Gabriele Montefalcone, Paul J. Steinhardt, Daniel H. Wesley

Perhaps the greatest challenge for fundamental theories based on compactification from extra dimensions is accommodating a period of accelerated cosmological expansion. Previous studies have identified constraints imposed by the existence of dark energy for two overlapping classes of compactified theories: (1) those in which the higher dimensional picture satisfies certain metric properties selected to reproduce known low energy phenomenology; or (2) those derived from string theory assuming they satisfy the Swampland conjectures. For either class, the analyses showed that dark energy is only possible if it takes the form of quintessence. In this paper, we explore the consequences for theories that belong to both classes and show that the joint constraints are highly restrictive, leaving few options.

Ivan Agullo, Dimitrios Kranas, V. Sreenath

We explore a model of the early universe in which the inflationary epoch is preceded by a cosmic bounce, and argue that this scenario provides a common origin to several of the anomalous features that have been observed at large angular scales in the cosmic microwave background (CMB). More concretely, we show that a power suppression, a dipolar asymmetry, and a preference for odd-parity correlations, with amplitude and scale dependence in consonance with observations, are expected from this scenario. The model also alleviates the tension in the lensing amplitude. These signals originate from the indirect effect that non-Gaussian correlations between CMB modes and super-horizon wavelengths induce in the power spectrum. We do not restrict to any specific theory, but rather derive features common to a family bouncing models.

Aaron Glanville, Tamara Davis

Does space stretch its contents as the universe expands? Usually we say the answer is no - the stretching of space is not like the stretching of a rubber sheet that might drag things with it. In this paper we explore a potential counter example - namely we show that is is impossible to make an arbitrarily long ruler in an expanding universe, because it is impossible to hold the distant end of the ruler "stationary" with respect to us (as defined in the Friedmann-Lemaitre-Robertson-Walker metric). We show that this does not mean that expanding space has a force associated with it, rather, some fictitious forces arise due to our choice of a non-inertial reference frame. By choosing our usual time-slice (where all comoving observers agree on the age of the universe), we choose a global frame in which special relativity does not hold. As a result, simple relativistic velocity transforms generate an apparent acceleration, even where no force exists. This effect is similar to the fictitious forces that arise in describing objects in rotating reference frames, as in the case of the Coriolis effect.

Gabriel German

We draw a figure from where it is possible to measure the number of e-folds of expansion of the universe with a ruler.