assignments - bartol.udel.eduowocki/phys333/lec20-07may.pdfthe cosmic microwave background— the...

Assignments

• Read Sec. 29-31 in Cosmology Part V of DocOnotes.pdf

• Do HW6.pdf by 5 p.m. Thursday 07-May• submit filename: Lastname-HW6.pdf to

astroprof@bartol.udel.edu

• Term project due 5 p.m.Tues. 12 May• submit filename: Lastname-Term.pdf to

astroprof@bartol.udel.edu

• Ordinary Dark Matter (MACHOS) – Massive Compact Halo Objects: dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS) – Weakly Interacting Massive Particles: mysterious neutrino-like particles

Two scenarios for dark matter

Maps of galaxy positions reveal extremely large structures: superclusters and voids.

Models show that gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time.

Time in billions of years

0.5 2.2 5.9 8.6 13.7

Size of expanding box in millions of light-years

13 35 70 93 140

Large-scale structure:

Obs.vs.

Sims

Evidence for the Big Bang

1. Universe not infinite2. We have detected the leftover radiation

from the Big Bang.3. The Big Bang theory correctly predicts the

abundance of helium and other light elements in the universe.

Olbers' ParadoxIf the universe were

1.infinite2.unchanging3.everywhere the same

then stars would cover the night sky.

The night sky is dark because the universe changes with time.

As we look out in space, we can look back to a time when there were no stars.

large-redshift quasars

• largest redshift quasar has z=7.1

• how can there be z > 1??

• does it imply v>c??

• no, cosmological redshift really just expansion of space

Redshift, Scale factor & Hubble law

z+1= λobsλem

=rtodayrpast

≡1R

R= 1− Hot = 1−Hodc

z= 1R−1= 1

1− Hodc

−1 ≈ Hodc

=Vc⇒V = Hod

What is the redshift z from an object that emitted its light when the universe had a scale factor R=1/2.

A. ¼B. 1/2C. 1D. 2E. 4

What is the redshift z from an object that emitted its light when the universe had a scale factor R=1/2.

A. ¼B. 1/2C. 1 1+z=1/R => z = -1+1/RD. 2E. 4

What was the scale factor R when the quasar with z=7 emitted the light we see today.

A. 1/8B. 1/4C. 1/2D. 2E. 4

What was the scale factor R when the quasar with z=7 emitted the light we see today.

A. 1/8 R=1/(1+z) B. 1/4C. 1/2D. 2E. 4

Running the Expansion Backward

•Early universe very dense •And very hot!

The cosmic microwave background— the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965.

Recombination epoch• Universe immersed in microwave radiation.• Unleashed at the end of the Era of Nuclei.

• 380,000 years after Big Bang, Universe had cooled to 3000 K• Free electrons became bound into atoms of H & He• “Recombination epoch” • without free electrons to scatter photons, Universe became transparent

The temperature at recombination was about 3000 K.About what is the peak wavelength of its blackbody emission?

A. 250 nmB. 500 nmC. 1000 nmD. 1500 nmE. 2000 nm

The temperature at recombination was about 3000 K.About what is the peak wavelength of its blackbody emission?

A. 250 nmB. 500 nmC. 1000 nmD. 1500 nmE. 2000 nm

CMB Blackbody Spectrum

Running the Expansion Backward

•Early universe very dense •And very hot!

The cosmic background radiation has a temperature of about 3K. By about what factor has the universe expanded since it was emitted.

A. 7B. 10C. 100D. 1000E. 2000

The cosmic background radiation has a temperature of about 3K. By about what factor has the universe expanded since it was emitted.

A. 7B. 10C. 100D. 1000E. 2000

Background has perfect thermal radiation spectrum at temperature 2.73 K.

Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.