Browsing by Subject "early universe"
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Applying fluctuations to simulations of early universe bubble collisions in O(N) scalar field theory (2023)Many beyond the Standard Model theories include a first order phase transition in the early universe. A phase transition of this kind is presumed to be able to source gravitational waves that might be be observed with future detectors, such as the Laser Interferometer Space Antenna. A first order phase transition from a symmetric (metastable) minimum to the broken (stable) one causes the nucleation of broken phase bubbles. These bubbles expand and then collide. It is of importance to examine how the bubbles collide in depth, as the events during the collision affect the gravitational wave spectrum. We assume the field to interact very weakly or not at all with the particle fluid in the early universe. The universe also experiences fluctuations due to thermal or quantum effects. We look into how these background fluctuations affect the field evolution and bubble collisions during the phase transition in O(N) scalar field theory. Specifically, we numerically simulate two colliding bubbles nucleated on top of the background fluctuations, with the field being a Ndimensional vector in the O(N) group. Due to the symmetries present, the system can be examined in cylindrical coordinates, lowering the number of simulated spatial dimensions. In this thesis, we perform the calculation of initial state fluctuations and simulate them and two bubbles numerically. We present results of the simulation of the field, concentrating on the effects of fluctuations on the O(N) scalar field theory.

(2024)Gravitational waves from cosmological phase transitions are a promising probe of the early universe. Many theories beyond the Standard Model predict the early universe to have undergone a cosmological firstorder phase transition at the electroweak scale. This transition would have produced gravitational waves potentially detectable with the future spacebased detector Laser Interferometer Space Antenna (LISA). We study the gravitational wave power spectrum generated by sound waves, which are a dominant source of gravitational waves from firstorder phase transitions. We compare two methods for calculating the sound wave power spectrum: a simulationmotivated broken powerlaw fit of the shape of the spectrum, and a wider theoretical framework called the Sound Shell Model, which includes hydrodynamic calculations of the phase transition. We present an implementation of the Sound Shell Model into the PTPlot tool, which is currently based on the broken powerlaw fit. With PTPlot, we calculate the signaltonoise ratios of LISA for the sound wave power spectrum of each method. The signaltonoise ratio allows us to estimate the detectability of gravitational wave signals with LISA. We analyse how the detectability of certain particle physics models changes between the two different methods. Our results show that the Sound Shell Model has a potentially significant impact on the signaltonoise ratio predictions, but it does not uniformly improve or worsen the detectability of the gravitational wave signals compared to the broken power law. The code implementation is overall successful and lays the foundation for an updated release of PTPlot and future work within this topic.

(2023)Many extensions to the Standard Model of particle physics feature a firstorder phase transition in the very early universe. This kind of a phase transition would source gravitational waves through the collision of nucleation bubbles. These in turn could be detected e.g. with the future spacebased gravitational wave observatory LISA (Laser Interferometer Space Antenna). Cosmic strings, on the other hand, are linelike topological defects. In this work, we focus on global strings arising from the spontaneous breakdown of a global symmetry. One example of global strings are axionic strings, which are a popular research topic, owing to the role of the axion as a potential dark matter candidate and a solution to the strong CP problem. In this work, our aim is to combine these two sets of earlyuniverse phenomena. We investigate the possibility of creating global strings through the bubble collisions of a firstorder phase transition. We use a simplified model with a twocomponent scalar field to nucleate the bubbles and simulate their expansion, obtaining a shortlived network of global strings in the process. We present results for string lifetime, mean string separations corresponding to different mean bubble separations, and gravitational wave spectra.

(2023)The purpose of this work is to investigate the scaling of ’t HooftPolyakov monopoles in the early universe. These monopoles are a general prediction of a grand unified theory phase transition in the early universe. Understanding the behavior of monopoles in the early universe is thus important. We tentatively find a scaling for monopole separation which predicts that the fraction of the universe’s energy in monopoles remains constant in the radiation era, regardless of initial monopole density. We perform lattice simulations on an expanding lattice with a cosmological background. We use the simplest fields which produce ’t HooftPolyakov monopoles, namely the SU(2) gauge fields and a Higgs field in the adjoint representation. We initialize the fields such that we can control the initial monopole density. At the beginning of the simulations, a damping phase is performed to suppress nonphysical fluctuations in the fields, which are remnants from the initialization. The fields are then evolved according to the discretized field equations. Among other things, the number of monopoles is counted periodically during the simulation. To extend the dynamical range of the runs, the PressSpergelRyden method is used to first grow the monopole size before the main evolution phase. There are different ways to estimate the average separation between monopoles in a monopole network, as well as to estimate the root mean square velocity of the monopoles. We use these estimators to find out how the average separation and velocity evolve during the runs. To find the scaling solution of the system, we fit the separation estimate on a function of conformal time. This way we find that the average separation ξ depends on conformal time η as ξ ∝ η^(1/3) , which indicates that the monopole density scales in conformal time the same way as the critical energy density of the universe. We additionally find that the velocity measured with the velocity estimators depends on the separation as approximately v ∝ dξ/dη. It’s been shown that a possible grand unified phase transition would produce an abundance of ’t HooftPolyakov monopoles and that some of these would survive to the present day and begin to dominate the energy density of the universe. Our result seemingly disagrees with this prediction, though there are several reasons why the predictions might not be compatible with the model we simulate. For one, in our model the monopoles do not move with thermal velocities, unlike what most of the predictions assume happens in the early universe. Thus future work of simulations with thermal velocities added would be needed. Additionally we ran simulations only in the radiation dominated era of the universe. During the matter domination era, the monopoles might behave differently.

(2020)First order electroweak phase transitions (EWPTs) are an attractive area of research. This is mainly due to two reasons. First, they contain aspects that could help to explain the observed baryon asymmetry. Secondly, strong first order PTs could produce gravitational waves (GWs) that could be detectable by the Laser Interferometer Space Antenna (LISA), a future spacebased GW detector. However, the electroweak PT in the Standard Model (SM) is not a first order transition but a crossover. In socalled beyond the SM theories the first order transitions are possible. To investigate the possibility of an EWPT and the detection by LISA, we must be able to parametrise the nature of the PT accurately. We are interested in the calculation of the bubble nucleation rate because it can be used to estimate the properties of the possible GW signal, such as the duration of the PT. The nucleation rate essentially quantifies how likely it is for a point in space to tunnel from one phase to the other. The calculation can be done either using perturbation theory or simulations. Perturbative approaches however suffer from the socalled infrared problem and are not free of theoretical uncertainty. We need to perform a nonperturbative calculation so that we can determine the nucleation rate accurately and test the results of perturbation theory. In this thesis, we will explain the steps that go into a nonperturbative calculation of the bubble nucleation rate. We perform the calculation on the cubic anisotropy model, a theory with two scalar fields. This toy model is one of the simplest in which a radiatively induced transition occurs. We present preliminary results on the nucleation rate and compare it with the thinwall approximation.

(2024)Cosmological firstorder phase transitions (FOPTs) are a hypothetical scenario occurring in the early universe in which bubbles nucleate and expand, generating gravitational waves (GWs). These transitions interest scientists due to their occurrence in extensions to the Standard Model of particle physics, their potential for providing insight into open questions in particle physics and cosmology, and the possibility of observing their signature with the planned Laser Interferometer Space Antenna (LISA). Modeling GW production from FOPTs is thus a topic of active research. In FOPT models, GW production is split into three sources: collisions between bubble walls Ωenv, overlapping fluid shells Ωsw, and fluid turbulence Ωturb. When modeling the contribution from Ωsw in 1D spherical simulations, a sound shell model is often employed which assumes that fluid shells reach a calculable selfsimilar state of expansion before overlapping. In this thesis, I determine when this asymptotic expansion state is reached by defining and calculating a relaxation time ts and transition rate βs for 1D expanding fluid shells. I model two scenarios, a thin and a thickwalled perturbed nucleation bubble expanding in a relativistic fluid, in the limit of fast detonations and weak coupling. In each case, respectively, relaxation temperature and transition rate are determined to be: tsTc = 7.422(21) × 103, βs/Tc = 1.3474(38) × 10−4; and tsTc = 9.901(33) × 105, βs/Tc = 1.011(35) × 10−5. When fixing the critical temperature Tc below which bubbles can nucleate, these results predict that when the transition rate β > βs, the GW spectrum produced assuming relaxed fluid shells may be inaccurate. In addition to this main result, I also compare various methods for estimating bubble wall expansion velocity. These results are useful for 3D simulations, in which direct methods for determining wall velocity are unwieldy.
Now showing items 16 of 6