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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.