arXiv subjects

Sohrab Rahvar

Publications and source records attributed to Sohrab Rahvar.

At least 91 records · Page 5Linked to original sources

arXiv2007

The Magellanic Stream in Modified Newtonian Dynamics

The dynamics of the Magellanic Stream (MS) as a series of clouds extending from the Magellanic Clouds (MCs) to the south Galactic pole is affected by the distribution and the amount of matter in the Milky Way. We calculate the gravitational effect of the Galactic disk on the MS in the framework of modified Newtonian dynamics(MOND) and compare with observations of the Stream's radial velocity. We consider the tidal force of the Galaxy, which strips material from the MCs to form the MS, and, using a no-halo model of the Galaxy, we ignore the effect of the drag of the Galactic halo on the MS. We also compare the MONDian dynamics with that in logarithmic and power-law dark halo models and show that the MOND theory seems plausible for describing the dynamics of satellite galaxies such as the MCs. Finally, we perform a maximum likelihood analysis to obtain the best MOND parameters for the Galactic disk.

astro-ph↗

arXiv2006

Modified gravity with $f(R) = \sqrt{R^2 - R_0^2}$

Here in this work we propose a modified gravity with the action of $f(R) = \sqrt{R^2 - R_0^2}$ instead of Einstein-Hilbert action to describe the late time acceleration of the universe. We obtain the equation of the modified gravity both in the metric and Palatini formalisms. An asymptotic solution of gravity equations corresponding to a constant Ricci scalar causes a late time acceleration of the universe. We do a conformal transformation in the action of the modified gravity and obtain the equivalent minimally coupled scalar-tensor gravity. The equivalent Brans-Dicke gravity of this model is also studied. To examine this model with the observation, the perihelion Precession of the Mercury is compared with our prediction and we put an upper constraint of $R_0<H_0^2$. This range for $R_0$ is also in agreement with the cosmological acceleration at the present time. Finally we show that this action has instability for the small perturbations of the metric in vacuum solution in which adding a quadratic term of Ricci scalar can stabilize it.

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arXiv2006

Characteristic Angular Scales in Cosmic Microwave Background Radiation

We investigate the stochasticity in temperature fluctuations in the cosmic microwave background (CMB) radiation data from {\it Wilkinson Microwave Anisotropy Probe}. We show that the angular fluctuations of the temperature is a Markov process with a {\it Markov angular scale}, $Θ_{\rm Markov}=1.01^{+0.09}_{-0.07}$. We characterize the complexity of the CMB fluctuations by means of a Fokker-Planck or Langevin equation and measure the associated Kramers-Moyal coefficients for the fluctuating temperature field $T(\hat n)$ and its increment, $ΔT =T(\hat n_1) - T(\hat n_2)$. Through this method we show that temperature fluctuations in the CMB has fat tails compared to a Gaussian distribution.

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arXiv2006

Power-law Parameterized Quintessence Model

We introduce a power-law parameterized quintessence model for the dark energy which accelerate universe at the low redshifts while behaves as an ordinary matter for the early universe. We construct a unique scalar potential for this parameterized quintessence model. As the observational test, the Supernova Type Ia (SNIa) Gold sample data, size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), the position of the acoustic peak from the CMB observations and structure formation from the 2dFGRS survey are used to constrain the parameters of the quintessence model. The best fit parameters indicates that the equation of state of this model at the present time is less than one $(w_0<-1)$ which violates the energy condition in General Relativity. Finally we compare the age of old objects with age of universe in this model.

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arXiv2006

A Parameterized Variable Dark Energy Model: Structure Formation and Observational Constraints

In this paper we investigate a simple parameterization scheme of the quintessence model given by Wetterich (2004). The crucial parameter of this model is the bending parameter $b$, which is related to the amount of dark energy in the early universe. Using the linear perturbation and the spherical infall approximations, we investigate the evolution of matter density perturbations in the variable dark energy model, and obtain an analytical expression for the growth index $f$. We show that increasing $b$ leads to less growth of the density contrast $δ$, and also decreases the growth index. Giving a fitting formula for the growth index at the present time we verify that the approximation relation $f\simeqΩ_m^α$ also holds in this model. To compare predictions of the model with observations, we use the Supernovae type Ia (SNIa) Gold Sample and the parameters of the large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS). The best fit values for the model parameters by marginalizing on the remained ones, are $Ω_m=0.21_{-0.06}^{+0.07}$, $w_0=-2.05_{-2.05}^{+0.65}$ and $b=4.05^{+7.05}_{-2.25}$ at $1σ$ confidence level. As a final test we calculate the age of universe for different choices of the free parameters in this model and compare it with the age of old stars and some high redshift objects. Then we show that the predictions of this variable dark energy model are consistent with the age observation of old star and can solve the "age crisis" problem.

astro-ph↗

arXiv2006

Observational Constraints on a Variable Dark Energy Model

We study the effect of a phenomenological parameterized quintessence model on low, intermediate and high redshift observations. At low and intermediate redshifts, we use the Gold sample of supernova Type Ia (SNIa) data and recently observed size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), to put constraint on the parameters of the quintessence model. At the high redshift, the same fitting procedure is done using WAMP data, comparing the location of acoustic peak with that obtain from the dark energy model. As a complementary analysis in a flat universe, we combine the results from the SNIa, CMB and SDSS. The best fit values for the model parameters are $Ω_m = 0.27^{+0.02}_{-0.02}$ (the present matter content) and $w_0=-1.45^{+0.35}_{-0.60}$ (dark energy equation of state). Finally we calculate the age of universe in this model and compare it with the age of old stars and high redshift objects

astro-ph↗

arXiv2006

Cooling in the Universe

One of the questions in the cosmology courses is the cooling mechanism of cosmic fluid during it expansion according to classical concepts of the thermodynamics. In this short pedagogical paper, we quote the questions and give a natural approach dealing with this problem by measuring the dispersion velocity of the particles in the cosmic fluid by a comoving observer. We show that the thermal motion of the particles in the cosmic fluid deviates from the Hubble flow and follows the geodesics governed by the gravity of the homogeneous universe. The dynamics of the "thermal peculiar velocity" of the particles leads in an expanding universe, momentum of the particles relax by the inverse of the expansion factor and the result is losing the energy of particles, hence cooling the universe.

physics.ed-ph↗

arXiv2006

Multifractal Detrended Fluctuation Analysis of Sunspot Time Series

We use multifractal detrended fluctuation analysis (MF-DFA), to See query 1 study sunspot number fluctuations. The result of the MF-DFA shows that there are three crossover timescales in the fluctuation function. We discuss how the existence of the crossover timescales is related to a sinusoidal trend. Using Fourier detrended fluctuation analysis, the sinusoidal trend is eliminated. The Hurst exponent of the time series without the sinusoidal trend is $0.12\pm 0.01$. Also we find that these fluctuations have multifractal nature. Comparing the MF-DFA results for the remaining data set to those for shuffled and surrogate series, we conclude that its multifractal nature is almost entirely due to long range correlations.

physics.data-an↗

arXiv2005

Gravitational Microlensing by Rotating Stars

Using the astrometry of microlensing events we study the effect of angular momentum as compared to that of the parallax. For a rotating lens it is shown that the effect of the angular momentum deviates the center of images from that in the simple standard microlensing. This effect could be observed by future astrometry missions such as GAIA and SIM for lenses with angular momentum $S\gtrsim \times 10^{48} kg m^2 sec^{-1}$. It is shown that for extreme black hole lenses the corresponding mass should be more than $10^3 M_{\odot}$.

astro-ph↗

arXiv2005

Dynamics of inflationary cosmology in TVSD model

Within the framework of a model Universe with time variable space dimensions (TVSD), known as decrumpling or TVSD model, we study TVSD chaotic inflation and obtain dynamics of the inflaton, scale factor and spatial dimension. We also study the quantum fluctuations of the inflaton field and obtain the spectral index and its running in this model. Two classes of examples have been studied and comparisons made with the standard slow-roll formulae. We compare our results with the recent Wilkinson Microwave Anisotropy Probe (WMAP) data.

astro-ph↗

arXiv2005

Constraining Galactic Models Through Parallax and Astrometry of Microlensing Events

In recent years various models for the Galactic distribution of massive compact halo objects (MACHOs) have been proposed for the interpretation of microlensing toward the Large Magellanic Cloud (LMC). A direct way to fit the best model is by measuring the lens parameters, which can be obtained by measuring the Einstein crossing time and the parallax effect on the microlensing light curve and by astrometry of centroids of images. In this work, the theoretical distribution of these parameters is obtained for the various power-law Galactic dark halo models and MACHO mass functions (MF). For self-lensing as one of the models for the interpretation of LMC events, the maximum shift of image centroids and the parallax parameter are one order of magnitude smaller than for models with dark halos. This can be used as a test for the self-lensing, although the shifting of image centroids 0.02 {\it mas}, for the LMC events is unlikely to be observed by the astrometric missions such as FAME, GAIA and SIM.

astro-ph↗

arXiv2005

Microlensing by the Halo MACHOs with Spatially Varying Mass Function

The main aim of microlensing experiments is to evaluate the mean mass of massive compact halo objects (MACHOs) and the mass fraction of the Galactic halo made by this type of dark matter. Statistical analysis shows that by considering a Dirac-Delta mass function (MF) for the MACHOs, their mean mass is about that of a white dwarf star. This result is, however, in discrepancy with other observations such as those of non-observed expected white dwarfs in the Galactic halo which give rise to metal abundance, polluting the interstellar medium by their evolution. Here we use the hypothesis of the spatially varying MF of MACHOs, proposed by Kerins and Evans to interpret microlensing events. In this model, massive lenses with a lower population contribute to the microlensing events more frequently than do dominant brown dwarfs. This effect causes the mean mass of the observed lenses to be larger than the mean mass of all the lenses. A likelihood analysis is performed to find the best parameters of the spatially varying MF that are compatible with the duration distribution of Large Magellanic Cloud microlensing candidates of the MACHO experiment.

astro-ph↗

arXiv2004

Spatially varying mass function of MACHOs in the galactic halo and interpretation of microlensing results

The gravitational microlensing experiments in the direction of Large Magellanic Cloud (LMC) predict a large amount of white dwarfs ($\sim 20%$) filling the galactic halo. However, the predicted white dwarfs have not been observed at the galactic halo. To interpret the microlensing results and resolving the mentioned problems, we use the hypothesis of spatially varying mass function of MACHOs, proposed by \cite{ker98} (hereafter KE). However the KE model is not compatible with the duration distribution of events \cite{rah04a}. Here we use more realistic power-law model of MF, $dn/dm\propto m^α$ for the MACHOs of halo. The index of MF in this model changes from -2.7 for stars with $m>1 M_{\bigodot}$ at the central part of galactic halo to the substellar regime with an upper limit of -1 at the edge of halo. We show that in contrast to the abundant brown dwarfs of galactic halo, heavy MACHOs can be responsible for the microlensing events in the direction of LMC.

astro-ph↗

arXiv2004

Possibility of Magnetic Mass Detection by the Next Generation of Microlensing Experiments

We study the possibility of magnetic mass detection by the gravitational microlensing technique. Recently the theoretical effect of magnetic mass in the NUT space on the microlensing light curve has been studied. It was shown that in the low photometric signal to noise and sampling rate of MACHO experiment light curves, no signature of NUT factor has been found. In order to increase the sensitivity of magnetic mass detection, we propose a systematic search for microlensing events, using the currently running alert systems and complementary telescopes for monitoring the Large Magellanic Clouds stars. In this strategy of observation, we obtain the magnetic mass detection efficiency and also the lowest observable limit of the NUT factor. This method of survey for gravitational microlensing detection can also be used as a tool for searching other exotic space-times.

astro-ph↗

arXiv2003

Statistical Interpretation of LMC Microlensing Candidates

After a decade of gravitational microlensing experiments, a dozen of microlensing candidates in the direction of the stars of the Large Magellanic Cloud (LMC) have been detected by the EROS and MACHO groups. Recently it was shown that the distribution of the duration of the observed LMC microlensing events is significantly narrower than what is expected from the standard halo model. In this article we make the same comparison, using non-standard halo models and considering the contribution of non-halo components of the Milky Way such as the disc, spheroid and LMC itself in the microlensing events. Comparing the theoretical and experimental widths of distributions of the duration of events shows that neither standard nor non-standard halo models are compatible with the microlensing data at least with 95 per cent of confidence. This results maybe explained if (i) the MACHOs in the Galactic halo reside in clumpy structures or (ii) the durations of events have been underestimated due to the blending effect.

astro-ph↗

arXiv2003

Gravitational Microlensing in NUT Space

We study the theoretical signature of magnetic masses on the light curve of gravitational microlensing effect in NUT space. The light curves for microlensing events in NUT space are presented and contrasted with those due to lensing produced by normal matter. In the next step, associating magnetic mass to massive astrophysical compact objects (MACHOs), we try to see its effect on the light curves of microlensing candidates observered by the MACHO group. Presence or absence of this feature in the observed microlensing events can shed light on the question of the existence of magnetic masses in the Universe.

astro-ph↗

arXiv2002

EROS/MACHO Gravitational Microlensing Events Toward LMC in Evans Halo Model

After a decade of gravitational microlensing experiments, 13 to 17 events by MACHO (depending on quality) and two events by EROS have been detected. All of those have been observed in the direction of Large Magellanic Cloud. We use Evans spherically symmetric halo model to study the rate of microlensing events. The expected number of events in this model obtain by using EROS and MACHO observational efficiencies. We compare our numbers with the observed events to obtain the fraction of halo that is made by compact objects. It is shown that results derived from two experiments are in good agreement with each other and MACHOs, comprise only a fraction (depending on the model) of Milky Way halo. The results are also compatible with the White Dwarfs population studies in the Hubble Deep Field.

astro-ph↗

arXiv2002

Density Contrast-Peculiar Velocity Relation in the Newtonian Gauge

In general relativistic framework of the large scale structure formation theory in the universe, we investigate the relation between density contrast and peculiar velocity in the Newtonian gauge. According to the gauge-invariant property of the energy-momentum tensor in the Newtonian gauge, we consider the perturbation of velocity in the energy-momentum tensor behaves as the Newtonian peculiar velocity. It is shown that in the relativistic framework, the relation between peculiar velocity and density contrast has an extra correction term with respect to the Newtonian Peebles formula which in small scales, can be ignorable . The relativistic correction of peculiar velocity for the structures with the extension of few hundred mega parsec is about few percent which is smaller than the accuracy of the recent observations for measuring peculiar velocity. The peculiar velocity in the general relativistic framework also changes the contribution of Doppler effect on the anisotropy of CMB.

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