Chapter 20 Cosmology

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Chapter 20Cosmology
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Hubble Ultra Deep Field
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Galaxies and Cosmology

• A galaxys age, its distance, and the age of the
  universe are all closely related
• Galaxies formed when the universe was young and
  have aged along with the universe

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Parallax
Measure the distances to nearby stars
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Star clusters
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• Luminosity
• Brightness
• 4p (distance)2
• Properties you can directly observe and measure
• Brightness
• Change in brightness over time
• Color
• Rotation speed
• A standard candle is an object whose luminosity
  we can determine without measuring its distance

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Cepheid variable stars are very luminous standard
candles
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White-dwarf supernovae all have same peak
luminosity standard candles
Can be seen up to 10 billion light years away!
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Tully-Fisher Relation Entire galaxies can also
be used as standard candles faster rotation
greater total luminosity
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Giant ellipticals if youve seen one, youve
seen them all
Homework assignment
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Hubble measured the distance to nearby galaxies
using Cepheid variables as standard candles
(1927, Mt Wilson Obs)
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Hubble found that the spectral features of
virtually all galaxies are redshifted ? Theyre
all moving away from us
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Hubble found that the further away a galaxy is,
the faster it is receding from us!
Slope y / x
Time age of the universe!
Hubbles Law velocity H0 x distance
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Distances of farthest galaxies are now measured
from their redshifts!!
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A balloons surface expands but has no center or
edge
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Cosmological Principle

• The universe looks about the same no matter
  where you are within it
• Matter is evenly distributed on very large scales
  in the universe
• No center no edges
• Not proved but consistent with all observations
  and predictions of the Big bang theory

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Distances between faraway galaxies changes
because the space between them expands! Think
of lookback time rather than distance
distance?
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Redshift is NOT the Doppler shift!
Expansion stretches photon wavelengths causing a
cosmological redshift directly related to
lookback time
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observations show us very distant galaxies as
they appeared a long time ago (Old light from
young galaxies)
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Galaxies of different ages look different from
one another
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Collisions play an important role in galaxy
evolution
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Collisions were much more common when U. was
young, because galaxies were closer together
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Many of the galaxies we see at great distances
(when U. was young) look violently disturbed
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Giant elliptical galaxies at the centers of
clusters seem to have consumed a number of
smaller galaxies
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Collisions may explain why giant elliptical
galaxies tend to be found where galaxies are
closer together
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Quasars are the most luminous galaxies
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• The highly redshifted spectra of quasars indicate
  large distances
• Redshift --gt distance --gt luminosities of some
  quasars are gt1012 LSun
• Variability shows that all this energy comes from
  region smaller than solar system active nucleus
  with supermassive black hole!!

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Galaxies around quasars often appear disturbed by
collisions
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Dark Matter, Dark Energy, and the Fate of the
Universe
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Mass within Suns orbit 1011 MSun
Observable stars and gas clouds few 109
MSun
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Dark matter and dark energy

• Dark Matter An undetected form of mass that
  emits little or no photons, but we know it must
  exist because we observe the effects of its
  gravity
• Dark Energy An unknown form of energy that is
  causing the universe to expand faster over time

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What is the Universe made of?

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 25
• Dark Energy 71

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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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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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measure the velocities of galaxies in a cluster
from their Doppler shifts
Mass is 50 x larger than the mass in stars!
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Clusters contain large amounts hot gas emits x
rays Temperature of hot gas tells us cluster
mass 85 dark matter 13 hot gas 2
stars
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Gravitational lensing of background galaxies also
tells us the mass
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What is dark matter made of?

• 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 do not cause 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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?
Gravity of dark matter is what caused
protogalactic clouds to contract early in time
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WIMPs dont contract to center because they dont
emit photons, so they can not radiate away their
orbital energy
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Maps of galaxy positions reveal extremely large
structures superclusters and voids
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WIMP models agree better with observations
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Fate of universe depends on the amount of dark
matter
Critical density of matter
Lots of dark matter
Not enough dark matter
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Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
Not enough dark matter
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But expansion appears to be speeding up!
Dark Energy?
Not enough dark matter
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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