Seminars

  • 23/01/96 (12 April 2017)
  • Dr. Javad Taghizadeh (IPM)
  • From theoretical black holes to cosmological black holes
  • Abstract:

    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.

  • 30/01/96 (19 April 2017)
  • Dr. MohammadAli Gorji (IPM)
  • Minimum Length Scale and Quantum Gravity Phenomenology
  • Abstract:

    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.

  • 6/02/96 (26 April 2017)
  • Dr. Hesam Arabi (INO)
  • Integral Field Unit in Astronomy
  • Abstract:

    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.

  • 13/02/96 (3 May 2017)
  • Sadra Jazayeri (IPM)
  • On Adiabatic Modes and violation of consistency relations in Cosmology
  • Abstract:

    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.

  • Monday, 8 May (18 Ordibehesht), 11:00 a.m.
  • Elyar Sedaghati (German Space Agency & VLT)
  • Exoplanetary Atmospheric detection with ground-based Transmission Spectroscopy
  • Place: Particles and accelerators seminar room, Larak garden
  • Abstract:

    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.

  • Wednesday, 17 May (27 Ordibehesht), 13:30 pm
  • Dr. Sadegh Movahed (SBU)
  • Looking for exotic features in cosmological stochastic fields
  • Place: Particles and accelerators seminar room, Larak garden
  • Abstract:

    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.

  • 10/03/96 (31 May 2017)
  • Dr. Mahmood Roshan (UM)

  • Sunday, 20 August (29 Mordad), 14:00 p.m
  • Masoumeh Ghasemi-Nodehi (Beijing Observatory)
  • Testing the Kerr Black Hole Hypothesis: X-ray Reflection Spectroscopy
  • Place: Particles and accelerators seminar room, Larak garden
  • Abstract:

    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.

  • Monday, 21 August (30 Mordad), 14:00 p.m
  • Hadis Goodarzi (University of Tabriz)
  • Sunspot: structure, importance and fine structures (UDs)
  • Place: Particles and accelerators seminar room, Larak garden
  • Abstract:

    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.

  • Wednesday, 23 August (1 Shahrivar), 14:00 p.m
  • Asieh Karami Majoomerd (SoA, IPM)
  • Bubble nucleation during inflation
  • Place: Particles and accelerators seminar room, Larak garden
  • Abstract:

    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.

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