Motohashi, Hayato, Starobinsky, Alexei A., Yokoyama, Jun'ichi
For a class of viable cosmological models in $f(R)$ gravity which deviation from the Einstein gravity decreases as a inverse power law of the Ricci scalar $R$ for large $R$, an analytic solution for...
Is cosmic acceleration slowing down? (2009)
Shafieloo, Arman, Sahni, Varun, Starobinsky, Alexei A.
We investigate the course of cosmic expansion in its `recent past' using the Constitution SN Ia sample (which includes CfA data at low redshifts), jointly with signatures of baryon acoustic...
Once again on thin-shell wormholes in scalar-tensor gravity (2009)
Bronnikov, Kirill A., Starobinsky, Alexei A.
It is proved that all thin-shell wormholes built from two identical regions of vacuum static, spherically symmetric space-times have a negative shell surface energy density in any scalar-tensor...
Reconstructing Cosmological Matter Perturbations using Standard Candles and Rulers (2008)
Alam, Ujjaini, Sahni, Varun, Starobinsky, Alexei A.
For a large class of dark energy (DE) models, for which the effective gravitational constant is a constant and there is no direct exchange of energy between DE and dark matter (DM), knowledge of the...
Two new diagnostics of dark energy (2008)
Sahni, Varun, Shafieloo, Arman, Starobinsky, Alexei A.
We introduce two new diagnostics of dark energy (DE). The first, Om, is a combination of the Hubble parameter and the cosmological redshift and provides a "null test" of dark energy being a...
Is a step in the primordial spectral index favored by CMB data ? (2008)
Joy, Minu, Shafieloo, Arman, Sahni, Varun, Starobinsky, Alexei A.
A rapid second order phase transition during inflation can result in a universal local feature in the spectrum of primordial perturbations[1]. This feature is characterized by a step in the spectral...
Jaan Einasto, Maret Einasto, Erik Tago, Alexei A. Starobinsky, Volker Muller, Alexander Knebe, ...
The primordial spectrum
New Universal Local Feature in the Inflationary Perturbation Spectrum (2007)
Joy, Minu, Sahni, Varun, Starobinsky, Alexei A.
A model is developed in which the inflaton potential experiences a sudden small change in its second derivative (the effective mass of the inflaton). An exact treatment demonstrates that the...
What do WMAP and SDSS really tell about inflation? (2007)
Lesgourgues, Julien, Starobinsky, Alexei A., Valkenburg, Wessel
We derive new constraints on the Hubble function H(phi) and subsequently on the inflationary potential V(phi) from WMAP 3-year data combined with the Sloan Luminous Red Galaxy survey (SDSS-LRG),...
Disappearing cosmological constant in f(R) gravity (2007)
For higher-derivative f(R) gravity where R is the Ricci scalar, a class of models is proposed which produce viable cosmology different from the LambdaCDM one at recent times and satisfy cosmological,...
Exploring the Properties of Dark Energy Using Type Ia Supernovae and Other Datasets (2006)
Alam, Ujjaini, Sahni, Varun, Starobinsky, Alexei A.
We reconstruct dark energy properties from two complementary supernova datasets -- the newly released Gold+HST sample and SNLS. The results obtained are consistent with standard $\Lambda$CDM model...
No realistic wormholes from ghost-free scalar-tensor phantom dark energy (2006)
Bronnikov, Kirill A., Starobinsky, Alexei A.
It is proved that no wormholes can be formed in viable scalar-tensor models of dark energy admitting its phantom-like ($w < -1$) behaviour in cosmology, even in the presence of electric or magnetic...
Scalar-Tensor Models of Normal and Phantom Dark Energy (2006)
Gannouji, Radouane, Polarski, David, Ranquet, Andre, Starobinsky, Alexei A.
We consider the viability of dark energy (DE) models in the framework of the scalar-tensor theory of gravity, including the possibility to have a phantom DE at small redshifts $z$ as admitted by...
Inflaton field potential producing the exactly flat spectrum of adiabatic perturbations (2005)
Presented is the exact solution of the problem of finding a potential of an inflaton scalar field for which adiabatic perturbations generated during a de Sitter (inflationary) stage in the early...
Smoothing Supernova Data to Reconstruct the Expansion History of the Universe and its Age (2005)
Shafieloo, Arman, Alam, Ujjaini, Sahni, Varun, Starobinsky, Alexei A.
We propose a non-parametric method of smoothing supernova data over redshift using a Gaussian kernel in order to reconstruct important cosmological quantities including H(z) and w(z) in a model...
Cosmological perturbations from multi-field inflation in generalized Einstein theories (2001)
Starobinsky, Alexei A., Tsujikawa, Shinji, Yokoyama, Jun'ichi
We study cosmological perturbations generated from quantum fluctuations in multi-field inflationary scenarios in generalized Einstein theories, taking both adiabatic and isocurvature modes into...
Sine-Gordon Parametric Resonance (1998)
Greene, Patrick B., Kofman, Lev, Starobinsky, Alexei A.
We consider the instability of fluctuations in an oscillating scalar field which obeys the Sine-Gordon equation. We present simple closed-form analytic solutions describing the parametric resonance...
On the Phase-Space Volume of Primordial Cosmological Perturbations (1998)
Lesgourgues, Julien, Polarski, David, Starobinsky, Alexei A.
We show how to determine the typical phase space volume $\Gamma$ for primordial gravitational waves produced during an inflationary stage, which is invariant under squeezing. An expression for...
The Coherence of Primordial Fluctuations Produced During Inflation (1998)
Kiefer, Claus, Lesgourgues, Julien, Polarski, David, Starobinsky, Alexei A.
The behaviour of quantum metric perturbations produced during inflation is considered at the stage after the second Hubble radius crossing. It is shown that the classical correlation between...
Constraining topology with the CMB (1997)
Smoot, George F., Starobinsky, Alexei A.
We present a new data analysis method to study rectangular T^3 ``small universes" with one or two of its dimensions significantly smaller than the present horizon (which we refer to as T^1- and...
Quantum-to-classical Transition of Cosmological Perturbations for Non-vacuum Initial States (1996)
Lesgourgues, Julien, Polarski, David, Starobinsky, Alexei A.
Transition from quantum to semiclassical behaviour and loss of quantum coherence for inhomogeneous perturbations generated from a non-vacuum initial state in the early Universe is considered in the...
Can the lack of symmetry in the COBE/DMR maps constrain the topology of the universe? (1995)
Smoot, George F., Starobinsky, Alexei A.
Although the cubic T^3 "small universe" has been ruled out by COBE/DMR results as an interesting cosmological model, we still have the possibility of living in a universe with a more anisotropic...
Non-Thermal Phase Transitions after Inflation (1995)
Kofman, Lev, Linde, Andrei, Starobinsky, Alexei A.
At the first stage of reheating after inflation, parametric resonance may rapidly transfer most of the energy of an inflaton field $\phi$ to the energy of other bosons. We show that quantum...
Structure of Primordial Gravitational Waves Spectrum in a Double Inflationary Model (1995)
Polarski, David, Starobinsky, Alexei A.
The gravitational waves (GW) background generated in a double inflationary model, with two scalar fields mutually interacting through gravity only, and its relative contribution $T/S$ to large-angle...
Semiclassicality and Decoherence of Cosmological Perturbations (1995)
Polarski, David, Starobinsky, Alexei A.
Transition to the semiclassical behaviour and the decoherence process for inhomogeneous perturbations generated from the vacuum state during an inflationary stage in the early Universe are considered...
Density fluctuations in Brans-Dicke inflation (1995)
Starobinsky, Alexei A., Yokoyama, Jun'ichi
Spectrum of density perturbations in the Universe generated from quantum-gravitational fluctuations in slow-roll-over inflationary scenarios with the Brans-Dicke gravity is calculated. It is shown...
Munshi, Dipak, Souradeep, Tarun, Starobinsky, Alexei A.
Skewness of temperature fluctuations of the cosmic microwave background (CMB) produced by initially Gaussian adiabatic perturbations with the flat (Harrison-Zeldovich) spectrum, which arises due to...
Equilibrium State of a Massless Self-Interacting Scalar Field in the De Sitter Background (1994)
Starobinsky, Alexei A., Yokoyama, Jun'ichi
Behaviour of a weekly self-interacting scalar field with a small mass in the de Sitter background is investigated using the stochastic approach (including the case of a double-well interaction...
Munshi, Dipak, Sahni, Varun, Starobinsky, Alexei A.
We compare different nonlinear approximations to gravitational clustering in the weakly nonlinear regime, using as a comparative statistic the evolution of non-Gaussianity which can be characterised...