Tensions between the early and late Universe

Abstract

The standard cosmological model successfully describes many observations from widely different epochs of the Universe, from primordial nucleosynthesis to the accelerating expansion of the present day. However, as the basic cosmological parameters of the model are being determined with increasing and unprecedented precision, it is not guaranteed that the same model will fit more precise observations from widely different cosmic epochs. Discrepancies developing between observations at early and late cosmological time may require an expansion of the standard model, and may lead to the discovery of new physics. The workshop "Tensions between the Early and the Late Universe" was held at the Kavli Institute for Theoretical Physics on July 15-17 2019 † to evaluate increasing evidence for these discrepancies, primarily in the value of the Hubble constant as well as ideas recently proposed to explain this tension. Multiple new observational results for the Hubble constant were presented in the time frame of the workshop using different probes: Cepheids, strong lensing time delays, tip of the red giant branch (TRGB), megamasers, Oxygen-rich Miras and surface brightness fluctuations (SBF) resulting in a set of six new ones in the last several months. Here we present the summary plot of the meeting that shows combining any three independent approaches to measure H0 in the late universe yields tension with the early Universe values between 4.0σ and 5.8σ. This shows that the discrepancy does not appear to be dependent on the use of any one method, team, or source. Theoretical ideas to explain the discrepancy focused on new physics in the decade of expansion preceding recombination as the most plausible. This is a brief summary of the workshop.

The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints

“…We test the prior empirical claim that the local value of H 0 exceeds the early-Universe value (e.g., Riess et al 2019;Wong et al 2019;Verde et al 2019) by calculating the probability of rejecting the null hypothesis that our measurement does not exceed Planck's, i.e.,…”

“…For example, distance ladder measurements using Cepheid variables to calibrate the absolute luminosities of Type Ia supernovae find 74.03 ± 1.42 km s −1 Mpc −1 , and time-delay strong lensing measurements from multiply-imaged quasars currently yield 73.3 +1.7 −1.8 km s −1 Mpc −1 (Wong et al 2019). Though this discrepancy between the earlyand late-Universe H 0 measurements is being taken increasingly seriously by the cosmological community (see Verde et al 2019), the tantalizing prospects that it holds for heralding physics beyond ΛCDM (e.g., Raveri et al 2017;Poulin et al 2018) require that we subject it to a correspondingly strict evidence threshold. Indepen-dent avenues for constraining H 0 are thus necessary to provide cross-checks against unrecognized systematics in the measurements.…”

The expansion of the Universe is faster than expected

“…Miras are variable, red giant stars recently pressed into service to measure H0 as a check on both Cepheids and TRGB and yield H0=73.3±3.9 km s -1 Mpc -1 using new HST observations and the previously discussed geometric distances [8]. Two other updates to the local measurement of H0 come from water masers (sources of microwave stimulated emission) in four galaxies at great distances (H0=74.8±3.1 km s -1 Mpc -1 ) and the use of a method called Surface Brightness Fluctuations (H0=76.5±4.0 km s -1 Mpc -1 ) [9].…”

Degenerate Sub-keV fermion dark matter from a solution to the Hubble tension

“…If the theory allows the vector-like fermions in the dark sector, values of their mass parameters are naturally close to a cut-off scale. 4 This point makes the vector-like fermions become irrelevant to the low-energy physics once integrating out heavy degrees of freedom is done. Thus, we take an option to exclude presence of the vector-like fermions in the dark sector.…”

“…5 Apart from the gauge field and the five Weyl fields, we introduce two complex scalar fields for the spontaneous breaking of U (1) X and to make fermions massive. Among several possibilities for a set of U (1) X charges 4 Of course, mass of a vector-like fermion is a free parameter and in principle it can be any value from zero to a UV-cut off of the theory. However, small mass regime is less appealing from the theoretical point of view unless there is any reasonable supporting explanation like restoration of certain symmetries in zero-limit.…”