Physical Cosmology 2011/2012

Lecture 2

• Galaxies and galaxy clusters astracers of cosmic structure

• Hubble expansion

• CMB basics

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Physical Cosmology 2011/2012

Galaxies and galaxy clusters as tracers of cosmic structure

galaxies: 108-1011 starsplus dark matterplus diffuse gas

~ 1 - 60 kpc

1kpc = 3.08 x 1019 m

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Physical Cosmology 2011/2012

Hubble sequence of galaxies

elliptical galaxies

spiral galaxies

barred spirals

“Early” types “Late” types

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Physical Cosmology 2011/2012

Galaxy clusters

30 - 1000 galaxies

plus hot gas (107-108 K)plus dark matter

bright X-ray sources

~ 1-5 Mpc

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Physical Cosmology 2011/2012

SDSS survey

1 million galaxyspectra

100,000 QSOspectra

SDSS = SloanDigital SkySurvey

http://www.sdss.orghttp://www.sdss.org

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Physical Cosmology 2011/2012

blue red

I-ve v +ve v

Doppler effect and velocities

Tc

T period

wavelength

TT

v

cc

z

vv

Get same result from specialrelativity in the limit v<<c

Classical:

I

x

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Physical Cosmology 2011/2012

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

The 2dF survey

250 000 galaxyspectra

25 000 QSOspectra

2dF = twodegree field

www.aao.gov.au/2dfwww.aao.gov.au/2df

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

2011 © University of Cambridge. Not to be quoted or reproduced without permission.

Physical Cosmology 2011/2012

All but some of the closest galaxies have redshifted spectra.

The “recession” velocity increases linearly with distance

dHv Hubble’s law

For a homogeneous Universe this means expansion.

As the Universe expands it cools Hot Big Bang

Hubble flow peculiar velocity

v

d

pecvv dH

Mpc10vpec ddH

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Physical Cosmology 2011/2012

The Expansion of the UniverseThe expansion of the universe is described by Hubble’s law

)()(

)()(

)(

1)()()(

00 tHnowHH

trdt

tdR

tRtrtHtv

where R(t) is a dimensionless scaling factor

)(

)()(

0tr

trtR

H0 ~ 70 km/sec/Mpc is the Hubble constant. The dimensionlessHubble constant h is defined as

100/0Hh

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Physical Cosmology 2011/2012

Hubble’s Original Diagram

redshift

standard candleapparent mag

Hubble estimatedH0~500 km/s/Mpc

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Physical Cosmology 2011/2012

Cosmological redshift:

RT

1 For radiation temperature scales

inversely with the scale factor

em

obs

em

obs

em

obs

em

obs

factorscale

const1

R

R

T

T

R

TR

Rz

obs

em

R

R

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Physical Cosmology 2011/2012

4rad,0

4

0rad,0rad 1

)()( z

tR

Rt

At high redshift/early times the Universe was radiation-dominated.

Radiation and matter energy density were equal at:

30001rad,0

mat,0eq

z

at z =0:

3144rad Jm107

CMB

2 Taccontributionfrom relativisticneutrinos

65.0,3.0forJm102 mat310

mat2 hc

At present the Universe is “matter-dominated”.

radiation constant

3mat,0

3

0mat,0mat )1(

)()( z

tR

Rt

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Physical Cosmology 2011/2012

The Cosmic Microwave Background

• Discovered in 1965 by Penzias and Wilson.

• Has T=2.73 K and is believed to be the radiationpresent in the universe at the time of recombination.

• Was extremely hot (T~109 K) but now appears cold dueto the expansion of the universe.

• Corresponds to the “recombination surface” at aredshift of ~1100.

• Observed temperature fluctuations (~20-70 K) in theCMB are thought to originate from the densityfluctuations at z~1100.

• These density fluctuations should be responsible forobserved large scale structures (galaxies, clusters, etc.)

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Physical Cosmology 2011/2012

log I (

log

1

2 23bodyblack

kThe

chI

The Cosmic Microwave Background

The CMB dominates the extragalactic background radiationin an Idiagram by two orders of magnitude.

1.36mm220 GHz

radio

Tk2sub-mmfar IR

kThe 3

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Physical Cosmology 2011/2012

The COBE satellite

http://space.gsfc.nasa.gov/astro/cobe/cobe_home.html

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Physical Cosmology 2011/2012

COBECMB

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Physical Cosmology 2011/2012

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Physical Cosmology 2011/2012

COBE satellite (1990-1994):

T = 2.728 +/- 0.004 K

best black body known

4

bodyblack

bodyblackmeas

10

I

II

Energy density 4Ta

Compare that to mean density of baryons 31bar m102.2 n

photon density 383

BB

m106.333

Tk

a

Tkn

typical photonenergy

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Physical Cosmology 2011/2012

10bar 106

n

n

There are 2×109 photons for every baryon.in the Universe.

The Universe has high entropy!

is a fundamental parameter. It is important fornucleosynthesis in the early Universe.

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Physical Cosmology 2011/2012

Surface of last scattering

The most abundant element, hydrogen, recombined whenT ~ 3000K at zrec ~1100.

The Universe changes from optically thick to optically thinat recombination.

Optical depth for Thomson scattering

23)(

0100

1~

ZXdln e

zl

Te

electrondensity

electronfraction

Thomson cross section

rec4

rec

10

1

zzX

zzX

e

e

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Physical Cosmology 2011/2012

Us surface of lastscattering atz~1100

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Physical Cosmology 2011/2012

Measurements of the Anisotropies in the CosmicMicrowave Background (CMB)

• COBE satellite, whole sky, DifferentialMicrowave Radiometer (DMR) 31, 53 and90GHz with 10° resolution.

• WMAP satellite was launched, June 2001, 4year mission. First results came out 2003.

• Boomerang, Antarctic balloon, 90, 150, 240 and400 GHz, 0.2° resolution.

• Planck Satellite, 30-900 GHz, launched 2009.

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Physical Cosmology 2011/2012

CMB anisotropies from COBE

29 micro-Kelvin fluctuations, 1 part in 100,000

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Physical Cosmology 2011/2012

Dipole + Galaxy + Cosmicanisotropies. 365 km/sapparent velocity. Pec. vel.of local group ~600 km/s

Cosmic anisotropies

Galaxy + Cosmicanisotropies

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Physical Cosmology 2011/2012

CMB fluctuationspower spectrum

before 1997.

COBE didn’t haveenough spatial

resolution to detectthe first peak

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CMB anisotropies from Boomerang

Image is ~40x45 degrees

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Physical Cosmology 2011/2012

Power spectrumfrom Boomerang

data

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Physical Cosmology 2011/2012

WMAP measured theCMB with much greaterresolution and sensitivity(one millionth of a degreein temperature) thanCOBE.

WMAP(WilkinsonMicrowaveAnisotropy

Probe)

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Physical Cosmology 2011/2012

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