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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