The Big Band

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The Big Band
Lecture 28 NATS102-13 3 December 2009
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Announcements

• Essays and homework are due today
• Exam is next Tuesday. Same format as others.
• Grades will be posted next Wednesday
• I will be impossible to locate (I hope) from Dec
  10 to Jan 3.
• There will be a review session from 200 to 400
  PM next Monday in room 330.

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Review A Look Ahead
The fact that all galaxies seem to be moving away
with a velocity that is proportional to their
distance implies that they were once close
together. Well also talk about some other
evidence for the Big Bang, especially the Comic
Microwave Background, and address the question of
whether the Universe will continue to expand
forever, or will someday collapse. This latter
point is related to current questions about dark
matter in the Universe.
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Expansion of the Universe
Remember, Its not that galaxies are flying apart
into some fixed Universe, its the Universe
itself that is expanding.
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The cosmic microwave background the radiation
left over from the Big Bang was detected by
Penzias Wilson in 1965
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Universe must have been much hotter and denser
early in time
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The early universe must have been extremely hot
and dense
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Background radiation from Big Bang has been
freely streaming across universe since atoms
formed at temperature 3,000 K visible/IR
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Background has perfect thermal radiation spectrum
at temperature 2.73 K
The spectrum of the CBR
This is a near perfect blackbody spectrum for an
object with a temperature of 2.73 Kelvin.
Expansion of universe has redshifted thermal
radiation from that time to 1000 times longer
wavelength microwaves
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Two Ways to Think About Red Shifts
From one point of view, distant galaxies are red
shifted because of the Doppler shift and the fact
that they are moving away from us. From the
other point of view, the galaxies appear to be
moving away from us because the Universe is
expanding and their spectra are red shifted
because wavelengths get longer as the Universe
expands. According to this point of view, the
wavelength of radiation gets longer with time,
so, for a distant object whose light takes a long
time to reach us, the wavelength of the light we
observe was shorter when it was emitted.
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The Latest All-Sky Map of the CMB
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How do the abundances of elements support the Big
Bang theory?
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Protons and neutrons combined to make
long-lasting helium nuclei when universe was 3
minutes old
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Big Bang theory prediction 75 H, 25 He (by
mass) Matches observations of nearly primordial
gases
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Abundances of other light elements agree with Big
Bang model having 4.4 normal matter
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Three Pieces of Evidence for the Big Band

• Galaxies are flying apart with relative
  velocities that are proportional to their
  distance apart.
• The Cosmic Microwave Background implies that the
  Universe was hot and dense 14 Billion years ago.
• The abundance of light elements (elements not
  made in supernova explosions) is consistent with
  what we expect from a hot, dense early Universe.

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Planck Era Before Planck time (10-43 sec) No
theory of quantum gravity
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GUT Era Lasts from Planck time (10-43 sec) to
end of GUT force (10-38 sec)
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Electroweak Era Lasts from end of GUT force
(10-38 sec) to end of electroweak force (10-10
sec)
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Particle Era Amounts of matter and antimatter
nearly equal (Roughly 1 extra proton for every
109 proton-antiproton pairs!)
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Photons converted into particle-antiparticle
pairs and vice-versa E mc2 Early
universe was full of particles and radiation
because of its high temperature
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Era of Nucleosynthesis Begins when matter
annihilates remaining antimatter at 0.001
sec Nuclei begin to fuse
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Era of Nuclei Helium nuclei form at age 3
minutes Universe has become too cool to blast
helium apart
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Era of Atoms Atoms form at age 380,000
years Background radiation released
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Era of Galaxies Galaxies form at age 1 billion
years
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Why does the Cosmic Microwave Background tell us
that the early Universe was hot and dense?
Photons decoupled from matter when atoms began to
form from electrons and nuclei. This happened at
a temperature 3000 K, comparable to the
temperature of a cool star. Using Weins law, the
peak wavelength of this radiation would be close
to 1 µm. Since that time, photons in the CMB
have been traveling freely through the Universe,
with wavelengths that increase as the Universe
expands. The current peak wavelength that we
observe is close to 1 mm. So, the Universe has
expanded by a factor of 1000 since the formation
of atoms.
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Mysteries Needing Explanation

• Where does structure come from?
• Why is the overall distribution of matter so
  uniform?
• Why is the density of the universe so close to
  the critical density?

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Will the universe continue expanding forever?
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Does the universe have enough kinetic energy to
escape its own gravitational pull?
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Fate of universe depends on the amount of dark
matter
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22.2 Evidence for Dark Matter

• Our goals for learning
• What is the evidence for dark matter in galaxies?
• What is the evidence for dark matter in clusters
  of galaxies?
• Does dark matter really exist?
• What might dark matter be made of?

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What is the evidence for dark matter in galaxies?
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• We measure the mass of the solar system using the
  orbits of planets
• Orb. Period
• Avg. Distance
• Or for circles
• Orb. Velocity
• Orbital Radius

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Rotation curve A plot of orbital velocity versus
orbital radius Solar systems rotation curve
declines because Sun has almost all the mass
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Who has the largest orbital velocity? A, B, or C?
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Who has the largest orbital velocity? A, B, or C?
Answer C
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Rotation curve of merry-go-round rises with radius
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Rotation curve of Milky Way stays flat with
distance Mass must be more spread out than in
solar system
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Mass in Milky Way is spread out over a larger
region than the stars Most of the Milky Ways
mass seems to be dark matter!
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Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
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The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
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We can measure rotation curves of other spiral
galaxies using the Doppler shift of the 21-cm
line of atomic H
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Spiral galaxies all tend to have flat rotation
curves, indicating large amounts of dark matter
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Broadening of spectral lines in elliptical
galaxies tells us how fast the stars are
orbiting These galaxies also have dark matter
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What is the evidence for dark matter in clusters
of galaxies?
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We can measure the velocities of galaxies in a
cluster from their Doppler shifts
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The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
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Clusters contain large amounts of X-ray emitting
hot gas Temperature of hot gas (particle
motions) tells us cluster mass 85 dark
matter 13 hot gas 2 stars
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Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
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All three methods of measuring cluster mass
indicate similar amounts of dark matter
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Does dark matter really exist?
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Our Options

• Dark matter really exists, and we are observing
  the effects of its gravitational attraction
• Something is wrong with our understanding of
  gravity, causing us to mistakenly infer the
  existence of dark matter

Currently, 1 is the option preferred by most
astronomers.
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What might dark matter be made of?
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What might dark matter be made of?
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How dark is it?
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How dark is it?

• not as bright as a star.

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

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Two Basic Options

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

The Best Bet
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MACHOs occasionally make other stars appear
brighter through lensing
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MACHOs occasionally make other stars appear
brighter through lensing but not enough
lensing events to explain all the dark matter
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Why Believe in WIMPs?

• Theres not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
  formation works

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Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
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But expansion appears to be speeding up!
Dark Energy?
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Estimated age depends on both dark matter and
dark energy
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Is the expansion of the universe accelerating?
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Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded
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Accelerating universe is best fit to supernova
data
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Unseen Influences

• Dark Matter An undetected form of mass that
  emits little or no light but whose existence we
  infer from its gravitational influence
• Dark Energy An unknown form of energy that
  seems to be the source of a repulsive force
  causing the expansion of the universe to
  accelerate

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Summary

• Overall geometry is flat
• Total massenergy 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

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Contents of Universe

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 23
• Dark Energy 73