KamLAND has measured the flux of nu;(e)'s from distant nuclear reactors. We find fewer nu;(e) events than expected from standard assumptions about nu;(e) propagation at the 99.95% C.L. In a 162 ton.yr exposure the ratio of the observed inverse beta-decay events to the expected number without nu;(e) disappearance is 0.611+/-0.085(stat)+/-0.041(syst) for nu;(e) energies >3.4 MeV. In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrino problem except for the "large mixing angle" region are excluded.

First Results from KamLAND: Evidence for Reactor Anti-Neutrino Disappearance

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 multiband photometry obtained at CFHT. Cosmological fits to this first year SNLS Hubble diagram give the following results: {Omega}{sub M} = 0.263 {+-} 0.042 (stat) {+-} 0.032 (sys) for a flat {Lambda}CDM 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.

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

Exact interior and exterior solutions to Einstein's field equations are derived for vacuum strings. The exterior solution for a uniform density vacuum string corresponds to a conical space while the interior solution is that of a spherical cap. For Mu equals 0-1/4 the external metric is ds-squared = -dt-squared + dr-squared + (1-4 Mu)-squared r-squared dphi-squared + dz-squared, where Mu is the mass per unit length in the string in Planck masses per Planck length. A maximum mass per unit length for a string is 6.73 x 10 to the 27th g/cm. It is shown that strings cause temperature fluctuations in the cosmic microwave background and produce equal brightness double QSO images separated by up to several minutes of arc. Formulae for lensing probabilities, image splittings, and time delays are derived for strings in a realistic cosmological setting. String searches using ST, the VLA, and the COBE satellite are discussed.

Gravitational Lensing Effects of Vacuum Strings: Exact Solutions

To date, there has been no general way of determining if the Copernican principle--that we live at a typical position in the Universe--is in fact a valid assumption, significantly weakening the foundations of cosmology as a scientific endeavor. Here we present an observational test for the Copernican assumption which can be automatically implemented while we search for dark energy in the coming decade. Our test is entirely independent of any model for dark energy or theory of gravity and thereby represents a model-independent test of the Copernican principle.

A General Test of the Copernican Principle

Motivated by recent attempts to solve the cosmological constant problem, we examine the observational consequences of a vacuum energy which decays in time. In both radiation and matter dominated eras, the ratio of the vacuum to the total energy density of the universe must be small. Although the vacuum cannot provide the “missing mass” required to close the universe today, its presence earlier in the history of the universe could have important consequences. Element abundances from primordial nucleosynthesis require the ratio x = ϱ vac /( ϱ vac + ϱ rad ) ⩽ 0.1 of neutrino (or equivalent light) species to exceed N ν > 4, a case ruled out in the standard cosmological model. If the vacuum decays into low energy photons, the lack of observed spectral distortions in the microwave background gives tighter bounds, x −4 . In the matter-dominated era, the presence of a vacuum term may allow more time for growth of protogalactic perturbations.

Cosmology with Decaying Vacuum Energy

We perform a study of cosmic evolution with an equation of state parameter ω(t) = ω 0 +ω 1 (tH/H˙ ) by selecting a phenomenological Λ model of the form, Λ˙ ∼ H 3 . This simpleproposition explains both linearly expanding and inﬂation ary Universes with a single set ofequations. We notice that the inﬂation leads to a scaling in t he equation of state parameter,ω(t), and hence in equation of state. In this approach, one of its two parameters have been pinpointed and the other have been delineated. It has been possible to show a connection betweendark energy and Higgs-Boson.Key words: gravitation - cosmological parameters - cosmology: theory- early Universe. 1 INTRODUCTIONCosmological research is mainly concerned with time (and in somecases space as well) evolution of various physical parameters likescale factor, Hubble parameter, matter-energy density etc. Alongwith these parameters, in recent years a new physical entity Λ hasresurrected in the foreground of cosmology. In fact, Λ has becomean essential part of the ﬁeld equations of Einstein after som e ob-servational results (Riess et al. 1998; Perlmutter et al. 1999) indi-cated towards an accelerating Universe. It is believed by most ofthe physicists that the cosmological parameter Λ is responsible fordriving the present acceleration because it can exert negative pres-sure. Moreover, due to some ﬁne-tuning problem (known as cos-mological constant problem), Λ is regarded as a variable quantityrather than a constant.Now, in order to specify exact time-dependence of the un-known physical quantities including Λ, one has to take recourse ofa relationship between cosmic pressure p and matter-energy den-sity ρ involving the equation of state parameter ω. Mathemati-cally speaking, one variable quantity can depend on the productof two other variable quantities. So, one may construct ω as a func-tion of time, red-shift or scale factor (Chervon & Zhuravlev2000;Zhuravlev 2001; Peebles & Ratra 2003). In fact, values of ω at dif-ferent stages of cosmic evolution suggest that it may evolve withtime. As an instance, for the present pressure-less Universe, thevalue of ω is considered as zero, whereas its value was 1/3 in theearly radiation dominated Universe. However, it is convenient toconsider ω as a constant quantity because observational data canhardly distinguish between a varying and a constant equation of

The dark energy equation of state

Two phenomenological models of Λ, viz. and , are studied under the assumption that G is a time-variable parameter. Both models show that G is inversely proportional to time, as suggested earlier by others, including Dirac. The models considered here can be matched with observational results by properly tuning the parameters of the models. Our analysis shows that the model corresponds to a repulsive situation and hence correlates with the present status of the accelerating Universe. The other model, , is in general attractive in nature. Moreover, it is seen that due to the combined effect of time-variable Λ and G the Universe evolved with acceleration as well as deceleration. Deceleration indicates a "big crunch".

Dark energy models with a time-dependent gravitational constant

We perform a study of cosmic evolution with an equation of state parameter $\omega(t)=\omega_0+\omega_1(t\dot H/H)$ by selecting a phenomenological $\Lambda$ model of the form, $\dot\Lambda\sim H^3$. This simple proposition explains both linearly expanding and inflationary Universes with a single set of equations. We notice that the inflation leads to a scaling in the equation of state parameter, $\omega(t)$, and hence in equation of state. In this approach, one of its two parameters have been pin pointed and the other have been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.

Two phenomenological variable Λ models, viz.Λ ~ (ȧ/a)2 and Λ ~ ρ, have been studied under the assumption that the equation of state parameter ω is a function of time. The selected Λ models are found to be equivalent both in four and five dimensions. The possibility of signature flip of the deceleration parameter is also shown.

Dark energy models with variable equation of state parameter

Two phenomenological models of $\Lambda$, viz. $\Lambda \sim (\dot a/a)^2$ and $\Lambda \sim \ddot a/a$ are studied under the assumption that $G$ is a time-variable parameter. Both models show that $G$ is inversely proportional to time as suggested earlier by others including Dirac. The models considered here can be matched with observational results by properly tuning the parameters of the models. Our analysis shows that $\Lambda \sim \ddot a/a$ model corresponds to a repulsive situation and hence correlates with the present status of the accelerating Universe. The other model $\Lambda \sim (\dot a/a)^2$ is, in general, attractive in nature. Moreover, it is seen that due to the combined effect of time-variable $\Lambda$ and $G$ the Universe evolved with acceleration as well as deceleration. This later one indicates a Big Crunch.

Utpal Mukhopadhyay

Papers

The dark energy equation of state

Dark energy models with a time-dependent gravitational constant

The dark energy equation of state

Dark energy models with variable equation of state parameter

Dark energy models with time-dependent gravitational constant