Conclusion

Back to: Expansion of the Universe

• 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

• Physical origin of 0

is unclear• Einstein’s biggest

blunder – or not !• Appears to be small

but not quite zero!• Particle Physics’

biggest failure

• Usually denoted 0, it represents a uniform pressure which either helps or slows down the expansion (depending on its sign)

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 universeSupernovae are further away than expected for any decelerating (“standard”) universe

Pie in the Sky: Content of the Universe

We know almost everything about almost nothing!

1

2

3

23%

5%

72%

Dark EnergyDark MatterSM MatterSM Matter

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 ?????? (“Planck Era”)

10-43 s T=1032 K gravity splits from other forces

10-43 to 10-35 s Grand Unification era

10-35 s T=1028 K Strong force splits from others. Epoch of inflation?

10-35 s to 10-10 s “Electroweak era”

10-10 s T=1015 KElectromagnetic force splits from others

10-10 to 10-4 s “Quark era”

10-4 s T=1013 K Quarks combine to form protons and neutrons

10-4 to 500,000 years Radiation era

180 s (3 minutes) T=109 K Protons and neutrons combine to form nuclei (mainly Helium,

deuterium)

500,000 years T=3,000 K Nuclei and electrons combine toform atoms – Decoupling

500,000 years to present 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:

– 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 – 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 Sagittarius2)in the constellation Scorpius 3)in the constellation Libra4)west (right) of Libra

Math

c = λ f

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…

• … 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 4xBe the same (Newton III) None of the above