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Chapter 20 Cosmology

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Chapter 20 Cosmology – PowerPoint PPT presentation

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Title: Chapter 20 Cosmology


1
Chapter 20Cosmology
2
Hubble Ultra Deep Field
3
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

4
Parallax
Measure the distances to nearby stars
5
Star clusters
6
  • 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

7
Cepheid variable stars are very luminous standard
candles
8
White-dwarf supernovae all have same peak
luminosity standard candles
Can be seen up to 10 billion light years away!
9
Tully-Fisher Relation Entire galaxies can also
be used as standard candles faster rotation
greater total luminosity
10
Giant ellipticals if youve seen one, youve
seen them all
Homework assignment
11
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12
Hubble measured the distance to nearby galaxies
using Cepheid variables as standard candles
(1927, Mt Wilson Obs)
13
Hubble found that the spectral features of
virtually all galaxies are redshifted ? Theyre
all moving away from us
14
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
15
Distances of farthest galaxies are now measured
from their redshifts!!
16
A balloons surface expands but has no center or
edge
17
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

18
Distances between faraway galaxies changes
because the space between them expands! Think
of lookback time rather than distance
distance?
19
Redshift is NOT the Doppler shift!
Expansion stretches photon wavelengths causing a
cosmological redshift directly related to
lookback time
20
observations show us very distant galaxies as
they appeared a long time ago (Old light from
young galaxies)
21
Galaxies of different ages look different from
one another
22
Collisions play an important role in galaxy
evolution
23
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24
Collisions were much more common when U. was
young, because galaxies were closer together
25
Many of the galaxies we see at great distances
(when U. was young) look violently disturbed
26
Giant elliptical galaxies at the centers of
clusters seem to have consumed a number of
smaller galaxies
27
Collisions may explain why giant elliptical
galaxies tend to be found where galaxies are
closer together
28
Quasars are the most luminous galaxies
29
  • 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!!

30
Galaxies around quasars often appear disturbed by
collisions
31
Dark Matter, Dark Energy, and the Fate of the
Universe
32
Mass within Suns orbit 1011 MSun
Observable stars and gas clouds few 109
MSun
33
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

34
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

35
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36
Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
37
The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
38
measure the velocities of galaxies in a cluster
from their Doppler shifts
Mass is 50 x larger than the mass in stars!
39
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
40
Gravitational lensing of background galaxies also
tells us the mass
41
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42
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43
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44
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

45
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
46
MACHOs do not cause enough lensing events to
explain all the dark matter
47
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

48
?
Gravity of dark matter is what caused
protogalactic clouds to contract early in time
49
WIMPs dont contract to center because they dont
emit photons, so they can not radiate away their
orbital energy
50
Maps of galaxy positions reveal extremely large
structures superclusters and voids
51
WIMP models agree better with observations
52
Fate of universe depends on the amount of dark
matter
Critical density of matter
Lots of dark matter
Not enough dark matter
53
Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
Not enough dark matter
54
But expansion appears to be speeding up!
Dark Energy?
Not enough dark matter
55
Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded
56
Accelerating universe is best fit to supernova
data
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