arXiv subjects

Tirthabir Biswas

Publications and source records attributed to Tirthabir Biswas.

At least 37 records · Page 2Linked to original sources

arXiv2012

Could our Universe have begun with Negative Lambda?

In this paper we present an informal description of the Cyclic Inflation scenario which allows our universe to "start" with a negative potential energy, inflate, and then gracefully exit to a positive potential energy universe. We discuss how this scenario fares in comparison with the standard inflationary paradigm with respect to the classic cosmological puzzles associated with the horizon, flatness and isotropy of our current universe. We also discuss some of the most debilitating problems of cyclic cosmologies, Tolman's entropy problem, and the problem with the overproduction of blackholes. We also sketch the calculation of the primordial spectrum in these models and possible observable signatures. We end with a special focus on the exit mechanism where the universe can transition from the negative to a positive potential region. The treatise is based on an ongoing collaboration between the authors and closely follows conference presentations given on the subject by TB.

astro-ph.CO↗

arXiv2011

Towards singularity and ghost free theories of gravity

We present the most general covariant ghost-free gravitational action in a Minkowski vacuum. Apart from the much studied f(R) models, this includes a large class of non-local actions with improved UV behavior, which nevertheless recover Einstein's general relativity in the IR.

gr-qc↗

arXiv2010

Towards a Resolution of the Cosmological Singularity in Non-local Higher Derivative Theories of Gravity

One of the greatest problems of standard cosmology is the Big Bang singularity. Previously it has been shown that non-local ghostfree higher-derivative modifications of Einstein gravity in the ultra-violet regime can admit non-singular bouncing solutions. In this paper we study in more details the dynamical properties of the equations of motion for these theories of gravity in presence of positive and negative cosmological constants and radiation. We find stable inflationary attractor solutions in the presence of a positive cosmological constant which renders inflation {\it geodesically complete}, while in the presence of a negative cosmological constant a cyclic universe emerges. We also provide an algorithm for tracking the super-Hubble perturbations during the bounce and show that the bouncing solutions are free from any perturbative instability.

hep-th↗

arXiv2010

Testing the Void against Cosmological data: fitting CMB, BAO, SN and H0

In this paper, instead of invoking Dark Energy, we try and fit various cosmological observations with a large Gpc scale under-dense region (Void) which is modeled by a Lemaitre-Tolman-Bondi metric that at large distances becomes a homogeneous FLRW metric. We improve on previous analyses by allowing for nonzero overall curvature, accurately computing the distance to the last-scattering surface and the observed scale of the Baryon Acoustic peaks, and investigating important effects that could arise from having nontrivial Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE), Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a nonzero overall curvature drastically improves the goodness of fit of the Void model, bringing it very close to that of a homogeneous universe containing Dark Energy, while by varying the profile one can increase the value of the local Hubble parameter which has been a challenge for these models. We also try to gauge how well our model can fit the large-scale-structure data, but a comprehensive analysis will require the knowledge of perturbations on LTB metrics. The model is consistent with the CMB dipole if the observer is about 15 Mpc off the centre of the Void. Remarkably, such an off-center position may be able to account for the recent anomalous measurements of a large bulk flow from kSZ data. Finally we provide several analytical approximations in different regimes for the LTB metric, and a numerical module for CosmoMC, thus allowing for a MCMC exploration of the full parameter space.

astro-ph.CO↗

arXiv2010

Wiggles in the cosmic microwave background radiation: echoes from non-singular cyclic-inflation

In this paper we consider a unique model of inflation where the universe undergoes rapid asymmetric oscillations, each cycle lasting millions of Planck time. Over many-many cycles the space-time expands to mimic the standard inflationary scenario. Moreover, these rapid oscillations leave a distinctive periodic signature in ln(k) in the primordial power spectrum, where k denotes the comoving scale. The best fit parameters of the cyclic-inflation model provides a very good fit to the 7-year WMAP data.

hep-th↗

arXiv2010

Thermodynamics and Cosmological Constant of Non-Local Field Theories from p-Adic Strings

We develop the thermodynamics of field theories characterized by non-local propagators. We analyze the partition function and main thermodynamic properties arising from perturbative thermal loops. We focus on the p-adic models associated with the tachyon phenomenology in string theories. We reproduce well known features of these theories, but also obtain many new results. In particular, we explain how to maintain consistency of such non-local theories by avoiding the appearance of ghosts at finite temperature. As a consequence of this fact, the vacuum energy in p-adic theories becomes positive. It is also hierarchically suppressed, and we explore the parameter space where it is consistent with the observed value of the cosmological constant.

hep-th↗

arXiv2010

Finite Temperature Solitons in Non-Local Field Theories from p-Adic Strings

Non-local field theories which arise from p-adic string theories have vacuum soliton solutions. We find the soliton solutions at finite temperature. These solutions become important for the partition function when the temperature exceeds m_s/g_o^2 where m_s is the string mass scale and g_o is the open string coupling.

hep-th↗

arXiv2010

Cosmological Bardeen-Cooper-Schrieffer condensate as dark energy

We argue that the occurrence of late-time acceleration can conveniently be described by first-order general relativity covariantly coupled to fermions. Dark energy arises as a Bardeen-Cooper-Schrieffer condensate of fermions which forms in the early universe. At late times, the gap and chemical potential evolve to have an equation of state with effective negative pressure, thus naturally leading to acceleration.

astro-ph.CO↗

arXiv2009

Thermal Duality and Hagedorn Transition from p-adic Strings

We develop the finite temperature theory of p-adic string models. We find that the thermal properties of these non-local field theories can be interpreted either as contributions of standard thermal modes with energies proportional to the temperature, or inverse thermal modes with energies proportional to the inverse of the temperature, leading to a "thermal duality" at leading order (genus one) analogous to the well known T-duality of string theory. The p-adic strings also recover the asymptotic limits (high and low temperature) for arbitrary genus that purely stringy calculations have yielded. We also discuss our findings surrounding the nature of the Hagedorn transition.

hep-th↗

arXiv2009

Are we seeing the beginnings of Inflation?

Phantom Cosmology provides an unique opportunity to "connect" the phantom driven (low en- ergy meV scale) dark energy phase to the (high energy GUT scale) inflationary era. This is possible because the energy density increases in phantom cosmology. We present a concrete model where the energy density, but not the scale factor, cycles through phases of standard radiation/matter domi- nation followed by dark energy/inflationary phases, and the pattern repeating itself. An interesting feature of the model is that once we include interactions between the "phantom fluid" and ordinary matter, the Big rip singularity is avoided with the phantom phase naturally giving way to a near exponential inflationary expansion.

astro-ph.CO↗

arXiv2009

Emergence of a Cyclic Universe from the Hagedorn Soup

One of the challenges of constructing a successful cyclic universe scenario is to be able to incorporate the second law of thermodynamics which typically leads to Tolman's problem of ever shrinking cycles. In this paper we construct a non-singular toy model where as the cycles shrink in the past they also spend more and more time in the entropy conserving Hagedorn phase. Thus in such a scenario the entropy asymptotes to a finite non-zero constant in the infinite past. The universe ``emerges'' from a small (string size) geodesically complete quasi-periodic space-time. This paradigm also naturally addresses some of the classic puzzles of Big Bang cosmology, such as the largeness, horizon and flatness problems.

hep-th↗

arXiv2009

The cosmological BCS mechanism and the Big Bang Singularity

We provide a novel mechanism that resolves the Big Bang Singularity present in FRW space-times without the need for ghost fields. Building on the fact that a four-fermion interaction arises in General Relativity when fermions are covariantly coupled, we show that at early times the decrease in scale factor enhances the correlation between pairs of fermions. This enhancement leads to a BCS-like condensation of the fermions and opens a gap dynamically driving the Hubble parameter $H$ to zero and results in a non-singular bounce, at least in some special cases.

hep-th↗

arXiv2009

Inflation with a Negative Cosmological Constant

We find a unique way of realizing inflation through cyclic phases in an universe with negative vacuum energy. According to the second law of thermodynamics entropy monotonically increases from cycle to cycle, typically by a constant factor. This means that the scale factor at the same energy density in consecutive cycles also increases by a constant factor. If the time period of the oscillations remain approximately constant then this leads to an ``over all'' exponential growth of the scale factor, mimicking inflation. A graceful exit from this inflationary phase is possible as a dynamical scalar field can take us from the negative to a positive energy vacuum during the last contracting phase.

hep-th↗

arXiv2009

Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae

It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an "apparent acceleration", even when any form of dark energy is absent. In this paper, we present a "Minimal Void" scenario, i.e. a "void" with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only explain the supernovae data, but also be consistent with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter, and also point out possible observable signatures.

astro-ph↗

arXiv2008

Cyclic Inflation

We present an inflationary model that is geodesically complete and does not suffer from the transplanckian problem. In most inflationary models, massless (conformal) scalar field fluctuations in a deSitter background gives rise to a scale-invariant spectrum. In this work, we realize scale invariant perturbations from thermal fluctuations in (conformal) radiation during a radiation dominated contraction era prior to inflation. As the modes exit the Hubble radius during the contraction phase, scale invariant fluctuations are indeed generated. After many cycles, we enter into a power-law inflationary phase, that stretches the modes produced in the previous contraction phase to scales that we observe today.

hep-th↗

arXiv2008

On the Transfer of Adiabatic Fluctuations through a Nonsingular Cosmological Bounce

We study the transfer of cosmological perturbations through a nonsingular cosmological bounce in a special model in which the parameters of the bounce and the equation of state of matter are chosen such as to allow for an exact calculation of the evolution of the fluctuations. We find that the growing mode of the metric fluctuations in the contracting phase goes over into the growing mode in the expanding phase, a result which is different from what is obtained in analyses in which fluctuations are matched at a singular hypersurface. Consequences for Ekpyrotic cosmology are discussed in a limit when the equation of state of a fluid becomes large.

hep-th↗

arXiv2007

p-adic Inflation

We construct approximate inflationary solutions rolling away from the unstable maximum of p-adic string theory, a nonlocal theory with derivatives of all orders. Novel features include the existence of slow-roll solutions even when the slow-roll parameters, as usually defined, are much greater than unity, as well as the need for the Hubble parameter to exceed the string mass scale m_s. We show that the theory can be compatible with CMB observations if g_s / \sqrt{p} ~ 10^{-7}, where g_s is the string coupling, and if m_s < 10^{-6} M_p. A red-tilted spectrum is predicted, and the scalar-to-tensor ratio is bounded from above as r < 0.006. The p-adic theory is shown to have identical inflationary predictions to a local theory with superPlanckian parameter values, but with the advantage that the p-adic theory is ultraviolet complete.

hep-th↗

arXiv2007

"Swiss-Cheese" Inhomogeneous Cosmology & the Dark Energy Problem

We study an exact swiss-cheese model of the Universe, where inhomogeneous LTB patches are embedded in a flat FLRW background, in order to see how observations of distant sources are affected. We find negligible integrated effect, suppressed by (L/R_{H})^3 (where L is the size of one patch, and R_{H} is the Hubble radius), both perturbatively and non-perturbatively. We disentangle this effect from the Doppler term (which is much larger and has been used recently \cite{BMN} to try to fit the SN curve without dark energy) by making contact with cosmological perturbation theory.

astro-ph↗