arXiv subjects

Scott Watson

Publications and source records attributed to Scott Watson.

At least 55 records · Page 3Linked to original sources

arXiv2012

Decoupling Survives Inflation: A Critical Look at Effective Field Theory Violations During Inflation

We investigate the validity of effective field theory methods and the decoupling of heavy fields during inflation. Considering models of inflation in which the inflaton is coupled to a heavy (super-Hubble) degree of freedom initially in its vacuum state, we find that violations of decoupling are absent unless there is a breakdown of the slow-roll conditions. Next we allow for a temporary departure from inflation resulting in a period of non-adiabaticity during which effective field theory methods are known to fail. We find that the locality of the event and energy conservation lead to a tight bound on the size of the effects of the heavy field. We discuss the implications for the power spectrum and non-gaussianity, and comment on the connection with recent studies of the dynamics of multi-field inflation models. Our results further motivate the use of effective field theory methods to characterize cosmic inflation, and focus the question of observability of additional degrees of freedom during inflation to near the Hubble scale or below - as anticipated from the Wilsonian notions of decoupling and naturalness.

hep-th↗

arXiv2012

The Importance of Slow-roll Corrections During Multi-field Inflation

We re-examine the importance of slow-roll corrections during the evolution of cosmological perturbations in models of multi-field inflation. We find that in many instances the presence of light degrees of freedom leads to situations in which next to leading order slow-roll corrections become significant. Examples where we expect such corrections to be crucial include models in which modes exit the Hubble radius while the inflationary trajectory undergoes an abrupt turn in field space, or during a phase transition. We illustrate this with two examples -- hybrid inflation and double quadratic inflation. Utilizing both analytic estimates and full numerical results, we find that corrections can be as large as 20%. Our results have implications for many existing models in the literature, as these corrections must be included to obtain accurate observational predictions -- particularly given the level of accuracy expected from CMB experiments such as Planck

astro-ph.CO↗

arXiv2011

The Baryon-Dark Matter Ratio Via Moduli Decay After Affleck-Dine Baryogenesis

Low-scale supersymmetry breaking in string motivated theories implies the presence of O(100) TeV scale moduli, which generically lead to a significant modification of the history of the universe prior to Big Bang Nucleosynthesis. Such an approach implies a non-thermal origin for dark matter resulting from scalar decay, where the lightest supersymmetric particle can account for the observed dark matter relic density. We study the further effect of the decay on the baryon asymmetry of the universe, and find that this can satisfactorily address the problem of the over-production of the baryon asymmetry by the Affleck-Dine mechanism in the MSSM. Remarkably, there is a natural connection between the baryon and dark matter abundances today, which leads to a solution of the `Cosmic Coincidence Problem'.

hep-ph↗

arXiv2011

Constraints on a Non-thermal History from Galactic Dark Matter Spikes

In this paper we examine whether indirect detection constraints on dark matter associated with a non-thermal history may be significantly improved when accounting for the presence of galactic substructure in the form of dark matter spikes. We find that significant constraints may be derived from the non-observation of an excess of diffuse gamma-rays and from the properties of bright gamma-ray point sources observed by the Fermi Gamma-Ray Space Telescope, but these constraints depend sensitively on the details of the formation of the first stars and their subsequent black hole remnants. However, we also find that, especially if WIMPs annihilate primarily to quarks or gauge bosons, it is possible to extract meaningful and conservative bounds on the annihilation cross section.

astro-ph.CO↗

arXiv2010

A Unified Approach to Cosmic Acceleration

We present a unified framework for the study of late time cosmic acceleration. Using methods of effective field theory, we show that existing proposals for late time acceleration can be subsumed in a single framework, rather than many compartmentalized theories. We construct the most general action consistent with symmetry principles, derive the background and perturbation evolution equations, and demonstrate that for special choices of our parameters we can reproduce results already existing in the literature. Lastly, we lay the foundation for future work placing phenomenological constraints on the parameters of the effective theory. Although in this paper we focus on late time acceleration, our construction also generalizes the effective field theory of inflation to the scalar-tensor and multi-field case for perturbatively constructed backgrounds.

hep-th↗

arXiv2009

Reevaluating the Cosmological Origin of Dark Matter

The origin of dark matter as a thermal relic offers a compelling way in which the early universe was initially populated by dark matter. Alternative explanations typically appear exotic compared to the simplicity of thermal production. However, recent observations and progress from theory suggest that it may be necessary to be more critical. This is important because ongoing searches probing the microscopic properties of dark matter typically rely on the assumption of dark matter as a single, unique, thermal relic. On general grounds I will argue that non-thermal production of dark matter seems to be a robust prediction of physics beyond the standard model. However, if such models are to lead to realistic phenomenology, they must sit in a restrictive theoretical framework. As we will show, as a consequence of such restrictions, viable models will result in concrete and testable predictions. Although many challenges remain, the non-thermal component of such models may offer a new way to test string theories that are formulated to provide realistic particle physics near the electroweak scale.

hep-th↗

arXiv2009

A Non-thermal WIMP Miracle

Light scalar fields with only gravitational strength couplings are typically present in UV complete theories of physics beyond the Standard Model. In the early universe it is natural for these fields to dominate the energy density, and their subsequent decay, if prior to BBN, will typically yield some dark matter particles in their decay products. In this paper we make the observation that a Non-thermal WIMP `Miracle' may result: that is, in the simplest solution to the cosmological moduli problem, non-thermally produced WIMPs can naturally account for the observed dark matter relic density. Such a solution may be generic in string theory compactifications.

astro-ph.CO↗

arXiv2009

PAMELA Satellite Data as a Signal of Non-Thermal Wino LSP Dark Matter

Satellite data is accumulating that suggests and constrains dark matter physics. We argue there is a very well motivated theoretical preexisting framework consistent with dark matter annihilation being observed by the PAMELA satellite detector. The dark matter is (mainly) the neutral W boson superpartner, the wino with mass below 200 GeV. Using the program GALPROP we study the annihilation products and backgrounds together. Antimatter and gammas from annihilating winos contribute below this energy. We explain why PAMELA data does not imply no antiproton signal was observed by PAMELA or earlier experiments, and explain why the antiproton analysis was misunderstood by earlier papers. Wino annihilation does not describe the Fermi e+ + e- data (except partially below ~ 100 GeV). At higher energies we expect astrophysical mechanisms, and we simply parameterize them so the combination can describe all the data. We emphasize several predictions for satellite data to test the wino interpretation, particularly the turndown of the positron and antiproton spectra above 100 GeV. Most other interpretations require a large rise in the positron rates above 100 GeV. We focus on studying this well-motivated and long predicted wino interpretation, rather than comparisons with other interpretations. We emphasize that interpretations also depend very strongly on assumptions about the cosmological history of the universe, and on propagation of antiprotons and positrons in the galaxy. The winos PAMELA is observing arose from some non-thermal sources such as moduli decay rather than a universe that cooled in thermal equilibrium after the big bang. Then it is appropriate to normalize the wino density to the local relic density, and no "boost factors" are needed to obtain the reported PAMELA rates.

astro-ph.HE↗

arXiv2009

CMBPol Mission Concept Study: Probing Inflation with CMB Polarization

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale-dependence and non-Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.

astro-ph↗

arXiv2008

Is the PAMELA Positron Excess Winos?

Recently the PAMELA satellite-based experiment reported an excess of galactic positrons that could be a signal of annihilating dark matter. The PAMELA data may admit an interpretation as a signal from a wino-like LSP of mass about 200 GeV, normalized to the local relic density, and annihilating mainly into W-bosons. This possibility requires the current conventional estimate for the energy loss rate of positrons be too large by roughly a factor of five. Data from anti-protons and gamma rays also provide tension with this interpretation, but there are significant astrophysical uncertainties associated with their propagation. It is not unreasonable to take this well-motivated candidate seriously, at present, in part because it can be tested in several ways soon. The forthcoming PAMELA data on higher energy positrons and the FGST (formerly GLAST) data, should provide important clues as to whether this scenario is correct. If correct, the wino interpretation implies a cosmological history in which the dark matter does not originate in thermal equilibrium.

hep-ph↗

arXiv2008

Neutralino Dark Matter from Indirect Detection Revisited

We revisit indirect detection possibilities for neutralino dark matter, emphasizing the complementary roles of different approaches. While thermally produced dark matter often requires large astrophysical "boost factors" to observe antimatter signals, the physically motivated alternative of non-thermal dark matter can naturally provide interesting signals, for example from light wino or Higgsino dark matter. After a brief review of cosmic ray propagation, we discuss signals for positrons, antiprotons, synchrotron radiation and gamma rays from wino annihilation in the galactic halo, and examine their phenomenology. For pure wino dark matter relevant to the LHC, PAMELA and GLAST should report signals.

hep-ph↗

arXiv2008

Dark Matter and LHC: What is the Connection?

We review what can (and cannot) be learned if dark matter is detected in one or more experiments, emphasizing the importance of combining LHC data with direct, astrophysical and cosmological probes of dark matter. We briefly review the conventional picture of a thermally produced WIMP relic density and its connection with theories of electroweak symmetry breaking. We then discuss both experimental and theoretical reasons why one might generically expect this picture to fail. If this is the case, we argue that a combined effort bringing together all types of data -- combined with explicitly constructed theoretical models -- will be the only way to achieve a complete understanding of the dark matter in our universe and become confident that any candidate actually provides the relic density.

hep-ph↗

arXiv2008

Inflation without Inflaton(s)

We propose a model for early universe cosmology without the need for fundamental scalar fields. Cosmic acceleration and phenomenologically viable reheating of the universe results from a series of energy transitions, where during each transition vacuum energy is converted to thermal radiation. We show that this `cascading universe' can lead to successful generation of adiabatic density fluctuations and an observable gravity wave spectrum in some cases, where in the simplest case it reproduces a spectrum similar to slow-roll models of inflation. We also find the model provides a reasonable reheating temperature after inflation ends. This type of model may also be relevant for addressing the smallness of the vacuum energy today.

hep-th↗

arXiv2008

Non-thermal Dark Matter and the Moduli Problem in String Frameworks

We address the cosmological moduli/gravitino problems and the issue of too little thermal but excessive non-thermal dark matter from the decays of moduli. The main examples we study are the G2-MSSM models arising from M theory compactifications, which allow for a precise calculation of moduli decay rates and widths. We find that the late decaying moduli satisfy both BBN constraints and avoid the gravitino problem. The non-thermal production of wino LSPs, which is a prediction of G2-MSSM models, gives a relic density of about the right order of magnitude.

hep-ph↗

arXiv2008

Geometric Precipices in String Cosmology

We consider the effects of graviton multiplet fields on transitions between string gas phases. Focusing on the dilaton field, we show that it may obstruct transitions between different thermodynamic phases of the string gas, because the sign of its dimensionally reduced, $T$-duality invariant, part is conserved when the energy density of the universe is positive. Thus, many interesting solutions for which this sign is positive end up in a future curvature singularity. Because of this, some of the thermodynamic phases of the usual gravitating string gases behave like superselection sectors. For example, a past-regular Hagedorn phase and an expanding FRW phase dominated by string momentum modes cannot be smoothly connected in the framework of string cosmology with positive sources. The singularity separates them like a geometric precipice in the moduli space, preventing the dynamics of the theory from bridging across. Sources which simultaneously violate the positivity of energy and NEC could modify these conclusions. We provide a quantitative measure of positivity of energy and NEC violations that would be necessary for such transitions. These effects must dominate the universe at the moment of transition, altering the standard gas pictures. At present, it is not known how to construct such sources from first principles in string theory.

hep-th↗

arXiv2007

Cosmological Moduli Dynamics

Low energy effective actions arising from string theory typically contain many scalar fields, some with a very complicated potential and others with no potential at all. The evolution of these scalars is of great interest. Their late time values have a direct impact on low energy observables, while their early universe dynamics can potentially source inflation or adversely affect big bang nucleosynthesis. Recently, classical and quantum methods for fixing the values of these scalars have been introduced. The purpose of this work is to explore moduli dynamics in light of these stabilization mechanisms. In particular, we explore a truncated low energy effective action that models the neighborhood of special points (or more generally loci) in moduli space, such as conifold points, where extra massless degrees of freedom arise. We find that the dynamics has a surprisingly rich structure - including the appearance of chaos - and we find a viable mechanism for trapping some of the moduli.

hep-th↗

arXiv2007

Living on a dS brane: Effects of KK modes on inflation

We develop a formalism to study non-local higher-dimensional effects in braneworld scenarios from a four-dimensional effective theory point of view and check it against the well-known Garriga-Tanaka result in the appropriate limit. We then use this formalism to study the spectrum of density perturbations during inflation as seen from the lower-dimensional effective theory. In particular, we find that the gravitational potential is greatly enhanced at short wavelengths. The consequences to the curvature perturbations are nonetheless very weak and will lead to no characteristic signatures on the power spectrum.

hep-th↗

arXiv2006

Moduli Stabilization with Long Winding Strings

Stabilizing all of the modulus fields coming from compactifications of string theory on internal manifolds is one of the outstanding challenges for string cosmology. Here, in a simple example of toroidal compactification, we study the dynamics of the moduli fields corresponding to the size and shape of the torus along with the ambient flux and long strings winding both internal directions. It is known that a string gas containing states with non-vanishing winding and momentum number in one internal direction can stabilize the radius of this internal circle to be at self-dual radius. We show that a gas of long strings winding all internal directions can stabilize all moduli, except the dilaton which is stabilized by hand, in this simple example.

hep-th↗