Browsing by Subject "early universe"

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 first-order phase transition at the electroweak scale. This transition would have produced gravitational waves potentially detectable with the future space-based 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 first-order phase transitions. We compare two methods for calculating the sound wave power spectrum: a simulation-motivated broken power-law 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 power-law fit. With PTPlot, we calculate the signal-to-noise ratios of LISA for the sound wave power spectrum of each method. The signal-to-noise 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 signal-to-noise 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.

The purpose of this work is to investigate the scaling of ’t Hooft-Polyakov 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 Hooft-Polyakov 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 Press-Spergel-Ryden 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 Hooft-Polyakov 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.

Cosmological first-order 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 self-similar 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 thick-walled 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.