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

1
Seeing Dark Energy(or the cosmological constant
which is the simplest form of DE)

• Professor Bob Nichol
• (ICG, Portsmouth)

2
Overview

• Cosmology Primer
• Standard Candles (Supernovae)
• Standard Rulers (CMB)
• My role in all this (SDSS)
• ISW effect
• BAO

3
COSMOLOGY PRIMER
4
1916 - 1929

• This decade saw the birth of cosmology
• Einstein gave us a new theory of gravity (GR),
  that works for the whole Universe
• Hubble discovered the Universe, and its expands!

5
FRW Equation

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

Cosmological constant, but could be fn of time
space wp/?-1
Average density of matter
a is the scale factor (radius) of the
Universe relative to today
Hubble Parameter
k is the curvature of space-time of the Universe
(a constant)
6
3 Solutions to FRW equation (?0)
Never stop!
Stop at infinity
R
Larger universe
Big crunch!
Bang
time
Later in Universe
Value of ? decides the fate of Universe! Like
throwing a stone into space
7
Search for two numbers (H0 and ?0)
Subscript 0 means today (R1), but formula
holds at other cosmic times. Total energy density
(?)
8
Standard Candles
9
Luminosity Distance

• We cant 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

10
Supernovae II
11
(No Transcript)
12
(No Transcript)
13
(distance)
Supernova are 20 fainter than they should be
(lookback time)
14
(No Transcript)
15
Standard Rulers
16
Baryon Acoustic Oscillations
Photons
Baryons
Initial fluctuation in DM. Sound wave driven out
by intense pressure at 0.57c.
17
CMB
Preferred scale imprinted on CMB
After 105 years, we reach recombination and
photons stream away leaving the baryons behind
18
0.57t
19
(No Transcript)
20
My research
21
Sloan Digital Sky Survey
22
Integrated Sachs-Wolfe Effect
23
My Experiment SDSS
24
CMB as seen by WMAP
25
Combine them
26
Lens Experiment
27
What we measure
28
WMAP
SDSS
29
Most direct evidence yet that dark energy exists
we see its repulsive force counteracting gravity
directly
Positive Signal - DE!
No Signal - No DE
30
2003
It is one of the ultimate discoveries in basic
science,
31
baryons
photons
Today
32
(No Transcript)
33
(No Transcript)
34
Percival et al. (2006)
Sullivan et al. (2003)
?m0.2560.019-0.023
35

• Supernovae ?m lt ??
• ?CMB ?m ??????
• SDSS/LSS ?m ??0.25

36
So is w-1?
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)
99.74 detection
143k 465k
z0.2
79k
z0.35
Percival et al. (2006)
Percival et al. 2007
37
(No Transcript)
38
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, ADEPT