Arefe Rasouli
Department of Physics, Sharif University of Technology
Seminar 1: Investigating the Homogeneity and Isotropy of the Universe with Large-scale Structure Data and the Consistency of our Motion to CMB
Haniyeh Tadayoni
Department of Physics, Sharif University of Technology
Seminar 2: The study of the Kinematic and Clustering dipole using cosmological large-scale structure data
Nooshin Torabi
Department of Physics, Sharif University of Technology
Seminar 3: How PBHs and NFW dark matter halos change the number of strongly lensed GW events?
یکشنبه 24 تیر 1403، ساعت 10:00
Sunday 14 July 2024 – 10:00 Tehran Time
Hybrid Seminar
دانشکده فیزیک – طبقه پنجم – کلاس 512 Physics Department fifth floor – Room 512 /
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Abstract of the Seminar 1: The standard ΛCDM model is built upon the cosmological principle, which states that the universe on large scales is homogeneous and isotropic when averaged over sufficiently large scales. However, the cosmic microwave background (CMB) displays a dipole anisotropy at a level of ΔT/T ∼ 10^-3. This dipole is commonly interpreted as owing to our motion with respect to the CMB rest frame. A model-independent approach to validate this kinematic hypothesis is to determine the dipole moment in the angular distribution of the large-scale structure at lower redshifts. Previous observations of the dipole anisotropy in the sky distribution of radio galaxies and quasars drawn from NVSS and WISE catalogs indicate a discrepancy with the CMB dipole, both in terms of direction and amplitude.
In this study, we investigate this inconsistency using large-scale cosmic data. Along with analyzing the Doppler effect, we will also explore the average peculiar velocity of structures and compare it with the standard model. In order to determine the true peculiar velocity, we are trying to distinguish between the kinematic and clustering dipoles. Additionally, we will look into new methods for measuring kinematic velocity and examine the impact of alternative models in resolving this discrepancy.
Abstract of the Seminar 2: The ΛCDM Standard Model of Cosmology relies on the cosmological principle, which asserts that the universe is homogeneous and isotropic on large scales, around 100 megaparsecs, regardless of the observer’s location. Observational evidence from the Cosmic Microwave Background (CMB) radiation, despite minor temperature fluctuations of 1 part in 10,000 on small angular scales, supports this principle. However, the CMB exhibits a dipole anisotropy at a level of which is significantly larger than the smaller-scale anisotropies. This dipole is often interpreted as the result of our motion relative to the CMB rest frame, making its study essential for understanding the universe’s dynamics and the cosmological principle’s validity.
The CMB dipole’s origin is debated, with some attributing it to the Solar System’s motion relative to the CMB rest frame and others to early universe phenomena. Within the ΛCDM framework, the CMB dipole’s characteristics should align with model predictions, helping verify assumptions about dark matter, dark energy, and the universe’s large-scale structure.
Recent studies have explored the origin of the Cosmic Microwave Background (CMB) dipole and its implications for the ΛCDM model. Some research attributes the CMB dipole to the Doppler effect from our movement relative to the CMB frame, while observations of large-scale structures (LSS) have shown deviations from this kinematic interpretation. Additionally, a study using the CatWISE2020 catalog found the dipole amplitude to be more than twice the expected value, raising questions about the cosmological principle in the ΛCDM model and emphasizing the need for a better understanding of the CMB dipole’s origin. Moreover, recent studies have introduced the concept of a clustering dipole, suggesting it may explain discrepancies between the large-scale structure dipole and the CMB dipole, and should be considered in measurements.
In this seminar, we will examine both dipoles, using large-scale structure data such as NVSS to evaluate their magnitudes and analyze the differences in results between linear and non-linear regimes.
Abstract of the Seminar 3: Massive objects in the universe cause deflection of the light rays on their path from the source to the observer; this phenomenon called Gravitational Lensing happens to gravitational waves similar to light rays.
In the case of Strong lensing, creating two or even multiple images with different magnifications of an event is possible. The images could reach the observer with a time delay. Cross section of Strong lensing depends on the lens model and the corresponding Einstein radius.
Strong lensing statistics could provide us information about the expected number of lensed events that reach an observer, and the time delay distribution, which helps us determine if we could detect separate images with a given detector and observation duration.
To determine the expected number of lensed events we need to calculate the merger rate of black holes, the probability of lensing which is capsulated in optical depth, and the detectors’ characteristics. Different lens models could lead to different optical depths and thus change the number of lensed events. Considering dark matter halos as diffuse objects with NFW profile mass density would reduce the probability of lensing compared to the point mass approximation.
The existence of Primordial black holes, which has been a subject of debate in the field, could affect the lensing rate due to acting like a point mass lens and also being a source as an outcome of mergers. Furthermore, because of the range of frequencies that we detect gravitational waves, wave optics can play an important role in lensing.
In this talk, we investigate how considering PBHs as a fraction of dark matter in the universe would change the expected number of lensed GW events and compare the effect of halos by considering different mass profiles for them. We also review the current research in this field.