 11 dimensions (10 spatial + 1 time).
Lec 30: The Early Universe 13
Epochs
10-50 10-30 10-10 1 1010 10-30 10-10 1 1010
Radius (cm)
GUT GUT = E-M, Weak, & Strong forces unified
Planck All four forces unified (Quantum Gravity)
Hadron
Inflation
“Heavy particles in equilibrium with
the radiation field (Pair Production)
10-45
10-35
10-25
10-15
10-5
105
1015
1032
1027
1022
1017
1012
107
102
Present t
(sec)
T (
K)
Lec 30: The Early Universe 14
Inflation (Part I)
At 10-35 sec after the Big Bang the Universe cooled to 1027 K!
This caused a “phase transition” Like ice changing into water
The strong force split from the other forces releasing tremendous amounts of energy
The Universe expanded by a factor of 1050 in 10-33 sec!
Lec 30: The Early Universe 15
Inflation (Part II) This rapid expansion phase is called inflation.
Universe causally connected before inflation CMB will be the same in all directions afterward
Solves Horizon Problem!
Universe becomes flat Because of stretching of space
Solves Flatness Problem!
Space will now be flat due to inflation
Lec 30: The Early Universe 16
Epochs
10-50 10-30 10-10 1 1010 10-30 10-10 1 1010
Radius (cm)
GUT GUT = E-M, Weak, & Strong forces unified
Planck All four forces unified (Quantum Gravity)
Hadron
Inflation
“Heavy particles in equilibrium with
the radiation field (Pair Production)
Lepton Electron, muons, etc. in equilibrium
(Pair Production for low mass particles)
10-45
10-35
10-25
10-15
10-5
105
1015
1032
1027
1022
1017
1012
107
102
Present t
(se
c)
T (
K)
Lec 30: The Early Universe 17
What is Anti-Matter?
A particle and its anti-particle have the
same mass but opposite charge.
Many antiparticles can be created in
laboratories.
Positrons (anti-electrons) are used
routinely in medicine to imagine internal
organs using Positron Emission
Tomography (PET).
Lec 30: The Early Universe 18
Pair Production Particle-antiparticle annihilation occurs when
matter and anti-matter destroy each other in a
burst of g-rays.
The reverse is called pair production:
2 g particle + anti-particle
Pair production happens spontaneously, and
depends upon the temperature.
Higher T more energetic photons
more massive particles produced
Lec 30: The Early Universe 19
T ~ 1013 K proton, anti-proton
T ~ 6109 K electron, positron
T < 109 K no pair production
Pair production (cont’d) In the early universe temperatures were high
enough for pair production to take place.
There was a “sea” of photons, particles and
anti-particles.
The “threshold” temperatures are:
Temperature Particles Pairs
Lec 30: The Early Universe 20
Pair Production (cont’d) Above these threshold temperatures,
particles and anti-particles will exist in equilibrium (as many created as destroyed).
As the universe expands and the “plasma” cools, we expect particles and anti-particles to annihilate one another leaving just photons.
This didn’t happen! We are here.
This is because there are asymmetries between matter and anti-matter. Still not fully understood
Lec 30: The Early Universe 21
Epochs
10-50 10-30 10-10 1 1010 10-30 10-10 1 1010
Radius (cm)
GUT GUT = E-M, Weak, & Strong forces unified
Planck All four forces unified (Quantum Gravity)
Hadron
Inflation
“Heavy particles in equilibrium with
the radiation field (Pair Production)
Lepton Electron, muons, etc. in equilibrium
(Pair Production for low mass particles)
Nuclear Formation of light elements
10-45
10-35
10-25
10-15
10-5
105
1015
1032
1027
1022
1017
1012
107
102
Present t
(se
c)
T (
K)
Lec 30: The Early Universe 22
Deuterium
7Li
4He
10-32 10-31 10-30 10-29 10-28 10-12
10-9
10-5
10-1
Present density of baryons (g/cm3)
Fra
ction o
f to
tal m
ass in the u
niv
ers
e
3He
Big Bang Nucleosynthesis Predictions
Observations
Expected from CMB
Lec 30: The Early Universe 23
Epochs Stellar
10-50 10-30 10-10 1 1010 10-30 10-10 1 1010
Radius (cm)
GUT GUT = E-M, Weak, & Strong forces unified
Planck All four forces unified (Quantum Gravity)
Hadron
Inflation
“Heavy particles in equilibrium with
the radiation field (Pair Production)
Lepton Electron, muons, etc. in equilibrium
(Pair Production for low mass particles)
Nuclear Formation of light elements
Atomic Atoms form, matter photon decoupling
Galactic First Galaxies
10-45
10-35
10-25
10-15
10-5
105
1015
1032
1027
1022
1017
1012
107
102
Present t
(se
c)
T (
K)
Radiation
Dominated
Matter
Dominated
Dark Energy
Dominated
Lec 30: The Early Universe 25
What were two problems with the Big Bang theory?
a) Horizon and Bigness
b) Flatness and Expansion
c) Expansion and Bigness
d) Horizon and Flatness
e) There were no problems
In-Class Question
Lec 30: The Early Universe 26
What were two problems with the Big Bang theory?
a) Horizon and Bigness
b) Flatness and Expansion
c) Expansion and Bigness
d) Horizon and Flatness
e) There were no problems
What is the answer to these problems?
a) Cosmic string theory
b) Inflation
c) Accelerating Universe
d) All of the above
e) None of the above
In-Class Question
Sky is more uniform
than it should be (not
causally connected)
We are very near a flat universe (W ~ 1)
Lec 30: The Early Universe 27
What were two problems with the Big Bang theory?
a) Horizon and Bigness
b) Flatness and Expansion
c) Expansion and Bigness
d) Horizon and Flatness
e) There were no problems
What is the answer to these problems?
a) Cosmic string theory
b) Inflation
c) Accelerating Universe
d) All of the above
e) None of the above
In-Class Question
Sky is more uniform
than it should be (not
causally connected)
We are very near a flat universe (W ~ 1)
Expansion of space due to “phase transition” in the early universe.
Lec 30: The Early Universe 29
Pillars of the Big Bang
Hubble Expansion
Universe expanding in all directions
(necessary but not sufficient)
Cosmic Background Radiation
probes T ~ 1 eV, t ~ 105 years
Light Element Abundances
probes T ~ 1 MeV, t ~ 10 mins
These two
agree!!!
But how did
it all begin? Quantum gravity emergence:
Universe derives from
quantum fluctuations
The seeds of galaxies
cannot be infinitely close
together
Multiverse:
Maybe we are one of many
universes continuously
being created
As above or intersection of
higher dimensional spaces
Laws of physics may be
different in each universe
See Michal Turner article in
Sep 2009 Scientific American
The Multiverse
Our observable Universe extends
out to a distance of about 42 billion
light-years.
Our cosmic horizon, which
represents how far light has been
able to travel since the big bang
(as well as how much the universe
has expanded in size since then).
Assuming that space does not just
stop there and may well be
infinitely big
Cosmologists make educated
guesses as to what the rest of it
looks like. Observable Universe
Us
42 billion light-years
See Scientific American - Aug. 2011
article by George Ellis
Level 1 Multiverse: Plausible:
Our volume of space is a
representative sample of the whole.
Distant alien beings see different
volumes‚ but all of these look
basically alike but we can’t see each
other.
Level 2 Multiverse: Questionable
Sufficiently far away, things look
quite different from what we see.
The laws of physics would differ
from bubble to bubble, leading to an
almost inconceivable variety of
outcomes.
Lec 30: The Early Universe 34
The Anthropic Principle
Philosophical position, rather than hard
science. (Not universally agreed on.)
Essential it states “we are here, so the
Universe must have formed in such a way as
to allow life”.
Can have powerful reasoning implications.
Lec 30: The Early Universe 35
The Anthropic Principle
If any of a number of fundamental constants
were altered just slightly, the Universe
wouldn’t be capable of sustaining life.
It may seem that the Universe is very well
suited to us, but if it wasn’t then there
wouldn’t be anyone around to ask the
question, why is the Universe the way it is?
Lec 30: The Early Universe 36
Theory Report Card
From James Peebles (noted cosmologist), Sci. Am, Jan. 2001
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