Sharif Digital Repository

The inflation paradigm solves the so-called flatness problem of the standard cosmo- logical model and explains the homogeneity of the Universe by adding a period of exponential expansion at early times to the standard scenario. Moreover, it provides a mechanism for generating density perturbations, serving as the seed of large-scale structures. One of the successes of inflation is predicting primordial scale-invariant perturbations that agree with the observations of cosmic background radiation. Nev- ertheless, there will be a cold, matter-free universe after exponential expansion, so one needs a process that leads to a hot cosmos containing standard particles after the end of inflation, the... 

Inflation first introduced for solving the puzzles of the standard model of cosmology. Soon it became clear that this paradigm can provide a natural way for production of Initial conditions for the birth of large-scale structures: cosmological perturbations. But inflationary models are usually phenomenological models. They are constructed to solve the problems of the standard model of cosmology and explain the properties of cosmological fluctuations. Where do they come from? A property of a big family of inflationary models is the flatness of their potential. What would may be the cause of this flatness? . . .. In this thesis our purpose is searching about one probable field theoretic root... 

In this project we investigate the effect of coupling of the inflaton with light particles which have oscillating mass that depends on inflaton. As the inflaton slowly rolls towards the minimum of its potential, the mass of other particles changes periodically(monodromy). The monodromy structure with tuning the parameters of model allows us to produce ultrarelativistic particles during inflation. These particles dilute with the expansion of the universe but continuous particle production leads to burst of particles at the end of inflation. It should be noted that other Warm Inflation models are phenomenological models which is assumed that during the inflation, the inflaton energy is... 

Standard model of cosmology successfully describes the properties of the universe on large scales. General relativistic perturbation theory accurately describes how the initial fluctuations created in the inflation era grew larger and became the seeds of structure formation later in the radiation and matter dominated era. However, linear perturbation theory breaks at the scales of approximately 10Mpc. In such scales, the gravitational effects are non-linear and we should use standard pertubation theory to find the corrections to the linear power spectrum. In this thesis, I calculated these non-linear contributions analytically and proved the stability of the linear point which is believed to... 

n this thesis, the formation of cosmic structures in the presence of fuzzy dark matter (FDM) is investigated using the Gevolution code. In the FDM model, dark matter is composed of ultralight bosons. In fact, the mentioned bosons are the remnants of a scalar field present in the early universe. Simulations of structure formation in the presence of FDM that have been carried out so far (for example with Gadget and RAMSES) are based on Newtonian gravity, which have a number of limitations. The Gevolution code is an N-Body simulation that, unlike other existing simulations, is based on General Relativity, which can lead to interesting results. In fact, in this simulation, the weak field limit... 

A variety of early universe models predict the formation of primordial black holes. Primordial black holes can be a candidate for a fraction or total dark matter in the universe. Moreover, mergers of these black holes can produce gravitational waves. After re-entering the horizon, having a sufficiently large amplitude, the initial fluctuations of cosmic matter can collapse to primordial black holes. This thesis studies an inflationary potential that can amplify the power spectrum of curvature perturbations up to several orders of magnitude via imposing some features on the potential. This power amplification on small scales can lead to the formation of primordial black holes. For simplicity,... 

The title of this thesis is cosmological perturbation theory in brane cosmology. In this thesis, we investigate brane cosmology, specifically we will mainly investigate super-horizon curvature perturbation during inflation, radiation-dominated and matter-dominated eras. The areas to be reviewed and the steps necessary to reach the final conclusion of this thesis are as follows: Firstly, we will have an overview of brane cosmology. Then, we will investigate the inflation within the brane cosmology framework. Furthermore, by investigating the cosmological perturbation theory during inflation, radiation-dominated and matter-dominated eras, we investigate the evolution of curvature perturbation... 

From 1990’s the Cosmological Constant Problem was brought back to spotlight and physicists have tried to find out its source. But the value that particle physic anticipates for cosmological constant is not harmonic with cosmological considerations. This thesis investigates the studies in this field  

Primordial black holes have attracted attention because of their possible role in explaining phenomena such as dark matter and gravitational waves. These observations require the primordial black holes to be in different mass ranges. We investigated the inflaton potentials in which the stochastic sequence of sharp step-like features on the slow-roll potential can produce primordial black holes in different mass ranges. To this end, we first studied the cumulative effects of consecutive steps and showed that there is a resonance mechanism that leads to an exponential enhancement of the power spectrum of certain wavelengths.
 

During the past 20 years, there has been an ever¬increasing interest in searching for anomalies or features beyond the standard scale¬invariant Gaussian spectrum of the CMB in the hope of finding signals of new physics. In this study, a new type of anomaly on the cosmic microwave background called the Inflationary Cherenkov effect is studied. By first simulating the expected effects and using machine learning methods such as convolutional neural networks, we obtain methods to detect this effect. Afterward, with the help of the detector, we study real data, namely temperature anisotropies of the CMB. Finally, we infer the speed of propagation of scalar field perturbations whose energy runs... 

Titanium alloy Ti-6Al-4V ELI with high biocompatibility and corrosion resistance, has a lot of applications in biomedical engineering. Disadvantage of this alloy is it’s disability to create a fast and good contact with the host/bone environment, after implanting in the body. Beside that it has low wear resistance. Nowadays to optimize the wear resistance, bioactivity and osteoconduction of surface of implants which are made from this alloy, the surface morphology are optimized in size and distribution. Different surface treatments are used for producing rough and porous surfaces to improve bioactivity along with wear resistance. In this study, surface modification of Ti-6Al-4V ELI was done... 

Commercially pure titanium (CP-Ti) is an important groups of titanium family and because of high strength, low density, high corrosion resistance and biocompatibility, this group of titanium is a suitable choice for application at chemical, petrochemical and medicine industry. Nowadays surgeons have a tendency toward use of CP-Ti instead of Ti-6Al-4V alloy. Moreover, because of body anatomy limitation, use of thick sheets result in high volume and damage soft tissue. Therefore, for sheet thickness reduction, study of the effects of metallurgical and geometrical parameter on fracture behavior of thin sheets is important. In this study, the fracture behavior of CP-Ti thin sheets investigated.... 

The introduction of multi phase steels in recent years resulted in a considerable increase in development of some industries such as automobiles. By inducing soft and hard phases together, these steels could bring better mechanical properties than classical ones. Recent studies on 4340 ferritic- bainitic and ferritic- martensitic dual phase steels, indicate that 34 volume percent ferrite with hard phase, has the best combination of strength, toughness and fatigue properties. In present research, by proper heat treatment on a 4340 steel, it is tried to prepare specimens ferritic- bainitic- martensitic triple phase steels with 34 volume percent of ferrite and different percents of martensite.... 

In the research transient liquid phase bonding of AL6061-SiC composite was performed using copper as interlayer. The joining praocess was performed in various tetemperatures (560c and 580c and 600c) And times (6 and 8 hours). Optimum Joining condition was determined by using optical and of the joint, micro hardness test was performed from the bonding zone to the bulk of the base composite. The fatigue endurance limit of Al6061-SiC composite was determined by several fatigue tests and TLP bonded samples were loaded under achieved endurance limit stress. Finally fatigue fracture surface of both base composite and TLP bonded samples were studied by using of scanning electronic microscope  

In this research, transient liquid phase (TLP) bonding of FSX-414 superalloy was investigated using MBF-80 interlayer. The bonding was performed at different temperatures (1050-1200oC) for various times (1-120min). Also, different gap sizes were studied using the interlayer with various thicknesses (25-100µm). Homogenizing treatment was performed at different temperatures (1175-1225oC) and times (1-6h) for the samples with complete isothermal solidification. The microstructure of the samples were studied using the optical, scanning electron and transmission electron microscopes. Also, XRD, SEM/EDS, SEM/WDS and TEM/EDS analyses were used to analyse the observed phases at the joints. To... 

AISI 4340 steel bars were austenitized at 900°C for 1 hour followed by heating at 740°C (ferrite and austenite region) for 100 min and quenching into a salt bath at 300°C for different times followed by quenching into water to obtain triple phase microstructures with 34 Vol.% ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent of each other confirmed by metallographic analysis and TEM technique. Volume fraction of different phases was measured by image analyser. The results of optical microscopy showed that by increasing VM, morphology of martensite varies from uniform distribution of small spherical network particles to large blocky islands with... 

Interlayer composition as one of the most important factors of TLP bonding could affect bonding region composition and microstructure that both of them govern the mechanical properties of the joint. Due to the risk of substrate microstructure degradation , welding of dual phase steel needs more cautions. TLP bonding ideally acquires joints having more similar microstructure to substrate. Moreover , using this method could avoid the change of base metal microstructure when heat treatment of carbon steel is postponed until after bonding. Fe-based , Ni-based and commercially pure Cu are three different composition has been studied in this work. The bonded samples were investigated by optic and... 

Dual Phase ferritic-martensitic steels are kind of high strength low alloy steels (HASLA) which are widely used in industry because of their strength and formability. Since welding of dual phase steels in conventional welding methods causes structural change and Subsequently alters mechanical properties of both weld zone and heat affected zone(HAZ), additional operations which are not commercially viable is needed to achieve a dual phase structure in structurally changed regions. In this study, transient liquid phase (TLP) bonding method is used during dual phasing process in order to preserve dual phase ferritic-martensitic structure and avoid extra heat treatment. Applied pressure during... 

Stainless steels have many applications in different industries like chemistry, medicine and food production. So for using these alloys it is necessary to join them with appropriate method. In this research, microstructure-mechanical properties relationship of 304LC austenitic stainless steel transient liquid phase bonded using a Co-based interlayer was investigated. For this purpose, bonding was conducted at 1180 ℃ for different holding times. Microstructure of the joints was evaluated by optical microscopy (OM) and Scanning Electron Microscopy (SEM). The results showed that isothermal solidification (IS) was completed within 1800s and no intermetallic compound was formed in the joint area... 

After World War II, steel bearings containing copper element have been developed by the Navy of the United States of America with the HSLA-100 brand. Since 1890 and in order to manufacture tanks and tools for the body of navy equipment, the steel HY 80 was used instead of HSLA-100. Because of the high carbon content, the welding properties of the HY 80 are low. Furthermore, increasing of hardness and decreasing of toughness in the heat-affected zone generated during the welding process lead to susceptibility of this alloy to hydrogen cracking and brittle fracture. Therefore, in order to minimize the cost of welding, low-alloy high strength steel (HSLA) is used which contain copper. Reducing...