Observational cosmology

David Bacon Institute of cosmology and Gravitation

Portsmouth

The visible Universe

W H E R E A R E W E ?

W H E R E A R E W E ?

S O L A R N E I G H B O U R H O O D

NASA/JPL

T H E L O C A L G R O U P

T H E V I R G O S U P E R C L U S T E R

53 million light yearsto centre

N E A R B Y S U P E R C L U S T E R S

Measure redshifts and angular positions of galaxies

Bolton et al 2012 BOSS spectra

M A P O F T H E G A L A X I E S

One dot = one galaxy.

Typical image

arcminutes (60th of a degree)

Almost all the objects on this image aregalaxies, each made of hundreds of billions of stars

A R E W E T O O F O C U S E D O N T H I N G S T H AT G L O W ?

Dark matter

Dark energy

Observational cosmology

David Bacon Institute of cosmology and Gravitation

Portsmouth

2Cosmology Framework

The Cosmological Principle

Copernican principle: the Earth does not inhabit a special location in the Universe

• Depends on definition• It is possibly not true in detail• It needs testing as far as possible.

Cosmological principle: The Universe is homogeneous and isotropic on very large scales

• Homogeneous = same everywhere• Isotropic = same in all directions• Can be tested (with care) by large-scale surveys• Scales > 10Mpc

We observe isotropy, and assuming isotropy everywhere via the Copernican principle implies homogeneity

0 500 km/s/Mpc ??H =

Velocity

Distance

Define stretching factor of light due to cosmological expansion as redshift

For low redshifts, z ≈ v/c, so redshift directly measures recession velocity

Original Hubble diagram (Hubble 1929)

Huchra

What does the expansion mean?

• Big Bang model: the Universe is expanding now, so we might suppose it was very small in the past, and had a “beginning”

• Steady State model: matter is continually created as space expands, so the Universe is homogeneous in time as well as space

Either interpretation valid until the discovery of the microwave background radiation, which favoured BB

Geometry• A metric allows us to calculate the spacetime

separation between 2 (4-D) points in a given coordinate system

• 3-dimensional space has three possible geometries:• Spherical• Hyperbolic • Euclidean

Possible geometries of 2-D surfaces

Geometry

 

Comoving distance

t

Note that r is a co-moving coordinate:

0r 1r

Physical distancesbecome smallerin the past byfactor a

Dynamics

 

geometry matter vacuum energy

Dynamics

 

Equation of state

 

p

Critical density

 

-> BlackboardHubble 1

Redshift

 

0r r=

Luminosity distance

 

(otherwise it is very complicated – see Hogg, “Distance measures in cosmology”, astro-ph/9905116)

Angular diameter distance

 

2 1(1 ) (1 )A L pd z d z d− −= + = +

-> Blackboarddadl

Summary

The Universe is (rather) homogeneous and isotropic, so we arrive at a simple metric.

General Relativity gives us the dynamics of the spacetime - depends on density and vacuum energy.

Photons redshift during the expansion.

Several distance measures are useful to us.