Chapter 17
The Beginning of Time
Running the Expansion Backward
Temperature of the Universe
from the Big Bang to the
present (1010 years ~ 3 x 1017
seconds)
A Scientific History of the Universe
• The Big Bang Theory
– The scientific theory of the universe’s earliest
moments.
– It presumes that all we see today, from the Earth out
to the cosmic horizon, began as a tiny, hot and dense
collection of radiation and matter.
– It describes how expansion and cooling has led to the
present universe of stars and galaxies.
– It explains some current observational facts.
Eras of the Universe
Conditions of the Earlier Universe
• It was so hot, that photons (light energy) could transform
into matter, and vice - versa, according to Einstein’s
equation:
E=mc2
where
E = energy
m = mass
c = speed of light.
• Antimatter is common
Electron-positron Creation And Annihilation Reactions
The First Instant – Planck Era
• Occurs within the first 10-43 seconds.
• ?????- current theories fail to explain what
might have been happening during this
era.
• A theory that links quantum mechanics
and general relativity is required, but does
not yet exist…perhaps someday in the
future, we will have such a theory.
The GUT (Grand Unified Theories) Era
•
Occurs after 10-43 seconds and lasts to 10-38 seconds.
•
Today, the universe is governed by four forces that we know of:
1.
2.
3.
4.
Gravity.
Electromagnetism.
Strong nuclear force.
Weak nuclear force.
•
The number of forces governing the universe were fewer during
the GUT era.
•
GUT explains the merger of the four forces into only two forces.
•
electromagnetism+weak force  electroweak force.
•
electroweak force + strong nuclear force  GUT force
The Four forces are distinct at low temperatures
but may merge at very high temperatures
At the end of the GUT era…
• Strong nuclear forces become distinct
• This released an enormous amount of
energy which caused a sudden and
dramatic expansion called
Inflation
Electroweak Era
• Occurs from 10-38 seconds to 10-10 seconds.
(after the GUT era)
• Intense radiation filled all of space, creating
particle/antiparticle pairs.
• These newly formed particles would transform
back into energy through particle annihilation.
• The universe continues to expand and cool.
At the end of the electroweak era…
• Electromagnetic and weak forces become
distinct.
• Two particles predicted to exist then were the W
and Z bosons. These particles exist at
temperatures ~ 1015 K.
• In 1983, the particle accelerator at CERN
produced energies equivalent to 1015 K., and
they discovered the W and Z particles.
The particle era
• Occurs from 10-38 seconds to 0.001 seconds.
• Spontaneous creation and annihilation of
particles continues.
• Electrons
• Neutrinos
• Quarks – building blocks of protons and
neutrons.
• Antimatter is common
The era of nucleosynthesis
• Occurs from 0.001 seconds to 3 minutes.
• After matter annihilates antimatter near the
end of the electroweak era, protons,
neutrons, electrons and neutrinos survive.
• By this time, antimatter is rare.
Era of Nuclei
• Occurs after 3 minutes and continues to
500,000 years after the Big Bang.
• By fusion, hydrogen nuclei have formed
into helium nuclei.
• Most of the matter in the universe is in a
state of plasma made up of hydrogen and
helium nuclei + electrons.
Era of Atoms
• Occurs after 500,000 years to 1 billion
years after the Big Bang.
• The universe consists of a mixture of
neutral atoms and plasma.
• Protogalactic clouds form
• At the end of this era, the first galaxies and
stars have formed.
Era of Galaxies
• Occurs after 1 billion years to present day
after the Big Bang.
• Galaxies, galactic clusters and stars have
formed.
• New galaxies and stars continue to form
out of atoms, molecules and plasma.
Evidence for the Big Bang
•
The Big Bang Model has gained widespread
acceptance in the scientific community for two
main reasons:
1. Cosmic Microwave Background Radiation:
•
The Big Bang model predicts that the radiation produced at
the end of the era of nuclei should still be present today.
The discovery of the cosmic microwave background
verified this.
2. Observations of actual helium content of the
universe closely match the amount predicted
by the Big Bang model.
Photons released after the era of nuclei
make up the cosmic microwave background
Spectrum of the Cosmic Microwave
Background - COBE Mission data.
Data points
Theoretically
calculated curve
All-Sky map shows slight temperature differences in the cosmic
microwave background
GUT theory and Inflation – possible
explanation for the following problems
• Where does the structure come from?
– Density enhancements would be required for galaxy
formation to occur.
• Why is the large scale universe so smooth?
– Big Bang Theory cannot yet explain why distant reaches of the
universe look so similar.
• Why is the density almost critical?
– Density of matter is ~ (20-100)% of critical density.
Big Bang Theory does not say anything about what
the density should be.
End of Section