arXiv subjects

Martin Kunz

Publications and source records attributed to Martin Kunz.

171 records · Page 10Linked to original sources

arXiv2002

Did the dark energy always have a large Compton wavelength?

Until now all well-studied dark energy models proposed have very large Compton wavelengths, $λ_c > 100 Mpc$ at all time and hence show no clustering on small scales. Evidence against this would disfavour both $Λ$CDM and quintessence. We show however, that large $λ_c$ is actually favoured by current CMB, LSS and SN1a data by comparing quintessence with a condensate scenario with the same background dynamics. The condensate scenario has a very small $λ_c$ before making a transition to negative pressure and large $λ_c$ and therefore exhibits a rich CMB phenomenology which roughly interpolates between $Λ$CDM and flat CDM models.

astro-ph↗

arXiv2002

Observational constraints on the inflaton potential combined with flow-equations in inflaton space

Direct observations provide constraints on the first two derivatives of the inflaton potential in slow roll models. We discuss how present day observations, combined with the flow equations in slow roll parameter space, provide a non-trivial constraint on the third derivative of the inflaton potential. We find a lower bound on the third derivative of the inflaton potential V'''/V > -0.2. We also show that unless the third derivative of the inflaton potential is unreasonably large, then one predicts the tensor to scalar ratio, r, to be bounded from below r > 3 * 10^{-6}.

hep-ph↗

arXiv2002

A late-time transition in the cosmic dark energy?

We study constraints from the latest CMB, large scale structure (2dF, Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in their equation of state, w(z). Such a transition is motivated by models like vacuum metamorphosis where non-perturbative quantum effects are important at late times. We allow the transition to occur at a specific redshift, z_t, to a final negative pressure -1 < w_f < -1/3. We find that the CMB and supernovae data, in particular, prefer a late-time transition due to the associated delay in cosmic acceleration. The best fits (with 1 sigma errors) to all the data are z_t = 2.0^{+2.2}_{-0.76}, Ω_Q = 0.73^{+0.02}_{-0.04} and w_f = -1^{+0.2}.

astro-ph↗

arXiv2002

The role of point-like topological excitations at criticality: from vortices to global monopoles

We determine the detailed thermodynamic behavior of vortices in the O(2) scalar model in 2D and of global monopoles in the O(3) model in 3D. We construct new numerical techniques, based on cluster decomposition algorithms, to analyze the point defect configurations. We find that these criteria produce results for the Kosterlitz-Thouless temperature in agreement with a topological transition between a polarizable insulator and a conductor, at which free topological charges appear in the system. For global monopoles we find no pair unbinding transition. Instead a transition to a dense state where pairs are no longer distinguishable occurs at T 3 are drawn, based on the observed behavior.

cond-mat↗

arXiv2000

Skewness as a probe of non-Gaussian initial conditions

We compute the skewness of the matter distribution arising from non-linear evolution and from non-Gaussian initial perturbations. We apply our result to a very generic class of models with non-Gaussian initial conditions and we estimate analytically the ratio between the skewness due to non-linear clustering and the part due to the intrinsic non-Gaussianity of the models. We finally extend our estimates to higher moments.

astro-ph↗

arXiv1999

Cosmic Microwave Background Anisotropies from Global Texture

We investigate the global texture model of structure formation in cosmogonies with non-zero cosmological constant for different values of the Hubble parameter. We find that the absence of significant acoustic peaks and little power on large scales are robust predictions of these models. However, from a careful comparison with experiments we conclude that at present we cannot safely reject the model on the grounds of published CMB anisotropy data. If bias is close to one on large scales, galaxy correlation data rules out the models. New, very stringent constraints come from peculiar velocities. Investigating the large-N limit, we argue that our main conclusions apply to all global O(N) models of structure formation.

astro-ph↗

arXiv1998

Cosmic Microwave Background Anisotropies from Scaling Seeds: Global Defect Models

We investigate the global texture model of structure formation in cosmogonies with non-zero cosmological constant for different values of the Hubble parameter. We find that the absence of significant acoustic peaks and little power on large scales are robust predictions of these models. However, from a careful comparison with data we conclude that at present we cannot safely reject the model on the grounds of present CMB data. Exclusion by means of galaxy correlation data requires assumptions on biasing and statistics. New, very stringent constraints come from peculiar velocities. Investigating the large-N limit, we argue that our main conclusions apply to all global O(N) models of structure formation.

astro-ph↗

arXiv1997

Cosmic Microwave Background Anisotropies from Scaling Seeds: Generic Properties of the Correlation Functions

In this work we present a partially new method to analyze fluctuations which are induced by causal scaling seeds. We show that the power spectra due to this kind of seed perturbations are determined by five analytic functions, which we determine numerically for a special example. We put forward the view that, even if recent work disfavors the models with cosmic strings and global O(4) texture, causal scaling seed perturbations merit a more thorough and general analysis, which we initiate in this paper.

astro-ph↗

arXiv1996

Microwave Background Anisotropies Induced by Global Scalar Fields: The Large N Limit

We present an analysis of CMB anisotropies induced by global scalar fields in the large N limit. In this limit, the CMB anisotropy spectrum can be determined without cumbersome 3D simulations. We determine the source functions and their unequal time correlation functions and show that they are quite similar to the corresponding functions in the texture model. This leads us to the conclusion that the large N limit provides a 'cheap approximation' to the texture model of structure formation.

astro-ph↗