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Galaxy NGC4414 from HST Astronomy 162:
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

Key Ideas

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:

The Universe 14 Gyr Ago:

How far back into the Universe's past can we go?

Binding & Loosing

Binding Energy:

Can define a Binding Temperature:


Typical Sizes & Binding Energies

Binding Energy
~10-10 meters
103 K
~10-14 meters
1010 K
Protons & Neutrons
~10-15 meters
1011 K
~10-18 meters
1013 K

The Fundamental Forces of Nature

Gravitation: Electromagnetic Force:

Weak Nuclear Force:

Strong Nuclear Force:

Unification of the Forces

Electro-weak Force:

Grand Unified Theory (GUTs):

"Dreams of a Final Theory"

What about Gravity? Problem: We have no quantum theory of Gravity!

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.

The Planck Epoch

Before t=10-43 seconds, immediately after the Big Bang (t=0):

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

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.

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:

Inflation helps to explain why the Universe is so smooth and flat:

Four Forces at Last!

At t=10-12 sec, T=1015 K: 4 forces rule physics: Gravity, Strong, Weak, & Electromagnetic

Conditions now become right for matter to exist separate from photons, instead of as a hot soup of matter and photons in equilibrium.

Quark Freeze-out

At t=10-6 sec, T=1013 K: No more free quarks in the Universe.

Matter as we would recognize it today begins to emerge.

Nucleon Freeze-out

At t=0.01 sec, T=1011 K:

Free neutrons are stable during this epoch.

Neutrino Decoupling

At t=1 sec, T=1010 K:

Free neutrons are no longer stable:

The Epoch of Nucleosynthesis

At t=3 minutes, T=109 K:

Fusion of protons and the remaining free neutrons:

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:

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":

Time of rapid evolution:

At t= 500 Myr - 1 Gyr, T=30 K:

Present: t=14 Gyr, T=2.725 K

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 Astronomy 162.

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Updated: 2006 February 28
Copyright Richard W. Pogge, All Rights Reserved.