Holographic dark energy model with linearly varying deceleration parameter and generalised Chaplygin gas dark energy model in Bianchi type-I universe     

Sanjay Sarkar 《Astrophysics and Space Science》2014,349(2):985-993

The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.  相似文献   

Derivations of coupled equations appropriate to the lower regions of the ionosphere     

S. S. De  B. K. Sarkar  Bithika Ghosh  Manasi Mal  B. Ghosh  S. Bandyopadhyay  S. K. Adhikari  A. C. Sen 《Astrophysics and Space Science》1991,184(2):281-285

Coupled equations are derived for different ionospheric parameters through susceptibility dyadic appropriate to the medium. The nature of dispersion within the medium is studied.  相似文献   

Quasi-spherical collapse with cosmological constant     

Ujjal Debnath  Soma Nath  Subenoy Chakraborty 《Monthly notices of the Royal Astronomical Society》2006,369(4):1961-1964

The junction conditions between static and non-static space–times are studied for analysing gravitational collapse in the presence of a cosmological constant. We have discussed about the apparent horizon and their physical significance. We also show the effect of cosmological constant in the collapse and it has been shown that cosmological constant slows down the collapse of matter.  相似文献   

A multivariate statistical study of copper mineralization in the central section of Mosaboni Mine, Eastern Singhbhum, India     

A. K. Saha  S. N. Sarkar  Swapna Basu  Dhruba Ganguly 《Mathematical Geology》1986,18(2):215-235

A multivariate statistical analysis was carried out with log-transformed values of Cu, Ni, Co, Pb, Zn, Ag, Cr, Mn, Ca, and Sr in several sets of samples collected across the mineralized base metal zone in sheared soda granite, feldspathic schist, and chlorite schist from the central section of Mosaboni Mine of the famous Singhbhum Copper Belt of eastern India. Linear correlation coefficient matrices of two sets of ore samples (>0.5% Cu)—one from levels 18 and 21 and the other from levels 25 and 28—indicate two well-defined and distinct clusters comprising Cu, Ni, Co, Pb, and Zn on one hand and Ca, Sr, and Mn on the other. Varimax-rotatedR-mode factor analysis of two above-noted sample sets, taken along with available geologic information, indicates that over 80% of the variability in data matrices for 9–10 elements can be accounted for by four distinct processes: (a) an early phase of copper mineralization which apparently replaced Mn, Ca, and Sr in the host rock; (b) a silicate-cum-oxide phase of crystallization/recrystallization of host rock; (c) remobilization of sulfide-forming ore elements (Cu, Ni, Co, Pb, and Zn); and (d) a phase of mineralization of Ag which appears to have replaced Cr, Ca and Cu. Process (c) was quantitatively most important. Factor score studies are suggestive of preferred introduction of Ni, Co, Pb, and Zn along central parts of preexisting copper-mineralized zones.  相似文献   

Influence of plastic fines content on the liquefaction susceptibility of sands: monotonic loading     

Goudarzy  Meisam  Sarkar  Debdeep  Lieske  Wolfgang  Wichtmann  Torsten 《Acta Geotechnica》2022,17(5):1719-1737

Acta Geotechnica - The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand–fines mixtures under...  相似文献   

EChOSim: The Exoplanet Characterisation Observatory software simulator     

E. Pascale  I. P. Waldmann  C. J. MacTavish  A. Papageorgiou  A. Amaral-Rogers  R. Varley  V. Coudé du Foresto  M. J. Griffin  M. Ollivier  S. Sarkar  L. Spencer  B. M. Swinyard  M. Tessenyi  G. Tinetti 《Experimental Astronomy》2015,40(2-3):601-619

EChOSim is the end-to-end time-domain simulator of the Exoplanet Characterisation Observatory (EChO) space mission. EChOSim has been developed to assess the capability of the EChO mission concept to detect and characterise the atmospheres of transiting exoplanets. Here we discuss the details of the EChOSim implementation and describe the models used to represent the instrument and to simulate the detection. Software simulators have assumed a central role in the design of new instrumentation and in assessing the level of systematics affecting the measurements of existing experiments. Thanks to its high modularity, EChOSim can simulate basic aspects of several existing and proposed spectrometers including instruments on the Hubble Space Telescope and Spitzer, ground-based and balloon-borne experiments. A discussion of different uses of EChOSim is given, including examples of simulations performed to assess the EChO mission.  相似文献   

Interacting holographic dark energy with variable deceleration parameter and tachyon scalar field dark energy model in LRS Bianchi type-II universe     

Sanjay Sarkar 《Astrophysics and Space Science》2014,350(2):821-829

In this work, we have considered the spatially homogeneous and anisotropic Bianchi type-II universe filled with two interacting fluids; dark matter and holographic dark energy components. Assuming the proportionality relation between one of the components of shear scalar and expansion scalar which yields time dependent deceleration parameter, an exact solution to Einstein’s field equations in Bianchi type-II line element is obtained. We have investigated geometric and kinematics properties of the model and the behaviour of the holographic dark energy. It is observed that the mean anisotropic parameter is uniform through the whole evolution of the universe and the coincidence parameter increases with increasing time. The solutions are also found to be in good agreement with the results of recent observations. We have applied the statefinder diagnostics method to study the behaviour of different stages of the universe and to differentiate the proposed dark energy model from the ΛCDM model. We have also established a correspondence between the holographic dark energy model and the tachyon scalar field dark energy model. We have reconstructed the potential and the dynamics of the tachyon scalar field, which describes accelerated expansion of the universe.  相似文献   

Effect of sample pretreatment and size fraction on the δ¹⁸O and δ¹³C values of foraminifera in Arabian Sea sediments     

A. Sarkar  R. Ramesh  S.K. Bhattacharya 《地学学报》1990,2(5):488-493

Stable oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C, respectively) of a planktonic foraminiferal species (Globigerinoides sacculifer) and marble carbonate were measured with and without a variety of pretreatments like ultrasonication, soaking in H₂O₂, methanol and roasting under vacuum. Additionally, organic matter - carbonate mixtures were also analysed with and without the above pretreatments. No significant difference was found in the isotope ratios of treated and untreated samples. It appears that most of the pretreatments may not be necessary when dealing with planktonic foraminifera, especially G. sacculifer. Analysis of two different size fractions (> 400 μm and 250–400 μm) of this species reveals that the smaller size fraction is enriched in ¹⁸O and depleted in ¹³C relative to the bigger size fraction. It is therefore necessary to choose a proper size fraction for isotopic analyses.  相似文献   

Some studies on the solar microwave bursts in relation to the slowly varying component     

M. K. Das Gupta  S. K. Sarkar 《Solar physics》1972,26(2):378-385

The occurrences of 5772 microwave bursts recorded by the Sagamore Hill and Manilla Solar Radio Observatories over the period January 1968 to July 1970, covering the maximum phase of the current solar cycle at frequencies 2695, 4995 and 8800 MHz and their energy excesses have been examined in relation to the S-component of solar radio emission. The average slowly varying component has been determined by the superposed epoch method commonly known as the Chree analysis. Similar treatment of the bursts, data, mentioned above has been made to examine any probable 27-day variation and the results obtained have been compared with that of the S-component. Further, spectra of the microwave bursts under the so-called spectral type - inverted U, particularly those having a peak at 4995 MHz, have also been examined and compared with the average spectrum of the S-component. Some of the important results obtained from the present analysis are: (1) the nature of variation of both the average number of occurrences and energy excesses of the microwave bursts follow in general the average 27-day variation of the S-component, (2) the number of occurrences and energy excesses of the microwave bursts are comparatively greater in the ascending phase of the 27-day cycle than those in the descending phase, (3) bursts at progressively higher frequencies originate at lower levels in the solar atmosphere than those of the associated S-component, and (4) the average spectrum of the microwave bursts of inverted U spectral type having a peak at 4995 MHz is quite identical in nature to that of the S-component.  相似文献   

Some studies on solar microwave bursts in relation to the phases of the associated Hα-flares and their spectral nature     

S. K. Sarkar  T. Chattopadhyay  M. K. Das Gupta 《Solar physics》1975,40(2):411-415

Occurrences of the flare-associated microwave bursts as well as their peak flux and energy excess spectra have been examined in relation to the pre- and post-maximum phases of the respective flares during the period 1969–72. Results obtained are: (i) about 76% of the flare-associated bursts occur in the pre-maximum phase and the remaining 24% occurs in the post-maximum phase irrespective of the flare classification, intensity-wise or area-wise; (ii) ‘impulsive’ and ‘gradual rise and fall’ bursts are relatively more important in the pre-maximum phase while ‘post burst increase’ bursts show comparatively higher occurrences in the post-maximum phase; (iii) peak flux and energy excess spectra of the concurrent microwave bursts in the pre-maximum phase of the flare are mostly of ‘inverted U’ and ‘increasing with frequency’ spectral types. Of these, ‘impulsive’ bursts are predominantly of the ‘inverted U’ and the ‘grf’ bursts are of the ‘increasing with frequency’ spectral type.  相似文献