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Astronomy 101 Final exam:


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Title: Astronomy 101 Final exam:

Astronomy 101 Final exam Time Monday May 19 at
800 AM Place Gym (not the Sports Complex, and
not the Dance Studio) Final grades will be
assigned on the basis of the total course
grade Each midterm 15 Final
Exam 30 Recitation 30 Projects 10 We
will apply a curve to this final score to obtain
the course grade.
Observing project reports returned today at
entrance to lecture hall
Optional review, Monday May 12 in lecture hour
(1030 to 1125) here
See a total eclipse of the moon, Thursday May 15,
around midnight info at http//skyandtelescope.c
om/observing/objects/eclipses/article_923_1.asp On
August 27, Mars and Earth will be closer
together than for 57,000 years. The next record
approach is in about 280 years , so plan to catch
this one ! http//
Astronomy 101
Lecture 28, May 7 2003
The early universe (Chapter 27 in text)
OK, we believe in the Big Bang around 13.7
billion years ago. What happened in the very
earliest moments, days, years of the universe,
and how did those events shape our world
today? The environment in the early universe
was changing very rapidly from the initial
point singularity of the big bang, as the
universe expanded, the Temperature dropped and
the density of matter and radiation (photons)
decreased. The universe passed through a
series of epochs with differing conditions,
depending on the Temperature.
Nowadays, we see density of about 2 x 10-27 kg/m3
of matter. The radiation is dominated by the
blackbody cosmic microwave background (starlight
is negligible), and the density in mass terms
(Emc2) is only about 10-32 kg/m3. So we are
in presently in a matter-dominated universe
matter density exceeds radiation density by about
100,000. Going back toward the Big Bang, the
smaller universe means that both the density of
atoms/protons/electrons and the density of
photons increases the number stays the same,
but the volume decreases (density
mass/volume). But in addition, each photon
carried more energy in past since the wavelength
becomes smaller as we go back in time to when
space was more compressed.
This means that going back, the radiation becomes
more dense faster than the matter, and ultimately
we go back to a time when radiation dominated the
universe. The cross-over point was at a few
thousand years after the Big Bang. Before then,
universe is mainly a bath of hot photons.
(Dark energy was presumably less important in the
early universe than now.)
There were particles (protons, neutrons,
electrons etc.) present in the early universe,
but they were continually appearing and
disappearing. Pair creation reactions and photon
annihilation reactions can transform particles
into photons and vice versa. The pair creation
reactions are the source of all particles now in
the universe.
At high Temp, the two reactions are in
equilibrium they occur at same rate and number
of electrons and photons remains the same until
the time when the temperature drops and the
energy of the photons is not enough to create the
mass associated with the electron and positron.
Occurs at a few billion degrees. Population of
electrons declines drastically.
e e- ? g g
g g ? e e-
create electrons
destroy electrons
Similar creation and annihilation reactions for
proton/antiproton pairs and neutron/antineutron
pairs. At a temperature of about 1013 K, these
pair creation reactions become impossible, and
most protons/neutrons disappear through
annihilation. Passing these temperature
thresholds is called Freeze out.
We know a great deal about the particle
interactions from our experiments at
accelerators, so this aspect of understanding the
early universe is well understood. This is a
particle collision seen in Stony Brooks D0
2 km
The accelerator at Fermi National Laboratory is a
four mile circumference race track for protons
and antiprotons.
Four fundamental forces among the particles
In order of increasing size of the
forces Gravity all particles feel this, in
proportion to their mass Weak nuclear force the
force that operates in nuclear decay reactions
like 15O ? 15N positron neutrino that
operates in the sun to fuse hydrogen into helium.
Most particles feel this. Electromagnetic force
force between charges like signs repel
causing it to be hard to get two nuclei together
to react in the center of a star. Only charged
particles and photons participate. Strong nuclear
force causes reactions like 12C p ? 13N
energy in stars. A host of particles experience
these forces in different combinations quarks
(which make protons and neutrons), electrons,
photons etc. The 4 forces in our laboratory
experiments are quite distinct and have very
different strengths. But at very high energy or
Temperature, the Strong, Electromagnetic and Weak
forces are known to fuse into one and share
common properties. We suspect that gravity may
also merge with the other three at even higher
Temperatures. As the universe cools (energy
decreases), one by one, the forces drop out of
the unified whole. Called a phase change like
water freezing. Energy is liberated when the
phase transition occurs.
A brief history of the universe (times, density
and Temp. are for end of epochs) Epoch time
density Temp character Planck 10-43
s 1095 kg/m3 1032 K all 4 forces unified,
vigorous particle production Grand
unification 10-35 s 1075 1027 only
strong, electromag, weak unified
Inflation occurs at the end of epoch Hadron 10-4
s 1016 1012 all particles in
equilibrium with photons Lepton 102 s 104
109 only electrons/photons in equil.
Nuclear 103 yr 10-13 6x104 form
helium and deuterium transition
between radiation dominated and matter dominated
Atomic 106 yr 10-19 103 atoms
form at 380,000 yrs and CMB is
born clouds of atoms in universe Galactic 109
yr 10-25 10 galaxies and clusters
form Stellar present 2x10-27 2.7
complex galaxies with bright stars
and planets evolving some intelligent (!)
(No Transcript)
What happened in each successive epoch?
In the Planck epoch, gravity, weak,
electromagnetic and strong forces are presumed to
be unified (we dont understand just how because
we dont have a theory of quantum gravity yet).
All particles known (and others to heavy to be
seen yet) were present in equilibrium with the
very hot bath of photons. Universe extremely
tiny, hot and dense. In the Grand unification
epoch, gravity drops out of unification, leaving
unified strong nuclear, electromagnetic and weak
nuclear forces unified. Near the end of this
epoch, when T drops to 1028 K, the strong nuclear
force also goes out of unification. At this
point, some of the heavy particles associated
with the combined unified forces freeze out of
the equilibrium and propagate freely. One of
these particles (supersymmetric partners to the
photon) is one of the best candidates for the
dark matter, seen to fill the universe today. At
the end of the Grand unification epoch when the
universe was around 10-35 seconds old, a
mysterious phenomenon called Inflation seems to
have occurred.
Going from a universe in which strong, EM and
weak forces are unified to one where strong
differs from EM weak is a phase transition. A
familiar phase transition is water freezing to
ice. It is possible in water be supercooled and
to exist as liquid even below the usual freezing
temperature. When the supercooled water finally
turns into ice, it releases a great deal of
energy suddenly. (A similar phenomenon occurs
in reverse when you heat water very gently to
above its boiling point eventually any little
speck of dust will initiate vigorous
boiling.) The universe seems to have supercooled
in the symmetric state of all 3 forces unified
for longer than it should have. When it popped
into the broken symmetry phase, a tremendous
burst of energy was emitted that inflated the
universe extremely rapidly. Inflation lasted from
about 10-35 to 10-32 seconds Radius of universe
expanded from 10-36 times the size of a proton to
the size of a grapefruit by a factor of more
than 1050 ! At 10-32 s, after the energy was
dissipated, the evolution of the universe resumed
its more normal pace and the grand unification of
forces was gone, leaving the separate strong and
combined EM weak forces. Not necessarily all
the universe at the time inflated, but all that
is visible to us did so. We may be in one bubble
among many.
Inflation blew up the universes size by a factor
of 1050.
Inflation solves the flatness problem just as
if you blew a balloon up enormously, the previous
spherical surface would look flat. And flat
means that W0 1 (so the universe has
critical density). It also solves the horizon
problem two points on opposite sides of visible
universe were close and in contact before
inflation, and lost touch only after it.
By the end of the Hadron epoch at 10-4 sec, it
became impossible for the reaction gg ? p p to
occur, since not enough energy was available to
the photons to make the mass energy (Emc2) of
the proton and antiproton. This meant that the
protons mostly disappeared, leaving only a very
tiny remnant density that escaped before
annihilating with antiprotons. In the universe
now, we see matter (protons, neutrons) and not
antimatter (antiprotons, antineutrons). The
laws of physics seem to require equal densities
of each. We guess that some asymmetry in the
laws of physics in the universe during the hadron
epoch caused this, but it remains a puzzle how
this works. Until about 100 seconds, the
electron and anti-electron (positron) remained in
equilibrium with the photon bath. At the end of
the Lepton epoch, the photon energies were too
small to make e e- , and electrons mostly
disappeared leaving a tiny remnant
population. The small fraction of remaining
electrons and protons after their freezeouts
remain today, and determine the ratio of matter
to photons in the universe. Cosmology can
calculate this ratio, so it is a good check on
the validity of our Big Bang model.
The protons and neutrons collide in the reaction
p n ? 2H (deuteron d ) g . But at high
temperatures, the reverse reaction g d ? p n
occurs as rapidly and keeps the deuteron density
low. In the Nuclear epoch, when the
temperature drops below a billion degrees, the
reverse reaction stops (not enough energy to make
it occur), and free deuterons appear. Rapid
formation of helium then occurs by combining
deuterons, protons and neutrons for example d
d ? 4He energy. About 25 of the matter is
helium nuclei about 75 is protons (hydrogen
nucleus), and only about 0.01 remains as
deuterons. The electrons are still too
energetic to be captured by the nuclei.
The helium and deuterium formed in the Nuclear
epoch remain today the helium in a balloon was
made in the first few minutes of the universe
(plus a little made later in burning in
stars.) The deuterium abundance depends on the
total density of matter the more matter
present, the less deuterium remaining. The
observed density of deuterium confirms that about
4 of the universe is composed of ordinary atoms.
The universe has now passed into its matter
dominated phase. In the Atomic epoch, the
temperature drops to the point that electrons can
be captured by nuclei to form neutral atoms. Up
until this point, the hot gas of photons
interacted strongly with electrons, so was in
equilibrium with the matter. From this
decoupling of radiation point onward, the
photons were free to roam without interaction.
We see them, red shifted by a factor of 1000, as
the Cosmic Microwave Background.
In the Galactic epoch, the small fluctuations in
density seen in the CMB pattern, reflecting also
the matter fluctuations, seed the formation of
clumps of gas from which the galaxies form.
Actually the visible matter fluctuations revealed
in the CMB pattern is insufficient to make the
galaxies seen after about 1 billion years but
the dark matter clumping in the early universe
could account for the early galactic formation.
Simulations of gravitationally induced structure
using visible matter (atoms) and dark matter
agree with the structure of galaxies and galactic
clusters that we see. The dark matter is a
necessary ingredient !
Temperature fluctuations from decoupling
The evolution of structure cosmic microwave
background to clumps of matter to stars and
protogalaxies to our present day universe.
Matter clumping due to fluctuations
First stars at 200 million years
Galaxy forming along lines of density
fluctuations in frame 2
Modern era
Powers of 10 in distance a 10 minute tour of
the universe to the very large and to the very
small. Lets see in 10 minutes the whole span of
this course.
1 m The picture of the sleeping picknicker in
Golden Gate Park in San Francisco is 1 meter
square. The small blue square is 0.1m 10 cm on
each side. Successive pictures will enlarge the
frame by a factor of 10. The current outer frame
will be shown as the small frame in the next
10 m On the grass at the park
102 m The Music Concourse at Golden Gate Park
103 m 1 km Golden Gate Park and its roads
104 m 10 km San Francisco
105 m 100 km San Francisco Bay area
106 m 1000 km California
107 m 10,000 km North America
108 m 100,000 km Earth
109 m 1 million km Earth and Moons orbit
1010 m Four days on Earths orbit
1011 m Venus, Earth and Mars orbits
1012 m Planetary orbits out to Jupiter
1013 m The solar system
1014 m Our sun and its little solar system
1015 m Sol our lonely sun
1016 m 1 light year The Oort cloud birthplace
of comets
1017 m 10 light years The sun and the nearest
Alpha Centauri
1018 m 100 light years Nearby stars in the
galaxy spiral arm
1019 m 1000 light years The stars of the Orion
spiral arm of the galaxy
1020 m 10,000 light years The full spiral arm
1021 m 100,000 light years The Milky Way
1022 m 1 million light years The local group
1023 m 10 million light years Within the Virgo
1024 m 100 million light years Clusters of
1025 m billion light years Large scale
structure in the universe
1 m The picture of the sleeping picknicker in
Golden Gate Park in San Francisco is 1 meter
square. The small blue square is 0.1m 10 cm on
each side. Succeeding pictures will magnify the
small square to the full frame.
10-1 m 10 cm A hand
10-2 m 1 cm Skin
10-3 m 1 mm A pore on the skin
10-4 m 100 microns Micro-organisms
10-5 m 10 microns A lymphocyte
10-6 m 1 micron Nucleus of a cell
10-7 m 100
nanometers Strands of DNA
10-8 m 10 nanometers The
structure of DNA
10-9 m 1 nanometer Molecules of DNA
10-10 m 0.1 nanometer
1 angstrom Carbon atoms outer shell
of electrons
10-11 m 10 picometers The
inner cloud of electrons in carbon
10-12 m 1 picometer Inside the electron cloud
the nucleus is just visible
10-13 m 100 femtometers A
carbon nucleus
10-14 m 10
femtometers Carbon nucleus up close six protons
and six neutrons
10-15 m 1
femtometer Inside the proton a swarm of quarks
10-16 m 100 attometer The
The laws of physics at the level of quarks and
photons control the structure and evolution of
the universe at large. Inner space and
outer space are intertwined.
Element formation in stars
Element formation in stars
Fundamental particle forces shape the future
The big bang
Dark matter shapes the galaxies
Laboratory particle physics and cosmology let us
look back to the big bang
Forming Earth-like planets
Chemistry of life