Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 38: "The First Three Minutes"
Readings: Chapter 29, sections 29-1 through 29-5
- Physics of the Early Universe
- Informed by experimental & theoretical physics
- Later stages confirmed by observations
- The Cosmic Timeline
- Unification of the forces just after the Big Bang
- Separation of the forces as the Universe expands & Cools
- Inflationary Epoch explains smoothness & flatness
- Emergence of Matter beginning at t=10-6 seconds
- Recombination & the emergence of the visible Universe
The Big Bang's Hot Past
The Universe Today:
- Today the Universe is low density, dark, and rather cool.
- We see that it is continuing to expand
The Universe 14 Gyr Ago:
- The Universe was smaller, denser, & hotter
- The Universe was Opaque and filled with radiation (photons)
How far back into the Universe's past can we go?
Binding & Loosing
- Energy needed to unbind (break up) matter.
Can define a Binding Temperature:
- Temperature equivalent to the binding energy
- Matter at this temperature "melts" (unbinds)
- In massive star cores, nuclei melt at temperature of ~10 Billion K.
Typical Sizes & Binding Energies
|Protons & Neutrons||~10-15 meters
The Fundamental Forces of Nature
- Long-Range Force, weakest in Nature
- Long-Range, 1039 times stronger than gravity
Weak Nuclear Force:
- Range <10-17 meters, 1028 times stronger than
Strong Nuclear Force:
- Range <10-15 meters, 1041 times stronger than
Unification of the Forces
- EM & Weak forces unify at high energies (1015K)
- Verified in particle accelerator experiments.
Grand Unified Theory (GUTs):
- Strong + Electroweak Force.
- Predicted, but no experimental basis (yet?)
"Dreams of a Final Theory"
What about Gravity?
Problem: We have no quantum theory of Gravity!
- Gravity should unify with the GUTs force at very high energies.
- Much higher than in any possible accelerator we might build.
- However, these energies could occur in the Early Universe.
The Cosmic Timeline
Physics gives us a framework within which to describe the events
of the Big Bang from the earliest phases to the present.
History of the hot early phases of the Big Bang imprints itself
upon the visible Universe.
- Particle accelerators probe matter at states similar to some of
these early phases.
- Astronomers can look for the evidence of these early
phases in the present Universe (e.g., Cosmic Background, primordial
amounts of deuterium & helium)
The Planck Epoch
Before t=10-43 seconds, immediately after
the Big Bang (t=0):
- All 4 forces unified into a single Superforce
- 1 force rules all of physics
We cannot say much else about the time before this, as we as do not
yet have a quantum theory of gravity to guide us.
The Grand Unification Epoch
At t=10-43 sec, T=1032 K:
2 forces rule physics: Gravity & the GUTs force
- Gravity separates from the Superforce
- Strong & Electroweak Forces unified into the GUTs force.
The Universe at this phase is a hot, dense particle soup of quarks,
antiquarks, & photons in equilibrium with each other.
The Inflationary Epoch
At t=10-35 sec, T=1027 K:
3 forces rule physics: Gravity, Strong & Electroweak forces.
- Strong Force separates from the GUTs force
- EM and Weak forces are still unified
The rapid separation of the Strong Force from the GUTs Force triggers a
rapid inflation of the Universe.
The Inflationary Universe
Universe grows exponentially by a factor of about 1043 in
size between 10-36 to 10-34 seconds:
- The expansion greatly slows down (back to normal) after Inflation.
Inflation helps to explain why the Universe is so smooth and flat:
- Smooth: Cosmic Background Radiation is smooth to 1 part in 105
- Flat: current observations suggest W0=1
Four Forces at Last!
At t=10-12 sec, T=1015 K:
4 forces rule physics: Gravity, Strong, Weak, & Electromagnetic
- Electroweak force separates into the Electromagnetic & Weak forces.
- All forces are now separate.
Conditions now become right for matter to exist separate from photons,
instead of as a hot soup of matter and photons in equilibrium.
At t=10-6 sec, T=1013 K:
No more free quarks in the Universe.
- Free quarks combine into hadrons (primarily protons &
- Equilibrium between particle-antiparticle pairs and photons
Matter as we would recognize it today begins to emerge.
At t=0.01 sec, T=1011 K:
- Protons & neutrons decouple from photons and exist
as free particles.
- electrons & positrons in equilibrium with photons.
- neutrinos & nucleons in equilibrium
Free neutrons are stable during this epoch.
At t=1 sec, T=1010 K:
- Neutrinos decouple from matter and radiation, and stream out into space
- These form a Cosmic Neutrino Background (predicted but not yet
observable with current technology)
Free neutrons are no longer stable:
- Decay into protons, electrons, and neutrinos
- Left with about 1 neutron for every 5 protons.
The Epoch of Nucleosynthesis
At t=3 minutes, T=109 K:
Fusion of protons and the remaining free neutrons:
- Formation of 2H (Deuterium) & 4He
- End up with ~75% 1H, 25% 4He
- Also end up with traces of 2H, 3He, Li, Be, B
We cannot observe this epoch directly, but we can look for the
products of primordial nucleosynthesis in the present-day
Universe, as described in the previous lecture.
The Epoch of Recombination
At t=300,000 years, T=3000 K:
Electrons & nuclei combine into neutral atoms:
- Universe becomes transparent
- Photons stream out into space
- Origin of the Cosmic Background Radiation.
This represents the earliest epoch of the Universe we can observe
directly using photons.
Previous to this, the Universe is opaque to photons.
The "Dark Ages" and Emergence of Galaxies
After the end of Recombination but before the first generation
of stars formed is the Epoch known as "The Dark Ages":
- No visible or infrared light because there were no stars ("dark")
- The hydrogen and helium in the Universe are neutral
- Universe is mostly opaque to UV photons because of absorption
by neutral H and He.
Time of rapid evolution:
- Matter density drops by factor of ~10 Million.
- Matter starts organizing itself into large-scale structures via
At t= 500 Myr - 1 Gyr, T=30 K:
- First generation of stars form, ending the "Dark Ages"
- Quasars first form
- First heavy metals made by the first supernovae
Present: t=14 Gyr, T=2.725 K
- Galaxies, stars, planets, us...
- Metals from supernovae of massive stars.
What about the Beginning?
Our physics can not yet ask questions about times earlier than the start
of the Planck Epoch (t=10-43 sec).
The current frontier appears to be before the Electroweak Epoch
(t=10-12 sec), during the period of rapid Inflation.
This will be the astrophysics theory of the 21st Century (or maybe
the 22nd ...)
- The title of this lecture is borrowed from the title of an
outstanding popular-science book by physicist Steven Weinberg (who
shared the 1989 Nobel Prize in Physics for his work on Electroweak
Unification). This book and Weinberg's Dreams of a Final
Theory will greatly reward reading by students who have taken
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Updated: 2006 February 28
Copyright © Richard W. Pogge, All Rights Reserved.