[Paper Review] Looking the void in the eyes - the kSZ effect in LTB models

J. García-Bellido, Troels Haugboelle|ArXiv.org|Jul 9, 2008

Cosmology and Gravitation TheoriesPhysics and Astronomy22 references69 citations

TL;DR

This paper investigates the kinematic Sunyaev-Zeldovich (kSZ) effect as a probe of Lemaître-Tolman-Bondi (LTB) void models, which propose that cosmic acceleration is due to a large underdense void rather than dark energy. It shows that kSZ observations from off-centre galaxy clusters impose stringent constraints on void size and density profile, ruling out giga-parsec-sized voids at high significance with current data and predicting near-future surveys will confirm or rule out such models definitively.

ABSTRACT

As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric. The Universe is no longer homogeneous and isotropic and the apparent late time acceleration is actually a consequence of spatial gradients in the metric. If we did not live close to the centre of the void, we would have observed a Cosmic Microwave Background (CMB) dipole much larger than that allowed by observations. Hence, until now it has been argued, for the model to be consistent with observations, that by coincidence we happen to live very close to the centre of the void or we are moving towards it. However, even if we are at the centre of the void, we can observe distant galaxy clusters, which are off-centre. In their frame of reference there should be a large CMB dipole, which manifests itself observationally for us as a kinematic Sunyaev-Zeldovich (kSZ) effect. kSZ observations give far stronger constraints on the LTB model compared to other observational probes such as Type Ia Supernovae, the CMB, and baryon acoustic oscillations. We show that current observations of only 9 clusters with large error bars already rule out LTB models with void sizes greater than approximately 1.5 Gpc and a significant underdensity, and that near future kSZ surveys like the Atacama Cosmology Telescope, South Pole Telescope, APEX telescope, or the Planck satellite will be able to strongly rule out or confirm LTB models with giga parsec sized voids. On the other hand, if the LTB model is confirmed by observations, a kSZ survey gives a unique possibility of directly reconstructing the expansion rate and underdensity profile of the void.

Motivation & Objective

• To test the viability of LTB void models as an alternative to dark energy for explaining supernova dimming.
• To assess whether the observed CMB dipole is consistent with a local void model, given that off-centre clusters should exhibit large kSZ signals.
• To evaluate the power of kSZ observations in constraining void parameters such as size and underdensity.
• To predict the sensitivity of upcoming kSZ surveys (ACT, SPT, Planck) in confirming or ruling out LTB void models.

Proposed method

• Modeling the LTB metric with a spherically symmetric, inhomogeneous matter distribution, assuming a homogeneous Big Bang (GBH) to reduce free parameters.
• Computing the CMB dipole in the rest frame of off-centre galaxy clusters due to photon trajectory differences in the void's inhomogeneous expansion.
• Deriving the apparent cluster velocity relative to the CMB rest frame, which induces a kSZ effect detectable in microwave background anisotropies.
• Using current kSZ data from 9 clusters with large error bars to constrain void parameters, particularly radius and inner density contrast.
• Projecting future sensitivity of kSZ surveys (ACT, SPT, APEX, Planck) to detect or rule out voids of various sizes.
• Combining kSZ constraints with existing data from SNe Ia, BAO, and CMB to test model consistency.

Experimental results

Research questions

• RQ1 Can the kSZ effect provide stronger constraints on LTB void models than other cosmological probes like SNe Ia or CMB anisotropies?
• RQ2 What is the maximum allowed void radius in LTB models given current kSZ observations of galaxy clusters?
• RQ3 How do future kSZ surveys like ACT and SPT compare in their ability to rule out giga-parsec-sized voids?
• RQ4 To what extent do kSZ observations rule out the GBH-constrained LTB model when combined with other data sets?
• RQ5 Can kSZ surveys directly reconstruct the expansion rate and underdensity profile of a void if the LTB model is confirmed?

Key findings

• Current kSZ observations of 9 clusters with large error bars already rule out LTB models with void sizes greater than approximately 1.5 Gpc and significant underdensity.
• The constrained-GBH LTB model is practically ruled out at the 3-σ level when combining kSZ data with SNe Ia, BAO, and CMB data.
• Small voids (r₀ ≤ 800 Mpc) remain unconstrained by current kSZ data, as they do not affect the lowest-redshift clusters in the sample.
• Future surveys with 10 well-observed clusters could rule out 800 Mpc radius voids at the 3-σ level, while 100 clusters would extend this to 500 Mpc radius.
• If LTB models are confirmed, kSZ surveys would provide a unique, direct method to reconstruct the void's density and expansion rate profile.
• The kSZ effect is identified as the most powerful observational probe for constraining LTB void models, surpassing other probes in sensitivity to void structure.