Munch

We show that the current accelerated expansion of the Universe can be explained without resorting to dark energy. Models of generalized modified gravity, with inverse powers of the curvature can have late time accelerating attractors without conflicting with solar system experiments. We have solved the Friedman equations for the full dynamical range of the evolution of the Universe. This allows us to perform a detailed analysis of Supernovae data in the context of such models that results in an excellent fit. Hence, inverse curvature gravity models represent an example of phenomenologically viable models in which the current acceleration of the Universe is driven by curvature instead of dark energy. If we further include constraints on the current expansion rate of the Universe from the Hubble Space Telescope and on the age of the Universe from globular clusters, we obtain that the matter content of the Universe is 0.07 <= omega_m <= 0.21 (95% Confidence). Hence the inverse curvature gravity models considered can not explain the dynamics of the Universe just with a baryonic matter component.

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Cosmological constraints on f(R) gravity theories within the Palatini approach

Authors: M. Amarzguioui, O. Elgaroy, D.F. Mota, T. Multamaki
Comments: 11 pages (REVTeX), 7 figures

We investigate f(R) theories of gravity within the Palatini approach and show how one can determine the expansion history, H(a), for an arbitrary choice of f(R). As an example, we consider cosmological constraints on such theories arising from the supernova type Ia, large scale structure formation and cosmic microwave background observations. We find that best fit to the data is a non-null leading order correction to the Einstein gravity, but the current data exhibits no significant preference over the concordance LCDM model. Our results show that the often considered 1/R models are not compatible with the data. The results demonstrate that the background expansion alone can act as a good discriminator between modified gravity models when multiple data sets are used.

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Conjecture on the Physical Implications of the Scale Anomaly

Authors: Christopher T. Hill
Comments: Invited talk delivered at the Santa Fe Institute on the Occasion of the Celebration of the 75th Birthday of Murray Gell-Mann. July 23, 2005
Report-no: FERMILAB-CONF-05/482-T

Murray Gell-Mann, after co-inventing QCD, recognized the interplay of the scale anomaly, the renormalization group, and the origin of the strong scale, Lambda_{QCD}. I tell a story, then elaborate this concept, and for the sake of discussion, propose a conjecture that the physical world is scale invariant in the classical, \hbar -> 0, limit. This principle has implications for the dimensionality of space-time, the cosmological constant, the weak scale, and Planck scale.

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The Supernova Legacy Survey: Measurement of Omega_M, Omega_Lambda and w from the First Year Data Set

Authors: P. Astier, J. Guy, N. Regnault, R. Pain, E. Aubourg, D. Balam, S. Basa, R.G. Carlberg, S. Fabbro, D. Fouchez, I.M. Hook, D.A. Howell, H. Lafoux, J.D. Neill, N. Palanque-Delabrouille, K. Perrett, C.J. Pritchet, J. Rich, M. Sullivan, R. Taillet, G. Aldering, P. Antilogus, V. Arsenijevic, C. Balland, S. Baumont, J. Bronder, H. Courtois, R.S. Ellis, M. Filiol, A.C. Goncalves, A. Goobar, D. Guide, D. Hardin, V. Lusset, C. Lidman, R. McMahon, M. Mouchet, A. Mourao, S. Perlmutter, P. Ripoche, C. Tao, N. Walton
Comments: (The SNLS Collaboration) 24 pages, 13 figures, Accepted in A&A. Computer readable tables at this http URL

We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of the 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color light-curves measured using the MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands. Follow-up spectroscopy was performed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshift. With this data set, we have built a Hubble diagram extending to z=1, with all distance measurements involving at least two bands. Systematic uncertainties are evaluated making use of the multi-band photometry obtained at CFHT. Cosmological fits to this first year SNLS Hubble diagram give the following results : Omega_M = 0.263 +/- 0.042(stat) +/- 0.032(sys) for a flat LambdaCDM model; and w = -1.023 +/- 0.090(stat) +/- 0.054(sys) for a flat cosmology with constant equation of state w when combined with the constraint from the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations.

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New constraint on the cosmological background of relativistic particles

Authors: Steen Hannestad (University of Aarhus)
Comments: 9 pages, 3 figures

We have derived new bounds on the relativistic energy density in the Universe from cosmic microwave background (CMB), large scale structure (LSS), and type Ia supernova (SNI-a) observations. In terms of the effective number of neutrino species a bound of N_\nu = 4.2^{+1.2}_{-1.7} is derived at 95% confidence. This bound is significantly stronger than previous determinations, mainly due to inclusion of new CMB and SNI-a observations. The absence of a cosmological neutrino background (N_\nu = 0) is now excluded at 5.4 \sigma. The value of N_\nu is compatible with the value derived from big bang nucleosynthesis considerations, marking one of the most remarkable successes of the standard cosmological model. In terms of the cosmological helium abundance, the CMB, LSS, and SNI-a observations predict a value of 0.240 < Y < 0.281.

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Back-Reaction: A Cosmological Panacea

Authors: P. Martineau, R. Brandenberger
Comments: 6 pages

We present a solution to the dark energy problem in terms of the Effective Energy Momentum Tensor (EMT) of cosmological perturbations. The approach makes use of the gravitational back-reaction of long wavelength (super-Hubble) fluctuation modes on the background metric. Our results indicate that, following preheating, the energy density associated with back-reaction is sub-dominant and behaves as a tracker during the radiation era. At the onset of matter domination, however, the effects of back-reaction begin to grow relative to the matter density and the associated equation of state quickly approaches that of a cosmological constant. Using standard values for the preheating temperature and the amplitude of the inflaton following preheating, we show that this mechanism leads to a very natural explanation of dark energy. We comment on other recent attempts to explain the dark energy using back-reaction and their relation to our work.

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Caustics in Dark Matter Haloes

Authors: Roya Mohayaee, Stephane Colombi, Bernard Fort, Raphael Gavazzi, Sergei Shandarin, Jihad Touma
Comments: 6 pages, 3 figures, to appear in Proc. 21st IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005 (EAS Publications Series, G. Mamon, F. Combes, C. Deffayet, B. Fort eds

Caustics are formally singular structures, with infinite density, that form in collisionless media. The non-negligible velocity dispersion of dark matter particles renders their density finite. We evaluate the maximum density of the caustics within the framework of secondary infall model of formation of dark matter haloes. The result is then used to demonstrate that caustics can be probed by properly stacking the weak-lensing signal of about 600 haloes. CFHTLS accompanied by X-ray observations and the space-based experiments like SNAP or DUNE can provide us with the required statistics and hence a way of distinguishing between the viable dark matter particle candidate. The extension of our results to more realistic models including the effects of mergers of haloes is briefly outlined.

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Some Systematic Properties of Rotation Curves

Authors: Stacy McGaugh (University of Maryland)
Comments: 8 pages including 7 figures. Invited review for the 21st IAP Colloquium: Mass Profiles and Shapes of Cosmological Structures, eds. G. A. Mamon, F. Combes, C. Deffayet, B. Fort

The rotation curves of spiral galaxies obey strong scaling relations. These include the Tully-Fisher and baryonic Tully-Fisher relations, and the mass discrepancy--acceleration relation. These relations can be used to place constraints on the mass-to-light ratios of stars. Once the stellar mass is constrained, the distribution of dark matter follows. The shape of the dark matter distribution is consistent with the expectations of NFW halos exterior to 1 kpc, but the amplitude is wrong. This is presumably related to the long-standing problem of the normalization of the Tully-Fisher relation and may imply a downturn in the amplitude of the power spectrum at small scales. More fundamentally, the persistent success of MOND remains a troubling fact.

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Contraction of Dark Matter Halos in Response to Condensation of Baryons

Authors: Oleg Y. Gnedin
Comments: 6 pages, invited talk at the XXI IAP Colloquium, Paris, July 2005

The cooling of baryons in the centers of dark matter halos leads to a more concentrated dark matter distribution. This effect has traditionally been calculated using the model of adiabatic contraction, which assumes spherical symmetry, while in hierarchical formation scenarios halos grow via multiple violent mergers. We test the adiabatic contraction model in high-resolution cosmological simulations and find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile compared to the case without cooling. Although the standard model systematically overpredicts the increase of dark matter density, a simple modification of the assumed invariant from M(r)r to M(<r>)r, where <r> is the orbit-averaged particle position, reproduces the simulated profiles within 10%.

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Message in the Sky

Authors: S. Hsu, A. Zee
Comments: 3 pages, revtex
Subj-class: Popular Physics

We argue that the cosmic microwave background (CMB) provides a stupendous opportunity for the Creator of our universe (assuming one exists) to have sent a message to its occupants, using known physics. The medium for the message is unique. We elaborate on this observation, noting that it requires only careful adjustment of the fundamental Lagrangian, but no direct intervention in the subsequent evolution of the universe.

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Inflation models and observation

Authors: David H. Lyth, Laila Alabidi
Comments: 13 pages LaTeX, 10 figures

We consider small-field models which invoke the usual framework for the effective field theory, and large-field models which go beyond that. Present and future possibilities for discriminating between the models are assessed, on the assumption that the primordial curvature perturbation is generated during inflation. The prediction for the trispectrum in a generic multi-field inflation model is given for the first time.

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Where are the missing cosmic metals ?

Authors: A. Ferrara (1), E. Scannapieco (2), J. Bergeron (3) ((1) SISSA/ISAS, Trieste, Italy (2) Kavli Institute, Santa Barbara, USA (3) IAP, Paris, France)
Comments: 9 pages, 2 figures, ApJ Letters, in press

The majority of the heavy elements produced by stars 2 billion years after the Big Bang (redshift z~3) are presently undetected at those epochs. We propose a solution to this cosmic `missing metals' problem in which such elements are stored in gaseous halos produced by supernova explosions around star-forming galaxies. By using data from the ESO/VLT Large Program, we find that:(i) only 5%-9% of the produced metals reside in the cold phase, the rest being found in the hot (log T=5.8-6.4) phase; (ii) 1%-6% (3%-30%) of the observed CIV (OVI) is in the hot phase. We conclude that at z~3 more than 90% of the metals produced during the star forming history can be placed in a hot phase of the IGM, without violating any observational constraint. The observed galaxy mass-metallicity relation, and the intergalactic medium and intracluster medium metallicity evolution are also naturally explained by this hypothesis.

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Detecting Reionization in the Star Formation Histories of High-Redshift Galaxies

Authors: Rennan Barkana (Tel Aviv U.), Abraham Loeb (Harvard U.)
Comments: 7 pages, 5 figures, MNRAS, submitted

The reionization of cosmic hydrogen, left over from the big bang, increased its temperature to >~ 1.e4 K. This photo-heating resulted in an increase of the minimum mass of galaxies and hence a suppression of the cosmic star formation rate. The affected population of dwarf galaxies included the progenitors of massive galaxies that formed later. We show that a massive galaxy at a redshift z >~ 6 should show a double-peaked star formation history marked by a clear break. This break reflects the suppression signature from reionization of the region in which the galaxy was assembled. Since massive galaxies originate in overdense regions where cosmic evolution is accelerated, their environment reionizes earlier than the rest of the universe. For a galaxy of ~ 1.e12 M_solar in stars at a redshift of z ~ 6.5, the star formation rate should typically be suppressed at a redshift z >~ 10 since the rest of the universe is known to have reionized by z >~ 6.5. Indeed, this is inferred to be the case for HUDF-JD2, a massive galaxy which is potentially at z ~ 6.5 but is inferred to have formed the bulk of its 3.e11 M_solar in stars at z >~ 9.

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