The Early Universe
Toward the Beginning of Time

May 12, 2015, jeb

Outline The Big Bang Evolution of the Universe Formation of Nuclei and Atoms The Inflationary Universe Formation of Structure in the Universe Cosmic Structure and the Microwave Background Summary

The Big Bang

Today the Universe appears to be dominated by dark energy
• About 3/4 of the total mass-energy of the universe is dark energy
• The remaining quarter is virtually all matter (dark matter and normal matter)
• The radiation in the universe is a small fraction
  • Mostly the cosmic microwave background
  • The radiation from stars and galaxies is much weaker

It was not always this way
• The early universe was radiation dominated
  • density of radiation exceeded density of matter
• After about 50,000 years, the density of matter exceeded the density of radiation for the first time, eventually dominating the universe.
• Today, it appears, dark energy dominates as the matter density has fallen

In the early universe, matter and anti-matter were being created equally out of the radiation
• pair production
  • pair production is the production of matter and anti-matter in pairs
  • two photons can produce a pair
  • particle-antiparticle annihilation (the reverse process) is also possible

Anti-Matter
• What is anti-matter (anti-particles)?
  • A type of matter which has the same mass as normal matter, but opposite charge



particle	charge of particle	anti-particle	charge of anti-particle
proton	positive	anti-proton	negative
neutron	neutral	anti-neutron	neutral
electron	negative	anti-electron or positron	positive

Matter and anti-matter can be created in pairs from energy (or electromagnetic radiation)
E = m c²

E = energy
m = mass
c² = speed of light squared (here just a constant of proportionality)
For example
energy -------->proton + anti-proton
energy --------> electron + positron
OR matter can annihilate in pairs
proton + anti-proton ----------> energy
electron + positron (anti-electron) ---------> energy

The Early Universe
• We've already discussed the change at about 400,000 years after the Big Bang.
  • Light is free to pass through the universe as expansion of the universe changes it from opaque to transparent
• How about before that?
  • Very early universe (when temperature was 10 billion K)
    • Due to high temperature photons had enough energy to create electron-positron pairs
    • Great numbers of electrons and positrons exist in thermal equilibrium with the radiation
  • As universe expanded, it cooled
    • Universe when temperature was 1 billion K
    • Photons now have too little energy to create pairs, so electrons and positrons are no longer in thermal equilibrium
• The fundamental question: Why did matter (particles) eventually dominate over anti-matter (anti-particles)
  • Eventually matter (which initially was equal to anti-matter) came to dominate over anti-matter
    • Experiments at high energy accelerators have found that if you start with equal amounts of matter and anti-matter, the interactions between them will cause a slight excess of matter to develop
  • The higher the temperature of radiation, the greater the energy of photons, and the greater the mass of particles created in pairs
  • As temperature decreased, eventually even the lightest particles could not be created
    • this occurred about a minute after the Big Bang

Evolution of the Universe

• Radiation Era
  (The radiation era lasted for about 50,000 years)
  • Planck Epoch
    • First 10^-43 seconds after the Big Bang
    • No current theory of physics (quantum gravity) exists
  • GUT (Grand Unified Theory) Epoch
    • After 10^-43 seconds, temperature fell to 10³² K
  • Quark Epoch
    • Creation of protons and neutrons continued for about 10^-4 seconds
    • Temperature drops below 10¹³ K, and protons and neutrons are no longer produced in pairs
  • Lepton Epoch
    • Ends when the universe is about 100 seconds old
    • During this epoch, the leptons (electrons, neutrinos, and other light particles) are still produced in pairs, because they are light
    • Ends when temperature drops below 1 billion K
  • Nuclear Epoch
    • Protons and neutrons fuse into nuclei
    • By the time the universe is about 15 minutes old, much of the helium had been formed
• Matter Era
  • Atomic Epoch
    • Begins about 50,000 years after the Big Bang
    • Atoms form and remain intact (electrons attached to nuclei)
    • Ends 200,000,000 years after Big Bang
  • Galactic Epoch
    • Large scale structure and bulk of most galaxies form
    • Lasts from 200,000,000 years to 3,000,000,000 after Big Bang
  • Stellar Epoch
    • Stars continue to form up to today
    • Extends into the Dark Energy Era
• Dark Energy Era
  • Today

Formation of Nuclei and Atoms

Why is there so much Helium?
• Hydrogen - simplest atom
  • proton and electron
• Helium - second simplest
  • 2 protons, 2 neutrons, and 2 electrons

Example: Sun's Composition (as percentage of mass)
• 71% Hydrogen
• 27% Helium
• 2% Heavier elements

In 1940's George Gamow realized the helium could have been produced just after the Big Bang due to extreme temperature in the Primordial Fireball

Present abundances (like deuterium) tell us what was going on very early in the evolution of the universe

History of creation of elements in the early universe
Primordial nucleosynthesis
• As universe cools the proton and neutrons begin to stick together
  • when universe is 100 seconds old
    • cooled to 1 billion K -----> deuterium (p+n) forms
  • as universe continues to cool Helium (2p +2n) forms
  • after a few minutes
    • cools to 100 million K
    • now too cool for nuclear reaction so we're stuck with about 25% He (by mass)
  • many heavier elements had to await formation of stars where elements are formed

We get:

ELEMENT	NUCLEUS
Deuterium (heavy hydrogen)	p + n
Helium -3	p + p +n
Helium -4	p + p + n + n
Beryllium -7	4 p + 3 n
Lithium -7	3 p + 4 n
Lithium -6	3 p + 3 n


Deuterium Cosmology
• The amount of deuterium reveals the density of the early universe
  • deuterium is not formed in stars in much quantity

About 400,000 years after the Big Bang, the universe cooled sufficiently for atoms to form (The next 100,000,000 years are the Atomic Epoch)
• electrons stick to nuclei to form atoms
  • called recombination, although actually first combination
• radiation no longer trapped by plasma after 400,000 years(free electrons bound to atoms)
• decoupling occurs and universe becomes transparent

Heavier nuclei were formed later in burning stars and supernovae explosions.

The Inflationary Universe

Horizon Problem
• The cosmic microwave background is remarkably isotropic

Flatness Problem
• Any deviation from flatness early in the Big Bang would have quickly increased
• Therefore, the early universe must have been very, very, very flat for it still to be nearly flat now
• WHY?

Puzzles of the Early Universe
1. Why does the universe appear so flat?
   • not clearly open or closed
   • Flatness Problem
2. How did the universe become so homogeneous and isotropic?
   Horizon problem
   • Background radiation nearly the same in all directions
   • Therefore - entire universe must have been at uniform temperature near beginning
   • But - different regions are not in contact and never have been
     • universe expanded before this could happen

Answers from Grand Unified Theory (GUTS).
• Inflationary Universe
  • At very early times, all fundamental forces were unified
  • Giving rise to vacuum energy
  • Causing rapid acceleration of expansion of the universe
  • Epoch of inflation

Grand Unified Theory (GUTS)
• Quest of science is always to unify seemingly different things.
  • 1600's Newton - unified gravity in space and on earth.
    • "celestial and terrestrial gravity the same"
    • apples and the moon
  • 1800's Maxwell - unified electricity and magnetism
    • same force causes lightning and magnets
  • 1970's Glashow, Weinberg, and Salam - unified electricity and magnetism with weak nuclear force
• NOW GUTS - unifies further.
  • strong, electromagnetic, and weak are all the same force

Inflation Solves Mysteries
1. Why is universe so flat? (flatness problem)
   • Expansion flattens it
   • Like balloon suddenly expanding becomes very flat locally
2. Why so homogeneous and isotropic? (horizon problem)
   • Universe was in contact before inflation
   • but not now

Formation of Structure in the Universe

Dark Matter
• Recall, most of the mass of the universe is dark matter.
• So fluctuations in dark matter should be more important than baryonic.
• Dark matter will interact very weakly with baryonic matter (just gravity) ( fig. 27.15)
• How did we get from homogeneous, isotropic early universe to today's clumpy universe?
  • Initially universe was extremely uniform (see cosmic microwave background).
  • But if there are any matter fluctuations, they will grow with time (recall star formation in galaxies spiral arm).
  • Quasars indicate galaxies were forming about 100 million years after Big Bang.
  • Calculations cannot explain this fast of formation from baryonic (matter made of protons and neutrons) fluctuations.
    • But they can from dark matter fluctuations

What is dark matter?
• Hot dark matter
• Cold dark matter

Hot Dark Matter
• lightweight particles
  • such as neutrinos (which have very small mass)
• Theory of hot dark matter on computer
  • Large structures (super clusters, voids)
  • Small scale structures don't form (no galaxies!)
  • Hot dark matter seems unable to explain structure of the universe

Cold Dark Matter
• heavy particles
  • Such as hypothesized supersymmetric particles being searched for at the Large Hadron Collider
• Theory of cold dark matter on computer
  • Small scale structures easy (galaxies)
  • Also can form some large scale structures
  • Cold dark matter favored as it best explains the structure

Best model of actual universe:
• Cold Dark Matter
  • 100 × 100 × 100 Mpc cube
  • at 1 billion, 4 billion, and 13.8 billion years
  • Ω₀ = 1
  • Notice the filamentary structure on the large scale,
    and the halos surrounding galaxies,
    as we observe in the real universe

Cosmic Structure and the Microwave Background

• Cosmological models predict there should be tiny "ripples" on the cosmic microwave background (few parts per million) due to dark matter
  • Dark matter does not act directly with photons
  • But radiation is affected a small amount through gravity by the dark matter clumps (gravitational redshift)
• Cosmic Microwave Background Map of COBE
  • COBE launched in 1989
  • Reported discovery of "ripples" in 1992
• Early Structure revealed by WMAP
  • Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001
• More Detailed Early Structure measured by Planck
  • Planck launched in 2009
• Structure in comic microwave background at different angles
  • Peak at 1 degree
  • Flat universe (Ω₀ = 1)
  • 68% dark energy
  • 32% matter (27% dark matter)

Summary

• Current density of the universe is dominated by dark energy
  • Matter density greatly exceeds the density of radiation
• The very early universe was radiation dominated
• After the early radiation dominance, and before the current dark energy dominance, the universe was matter dominated
• During the first few minutes of the Big Bang, the primordial fireball created matter by pair production
• A period of primordial nucleosynthesis created all of the hydrogen of the universe, and most of the helium observed today
• As the universe expanded and cooled, atoms were formed, and the background radiation decoupled from matter
• The very early universe went through a period of inflation, where the universe rapidly expanded
  • Inflation solved the horizon and flatness problems
• Large-scale structure of the universe was formed by early density fluctuations in dark matter
• Cold dark matter best explains the large-scale structure
• "Ripples" in the cosmic microwave background strongly support the predictions of inflation of the flat universe