Matching Items (14)

129138-Thumbnail Image.png

Cosmic strings in hidden sectors: 2. Cosmological and astrophysical signatures

Description

Cosmic strings can arise in hidden sector models with a spontaneously broken Abelian symmetry group. We have studied the couplings of the Standard Model fields to these so-called dark strings

Cosmic strings can arise in hidden sector models with a spontaneously broken Abelian symmetry group. We have studied the couplings of the Standard Model fields to these so-called dark strings in the companion paper. Here we survey the cosmological and astrophysical observables that could be associated with the presence of dark strings in our universe with an emphasis on low-scale models, perhaps TeV . Specifically, we consider constraints from nucleosynthesis and CMB spectral distortions, and we calculate the predicted fluxes of diffuse gamma ray cascade photons and cosmic rays. For strings as light as TeV, we find that the predicted level of these signatures is well below the sensitivity of the current experiments, and therefore low scale cosmic strings in hidden sectors remain unconstrained. Heavier strings with a mass scale in the range 10[superscript 13] GeV to 10[superscript 15] GeV are at tension with nucleosynthesis constraints.

Contributors

Agent

Created

Date Created
  • 2014-12-01

129126-Thumbnail Image.png

Implications of a primordial magnetic field for magnetic monopoles, axions, and Dirac neutrinos

Description

We explore some particle physics implications of the growing evidence for a helical primordial magnetic field (PMF). From the interactions of magnetic monopoles and the PMF, we derive an upper

We explore some particle physics implications of the growing evidence for a helical primordial magnetic field (PMF). From the interactions of magnetic monopoles and the PMF, we derive an upper bound on the monopole number density, nðt0Þ < 1 × 10−20 cm−3, which is a “primordial” analog of the Parker bound for the survival of galactic magnetic fields. Our bound is weaker than existing constraints, but it is derived under independent assumptions. We also show how improved measurements of the PMF at different redshifts can lead to further constraints on magnetic monopoles. Axions interact with the PMF due to the gaγφE · B=4π interaction. Including the effects of the cosmological plasma, we find that the helicity of the PMF is a source for the axion field. Although the magnitude of the source is small for the PMF, it could potentially be of interest in astrophysical environments. Earlier derived constraints from the resonant conversion of cosmic microwave background photons into axions lead to gaγ ≲ 10−9 GeV−1 for the suggested PMF strength ∼10−14 G and coherence length ∼10 Mpc. Finally, we apply constraints on the neutrino magnetic dipole moment that arise from requiring successful big bang nucleosynthesis in the presence of a PMF, and we find μν ≲ 10−16 μB.

Contributors

Agent

Created

Date Created
  • 2015-05-20

129350-Thumbnail Image.png

Search for CP violating signature of intergalactic magnetic helicity in the gamma-ray sky

Description

The existence of a cosmological magnetic field could be revealed by the effects of non-trivial helicity on large scales. We evaluate a CP (conjugation plus parity) odd statistic, Q, using

The existence of a cosmological magnetic field could be revealed by the effects of non-trivial helicity on large scales. We evaluate a CP (conjugation plus parity) odd statistic, Q, using gamma-ray data obtained from Fermi satellite observations at high galactic latitudes to search for such a signature. Observed values of Q are found to be non-zero; the probability of a similar signal in Monte Carlo simulations is ∼0.2 per cent. Contamination from the Milky Way does not seem to be responsible for the signal since it is present even for data at very high galactic latitudes. Assuming that the signal is indeed due to a helical cosmological magnetic field, our results indicate left-handed magnetic helicity and field strength ∼10[superscript −14] G on ∼10 Mpc scales.

Contributors

Agent

Created

Date Created
  • 2014-11-21

151369-Thumbnail Image.png

Holography in Rindler space

Description

This thesis addresses certain quantum aspects of the event horizon using the AdS/CFT correspondence. This correspondence is profound since it describes a quantum theory of gravity in d + 1

This thesis addresses certain quantum aspects of the event horizon using the AdS/CFT correspondence. This correspondence is profound since it describes a quantum theory of gravity in d + 1 dimensions from the perspective of a dual quantum field theory living in d dimensions. We begin by considering Rindler space which is the part of Minkowski space seen by an observer with a constant proper acceleration. Because it has an event horizon, Rindler space has been studied in great detail within the context of quantum field theory. However, a quantum gravitational treatment of Rindler space is handicapped by the fact that quantum gravity in flat space is poorly understood. By contrast, quantum gravity in anti-de Sitter space (AdS), is relatively well understood via the AdS/CFT correspondence. Taking this cue, we construct Rindler coordinates for AdS (Rindler-AdS space) in d + 1 spacetime dimensions. In three spacetime dimensions, we find novel one-parameter families of stationary vacua labeled by a rotation parameter β. The interesting thing about these rotating Rindler-AdS spaces is that they possess an observer-dependent ergoregion in addition to having an event horizon. Turning next to the application of AdS/CFT correspondence to Rindler-AdS space, we posit that the two Rindler wedges in AdSd+1 are dual to an entangled conformal field theory (CFT) that lives on two boundaries with geometry R × Hd-1. Specializing to three spacetime dimensions, we derive the thermodynamics of Rindler-AdS space using the boundary CFT. We then probe the causal structure of the spacetime by sending in a time-like source and observe that the CFT “knows” when the source has fallen past the Rindler horizon. We conclude by proposing an alternate foliation of Rindler-AdS which is dual to a CFT living in de Sitter space. Towards the end, we consider the concept of weak measurements in quantum mechanics, wherein the measuring instrument is weakly coupled to the system being measured. We consider such measurements in the context of two examples, viz. the decay of an excited atom, and the tunneling of a particle trapped in a well, and discuss the salient features of such measurements.

Contributors

Agent

Created

Date Created
  • 2012

158152-Thumbnail Image.png

Phenomenology of Topological Solitons

Description

In this dissertation, I present the results from my recent

investigations into the interactions involving topological defects, such as

magnetic monopoles and strings, that may have been produced in

In this dissertation, I present the results from my recent

investigations into the interactions involving topological defects, such as

magnetic monopoles and strings, that may have been produced in the early

universe. I performed numerical studies on the interactions of twisted

monopole-antimonopole pairs in the 't Hooft-Polyakov model for a range of

values of the scalar to vector mass ratio. Sphaleron solution predicted by

Taubes was recovered, and I mapped out its energy and size as functions of

parameters. I also looked into the production, and decay modes of $U(1)$ gauge

and global strings. I demonstrated that strings can be produced upon evolution

of gauge wavepackets defined within a certain region of parameter space. The

numerical exploration of the decay modes of cosmic string loops led to the

conclusions that string loops emit particle radiation primarily due to kink

collisions, and that their decay time due to these losses is proportional to

$L^p$, where $L$ is the loop length and $p \approx 2$. In contrast, the decay

time due to gravitational radiation scales in proportion to $L$, and I

concluded that particle emission is the primary energy loss mechanism for loops

smaller than a critical length scale, while gravitational losses dominate for

larger loops. In addition, I analyzed the decay of cosmic global string loops

due to radiation of Goldstone bosons and massive scalar ($\chi$) particles.

The length of loops I studied ranges from 200-1000 times the width of the

string core. I found that the lifetime of a loop is approximately $1.4L$. The

energy spectrum of Goldstone boson radiation has a $k^{-1}$ fall off, where $k$

is the wavenumber, and a sharp peak at $k\approx m_\chi/2$, where $m_\chi$ is

the mass of $\chi$. The latter is a new feature and implies a peak at high

energies (MeV-GeV) in the cosmological distribution of QCD axions.

Contributors

Agent

Created

Date Created
  • 2020

155435-Thumbnail Image.png

Topics in cosmology and gravitation

Description

Two ideas that extends on the theory of General Relativity are introduced and then the phenomenology they can offer is explored. The first idea shows how certain types of $f(R)$

Two ideas that extends on the theory of General Relativity are introduced and then the phenomenology they can offer is explored. The first idea shows how certain types of $f(R)$ gravity allows for traversable wormholes among its vacuum solutions. This is surprising to find in such simple setting as these type of solutions usually requires fairly complex constructions to satisfy the equations of motion of a gravitational theory. The second idea is the matter bounce description of the early universe where a fairly unique feature of the model is identified. Consequences of this feature could allow the paradigm to distinguish itself from other alternative descriptions, such as inflation, through late time observations. An explicit example of this claim is worked out by studying a model involving an interaction in the dark sector. Results of a more astrophysical nature, where a careful analysis of the morphology of blazar halos is performed, are also presented in the Appendix. The analysis determined that the $Q$-statistic is an appropriate tool to probe the properties of the intergalactic magnetic fields responsible for the halos formation.

Contributors

Agent

Created

Date Created
  • 2017

156309-Thumbnail Image.png

Techniques for the analysis and understanding of cosmic evolution

Description

The Cosmic Microwave Background (CMB) has provided precise information on the evolution of the Universe and the current cosmological paradigm. The CMB has not yet provided definitive information on the

The Cosmic Microwave Background (CMB) has provided precise information on the evolution of the Universe and the current cosmological paradigm. The CMB has not yet provided definitive information on the origin and strength of any primordial magnetic fields or how they affect the presence of magnetic fields observed throughout the cosmos. This work outlines an alternative method to investigating and identifying the presence of cosmic magnetic fields. This method searches for Faraday Rotation (FR) and specifically uses polarized CMB photons as back-light. I find that current generation CMB experiments may be not sensitive enough to detect FR but next generation experiments should be able to make highly significant detections. Identifying FR with the CMB will provide information on the component of magnetic fields along the line of sight of observation.

The 21cm emission from the hyperfine splitting of neutral Hydrogen in the early universe is predicted to provide precise information about the formation and evolution of cosmic structure, complementing the wealth of knowledge gained from the CMB.

21cm cosmology is a relatively new field, and precise measurements of the Epoch of Reionization (EoR) have not yet been achieved. In this work I present 2σ upper limits on the power spectrum of 21cm fluctuations (Δ²(k)) probed at the cosmological wave number k from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) 64 element deployment. I find upper limits on Δ²(k) in the range 0.3 < k < 0.6 h/Mpc to be (650 mK)², (450 mK)², (390 mK)², (250 mK)², (280mK)², (250 mK)² at redshifts z = 10.87, 9.93, 8.91, 8.37, 8.13 and 7.48 respectively

Building on the power spectrum analysis, I identify a major limiting factor in detecting the 21cm power spectrum.

This work is concluded by outlining a metric to evaluate the predisposition of redshifted 21cm interferometers to foreground contamination in power spectrum estimation. This will help inform the construction of future arrays and enable high fidelity imaging and

cross-correlation analysis with other high redshift cosmic probes like the CMB and other upcoming all sky surveys. I find future

arrays with uniform (u,v) coverage and small spectral evolution of their response in the (u,v,f) cube can minimize foreground leakage while pursuing 21cm imaging.

Contributors

Agent

Created

Date Created
  • 2018

158509-Thumbnail Image.png

Emergence of Spacetime: From Entanglement to Einstein

Description

Here I develop the connection between thermodynamics, entanglement, and gravity. I begin by showing that the classical null energy condition (NEC) can arise as a consequence of the second law

Here I develop the connection between thermodynamics, entanglement, and gravity. I begin by showing that the classical null energy condition (NEC) can arise as a consequence of the second law of thermodynamics applied to local holographic screens. This is accomplished by essentially reversing the steps of Hawking's area theorem, leading to the Ricci convergence condition as an input, from which an application of Einstein's equations yields the NEC. Using the same argument, I show logarithmic quantum corrections to the Bekenstein-Hawking entropy formula do not alter the form of the Ricci convergence condition, but obscure its connection to the NEC. Then, by attributing thermodynamics to the stretched horizon of future lightcones -- a timelike hypersurface generated by a collection of radially accelerating observers with constant and uniform proper acceleration -- I derive Einstein's equations from the Clausius relation. Based on this derivation I uncover a local first law of gravity, connecting gravitational entropy to matter energy and work. I then provide an entanglement interpretation of stretched lightcone thermodynamics by extending the entanglement equilibrium proposal. Specifically I show that the condition of fixed volume can be understood as subtracting the irreversible contribution to the thermodynamic entropy. Using the AdS/CFT correspondence, I then provide a microscopic explanation of the 'thermodynamic volume' -- the conjugate variable to the pressure in extended black hole thermodynamics -- and reveal the super-entropicity of three-dimensional AdS black holes is due to the gravitational entropy overcounting the number of available dual CFT states. Finally, I conclude by providing a recent generlization of the extended first law of entanglement, and study its non-trivial 2+1- and 1+1-dimensional limits. This thesis is self-contained and pedagogical by including useful background content relevant to emergent gravity.

Contributors

Agent

Created

Date Created
  • 2020

154965-Thumbnail Image.png

Faint relics of violent high energy physics in the early universe

Description

The work presented in this dissertation examines three different nonequilibrium particle physics processes that could play a role in answering the question “how was the particle content of today’s universe

The work presented in this dissertation examines three different nonequilibrium particle physics processes that could play a role in answering the question “how was the particle content of today’s universe produced after the big bang?” Cosmic strings produced from spontaneous breaking of a hidden sector $U(1)_{\rm X}$ symmetry could couple to Standard Model fields through Higgs Portal or Kinetic Mixing operators and radiate particles that contribute to the diffuse gamma ray background. In this work we calculate the properties of these strings, including finding effective couplings between the strings and Standard Model fields. Explosive particle production after inflation, known as preheating, would have produced a stochastic background of gravitational waves (GW). This work shows how the presence of realistic additional fields and interactions can affect this prediction dramatically. Specifically, it considers the inflaton to be coupled to a light scalar field, and shows that even a very small quartic self-interaction term will reduce the amplitude of the gravitational wave spectrum. For self-coupling $\lambda_{\chi} \gtrsim g^2$, where $g^2$ is the inflaton-scalar coupling, the peak energy density goes as $\Omega_{\rm GW}^{(\lambda_{\chi})} / \Omega_{\rm GW}^{(\lambda_{\chi}=0)} \sim (g^2/\lambda_{\chi})^{2}$. Finally, leptonic charge-parity (CP) violation could be an important clue to understanding the origin of our universe's matter-antimatter asymmetry, and long-baseline neutrino oscillation experiments in the coming decade may uncover this. The CP violating effects of a possible fourth ``sterile" neutrino can interfere with the usual three neutrinos; this work shows how combinations of various measurements can help break those degeneracies.

Contributors

Agent

Created

Date Created
  • 2016

157133-Thumbnail Image.png

Anomalous Chiral Plasmas in the Hydrodynamic Regime

Description

Chiral symmetry and its anomalous and spontaneous breaking play an important role

in particle physics, where it explains the origin of pion and hadron mass hierarchy

among other things.

Despite its

Chiral symmetry and its anomalous and spontaneous breaking play an important role

in particle physics, where it explains the origin of pion and hadron mass hierarchy

among other things.

Despite its microscopic origin chirality may also lead to observable effects

in macroscopic physical systems -- relativistic plasmas made of chiral

(spin-$\frac{1}{2}$) particles.

Such plasmas are called \textit{chiral}.

The effects include non-dissipative currents in external fields that could be present

even in quasi-equilibrium, such as the chiral magnetic (CME) and separation (CSE)

effects, as well as a number of inherently chiral collective modes

called the chiral magnetic (CMW) and vortical (CVW) waves.

Applications of chiral plasmas are truly interdisciplinary, ranging from

hot plasma filling the early Universe, to dense matter in neutron stars,

to electronic band structures in Dirac and Weyl semimetals, to quark-gluon plasma

produced in heavy-ion collisions.

The main focus of this dissertation is a search for traces of chiral physics

in the spectrum of collective modes in chiral plasmas.

I start from relativistic chiral kinetic theory and derive

first- and second-order chiral hydrodynamics.

Then I establish key features of an equilibrium state that describes many

physical chiral systems and use it to find the full spectrum of collective modes

in high-temperature and high-density cases.

Finally, I consider in detail the fate of the two inherently chiral waves, namely

the CMW and the CVW, and determine their detection prospects.

The main results of this dissertation are the formulation of a fully covariant

dissipative chiral hydrodynamics and the calculation of the spectrum of collective

modes in chiral plasmas.

It is found that the dissipative effects and dynamical electromagnetism play

an important role in most cases.

In particular, it is found that both the CMW and the CVW are heavily damped by the usual

Ohmic dissipation in charged plasmas and the diffusion effects in neutral plasmas.

These findings prompt a search for new physical observables in heavy-ion collisions,

as well as a revision of potential applications of chiral theories in

cosmology and solid-state physics.

Contributors

Agent

Created

Date Created
  • 2019