Are domain walls ruled out?

Abstract

Recent analysis of the combined data of cosmic microwave background, galaxy clustering and supernovae type Ia observations have set strong constraints on the equation of state parameter wX . The upper bound wX < −0.82 at 95% c.l. rules out an important class of models, the domain walls (−2/3 < wX < −1/3). Here we revisit the issue of domain walls as a possible alternative to the standard Λ-CDM model by questioning the assumptions made in the choice of priors of the data analysis. The results of our investigation show that domain walls can provide a good fit to the WMAP data for a different choice of priors with "lower" values of the Hubble parameter (h < 0.65), (as indicated by Sunyaev-Zeldovich and time delays for gravitational lensing observations), and "higher" values of the matter density (Ωm > 0.35), (in agreement with recent measurements of the temperature-luminosity relation of distant clusters observed with the XMM-Newton satellite). In this new perspective, their existence would lead to important implications for the CMB constraints on cosmological and inflationary parameters.

Frozen Rigging Model of the Energy Dominated Universe

“…Since the condition (4) seems [12] to be inconsistent with the available cosmological evidence, the conclusion to be drawn is that we need a model of a kind more general than a polytropic fluid. Whereas it is hard to see how to construct a viable theoretical model that violates the energy dominance postulate (1), it was pointed out by Bucher and Spergel [14] that it is easy to construct a continuum model that gets round the restriction (4) if, instead of assuming that it behaves as a fluid, one supposes that it will behave as a solid with a sufficiently large rigidity modulus µ. Generalising results that are well known in non-relativistic elasticity theory, it was shown many years ago by the present author [15] that, in a relativistic elastic solid, the speed, c say, of longitudinally polarised propagation modes will be given in terms of the value that it would have according to (2) in the absence of rigidity by…”

“…The kind of frozen (locally static) rigging system described in the preceding section will provide roughly what seems to be needed [12] to account for the cosmological appearance of dark energy provided the microscopic field theory parameters η and ε are such that the corresponding value of the coefficient c in (14), which according to (21) and (23) will be given by c ≈ |ε| 1/2 η 7/2 ,…”

“…The present situation [12] is that the available evidence does not seem to impose quite such a radically revolutionary step as abandonning the usual supposition (1), nor even to impose the marginal limit ρ = |P | = −P requiring invocation of a cosmological constant (whose magnitude would be hard to account for except by anthropic considerations [13]). What it does seem to call for, rather conclusively, is the abandonning of the more restrictive condition P > 0 that is usually postulated as a necessity for avoiding microscopic instability in a perfect fluid model.…”

Probing dark energy with supernovae: A concordant or a convergent model?

“…However, i) in spite of the precise HST result, the Hubble constant value is still controversial [11,12]. ii) We have shown in the previous section that the SN data analysis can only conclude that Ω M < 0.6 (see Fig.…”

“…Conversi et al [12] provide an interesting critical analysis on the present constraints on cosmological parameters, especially on Ω M , h, and w. Through the study of the degeneracies, they show that the result Ω M = 0.27 ± 0.04 is obtained under the assumption of the ΛCDM model, and provide specific examples with smaller h (h < 0.65) and higher Ω M (Ω M > 0.35) which are in perfect agreement with the most recent CMB and galaxy redshift surveys.…”