240 research outputs found
HI Fluctuations at Large Redshifts: II - the Signal Expected for GMRT
For the GMRT, we calculate the expected signal from redshifted HI emission at
two frequency bands centered at 610 and 325 MHz. The study focuses on the
visibility-visibility cross-correlations, proposed earlier as the optimal
statistical estimator for detecting and analyzing this signal. These
correlations directly probe the power spectrum of density fluctuations at the
redshift where the radiation originated, and thereby provide a method for
studying the large scale structures at large redshifts. We present detailed
estimates of the correlations expected between the visibilities measured at
different baselines and frequencies. Analytic fitting formulas representing the
salient features of the expected signal are also provided. These will be useful
in planning observations and deciding an optimal strategy for detecting this
signal.Comment: 16 pages including 7 figures, published in JAp
HI Fluctuations at Large Redshifts: I--Visibility correlation
We investigate the possibility of probing the large scale structure in the
universe at large redshifts by studying fluctuations in the redshifted 1420 MHz
emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen
content of the universe is known from absorption studies for z<4.5. The HI
distribution is expected to be inhomogeneous in the gravitational instability
picture and this inhomogeneity leads to anisotropy in the redshifted HI
emission. The best hope of detecting this anisotropy is by using a large
low-frequency interferometric instrument like the Giant Meter-Wave Radio
Telescope (GMRT). We calculate the visibility correlation function <V_nu(u)
V_nu'(u)> at two frequencies nu and nu' of the redshifted HI emission for an
interferometric observation. In particular we give numerical results for the
two GMRT channels centered around nu =325 and 610 MHz from density
inhomogeneity and peculiar velocity of the HI distribution. The visibility
correlation is ~10^-9 to 10^-10 Jy^2. We calculate the signal-to-noise for
detecting the correlation signal in the presence of system noise and show that
the GMRT might detect the signal for integration times ~ 100 hrs. We argue that
the measurement of visibility correlation allows optimal use of the
uncorrelated nature of the system noise across baselines and frequency
channels.Comment: 17 pages, 2 figures, Submitted to JA
Developing textile entrepreneurial inclination model by integrating experts mining and ISM-MICMAC
The Indian textile industry is lacking in an entrepreneurial inclination of a skilled young generation; because of this, the industry is facing a challenge to achieve sustainable development and growth. To overcome this problem, the goal of this work is to build an entrepreneurial inclination model in the context of the textile industry. For achieving this goal, a combined approach of an extensive literature review and experts mining has been used to establish the entrepreneurial inclination factors in phased of the study. In the second phase, an Interpretive Structural Modelling (ISM) with Matrice d'Impacts Croisés Multiplication Appliqués à un Classement (MICMAC) has been applied to build a structural model and to find the driving force factors and dependence power. The results show that effective entrepreneurship courses, institutional policy, training and internship, institutional corporation and the involvement of institutional heads play a very significant role in encouraging youth towards entrepreneurship. The outcomes of the study can help both the government and academic institutes to draw up effective policy and develop an entrepreneurial culture which can help to create more entrepreneurs in the textile field.N
Using HI to probe large scale structures at z ~ 3
The redshifted 1420 MHz emission from the HI in unresolved damped
Lyman-\alpha clouds at high z will appear as a background radiation in low
frequency radio observations. This holds the possibility of a new tool for
studying the universe at high-z, using the mean brightness temperature to probe
the HI content and its fluctuations to probe the power spectrum. Existing
estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK
at 320 Mhz. The cross-correlation between the temperature fluctuations across
different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to
10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40
Mpc for some of the currently favoured cosmological models. Comparing this with
the expected sensitivity of the GMRT, we find that this can be detected with
\~10 hrs of integration, provided we can distinguish it from the galactic and
extragalactic foregrounds which will swamp this signal. We discuss a strategy
based on the very distinct spectral properties of the foregrounds as against
the HI emission, possibly allowing the removal of the foregrounds from the
observed maps.Comment: 16 pages, includes 6 figures, accepted in JAA (minor revisions,
references added
System size dependence of nuclear modification and azimuthal anisotropy of jet quenching
We investigate the system size dependence of jet-quenching by analyzing
transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at
=200 GeV for different centralities. The fast partons
are assumed to lose energy by radiating gluons as they traverse the plasma and
undergo multiple collisions. The energy loss per collision, , is
taken as proportional to (where is the energy of the parton),
proportional to , or a constant depending on whether the formation
time of the gluon is less than the mean path, greater than the mean free path
but less than the path length, or greater than the path length of the partons,
respectively. NLO pQCD is used to evaluate pion production by modifying the
fragmentation function to account for the energy loss. We reproduce the nuclear
modification factor by treating as the only free
parameter, depending on the centrality and the mechanism of energy loss. These
values are seen to explain the nuclear modification of prompt photons, caused
by the energy lost by final state quarks before they fragment into photons.
These also reproduce the azimuthal asymmetry of transverse momentum
distribution for pions within a factor of two and for prompt photons in a fair
agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded.
Typographical corrections done, several references added. To appear in
Journal of Physics
Decaying Neutrinos and Large Scale Structure Formation
We study the growth of density perturbations in a universe with unstable dark
matter particles. The mass (m_\nu) range 30 eV < m_\nu < 10 keV with lifetimes
() in the range 10^7 sec < t_d < 10^16 sec are considered. We calculate
the COBE normalized matter power spectrum for these models. We find that it is
possible to construct models consistent with observations for masses $m_\nu >
50 eV by adjusting t_d so as to keep the quantity (m_\nu(keV))^2 t_d(yr)
constant at a value around 100. For m_\nu < 1 keV the power spectrum has extra
power at small scales which could result in an early epoch of galaxy formation.
We do not find any value of t_d which gives a viable model in the mass range
m_\nu < 50 eV. We also consider the implications of radiatively decaying
neutrinos---models in which a small fraction B << 1 of neutrinos decay into
photons-which could possibly ionize the intergalactic medium (IGM) at high
redshift. We show that the parameter space of decaying particles which
satisfies the IGM observations does not give viable models of structure
formation.Comment: 42 pages, 9 Postscript figures, Accepted in ApJ Supplemen
Collective and noncollective states in (120)Te
High-spin states in (120)Te were populated in the reaction (80)Se((48)Ca, alpha 4n)(120)Te at a beam energy of 207 MeV and gamma-ray coincidences were measured using the Gammasphere spectrometer. The previously known level scheme is extended to higher spin and new interband transitions and side-feeding branches are established. Five highly deformed rotational bands, extending up to almost I = 50, are observed for the first time. The bands are compared with similar structures found recently in neighboring nuclei. The experimental results are interpreted within the framework of the cranked Nilsson-Strutinsky model. Configuration assignments to several terminating states and to the high-spin bands are discussed
HI Fluctuations at Large Redshifts: III - Simulating the Signal Expected at GMRT
We simulate the distribution of neutral hydrogen (HI) at the redshifts z=1.3
and 3.4 using a cosmological N-body simulation along with a prescription for
assigning HI masses to the particles. The HI is distributed in clouds whose
properties are consistent with those of the damped Lyman-\alpha absorption
systems (DLAs) seen in quasar spectra. The clustering properties of these
clouds are identical to those of the dark matter. We use this to simulate the
redshifted HI emission expected at 610 MHz and 325 MHz, two of the observing
bands a the GMRT. These are used to predict the correlations expected between
the complex visibilities measured at different baselines and frequencies in
radio-interferometric observations with the GMRT. The visibility correlations
directly probe the power spectrum of HI fluctuations at the epoch when the HI
emission originated, and this holds the possibility of using HI observations to
study large-scale structures at high z.Comment: Submitted to JApA, 12 Latex pages including 6 figure
Skewness in the Cosmic Microwave Background Anisotropy from Inflationary Gravity Wave Background
In the context of inflationary scenarios, the observed large angle anisotropy
of the Cosmic Microwave Background (CMB) temperature is believed to probe the
primordial metric perturbations from inflation. Although the perturbations from
inflation are expected to be gaussian random fields, there remains the
possibility that nonlinear processes at later epochs induce ``secondary''
non-gaussian features in the corresponding CMB anisotropy maps. The
non-gaussianity induced by nonlinear gravitational instability of scalar
(density) perturbations has been investigated in existing literature. In this
paper, we highlight another source of non-gaussianity arising out of higher
order scattering of CMB photons off the metric perturbations. We provide a
simple and elegant formalism for deriving the CMB temperature fluctuations
arising due to the Sachs-Wolfe effect beyond the linear order. In particular,
we derive the expression for the second order CMB temperature fluctuations. The
multiple scattering effect pointed out in this paper leads to the possibility
that tensor metric perturbation, i.e., gravity waves (GW) which do not exhibit
gravitational instability can still contribute to the skewness in the CMB
anisotropy maps. We find that in a flat universe, the skewness in
CMB contributed by gravity waves via multiple scattering effect is comparable
to that from the gravitational instability of scalar perturbations for equal
contribution of the gravity waves and scalar perturbations to the total rms CMB
anisotropy. The secondary skewness is found to be smaller than the cosmic
variance leading to the conclusion that inflationary scenarios do predict that
the observed CMB anisotropy should be statistically consistent with a gaussian
random distribution.Comment: 10 pages, Latex (uses revtex), 1 postscript figure included. Accepted
for publication in Physical Review
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