گروه کیهان‌شناسی

گزینه ورود به صورت مهمان – Enter as a Guest

Tossin’ and Turnin’ in the Landscape

12 فوریه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Amjad Ashoorioon

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

Tossin’ and Turnin’ in the Landscape: Primordial Black Holes and Gravitational Waves

Abstract: The landscape of String Theory could not only contain a lot of metastable vacua, but also a lot of troughs connecting them. These troughs are separated by barriers. Primordial inflation could have happened through rolling on one of these troughs. While rolling slowly on the false vacuum trough during inflation, bubbles of true vacuum nucleate. These bubbles can collapse to primordial black holes (PBHs) after the end of slow-roll inflation. With this picture in mind, we explicitly construct double-field inflationary models at relatively small energy scales, which realizes this picture and satisfies the latest Planck constraints at the scales that contribute to the cosmic microwave background (CMB). We will show that the shape of the mass distribution of the PBHs is dependent on how inflation ends and the Universe settles from the false trough to the true one. The end of inflation can also be probed by examining the gravitational waves spectrum, which could be generated from both a first or second-order phase transition during or after the end of slow-roll inflation. The spectra that these two ways of settling to the true vacuum leave behind, could be distinguished from the shape, and sometimes from the amplitude. In particular, we apply this framework to PBHs in the mass range, 10^{-17}–10^{-13} M_{\odot}, which can constitute the whole dark matter budget and identify a correlated SGWB signal within the DECIGO and BBO sensitivity bands. Alternatively trying to interpret the recently claimed NANOGrav 12.5 year signal within this framework, lead us to LIGO mass and heavier PBHs. We discover that in the latter case, only a first-order phase transition can generate a signal as strong as NANOGrav.

یکشنبه 24 بهمن 1400، ساعت 19:00

Sunday 13 February 2022 – 19:00 Tehran Time

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Probing The Universe with Gravitational Waves

5 فوریه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Valeri Vardanyan

Kavli Institute for the Physics and Mathematics of the Universe (WPI),

UTIAS, The University of Tokyo, Japan

Probing The Universe with Gravitational Waves

Abstract: In this talk, I will summarize several recent results about gravitational wave cosmology in the context of dark energy and inflation. In the first part of the talk, I will concentrate on astrophysical gravitational waves and will argue that the spatial clustering of gravitational wave sources provides a wealth of invaluable information. I will present a new powerful method and will demonstrate its applications for testing dark energy models, and for identifying the potentially primordial origin of gravitational-wave black holes. In the second part of the talk, I will discuss gravitational waves produced during inflation and will revisit the implications of their possible near-future detection for inflationary models. I will particularly present a working proposal of resonant gravitational wave production during inflation due to non-linear effects and will discuss the implications for the well-known Lyth bound.

یکشنبه 17 بهمن 1400، ساعت 19:00

Sunday 6 February 2022 – 19:00 Tehran Time

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Testing Large-Scale Structure Measurements Against Fisher Matrix Predictions

29 ژانویه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Setareh Foroozan

Waterloo Centre for Astrophysics, University of Waterloo

Department of Physics and Astronomy, University of Waterloo

Testing Large-Scale Structure Measurements Against Fisher Matrix Predictions

Abstract: We compare Baryonic Acoustic Oscillation (BAO) and Redshift Space Distortion (RSD) measurements from recent galaxy surveys with their Fisher matrix based predictions. Measurements of the position of the BAO signal lead to constraints on the comoving angular diameter distance DM and the Hubble distance DH that agree well with their Fisher matrix based expectations.

However, RSD-based measurements of the growth rate fσ8 do not agree with the predictions made before the surveys were undertaken, even when repeating those predictions using the actual survey parameters. We show that this is due to a combination of effects including degeneracies with the geometric parameters DM and DH, and optimistic assumptions about the scale to which the linear signal can be extracted. We show that measurements using current data and large-scale modelling techniques extract an equivalent amount of signal to that in the linear regime for k ~< 0.08 h/Mpc, remarkably independent of the sample properties and redshifts covered.

یکشنبه 10 بهمن 1400، ساعت 19:00

Sunday 30 January 2022 – 19:00 Tehran Time

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Handling Observational Challenges in the era of Big Data Cosmology

25 ژانویه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Mehdi Rezaie

Department of Physics, Kansas State University

Handling Observational Challenges in the era of Big Data Cosmology

Abstract: During four distinct phases, the Sloan Digital Sky Survey (SDSS) collected images and spectra of thousands to millions of astronomical objects and created the largest three-dimensional map of the cosmic web to date. As part of SDSS-IV, the extended Baryon Oscillation Spectroscopic Survey (eBOSS) was the final galaxy redshift survey proposed to measure the expansion history and energy contents of the Universe. In this talk, I will present a careful assessment of observational challenges in the final sample of quasars from eBOSS. Then, I will briefly present accompanying studies using the same dataset for constraining the local-type primordial non-Gaussianity and for exploring the impact of observational effects on Baryon Acoustic Oscillations measurements. Finally, I conclude with my ongoing work within the Dark Energy Spectroscopic Instrument (DESI).

یکشنبه 3 بهمن 1400، ساعت 19:00

Sunday 23 January 2022 – 19:00 Tehran Time

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Testing gravity with relativistic effects in large-scale structure

15 ژانویه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Camille Bonvin

Université de Genève, Département de Physique Théorique and

Center for Astroparticle Physics

Testing gravity with relativistic effects in large-scale structure

Abstract: The distribution of galaxies is a power tool to test our cosmological model. In this talk I will discuss the different contributions that affect the observed distribution of galaxies. I will show that besides the standard density and redshift-space distortions, the number counts of galaxies is affected by gravitational lensing and relativistic effects. I will then discuss how one of these effects, called gravitational redshift, can be used to build new tests of gravity.

سه شنبه 28 دی 1400، ساعت 19:00

Tuesday 18 January 2022 – 19:00 Tehran Time

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Luminous and Dark Matter in Massive Early-type Galaxies with Weak Gravitational Lensing‪

8 ژانویه 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Maryam Tajalli

Leiden Observatory

Luminous and Dark Matter in Massive Early-type Galaxies with Weak Gravitational Lensing‪

Abstract: The stellar initial mass function (IMF) is one of the fundamental properties of stellar populations and detecting a variation, or lack thereof, of the IMF with galaxy properties can give us insight into the physics of star formation. However, determining the stellar IMF of massive galaxies is still one of the open problems in cosmology. Future space-based surveys will provide unprecedented weak lensing shape measurements on scales of a few tens of kpc, enabling us to probe the stellar masses of galaxies as well as the inner slope of the dark matter density profile.

In this talk, I will present forecasted constraints on the stellar IMF and stellar-to-halo mass relation of massive early-type galaxies from the future Euclid survey based on the Bayesian hierarchical inference formalism that we have developed. I will show that Euclid is expected to constrain the IMF normalization at the unprecedented level of ∼ 0.02 dex, allowing us to distinguish between a Salpeter and a Chabrier IMF. This is an important result and could lead to a solution to one of the open problems in cosmology, that is, whether the stellar IMF is a universal function or not.

یکشنبه 19 دی 1400، ساعت 19:00

Sunday 9 January 2022 – 19:00 Tehran Time

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Void Galaxy Distribution: A Challenge for ΛCDM

30 دسامبر 2021 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

‪Saeed Tavasoli

Physics Department, Kharazmi University

Void Galaxy Distribution: A Challenge for ΛCDM

Abstract: The under-dense regions of cosmic web provide powerful cosmological laboratories and have been widely used in recent years to extract cosmological information. Since these environments are in the linear regime, this makes them attractive to theoretical modeling and tests.

On the other hand, the low frequency of galaxy mergers inside these regions makes them an ideal environment for the study of the environmental dependence of galaxy formation and evolution with the minimal impact from mergers.

In this talk I will discuss properties of void environments and how void-galaxy distribution could make a possible challenge with LCDM model.

یکشنبه 12 دی 1400، ساعت 19:00

Sunday 2 January 2022 – 19:00 Tehran Time

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Weekly cosmology seminar

25 دسامبر 2021 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Sara Aliqolizadehsafari

Institute for Particle Physics and Astrophysics, ETH, Zurich

Separating Weak Gravitational Lensing and Galaxy Intrinsic Alignments in the Dark Energy Survey (DES)

Abstract: The assumption that galaxies’ orientations are randomly distributed in the universe is not sufficiently accurate when dealing with modern surveys and can lead to contamination of weak lensing measurements with a systematic bias called Intrinsic Alignment (IA).

This bias has to be dealt with properly so as to preserve the accuracy of recent weak lensing measurements. The alignment of elliptical galaxies, especially luminous red galaxies, with the local tidal field at large scales has been observed in recent years. At small scales, however, non-linearities can complicate this behavior. In this work, we have extracted the intrinsic alignment signal using two tomographic bins of DES Y1 source galaxies, 0.3<z<0.45 and 0.55<z<0.7, and the DES redMaGiC lens sample at 0.3<z<0.45. Our method is based on the fact that the measured lensing signal, ΔΣ, is the same for the two tomographic source bins since it depends only on the characteristics of the lens sample.

Statistically, the number of galaxies that contribute to the intrinsic alignment contamination signal is expected to reduce as the separation between lens and source sample increases. Error estimation is done via the Jackknife covariance method using 40 sub-samples. We have found IA signals using two different weighting schemes which both have a signal-to-noise ratio over 10. Finally, we have modeled the intrinsic alignment signal in the tidal alignment framework, using linear theory on large scales and a fully non-perturbative treatment of the one-halo scale. We have found that this model provides good agreement with the measured signal.

یکشنبه 5 دی 1400، ساعت 19:00

Sunday 26 December 2021 – 19:00 Tehran Time

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The Astrophysical Context of Gravitational Wave Events

18 دسامبر 2021 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Mohammad Safarzadeh

NASA-GSFC fellow

The Astrophysical Context of Gravitational Wave Events

Abstract: We live in an era of breakthrough discoveries in gravitational waves (GW) astronomy. Every month or so, such discoveries by LIGO/Virgo have been making headlines because these events’ nature has been far from the expectations. But why are we puzzled? And what is the road ahead for us to a deeper understanding? I discuss two of the puzzling events that LIGO has discovered: 1) The most massive binary black hole merger with masses above the pair-instability limit. After a deep dive into the physics of pair-instability supernova, I will discuss how a broader perspective on the host environment of binary black holes can hold the key to understanding the nature of such massive systems. 2) The most massive binary neutron star merger. I will present possible scenarios to explain why we have not detected such systems in the radio observations before and how the key to understanding these events might lie in the r-process enrichment in the early universe and magnetic field evolution of neutron stars. Through these two examples, I will conclude that the synergy of traditional electromagnetic-wave astronomy with gravitational wave astronomy is essential for gaining insights into the surprises that are being uncovered with these new observations.

یکشنبه 28 آذر 1400، ساعت 19:00

Sunday 19 December 2021 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room