Download Cosmology

H205
Cosmic Origins
APOD
The Beginning of Time (Ch. 23)
Gravity (Ch. 4) on Wednesday
EP2 Due Wednesday
Special
Lecture
“The Chemical Heritage of
Star and Planet Formation”
7:30 Tuesday, March 24
Fine Arts 015
Tom Hartquist
Olber’s
Paradox
You are lost in a very
large forest
The forest is
uniformly dense with
trees
What do you see?
IF:
•The universe is infinite
•The universe is filled with
galaxies, clusters of galaxies,
and superclusters
•The universe is eternal and
unchanging
Why Is the Sky Dark?
Olbers’ Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would
cover the night sky
everywhere
An Overview
Earth
Radius: 6400 km
Distance from Sun: 150,000,000 km
1 AU, 8 light minutes
Moon
Radius: ¼ Earth’s radius
Distance from Earth:
384,000 km
The
Sun
Radius: 700,000 km
Diameter: 1,400,000 km
(about 100 x the diameter of Earth)
Mass: 300,000 x mass of Earth
The
Nearest
Stars
The closest star to our Sun is Proxima
Centauri, about 4 light years distant.
Most of the stars
we see in the sky
are within 250
light years
Our Sector of the Galaxy
The Sun lies along one of
our Galaxy’s spiral arms,
known as the Orion Arm
View of the
Milky Way Galaxy
Our Milky Way galaxy
contains two hundred
billion stars.
The Sun is about
26,000 light
years from the
center.
Our Milky Way
Galaxy is part of
a small cluster of
about 3 dozen
galaxies.
Our Local Group of
galaxies is part of a
larger Supercluster
of galaxy groups.
Virgo
Supercluster
Galaxies and clusters
of galaxies collect
into vast streams,
sheets, and walls of
galaxies.
The Visible
Universe
On the largest
scales, the
universe seems
to be more or
less uniform
Our Goal
• How did the Universe begin?
• How did the Universe become
what we see today?
• What does the state of the
Universe today tell us about
the origin of the Universe?
A Concept We Need
• Lookback Time
– Astronomers can see into
the past
• How?
What are the properties of the
Universe?
• Size
• Distribution of matter
• Age
• Models of the Universe
– Steady State Model
– Big Bang Model
More Evidence for the Big Bang
The cosmic microwave
background radiation
In 1965, two engineers at AT&T’s Bell
Labs in New Jersey were puzzled by a
constant “noise” in a microwave antenna
they were calibrating
The cosmic
microwave
background –
the radiation
left over from
the Big Bang –
was detected
by Penzias &
Wilson in
1965
A Serendipitous Discovery and a
Nobel Prize
• The noise could not be explained by
terrestrial or extraterrestrial sources. It
seemed to be everywhere
• Penzias and Wilson shared the 1978 Nobel
Prize in physics for their discovery of the
Cosmic Microwave Background (CMB)
radiation
A uniform, faint signal
from all over the sky
COBE data points
Spectrum of a
2.726 K degree
blackbody
The CMB
The existence of the
CMB shows that the
universe was hot and
dense in the distant past
• In 1989, NASA launched the Cosmic Background
Explorer (COBE) satellite to study the spectrum of
the CMB
• COBE showed that the CMB has a perfect thermal
spectrum at a temperature of 2.726 K
• Cold! No natural object in the Universe can be
colder than this (why not?)
Wilkinson
MICROWAVE
Anisotropy Probe
The microwave light that we detect has been stretched
out as the universe has expanded, so light that was once
very short wavelength is now reaching us as microwaves.
Background has perfect
thermal radiation
spectrum at temperature
2.73 Kelvin… COLD!
Wilkinson Microwave Anisotropy Probe
WMAP, a later NASA mission, shows us the
beginning of cosmic structure in the Universe.
Structure began as tiny temperature differences
from place to place. The temperature differences
WMAP sees vary by only millionths of a degree.
The microwave light
captured in this
picture is from
380,000 years after
the Big Bang, over 13
billion years ago
A baby
picture of
the
Universe
• Imagine a temperature map of the
Earth in June 1992
• WMAP makes a similar map of the sky
What are we seeing in
the baby picture?
 The microwave
radiation comes from
the time when the
temperature of the
Universe became low
enough for atoms to
form (about 3000K)
 Matter became
transparent, allowing
light to travel great
distances
 It is like seeing the
bottom layer of clouds
on an overcast day.
Background
radiation
from Big
Bang has
been freely
streaming
across
universe
since atoms
formed at
temperature
~ 3,000 K:
visible/IR
What does the “baby picture” tell us?
On what angular scales do we
see variations in the CMB?
A “Power Spectrum”
How strong are variations on
different angular scales?
Different angular scales probe
different aspects the early Universe
• Astronomers compute
a complex physical
model of conditions
and adjust parameters
to match the observed
curve
• Parameters include an
expansion rate, a
composition, age, etc.
A simple “Big
Bang” doesn’t
explain all the
properties of the
Universe
1) Where does structure come from?
2) Why is the overall distribution of matter
so uniform?
3) Why is the density of the universe so
close to the critical density?
Explaining the Origin of Structure
• The simple Big
Bang model gives
a Universe with
no structure
• To explain why
the Universe has
structure, we
need “inflation”
What is Inflation?
• A period of
extremely rapid
expansion when the
Universe was very
young.
• 10-38 to 10-36
seconds
• Regions of the
Universe expanded
from the size of
an atomic nucleus
to the size of the
Solar System
Inflation can
make all the
structure by
stretching tiny
quantum ripples
to enormous size
These ripples in
density then
become the
seeds for all
structures in
the Universe
Testing
Inflation
Why is the Cosmic Microwave Background
Radiation almost perfectly isotropic?
The CMB is isotropic because regions now
on opposite sides of the sky were close
together before inflation pushed them far
apart
Testing
Inflation
 Patterns of structure
observed by WMAP
show the “seeds” of
universe
 Observed patterns of
structure in the
Universe agree (so far)
with the “seeds” that
inflation would produce
WMAP’s observation of the “seeds” of
structure inferred from the CMB confirm
the existence of dark energy
• Overall geometry is flat
– Total mass+energy has critical density
• Ordinary matter ~ 4.4% of total
• Total matter is ~ 27% of total
– Dark matter is ~ 23% of total
– Dark energy is ~ 73% of total
• Age of 13.7 billion years
More evidence for the Big Bang:
The Helium Fraction
The amount of helium compared to hydrogen
formed as hot, primordial energy cools and
forms into matter can be predicted by physical
laws
The fraction of helium in the Universe can be
measured with observations
The observed abundances of elements support
the predictions of the Big Bang theory
In the hot early universe, protons and neutrons combined to
make long-lasting helium nuclei when universe was ~ 3
minutes old
Nuclear physics shows us how to predict the amounts of
hydrogen and helium that will be produced
Big Bang theory prediction: 75% H, 25% He (by mass)
Matches observations of nearly primordial gases
Abundances of other light elements agree with Big Bang model
having 4.4% normal matter
The abundances
depend on the
density of the
Universe when
nuclei first
formed
–
–
–
–
Deuterium
Helium-4
Helium-3
Lithium-7
Conclusion:
The “Big Bang”
•
•
•
Expansion implies a beginning, assuming that
the universe has been expanding since it
came into being
Scientists call the beginning of our universe
the Big Bang. The term was coined by
British astronomer Sir Fred Hoyle in the
1940s (on a BBC broadcast!)
The Big Bang represents a hot, dense
primordial state of high energy (but not an
explosion)
Summary – Strong evidence supports
the Big Bang Theory
• The Universe is expanding (and cooling) from
an initial, dense state
• Radiation left over from the Big Bang is now
detected in the form of microwaves—the
cosmic microwave background—which we can
observe with a radio telescope
• Observations of helium and other light
elements agree with the predictions for
fusion in the Big Bang theory
Coping with a misconception…
If the Universe is expanding, what
it is expanding into????????
NOTHING!!!!
All of space came into being at the
moment of the Big Bang. Space
itself is expanding
Balloon Model
Will the Universe Keep Expanding…
Or Re-Collapse????
• It depends on the density of the
Universe
– The critical density is 10-29 g/cm3, about
one hydrogen atom per cubic meter
– About 25 times more than the observed
mass of stars and gas
– But what about dark matter?
• there isn’t quite enough to re-collapse the
Universe
But the universe isn’t even slowing down….
A brief history
of the Universe
Courtesy of Fred Adams
University of Michigan
The History of the Universe in 200 Words or Less
Quantum fluctuation. Inflation. Expansion. Strong nuclear interaction. Particleantiparticle annihilation. Deuterium and helium production. Density perturbations.
Recombination. Blackbody radiation. Local contraction. Cluster formation.
Reionization? Violent relaxation. Virialization. Biased galaxy formation? Turbulent
fragmentation. Contraction. Ionization. Compression. Opaque hydrogen. Massive star
formation. Deuterium ignition. Hydrogen fusion. Hydrogen depletion. Core
contraction. Envelope expansion. Helium fusion. Carbon, oxygen, and silicon fusion.
Iron production. Implosion. Supernova explosion. Metals injection. Star formation.
Supernova explosions. Star formation. Condensation. Planetesimal accretion.
Planetary differentiation. Crust solidification. Volatile gas expulsion. Water
condensation. Water dissociation. Ozone production. Ultraviolet absorption.
Photosynthetic unicellular organisms. Oxidation. Mutation. Natural selection and
evolution. Respiration. Cell differentiation. Sexual reproduction. Fossilization. Land
exploration. Dinosaur extinction. Mammal expansion. Glaciation. Homo sapiens
manifestation. Animal domestication. Food surplus production. Civilization! Innovation.
Exploration. Religion. Warring nations. Empire creation and destruction. Exploration.
Colonization. Taxation without representation. Revolution. Constitution. Election.
Expansion. Industrialization. Rebellion. Emancipation Proclamation. Invention. Mass
production. Urbanization. Immigration. World conflagration. League of Nations.
Suffrage extension. Depression. World conflagration. Fission explosions. United
Nations. Space exploration. Assassinations. Lunar excursions. Resignation.
Computerization. World Trade Organization. Terrorism. Internet expansion.
Reunification. Dissolution. World-Wide Web creation. Composition. Extrapolation?
Copyright 1996-1997 by Eric Schulman .
For Wednesday
Chapter 4 - Making Sense
Finish EP2 on Wednesday