Dina Mousavi

Department of Physics, Sharif University of Technology

Nonlinear Structure Formation in Beyond Cold Dark Matter Models with Nonzero Sound Speed

Supervisor: Shant Baghram

Abstract: The cold dark matter (CDM) paradigm has been successful in explaining large-scale structure formation in the universe, yet it faces significant challenges on length scales smaller than 1 Mpc and mass scales smaller than 10^11 M_sun. So the search for alternative models is ongoing. In this work, we investigate a novel dark matter model based on the dark gravitons in the Dark Dimension scenario, inspired by the Swampland program. We begin by discussing the theoretical underpinnings of the Dark Dimension scenario and how it modifies the properties of dark matter. We then calculate the growth function of matter perturbations in the linear regime, establishing the groundwork for understanding the evolution of density fluctuations. Using the growth functions, We calculate the matter power spectrum in this model. Extending our analysis to the nonlinear regime, we employ excursion set theory to determine the number density of dark matter halos. By analyzing the resulting halo mass function within this dark dimension framework, we evaluate its impact on the missing satellite problem. We also calculate how this model modifies dark matter halo bias.

یکشنبه 18 شهریور 1403، ساعت 10:00

Sunday 8 September – 10:00 Tehran Time

دانشکده فیزیک – طبقه پنجم – کلاس 512 Physics Department fifth floor – Room 512  / 

Behnam Pirayesh

Department of Physics, Sharif University of Technology

Seminar 1:  A Novel Mechanism for Early Dark Energy: Bifurcation Theory and Interacting Dark Matter-Dark Energy model to Resolve the H0 Tension

Rohina Hassan

Department of Physics, Sharif University of Technology 

Seminar 2: Revisiting the step-like features on the Vanilla Inflationary potentials 

Dina Mousavi

Department of Physics, Sharif University of Technology

Seminar 3: Dark matter from the Dark Dimension scenario and structure formation

Mahbod Khordbin

Department of Physics, Sharif University of Technology

 Seminar 4: Simulation of magneto hydrodynamic confinement in magnetic mirrors using Python code

 یکشنبه 31 تیر 1403، ساعت 10:00

Sunday 21 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 Hubble constant (H0) represents the rate at which the universe is expanding. Over the past decade, two primary methods of measuring H0 have resulted in significantly different values, leading to what is known as the “H0 tension.”

1. Direct Measurements (Local Universe): Observations of supernovae and Cepheid variables in the local universe suggest a higher H0 value, around 73-74 km/s/Mpc.

 2. Indirect Measurements (Early Universe): Data from the Cosmic Microwave Background (CMB) measured by the Planck satellite, combined with the standard ΛCDM model, yield a lower H0 value of about 67-68 km/s/Mpc.

This discrepancy has led to the exploration of new physics beyond the standard ΛCDM model to reconcile these differences. One proposed solution is Early Dark Energy (EDE). EDE posits that an additional component of dark energy was present in the early universe, especially around the time of recombination (when the CMB was formed). This early dark energy would have temporarily contributed a significant fraction of the total energy density of the universe, altering the expansion rate and leading to a higher inferred value of H0 from CMB data. While EDE models can potentially resolve the H0 tension, they come with their own set of challenges:

1. Fine-Tuning: EDE models often require fine-tuning of parameters to match observations, which can make them less attractive from a theoretical standpoint.

2. Consistency with Other Observations: Any new model must not only resolve the H0 tension but also remain consistent with a wide range of other cosmological observations, including large-scale structure, baryon acoustic oscillations (BAO), and Big Bang Nucleosynthesis (BBN).

 3. Physical Motivation: The physical origin of EDE is not well understood. The introduction of a new component necessitates a compelling theoretical framework that explains its properties and behavior.

4. Impact on the CMB and LSS: EDE models modify the dynamics of the early universe, which can impact the CMB anisotropies and the formation of large-scale structures. Ensuring that these modifications do not contradict existing data is a significant challenge.

We introduce a new mechanism for EDE via interaction between Dark Matter and Dark Energy (DE-DM interacting model) by using Bifurcation Theory. This theory is a mathematical framework used to study changes in the qualitative or topological structure of a given family of dynamical systems. Bifurcation occurs when a small change in the system parameters causes a qualitative change in its behavior. In our DE-DM interacting model, the number and stability of the equilibrium points are changed due to the Bifurcation phenomenon, and EDE arises naturally in this model.

Abstract of the Seminar 2: Phase transitions in cosmic Inflation have always been under scrutiny due to their natural occurrence in an Inflationary landscape and their ability to produce primordial black holes and gravitational waves.

These transitions can often be translated as local features of inflation potential. However various realizations that can be imagined for such features do not lead to the same results. This may affect the predictions of both large and small scales and recent loop corrections debates.

In the initial steps of this project, we tried to categorize these models by limiting ourselves to local step-like features and identifying the parameters related to the shape of the final power spectrum.

Abstract of the Seminar 3: The cold dark matter (CDM) paradigm has been successful in explaining large-scale structure formation in the universe, yet it faces significant challenges on small scales, including the missing satellite problem. This discrepancy arises from the overprediction of small satellite galaxies around larger galaxies like the Milky Way, as simulations predict more satellites than are observed. In this talk, we investigate a novel dark matter model based on the dark gravitons in the Dark Dimension scenario, inspired by the Swampland program, which offers a promising resolution to this small-scale issue.

We begin by discussing the theoretical underpinnings of the dark dimension scenario and how it modifies the properties of dark matter. We then calculate the growth function of matter perturbations in the linear regime, establishing the groundwork for understanding the evolution of density fluctuations. Extending our analysis to the nonlinear regime, we employ excursion set theory to determine the number density of dark matter halos. By analyzing the resulting halo mass function within this dark dimension framework, we evaluate its impact on the missing satellite problem. Our results indicate that the dark dimension graviton model can significantly alter the predicted abundance of smaller dark matter halos, providing a potential solution to the small-scale challenges faced by the CDM paradigm.

Abstract of the Seminar 4: We know that most of the material world is composed of plasma. Plasma, or the fourth state of matter, is a quasi-neutral gas that consists of charged and neutral particles and exhibits collective behavior. Magnetic mirrors are a method of plasma confinement in which the amount of magnetic field increases and decreases in the direction of the field. Due to the complexity and multiplicity of equations required to investigate plasma, computer simulation is one of our most important tools to study it, which we can study by simulating magnetic mirrors. For simulation, we use pencil code. The pencil code is a modular simulation code with MPI capability for solving partial differential equations and particles. In this research, the shape and behavior of the flow in β is approximately equal to one and the frozen field regime has been investigated and plotted. Also, the rate of changes in the mass of the fluid due to its exit from the two ends of the magnetic mirrors in different regimes has been obtained and analyzed.

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.

Haidar Sheikhahmadi

School of Astronomy, Institute for Research in Fundamental Sciences-IPM

Self-Interacting QFT in Curved Backgrounds, the Bubble Loop Corrections

Abstract:  We consider a general Lagrangian for a massive scalar field with a conformal coupling in the dS background and study the quantum corrections from bubble loop diagrams. Employing a dimensional regularization scheme, we calculate the regularized zero-point energy density, pressure, and the trace of the energy-momentum tensor. It is shown that the classical relation of trace for the vacuum stress-energy tensor receives quantum anomaly correction which depends on the mass and the conformal coupling, while the consistency relation of zero-point energy does hold. We calculate the density contrast associated with the vacuum zero-point energy and show that perturbations are of the order of background values indicate an inhomogeneous and non-perturbative distribution of the zero-point energy. Even more, we calculate the bispectrum associated with the distribution of the zero-point energy and pressure and show that they are highly non-Gaussian. In the next step, incorporating the perturbative in-in formalism, we extend our analysis to self-interacting fields with a general potential of even orders of the field. We again calculate the quantum corrections in the vacuum zero-point energy and pressure for these fields as well. We then investigate the equation of state corresponding to these quantum corrections and examine the scaling of the divergent terms in the vacuum zero-point energy and pressure associated with the dimensional regularization scheme. We discuss some interesting aspects of free and interacting vacua and their relation as well.

یکشنبه 6 خرداد 1403، ساعت 17:00

Sunday 26 May 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

Raihaneh Moti

School of Astronomy, Institute for Research in Fundamental Sciences-IPM

On the GravoThermo Memory 

Abstract: In this presentation, the concepts discussed in arXiv:2307.04151 will be explored. First, I will review the gravitational memory effect. Then, the thermodynamic properties of a freely falling ensemble of gyroscopes after the passage of a weak gravitational wave will be discussed. Due to the precession memory effect, the thermodynamic quantities will experience a change because of the space-time perturbation. This GravoThermo memory effect potentially can be used for the detection of gravitational waves.

  یکشنبه 30 اردیبهشت 1403، ساعت 17:00

Sunday 19 May 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

Amirmohammad Chegeni

Department of Physics and Astronomy “Galileo Galilei”, University of Padova

Clusternets: A deep learning approach to probe clustering dark energy

Abstract: Machine Learning (ML) algorithms are becoming popular in cosmology for extracting valuable information from cosmological data. In this paper, we evaluate the performance of a Convolutional Neural Network (CNN) trained on matter density snapshots to distinguish clustering Dark Energy (DE) from the cosmological constant scenario and to detect the speed of sound ($c_s$) associated with clustering DE. We compare the CNN results with those from a Random Forest (RF) algorithm trained on power spectra. Varying the dark energy equation of state parameter $w_{\rm{DE}}$ within the range of -0.7 to -0.99, while keeping $c_s^2 = 1$, we find that the CNN approach results in a significant improvement in accuracy over the RF algorithm. The improvement in classification accuracy can be as high as 40\% depending on the physical scales involved. We also investigate the ML algorithms’ ability to detect the impact of the speed of sound by choosing $c_s^2$ from the set $\{1, 10^{-2}, 10^{-4}, 10^{-7}\}$ while maintaining a constant $w_{\rm DE}$ for three different cases: $w_{\rm DE} \in \{-0.7, -0.8, -0.9\}$. Our results suggest that distinguishing between various values of $c_s^2$ and the case where $c_s^2=1$ is challenging, particularly at small scales and when $w_{\rm{DE}}\approx -1$. However, as we consider larger scales, the accuracy of $c_s^2$ detection improves. Notably, the CNN algorithm consistently outperforms the RF algorithm, leading to an approximate 20\% enhancement in $c_s^2$ detection accuracy in some cases.

یکشنبه23 اردیبهشت 1403، ساعت 17:00

Sunday 12 May 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 Ansari

Department of Physics, K.N. Toosi University of Technology, Tehran, Iran;

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

The nearest neighbor statistics in cosmology

Abstract: The statistic of matter in non-linear regimes is non-Gaussian, and the two-point correlation function doesn’t have all the information in it. Introducing innovative quantities, along with the two-point correlation function is useful for non-linear regimes. In this direction we introduce the nearest neighbor statistics and show how this function could help us to constrain parameters and cosmological models (such as neutrino mass and fuzzy dark matter). In addition, for the first time we introduce the angle between nearest neighbors to cosmology. Noting that this quantity depends on angles and geometry (and not distances), exploring it may open up new horizons to cosmology. For example, by this quantity we can classify different cosmic web finder approaches.

یکشنبه16 اردیبهشت 1403، ساعت 17:00

Sunday 5 May 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

Farnik Nikakhtar

Department of Physics, Yale University, USA

Year 1 results of the Dark Energy Spectroscopic Instrument with a focus on Baryon Acoustic Oscillations 

Abstract: The Dark Energy Spectroscopic Instrument (DESI) collaboration is conducting a five-year redshift survey of 40 million extragalactic sources across 14,000 square degrees of the northern sky, up to a redshift of 4, using the Mayall 4-meter telescope at Kitt Peak National Observatory. One of its primary goals is to precisely and accurately measure the cosmic expansion history through measurements of baryon acoustic oscillations (BAO). In this talk, I will discuss the measurements from DESI’s first-year Baryon Acoustic Oscillations, utilizing the distributions of galaxies (and quasars) over a redshift range of 0.1-2. For the first time, the DESI team employed catalog-level blinding in the BAO analysis to prevent confirmation bias in determining the expansion history. The resultant aggregate precision of the DESI Year 1 BAO analysis surpasses that of all previous galaxy surveys combined, prior to DESI.

یکشنبه  9 اردیبهشت 1403، ساعت 17:00

Sunday 28 April 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – آمفی تئاتر (تالار جناب) /Physics Department – Amphitheater  (Jenab Hall)   

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

گزینه ورود به صورت مهمان – Enter as a Guest

Ebrahim Siri

Department of Physics, Sharif University of Technology

Thermodynamic properties of a relativistic Bose gas under rigid rotation

Abstract: The investigation of the impact of rotation on fermionic and bosonic systems has recently become a significant area of research in finite temperature field theory. In this work, we focus on a system of relativistic Bose gas subjected to a rigid rotation and analyze its thermodynamic properties. First, we introduce the rigid rotation metric and derive the solutions of the Klein-Gordon equation in cylindrical coordinates. We then calculate the propagator of a rotating bosonic system by making use of the generalized Fock-Schwinger proper-time method. Using the imaginary time formalism at finite temperature, we compute the thermodynamic potential of the system up to the first order of perturbative expansion. Additionally, we determine the nonperturbative ring contribution to the thermodynamic potential. Utilizing this potential, we calculate some thermodynamic quantities, such as pressure, energy density, entropy density, and angular momentum density. The obtained results show that the moment of inertia becomes negative in certain regimes of the parameter space. Our results are similar to the recent findings for the moment of inertia of a rotating gluonic plasma, where a supervortical temperature is introduced.

یکشنبه  2 اردیبهشت 1403، ساعت 17:00

Sunday 21 April 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