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.
The importance of non-linearities in dark energy models
فربد حسنی
(Department of Theoretical Physics of the University of Geneva)
Abstract: I’m going to introduce k-evolution, a relativistic N-body code, which includes clustering dark energy among its cosmological components. I’ll show the effect of dark energy non-linearities on the power spectrum of dark matter, dark energy and the gravitational potential using k-evolution results. I’ll also discuss about the importance of taking dark energy non-linearities into account for doing precision cosmology and I’ll show that for some choice of parameters the dark energy equation suffer from a new instability and blowup in finite time. My talk will be based on arXiv: 1910.01104, arXiv: 1910.01105 and arXiv:1906.04748.
We are pleased to announce a joint program between Sharif and Beheshti universities. Farnik Nikakhtar (University of Pennsylvania) will give four lectures on the topic of structure formation. This workshop is scheduled as follow:– Wednesday 11th Dey @ Shahid Beheshti University and– Wednesday 18th Dey @ Sharif University of Technology. The poster is attached and please register if you like to participate.
جای بسی خرسندی است که برای دومین سال پیاپی روز کیهان شناسی را در شریف برگزار می کنیم. در فایل پیوست، لطفا برنامه روز کیهان شناسی دانشکده فیزیک دانشگاه شریف را بیابید.
این برنامه روز یکشنیه 8 دی ماه 1398 / 29 دسامبر 2019 در آمفی تئاتر دانشکده فیزیک (تالار جناب) از ساعت 15:00 الی 18:30 برگزار می گردد.
Abstract: Recent rapid progress in time domain surveys makes it possible to detect various types of explosive transients (Gamma-ray bursts, Gravitational wave, supernovae, and Fast radio bursts) in the Universe in large numbers, some of which will gravitationally lensed into multiple images. Although a large number of strongly lensed distant galaxies and quasars have already been discovered, strong lensing of explosive transients opens up new applications, including improved measurements of cosmological parameters and powerful probes of small scale structures of the Universe, thanks to their rapidly evolving light curves as well as their compact sizes. Recently we have witnessed first discoveries of strongly lensed supernovae, and strong lensing events of other types of explosive transients are expected to be observed soon. In this presentation, I will review a recent paper on the current state of research on strong gravitational lensing of explosive transients and discuss future prospects.
Swampland Conjectures and Inflating Phases of the Universe (II)
مهدی ترابیان (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: The swampland conjectures that are motivated by string theory offer a list of criteria that every field theory consistent with quantum gravity must admit. In particular, they constrain the inflating phases of the early and late universe. In this talk, we discuss the recently proposed trans-Planckian censorship conjecture and its consequences for inflationary cosmology. It excludes many models of inflation. Specifically, it implies that inflation must be low scale that implies severe fine-tuning of initial conditions and no primordial gravitation waves. We discuss how multiple inflationary models can explain the present horizon problem, alleviate the initial condition problem and accommodate possible detection of gravitational waves.
Swampland Conjectures and Inflating Phases of the Universe
مهدی ترابیان (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: The swampland conjectures that are motivated by string theory offer a list of criteria that every field theory consistent with quantum gravity must admit. In particular, they constrain the inflating phases of the early and late universe. In this talk, we discuss the recently proposed trans-Planckian censorship conjecture and its consequences for inflationary cosmology. It excludes many models of inflation. Specifically, it implies that inflation must be low scale that implies severe fine-tuning of initial conditions and no primordial gravitation waves. We discuss how multiple inflationary models can explain the present horizon problem, alleviate the initial condition problem and accommodate possible detection of gravitational waves.
Gravitational Microlensing and Degeneracy Problem: New Method for Breaking the Degeneracy Using Gaia Catalogue
امیرحسین دهقانی (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: Detection of light blending in a gravitational field, previously predicted by general relativity, stimulated a new line of research for astronomers. In 1986, Paczynski, regarding the dark matter problem, investigated the possibility of observing lensing of a source star light by gravitational field of another star in the Milky Way galaxy. In a simple microlensing event, the only observable parameter that has physical information is the Einstein crossing time; a characteristic time scale of microlensing events. This parameter is a function of lens mass, lens and source distance and their relative velocity. Several methods have been proposed to overcome this degeneracy problem. In this talk, after presenting a brief review of basic formalism and physics, I will show different methods of microlensing degeneracy breaking and will finally present my proposal which is mainly based on Gaia observations. The results show a continuum in mass distribution of lenses with some candidates for neutron stars and black holes.
به مناسبت هفته پژوهش، رویداد روز پژوهش در دانشکده فیزیک برگزار می‌شود و سمینار کیهانشناسی تشکیل نخواهد شد. برنامه رویداد را در فایل پیوست بیابید.
