Armin Sadeghi

Department of Physics, University of Tehran

Collapse into regular black holes 

Abstract: In this talk, we explore the formation of regular black holes with de Sitter cores, from the gravitational collapse of a massive star based on a generalized Oppenheimer–Snyder collapse model. The collapse scenario leads to a class of general regular black holes with de Sitter cores. We will examine key properties of these black holes, including their horizons, energy conditions, and surface gravity.

یکشنبه  4 آذر 1403، ساعت 17:00

Sunday 24 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Golshan Ejlali

School of Astronomy, Institute for Research in Fundamental Sciences (IPM)

 Investigating Millimeter Emission in Nearby Galaxies 

Abstract:D ust plays a crucial role in the interstellar medium of galaxies, influencing their thermal, chemical, and radiative properties. It absorbs and scatters starlight, leading to extinction and reddening effects, and re-emits the absorbed energy in the far-infrared. Dust also cools the interstellar medium by emitting radiation in infrared and submillimeter wavelengths and acts as a catalyst for molecular hydrogen formation, essential for star formation.  The IMEGIN project (Interpreting Millimeter Emission of Galaxies with IRAM and NIKA2) leverages the NIKA2 camera on the IRAM 30m telescope to explore millimeter properties of galaxies, focusing on dust emission beyond 500 µm to detect cold dust components. This project, led by Dr. Suzanne Madden and involving 14 European institutes in addition to IPM from Iran, aims to disentangle contributions from dust, synchrotron, and free-free emissions. High spatial resolution maps of these emissions will advance our understanding of star formation and dust evolution in galaxies. We utilized multi-wavelength data from Herschel, Spitzer, and Planck in infrared and submillimeter, and from VLA, Effelsberg and WSRT in radio regime to complement NIKA2 observations, constructing detailed spectral energy distributions (SEDs) for a diverse sample of galaxies. This enables the separation of different emission components and derivation of key physical parameters such as dust temperature, mass, and composition. We investigated the relationship between dust properties and galactic characteristics like star formation rate, and morphology, providing insights into dust evolution and galaxy evolution. We also suggest millimeter emission as a powerful extinction-free tracer of molecular gas across various galactic environments.

یکشنبه  27 آبان 1403، ساعت 17:00

Sunday 17 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Ali Rostami

Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan

Black Holes as Catalysts: The Far-Reaching Effects of Black Hole Retention in Evolution of Star Clusters 

Abstract: Massive stars, upon exhausting their thermonuclear fuel, undergo gravitational collapse followed by supernova explosions, leaving behind black holes (BHs) as remnants. These explosions aren’t perfectly symmetrical, and the uneven ejection of mass imparts a “natal kick” to the newborn BH. The magnitude of these kicks remains a subject of debate, creating uncertainty about the initial population of BHs retained in star clusters. High natal kicks lead to the immediate ejection of BHs from their host clusters, while gentler kicks allow them to stay put, forming a sub-system of BHs right in the cluster’s heart. This BH sub-system (BHSub) acts like a bustling, dynamic factory at the cluster’s core, pumping energy into its surroundings and dramatically influencing its evolutionary path.

In this talk, I’ll first address the dynamical processes governing clusters harboring a BHSub, followed by an examination of the emergence and evolution of the dark cluster phase. But that’s not all – I’ll demonstrate how assuming low natal kicks for BHs and the subsequent formation of a BHSub in the cluster’s heart can provide explanations for several outstanding open questions regarding globular clusters:

1. The origin of high dark remnant fractions in Milky Way globular clusters
2. The dichotomy between metal-rich and metal-poor globular clusters in the Milky Way
3. The mysterious lack of metal-poor globular clusters in the inner regions of early-type galaxies

By the end, you’ll see just how far-reaching the effects of BH retention can be in the grand story of star cluster evolution.

یکشنبه  20 آبان 1403، ساعت 17:00

Sunday 10 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Saeed Pourojaghi

Department of Physics, Bu-Ali Sina University, Hamedan.

Recent Challenges to the Standard Cosmological Model 

Abstract: The standard cosmological model (ΛCDM) is considered the first and simplest model to describe the accelerated expansion of the universe. Despite its good compatibility with observational data, this model faces various theoretical and observational challenges. In this talk, I intend to discuss the latest challenges to this model. First, we will examine the redshift evolution of the free parameters of the ΛCDM model using observational data binning. This approach is significant because these parameters are mathematically and observationally considered constants, but our results show different values for them in various redshift bins. In the next part of my talk, we will present a study on the cosmological constraints of dark energy parametrizations post-DESI 2024, suggesting potential deviations from the standard ΛCDM cosmology. Our main goal is to examine the results of DESI collaborations on some extensions of the CPL approximation, known as the BA and Pade parametrizations. Finally, we will provide an analysis of the ΛCDM model using a cosmography approach, which includes recent DESI BAO observations and Type Ia supernova catalogs, including the DES-SN5YR and Pantheon+ sets.

یکشنبه  13 آبان 1403، ساعت 17:00

Sunday 3 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 /Physics Department – first floor – Room Physics 3   

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Samaneh Sarbaz

Department of Physics, Sharif University of Technology

Frequency shift in binary microlensing 

Abstract: Gravitational microlensing with binary lensing is one of the channels for detecting exoplanets. Due to the degeneracy of the lens parameters for the binary microlensing, additional features such as parallax and finite-size effects need to identify the lens parameters. The frequency-shift effect as the relativistic analogy of the gravity assist for the photons, is an extra observation that provides additional constraint between the lens parameters. In this work, we extend the application of the frequency shift effect to binary microlensing and derive the frequency shift during the lensing and caustic crossing. The frequency shift for the binary lens is of the order of $\Delta\nu/\nu∼10^{-12}$. We also investigate the feasibility of detecting this effect by employing Cross-Correlation methods.

یکشنبه  6 آبان 1403، ساعت 17:00

Sunday 27 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Hedieh Abdollahi

School of Astronomy, Institute for Research in Fundamental Sciences (IPM)

The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies VIII, Detection of the Long Period Variable Stars of Andromeda Dwarf Satellite

Abstract: We present an extensive analysis of spheroidal dwarf satellite galaxies of the Andromeda galaxy, aiming to provide a comprehensive catalog of long-period variable (LPV) stars. This study is part of an optical monitoring survey targeting 55 dwarf galaxies and four globular clusters in the Local Group. Observations were conducted using the Wide Field Camera (WFC) on the 2.5-meter Isaac Newton Telescope (INT) over nine epochs, employing the i-band (Sloan) and V-band (Harris) filters. Our focus is on detecting asymptotic giant branch (AGB) stars with magnitude amplitudes exceeding 0.2 mag to trace the evolution of these dwarf galaxies. The resulting catalog of LPV candidates in Andromeda’s satellite galaxies provides updated characteristics such as half-light radii, TRGB magnitudes, and distance moduli. By applying the Sobel filter, we determine distance moduli for these satellites, ranging from 23.57±0.08 to 25.62±0.17 mag. Given that LPV stars in dwarf galaxies span ages from 30 Myr to 10 Gyr, they serve as valuable probes for reconstructing star formation histories (SFHs). The relationship between LPV birth mass and luminosity at the end of their evolution makes them ideal for studying galaxy properties. This catalog will also facilitate the estimation of quenching times, galaxy masses, and the assessment of mass-loss and dust production in these systems.

یکشنبه  29 مهر 1403، ساعت 17:00

Sunday 20 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Sina Hooshangi

School of Astronomy, Institute for Research in Fundamental Sciences (IPM)

Tail Diversity from Inflation

Abstract: Primordial black holes (PBHs) hold significant importance in modern cosmology as possible candidates for dark matter and as potential explanations for the origin of some observed gravitational waves. The primary mechanism for the formation of these black holes is the large fluctuations generated during cosmic inflation. However, conventional perturbative methods used to calculate the statistics of these primordial fluctuations may lead to inaccurate estimates of PBH abundance. In this presentation, we examine a non-perturbative method for describing large fluctuations during the inflationary period. Initially, the concept of non-attractor inflation is introduced, which can potentially produce large perturbations with low but non-zero probability during cosmic inflation. Subsequently, the $\delta N$ formalism is discussed as an effective tool for non-perturbative treatment of primordial fluctuations. We then analyze a set of inflationary models in which the non-perturbative treatment of fluctuations shows significant differences from perturbative calculations. This difference is particularly evident in the tail of the probability distribution of primordial fluctuations. The results of this research indicate that the non-linear behavior of curvature perturbations in inflationary models can lead to the formation of non-Gaussian tails in the probability distribution, which may result in more accurate estimates of PBH abundance. These findings highlight the importance of employing non-perturbative methods in studying PBH formation and demonstrate that conventional linear approximations may not be sufficient for accurately predicting the abundance of these objects.

یکشنبه  22 مهر 1403، ساعت 17:00

Sunday 13 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 /Physics Department – first floor – Room Physics 3   

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Mohammad Hossein Jalali Kanafi

Department of Physics, Shahid Beheshti University

Imprint of Massive Neutrinos on Persistent Homology (Morphology) of Large-Scale Structure 

Abstract: The morphology of the cosmos provides valuable information about the structure of the cosmic web, which can be used for cosmological inferences. In this talk, I will discuss the capability of Persistent Homology (PH) as a morphological tool to investigate the impact of massive neutrinos on the Large-Scale Structure (LSS). PH naturally addresses the multi-scale topological characteristics of the LSS as a distribution of clusters, loops, and voids. Utilizing the Fisher forecast formalism, I will provide a quantitative assessment about the constraining power of some PH vectorizations to estimate the parameters of the $\nu \Lambda CDM$ model.

یکشنبه 15 مهر 1403، ساعت 17:00

Sunday 6 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Nima Khosravi

Department of Physics, Sharif University of Technology,

Department of Physics, Shahid Beheshti University

Random Matrix Gravity and Double Hierarchy Problem 

Abstract: Why are the cosmological constant, electroweak and Planck scales so different? This “double hierarchy” problem, where Λ≪M^2_EW≪M^2_p, is one of the most pressing in fundamental physics. We show that in a theory of N randomly coupled massive gravitons at the electroweak scale, these scales are linked precisely by such a double hierarchy for large N, with intriguing cosmological consequences. Surprisingly, in all the physical scales, only one massless graviton emerges which is also, effectively, the only one that is coupled to matter, giving rise to standard Einstein gravity, with M^2_pGμν=Tμν at large N. In addition, there is a tower of massive gravitons, the lightest of which can drive late-time acceleration. In this scenario, the observed empirical relation ΛMp^2∼M_EW^4 as well as the double hierarchy, arise naturally since since Λ ∼ M_EW^2/√(N) and Mp^2 ∼ √(N)M_EW^2.

یکشنبه  8 مهر 1403، ساعت 17:00

Sunday 29 September 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3   /

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest