Transcript Monday, December 8

Conclusion
Back to: Expansion of the Universe
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Either it grows forever
Or it comes to a standstill
Or it falls back and collapses (“Big crunch”)
In any case: Expansion slows down!
Surprise of the year 1998
(Birthday of Dark Energy):
All wrong! It accelerates!
Enter: The Cosmological Constant
• Usually denoted 0, it represents a uniform
pressure which either helps or slows down the
expansion (depending on its sign)
• Physical origin of 0
is unclear
• Einstein’s biggest
blunder – or not !
• Appears to be small
but not quite zero!
• Particle Physics’
biggest failure
Triple evidence for Dark Energy
• Supernova data
• Large scale structure
of the cosmos
• Microwave
background
Microwave Background:
Signal from the Big Bang
• Heat from the Big Bang should still be around,
although red-shifted by the subsequent expansion
• Predicted to be a blackbody spectrum with a
characteristic temperature of 2.725 Kelvin by
George Gamow (1948)
 Cosmic Microwave Background
Radiation (CMB)
Discovery of Cosmic Microwave
Background Radiation (CMB)
• Penzias and Wilson
(1964)
• Tried to “debug” their
horn antenna
• Couldn’t get rid of
“background noise”
 Signal from Big Bang
• Very, very isotropic (1
part in 100,000)
CMB: Here’s how it looks like!
Peak as expected from 3 Kelvin warm object
Shape as
expected
from black
body
CMB measurements improve
Latest Results: PLANCK
• Measure fluctuations in microwave background
• Expect typical size of fluctuation of ½ degree if universe
is flat
• Result:
Universe is flat !
Experiment and Theory
Expect
“accoustic
peak” at l=200
 There it is!
Supernova Data
• Type Ia Supernovae are
standard candles
• Can calculate distance
from brightness
• Can measure redshift
• General relativity gives us distance as a
function of redshift for a given universe
Supernovae are further away than
expected for any decelerating (“standard”)
universe
Pie in the Sky: Content of the
Universe
5%
23%
1
2
3
72%
Dark Energy
Dark Matter
SM Matter
We know almost everything about almost nothing!
Properties of Dark Energy
• Should be able to explain acceleration of
cosmic expansion  acts like a negative
pressure
• Must not mess up structure formation or
nucleosynthesis
• Does not dilute as the universe expands 
will be different % of content of universe as
time goes by
Threefold
Evidence
Three independent
measurements
agree:
•Universe is flat
•28 % Matter
•72 % dark energy
History of the Universe: Hot & small  cold & big
before 10-43 s
10-43 s
10-43 to 10-35 s
10-35 s
10-35 s to 10-10 s
10-10 s
10-10 to 10-4 s
10-4 s
T=1032 K
T=1028 K
T=1015 K
T=1013 K
10-4 to 500,000 years
180 s (3 minutes)
T=109 K
500,000 years
T=3,000 K
500,000 years to present
?????? (“Planck Era”)
gravity splits from other forces
Grand Unification era
Strong force splits from others. Epoch
of inflation?
“Electroweak era”
Electromagnetic force splits from others
“Quark era”
Quarks combine to form protons and
neutrons
Radiation era
Protons and neutrons combine to
form nuclei (mainly Helium, deuterium)
Nuclei and electrons combine to
form atoms – Decoupling
Matter era
History of the universe
Thus ends the story of the doubly
expanding universe for now…
• Thanks for your attention, patience,
persistence, and interest!
Final Exam
• Comprehensive
– Most questions from Ch. 15-18, some from Ch. 4-14,
few from Ch. E-2 (What we did not cover of some of
these chapters or sections will NOT be on the exam)
• Multiple choice plus some short answer questions
• Please study:
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Midterm exams (available on homepage)
Homework
Activities
Textbook
Powerpoint slides
Daily Rising and Setting
• Due to the rotation of the
Earth around its axis
• Period of rotation:
1 siderial day= 23h56m4.1s
• 1 solar day (Noon to Noon) =24h
• Stars rotate around the
North Star – Polaris
• Why are these different?
Daily and yearly motion intertwined
Solar vs Siderial Day
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Earth rotates in 23h56m
also rotates around sun
 needs 4 min. to “catch
up”
Consequence: stars rise 4
minutes earlier each
night (or two hours
per month, or 12
hours in ½ year)
After 1/2 year we see a completely different sky at night!
Figure 2 shows a horizon view of what you would see when facing south at midnight
on the night of December 1 in the northern hemisphere. How would this view change
if you were to look towards south at midnight a month earlier?
a. You would have the same view as on December 1 because it still is autumn.
b. Aries would have been in the South because the stars rise earlier in the East
every day.
c. Cancer would be in the South because the seasons were closer to summer.
d. Gemini would have been highest in the South because the stars set earlier in the
West.
Consider Figure 2 again. How would this view change if you were to look
towards south at 2am, i.e. two hours later?
a. You would have the same view since the Earth barely moves around the
Sun in two hours.
b. Aries would be in the South because the stars shift by one constellation.
c. Pisces would be in the South because the stars shift a constellation per
hour.
d. Gemini would be highest in the South because the Earth rotates 30
degrees in 2 hours.
On December 1, at noon, you are looking toward the
south and see the Sun among the stars of the
constellation Scorpius as shown in Figure 1. At 4 PM
that afternoon, where will the Sun be with respect to the
stars shown in this diagram?
1)in the constellation Sagittarius
2)in the constellation Scorpius
3)in the constellation Libra
4)west (right) of Libra
Math
c=λf
E= hf
T λ = 0.0029 m K
P = A σ T4
B = L/ (4πd2)
d = 1/p
F = G mM/d2
As the wavelength of EM
increases…
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… the frequency increases
… the energy decreases
… the intensity increases
None of the above
Two stars have the same chemical composition,
spectral type, and luminosity class, but one is 5 light
years from the Earth and the other is 50 light years
from the Earth. The farther star appears to be …
a) 100 times fainter.
b) 10,000 times fainter.
c) the same brightness since the stars are
identical.
d) None of the above
Two stars have the same chemical composition,
spectral type, and luminosity class, but one is 2000
light years from the Earth and the other is 20 light
years from the Earth. The farther star appears to be
…
a) 100 times fainter.
b) 10,000 times fainter.
c) the same brightness since the stars are
identical.
d) None of the above
Two stars have the same radius, but one
has two times the temperature of the
other star. How much brighter is the
hotter star?
4 times
16 times
64 times
None of the above
Two stars have the same radius, but one
has four times the temperature of the
other star. How much brighter is the
hotter star?
4 times
16 times
64 times
None of the above
Two stars have the same temperature,
but one has four times the radius of the
other star. How much brighter is the
bigger star?
4 times
16 times
64 times
None of the above
Two stars have the same temperature,
but one has four times the radius of the
other star. How much larger is the peak
wavelength of the bigger star?
2 times
4 times
16 times
None of the above
If the moon would be twice as far away,
the force of gravity exerted by it on the
Earth would …
Increase 2x
Decrease 4x
Be the same (Newton III)
None of the above