Seeing Dark Energy (or the cosmological constant which is the simplest form of DE) Professor Bob Nichol (ICG, Portsmouth)

Seeing Dark Energy(or the cosmological constant which is the simplest form of

DE)

Professor Bob Nichol(ICG, Portsmouth)

Overview

1. Cosmology Primer2. Standard Candles (Supernovae)3. Standard Rulers (CMB)4. My role in all this (SDSS)

ISW effect BAO

COSMOLOGY PRIMER

FRW Equation

• Assuming homogeneous and isotropic universe (RW metric), then GR gives:

Hubble Parameter

Average density of

matter

‘a’ is the scale factor (“radius”) of the Universe relative to

today

k is the curvature of space-time of the

Universe (a constant)€

H 2 ≡a•

a

⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟

2

=8π

3ρ −

k

a2−

Λ

3

Cosmological constant, but could be fn of time & space

w=p/=-1

3 Solutions to FRW equation (=0)

R

timeBangBang

Never stop!

Stop at infinity

Big crunch!

Value of decides the fate of Universe! Like throwing a stone into space

Larg

er

univ

ers

e

Later in Universe

Search for two numbers (H0 and 0)

Subscript “0” means today (R=1), but formula holds at other cosmic times. Total

energy density ()

€

0 ≡ρ 0

ρ c=

8πG

3H02ρ 0

€

0 = Ωm +ΩΛ +Ωk =1

Standard Candles

Luminosity Distance

• We can’t measure distances in the Universe directly, so hard to measure geometry and expansion rate directly

• dL is the luminosity distance and depends on the cosmological parameters, z is the redshift

€

dL =cz

H0

(1+1

2(1−

Ω0

2)z+.....)€

L =f

4πdL2

Supernovae II

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

(lookback time)(lookback time)(lookback time)(lookback time)

(dis

tan

ce)

(dis

tan

ce)

Supernova are Supernova are 20% fainter 20% fainter than they than they should beshould be

Supernova are Supernova are 20% fainter 20% fainter than they than they should beshould be

Standard Rulers

Baryon Acoustic Oscillations

QuickTime™ and aTIFF (Uncompressed) decompressor


Initial fluctuation in DM. Sound wave driven out by intense pressure at 0.57c.

BaryonsBaryons PhotonsPhotons

CMB



After 105 years, we reach recombination and photons stream away leaving the

baryons behind



Preferred scale imprinted on CMBPreferred scale imprinted on CMB


are needed to see this picture.0.57t0.57t

€

dA = dL (1+ z)2

My research



Sloan Digital Sky Survey



Integrated Sachs-Wolfe Effect

What we measure

SDSSSDSSSDSSSDSS

WMAPWMAPWMAPWMAP

No Signal - No DENo Signal - No DE

Positive Signal - DE!Positive Signal - DE!

Most direct evidence yet

that dark energy exists we see it’s repulsive

force counteracting gravity directly





baryons photons





TodayToday

Sullivan et al. (2003)

m=0.256+0.019-0.023

Percival et al. (2006)

• Supernovae

•CMB • SDSS/LSS

• Supernovae

•CMB • SDSS/LSS

So is w=-1?So is w=-1?

99.74% detection

Percival et al. (2006)

143k + 465k

79kz~0.35

z~0.2

Percival et al. 2007

Measure ratio of angular-diameter distance between these redshifts (D0.35 /D0.2)

D0.35 /D0.2 = 1.812 ± 0.060

(ratio should be 1.67 for cosmological constant)

Future Questions

• Is it a Cosmological Constant? Better measurements, specifically control of systematics (new experiments)

• Is it just a breakdown of GR on large scales? Probe universe using different measures (growth of structure). Again limited by systematics

• Better theory (any theory!)• Parallels with HEP - large careful experiments

worrying about large datasets and systematics

DES, SDSS-III, WFMOS, DUNE, SPACE, SNAP, ADEPTDES, SDSS-III, WFMOS, DUNE, SPACE, SNAP, ADEPT

Documents

Seeing Dark Energy (or the cosmological constant which is the simplest form of DE) Professor Bob Nichol (ICG, Portsmouth)