arXiv subjects

Mark Wyman

Publications and source records attributed to Mark Wyman.

At least 19 recordsLinked to original sources

arXiv2016

Higgsed Chromo-Natural Inflation

We demonstrate that Chromo-Natural Inflation can be made consistent with observational data if the SU(2) gauge symmetry is spontaneously broken. Working in the Stueckelberg limit, we show that isocurvature is negligible, and the resulting adiabatic fluctuations can match current observational constraints. Observable levels of chirally-polarized gravitational radiation ($r\sim 10^{-3}$) can be produced while the evolution of all background fields is sub-Planckian. The gravitational wave spectrum is amplified via linear mixing with the gauge field fluctuations, and its amplitude is not simply set by the Hubble rate during inflation. This allows observable gravitational waves to be produced for an inflationary energy scale below the GUT scale. The tilt of the resulting gravitational wave spectrum can be either blue or red.

hep-th↗

arXiv2015

Irruption of massive particle species during inflation

All species of (non-conformally-coupled) particles are produced during inflation so long as their mass $M$ is not too much larger than $H$, the expansion rate during inflation. It has been shown that if a particle species that is normally massive ($M\gg H$) couples to the inflaton field in such a way that its mass vanishes, or at least becomes small ($M < H$), for a particular value of the inflaton field, then not only are such particles produced, but an irruption of that particle species can occur during inflation. In this paper we analyze creation of a massive particle species during inflation in a variety of settings, paying particular attention to models which realize such an irruptive production mechanism.

astro-ph.CO↗

arXiv2014

Neutrinos help reconcile Planck measurements with both Early and Local Universe

In light of the recent BICEP2 B-mode polarization detection, which implies a large inflationary tensor-to-scalar ratio r_{0.05}=0.2^{+0.07}_{-0.05}, we re-examine the evidence for an extra sterile massive neutrino, originally invoked to account for the tension between the cosmic microwave background (CMB) temperature power spectrum and local measurements of the expansion rate H0 and cosmological structure. With only the standard active neutrinos and power-law scalar spectra, this detection is in tension with the upper limit of r<0.11 (95% confidence) from the lack of a corresponding low multipole excess in the temperature anisotropy from gravitational waves. An extra sterile species with the same energy density as is needed to reconcile the CMB data with H0 measurements can also alleviate this new tension. By combining data from the Planck and ACT/SPT temperature spectra, WMAP9 polarization, H_0, baryon acoustic oscillation and local cluster abundance measurements with BICEP2 data, we find the joint evidence for a sterile massive neutrino increases to DeltaNeff=0.98\pm 0.26 for the effective number and ms= 0.52\pm 0.13 eV for the effective mass or 3.8 sigma and 4 sigma evidence respectively. We caution the reader that these results correspond to a joint statistical evidence and, in addition, astrophysical systematic errors in the clusters and H0 measurements, and small-scale CMB data could weaken our conclusions.

astro-ph.CO↗

arXiv2014

Detectability of Weak Lensing Modifications under Galileon Theories

Theories of modified gravity attempt to reconcile physics at the largest and the smallest scales by explaining the accelerated expansion of our universe without introducing the cosmological constant. One class of such theories, known as Galileon theories, predict lensing potentials of spherically symmetric bodies, such as dark matter halos, to receive a feature-like modification at the 5% level. With the advent of next-generation photometric surveys, such modifications can serve as novel probes of modified gravity. Assuming an LSST-like fiducial dataset, we produce halo-shear power spectra for LCDM and Galileon scenarios, and perform a Fisher analysis including cosmological, nuisance, and Galileon parameters to study the detectability of the aforementioned modifications. With the LCDM scenario as our null hypothesis, we conclude that it is possible to detect the Galileon modifications at up to 4-σ if present, or strongly exclude the model in a non-detection, with a tomography of four redshift bins and four mass bins, an LSST-like set of survey parameters, and Planck priors on cosmological parameters.

astro-ph.CO↗

arXiv2014

$νΛ$CDM: Neutrinos help reconcile Planck with the Local Universe

Current measurements of the low and high redshift Universe are in tension if we restrict ourselves to the standard six parameter model of flat $Λ$CDM. This tension has two parts. First, the Planck satellite data suggest a higher normalization of matter perturbations than local measurements of galaxy clusters. Second, the expansion rate of the Universe today, $H_0$, derived from local distance-redshift measurements is significantly higher than that inferred using the acoustic scale in galaxy surveys and the Planck data as a standard ruler. The addition of a sterile neutrino species changes the acoustic scale and brings the two into agreement; meanwhile, adding mass to the active neutrinos or to a sterile neutrino can suppress the growth of structure, bringing the cluster data into better concordance as well. For our fiducial dataset combination, with statistical errors for clusters, a model with a massive sterile neutrino shows 3.5$σ$ evidence for a non-zero mass and an even stronger rejection of the minimal model. A model with massive active neutrinos and a massless sterile neutrino is similarly preferred. An eV-scale sterile neutrino mass -- of interest for short baseline and reactor anomalies -- is well within the allowed range. We caution that 1) unknown astrophysical systematic errors in any of the data sets could weaken this conclusion, but they would need to be several times the known errors to eliminate the tensions entirely; 2) the results we find are at some variance with analyses that do not include cluster measurements; and 3) some tension remains among the datasets even when new neutrino physics is included.

astro-ph.CO↗

arXiv2013

Self-accelerating Massive Gravity: How Zweibeins Walk through Determinant Singularities

The theory of massive gravity possesses ambiguities when the spacetime metric evolves far from the non-dynamical fiducial metric used to define it. We explicitly construct a spherically symmetric example case where the metric evolves to a coordinate-independent determinant singularity which does not exist in the initial conditions. Both the metric and the vierbein formulation of the theory are ill-defined at this point. In unitary gauge, the chart of the spacetime ends at this point and does not cover the full spacetime whereas the spherically symmetric vierbeins, or zweibeins, of the fiducial metric become non-invertible and do not describe a valid metric. Nonetheless it is possible to continuously join a zweibein solution on the other side of the singularity which picks one of the degenerate solutions of the metric square root. This continuous solution is not the choice conventionally made in the previous literature. We also show that the Stueckelberg equations of motion on the self-accelerating branch prevent solutions from evolving to a more pathological situation in which the spacetime vierbeins lack a crucial symmetry with the fiducial vierbeins and real square roots fail to exist.

hep-th↗

arXiv2013

Galaxy Infall Kinematics as a Test of Modified Gravity

Infrared modifications of General Relativity (GR) can be revealed by comparing the mass of galaxy clusters estimated from weak lensing to that from infall kinematics. We measure the 2D galaxy velocity distribution in the cluster infall region by applying the galaxy infall kinematics (GIK) model developed by Zu and Weinberg (2013) to two suites of f(R) and Galileon modified gravity simulations. Despite having distinct screening mechanisms, namely, the Chameleon and the Vainshtein effects, the f(R) and Galileon clusters exhibit very similar deviations in their GIK profiles from GR, with ~ 100-200 k/s enhancement in the characteristic infall velocity at r=5 Mpc/h and 50-100 km/s broadening in the radial and tangential velocity dispersions across the entire infall region, for clusters with mass ~ 10^{14} Msol/h at z=0.25. These deviations are detectable via the GIK reconstruction of the redshift--space cluster-galaxy cross-correlation function, xi_cg^s(r_p,r_π), which shows ~ 1-2 Mpc/h increase in the characteristic line-of-sight distance r_π^c at r_p<6 Mpc/h from GR predictions. With overlapping deep imaging and large redshift surveys in the future, we expect that the GIK modelling of xi_cg^s, in combination with the stacked weak lensing measurements, will provide powerful diagnostics of modified gravity theories and the origin of cosmic acceleration.

astro-ph.CO↗

arXiv2013

Sex-specific recombination rates and allele frequencies affect the invasion of sexually antagonistic variation on autosomes

The introduction and persistence of novel sexually antagonistic alleles can depend upon factors that differ between males and females. Understanding the conditions for invasion in a two-locus model can elucidate these processes. For instance, selection can act differently upon the sexes, or sex-linkage can facilitate the invasion of genetic variation with opposing fitness effects between the sexes. Two factors that deserve further attention are recombination rates and allele frequencies -- both of which can vary substantially between the sexes. We find that sex-specific recombination rates in a two-locus diploid model can affect the invasion outcome of sexually antagonistic alleles and that the sex-averaged recombination rate is not necessarily sufficient to predict invasion. We confirm that the range of permissible recombination rates is smaller in the sex benefitting from invasion and larger in the sex harmed by invasion. However, within the invasion space, male recombination rate can be greater than, equal to, or less than female recombination rate in order for a male-benefit, female-detriment allele to invade (and similarly for a female-benefit, male-detriment allele). We further show that a novel, sexually antagonistic allele that is also associated with a lowered recombination rate can invade more easily when present in the double heterozygote genotype. Finally, we find that sexual dimorphism in resident allele frequencies can impact the invasion of new sexually antagonistic alleles at a second locus. Our results suggest that accounting for sex-specific recombination rates and allele frequencies can determine the difference between invasion and non-invasion of novel sexually antagonistic alleles in a two-locus model.

q-bio.PE↗

arXiv2013

Novel Probes of Gravity and Dark Energy

The discovery of cosmic acceleration has stimulated theorists to consider dark energy or modifications to Einstein's General Relativity as possible explanations. The last decade has seen advances in theories that go beyond smooth dark energy -- modified gravity and interactions of dark energy. While the theoretical terrain is being actively explored, the generic presence of fifth forces and dark sector couplings suggests a set of distinct observational signatures. This report focuses on observations that differ from the conventional probes that map the expansion history or large-scale structure. Examples of such novel probes are: detection of scalar fields via lab experiments, tests of modified gravity using stars and galaxies in the nearby universe, comparison of lensing and dynamical masses of galaxies and clusters, and the measurements of fundamental constants at high redshift. The observational expertise involved is very broad as it spans laboratory experiments, high resolution astronomical imaging and spectroscopy and radio observations. In the coming decade, searches for these effects have the potential for discovering fundamental new physics. We discuss how the searches can be carried out using experiments that are already under way or with modest adaptations of existing telescopes or planned experiments. The accompanying paper on the Growth of Cosmic Structure describes complementary tests of gravity with observations of large-scale structure.

astro-ph.CO↗

arXiv2013

Self-accelerating Massive Gravity: Bimetric Determinant Singularities

The existence of two metrics in massive gravity theories in principle allows solutions where there are singularities in new scalar invariants jointly constructed from them. These configurations occur when the two metrics differ substantially from each other, as in black hole and cosmological solutions. The simplest class of such singularities are determinant singularities. We investigate whether the dynamics of bimetric massive gravity -- where the second metric is allowed to evolve jointly with the spacetime metric -- can avoid these singularities. We investigate whether the dynamics of bimetric massive gravity, where the second metric is allowed to evolve jointly with the spacetime metric, averts the simplest class of such singularities, namely determinant singularities. We show that it is still possible to specify non-singular initial conditions that evolve to a determinant singularity. Determinant singularities are a feature of massive gravity of both fixed and dynamical metric type.

hep-th↗

arXiv2013

Perturbations in Chromo-Natural Inflation

Chromo-Natural Inflation is the first worked example of a model of inflation in which slow-roll inflation is achieved by "magnetic drift" as opposed to Hubble friction. In this work, we give an account of the perturbations at linear order in this model. Our analysis uncovers two novel phenomena. First, the amplitude of scalar curvature perturbations is not directly tied to the shape of the inflationary potential. This allows the theory to violate naïve formulations of the Lyth bound. Second, the tensor sector of the theory is significantly altered from the usual case: the non-Abelian gauge field perturbations have a tensor degree of freedom. One chirality of the this tensor can be exponentially enhanced by a temporary instability near horizon crossing; this chiral instability exists because of the classical gauge field background, which violates parity. These tensor fluctuations of the gauge field also couple to gravitational waves at linear order in perturbation theory and source a chiral spectrum of gravitational waves. This spectrum can be exponentially enhanced over the usual inflationary spectrum due to the instability in the gauge sector. These new features cause the theory in its present form to be in significant tension with current observational data. This is because the new scalar physics leads to a significant reddening of the spectral tilt in the same region of parameter space where the exponential enhancement of the gravitational wave amplitude is small enough to satisfy current constraints on the tensor-to-scalar index. Hence, the model either predicts a spectral tilt that is too red, or it overproduces gravitational waves, or both.

hep-th↗

arXiv2013

Gauge Fields and Inflation: Chiral Gravitational Waves, Fluctuations and the Lyth Bound

Models of inflation involving non-Abelian gauge field backgrounds can produce gravitational waves at an observable level with a preferred handedness. This asymmetry comes about because the non-Abelian background generates parity-violation in the action for perturbations. In the specific model we study, Chromo-Natural Inflation, these gravitational waves can be produced at observable levels even when no field makes a super-Planckian field excursion, thus evading a common formulation of the Lyth bound. Unfortunately, when considered in concert with the scalar fluctuations, this chiral enhancement of the gravitational waves makes the model observationally inviable.

hep-th↗

arXiv2013

Self-accelerating Massive Gravity: Time for Field Fluctuations

The ghost-free theory of massive gravity has exact solutions where the effective stress energy generated by the graviton mass term is a cosmological constant for any isotropic metric. Since they are exact, these solutions mimic a cosmological constant in the presence of any matter-induced isotropic metric perturbation. In the Stueckelberg formulation, this stress energy is carried entirely by the spatial Stueckelberg field. We show that any stress energy carried by fluctuations in the spatial field away from the exact solution always decays away in an expanding universe. However, the dynamics of the spatial Stueckelberg field perturbation depend on the background temporal Stueckelberg field, which is equivalent to the unitary gauge time coordinate. This dependence resolves an apparent conflict in the existing literature by showing that there is a special unitary time choice for which the field dynamics and energy density perturbations vanish identically. In general, the isotropic system has a single dynamical degree of freedom requiring two sets of initial data; however, only one of these initial data choices will affect the observable metric. Finally, we construct cosmological solutions with a well-defined perturbative initial value formulation and comment on alternate solutions that evolve to singularities.

hep-th↗

arXiv2013

Simulations of Galileon modified gravity: Clustering statistics in real and redshift space

We use N-body simulations to study the statistics of massive halos and redshift space distortions for theories with a standard ΛCDM expansion history and a galileon-type scalar field. The extra scalar field increases the gravitational force, leading to enhanced structure formation. We compare our measurements of the real space matter power spectrum and halo properties with fitting formula for estimating these quantities analytically. We find that a model for power spectrum, halo mass-function and halo bias, derived from ΛCDM simulations can fit the results from our simulations of modified gravity when σ_8 is appropriately adjusted. We also study the redshift space distortions in the two point correlation function measured from these simulations, finding a difference in the ratio of the redshift space to real space clustering amplitude relative to standard gravity on all scales. We find enhanced clustering on scales r>10 Mpc/h and increased damping of the correlation function for scales r<9 Mpc/h. The boost in the clustering on large scales due to the enhanced gravitational forces cannot be mimicked in a standard gravity model by simply changing σ_8. This result illustrates the usefulness of redshift space distortion measurements as a probe of modifications to General Relativity.

astro-ph.CO↗

arXiv2012

Self-accelerating Massive Gravity: Exact solutions for any isotropic matter distribution

We present an exact solution to the equations of massive gravity that displays cosmological constant-like behavior for any spherically symmetric distribution of matter, including arbitrary time dependence. On this solution, the new degrees of freedom from the massive graviton generate a cosmological constant-like contribution to stress-energy that does not interact directly with other matter sources. When the effective cosmological constant contribution dominates over other sources of stress energy the cosmological expansion self-accelerates, even when no other dark-energy-like ingredients are present. The new degrees of freedom introduced by giving the graviton the mass do not respond to arbitrarily large radial or homogeneous perturbations from other matter fields on this solution. We comment on possible implications of this result.

hep-th↗

arXiv2012

Gauge-flation trajectories in Chromo-Natural Inflation

We provide a detailed discussion of the multifield trajectories and inflationary dynamics of the recently proposed model of Chromo-Natural inflation, which allows for slow roll inflation on a steep potential with the aid of classical non-Abelian gauge fields. We show that slow roll inflation can be achieved across a wide range of the parameter space. We demonstrate that Chromo-Natural Inflation includes trajectories that match those found in Gauge-flation and describe how the theories are related.

hep-th↗

arXiv2012

Chern-Simons EM-flation

We propose a new, generic mechanism of inflation mediated by a balance between potential forces and a Chern-Simons interaction. Such quasi-topological interactions are ubiquitous in string theory. In the minisuperspace approximation, their effect on the dynamics can be mapped onto the problem of a charged particle in an electromagnetic field together with an external potential; slow roll arises when the motion is dominated by the analogue of `magnetic drift'. This mechanism is robust against radiative corrections. We suggest a possible experimental signature which, if observed, might be considered strong evidence for string theory.

hep-th↗

arXiv2012

Chromo-Natural Inflation: Natural inflation on a steep potential with classical non-Abelian gauge fields

We propose a model for inflation consisting of an axionic scalar field coupled to a set of three non-Abelian gauge fields. Our model's novel requirement is that the gauge fields begin inflation with a rotationally invariant vacuum expectation value (VEV) that is preserved through identification of SU(2) gauge invariance with rotations in three dimensions. The gauge VEV interacts with the background value of the axion, leading to an attractor solution that exhibits slow roll inflation even when the axion decay constant has a natural value ($<M_{\rm Pl}$). Assuming a sinusoidal potential for the axion, we find that inflation continues until the axionic potential vanishes. The speed at which the axion moves along its potential is modulated by its interactions with the gauge VEV, rather than being determined by the slope of its bare potential. For sub-Plankian axion decay constants vanishingly small tensor to scalar ratios are predicted, a direct consequence of the Lyth bound. The parameter that controls the interaction strength between the axion and the gauge fields requires a technically natural tuning of $\mathcal{O}$(100).

hep-th↗