Research Papers (TP)
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Item Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks(Astrophys Space Sci, 2008-04-21) Bazarghan, Mahdi; Gupta, RanjanAutomated techniques have been developed to automate the process of classification of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classification of about 5000 SDSS spectra into 158 spectral type of aItem A 3D Automated Classification Scheme for the TAUVEX data pipeline(Mon. Not. R. Astron. Soc, 2007-01-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; etIn order to develop a pipeline for automated classification of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an artificial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250°A to 3220°A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classified 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two different simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classified the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classification scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.Item Cosmology with tachyon field as dark energy(American Physical Society, 2003-03-14) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with nonrelativistic matter and radiation, concentrating on the inverse square potential and the exponential potential for the tachyonic scalar field. A distinguishing feature of these models ~compared to other cosmological models! is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t)}t2/3 as t!`. This eliminates the future event horizon present in cold dark matter models with a cosmological constant (LCDM) and is an attractive feature from the string theory perspective. A comparison with supernova type Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts which are also consistent with the requirements of structure formation. They do require fine-tuning of parameters but not any more than in the case of L CDM models or quintessence models.Item Cosmological parameters from supernova observations: A critical comparison of three data sets(EDP Sciences, 2004-09-15) Choudhury, T. Roy; Padmanabhan, T.We extend our previous analysis of cosmological supernova type Ia data (Padmanabhan & Choudhury 2003) to include three recent compilation of data sets. Our analysis ignores the possible correlations and systematic effects present in the data and concentrates mostly on some key theoretical issues. Among the three data sets, the first set consists of 194 points obtained from various observations while the second discards some of the points from the first one because of large uncertainties and thus consists of 142 points. The third data set is obtained from the second by adding the latest 14 points observed through HST. A careful comparison of these different data sets help us to draw the following conclusions: (i) All the three data sets strongly rule out non-accelerating models. Interestingly, the first and the second data sets favour a closed universe; if Ωtot ≡ Ωm + ΩΛ, then the probability of obtaining models with Ωtot > 1 is >∼0.97. Hence these data sets are in mild disagreement with the “concordance” flat model. However, this disagreement is reduced (the probability of obtaining models with Ωtot > 1 being ≈0.9) for the third data set, which includes the most recent points observed by HST around 1 < z < 1.6. (ii) When the first data set is divided into two separate subsets consisting of low (z < 0.34) and high (z > 0.34) redshift supernova, it turns out that these two subsets, individually, admit non-accelerating models with zero dark energy because of different magnitude zero-point values for the different subsets. This can also be seen when the data is analysed while allowing for possibly different magnitude zero-points for the two redshift subsets. However, the non-accelerating models seem to be ruled out using only the low redshift data for the other two data sets, which have less uncertainties. (iii) We have also found that it is quite difficult to measure the evolution of the dark energy equation of state wX(z) though its present value can be constrained quite well. The best-fit value seems to mildly favour a dark energy component with current equation of state wX < −1, thus opening the possibility of existence of more exotic forms of matter. However, the data is still consistent with the the standard cosmological constant at 99 per cent confidence level for Ωm >∼ 0.2.Item Cosmological constant—the weight of the vacuum(Elsevier Science Publishers, 2003-03-01) Padmanabhan, T.Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(Gℏ/c3)≈10−123. This review discusses several aspects of the cosmological constant both from the cosmological (Sections 1–6) and field theoretical (Sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, …) from different perspectives. The latter part of the review (Sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter spacetime, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.Item Constraints on ΩB, Ωm, and h from MAXIMA and BOOMERANG(American Astronomical Society, 2001-07-01) Padmanabhan, T.; Sethi, Shiv K.We analyze the BOOMERANG and MAXIMA results in the context of simplest inflationary universes: ΩTotal = 1, ns~ 1. We attempt to constrain three other parameters-h, ΩB, and Ωm from these observations. We show that (1) the data are consistent with the values of Ωm and h inferred from other observations and (2) the value of ΩBh² is too high to be compatible with big bang nucleosynthesis observations at the 2 σ level for ns = 1. We also include two cosmic background imager(CBI) band powers in our analysis. However, the inclusion of CBI band powers doesn't affect our conclusions.Item Isochronous Potentials(Indian Academy of Sciences, 2008-11-12) Padmanabhan, T.Oscillatory motion of a particle in a one dimensional potential belongs to a class of exactly solvable problems in classical mechanics. In this installment, we examine some lesser known aspects of the oscillations in so me potentials.Item Is gravity an intrinsically quantum phenomenon ? Dynamics of Gravity from the Entropy of Spacetime and the Principle of Equivalence(World Scientific Publishing Company, 2002-05-21) Padmanabhan, T.The two surprising features of gravity are (a) the principle of equivalence and (b) the connection between gravity and thermodynamics. Using principle of equivalence and special relativity in the local inertial frame, one could obtain the insight that gravity must possess a geometrical description. We show that, using the same principle of equivalence, special relativity and quantum theory in the local Rindler frame one can obtain the Einstein{Hilbert action functional for gravity and thus the dynamics of the space{ time. This approach, which essentially involves postulating that the horizon area must be proportional to the entropy, uses the local Rindler frame as a natural extension of the local inertial frame and leads to the interpretation that the gravitational action represents the free energy of the space{time geometry. As an aside, one also obtains a natural explanation as to: (i) why the covariant action for gravity contains second derivatives of the metric tensor and (ii) why the gravitational coupling constant is positive. The analysis suggests that gravity is intrinsically holographic and even intrinsically quantum mechanical.Item Is gravitational entropy quantized?(American Physical Society, 2008-11-18) Kothawala, Dawood; Padmanabhan, T.; Sarkar, SudiptaIn Einstein’s gravity, the entropy of horizons is proportional to their area. Several arguments given in the literature suggest that, in this context, both area and entropy should be quantized with an equallyspaced spectrum for large quantum numbers. But in more general theories (like, for example, in the black hole solutions of Gauss-Bonnet or Lanczos-Lovelock gravity) the horizon entropy is not proportional to area and the question arises as to which of the two (if at all) will have this property. We give a general argument that in all Lanczos-Lovelock theories of gravity, it is the entropy that has an equally-spaced spectrum. In the case of Gauss-Bonnet gravity, we use the asymptotic form of quasinormal mode frequencies to explicitly demonstrate this result. Hence, the concept of a quantum of area in Einstein- Hilbert gravity needs to be replaced by a concept of quantum of entropy in a more general context.Item Initial state of matter fields and trans-Planckian physics: Can CMB observations disentangle the two?(American Physical Society, 2005-05-17) Sriramkumar, L.; Padmanabhan, T.The standard, scale-invariant, inflationary perturbation spectrum will be modified if the effects of trans- Planckian physics are incorporated into the dynamics of the matter field in a phenomenological manner, say, by the modification of the dispersion relation. The spectrum also changes if we retain the standard dynamics but modify the initial quantum state of the matter field. We show that, given any spectrum of perturbations, it is possible to choose a class of initial quantum states which can exactly reproduce this spectrum with the standard dynamics.We provide an explicit construction of the quantum state which will produce the given spectrum. We find that the various modified spectra that have been recently obtained from ‘‘trans-Planckian considerations’’ can be constructed from suitable squeezed states above the Bunch- Davies vacuum in the standard theory. Hence, the CMB observations can, at most, be useful in determining the initial state of the matter field in the standard theory, but it can not help us to discriminate between the various Planck scale models of matter fields.We study the problem in the Schrodinger picture and determine the criterion for negligible back reaction due to modified initial conditions.