Munch

Standard model gauge bosons propagating in two universal extra dimensions give rise to heavy spin-1 and spin-0 particles. The lightest of these, carrying Kaluza-Klein numbers (1,0), may be produced only in pairs at colliders, whereas the (1,1) modes, which are heavier by a factor of \sqrt{2}, may be singly produced. We show that the cascade decays of (1,1) particles generate a series of closely-spaced narrow resonances in the t\bar{t} invariant mass distribution. At the Tevatron, s-channel production of (1,1) gluons and electroweak bosons will be sensitive to t\bar{t} resonances up to masses in the 0.5 - 0.7 TeV range. Searches at the LHC for resonances originating from several higher-level modes will further test the existence of two universal extra dimensions.

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Dark matter: A phenomenological existence proof

Authors: D. V. Ahluwalia-Khalilova
Comments: 8 pages, 3 figures (Latex: iopart style)
Report-no: ASGBG Preprint: 20.1.2006Ah

The non-Keplerian galactic rotational curves and the gravitational lensing data strongly indicate a significant dark matter component in the universe. Moreover, these data can be combined to deduce the equation of state of dark matter. Yet, the existence of dark matter has been challenged following the tradition of critical scientific spirit. In the process, the theory of general relativity itself has been questioned and various modified theories of gravitation have been proposed. Within the framework of the Einsteinian general relativity, here I make the observation that if the universe is described by a spatially flat Friedmann-Robertson-Walker cosmology with Einsteinian cosmological constant then the resulting cosmology predicts a significant dark matter component in the universe. The phenomenologically motivated existence proof refrains from invoking the data on galactic rotational curves and gravitational lensing, but uses as input the age of the universe as deciphered from studies on globular clusters.

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Galaxy Cluster Masses Without Non-Baryonic Dark Matter

Authors: J. R. Brownstein, J. W. Moffat
Comments: Submitted to MNRAS, July 8, 2005. 16 pages, 2 figures, 1 table, 106 galaxy clusters

We apply the modified acceleration law obtained from Einstein gravity coupled to a massive skew symmetric field, F_{\mu\nu\lambda}, to the problem of explaining X-ray galaxy cluster masses without exotic dark matter. Utilizing X-ray observations to fit the gas mass profile and temperature profile of the hot intracluster medium (ICM) with King beta-models, we show that the dynamical masses of the galaxy clusters resulting from our modified acceleration law fit the cluster gas masses for our sample of 106 clusters without the need of introducing a non-baryonic dark matter component. We are further able to show for our sample of 106 clusters that the distribution of gas in the ICM as a function of radial distance is well fit by the dynamical mass distribution arising from our modified acceleration law without any additional dark matter component. In previous work, we applied this theory to galaxy rotation curves and demonstrated good fits to our sample of 101 LSB, HSB and dwarf galaxies including 58 galaxies that were fit photometrically with the single parameter (M/L)_{stars}. The results there were qualitatively similar to those obtained using Milgrom's phenomenological MOND model, although the determined galaxy masses were quantitatively different and MOND does not show a return to Keplerian behavior at extragalactic distances. The results here are compared to those obtained using Milgrom's phenomenological MOND model which does not fit the X-ray galaxy cluster masses unless an auxiliary dark matter component is included.

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Big bang nucleosynthesis constraints on scalar-tensor theories of gravity

Authors: Alain Coc, Keith A. Olive, Jean-Philippe Uzan, Elisabeth Vangioni
Comments: 19 pages, 30 eps figures
Report-no: UMN--TH--2424/05, FTPI--MINN--05/52

We investigate BBN in scalar-tensor theories of gravity with arbitrary matter couplings and self-interaction potentials. We first consider the case of a massless dilaton with a quadratic coupling to matter. We perform a full numerical integration of the evolution of the scalar field and compute the resulting light element abundances. We demonstrate in detail the importance of particle mass thresholds on the evolution of the scalar field in a radiation dominated universe. We also consider the simplest extension of this model including a cosmological constant in either the Jordan or Einstein frame.

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The non-perturbative regime of cosmic structure formation

Authors: Thomas Buchert
Comments: 9 pages; submitted to Astron. Astrophys

This paper focusses on the barely understood gap in between the weakly nonlinear regime of structure formation and the onset of the virialized regime. While the former is accessed through perturbative calculations and the latter through virialization conditions incorporating dynamical stresses that arise in collisionless self-gravitating systems due to velocity dispersion forces, the addressed regime can only be understood through non-perturbative models. We here present an exact Lagrangian integral that provides a tool to access this regime. We derive a transport equation for the peculiar-gravitational field strength and integrate it along comoving trajectories of fluid elements. The so-obtained integral provides an exact expression that solves the longitudinal gravitational field equation in general. We argue why this integral provides a powerful approximation beyond the Lagrangian perturbative regime, and discuss its relation to known approximations, among them Lagrangian perturbation solutions including Zel'dovich's approximation and approximations for adhesive gravitational clustering including the adhesion approximation. Furthermore, we propose an iteration scheme for a systematic analytical and numerical construction of trajectory fields. The integral may also be employed to improve inverse reconstruction techniques.

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Small scale contributions to CMB: A coherent analysis

Authors: Marian Douspis (IAS, LATT), Nabila Aghanim (IAS), Mathieu Langer (IAS)
Comments: 9 pages

We reanalyse Cosmic Microwave Background data from experiments probing both large and small scales. We assume that measured anisotropies are due not only to primary fluctuations but also, especially at small scales, to secondary effects (namely the Sunyaev-Zel'dovich effect) and possible point source contaminations. We first consider primary and secondary anisotropies only. For the first time in such analyses, the cosmological dependence of secondary fluctuations is fully taken into account. We show in that case that a higher value of the normalisation $\sigma\_8$ is preferred, as found by previous studies, but also higher values of the optical depth $\tau$ and power spectrum index $n\_s$ are needed. In the second part of our analysis, we further include possible contaminations from unresolved and unremoved point sources. Under these considerations, we discuss the effects on the cosmological parameters. We further obtain the best combination of relative contributions of the three kinds of sources to the measured microwave power on small scales at each frequency. Our method allows us to simultaneously obtain cosmological parameters and explain the so-called small scale power excess in a consistent way.

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On the CMB large-scales angular correlations

Authors: Armando Bernui, Thyrso Villela, Carlos A. Wuensche, Rodrigo Leonardi, Ivan Ferreira
Comments: 7 pages, 11 color figures, accepted for publication in A&A

We study the large-scale angular correlation signatures of the Cosmic Microwave Background (CMB) temperature fluctuations from WMAP data in several spherical cap regions of the celestial sphere, outside the Kp0 or Kp2 cut-sky masks. We applied a recently proposed method to CMB temperature maps, which permits an accurate analysis of their angular correlations in the celestial sphere through the use of normalized histograms of the number of pairs of such objects with a given angular separation versus their angular separation. The method allows for a better comparison of the results from observational data with the expected CMB angular correlations of a statistically isotropic Universe, computed from Monte Carlo maps according to the WMAP best-fit Lambda CDM model. We found that the, already known, anomalous lack of large-scale power in full-sky CMB maps are mainly due to missing angular correlations of quadrupole-like signature. This result is robust with respect to frequency CMB maps and cut-sky masks. Moreover, we also confirm previous results regarding the unevenly distribution in the sky of the large-scale power of WMAP data. In a bin-to-bin correlations analyses, measured by the full covariance matrix chi^2 statistic, we found that the angular correlations signatures in opposite Galactic hemispheres are anomalous at the 98%-99% confidence level.

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Collisions of strings with Y junctions

Authors: E. J. Copeland, T. W. B. Kibble, D. A. Steer
Comments: 4 pages, 1 figure, uses revtex 4. Clarifying comments added to correct a conceptual error, reference updated

We study the dynamics of Nambu--Goto strings with junctions at which three strings meet. In particular, we exhibit one simple exact solution and examine the process of intercommuting of two straight strings, in which they exchange partners but become joined by a third string. We show that there are important kinematical constraints on this process. The exchange cannot occur if the strings meet with very large relative velocity. This may have important implications for the evolution of cosmic superstring networks and non-abelian string networks.

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Nongaussian and nonscale-invariant perturbations from tachyonic preheating in hybrid inflation

Authors: Neil Barnaby, James M. Cline
Comments: 27 pages, 14 figures

We show that in hybrid inflation it is possible to generate large second-order perturbations in the cosmic microwave background due to the instability of the tachyonic field during preheating. We carefully calculate this effect from the tachyon contribution to the gauge-invariant curvature perturbation, clarifying some confusion in the literature concerning nonlocal terms in the tachyon curvature perturbation; we show explicitly that such terms are absent. We quantitatively compute the nongaussianity generated by the tachyon field during the preheating phase and translate the experimental constraints on the nonlinearity parameter f_{NL} into constraints on the parameters of the model. We also show that nonscale-invariant second-order perturbations from the tachyon field can become larger than the inflaton-generated first-order perturbations, leading to stronger constraints than those coming from nongaussianity. The width of the excluded region in terms of the logarithm of the dimensionless coupling g, grows linearly with the log of the ratio of the Planck mass to the tachyon VEV, \log(M_p/v); hence very large regions are ruled out if the inflationary scale v is small. We apply these results to string-theoretic brane-antibrane inflation, and find a stringent upper bound on the string coupling, g_s < 10^{-4.5}.

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Bulk viscosity of a gas of neutrinos and coupled scalar particles, in the era of recombination

Authors: R. F. Sawyer
Comments: 6 pages, 4 figures

Bulk viscosity may serve to damp sound waves in a system of neutrinos coupled to very light scalar particles, in the era after normal neutrino decoupling but before recombination. We calculate the bulk viscosity parameter in a minimal scheme involving the coupling of the two systems. We add some remarks on the bulk viscosity of a system of fully ionized hydrogen plus photons.

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