arXiv subjects

M. V. Sazhin

Publications and source records attributed to M. V. Sazhin.

At least 19 recordsLinked to original sources

arXiv2019

Optical analysis of a CMB cosmic string candidate

The complexity of the cosmological scenario regarding cosmic strings (CSs) stands still in the way of a complete understanding. We describe here a promising strategy for the possible detection of these elusive physical entities. It is based on the search of strong gravitational lensing events in the location area of the CS candidate (CSc-1), which was declared in a previous work by CMB analysis. Using photometric and geometric criteria, we identified pairs of candidates of lensed galaxies (LGCs) in the "string field" (SF), which were then compared with the average density of background galaxy pairs in a set of "control fields" (CFs). We found an excess of $22\%$ (per sq. deg.) of the LGCs in SF, which exceeds the estimated cosmic dispersion. We also found that the number of LGCs is in excess of $29.2\%$ in the angular separation bin $[8'', 9'']$. Finally, we analysed the possibility of a preferred orientation of the line connecting the centres of the LGCs. The orientation is statistically significant for an angular separation bin $[4'',6'']$. Therefore, we found two "windows" for the preferred angular separation for LGCs along the possible CS. However, the confirmation of the gravitational lensing origin of our LGCs requires spectroscopic observations which seem to be justified by the present results. We plan to acquire their spectra as well as to continue the study of the spectral and morphological features of the LGCs in the CSc-1 field and to analyse the other CS-candidates using the same strategy.

astro-ph.CO↗

arXiv2011

The Scale Factor in the Universe with Dark Energy

Friedmans cosmological equations for the scale factor are analyzed for the Universe containing dark energy. The parameter of the equation of state of the dark energy is treated as an arbitrary constant whose value lies within the interval $w \in [-1.5, -0.5]$, the limits of which are set by current observations. A unified analytic solution is obtained for the scale factor as a function of physical and conformal time. We obtain approximated solutions for scale factor to an accuracy of better then 1%. This accuracy is better then measurement errors of global density parameters and therefore is suitable for the approximated models of our Universe. An analytic solution is obtained for the scale factor in $Λ$CDM cosmological model both in physical and conformal time, for the description of the evolution of the Universe from the epoch of matter domination up to the infinite future.

astro-ph.CO↗

arXiv2009

Cosmological and kinematical criteria for the ICRF2 sources selection

The most precise realization of inertial reference frame in astronomy is the catalogue of 212 defining extragalactic radiosources with coordinates obtained during VLBI observation runs in 1979-1995. IAU decided on the development of the second realization of the ICRF2 catalogue. The criteria of best sources selection (in terms of coordinates stability) must be defined as the first aim. The selected sources have to keep stable the coordinate axes of inertial astronomical frame. Here we propose new criteria of source selection for the new ICRF catalogue. The first one we call as "cosmological" and the second one as "kinematical". The physical basis of these criteria is based on the assumption that apparent motion of quasars (at angular scale of the order of hundred microarcseconds) is connected with real motion inside quasars. Therefore apparent angular motion corresponds to real physical motion of a "hot spot" inside a radio source. It is shown that interval of redshift $0.8 ÷3.0$ is the most favorable in terms that physical shift inside such sources corresponds to minimal apparent angular shift of a "hot spot". Among "cosmologically" selected sources we propose to select motionless sources and sources with linear motion which are predictable and stable over long time interval. To select sources which satisfies such conditions we analyzed known redshifts of sources and time series obtained by our code and by different centers of analysis of VLBI data. As a result of these analyses we select 137 sources as a basis for the ICRF2 catalogue.

astro-ph.CO↗

arXiv2009

CMB anisotropy induced by tachyonic perturbations of dark energy

We study effects of possible tachyonic perturbations of dark energy on the CMB temperature anisotropy. Motivated by some models of phantom energy, we consider both Lorentz-invariant and Lorentz-violating dispersion relations for tachyonic perturbations. We show that in the Lorentz-violating case, the shape of the CMB anisotropy spectrum generated by the tachyonic perturbations is very different from that due to adiabatic scalar perturbations and, if sizeable, it would be straightforwardly distinguished from the latter. The tachyonic contribution improves slightly the agreement between the theory and data; however, this improvement is not statistically significant, so our analysis results in limits on the time scale of the tachyonic instability. In the Lorentz-invariant case, tachyonic contribution is a rapidly decaying function of the multipole number $l$, so that the entire observed dipole can be generated without conflicting the data at higher multipoles. On the conservative side, our comparison with the data places limit on the absolute value of the (imaginary) tachyon mass in the Lorentz-invariant case.

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arXiv2008

CMB Anisotropy Induced by a Moving Straight Cosmic String

We showed that the part of strings could be detected by optical method is only 20% from the total available amount of such objects, therefore the gravitational lensing method has to be "completed" by CMB one. We found the general structure of the CMB anisotropy generated by a cosmic string for simple model of straight string moving with constant velocity. For strings with deficit angle 1-2 arcsec the amplitude of generated anisotropy has to be 15-30 muK (the corresponding string linear density is (G mu) ~ 10^{-7} and energy is GUT one, 10^{15} GeV). To use both radio and optical methods the deficit angle has to be from 0.1 arcsec to 5-6 arcsec. If cosmic string can be detected by optical method, the length of corresponding brightness spot of anisotropy has to be no less than 100 degrees.

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arXiv2007

Gravitational lensing by cosmic strings: what we learn from the CSL-1 case

Cosmic strings were postulated by Kibble in 1976 and, from a theoretical point of view, their existence finds support in modern superstring theories, both in compactification models and in theories with extended additional dimensions. Their eventual discovery would lead to significant advances in both cosmology and fundamental physics. One of the most effective ways to detect cosmic strings is through their lensing signatures which appear to be significantly different from those introduced by standard lenses (id est, compact clumps of matter). In 2003, the discovery of the peculiar object CSL-1 (Sazhin et al.2003) raised the interest of the physics community since its morphology and spectral features strongly argued in favour of it being the first case of gravitational lensing by a cosmic string. In this paper we provide a detailed description of the expected observational effects of a cosmic string and show, by means of simulations, the lensing signatures produced on background galaxies. While high angular resolution images obtained with HST, revealed that CSL-1 is a pair of interacting ellipticals at redshift 0.46, it represents a useful lesson to plan future surveys.

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arXiv2006

Weak microlensing effect and stability of pulsar time scale

An influence of the weak microlensing effect on the pulsar timing is investigated for pulsar B1937+21. Average residuals of Time of Arrival (TOA) due to the effect would be as large as 10 ns in 20 years observation span. These residuals can be much greater (up to 1 ms in 20 years span) if pulsar is located in globular cluster (or behind it).

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arXiv2006

The true nature of CSL-1

On January 12 2006, the Hubble Space Telescope observed the peculiar double extragalactic object CSL-1, suspected to be the result of gravitational lensing by a cosmic string. The high resolution image shows that the object is actually a pair of interacting giant elliptical galaxies. In spite of the weird similarities of the energy and light distributions and of the radial velocities of the two components, CSL-1 is not the lensing of an elliptical galaxy by a cosmic string.

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arXiv2004

Image reconstruction technique and optical monitoring of the QSO2237+0305 from Maidanak Observatory in 2002 -- 2003

We have observed the gravitational lens system Q2237+0305 from the Maidanak Observatory over the period from August 2002 to November 2003. Here we report the results of our observations. We implemented a two-stage technique that has been developed specifically for the purpose of gravitational lens image reconstruction. The technique is based on the Tikhonov regularization approach and allows one to obtain astrometric and photometric characteristics of the gravitational lens system. Light curves with 78 data points for the four quasar components are obtained. Slow brightness variations over the observational period are found in all components. Images A, C, D have a tendency to decrease in brightness. Image B does not vary more than 0.05mag. The observations did not reveal evidence for large variations in brightness of the components due to microlensing effects. To provide an overall picture of the photometry behaviour, our data are combined with the Maidanak observations published for 1995 -- 2000.

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arXiv2004

Lens candidates in the Capodimonte Deep Field in the vicinity of the CSL1 object

CSL1 is a peculiar object discovered in the OACDF. Photometric and spectroscopic investigation strongly suggest that it may be the first case of gravitational lensing by cosmic string. In this paper we derive and discuss a statistical excess of a gravitational lens candidates present in OACDF region surrounding CSL1. This excess cannot be explained on the basis of conventional gravitational lens statistic alone, but is compatible with the proposed cosmic string scenario.

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arXiv2004

The Sky Polarization Observatory

SPOrt is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7 degrees. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 microK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

astro-ph↗

arXiv2003

Separation of foreground and background signals in single frequency measurements of the CMB Polarization

The polarization of the Cosmic Microwave Background (CMB)is a powerful observational tool at hand for modern cosmology. It allows to break the degeneracy of fundamental cosmological parameters one cannot obtain using only anisotropy data and provides new insight into conditions existing in the very early Universe. Many experiments are now in progress whose aim is detecting anisotropy and polarization of the CMB. Measurements of the CMB polarization are however hampered by the presence of polarized foregrounds, above all the synchrotron emission of our Galaxy, whose importance increases as frequency decreases and dominates the polarized diffuse radiation at frequencies below about 50 GHz. In the past the separation of CMB and synchrotron was made combining observations of the same area of sky at different frequencies. In this paper we show that the statistical properties of the polarized components of the synchrotron and dust foregrounds are different from the statistical properties of the polarized component of the CMB, therefore one can build a statistical estimator which allows to extract the polarized component of the CMB from single frequency data also when the polarized CMB signal is just a fraction of the total polarized signal. Our estimator improves the signal/noise ratio for the polarized component of the CMB and reduces from about 50 GHz to about 20 GHz the frequency above which the polarized component of the CMB can be extracted from single frequency maps of the diffuse radiation.

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arXiv2003

The BaR-SPOrt Experiment

BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an experiment to measure the linearly polarized emission of sky patches at 32 and 90 GHz with sub-degree angular resolution. It is equipped with high sensitivity correlation polarimeters for simultaneous detection of both the U and Q stokes parameters of the incident radiation. On-axis telescope is used to observe angular scales where the expected polarization of the Cosmic Microwave Background (CMBP) peaks. This project shares most of the know-how and sophisticated technology developed for the SPOrt experiment onboard the International Space Station. The payload is designed to flight onboard long duration stratospheric balloons both in the Northern and Southern hemispheres where low foreground emission sky patches are accessible. Due to the weakness of the expected CMBP signal (in the range of microK), much care has been spent to optimize the instrument design with respect to the systematics generation, observing time efficiency and long term stability. In this contribution we present the instrument design, and first tests on some components of the 32 GHz radiometer.

astro-ph↗

arXiv2002

SPOrt: an Experiment Aimed at Measuring the Large Scale Cosmic Microwave Background Polarization

SPOrt (Sky Polarization Observatory) is a space experiment to be flown on the International Space Station during Early Utilization Phase aimed at measuring the microwave polarized emission with FWHM = 7deg, in the frequency range 22-90 GHz. The Galactic polarized emission can be observed at the lower frequencies and the polarization of Cosmic Microwave Background (CMB) at 90 GHz, where contaminants are expected to be less important. The extremely low level of the CMB Polarization signal (< 1 uK) calls for intrinsically stable radiometers. The SPOrt instrument is expressly devoted to CMB polarization measurements and the whole design has been optimized for minimizing instrumental polarization effects. In this contribution we present the receiver architecture based on correlation techniques, the analysis showing its intrinsic stability and the custom hardware development carried out to detect such a low signal.

astro-ph↗

arXiv2002

Galactic Synchrotron Foreground and the CMB Polarization Measurements

The polarization of the Cosmic Microwave Background (CMB)is a powerful observational tool at hand for modern cosmology. It allows to break the degeneracy of fundamental cosmological parameters one cannot obtain using only anisotropy data and provides new insight into conditions existing in the very early Universe. Many experiments are now in progress whose aim is detecting anisotropy and polarization of the CMB. Measurements of the CMB polarization are however hampered by the presence of polarized foregrounds, above all the synchrotron emission of our Galaxy, whose importance increases as frequency decreases and dominates the polarized diffuse radiation at frequencies below $\simeq 50$ GHz. In the past the separation of CMB and synchrotron was made combining observations of the same area of sky made at different frequencies. In this paper we show that the statistical properties of the polarized components of the synchrotron and dust foregrounds are different from the statistical properties of the polarized component of the CMB, therefore one can build a statistical estimator which allows to extract the polarized component of the CMB from single frequency data also when the polarized CMB signal is just a fraction of the total polarized signal. This estimator improves the signal/noise ratio for the polarized component of the CMB and reduces from about 50 GHz to about 20 GHz the frequency above which the polarized component of the CMB can be extracted from single frequency maps of the diffuse radiation.

astro-ph↗

arXiv2002

Effects of quintessence on observations of Type Ia SuperNovae in the clumpy Universe

We discuss the amplification dispersion in the observed luminosity of standard candles, like supernovae (SNe) of type Ia, induced by gravitational lensing in a Universe with dark energy (quintessence). We derive the main features of the magnification probability distribution function (pdf) of SNe in the framework of on average Friedmann-Lemaitre-Robertson-Walker (FLRW) models for both lensing by large-scale structures and compact objects. The magnification pdf is strongly dependent on the equation of state, $w_Q$, of the quintessence. The dispersion increases with the redshift of the source and is maximum for dark energy with very large negative pressure; the effects of gravitational lensing on the magnification pdf, i.e. the mode biased towards de-amplified values and the long tail towards large magnifications, are reduced for both microscopic DM and quintessence with an intermediate $w_Q$. Different equations of state of the dark energy can deeply change the dispersion in amplification for the projected observed samples of SNe Ia by future space-born missions. The "noise" in the Hubble diagram due to gravitational lensing strongly affects the determination of the cosmological parameters from SNe data. The errors on the pressureless matter density parameter, $Ω_M$, and on $w_Q$ are maximum for quintessence with not very negative pressure. The effect of the gravitational lensing is of the same order of the other systematics affecting observations of SNe Ia. Due to the lensing by large-scale structures, in a flat Universe with $Ω_M =0.4$, at $z=1$ a cosmological constant ($w_Q=-1$) can be interpreted as dark energy with $w_Q <-0.84$ (at 2-$σ$ confidence limit).

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arXiv2001

Separation of the Galactic Synchrotron Foreground and the CMB Polarization Measurements

The polarization of the CMBR represents a powerful test for modern cosmology. It allows to break the degeneracy of fundamental cosmological parameters, and also to observe the contribution of gravitational waves background to the CMBR anisotropy. To observe the CMBR polarization several experiments are either in progress or planned and SPOrt is one of the most promising planned by ESA. At the same time the observation of the CMBR polarization is a difficult task and one of the reasons is the presence of polarized foreground emission. For instance, galactic polarized synchrotron emission (according to some estimates) can completely mimic the polarization of the CMBR. Nevertheless, one can use mathematical properties of the spherical harmonics of the distribution of radiation over the sky to separate different contributions. In this paper the mathematical properties of the polarized synchrotron foreground and the physical mechanism that produces it are discussed. The separation of synchrotron polarization from the polarization generated by density cosmological perturbations is discussed as well.

astro-ph↗

arXiv2000

The parallax distorsion via a weak microlensing effect

Parallax measurements allow distances to celestial objects to be determined. Coupled with measurement of their position on the celestial sphere, it gives a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a remote source affected by a weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. It is shown that the distortions to appear may change strongly the parallax values in case they amount to several microseconds of arc. In particular, at this accuracy many measured values of the parallaxes must be negative.

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