VERY Early Universe

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VERY Early Universe

• Tuesday, January 29
  (planetarium show tonight 7 pm, 5th floor Smith
  Lab)

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Its about time!
Different calendars have different starting times
(birth of Christ, hijra to Medina, etc.)
The Big Bang (start of expansion) provides an
absolute zero for time.
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Universe started expanding at a time t 0.
What is the current time t t0? (That is,
how much time has elapsed since the Big Bang?)
Weve already answered that question
(approximately).
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Flashback slide!
At a finite time in the past (t 1/H0), the
universe began in a very dense state.
1/H0, called the Hubble time, is the
approximate age of the universe in the Big Bang
Model.
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The Hubble time, 1/H0, is
approximately equal to t0 (time elapsed since
Big Bang).
If expansion has been slowing down, the universe
is younger than 1/H0.
If expansion has been speeding up, the universe
is older.
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Redshift (z) of a distant object measure of
how much the universe has expanded since light
was emitted.
Since universe has been expanding continuously,
each z corresponds to a unique time t.
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Looking at the Cosmic Microwave Background
z 1000, t 350,000 years
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As time (t) increases, density and temperature
decrease.
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What the _at_ do I mean by the temperature of
the early universe?
Today, the universe is full of things with many
different temperatures.
The early universe was dense particles
frequently collided, and came to the same
equilibrium temperature.
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The very early universe was a nearly homogeneous
soup of elementary particles.
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Particle Physics for Dummies
Electron low mass, negative charge
Proton higher mass, positive charge
Neutron proton mass, no charge
Neutrino VERY low mass, no charge
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Cosmic Gall (John Updike)
Neutrinos, they are very small. They
have no charge and have no mass And do not
interact at all. The
earth is just a silly ball
To them, through which they simply pass, Like
dustmaids down a drafty hall Or
photons through a pane of glass.
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A photon is a particle of light.
On very small scales, the laws of quantum
mechanics apply.
One of these laws states that a particle can have
the properties of a wave, and vice versa.
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This concept of wave-particle duality is
mind-bending but useful.
Light of a given color can be treated as 1)
waves of a given wavelength 2) photons
of a given energy
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Energy can be measured in BTUs, kilowatt-hours,
calories, ergs, etc
The energy of individual particles is usually
measured in electron-volts.
1 electron-volt (eV) is the energy gained by an
electron when its electrical potential increases
by 1 volt.
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An electron-volt is a tiny amount of energy,
appropriate for dealing with single particles and
atoms.
photon of red light energy 1.8 eV
photon of violet light energy 3.1 eV
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The temperature T of the early universe
determines the average particle energy E.
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t T E
30,000 yr 10,000 K 3 eV
12 days 10 million K 3 keV
1 second 10 billion K 3 MeV
10-6 sec 10 trillion K 3 GeV
1 GeV 1 billion electron-volts energy of a
gamma ray photon
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How far back in time dare we go?
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Looking again at the CMB
z 1000, t 350,000 years, T 3000 K
Universe became transparent because hydrogen went
from ionized to neutral.
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Hydrogen atom a proton and electron
held together by electrostatic attraction.
It takes 13.6 eV of energy to ionize a hydrogen
atom.
Any photon with E gt 13.6 eV (ultraviolet, X-ray,
gamma-ray) can ionize hydrogen.
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At T 3000 K, some photons are energetic enough
to ionize hydrogen.
At T lt 3000 K, hydrogen forms neutral atoms too
few ionizing photons!
13.6 eV photons
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Deuterium nucleus a proton and
neutron held together by strong nuclear
force.
It takes 2,200,000 eV of energy to dissociate a
deuterium nucleus.
Any photon with E gt 2.2 MeV (gamma-ray) can
dissociate deuterium.
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If hydrogen atoms are safe from ionization when T
lt 3000 K, then
deuterium nuclei will be safe from dissociation
when T lt ???
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The temperature of the universe fell below 480
million K when its age was t 7 minutes.
Photons were no longer energetic enough to blast
apart deuterium nuclei.
Deuterium nuclei could form and be safe from
destruction.
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Primordial nucleosynthesis
p n ? D ?
The very early universe was a nuclear fusion
reactor.
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Theres not a lot of deuterium in the universe
today. Why not?
Because fusion continued
D n ? T ?
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Theres not a lot of tritium in the universe
today. Why not?
For one thing, tritium is unstable. For another,
fusion continued
T p ? He ?
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Before primordial nucleosynthesis, there were 2
neutrons for every 14 protons. (Neutrons tend to
decay into protons.)
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2 neutrons combine with 2 protons to form 1
stable helium nucleus, with 12 lonely protons
(hydrogen nuclei) left over.
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25 of the initial protons neutrons (and hence
25 of their mass) should be in helium the rest
will be hydrogen.
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When we look at the spectra of the first stars
that formed, they consist of 25 helium by mass,
and 75 hydrogen.
TRIUMPH FOR PRIMORDIAL NUCLEOSYNTHESIS! Theres
just the amount of H He that was predicted.
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350,000 yr
7 min
1 billion yr
nucleo-synthesis
trans- parency
galaxy formation
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Gosh! We understand what the universe was like
when it was a few minutes old!
1) At t lt 1 minute, things get more speculative.
2) Cosmologists love to speculate.
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Thursdays Lecture
Gravity and the Expanding Universe
Reading
Chapter 5