COSMOLOGY

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COSMOLOGY
Largest Scales
Expansion
Fate of the Universe
Cosmological Constant
Group 36
Monica Chu Kate Mossbarger Erin Selawski Akaash
Gupta
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Cosmology Definition the study of the
structure and evolution of the entire universe
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Cosmological Principle
The two assumptions that the universe is
homogeneous and isotropic on sufficiently large
scales.
Homogeneity The universe is the same
everywhere. - On an imaginary large scale,
the number of galaxies is the same anywhere in
the universe.

- Homogeneity stands as
long as we assume that there can be no edge to
the universe.
Isotropy Appearing the same in all directions
from any given view. - From any observance
point, we count roughly the same number of
galaxies per square degree in any patch of the
sky. -There can be no center to the universe.
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Cosmic Expansion
Olbers Paradox A thought experiment suggesting
that if the universe were
homogeneous, infinite and unchanging, then the
entire night sky should be as bright as surface
of the Sun.
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The Birth of the Universe Many a star is born!
- Under the assumption that all recession
velocities have remained constant in time, we are
able to calculate how long it has taken for any
given galaxy to reach its present distance from
us by using the following formula
Time Distance
Velocity
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Hubbles Law
Recession Velocity H0 X Distance
Using this equation as well as the previous one,
we can now derive the following formula to
determine the age of the Universe
Time 1
H0
For H0 65 km/s /Mpc
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So what does all this mean?

• Hubbles Law implies that 15 billion years ago
  everything in the universe was confined to a
  single point.
• The point then exploded in a huge blast commonly
  known as the Big Bang

-The Big Bang resolves Olbers Paradox because
the question of the universe being infinite is
irrelevant as we can only see a finite part of
it. -In order for the Big Bang to work in
accordance with Hubbles Law and the cosmological
principle, we must remember that it was not the
matter in the universe that exploded, but the
universe itself.
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Cosmological Redshift

• A photons redshift is inversely proportional to
  the size of the universe at the time it was
  emitted.
• As the universe expands, the wavelength of the
  photon stretches and leads to a cosmological
  redshift.

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-Assuming universal expansion is constant, the
slope of the black line would be representative
of the recessional velocity. -In a decelerating
universe, the velocities of distant objects would
lie above the black line and deviation from the
curve is greater in a denser universe where
gravity has been more effective at slowing the
expansion.
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What will happen to the universe?

• 1) Distance between galaxies can increase
  forever forever expansion of the universe ? An
  unbound universe
• 2) Distance between galaxies will increase for a
  while and then begin to decrease ? A bound
  universe
• Whatever the outcome may be, it applies to the
  universe as a whole
• A marginally bound universe expands forever but
  at a decreasing rate.

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What determines the fate of the Universe?

• Density determines what will happen to the
  universe.
• Universe with high-density has enough mass to
  stop expansion and cause a recollapse ? leads to
  bounded universe.
• Universe with low-density is unbound and will
  expand forever

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Critical Density

• Definition Density of a marginally bound
  universe gravity is just sufficient to halt the
  present expansion.

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Density of the Universe

• Cosmic density parameter the ratio of the actual
  density of the universe to the critical value O0
• Critical universe has O0 1.0
• Universe with O0 lt 1.0 has insufficient gravity
  to halt and reverse the present cosmic expansion.
• - The lower the O0, the lower the gravity.

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To find the average density

• Measure the average mass of the galaxies residing
  within a large parcel of space.
• Calculate the volume of that space.
• Compute the total mass density.
• Result is O0 is about 0.01.
• This is the result from a space with luminous
  matter.
• One problem Most of the matter in the universe
  is dark. Dark matter does influence the density
  of the universe which affects the cosmic
  expansion. Including calculation of dark matter
  O0 is about 0.2 or 0.3.

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Global Measurements

• Observations of Type I supernovae are effective
  methods of measuring the universes overall
  density, not just in our local area.
• Because they are very bright and have a narrow
  spread in luminosities, they are useful as
  standard candles.
• By measuring their distances and redshifts, we
  can determine the rate of cosmic expansion in the
  past.
• Assuming the universe is decelerating because of
  gravity slowing its expansion, then objects at
  great distances that emitted their radiation long
  ago should be receding faster than Hubbles law
  predicts.

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An Accelerating Universe? GOOD NIGHT!

• In the late 1990s, two groups of astronomers
  announced the results of extensive surveys on
  distant supernovae.
• Instead of elaborating on the accepted notion of
  cosmic deceleration, their findings indicate that
  expansion is actually accelerating.
• According to their data, galaxies at large
  distances are receding less rapidly than Hubbles
  law would predict.
• This fact is inconsistent with the Big Bang
  theory and may require a thorough revision of our
  view of the cosmos.

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Why would the universe be accelerating?

• Cosmologists are not quite sure, but the leading
  theory is of an additional vacuum pressure
  associated with empty space and effective on only
  very large scales known as the cosmological
  constant.
• Although models including this data can fit the
  observational data, astronomers have no clear
  interpretation of what it means for it is not
  explained by any known law of physics.
• The mysterious cosmic field causing the universe
  to accelerate is referred to dark energy.

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Who wants to be a
Cosmological Millionaire?
Universe Milky Way Galaxy Our Solar
System Planet Earth
Question 4 Question 3 Question 2 Question 1
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Question 1
Which term describes the part of the Cosmological
Principle that the universe is the same
everywhere?
a. isotropic
b. redshift
c. homogeneous
d. Tytlers Law
c. homogeneous
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Question 2
What does Hubbles Law ultimately
determine? Recession velocity H0 x distance
a. The Age of the Universe
b. The Age of a Star
a. The Age of the Universe
c. The Age of the Milky Way Galaxy
d. The age of Professor Tytler
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Question 3
Which theory suggests that if the universe were
homogeneous and unchanging, then the entire night
sky should be as bright as the surface of the
Sun?
b. Olbers Paradox
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Question 4
What is the term used to describe a universe that
can expand forever?
d. unbound