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 ΛCDM 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

ΛCDM 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

There’s 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 0.46f�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

Caitlin Adams — Large Scale Structure and Galaxy Flows

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

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

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 (β)

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 β(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(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