Testing modified gravity beyond cosmic...
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Testing modified gravity beyond cosmic variance
Caitlin Adams Supervised by Professor Chris Blake
Large Scale Structure and Galaxy Flows, Quy Nhon — 08/07/16
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Aims
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How can we distinguish between ΛCDM and alternatives?
How can we best utilise low-redshift surveys?
Can we overcome cosmic variance in a self-consistent way?
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The standard model
Image from NASA/WMAP
ΛCDM attributes accelerated expansion to dark energy
but could there be an alternative explanation?
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Modified gravity
Many physicists are exploring extensions to GR
There are some well established models: f(R) models - extending functional form of GR Galileon models - addition of a scalar field Massive gravity models - graviton has mass
There’s also work on observational predictions from generalised forms of modified gravity (Baker et al. 2014)
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Scale-dependence
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Scale-dependence: 6dFGS
Data from Johnson et al. 2014
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Cosmic variance
Measurements on large scales at low redshift are limited by sample variance from the overdensity field
A single tracer is limited by this variance
But the ratio of tracers is not!
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Covariance model
we can construct
where
For data vector
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Covariance model
free parameters:
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0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f�8
0.8
1.0
1.2
1.4
1.6
1.8
b�8
0.8 1.0 1.2 1.4 1.6 1.8b�8
GiggleZ �g-vp Correlated
0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f�8
0.8
1.0
1.2
1.4
1.6
1.8
b�8
0.8 1.0 1.2 1.4 1.6 1.8b�8
GiggleZ �g-vp Independent
Results: GiggleZ simulation
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0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f�8
0.8
1.0
1.2
1.4
1.6
1.8
b�8
0.8 1.0 1.2 1.4 1.6 1.8b�8
GiggleZ �g-vp Independent
0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f�8
0.8
1.0
1.2
1.4
1.6
1.8
b�8
0.8 1.0 1.2 1.4 1.6 1.8b�8
GiggleZ �g-vp Correlated
Results: GiggleZ simulation
Caitlin Adams — Large Scale Structure and Galaxy Flows
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0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9f�8
0.1
0.2
0.3
0.4
0.5
0.6
0.7
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
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6dF �g-vp Independent
Results: 6dFGS
Preliminary
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Upcoming: Taipan
Koda et al. 2014
Constraint (β)
6dFGS 16%
Taipan 7.4%
WALLABY 3.8%
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Take home points
Scale-dependent measurements can distinguish between the standard model and modified gravity models
Modelling the covariance of velocities and overdensities will provide a measure of β(k)
Beating down cosmic variance through multiple probes will provide tighter constraints on f(k)
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Bonus Slide: Peculiar velocities
In Fourier Space:
1/k dependence means velocities are sensitive to large modes (small k)
Koda et al. 2014
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Bonus Slide: Peculiar velocities
Johnson et al. 2014
Peculiar velocities are non-Gaussian, so apply a transform:
For:
measured from Fundamental Plane
Hui & Greene 2006
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Bonus Slide: Exact equations
n
0
W (k)
j0(kr)
aH(r̂ · x̂v)
(aH)
2
1
3
cos↵ (j0(kr)� 2j2(kr)) +x�xv
r2j2(kr) sin
2 ↵
�
1
Nexpi
0
Caitlin Adams — Large Scale Structure and Galaxy Flows