Ch. 23 - Cosmology Part 2

The Beginning & the End

Skip Forward to 1964Penzias & Wilson discover isotropic emission at λ=7.35 cm. If a blackbody, T=3±0.5 K. Penzias talks on the phone to B. Burke, who heard from Ken Turner about a talk by P.J.E. Peebles (Princeton) who had predicted the universe would be filled with a 5 K radiation.

Discovery of the Cosmic Background Radiation (CBR).

How did we get to this point? History of Modern Cosmologies

1940s - Gamow, Alpher & Hermann, interested in nucleosynthesis, suggest that long ago the universe was hot & dense enough for the present heavy elements to be produced - BIG BANG,

1948 - Alpher & Hermann showed that a hot expanding universe should produce radiation redshifted to look like a T=5 K blackbody. However, this big bang model failed to correctly predict abundances of the heavies. Model largely forgotten by many, especially since….

1957 - Burbidge, Burbidge, Fowler & Hoyle (B2FH) demonstrated how heavies could be produced in stars.

1964 - Zel’dovich and (independently) Hoyle & Taylor realize that B2FH cannot make all the He we see today, but could have been made earlier.

1964-1965 (all unaware of Gamow, et al.)

R. Dicke (Princeton) realizes that the universe had to have been hotter in the past, and that the radiation could be detected today.

Peebles calculates necessary conditions for making lots of He but few heavies

Roll & Wilkinson make detector to observe 10 K radiation originally predicted by Dicke & Peebles

(Nucleosynthesis & radiation compete (γ-rays break up nuclei). Too little radiation ⇒too much nucleosynthesis. Too much radiation ⇒ too little nucleosynthesis. The CBR and He abundance today determine the state of the early universe.)

Before Dicke, Roll & Wilkinson complete their measurements, Dicke gets a call from Penzias regarding Peebles work.

Result: 2 Papers

Observations: “A measurement of excess antenna temperature at 4080 Mc/sec” Penzias & Wilson. (“… possible explanation is given by Dicke, Peebles, Roll & Wilkinson in a companion letter in this issue.”)

Theory: Dicke, Peebles, Roll & Wilkinson

1976 - Wagoner, Fowler & Hoyle work out “complete” Big Bang theory.

(Aside - Hoyle’s contribution here is “interesting” as he was a proponent of the Steady State model. He coined the term “Big Bang” as a derogatory term.)

What the CBR Should Look Like

1. When still ionized - optically thick free-free radiation - Blackbody

2. We see the blackbody spectrum at the “instant” of “recombination”, after which the gas was optically thin & radiation survived untouched except for ensuing redshift.

3. The universe, being “homogeneous” at the recombination epoch, should appear isotropic in brightness & temperature (remember surface brightness for blackbody σ T4). Note: but at some time, universe must become “lumpy”.

Is the CBR a Blackbody & Isotropic?

Earliest observations seemed to deviate from a single-T blackbody.

A puzzling but exciting period of theoretical fantasies ensued……..

Cosmic Background Explorer (COBE)

COBE Spectrum of the Universe - first 7 minutes of data (FAXed copy of version presented at AAS meeting)

Spectrum = Blackbody with T=2.726 K

Isotropy Measurement: after removal of Milky Way and Dipole Anisotropy (due to motion of Earth toward Virgo, etc.) Isotropic to within 1 part in 3x10-5.

Later (after years of accumulating data) higher-order anisotropies reported with ΔT/T~4.8±1.5x10-6.

The temperature and density fluctuations pointed toward the transition from a smooth big bang to the clumpy universe we have today.

Measuring the Decelerationm − M = 42 . 38 − 5 (log h ) + 5 (log z ) + 1 . 086 ( 1 − q

0

) z + .....

h =

H0

100

Measuring the Curvature - Angular Sizes (and number counts) of Galaxies

θ =

H0

D

c

q0

2

1 + z( )

2

q0

z − 1 − q0

( ) 1 + 2 q0

z − 1( )

Measuring DensityMeasured baryonic density ~ 0.05ρc.

Measured dark matter density ~ 0.3ρc

So, ρmatter~ ρc to within a factor of ~3 today.

However,

So at the time of recombination (z~1000) Ω=1 to within 1 part it 103, at the time of nucleosynthesis Ω=1 to within 1 part in 1012, and at the Planck time Ω=1 to within 1 part in 1060!

Coincidence?! Maybe Ω=1 precisely??? WHY???????

1

Ω z( )−1=

1Ω0

−1⎛

⎝⎜⎞

⎠⎟1+ z( )−1

SN Ia Programs:

Other SN Ia data ⇒ H0=74±4 implying t0=12 Gyr for the best-fit region.

2003 - Wilkinson Microwave Anisotropy Probe (WMAP)

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Acoustic Waves in Cosmic Microwave Background

the beginning of the baryonic structure seen in galaxy surveys - Baryon Acoustic Oscillations

What the Sound Wave Tells UsWhat the Sound Wave Tells Us

Distance to z~1100

Baryon-to-Photon Ratio

Matter-Radiation Equality Epoch

Dark Energy/New Physics? Text

slide “borrowed” from E. Komatsu’s Acoustic Wave Seminar (March 2,2007)

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Parameter X

Parameter Y

Observation A

Observation B

Degenerate Solutions to Observations

Parameter X

Parameter Y

Observation A

Observation B

Non-Degenerate Solutions to Observations

WMAP+BAO+H0

Ωbaryon =0.0456±0.0016

Ωdark matter=0.227±0.014

t0=13.75 ±0.11 Gyr

What is the Λ-term?

Time variable “Dark Energy” with equation of state ?

Pure cosmological constant Ω (w=-1)

Phantom Energy (w<-1) leading to “Big Rip”?

Current best estimate of w=-0.98±0.053

Results of WMAP+(Jarosik et al. 2001, ApJS, 192, 14)

Ex.: Prad/Urad=1/3

w ≡p ρ

Ωtot = 1.023−0.0054+0.0056

H0 =70.4−1.4+1.3 kms−1Mpc−1

28

= m-M

Unsolved problems:

Flatness problem: Why was Ωinitial so close to unity?

Isotropy (horizon) problem: How did distant locations of the universe, never in thermal contact, “know” how to be at the same temperature?

Monopole problem: GUTs predict production of magnetic monopoles, etc. - none ever detected.

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Reionization of the Universe

Ionized gas “in front of” the CMB - scatters light from it:has low but non-zero optical depth “blurs” CMB - reduces acoustic peak amplitudes & polarizes the CMB