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Why is the darkness of the night sky evidence that the Universe started sometime

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Why is the darkness of the night sky evidence that the Universe started sometime? ... Night sky is dark because the universe has a beginning and changes with time ... – PowerPoint PPT presentation

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Title: Why is the darkness of the night sky evidence that the Universe started sometime


1
Why is the darkness of the night sky evidence
that the Universe started sometime?
2
Why is the Night Sky Dark?
  • Suppose that the universe is an infinite and
    homogeneous collection of stars
  • every line of sight intercepts a star
  • sky should glow as brightly as the surface of an
    average star
  • but the night sky is dark
  • Olbers Paradox
  • Heinrich Olbers in 1826
  • Thomas Digges in 1576
  • Johannes Kepler in 1610
  • Edmund Halley in 1721
  • Therefore, Universe cannot be infinite and
    homogeneous!

Observer
3
Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
4
Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
5
Night sky is dark because the universe changes
with time
6
Dark Sky? No Problem in Big Bang Model
  • Universe has a finite age
  • Observable universe finite, even in infinite
    universe
  • If universe is 13 billion years old, then we
    cannot see light emitted by objects farther away
    than 13 billion light years
  • Expanding universe redshifts light
  • light emitted in the visible portion of the
    spectrum is shifted to longer wavelengths
  • more distant sources greater shifts
  • for very distant objects light is redshifted
    completely out of the visible

Radius
Observable Region of Universe
Infinite Universe That We Cannot Observe Yet
7
Night sky is dark because the universe has a
beginning and changes with time Or the Universe
is fractal!!
8
What do we mean by dark matter and dark energy?
9
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

10
Dark Stuff in the Universe
11
For groups of stars, galaxies, clusters of
galaxies, we measure the velocities of the member
objects, then assume there has to be enough mass
there for gravity to stop them from flying apart.
When we measure these velocities, it turns out
that more mass is needed! So we assume there
is dark stuff.
The first to observe this behavior was Fritz
Zwicky in 1933. He was looking at galaxies in
clusters. No one believed him.
He was quite a character!
12
What is the evidence for dark matter in clusters
of galaxies?
13
We can measure the velocities of galaxies in a
cluster from their Doppler shifts
14
The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
15
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
16
Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
17
All three methods of measuring cluster mass
indicate similar amounts of dark matter
18
Vera Rubin detected this lack of luminous stuff
in individual galaxies in the early seventies.
No one believed her! She is still a
character!
19
Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
20
Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
21
The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
22
What is the Dark Matter? Some Possibilities
23
Ordinary Matter Dust, Gas, Planets, Jupiters,
Brown Dwarfs, Faint Stars, White Dwarfs, bowling
balls, black holes, MACHOs Current
theories/observations say there just isnt enough
mass possible. Plus the Hot Big Bang idea
doesnt produce enough baryonic matter
(protons, neutrons, electrons etc.) to do it.
24
Exotic Matter Neutrinos are an example of a
particle that interacts with matter through the
weak nuclear force. There are so many neutrinos
in the universe, however, that even a small mass
would be important for dark matter a mass of 92
eV, one five-thousandth the mass of the electron,
would close the universe!
25
WIMPS Most exotic matter candidates fall into the
category of WIMPS, or Weakly Interacting Massive
Particles. These are heavy particles that only
interact weakly with other matter (or else they
would have been discovered by now). There are
many possible e.g., neutralinos, axions, etc.
26
Change Gravity Another possibility Perhaps
gravity on large scales, such as the size of
galaxies, doesn't work the same way as gravity
does on the small scales we can measure. MOND
MOdified Newtonian Dynamics Milgrom
Bekenstein Sanders
27
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28
In the past few years, astronomers have observed
that the light from very far supernovae is
fainter than it should be. The current
explanation for this is that they are more
distant than originally thought, which implies
that the expansion of the universe must be
accelerating.
29
Accelerating universe is best fit to supernova
data
30
This astonishing result can be explained if the
space between galaxies is filled with a
mysterious dark energy.
We have no idea what it is!
31
As the universe expands, it creates more dark
energy. The repulsive behavior could eventually
get badly out of hand. It is possible that
several billion years from now, the
ever-increasing dark energy could pull space
apart so rapidly that galaxy clusters would
scatter, followed by the disintegration of
individual galaxies such as the Milky Way, then
solar systems, until finally matter itself would
be shredded by accelerating space. The universe
would end in a "big rip."
32
  • What supplies the acceleration?
  • Two guesses
  • Cosmological Constant
  • Vacuum Energy Density of the Universe

33
Cosmological Constant The Einstein Field
Equations Geometry Physics
34
Einstein adds a term
The cosmological constant L ! Einsteins
biggest blunder but it can be added
with a or
35
2. Vacuum Energy Density of the Universe Current
high energy theories predict a sea of particles
Virtual particles and anti-particles are
constantly being produced out of the vacuum of
space!
36
The Repulsion may be gotten from the Vacuum
Energy Density of the Universe! In order
to check this and make predictions, we need a
theory of Quantum Gravity! String Theory?
Replace points with strings
37
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38
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39
So Astronomers know
96
nothing about
of the Universe -
we obviously need more funding!
40
Maps of galaxy positions reveal extremely large
structures superclusters and voids
41
Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
13
Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
42
Lots of dark matter
Fate of universe depends on amount of dark matter
43
Not enough dark matter
Lots of dark matter
Fate of universe depends on amount of dark matter
44
Not enough dark matter
Lots of dark matter
Critical density of matter
Fate of universe depends on amount of dark matter
45
Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
Not enough dark matter
46
But expansion appears to be speeding up!
Dark Energy?
Not enough dark matter
47
old
older
oldest
Estimated age depends on dark matter and dark
energy
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