The Early Universe
Toward the Beginning of Time

posted May 16, 2001, jeb

Outline

• The Big Bang
  
• Evolution of the Universe
  
• Formation of Atoms and Nuclei
  
• Inflation
  
• Fundamental Basis for Inflation
  
• Structure
  
• Cosmological Constant
  

The Big Bang

Today the Universe appears to be dominated by matter
(technically, we believe, it may actually now be dominated by dark energy)
• It was not always this way
• The early universe was radiation dominated
  • density of radiation exceeded density of matter
• About 1000 years after the Big Bang, the density of matter dominated, exceeding the density of radiation for the first time
• Today, it appears, Dark energy dominates

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

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 100,000 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?
  • Early universe
    Fundamental question: Why did matter (particles) eventually dominate over anti-matter (anti-particles)
    • continuous interactions
    • Matter/anti-matter
    • The early universe was very hot
  • Due to very high temperature matter and anti-matter was continuously created initially
    • Particles and anti-particles were equal in number very early
  • 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 1000 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
  • Hadron 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
    • Last until the Universe is about 15 minutes old
• Matter Era
  • Atomic Epoch
  • Galactic Epoch
  • Stellar Epoch
• Vacuum-energy Era (Dark Energy Era)
  • Today?
  • You won't read about this in the textbook; it is too new

Formation of Atoms and Nuclei

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

Example: Sun
• 73% Hydrogen
• 24% Helium
• 3% 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
• 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

A few thousand years after the Big Bang, the universe cooled sufficiently for atoms to form (The next million years are the Atomic Epoch)
• electrons stick to nuclei to form atoms
• radiation no longer trapped by plasma after 100,000 years(free electrons bound to atoms)
• decoupling occurs and universe becomes transparent

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

Inflation

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
3. Why no magnetic monopoles seen?
   • What is a magnetic monopole?
     • theoretical elementary particle containing only one magnetic pole

       All known magnetic particles are dipole in nature-that is, they contain both a "north" and a "south" magnetic pole
       
       -but the possible existence of magnetic monopoles was proposed by symmetry theories in the 1930s. This idea remains part of current grand unification theories that are attempting to bring together the electromagnetic and nuclear forces that, with gravitation, underlie the interactions of all matter.


Answers from Grand Unified Theory (GUTS).
• Inflationary Universe

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
3. Why no monopoles?
   • Since they are very heavy, they had to be created while universe was very hot
     • before their freeze-out time (the heaviest particles freeze out first)
     • this would have been before inflation
   • Inflation spread them out - so we won't see many now

Fundametal Basis for Inflation

Basic Building Blocks of the Universe (particles)



Matter can be probed to the fundamental building blocks

We are aware of three types of matter:
• quarks
  • protons, neutrons, and all nuclei of atoms are built from quarks

    This is a model of a neutron (above) and a proton (below)

• leptons
  • electrons and neutrinos are one examples of leptons
• dark matter
  • what is it?



We know about these fundamental particles from experiments at high energy accelerators
• Fermilab (near Chicago)
  • looking down at Fermilab
  • beamline at Fermilab
  • CDF experiment
  • more on Fermilab
• SLAC (Stanford Linear Accelerator Center)
  • SLD experiment
  • model of SLD
  • BaBar experiment at PEP II
  • model of BaBar Detector
  • more on SLAC
• CERN (Geneva, Switzerland)
  • more on CERN
• DESY (Hamburg, Germany)
  • more on DESY

Experiments in these high energy accelerators study particle collisions
• Bubble chamber event
• Collider event (OPAL)
• Future event at LHC

Learn more about fundamental particles at the Berkeley site The Particle Adventure

What is dark matter?
• just neutrinos?
• axions?
• photinos?
• some of each?
• just faint objects?
• ???
  More on this later

Matter interacts through four forces (see note below)
• gravity
• weak nuclear force
• electromagnetism
• strong nuclear force

  Note - these four forces can be thought of as just three, since the weak nuclear and electromagnetic force have been unified into the electroweak force



Forces of Nature

.	Relative Strength	Role	.
Strong Nuclear	1	Binds Nucleus	(short range)
Electromagnetic	10-2	Binds Atoms	(long range)
Weak Nuclear	10-13	Causes Radioactivity	(short range)
Gravitational	10-40	Binds earth to Sun Binds galaxies	(long range)


GUTS - Grand Unified Theories
States that strong, electromagnetic, and weak forces are manifestations of the same force.
• Mathematical Theory of GUTS ------> predicts Inflation
  • When universe cooled to point that strong force separated from electroweak force
  • From 10^-35 sec. to 10^-32 sec, universe expanded in size by 10⁵⁰
Inflation Solves:
• Horizon problem
• Flatness problem
• missing magnetic monopoles

Remnant of Inflation might be cosmic strings
• Inflation didn't occur in some region so matter packed in small tube
  • perhaps 10 ¹⁵ M_SUN (equivalent of 10,000 galaxies)

Structure

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 CMR).
  • 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
  • neutrinos? (if they have mass)
• Theory of hot dark matter on computer
  • Large structures (super clusters, voids)
  • Small scale structures don't form (no galaxies!)

Cold Dark Matter
• heavy particles
  • photinos?
• Theory of cold dark matter on computer
  • Small scale structures easy (galaxies)
  • Also can form some large scale structures

Best models:
• Cold Dark Matter
  and
• Hot Dark Matter
  • 20% hot dark matter (neutrinos), 80% cold dark matter, in this model
  • Notice the filamentary structure on the large scale,
    and the halos surrounding galaxies,
    as we observe in the real universe

How does Dark Matter relate to microwave background?
• Dark Matter does not interact directly with photons
• Density fluctuations in dark matter will not initially affect radiation.
• But there will be some ripples - small.
• 1992 - ripples finally observed by COBE
  • at about 1 part in 100,000.
  • Matches computer simulations

Cosmological Constant ( or )

What is the cosmological constant?
• When Einstein developed his general theory of relativity (1915) he realized that it showed the universe should be expanding
• At that time it was not known that the universe was expanding and Einstein assumed it was not
• To "correct" this feature of the theory he invented a constant to stop the expansion
• In 1929 Hubble discovered the universe was expanding, and Einstein called the cosmological constant the greatest blunder of his career
• Since then the cosmological constant has been thought to be zero

The expansion of the universe depends on the value of the cosmological constant

But now measurements by two different groups on the apparent brightness of supernovae with redshifts near z = 1 have generated new interest in the old idea of a cosmological constant.
• The groups look for supernovae in galaxies at a very large distance, that is with large redshifts
  • Astronomy 122 notes on Supernovae
• Data from one of these groups

The conclusions from this data:
• The supernovae are farther away than they should be with these redshifts
• The expansion of the universe is speeding up
• The cosmological constant is not zero
  • the vacuum energy of the universe is accelerating the expansion
• We have entered the Vacuum-energy Era

Where does the cosmological constant come from?
• vacuum energy density

These are recent discoveries and astrophysicists are still trying to understand what it all means.

Modern Cosmological Observations and Problems

by Professor Greg Bothun
University of Oregon