questions from reading activity? ib assessment statements olbers’ paradox e.4.1.describe...
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Questions From Reading Activity?TRANSCRIPT
DEVIL PHYSICSTHE BADDEST CLASS ON
CAMPUS
IB PHYSICS
LSN E-4: COSMOLOGY
Questions From Reading Activity?
IB Assessment Statements
Olbers’ ParadoxE.4.1. Describe Newton’s model of
the universe.E.4.2. Explain Olbers’paradox.
IB Assessment Statements
The Big Bang ModelE.4.3. Suggest that the red-shift of
light from galaxies indicates that the universe is expanding.
E.4.4. Describe both space and time as originating with the Big Bang.
E.4.5. Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.
IB Assessment Statements
The Big Bang ModelE.4.6. Explain how cosmic
radiation in the microwave region is consistent with the Big Bang model.
E.4.7. Suggest how the Big Bang model provides a resolution to Olbers’ Paradox.
IB Assessment Statements
The Development of the UniverseE.4.8. Distinguish between the terms
open, flat and closed when used to describe the development of the universe.
E.4.9. Define the term critical density by reference to a flat model of the development of the universe.
E.4.10. Discuss how the density of the universe determines the development of the universe.
E.4.11. Discuss problems associated with determining the density of the universe.
IB Assessment Statements
The Development of the UniverseE.4.12. State that the current
scientific evidence suggests that the universe is open.
E.4.13. Discuss an example of the international nature of recent astrophysics research.
E.4.14. Evaluate arguments related to investing significant resources into researching the nature of the universe.
Olber’s Paradox
Cosmological Principle – uniformity of the universe Near view - Appearance of hierarchy
and structure Planets in a solar system Stars in a galaxy Galaxy in a cluster of galaxies which
is part of a larger super-cluster of galaxies
Olber’s Paradox
Cosmological Principle – uniformity of the universe Beyond that, however, the universe
looks uniform Homogeneity principle – on a large
scale, the universe looks uniform Like comparing a serving spoon of
vegetable soup to the whole pot Isotropy principle – uniform in all
directions
Olber’s Paradox
Cosmological Principle – uniformity of the universe Implication that the universe has no
edges and no center – infinite in extent
Newton proposed that it was infinite and static – it has been uniform and isotropic at all times
This led to Olbers’ Paradox
Olber’s Paradox
Why is the night sky dark? Consider that the universe contains
an infinite number of stars, basically evenly distributed
Olber’s Paradox
Why is the night sky dark? Place an observer somewhere in the
universe The observer is at a distance, d, from a star
which has some luminosity L The apparent brightness (energy received
per area per second) of that star is
Now consider that the one star is in a shell of some thickness, t, that encircles the observer
24 dLb
Olber’s Paradox Why is the night sky
dark? Place an observer somewhere
in the universe The volume of that ring is
equal to the surface area of a sphere times the thickness of the shell, 4πd2t , that contains a number density (i.e., number of stars per unit volume) n
The number of the stars in the ring would be density times volume or, 4πd2t
Olber’s Paradox Why is the night sky dark?
Place an observer somewhere in the universe The energy received by the
observer per second per area from all the stars in the shell would be
This is a constant, if you consider an average number density of stars, that doesn’t depend on distance from the shell
LntntdxdL
22 4
4
Olber’s Paradox Why is the night sky
dark? Place an observer
somewhere in the universe If there are an infinite
number of shells containing stars that emit a constant amount of energy, the total energy received would be infinite which would make the night sky infinitely bright – This is Olbers’ Paradox
Olber’s Paradox
What happens to the energy? Is it absorbed by intervening stars
and other media? In an infinite timeline, this does not
hold up because eventually the media would heat up from the radiation to a point where it was emitting as much as it was receiving
Olber’s Paradox What happens to the energy?
The only explanation is that the universe is finite and expanding Stars are finite in number and have a
finite lifetime They have not been radiating forever and
won’t continue to radiate forever – finite radiation
If the age of the universe is finite, light from stars that are extremely far away haven’t even reached us
If the universe is expanding, radiation from stars is redshifted, the “Doppler effect for light”, so it contains less energy
Expanding universe
When analyzing the absorption spectra of distant galaxies, the dark lines are longer as compared to the same chemicals on earth
This means that they have a longer than expected wavelength
Expanding universe This can be explained by the star
moving away from the earth which causes the radiation to be redshifted – i.e. shifted toward the red end – similar to the Doppler effect
Hubble suggested that the redshift was evidence that the galaxies were moving away from us and away from each other
Expanding universe
Suggests that the universe was originally much smaller and was much more compact
Expansion must have been caused by some type of explosion –
Big Bang Theory
Cosmic Background Radiation In 1964, two radio astronomers from
Bell Laboratories set up an antenna to study radio signals from our galaxy
They kept picking up a microwave signal no matter where they pointed the antenna
Spectral analysis of the signal showed it to be a blackbody radiation corresponding to a temperature of 2.7 K
Cosmic Background Radiation The theory is that the
radiation is the remnant of a hot explosion that occurred at the beginning of time As the universe expanded, the
temperature dropped until it reached its current value of 2.7K
Big Bang Theory
The answer to Olbers’ Paradox combined with the discovery of cosmic background radiation and an abundance of helium in the universe led to the Big Bang Theory
At the beginning of time, about 14 billion years ago, the universe consisted of a single solid mass
Big Bang Theory
The mass exploded with matter flying outwards in all directions, creating space as it went along
The aftermath of the explosion continues as the universe continues to expand
Big Bang Theory
The explosion created tremendous heat with the residual radiation still observed
The theory predicts that about 25% of the mass in the universe would be helium Measurements in nearby galaxies
have shown that the values are never less than 25%
Development of the Universe – What’s Next?
The universe is expanding right now, but what happens next
Development of the Universe – What’s Next?
Consider two galaxies that are some distance apart, x0 At some future time, t, the separation
can be represented by,
R(t) is called the scale factor of the universe or sometimes just the radius of the universe
R(t) describes what the eventual size of the universe will be
0xtRtx
Development of the Universe – What’s Next?
Three possibilities: It will continue to expand forever at
an increasing rate, open universe It will continue to expand forever,
but at a rate that approaches zero, flat universe
Expansion will eventually stop, followed by a collapse, closed universe
Development of the Universe – What’s Next? What will happen next is
dependent on the density of the universe relative to its critical density Consider a mass that is expanding
outward It has kinetic energy due to its
expansion
Development of the Universe – What’s Next? What will happen next is dependent on
the density of the universe relative to its critical density But it has gravitational attraction that
opposes the expansion If kinetic energy is greater than gravitational
attraction, the universe will expand forever If kinetic energy is equal to gravitational
attraction, the universe will expand forever, but at a rate that approaches zero
If kinetic energy is less than the gravitational attraction, the universe will eventually stop expanding and then start to collapse in on itself
Development of the Universe – What’s Next? What will happen next is dependent
on the density of the universe relative to its critical density The equation for total energy of the
expanding universe is
H is the Hubble constant G is the gravitational constant
38
21 22 GHmrE
Development of the Universe – What’s Next? What will happen next is
dependent on the density of the universe relative to its critical density
The key variable here is density, ρ Critical density causes E=0 and is
estimated to be,
38
21 22 GHmrE
326
2
1083
mkg
GH
c
Development of the Universe – What’s Next? What will happen next is
dependent on the density of the universe relative to its critical density
ρ < ρc , the universe is open, the universe will expand forever
326
2
1083
mkg
GH
c
Development of the Universe – What’s Next? What will happen next is dependent
on the density of the universe relative to its critical density
ρ = ρc , the universe is flat, the universe will expand forever, but at a rate that approaches zero
326
2
1083
mkg
GH
c
Development of the Universe – What’s Next? What will happen next is dependent
on the density of the universe relative to its critical density
ρ > ρc , the universe is closed, the universe will eventually stop expanding and then start to collapse in on itself
326
2
1083
mkg
GH
c
Development of the Universe – What’s Next? Problems in determining the mass
density of the universe Dark matter – matter that we can’t see
because it is too cold to radiate (brown dwarfs)
Two hypotheses: WIMPS – weakly interacting massive
particles Neutrino masses are not yet determined so
contribution is unknown MACHOS – massive compact halo objects
Development of the Universe – What’s Next? Dark energy
Previous discussion on Big Bang Theory is the classic perspective that has been outdated since 1998 when it was discovered that distant supernovas are moving away from us at speeds much greater than expected
Theory that the universe is filled with an all-permeating vacuum energy called Dark Energy
Development of the Universe – What’s Next? Dark energy
Creates a repulsive force that opposes the force of gravity
Appears that dark energy started to dominate gravity about 5 billion years ago
Development of the Universe – What’s Next? Dark energy
Belief is that the density of the universe is equal to critical density, but instead of the rate of expansion approaching zero, dark energy is causing the expansion to accelerate????
Development of the Universe – What’s Next? Dark energy
Universe is now thought to be made of 73% dark energy and 27% matter Of the 27% matter, 85% is thought to be dark
matter and 15% (only 4% of the universe) to be ordinary matter
SO WHAT HAPPENS NEXT???
SO WHAT HAPPENS NEXT??? IB Exams
College Job Family Retirement Ashes to ashes, dust to dust
IB Assessment Statements
Olbers’ ParadoxE.4.1. Describe Newton’s model of
the universe.E.4.2. Explain Olbers’paradox.
IB Assessment Statements
The Big Bang ModelE.4.3. Suggest that the red-shift of
light from galaxies indicates that the universe is expanding.
E.4.4. Describe both space and time as originating with the Big Bang.
E.4.5. Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.
IB Assessment Statements
The Big Bang ModelE.4.6. Explain how cosmic
radiation in the microwave region is consistent with the Big Bang model.
E.4.7. Suggest how the Big Bang model provides a resolution to Olbers’ Paradox.
IB Assessment Statements
The Development of the UniverseE.4.8. Distinguish between the terms
open, flat and closed when used to describe the development of the universe.
E.4.9. Define the term critical density by reference to a flat model of the development of the universe.
E.4.10. Discuss how the density of the universe determines the development of the universe.
E.4.11. Discuss problems associated with determining the density of the universe.
IB Assessment Statements
The Development of the UniverseE.4.12. State that the current
scientific evidence suggests that the universe is open.
E.4.13. Discuss an example of the international nature of recent astrophysics research.
E.4.14. Evaluate arguments related to investing significant resources into researching the nature of the universe.
QUESTIONS?
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