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

Aims

Caitlin Adams Large Scale Structure and Galaxy Flows

How can we distinguish between CDM and alternatives?

How can we best utilise low-redshift surveys?

Can we overcome cosmic variance in a self-consistent way?

The standard model

Image from NASA/WMAP

CDM attributes accelerated expansion to dark energy

but could there be an alternative explanation?

Caitlin Adams Large Scale Structure and Galaxy Flows

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

Theres also work on observational predictions from generalised forms of modified gravity (Baker et al. 2014)

Caitlin Adams Large Scale Structure and Galaxy Flows

Scale-dependence

Caitlin Adams Large Scale Structure and Galaxy Flows

Scale-dependence: 6dFGS

Data from Johnson et al. 2014

Caitlin Adams Large Scale Structure and Galaxy Flows

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!

Caitlin Adams Large Scale Structure and Galaxy Flows

Covariance model

we can construct

where

For data vector

Caitlin Adams Large Scale Structure and Galaxy Flows

Covariance model

free parameters:

Caitlin Adams Large Scale Structure and Galaxy Flows

0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f8

0.8

1.0

1.2

1.4

1.6

1.8

b8

0.8 1.0 1.2 1.4 1.6 1.8b8

GiggleZ g-vp Correlated

0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f8

0.8

1.0

1.2

1.4

1.6

1.8

b8

0.8 1.0 1.2 1.4 1.6 1.8b8

GiggleZ g-vp Independent

Results: GiggleZ simulation

Caitlin Adams Large Scale Structure and Galaxy Flows

0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f8

0.8

1.0

1.2

1.4

1.6

1.8

b8

0.8 1.0 1.2 1.4 1.6 1.8b8

GiggleZ g-vp Independent

0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46f8

0.8

1.0

1.2

1.4

1.6

1.8

b8

0.8 1.0 1.2 1.4 1.6 1.8b8

GiggleZ g-vp Correlated

Results: GiggleZ simulation

Caitlin Adams Large Scale Structure and Galaxy Flows

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9f8

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

6dF g-vp Independent

Results: 6dFGS

Preliminary

Caitlin Adams Large Scale Structure and Galaxy Flows

Upcoming: Taipan

Koda et al. 2014

Constraint ()

6dFGS 16%

Taipan 7.4%

WALLABY 3.8%

Caitlin Adams Large Scale Structure and Galaxy Flows

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 (k)

Beating down cosmic variance through multiple probes will provide tighter constraints on f(k)

Caitlin Adams Large Scale Structure and Galaxy Flows

Bonus Slide: Peculiar velocities

In Fourier Space:

1/k dependence means velocities are sensitive to large modes (small k)

Koda et al. 2014

Caitlin Adams Large Scale Structure and Galaxy Flows

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

Caitlin Adams Large Scale Structure and Galaxy Flows

Bonus Slide: Exact equations

n

0

W (k)

j0(kr)

aH(r xv)

(aH)21

3

cos (j0(kr) 2j2(kr)) +xxvr2

j2(kr) sin2

1

Nexpi

0

Caitlin Adams Large Scale Structure and Galaxy Flows