Cosmology

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Cosmology

• Observe galaxies to try to learn
• does the universe have an edge?
• does it have a center?
• what was its past?
• what is its future?

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Evidence for a Big Bang

• What clues do we have that there was a BEGINNING
  to the Universe?
• Dark Sky at Night
• Expansion of the Universe (Hubbles Law)
• Cosmic Microwave Background Radiation
• Abundances of the Elements

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Expansion of the Universe (1928)
RED
BLUE
Almost all galaxies have redshifts ? appear to
be moving away from us ? more distant galaxies
have larger redshifts
BLUE
RED
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WHEW! DID SOMETHING DIE IN HERE?
COINCIDENCE? HMMM
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Hubbles Law
vr recession velocity d distance H0 Hubble
constant 22 km/s / Ml-y or 73 km/s /
Mpc (1 Ml-y 1 Mega light-year
106 light-years 1 Mpc 106 parsecs)
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Hubbles Law(fast forward)
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Consequence 1 If we see most galaxies moving
away from us, so will observers in other galaxies
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Thought Question

• Your observe a galaxy moving away from you at 0.1
  light-years per year, and it is now 1.2 billion
  light-years away from you. If the galaxy started
  very close to us, how long has it taken to get
  there?
• 1.2 million years
• 12 million years
• 12 billion years
• 120 billion years

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Hubbles Law
Consequence 2 Imagine running time backwards ?
galaxies would meet in same place at same time t
in past (BIG BANG)
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Why is the Sky Dark at Night?
Olbers Paradox

• IF the universe is
• infinite in size
• infinitely old
• homogeneous
• THEN
• the sky should be BRIGHT at night!

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How Does the Universe Look?
? Count galaxies in different directions! isotrop
ic looks same to us in all directions Does any
direction look special?
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Thought Question

• Which of these 4 universes is isotropic according
  to the stick figure?
• a
• b
• c
• d
• a, c
• c, d
• a, c, d

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Is the Universe Isotropic?
isotropic looks the same in all directions to
us All-sky surveys of millions of galaxies show
there are no special directions
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How Does the Universe Look?
I feel left out
? All directions may look the same to us, but do
other aliens see the same? homogeneous looks the
same to observers anywhere Are there any special
places?
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Thought Question

• Which of these 4 universes is homogeneous?
• a
• b
• c
• d
• a, c
• c, d
• a, c, d

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Is the Universe Homogeneous?
? harder to measure need to measure galaxy
distances and survey faint, distant galaxies

• some clumping, but there seem to be no special
  places

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Is the Universe Homogeneous?

• On small scales (lt108 light-yrs) galaxy clusters
  and voids

• On LARGER scales, things look more uniform

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Thought Question

• The universe is more uniform when you look at
  volumes more than 108 light years across. Roughly
  how many Milky Way diameters is that?
• 10
• 102
• 103
• 104
• 105

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The Observable Universe

• If universe is NOT infinitely old, then
• galaxies are only visible if their light has had
  time to reach us
• universe may be MUCH larger, but theres an edge
  (HORIZON) to what we see
• more distant galaxies look younger to us
  LOOKBACK TIME

UNOBSERVABLE UNIVERSE
OBSERVABLE UNIVERSE
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Thought Question

• Imagine you are observing a high-mass star
  located in a distant galaxy 100 million
  light-years away. By analyzing the starlight, you
  are able to tell that the star appears to be 10
  million years old. From its properties, you
  predict that the star has a total lifetime of 50
  million years.
• How old does the star appear to be to us on
  Earth?
• How long before we receive light from this stars
  supernova?
• When will or when did the supernova occur?

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Lookback
100 MILLION LIGHT-YEARS
40 MILLION LIGHT-YEARS
The star just blew up!!!
The star has been dead for 60 million years.
The star looks like it has 40 million years to
live.
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The Horizon
UNOBSERVABLE UNIVERSE (early in history)

• How far back in time can we possibly look?
• Galaxies and stars havent always been around
• DARK AGES
• Universe was once OPAQUE
• gas too dense to let light through

DARK AGES
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Cosmic Microwave Background Radiation (1965)
?Microwave and radio from all directions in sky
with nearly equal brightness
MAP OF ENTIRE SKY
T 2.728 K
?thermal radiation with very low temperature
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The Isotropy Problem
Opposite sides of observable universe somehow
knew to have almost the same temperature today
HORIZON
Choose a number between 1 and 1 million.
538,681
538,681
? takes longer than age of the universe for light
to travel from one side to the other
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Thought Question

• The cosmic microwave background is visible in
  every direction because
• we are at the center of the universe.
• we are looking back in every direction to when
  the Universe was young.
• we are looking back to when the universe was
  cool.
• it has reflected in every direction over the age
  of the Universe.

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Two Ways of Getting Redshifts

• Motion through Space
• redshifts from Doppler effect

• Stretching of Space
• galaxies dont have to move
• galaxies arent stretched (their own gravity
  holds them together)
• universe stretches light

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Balloon Universe

• Stretching of Space
• galaxies dont have to move
• galaxies arent stretched (their own gravity
  holds them together)
• universe stretches light

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Stretching and Redshifts
Redshift tells us about total amount of
stretching universe has done while light has been
traveling
STATIONARY (NOT EXPANDING) UNIVERSE
PAST
PRESENT
?NO REDSHIFT
EXPANDING UNIVERSE
DISTANT PAST
RECENT PAST
PRESENT
?MORE REDSHIFT FOR MORE DISTANT GALAXY
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Universe Scale
We cant measure size of universe (especially if
infinite), so compare distance at different
times Distances between non-moving galaxies
stretch in same way light wavelengths do
PAST
redshifts tell us how much universe has stretched
since light was released
PRESENT
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Thought Question

• The most distant object that has been seen had a
  redshift of about z 8. Roughly how far was it
  when it released the light we see today?
• 9 times its present distance from us.
• 8 times its present distance from us.
• 7 times its present distance from us.
• Its present distance from us.
• 1/7th its present distance from us.
• 1/8th its present distance from us.
• 1/9th its present distance from us.

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The universe was about as hot as a cool star at
380,000 yrs old3000 K and z?1100
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Cosmic Microwave Background Radiation
? thermal radiation with temperature T 2.728 K
Expansion of universe stretches light, makes
temperature lower
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Abundances of the Elements
In the oldest stars

• 1H 75 of mass
• 4He 25 of mass
• 2H (Deuterium), 3He, 7Li in trace amounts
  almost NO heavy elements
• Nuclear reactions had to occur in early universe
  high temperature and density required
• ?All other chemicals produced in stars!!

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Nuclear Reactions

• T 109 K
• light nuclei made
• When? Universe 3 minutes old

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Thought Question
Which observations provide evidence for important
pieces of the Big Bang model of the universe?