Tanguy Grall, Scott Melville

During inflation, there is a preferred reference frame in which the expansion of the background spacetime is spatially isotropic. In contrast to Minkowski spacetime, observables can depend on the velocity of the system with respect to this cosmic rest frame. We derive new constraints from radiative stability and unitarity on effective field theories with such spontaneously broken Lorentz symmetry. In addition to a maximum energy scale, there is now also a critical velocity at which the theory breaks down. The theory therefore has different resolving power in time and in space, and we show that these can only coincide if cubic Lorentz-violating interactions are absent. Applying these bounds to the Effective Field Theory of Inflation, we identify the region of parameter space in which inflation can be both single-field and weakly coupled on subhorizon scales. This can be implemented as a theoretical prior, and we illustrate this explicitly using Planck observational constraints on the primordial bispectrum.

Jiro Matsumoto, Teppei Okumura, Misao Sasaki

The observed accelerated expansion of the Universe may be explained by dark energy or the breakdown of general relativity (GR) on cosmological scales. When the latter case, a modified gravity scenario, is considered, it is often assumed that the background evolution is the same as the $\Lambda$CDM model but the density perturbation evolves differently. In this paper, we investigate more general classes of modified gravity, where both the background and perturbation evolutions are deviated from those in the $\Lambda$CDM model. We introduce two phase diagrams, $\alpha{\rm-}f\sigma _8$ and $H{\rm-}f\sigma _8$ diagrams; $H$ is the expansion rate, $f\sigma_8$ is a combination of the growth rate of the Universe and the normalization of the density fluctuation which is directly constrained by redshift-space distortions, and $\alpha$ is a parameter which characterizes the deviation of gravity from GR and can be probed by gravitational lensing. We consider several specific examples of Horndeski's theory, which is a general scalar-tensor theory, and demonstrate how deviations from the $\Lambda$CDM model appears in the $\alpha{\rm-}f\sigma _8$ and $H{\rm-}f\sigma _8$ diagrams. The predicted deviations will be useful for future large-scale structure observations to exclude some of the modified gravity models.

Charles L. Steinhardt, Albert Sneppen, Bidisha Sen

Redshifts used in current cosmological supernova samples are measured using two primary techniques, one based on well-measured host galaxy spectral lines and the other based on supernova-dominated spectra. Here, we construct an updated Pantheon catalog with revised redshifts, redshift sources and estimated uncertainties for the entire sample to investigate whether these two techniques yield consistent results. The best-fit cosmological parameters using these two measurement techniques disagree, and we explore several possible sources of bias which could result from using the lower-precision supernova-dominated redshifts. In a pilot study, we show that using a host redshift-only subsample of the Pantheon catalog will result in lower $\Omega_m$ and matter density $\Omega_m h^2$ and slightly higher $H_0$ than previous analysis which, among other possibilities, could result in supernova and CMB measurements agreeing on $\Omega_m h^2$ despite tension in $H_0$. To obtain rigorous results, though, the Pantheon catalog should be improved by obtaining host spectra for supernova that have faded and future surveys should be designed to use host galaxy redshifts rather than lower-precision methods.

Mikhail M. Ivanov, Yacine Ali-Haïmoud, Julien Lesgourgues

We study whether the discrepancy between the local and cosmological measurements of the Hubble constant $H_0$ can be reformulated as a tension in the cosmic microwave background (CMB) monopole temperature $T_0$. The latter is customarily fixed to the COBE/FIRAS best-fit value in CMB anisotropy data analyses. We show that the primary CMB anisotropies and the shape of the matter power spectrum are not directly sensitive to $T_0$. They depend only on the dark matter and baryon densities per CMB photon. Once these ratios are fixed, $T_0$ only measures the time elapsed since recombination until today. This results is a nearly perfect geometric degeneracy between $T_0$ and $H_0$. Taken at face value, this implies that removing the FIRAS prior on $T_0$ is enough to make the Planck CMB and SH0ES measurements consistent within the base $\Lambda$CDM model without introducing new physics. One may break the degeneracy by combining Planck with SH0ES, yielding an independent measurement of $T_0$, which happens to be in a $4\sigma$ tension with FIRAS. Therefore, the Hubble tension can be fully recast into the $T_0$ tension. The agreement with FIRAS can be restored if we combine Planck with the baryon acoustic oscillation data instead of SH0ES. Thus, the tension between SH0ES and cosmological measurements of $H_0$ within $\Lambda$CDM persists even if we discard the FIRAS $T_0$ measurement.