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27. Exploring the Early Universe
• Rapid inflation of the early Universe
• Mass & energy formed during inflation
• Most matter & antimatter annihilated each other
• Neutrinos & helium are primordial fireball relics
• Galaxies formed from early density variations
• Grand Unified Theories unite all physical forces
• Cosmic strings & other oddities may be relics
• Grand Unified Theories suggest 11 dimensions
Rapid Inflation of the Early Universe
• Two fundamental problems
– Theisotropy problem
• The cosmic microwave background is uniform to 1:10,000
• Opposite sides are much too far apart for this to occur
– The flatness problem
• What could have made r0 = rc to 50 decimal places?
• This is necessary to produce a flat Universe
– Too little mass would have ended up with no galaxies
– Too much mass would have ended up with a Big Crunch
• One possible cause
– Rapid inflation very shortly after the Big Bang
Possible Causes of Cosmic Inflation
• One possible solution to the problem of isotropy
– A very early & very brief period of inflation
• Lasted only ~ 10–24 seconds
• Universe expanded by a factor of ~ 1050
– During this time interval, the cosmological constant was huge
– About 10120 times larger than Einstein envisioned
• One possible solution to the problem of flatness
– We see only a tiny fraction of the Universe
• Our particle horizon is a sphere
• This sphere enlarged so much that its surface looks flat
– Similar to one acre of land on the Earth’s spherical surface
The Isotropy Problem
Inflation & the Observable Universe
Inflation Solves Flatness Problem
Mass & Energy Formed During Inflation
• Matter at two scales
– Super -atomic scale
Everyday experience
• Location & momentum can be precisely known
• Accuracy depends on measuring instruments
– Sub
-atomic scale
High-energy physics
• Location & momentum cannot be precisely known
• Accuracy depends on fundamental nature of matter
• Quantum mechanics
– Fundamental nature of matter at the smallest scale
• Heisenberg uncertainty principle for location & momentum
– EMR is needed to measure location & momentum of an electron
– Either location or momentum will be changed by the observation
• Heisenberg uncertainty principle for energy &
time
– Special relativity asserts that E = m . c2
• Heisenberg uncertainty principle for mass
Dm . Dt = h / (2 . p . c2)
&
time
Some Aspects of Quantum Mechanics
• Ambiguity of mass & time
– Uncertainty regarding mass over very short times
• “Empty space” might contain no
mass
• “Empty space” might containabundant mass
– Virtual pairs of particles spontaneously appear
• The more massive they are, the less time they exist
– Property of particle symmetry
• Two particles are always produced
• One particle has + charge & the other – charge
– The overall electrical charge of the Universe does not change
• Supporting observational evidence
– Lamb & Retherford detect H spectral irregularities
• Disturbing effects of virtual particles on H orbital electrons
– Extremely small shift in spectral line positions
Virtual Pairs Can Become Real Pairs
• Observational evidence
– Two highly energetic gamma rays collide
• Photon
pairs disappear
• Particle & antiparticle pairs appear
– Combined photon energy > m . c2
• More energy produces more massive particles
• Accepted interpretation of that evidence
– Photon collisions convert virtual pairs into real pairs
• Particle accelerators are used to study this phenomenon
– Annihilation occurs when particles recombine
• Photon
pairs appear
• Particle & antiparticle pairs disappear
• Relevance to cosmology
– Processes active during the inflationary period
Virtual Pairs Can Appear & Disappear
Annihilation in the Primordial Fireball
• Mass & energy formed as part of the Big Bang
– The mass was in the form of matter & antimatter
• Temperature & pressure were both extremely high
• Collisions were frequent & energetic
– A condition of thermal equilibrium existed
• Mass-to-energy & energy-to-mass processes in balance
• The primordial fireball cooled quickly
– By t = 10–4 sec, all protons & neutrons formed
• Annihilation decreased the Universe’s mass content
• Resulting energy contributed to the primordial fireball
– By t = 10
0
sec, all electrons & positrons formed
• Annihilation decreases the Universe’s mass content
• The resulting energy contributed to the primordial fireball
A Truly Remarkable Dilemma
• The symmetry problem
– Annihilation left an excess of matter over antimatter
• Perfect symmetry would produce only energy
– Any remaining antimatter would annihilate matter
• Gamma rays would be the result
• Gamma rays observed from some parts of the Universe
– Number & energy are both inconsistent with annihilation
• Symmetry-breaking somehow occurred
– The (proton + neutron) to photon ratio is ~ 1:109
• The “odds” were a highly unfavorable one billion to one!
Virtual Pair Production & Annihilation
Collision of Relativistic Gold Atoms
Neutrinos & Helium Are Fireball Relics
• Neutron decay
– Free neutrons are unstable
Radioactive
• Half-life of ~ 630 seconds
• Daughter products: 1 proton + 1 electron + 1 antineutrino
– By t = 2 sec, neutron decay had commenced
• Number of neutrons in the Universe decreased radically
• Nucleosynthesis
– The deuterium bottleneck prevented He formation
• Gamma rays too energetic for the synthesis process
– By t = 3 minutes, the Universe cooled even more
• Gamma rays too weak to prohibit the synthesis process
– Helium quickly formed
• The proton to neutron ratio stabilized a ~ 6:1
– By t = 15 minutes, too cool for nucleosynthesis
• Only H, He, Li & Be were present in appreciable numbers
Nucleosynthesis in the Early Universe
Galaxies Formed from Density Variations
• Recombination ~ 300,000 years after Big Bang
– The Universe was cool enough for neutral H
• Photon interactions became
very rare
– Matter decoupled from radiation
• The Universe thus
became transparent
– The neutral H was very uniformly distributed
• Very small density variations did exist
• The characteristics of density variations
– Gravity & pressure oppose each other
• The gravity increase tends to contract the gas cloud
• The pressure increase tends to expand the gas cloud
– Gravity & pressure balance at some point
• James Jeans
1902
– Density fluctuations larger than the Jeans length grow
– Density fluctuations smaller than the Jeans length dissipate
Globular Clusters & Jeans Length
• Conditions at recombination
– T = 3,000 K
rm = 10–15 g . m3
• Conditions in globular clusters
– Typical mass of ~ 5 . 105 MSun
– Typical diameter of ~ 100 ly
• Identical to the Jeans length for typical globular clusters
• Observations of globular clusters
– They contain the oldest known stars
• They may have been among the first structures formed
– Complicated by the discovery of dark matter
• Known only by its gravitational effects
Microwave Background Variations
The Growth of Density Fluctuations
Globular Clusters ~ Jeans Length
Cold & Hot Dark Matter
• The fundamental problem
– The nature of dark matter is unknown
• Many models have been suggested
• Computer models
– Cold dark matter
High mass particles, low speed
• Galaxies form from the bottom up
– Initial small clumps of matter coalesce into larger clumps
– Hot dark matter
Low mass particles, high speed
• Galaxies form from the top down
– Initial large clumps of matter break apart into smaller clumps
Cold Dark Matter Simulation
Grand Unified Theories
• Four basic forces
– Gravity
Weakest of all
• Only attractive
– Electromagnetism
Second strongest
• Both attractive & repulsive
– Strong
Strongest of all
• Only attractive
– Weak
Second weakest
• Only attractive
• Thought to be identical at very high energies
– Weak
& electromagnetic join > 10
2
GeV
• Easily achieved in particle accelerators
–W/E
&
–W/E/S &
strong
gravity
join > 1014 GeV
join > 1019 GeV
Table 29-1: The Four Forces
7 x 10–3
The Unification of the Four Forces
Unification of Forces: Another View
http://physics.angelo.edu/~msonntag/physics1301/forces.jpg
The Early History of the Universe
Cosmic Strings & Other Oddities
• Vacuum & symmetry
– Asymmetric true vacuum
Truly empty space
• Visualize pencils standing on their points
• The pencils do not point in
any
– Symmetric false vacuum
direction in XY plane
More energy
• Visualize pencils fallen on their sides
• The pencils
do
point in some direction in XY plane
• The possibility of cosmic strings
– Clusters of fallen pencils keep some pencils upright
• Symmetry remains intact at that location
• This is analogous to cosmic strings
– One possibility for dark matter
Symmetry Breaking & Cosmic Strings
Distribution of ~ 400,000 Galaxies
A Universe With 11 Dimensions?
• Hidden dimensions of space
– Einstein joined space & time into spacetime
1905
• Four dimensions
– Theodor Kaluza proposed a fifth dimension
1919
• Gravity & electromagnetism both warp spacetime
• Fifth dimension is curled up too tightly to be observed
– Oskar Klein
1926
• Make Kaluza spacetime compatible w/quantum mechanics
• Foundations of Kaluza-Klein theory
– Edward Witten
• All four forces best explained by 11 dimensions
– Ten dimensions of space & one dimension of time
– The seven extra dimensions are curled up very tightly
• This suggests the existence of very massive particles
– These have not yet been observed
Spatial Dimensions Too Small To See
Important Concepts
•
Cosmic inflation
•
– The isotropy & flatness problems
– Can be solved by cosmic inflation
• Lasted ~ 1–24 sec
• Universe grew by 1050
• Cosmological constant was huge
•
– Extremely delicate mass balance
– The Jeans length
• Variations must be quite large
• Globular clusters formed very early
•
The role of quantum mechanics
– Virtual pairs of particles
• Extremely high energies
•
• Extremely massive
• May help explain dark matter
– Extremely hot Universe
– Cooling Universe
• Production & annihilation unequal
• Nucleosynthesis of H, He, Li & Be
– Slight excess of matter over antimatter
• One extra particle per billion
Cosmic strings
– Remnants of primordial symmetry
• Can become real due to gamma-rays
• Production & annihilation equal
Grand Unified Theories (GUTs)
– All four natural forces are united
– Heisenberg uncertainty principle
• Location-energy & mass-time
Density variations & galaxy formation
•
An 11-dimensional Universe
– Best explains four unified forces
– Extra 7 dimensions are tightly wound
• Too small to be directly observed