Ignacy SawickiUniversité de Genève

Understanding Dark Energy

XXXV Physics in Collisions 2015, University of Warwick

Making Sense of the Sky

18 September 2015

Image credit: NASA and ESA

XXXV Physics in Collisions 2015, University of Warwick

Making Sense of the Sky

18 September 2015

Metric to define distances/causality

Geodesic motion for light

Distances: corrected by gravitational field Luminosity Angular diameter

𝑢em𝜇 𝑘𝜇

𝑢obs𝜇 𝑘𝜇

=𝜔em

𝜔obs

≡1+𝑧𝑘𝛼𝛻𝛼𝑘𝜇=0

Redshift

𝑑L=(1+𝑧 )2𝑑A

XXXV Physics in Collisions 2015, University of Warwick

Ordering by redshift

18 September 2015

Image credit: Ivo Labbé

XXXV Physics in Collisions 2015, University of Warwick

Use Standard Candles to Map

18 September 2015

XXXV Physics in Collisions 2015, University of Warwick

Use Standard Candles to Map

18 September 2015

Broader is intrinsically brighter

Standardise to some (unknown) intrinsic luminosity

Obtain luminosity distance as function of redshift

To interpret need a model Gravity Composition

XXXV Physics in Collisions 2015, University of Warwick

First Attempt at Model

Copernican Principle: here is not specialUniverse grossly homogeneous and

isotropic

Gravity is General Relativity

Matter non-relativistic

18 September 2015

d 𝑠2=d 𝑡 2−𝑎2 (𝑡 )𝛿𝑖𝑗d 𝑥𝑖d 𝑥 𝑗

XXXV Physics in Collisions 2015, University of Warwick

First Attempt at Model: FAIL

18 September 2015

From D. Huterer

Acceleration

Dim

mer

XXXV Physics in Collisions 2015, University of Warwick

Second Attempt: ΛCDM NOBEL

Cosmological Constant

68%

Baryons5%

Dark Matter27%

Composition of Universe IF ΛCDM

18 September 2015

(stars 0.4%, gas 4.6%)

Planck 2015

XXXV Physics in Collisions 2015, University of Warwick

Is the Universe fooling us?

Light dims/SNe evolve (“tired light”) matches : photons conserved (BAO)

Here is special (“inhomogenous universe”)Distant clusters do not see anisotropic sky (kSZ)

Non-linear fluctuations (“averaging problem”)True, but no prescription gets more than 0.1%

effect

18 September 2015

XXXV Physics in Collisions 2015, University of Warwick

Is the Universe fooling us?

18 September 2015

Acceleration detected at

64σ

XXXV Physics in Collisions 2015, University of Warwick

But is it actually Λ?

Old c.c. problem: new fine-tuning at every order

Coincidence problem:

Wishful thinking

18 September 2015

𝑀 Λ

𝑀weak

∼10−16

Alternatives to Λ must be dynamical:

search for time- and scale-dependence

XXXV Physics in Collisions 2015, University of Warwick

ΛCDM as Null Hypothesis

18 September 2015

Planck 2015

Background close to ΛCDM

XXXV Physics in Collisions 2015, University of Warwick

CMB is Gravitational Collapse

18 September 2015

gravitational collapse

Initial conditions (inflation)

Photon decoupling, Galaxy formation

Vacuum in quasi-de Sitter

(nearly) scale invariant Gaussian fluctuations

Planck 2015

XXXV Physics in Collisions 2015, University of Warwick

Dark Energy Changes Growth

18 September 2015

ΛCDM

Dynamical DE

𝑧=0𝑧=1𝑧=3

Snowmass Report 1309:5385

XXXV Physics in Collisions 2015, University of Warwick

Use Structure to Probe Gravity/DE

18 September 2015

SDSS/BOSS SDSS/BOSS DR10

2500 deg2 up to

XXXV Physics in Collisions 2015, University of Warwick

Redshift-Space Distortions

18 September 2015

Real space

Redshift space

Samushia et al. (2013)/BOSS

Monopole:

Quadrupole:

Kaiser (1985)

XXXV Physics in Collisions 2015, University of Warwick

Redshift-Space Distortions

Galaxy velocities tend to be lower than ΛCDM would imply

18 September 2015

Ruiz & Huterer (2014)

𝜕𝑖𝑣 g𝑖 ≡

�̇� g𝑖 =�̇�m

𝑖 =𝜕𝑖Ψ

XXXV Physics in Collisions 2015, University of Warwick

Weak Lensing

18 September 2015

𝜎 𝑖𝑗=∫𝑧 s

𝑧 o

d 𝑧 𝜕𝑖𝜕 𝑗 (Ψ +Φ )𝐾 (𝑧 , 𝑧 s)

d 𝑠2=− (1+2Ψ )d 𝑡2+𝑎2(𝑡) (1−2Φ )d 𝒙𝟐

XXXV Physics in Collisions 2015, University of Warwick

Is everything OK with ΛCDM?

18 September 2015

Weak Lensing (CFHTLeNS, Heymans 2012)

+ Distances

Clusters(Planck 2013)

CMB (Planck 2013)

+ Distances

𝜎 8

Pro

babili

ty

Densi

ty

XXXV Physics in Collisions 2015, University of Warwick

CMB Lensing: Smooths Peaks

Effect on CMB consistent with ΛCDM

Galaxy shear: same physics Kernel includes approx.

same redshifts

Must it be systematics? CMBL: WL:

18 September 2015

Planck: Ade et al. (2015)

𝜑 lens≡𝐴L (Φ+Ψ )

XXXV Physics in Collisions 2015, University of Warwick

The Takeaway

Dark energy is not going away

ΛCDM fits, but maybe first tensions are beginning to appearPower seems to be lacking in many probes of growth

It could well end up being other physicsMassive neutrinos can have similar physics

Caveat emptor: All cosmological probes sensitive only to gravity; cannot say anything direct about composition

18 September 2015

XXXV Physics in Collisions 2015, University of Warwick

Watch this Space!

18 September 2015

Credit: D. Huterer

XXXV Physics in Collisions 2015, University of Warwick

18 September 2015

THANK YOU!

XXXV Physics in Collisions 2015, University of Warwick

w is not an observable Distances only depend

on

We measure geometry only

DM/DE split is ambiguous

18 September 2015

Ωm0=0.4

3

Ωm0=0.2Ωm0=0

𝐻2=𝐻02 (Ωm0𝑎− 3+ΩΛ )

Kunz (2007)

𝐻2=𝐻02 (~Ωm0𝑎− 3+~ΩDE(𝑎))

XXXV Physics in Collisions 2015, University of Warwick

Scale-Dependent Growth Rate

18 September 2015

𝑃 (𝑘)

𝑘(hMpc− 1)

𝑘J=𝑎𝐻𝑐s

Exploit DE scale dependence:CMBL gives

ΔΩc0=0.1

BAO : SDSS vs Euclid

Distance-redshiftrelation moves P(k)

EUCLID expected SDSS today 0.7<z<2.0 0.15<z<0.5

Measuring shear in next generation wide field cosmic shear surveys