Cuscuton Bounce

غزال گشنیزجانی

Ghazal Geshnizjani

Department of Applied Mathematics,

Waterloo Centre for Astrophysics University of Waterloo

Affiliate and PSI Fellow, Perimeter Institute for Theoretical Physics

**لطفا  به ساعت نامتعارف سمینار توجه فرمایید**

Abstract: In general relativity producing a regular bounce entails violation of Null Energy Condition for a dynamical source in the model. That generically indicates the existence of ghosts or other instabilities. However, in cuscuton modification of gravity, the correspondence between a background bounce and violation of Null Energy Condition for dynamical sources is broken. Cuscuton action modifies equations of motion in InfraRed limit allowing the background to go through a regular bounce phase. At the same time, since it does not contain any dynamical degrees of freedom, it does not lead to ghosts or other instabilities. Here, I present a toy scenario of a regular bouncing cosmology and prove this claim.

یکشنبه 7 اردیبهشت 1399، ساعت 17:00

Sunday 26 April, 2020 – 17:00 Tehran Time

  اتاق سمینار مجازی –Virtual Seminar Room

https://vclass.ecourse.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

H_0 Tension: New Physics vs Systematics

آرمان شفیعلو

Arman Shafieloo

Faculty of Theoretical Astrophysics, Cosmology Group

Korea Astronomy and Space Science Institute [KASI]

یکشنبه 31 فروردین 1399، ساعت 15:00

  اتاق سمینار مجازی

https://vclass.ecourse.sharif.edu/ch/cosmology

Primordial Black Holes

انسیه عرفانی (دانشکده فیزیک – دانشگاه تحصیلات تکمیلی زنجان )

Abstract: With the discovery of gravitational waves (GW) from merging pairs of massive black holes, the interest in the question of whether Primordial Black Holes (PBHs) could constitute the Dark Matter (DM) has been revived. In this talk, I will review the different mechanisms for (DM) PBHs formation with a special focus on inflation which can source the required large density fluctuations for PBHs formation. I will explain in detail the possibility of PBHs as a candidate for DM. I will review the cosmological constraints on the abundance of PBHs after the discovery of GWs and the ways to distinguish PBHs from astronomical BHs.

یکشنبه 24 فروردین 1399، ساعت 15:00

  اتاق سمینار مجازی

https://vclass.ecourse.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان

دانشجویان گرامی، برای مراجعه به صفحه درس “آشنایی با کیهان” استاد بهرام مشحون در این وبگاه، کافی است به برگه کلاس درس مراجعه کنید.

On The Reliability of photometric and spectroscopic traces of halo relaxation

محمد حسین ژولیده حقیقی 

(پژوهشکده نجوم پژوهشگاه دانش های بنیادی IPM )

Abstract: Characterizing the relaxation state of galaxy systems is a necessary step for probing the role of environment and halo dynamics in the evolution of galaxies. In this presentation, we will provide an assessment of the reliability of both the photometric and spectroscopic probes. We quantify the correlations between the halo relaxation or the dynamical age, which is defined based on the halo mass assembly in the cosmological simulations, with two spectroscopic indicators, namely the Anderson Darling statistic of the galaxy velocity distribution, A2, and the velocity-segregation, ∆V. We also talk about the correlation of the halo relaxation with photometric indicators which are primarily the luminosity gap between the two most luminous galaxies within half the virial radius of a group, complemented with the off-set between brightest group galaxies and the luminosity centroid. We will explain how a combination of ∆m12 and off-set, correlates more strongly with the dynamical age of the halo than the A2, thus offering a reliable yet fast and economical method of quantifying the relaxation of galaxy systems.

یکشنبه ۴ اسفند ۱۳۹۸، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه چهارم تالار پرتوی

Speculations on an early geometrical phase transition: de Sitter–Gödel–de Sitter model

شیرین خدابخشی (دانشکده فیزیک دانشگاه تهران )

Abstract: De Sitter–Gödel–de Sitter (dGd) model is a possible geometrical phase transition from de Sitter to Gödel and back to de Sitter, controlled by a temperature dependent effective potential of a scalar field living on these backgrounds in early times. It is shown that due to a Casimir force in the Gödel phase, this phase transition could induce fluctuations on the matter and radiation fields with possibly observed traces. In this talk, After introducing the theoretical aspects and motivations of this model, a simulation of dGd-induced inhomogeneities and their power spectrum, parameterized by the parameter pair (p1, p2), as possible seeds of CMB anisotropies along with the standard inflationary perturbations will be presented. With the Planck 2018 observations, we find consistent results with pure inflationary power spectrum and no hint for the dGd transition. Future large scale surveys will be also considered to further tighten the constraints. Forecasts with the Euclid survey shows errors would improve by at least an order of magnitude compared to the constraints from CMB temperature and polarization anisotropies.

یکشنبه 27 بهمن 1398، ساعت 15:00

دانشکده فیزیک، طبقه چهارم تالار پرتوی

انتقال فرکانسی در ریز همگرایی گرانشی

سهراب راهوار

 (دانشکده فیزیک دانشگاه صنعتی شریف)

The talk is based on : “Frequency-shift in the gravitational microlensing”, Sohrab Rahvar Phys. Rev. D 101, 024015 – 3 January 2020

یکشنبه 20 بهمن 1398، ساعت 15:00

دانشکده فیزیک، طبقه چهارم تالار پرتوی