高能暂现源 · 每日文献综述

The inflationary Lyth bound relates the primordial tensor amplitude to the inflaton field excursion. In ekpyrotic, bouncing, and cyclic cosmologies, there is no analogous universal tensor-to-field-distance relation because scalar and tensor perturbations depend on entropy conversion, matching through the bounce, and the specific mechanism that violates or evades the null energy condition. Here, we propose a phase-resolved criterion for the accumulated scalar field trajectory length in a noninflationary smoothing history. The quantity constrained in this study is not, in general, the geodesic distance between the initial and final field-space points. Rather, it is the invariant path length accumulated along the actual cosmological trajectory. Therefore, the resulting inequalities should be understood as sufficient, conservative trajectory length criteria for small-field control, not as model-independent necessary exclusions based on geodesic distance swampland reasoning. We also impose Belinski-Khalatnikov-Lifshitz (BKL) anisotropy suppression as an additional constraint on the ekpyrotic phase. In the canonical phase of the ekpyrotic contraction, we recover the known small-field scaling and embed it in a total trajectory length budget inequality. We impose three requirements: a BKL anisotropy suppression that is parametrized separately, a phenomenological cutoff-corrected distance budget inspired by tower of states logic, and observational conversion windows from residual isocurvature and non-Gaussianity. Furthermore, we propose a new master formula that provides a conditional lower limit on the value of the parameter <inline-formula><mml:math><mml:msub><mml:mi>∊</mml:mi><mml:mrow><mml:mi>ek</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> that depends on the remaining distance available after conversion and the cosmological bounce. We also derive a curvature constraint for scale-invariant entropy perturbations in curved field space which shows that the small total distance and the observed red tilt seem to indicate ultrafast-roll ekpyrosis, sharp turns, short or strongly modified bounces, and/or significant negative sectional curvature of the scalar manifold. Finally, we demonstrate methods for testing the distance budget against observational data. Examples include <inline-formula><mml:math><mml:msub><mml:mi>n</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula> running, primordial non-Gaussianity, isocurvature, a stochastic gravitational waves background, and late-time dark energy distance constraints. The observational discussion is organized into explicit diagnostic maps, which are used to identify and analyze specific features and patterns in the data. We also apply the formalism to exponential and cyclic ekpyrotic scalar field potentials, providing a mechanism-resolved interpretation of the bounce or crossover contribution for several specific realizations.

The density of the upper thermosphere of Venus was measured in situ using the attitude control system of Venus Express during twelve low-pericentre campaigns between 2008 and 2014. The spacecraft's counteraction to aerodynamic drag torque was used to derive total neutral densities for 91 orbits in the altitude range 160─200 km. The results agree with predictions from the widely used VTS3 model based on Pioneer Venus Orbiter data on the dayside, but shows a much more rapid decrease than expected in density beyond the terminator. Superimposed on the mean density, oscillations consistent with gravity wave activity are observed. The measured wavelengths vary from orbit to orbit, spanning 50─250 km along the spacecraft trajectory, and are present across all sampled latitudes (70°─90°N), solar zenith angles (70°─100°), local times (morning and evening), and altitudes (160─200 km). These results demonstrate that gravity waves are a persistent feature of the Venusian high-latitude thermosphere, providing a pathway for coupling between the lower atmosphere and the upper thermosphere.