KEK Cosmophysics Workshop DE2008


2008/12/8(Mon)
Ishibashi, A.	A brief overview of the ideas and issues of the effects of inhomogeneities on cosmic expansion

Vanderveld, R.A.	Voids and Supernova Cosmology
	Abstract: I will review the ways in which voids in the large scale matter distribution can impede supernova cosmology, with a focus on whether or not they can mimic accelerated expansion. This discussion will first focus on the scenario in which the observer lives within a void, and then it will turn to what happens when there are voids along the line of sight connecting observer and source.

Nakao, K.I.	Solving the inverse problem with inhomogeneous universes
	We construct the Lema\^itre-Tolman-Bondi (LTB) dust universe whose distance-redshift relation is equivalent to that in the concordance $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological model. In our model, the density distribution and velocity field are not homogeneous, whereas the big-bang time is uniform, which implies that the universe is homogeneous at its beginning. We also study the effects of local clumpiness in the density distribution as well as the effects of large-scale inhomogeneities on the distance-redshift relation, and show that these effects may reduce the amplitude of large-scale inhomogeneities necessary for having a distance-redshift relation that is the same as that of the concordance $\Lambda$CDM universe. We also study the temporal variation of the cosmological redshift and show that, by the observation of this quantity, we can distinguish our LTB universe model from the concordance $\Lambda$CDM model, even if their redshift-distance relations are equivalent to each other.

2008/12/9(Tue)
Kasai, M.	Dark Energy? Backreaction? Inhomogeneous viewpoint!
	I briefly review the backreaction of nonlinear inhomogeneities to the cosmic expansion, with some historical remarks. Then I re-analyze the observed m-z relation of type Ia supernovae, and propose an inhomogeneous viewpoint that the apparent acceleration of the cosmic expansion is a consequence of large-scale inhomogeneities in the universe.

Notari, A.	Can an Inhomogeneous Universe mimic Dark Energy?
	In this talk I will review recent attempts to explain cosmological observations without Dark Energy. First I will briefly review the approaches based on "backreaction" of inhomogeneities. The challenge here is to find a convincing theoretical argument and to compute reliably the size of the effect. Second I will talk about the possibility that we live near the center of a huge underdense region (Void). The question here is if this can fit all observations. We discuss Minimal voids, which can be consistent with the supernovae data, the WMAP data and the local Hubble parameter. We discuss possible variations of this scenario which can give better fits, and also possibly fit other observations such as the Baryon Acoustic Oscillations.

Starobinsky, A.A.	Strengthening arguments for quasi-homogeneous accelerated expansion of the Universe

	Recent numerous observational data obtained from such independent sources as the temperature angular anisotropy and polarization of the cosmic microwave background radiation, large-scale gravitational clustering of galaxies and their clusters and observations of supernovae explosions at high redshifts prove convincingly that the Universe expands with acceleration at the present time while it was decelerating in the past for redshifts larger than about 0.7. If interpreted in terms of the Einstein general relativity, this means that the about 70% of the total energy density of matter in the present Universe is due to a new (effective) kind of matter in the Universe ("dark energy") which is non-baryonic, has negative pressure which modulus is very close to its energy density, and remains unclustered at all scales where clustering of baryons and dust-like cold dark matter is seen. On the other hand, the question if dark energy is exactly a cosmological constant or something more complicated remains open. Present data do not exclude a possibility of dark energy phantom behaviour (breaking of the weak energy condition for it) at recent redshifts. However, the Hubble parameter does decrease with time, though this is proved at the 2-sigma confidence level only. I discuss such new recently proposed diagnostics of the accelerated expansion of the Universe and dark energy as Om and q-probe. For alternative explanations of these data without the introduction of dark energy but assuming strong inhomogeneity of the Universe, the very existence of acoustic oscillations in CMB and galaxy power spectra plays a very important role since they may occur in a universe with a special type of the cosmological singularity only. This greatly restricts such explanations, if not excludes them at all.



2008/12/10(Wed)
Inoue, K.T.	Local Supervoids and the Origin of the WMAP Cold Spot
	We discuss the origin of the WMAP Cold Spot. We have found that a quasi-linear supervoid with radius ~ 200h^{-1}Mpc at redshift z<1 toward the Spot could generate such a cold spot via the linear integrated Sachs-Wolfe (ISW) effect. However, in the standard LCDM scenarios, the chance of having such a supervoid is extremely rare. Due to a correlation between the first-order and the second-order ISW effect, the CMB temperature would be negatively biased if such superstructures are abundant in the local universes.


Wiltshire, D.	Cosmological equivalence principle and dark energy
(GCJS)	The problem of the synchronization of clocks and normalization of gravitational energy in general relativity does not have a unique or obvious solution in the absence of exact symmetries of the background. I return to first principles and extend the strong equivalence principle in application to averaged dynamical fields in cosmology, to include the role of the evolving average background density in the calibration of inertial frames. This leads to a "radically conservative" solution to the problem of dark energy in cosmology, using only general relativity and matter obeying the strong energy condition. The proposal yields a model universe which appears to be quantitatively viable, in terms of its fit to supernovae luminosity distances, the angular scale of the sound horizon in the cosmic microwave background anisotropy spectrum, and the baryon acoustic oscillation scale. I will briefly overview the observational status of the proposal, as well as discussing the foundational issues.

Futamase, T.	Recent developements of weak lensing
(GCJS)

2008/12/11(Thu)
Wiltshire, D.	Dark energy without dark energy: Average observational quantities
	I have proposed that the phenomenon of dark energy is a misidentification of gravitational energy gradients in a presently inhomogeneous universe dominated by voids, typically of diameter 30/h Mpc. In this talk I will further discuss the physical basis of the proposal in relation to the debate about averaging and backreaction in inhomogeneous cosmology. I will provide further details and outline future observational tests which will distinguish the proposal from standard dark energy models in a homogeneous universe.
2008/12/12(Fri)
Asada, H.	Toward understanding the light propagation in the clumpy universe--- Perturbation theory of N point mass gravitational lens

Roman, A.E.	Inhomogeneities as alternatives to dark energy

Tomita, K.	On astrophysical explanations due to inhomogeneities for the observational cosmological acceleration