Seminars

In this lecture, first we review the theoretical aspects of classical black holes from mass and their horizon definition to their gravitational lensing and Hawking radiation, and then extend our discussion to a real black hole which is located in the expanding FLRW background. Different phases of these black holes like their growth and isolation will be discussed.

Existence of minimum length scale, below which no other length can be observed, is the common feature of quantum gravity candidates such as loop quantum gravity and string theories. In this talk, after brief review of the history of minimum length scale, we introduce some phenomenological approaches to the issue including Generalized Uncertainty Principle, Noncommutative Spacetime, Doubly Special Relativity theories, and recently proposed Gravity's Rainbow formalism. We also discuss how taking a minimum length scale into account can solve the Big Bang singularity problem in the standard model of cosmology.

Integral field Unit (IFU) is an optical instrument enabling us to obtain a 3D view of the galaxy in one shot. The IFU data contain both spectrographic and imaging information of every single pixel of the field. Astronomers use IFU information to measure the motion of gases ripped out of galaxies, the distance of the galaxies and etc.. In this talk the IFU instrument and its different configurations are introduced. I further demonstrate experimental results derived from existing IFUs data. Finally our progress in Technology Development Division toward fabricating an IFU for INO340 telescope is reported.

There is a celebrated theorem by Steven Weinberg, stating that the perturbations in the curvature of the universe on large scales is almost constant. The conservation of the curvature must be seen as a cornerstone of Cosmology since otherwise, one can not relate the early universe initial condition on the statistics of cosmological perturbations to the late-time observations of CMB and LSS. Weinberg uses coordinate transformation invariance of General Relativity to prove this conservation. Consistency relations in Cosmology, on the other hand, are unique tools to discriminate between single field and multiple field models of inflation. CRs basically relates an n-point correlations function of curvature perturbations to its (n-1) point correlation function. Observational violation of CRs is a smoking gun for multiple field models enabling them to rule out all single field models. To prove these consistency relations, the same idea of coordinate invariance of GR must be exploited. The similar nature of these two problems, namely conservation of the curvature perturbations and consistency relations let us study the possibility of their violation in a single set up. In this talk, after reviewing the Weinberg theorem and CRs in Cosmology, I would explain some few known examples in the literature violating both of them. Then I would try to find the reason how is it possible for these models to bypass Weinberg's theorem and CRs. At the end, I would explain the impact of a background curvature on the evolution of curvature perturbations as well as CRs.

Transmission spectroscopy, which entails the detection of minute wavelength-dependent variations of the inferred planetary radius due to an optically thick atmosphere, provides us with a unique opportunity of detecting and characterizing atmospheres of exoplanets. This is essential to understanding the formation of these alien worlds and the evolutionary paths that they embark on.

Ground-based observatories (VLT and Gemini for instance) have played a vital role, and will continue to do so, in complementing results from space-bound telescopes (HST, JWST). I will present recent results from ESO's FORS2 instrument at UT1 of the VLT. These include the improvements made to differential spectrophotometric observations with this instrument after the upgrade of the atmospheric dispersion corrector prisms. I will present results such as those by Nikolov et al. (2016), Sedaghati et al. (2015,2016,2017) which show significant detections of atmospheric properties. Our on-going analysis of WASP-19b, from multi-epoch and multi-grism observations, allows us to make very detailed observations of its atmosphere covering the entire visible wavelength domain, including characterizing haze composition, detection of a large scattering slope, water absorption in the near-IR, as well as multiple other molecules. These results have been driving the theory of exoplanetary atmosphere science, where inclusion of a whole host of new molecular species into retrieval models, has lead to the first discovery of a number of molecules in the atmosphere of an exoplanet.

I will also present results from archival data analysis (WASP-80b; Sedagahti et al. 2017) and how to overcome systematic effects in the data that were taken prior to the upgrade in 2014. These results show that with careful consideration of the optical state parameters that cause systematic trends, one is able to model transit light curves that include correlated noise, and very precise transmission spectra can be obtained. For instance, in the case of WASP-80b, we report the detection of significant absorption in the broad wings of the potassium doublet and the more general deviation from a flat spectrum, as it has been reported previously.

Complexity is ubiquitous behavior in the nature. In cosmology, mainly due to the initial conditions and mostly because of other relevant phenomena such as foreground effects, stochasticity emerges in contrast to deterministic behavior. Mentioned property is widely experienced in cosmological fields in 1, 2 and 3 dimensions. In principle the probabilistic frame work according to statistical points of view, can introduce the robust method not only in data analysis but also in preparing reliable strategies to predict the evolution and for characterization of underlying processes. In this talk, I will rely on stochasticity nature of cosmological fields in various dimensions, and attempt to set up cross-correlation of some geometrical features like local extrema and up-crossing. I will show that mentioned statistics are able to find exotic features hidden in the underlying field and to discriminate between them. As an example I will focus on 2+1D CMB field and look for non-Gaussianity and cosmic strings networks.

Iron line method is a powerful technique to unveil properties of strong gravity regime and constrain non-Kerr spacetime but it is degenerate and high-quality data is necessary. More recently, there have been studies to use this technique to test the nature of astrophysical black holes (BHs) and verify the Kerr BH hypothesis. Geometrically thin and optically thick accretion disk emits fluorescence narrow lines by hard X-ray photon of optically thin comptonized corona. The strongest line is the iron K\alpha𝛼 line, which is at 6.4 keV for neutral atoms and shifts up to 6.97 KeV in the case of ionized H-like iron. This emission line is prominent feature in X-ray reflection spectra. This emission in the inner region of the accretion disk would be broadened and asymmetric due to special and general relativistic effects of compact object. This asymmetric broad profile provide us the opportunity to study the geometry of space time near stellar-mass/supermassive BHs and test strong gravity regime. We currently measure spin and inclination of BH candidates using this method.In this talk, I will discuss possible tests of BH candidates using iron line method.

The origin and evolution of solar sunspots in deep photospheric layers are not yet well understood. We use the best available observations of the partial Sun free of turbulent Earth atmospheric effects from the Solar Optical Telescope (SOT) onboard the Hinode spacecraft, after greatly improving the resolution with an optimum Max likelihood deconvolution with the deducing Point Spread Function (PSF). Both the instrumental smearing and the observed stray light are analyzed in order to improve the resolution. The analysis demonstrates the basic heterogeneous nature of the umbra, similar to a network of minute bright and dark round or elongated cells with a spacing of order of 0.35 ̎. For the first time a complete and detailed map of the color index and temperature deduced from the analysis of deeply corrected continuum images is provided, showing that tiny bright UDs can reach photospheric temperatures and even higher for the peripheral BUDs. In the umbra, there are some very dark small regions with temperatures as low as 3100K that we name them dark Umbral Dots (DUDs), these features can be seen in different wavelength images with different formation height. A set of 125 consecutive deconvolved images is assembled in a 45 min long movie illustrating the complexity of the dynamical behavior inside and around the sunspot. Also in order to study proper motion of Umbral features, velocity maps deduced from FLCT (Fourier local correlation tracking) technique were analysed.

In this talk, we study the gravitational effects of a bubble, formed during inflation via the tunneling of a spectator field from the false vacuum of its potential to its true vacuum, on the inflaton fluctuations and the corrections in the primordial curvature perturbations power spectrum. We assume the observable CMB sphere lies initially outside of the bubble and while the bubble wall expands, the CMB scales come into the bubble. In this setup, the modes which leave the horizon during inflation at the time when the bubble wall collides with the observable CMB sphere are affected the most. We show that the bubble wall induces non-trivial anisotropic and scale dependent corrections in the two point function of the curvature perturbation.

Gaia is the European Space Agency mission which is revolutionizing our knowledge of our Milky Way Galaxy, providing a census of positions, motions, colors, and properties of 2.5 billion stars. Gaia's data will revolutionist much of astronomy, from near-Earth asteroids, through stellar evolution, the structure, formation, and evolution of our Milky Way Galaxy, the distribution of Dark Matter in the Milky Way, the number of planetary systems around other stars, the cosmological distance scale, and fundamental tests of General Relativity. In addition to the wealth of position data, Gaia's camera repeatedly scanning the sky discovers variable and new sources. These are published for follow-up by professional astronomers and by amateur astronomers and school classes, using remotely controlled telescopes across the world. Gaia's first major data release happened on Sept 14, 2016. The second and first major data release will happen in April 2018 and deliver 5-D data for over one billion sources, and 6-8D data for many millions. You can learn more, follow the mission, and download the app at https://gaia.ac.uk [gaia.ac.uk].

Consider a dynamic general relativistic spacetime in which the proper infinitesimal interval along one spatial coordinate direction decreases monotonically with time, while the corresponding intervals increase along other spatial directions. In a system undergoing such complete anisotropic collapse/expansion, we look for the formation of a cosmic double-jet configuration: free test particles in the ambient medium, relative to the collapsing system, gain energy from the gravitational field and asymptotically line up parallel and antiparallel to the direction of collapse such that their Lorentz factors approach infinity. A strong burst of electromagnetic radiation is expected to accompany this event if some of the free test particles carry electric charge. Previous work in this direction involved mainly Ricci-flat spacetimes; hence, we concentrate here on inhomogeneous perfect fluid spacetimes. We briefly explore the possible connection between these theoretical cosmic jets and astrophysical jets. We also discuss other general relativistic scenarios for the formation of cosmic jets.

High resolution imaging of the astronomical objects plays a fundamental role in solving many important scientific problems in astronomy. However, the image quality of a ground-based telescope is seriously degraded by the atmospheric turbulence. To overcome this obstacle, adaptive optics has been used to obtain near diffraction-limited astronomical images. Adaptive optics system consists of a wavefront sensor, a corrector such as a deformable mirror and a control computer to perform real-time numerical calculations. In this talk, first the implementation of the astronomical adaptive optics system will be presented. Then, the technical problems of the wavefront sensor, wavefront correctors and control system will be discussed.

The standard model of cosmology has been understood to be reasonably successful in explaining many observations so far. However, it is faced with some problems on both large and small scales. On large scales, some “anomalies” observed in the Cosmic Microwave Background (CMB) data challenges the statistical isotropy of the Universe. Around 80% of my talk will be about testing the assumption of isotropy for the acceleration of the expansion of the Universe, and also for the distribution of galaxy types. On small scales, the predictions of the standard model of cosmology for the physical properties, number, and spatial distribution of the dwarf satellite galaxies of the Milky Way do not match the observations. In the remaining 20% of my talk I will introduce our dwarf galaxy survey around nearby Milky-Way-like galaxies which aims at gathering data to eventually study the small scale problems outside the Local Group.

Brightest cluster/group galaxies (BCGs/BGGs) are the most massive and luminous galaxies in the Universe. They are generally located close to, or at, the centerof their host haloes. We use a sample of 407 BGGs selected from X-ray galaxy groups with median halo mass of 10^13.8 solar mass detected in the COSMOS, XMM-LSS and AEGIS fields and quantify the relative contribution of the stellar mass of BGGs to the total baryonic mass of hosting haloes in the past 10 billion years. We also investigate the stellar-to-halo- mass relation and the lognormal scatter in the stellar mass at fixed halo mass. In this talk, I will present our recent results and predictions from different galaxy formation models.

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8M logM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the “look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at 2.5-2.9 significance level (corresponding to false detection probability of 1 in 100-270). We report the results of search for echoes in the new black hole merger event GW170104. Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.

Cosmic strings are predicted by various theories to have formed early in the Universe and expected to exist till today. Being a potentially powerful probe of the physics of early Universe, different approaches are proposed and applied to search for their signature on the cosmological data, in particular on the cosmic microwave background (CMB) radiation. In this work, we apply a multi-stage pipeline to the CMB maps to boost the chances of the string detectability and put tight constraints on their energy density. We also use tree-based machine learning algorithms to improve the search, through tightening the constraints on the energy density of the strings, as well as reducing the computational cost.

Within the framework of general relativity, I will discuss gravitational radiation and its recent detection.

The structural properties, kinematical properties and the M/L ratios of 163 GCs in the M31 galaxy in the near infrared (K-band) and optical (V-band) are derived by Strader etal. (2009, 2011). Their sample of GCs exhibits M/L ratios which are considerably lower than what is predicted from SSP models of GCs with a canonical IMF. In addition, one expects that the M/L ratios of SSP models show a positive correlation with metallicity. In this talk, I will address this discrepancy and will propose solutions which are mainly based on the depletion of low-mass stars either due to dynamical evolution, a metallicity and density-dependent top-heavy IMF, and the relation between the age and the metallicity of the GCs.

I report the results of some high-resolution N-body simulations in which the evolution of stellar bar is measured in isolated galactic models. Some models follow the standard view and possess rigid/live dark matter halos. On the other hand, we construct a numeric model in which there is no dark matter halo, and instead, the gravitational force is modified following the prescription introduced in MOG. MOG is a covariant generalization of Einstein's general relativity which has been introduced to address the dark matter problem without invoking exotic particles. I first review this theory from astrophysical and cosmological point of views, and discuss its current status among other alternative theories. Then I report the results of our N-body simulations on the global stability of the disk galaxies, and compare MOG and the standard picture. We show that the stellar bar morphology and kinematics is substantially different in MOG. In fact, MOG yields to weaker stellar bars, and the bar growth rate is smaller in MOG. Finally, I discuss the current observations of galactic bars in order to distinguish between dark matter and modified gravity.

In this seminar, I will talk about new interactions of CMB photons. Then, using quantum Boltzmann equation I calculate the effects of new interactions on a system of CMB photons. I will show that those new interactions that violate parity, can produce circular polarization. Measurement of CMB circular polarization will be signatures of new physics in the early universe.

We apply stochastic methods to single field inflationary models to study the boundary crossing probability for arbitrary potentials. This includes probability of falling from the top of a potential, or escaping from a local minimum. In a suitable regime, we use the method of steepest descent to obtain an approximate formula. We also discuss crossing several barriers in a chain process.

We have studied the structure and luminosity of the supercritical viscous, steady state and axisymmetric accretion flow in the presence of large scale of magnetic field (by considering of the toroidal and vertical components of magnetic field (B_\phi,B_z)) and outflow by using the self-similar solutions in the cylindrical coordinates (r,\phi,z). Our results represent that two components of magnetic field have the opposite effects on the thickness of the disc while the disc accretes and rotates faster by adding two components of magnetic field. Our results show that the luminosity of the slim discs for high values of mass accretion rate is kept constant.

Currently available models for magnetic braking of solar type stars, predict a braking that is too weak to explain the present-day spin of the Sun. That is why in our previously constructed model we needed to artificially increase the strength of that braking by an order of magnitude. Here we investigate whether having a more complex magnetic field topology or and increased intensity of magnetic braking or mass-loss could provide a more physically reliable model. In addition, we show how the model can be used to explain the scarcity of hot Jupiters and brown dwarfs around G-type stars while they are present around their F-type counterparts.

In this talk, first we will show that how the standard model of cosmology known as ΛCDM can describe almost all cosmological observations from cosmic microwave background radiation to the late time large scale structure observations. Then we discuss the observations that introduce tensions with the standard model such as missing baryon problem, missing satellite problem, the H_0 tension and sigma-8 problem. Accordingly, we propose some proposal for solving or relaxing these tensions. In this direction, we assert that the baryon-dark matter relation known as the bias parameter is an important corner-stone in cosmological studies to address these problems properly.

In this talk, first we consider light polarization as a useful probe for the vacuum structure in the presence of strong magnetic fields associated with pulsars. We will present clear predictions which can be tested via the upcoming X-ray polarimetric observations. Moreover, we will report our results about using high-intensity laser facilities in order to test our predictions in the ground-based laser experiments. Finally, we will discuss that it is possible to discriminate different models of Gamma Ray Bursts (GRBs) by using polarization data and even we can look for new physics during inflation.

In this model, we study an interacting dark energy model in the perturbation level. We are studying effect of this model on the structure growth in the universe and also large scale structures, especially ISW effect.

Understanding most interesting things in the universe from fluctuations on CMB, large scales structures to galaxies and stars needs studying universe beyond homogeneous and isotropic limit. This study is the subject of the so-called "cosmological perturbation theory". In this talk, I will start off by a brief review of the cosmological perturbation theory. Afterwards, I will turn to separate universe approach to the evolution of long wave length limit of these perturbations. Then I will make a detour to Weinberg theorem of the adiabatic modes. Finally, I will briefly present our heuristic approach to study soft gravitational perturbations in an-isotropic inflationary models.