The essence of quintessence and the cost of compression

Standard two-parameter compressions of the infinite dimensional dark energy model space show crippling limitations even with current SN-Ia data. Firstly they cannot cope with rapid evolution - the best-fit to the latest SN-Ia data shows late and very rapid evolution to w_0 = -2.85. However all of the standard parametrisations (incorrectly) claim that this best-fit is ruled out at more than 2-sigma, primarily because they track it well only at very low redshifts, z < 0.2. Further they incorrectly rule out the observationally acceptable region w 1. Secondly the parametrisations give wildly different estimates for the redshift of acceleration, which vary from z_{acc}=0.14 to z_{acc}=0.59. Although these failings are largely cured by including higher-order terms (3 or 4 parameters) this results in new degeneracies which open up large regions of previously ruled-out parameter space. Finally we test the parametrisations against a suite of theoretical quintessence models. The widely used linear expansion in z is generally the worst, with errors of up to 10% at z=1 and 20% at z > 2. All of this casts serious doubt on the usefulness of the standard two-parameter compressions in the coming era of high-precision dark energy cosmology and emphasises the need for decorrelated compressions with at least three parameters.Comment: 7 pages, 4 colour figures, EmulateApJ; v2: includes Bayesian evidence analysis and table that were only present in published version, because of increased interest in Bayesian model comparison (no new material beyond the one in the published ApJL of 2004

Massless Metric Preheating

Can super-Hubble metric perturbations be amplified exponentially during preheating ? Yes. An analytical existence proof is provided by exploiting the conformal properties of massless inflationary models. The traditional conserved quantity \zeta is non-conserved in many regions of parameter space. We include backreaction through the homogeneous parts of the inflaton and preheating fields and discuss the role of initial conditions on the post-preheating power-spectrum. Maximum field variances are strongly underestimated if metric perturbations are ignored. We illustrate this in the case of strong self-interaction of the decay products. Without metric perturbations, preheating in this case is very inefficient. However, metric perturbations increase the maximum field variances and give alternative channels for the resonance to proceed. This implies that metric perturbations can have a large impact on calculations of relic abundances of particles produced during preheating.Comment: 8 pages, 4 colour figures. Version to appear in Phys. Rev. D. Contains substantial new analysis of the ranges of parameter space for which large changes to the inflation-produced power spectrum are expecte

Chandra Observations of Radio-Loud Quasars at z > 4: X-rays from the Radio Beacons of the Early Universe

We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radio-quiet quasar (RQQ) at z = 4.1-4.4. These observations cover a representative sample of RLQs with moderate radio-loudness (R ~ 40-400), filling the X-ray observational gap between optically selected RQQs and the five known blazars at z > 4 (R ~ 800-27000). We study the relationship between X-ray luminosity and radio-loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic N_H, with a best-fit neutral intrinsic column density of N_H = 2.4^{+2.0}_{-1.8} x 10^{22} cm^{-2}, consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background (CMB) at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low-to-moderate redshifts. Overall, the z ~ 4-5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well.Comment: 12 pages, The Astronomical Journal, in pres

Testing for double inflation with WMAP

With the WMAP data we can now begin to test realistic models of inflation involving multiple scalar fields. These naturally lead to correlated adiabatic and isocurvature (entropy) perturbations with a running spectral index. We present the first full (9 parameter) likelihood analysis of double inflation with WMAP data and find that despite the extra freedom, supersymmetric hybrid potentials are strongly constrained with less than 7% correlated isocurvature component allowed when standard priors are imposed on the cosomological parameters. As a result we also find that Akaike & Bayesian model selection criteria rather strongly prefer single-field inflation, just as equivalent analysis prefers a cosmological constant over dynamical dark energy in the late universe. It appears that simplicity is the best guide to our universe.Comment: 7 pages, 6 figure

The complementarity of the redshift drift

We derive some basic equations related to the redshift drift and we show how some dark energy (DE) properties can be retrieved from it. We consider in particular three kinds of DE models which exhibit a characteristic signature in their redshift drift while no such signature would be present in their luminosity-distances: a sudden change of the equation of state parameter w_{DE} at low redshifts, oscillating DE and finally an equation of state with spikes at low redshifts. Accurate redshift drift measurements would provide interesting complementary probes for some of these models and for models with varying gravitational coupling. While the redshift drift would efficiently constrain models with a spike at z~1, the signature of the redshift drift for models with large variations at very low redshifts z<0.1 would be unobservable, allowing a large arbitrariness in the present expansion of the universe.Comment: Accepted for publication in Phys. Rev. D; 12 pages, 8 figure

IMPACT OF WII-FIT TRAINING ON NEURO-MUSCULAR CONTROL

INTRODUCTION: In the past year, the interactive exercise video game Wii Fit (Nintendo, Tokyo, Japan) has achieved worldwide popularity. This system could be a potential asset for both training and physical therapy purposes; however, there is a lack of scientific validation to justify such applications. As a first step in ascertaining the advantages of the Wii Fit system, the present study is focused on the neuromuscular control changes that occur after 8 weeks of daily training. METHOD: Two healthy subjects (25.5±2.1 years, 177.8±14.37 cm, 71.5±16.26 kg) trained for 30 minutes a day for 8 consecutive weeks using standard Wii Fit strength training, aerobic, and yoga exercises. Before and after the training period, a series of tests were performed (gait, hop, isometric, and one leg stability) while collecting EMG data from the quadriceps (rectus femoris, vastus lateralis, vastus medialis), the hamstrings (biceps femoris and semitendinosus), and the grastrocnemii (lateralis and medialis). The EMG data was linear-enveloped and normalized by a maximum isometric voluntary contraction (MVIC). Similarly to Lloyd et al. (2005), the electromyographic activations were then summed by muscle group to calculate the co-contraction ratio (CCR), which is a value between 0 and 1 that indicates equalizing activation as it increases. RESULTS: Only the right leg data is being reported in this paper. Table 1 displays the CCR for the antagonist coactivations of the hamstrings and quadriceps and also the synergistic activations of the knee flexor muscle groups. It is worth noting that during gait and hopping motions, the ratios are decreased after training, while during the stability tests they increased. Finally, no trend emerged for the isometric data. Table 1 Co-contraction Ratio Maximums (Ext 60 and Flex 60 refer to isometric extesion and flexion at 60°)– values in italics are reciprocals Ext 60 Flex 60 Gait Hop Stability Ham/Quad Coactivation Pre 0.21 0.41 0.17 0.20 0.06 Post 0.46 0.47 0.02 0.06 0.88 Ham/Gast Synergy Pre 0.34 0.47 0.02 0.08 0.04 Post 0.47 0.28 0.01 0.08 0.35 DISCUSSION: After training, the CCR data for dynamic activity indicated more focused muscle control. During the stability tests, much higher CCR values were reported, indicating the muscles were doing a better job achieving a intra-articular equilibrium. CONCLUSION: These preliminary results indicate a promising use of the Wii Fit system for training and physical therapy as on a small population they demonstrated neuromuscular control improvement during dynamic and static trials. REFERENCES: Lloyd, D. G., Buchanan, T. S., and Besier, T. F. (2005). Neuromuscular Biomechanical Modeling to Understand Knee Ligament Loading. Medicine & Science in Sports & Exercise, 37, 1939-1947