Probing Dark Matter with the CMB and Large-Scale Structure 1 Cora Dvorkin IAS (Princeton) Harvard (Hubble fellow) COSMO 2014 August 2014, Chicago

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Probing Dark Matter with the CMB and Large-Scale Structure

Cora Dvorkin

IAS (Princeton) Harvard (Hubble fellow)

COSMO 2014

August 2014, Chicago

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Outline

Dark Matter overview.

Effect of WIMP Dark Matter Annihilation on the CMB: Homogeneous scenario. Inhomogeneous scenario: boosted electron perturbations (Non-Gaussian signal).

• Other effects: enhanced matter temperature fluctuations; key observable: 21 cm radiation field; CMB B-mode polarization.

Effect of Dark Matter-baryon interactions on the CMB and the Large-Scale Structure.

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Evidence for Dark Matter

The cosmic microwave background

Galaxy rotation curves

Gravitational lensing

Overwhelming evidence for Dark Matter:Galactic scalesCluster scalesCosmic Microwave Background

multipole moment l

Pow

er s

pect

rum

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Looking for Dark Matter off the beaten track

Where do Dark Matter interactions matter?

Some well known avenues:

Excess high energy cosmic/gamma rays;Missing energy at colliders;Nucleon recoil deep underground;…

Important to look for new processes

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FINAL PRODUCTS

WIMP Dark Matter Annihilation

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Standard Recombination

Effective Boltzmann equation for the free electron density:

Peebles, ApJ (1968)Z’eldovich and Sunyaev, JETP (1969)

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Recombination rate


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Ionization rateRecombination rate


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DM Annihilation in Recombination

Ionization rateRecombination rate

Energy injected into the plasma per unit volume, per unit time:

(Majorana particle)

Chen and Kamionkowski (2004)Padmanabhan and Finkbeiner (2005)

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Time scales (Recombination, Ionization, Expansion)

C. Dvorkin, K. Blum, and M. Zaldarriaga, Phys. Rev. D (2013)

z (redshift)

1/M

pc

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Free electron fraction evolution

x e (io

niza

tion

frac

tion)

z (redshift)

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Effect on the CMB TemperatureA higher ionization suppresses the CMB temperature fluctuations

Current CMB constraints are GeV Complementary to direct detection searches, that are most sensitive to GeV, due to kinematical considerations.

Degeneracy:

Padmanabhan and Finkbeiner (2005)

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• Screening of the observed spectrum at l>100.

• Re-scattering of photon generates extra polarization at large scales.

A higher ionization enhances the polarization fluctuationsat large scales

Effect on the CMB Polarization

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Current and Future Dark Matter Annihilation Constraints

from the CMB

• Planck polarization data: coming this year.

• CMB “Stage IV” experiment is being planned now!W. Wu, J. Errard, C. Dvorkin, C. L. Kuo, A. Lee, et al., ApJ (2014)

Thermal cross section

M. Madhavacheril, N. Sehgal, T. Slatyer, Phys. Rev. D (2014)

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There are growing ionization modes that track the collapse of matter overdensities.

k=0.04 Mpc-1

Dark Matter AnnihilationInhomogeneous scenario

pert

urba

tion

pert

urba

tion

time [Mpc] time [Mpc] C. Dvorkin, K. Blum, and M. Zaldarriaga, Phys. Rev. D (2013)

k=0.04 Mpc-1 k=0.3 Mpc-1

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Order of magnitude boost!


elec

tron

per

turb

ation

wave number k [1/Mpc]

Comparison to standard first order electron perturbations

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Order of magnitude boost!

Comparison to standard first order electron perturbations

elec

tron

per

turb

ation

wave number k [1/Mpc]C. Dvorkin, K. Blum, and M. Zaldarriaga, Phys. Rev. D (2013)

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Can we observe electron density perturbations in the CMB?


CMB Bispectrum: probe of electron density perturbations.

• Signal-to-noise 0.5 for Planck; polarization will have more information (work in progress).

• The main boost in the electron perturbations by DM annihilation occurs on small scales, l>3000 (challenging to observe in temperature).

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Enhanced Matter Temperature fluctuations

There should be more information in the 21 cm radiation field (future work).

wave number k [1/Mpc]

Matt

er te

mpe

ratu

re fl

uctu

ation

s

Current and future 21 cm experiments:LOFAR, MWA, PAPER,SKA, etc, etc..

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Beyond the WIMP paradigm

• It has been pointed out that Dark Matter self-interactions and Dark Matter-Baryon interactions can significantly affect small-scale structure.

• Baryon processes such as star formation, supernova feedback, gas accretion, etc. can have important effects, but these processes are partially understood theoretically and poorly constrained observationally.

Spergel and Steinhardt (2000);Wandelt et al. (2000)

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Goal: to use observational probes of the CMB and matter fluctuations (where the theory is under better control) to know how much interaction between baryons and Dark Matter can occur today.

Dark Matter-Baryon Interactions

C. Dvorkin, K. Blum and M. Kamionkowski, Phys. Rev. D (2013)

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Dark Matter-Baryon Interactions

with

Dark Matter-baryon momentum exchange rate:

for for


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Effect on the Matter Power Spectrum


wave number k [h/Mpc]

Matt

er P

ower

Spe

ctru

m P

(k) [

(h-1

Mpc

)3 ]

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Lyman-alpha

Effect on the Matter Power Spectrum


wave number k [h/Mpc]

Matt

er P

ower

Spe

ctru

m P

(k) [

(h-1

Mpc

)3 ]

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Constraining Dark Matter-Baryon Scattering with Cosmology

All the curves ( ) are normalized to satisfy a mean free path of 1 Mpc in a system like the Milky Way,with ,and .


z (redshift)

(mom

entu

m e

xcha

nge

rate

)/(c

omov

ing

Hub

ble

expa

nsio

n)

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Minimal mean free path for baryons scattering on Dark Matter in the Milky Way

A baryon in the halo of a galaxy like our Milky Way does not scatter from Dark Matter particles during the age of the Universe.


Mean free path:

(CMB data: from Planck; Ly-alpha data: from the Sloan Digital Sky Survey)

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Conclusions We will be able to put strong constraints on Dark Matter Annihilation with future CMB observations (currently complementary to direct detection constraints).

WIMP Dark matter annihilation leads to growing ionization modes that track the collapse of dark matter overdensities (boosted by 1 to 2 orders of magnitude at small scales relative to standard model).

Electron perturbations source CMB Non-Gaussianities at recombination. Enhanced matter temperature fluctuations at late times (natural observational tool: 21 cm radiation – future work).

Using CMB data from Planck + Ly-alpha forest measurements from the Sloan Digital Sky Survey, we conclude that a baryon in the halo of a Galaxy like our Milky Way does not scatter from Dark Matter particles during the age of the Universe.

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CMB Bispectrum: probe of electron density perturbations

• From perturbed visibility: anisotropic optical depth.

The first and second order anisotropies today are given by the line of sightsolutions to the Boltzmann equation:

Seljak and Zaldarriaga (1996)

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CMB Bispectrum: probe of electron density perturbations

New anisotropies generated by electron perturbations:

• From perturbed visibility: anisotropic optical depth.

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The main boost in the electron perturbations by DM annihilation occurs on small scales, l>3000 (challenging to observe).

at peak visibility at half-maximum visibility

Signal-to-noise 0.5 for Planck; polarization will have more information (work in progress).



multipole moment l multipole moment l

New

ani

sotr

opie

s “g

l”

New

ani

sotr

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s “g

l”

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• Solve the perturbed Boltzmann equation up to second order in the tight coupling limit ( ) and identify the physical processes:

Perturbed Harmonic Oscillator

C. Dvorkin, K. Blum, and M. Zaldarriaga, in preparation

Silk dampingSound speed

• Solution given by WKB’s Green function.

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Imprints on the CMB Power Spectrumfrom Dark Matter-Baryon scattering


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Documents

Probing Dark Matter with the CMB and Large-Scale Structure 1 Cora Dvorkin IAS (Princeton) Harvard (Hubble fellow) COSMO 2014 August 2014, Chicago