SN 2005hj: Evidence for Two Classes of Normal-Bright SNe Ia and Implications for Cosmology

HET Optical spectra covering the evolution from about 6 days before to about 5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the "Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxy shows HII region lines at redshift of z=0.0574, which puts the peak unfiltered absolute magnitude at a somewhat over-luminous -19.6. The spectra show weak and narrow SiII lines, and for a period of at least 10 days beginning around maximum light these profiles do not change in width or depth and they indicate a constant expansion velocity of ~10,600 km/s. We analyzed the observations based on detailed radiation dynamical models in the literature. Whereas delayed detonation and deflagration models have been used to explain the majority of SNe Ia, they do not predict a long velocity plateau in the SiII minimum with an unvarying line profile. Pulsating delayed detonations and merger scenarios form shell-like density structures with properties mostly related to the mass of the shell, M_shell, and we discuss how these models may explain the observed SiII line evolution; however, these models are based on spherical calculations and other possibilities may exist. SN 2005hj is consistent with respect to the onset, duration, and velocity of the plateau, the peak luminosity and, within the uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun. Our analysis suggests a distinct class of events hidden within the Branch-normal SNe Ia. If the predicted relations between observables are confirmed, they may provide a way to separate these two groups. We discuss the implications of two distinct progenitor classes on cosmological studies employing SNe Ia, including possible differences in the peak luminosity to light curve width relation.Comment: ApJ accepted, 31 page

Scale Dependence of Dark Energy Antigravity

We investigate the effects of negative pressure induced by dark energy (cosmological constant or quintessence) on the dynamics at various astrophysical scales. Negative pressure induces a repulsive term (antigravity) in Newton's law which dominates on large scales. Assuming a value of the cosmological constant consistent with the recent SnIa data we determine the critical scale $r_c$ beyond which antigravity dominates the dynamics ($r_c \sim 1Mpc$) and discuss some of the dynamical effects implied. We show that dynamically induced mass estimates on the scale of the Local Group and beyond are significantly modified due to negative pressure. We also briefly discuss possible dynamical tests (eg effects on local Hubble flow) that can be applied on relatively small scales (a few $Mpc$) to determine the density and equation of state of dark energy.Comment: Contributed talk at the 2nd Hellenic Cosmology Workshop at NOA (Athens) Jan. 2001.To appear in the proceedings. Based on work done in collaboration with M. Axenides and E. Florato

Broad Brush Cosmos

An innovative approach to map the large-scale structure in the Universe sidesteps the conventional need to observe millions of galaxies individually, and holds promise for both astrophysical and cosmological studies.Comment: Invited Nature 'News and Views' Commentary on Chang et al. 2010, Nature, 466, 463; 6pages, 1 figur

The Friedmann-Lemaitre-Robertson-Walker Big Bang singularities are well behaved

We show that the Big Bang singularity of the Friedmann-Lemaitre-Robertson-Walker model does not raise major problems to General Relativity. We prove a theorem showing that the Einstein equation can be written in a non-singular form, which allows the extension of the spacetime before the Big Bang. The physical interpretation of the fields used is discussed. These results follow from our research on singular semi-Riemannian geometry and singular General Relativity.Comment: 10 pages, 5 figure

F(T) gravity and k-essence

Modified teleparallel gravity theory with the torsion scalar have recently gained a lot of attention as a possible explanation of dark energy. We perform a thorough reconstruction analysis on the so-called $F(T)$ models, where $F(T)$ is some general function of the torsion term, and derive conditions for the equivalence between of $F(T)$ models with purely kinetic k-essence. We present a new class models of $F(T)$-gravity and k-essence. We also proposed some new models of generalized gases and knot universes as well as some generalizations of $F(T)$ gravity.Comment: 25 page

Cosmological solutions with massive gravitons in the bigravity theory

We present solutions describing homogeneous and isotropic cosmologies in the massive gravity theory with two dynamical metrics recently proposed in arXiv:1109.3515 and claimed to be ghost free. These solutions can be spatially open, closed, or flat, and at early times they are sourced by the perfect fluid, while the graviton mass typically manifests itself at late times by giving rise to a cosmological term. In addition, there are also exotic solutions, for which already at early times, when the matter density is high, the contribution of the graviton mass to the energy density is negative and large enough to screen that of the matter contribution. The total energy can then be negative, which may result in removing the initial singularity. For special parameter values there are also solutions for which the two metrics effectively decouple and evolve independently of each other. In the limit where one of the gravitational coupling constant vanishes, such special solutions reduce to those found in arXiv:1107.5504 within the theory where one of the metrics is flat.Comment: 21 pages, 4 figure

Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications

The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda. We report here the first such measurements, with our discovery of a Type Ia supernova (SN 1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the most distant spectroscopically confirmed supernova. Over two months of photometry of SN 1997ap with the Hubble Space Telescope and ground-based telescopes, when combined with previous measurements of nearer SNe Ia, suggests that we may live in a low mass-density universe. Further supernovae at comparable distances are currently scheduled for ground and space-based observations.Comment: 12 pages and 4 figures (figure 4 is repeated in color and black and white) Nature, scheduled for publication in the 1 January, 1998 issue. Also available at http://www-supernova.lbl.go