Ruling Out New Physics at Low Redshift as a Solution to the H_{0} Tension

We make the case that there can be no low-redshift solution to the H_{0} tension. To robustly answer this question, we use a very flexible parametrization for the dark energy equation of state such that every cosmological distance still allowed by data exists within this prior volume. To then answer whether there exists a satisfactory solution to the H_{0} tension within this comprehensive parametrization, we constrained the parametric form using different partitions of the Planck cosmic microwave background, SDSS-IV/eBOSS DR16 baryon acoustic oscillation, and Pantheon supernova datasets. When constrained by just the cosmic microwave background dataset, there exists a set of equations of state which yields high H_{0} values, but these equations of state are ruled out by the combination of the supernova and baryon acoustic oscillation datasets. In other words, the constraint from the cosmic microwave background, baryon acoustic oscillation, and supernova datasets together does not allow for high H_{0} values and converges around an equation of state consistent with a cosmological constant. Thus, since this very flexible parametrization does not offer a solution to the H_{0} tension, there can be no solution to the H_{0} tension that adds physics at only low redshifts. This is directly related to the expansion history of the Universe and its geometrical properties and would include models beyond those parametrized by w(z).

What It Takes to Solve the Hubble Tension through Modifications of Cosmological Recombination

We construct data-driven solutions to the Hubble tension which are perturbative modifications to the fiducial ΛCDM cosmology, using the Fisher bias formalism. Taking as proof of principle the case of a time-varying electron mass and fine structure constant, and focusing first on Planck CMB data, we demonstrate that a modified recombination can solve the Hubble tension and lower S_{8} to match weak lensing measurements. Once baryonic acoustic oscillation and uncalibrated supernovae data are included, however, it is not possible to fully solve the tension with perturbative modifications to recombination.

Early Dark Energy from Massive Neutrinos as a Natural Resolution of the Hubble Tension

The Hubble tension can be significantly eased if there is an early component of dark energy that becomes active around the time of matter-radiation equality. Early dark energy models suffer from a coincidence problem-the physics of matter-radiation equality and early dark energy are completely disconnected, so some degree of fine-tuning is needed in order for them to occur nearly simultaneously. In this Letter, we propose a natural explanation for this coincidence. If the early dark energy scalar couples to neutrinos then it receives a large injection of energy around the time that neutrinos become nonrelativistic. This is precisely when their temperature is of order of their mass, which, coincidentally, occurs around the time of matter-radiation equality. Neutrino decoupling therefore provides a natural trigger for early dark energy by displacing the field just before matter-radiation equality. We discuss various theoretical aspects of this proposal, potential observational signatures, and future directions for its study.

Image Simulations for Strong and Weak Gravitational Lensing

Gravitational lensing has been identified as a powerful tool to address fundamental problems in astrophysics at different scales, ranging from exoplanet identification to dark energy and dark matter characterization in cosmology. Image simulations have played a fundamental role in the realization of the full potential of gravitational lensing by providing a means to address needs such as systematic error characterization, pipeline testing, calibration analyses, code validation, and model development. We present a general overview of the generation and applications of image simulations in strong and weak gravitational lensing.

Gravitational Waves, CMB Polarization, and the Hubble Tension

The discrepancy between the Hubble parameter inferred from local measurements and that from the cosmic microwave background (CMB) has motivated careful scrutiny of the assumptions that enter both analyses. Here we point out that the location of the recombination peak in the CMB B-mode power spectrum is determined by the light horizon at the surface of last scatter and thus provides an alternative early-Universe standard ruler. It can thus be used as a cross-check for the standard ruler inferred from the acoustic peaks in the CMB temperature power spectrum and to test various explanations for the Hubble tension. The measurement can potentially be carried out with a precision of ≲2% with stage-IV B-mode experiments. The measurement can also be used to measure the propagation speed of gravitational waves in the early Universe.

Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension

Large, nonstandard neutrino self-interactions have been shown to resolve the ∼4σ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force carrier with a large neutrino coupling (>10^{-4}) and mass in the keV-100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to τ-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana particles, and that a UV-complete model requires a nonminimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

Revisiting a Negative Cosmological Constant from Low-Redshift Data

Persisting tensions between high-redshift and low-redshift cosmological observations suggest the dark energy sector of the Universe might be more complex than the positive cosmological constant of the Λ CDM model. Motivated by string theory, wherein symmetry considerations make consistent AdS backgrounds (i.e., maximally-symmetric spacetimes with a negative cosmological constant) ubiquitous, we explore a scenario where the dark energy sector consists of two components: a negative cosmological constant, with a dark energy component with equation of state w ϕ on top. We test the consistency of the model against low-redshift baryon acoustic oscillation and Type Ia supernovae distance measurements, assessing two alternative choices of distance anchors: the sound horizon at baryon drag determined by the Planck collaboration and the Hubble constant determined by the SH0ES program. We find no evidence for a negative cosmological constant and mild indications for an effective phantom dark energy component on top. A model comparison analysis reveals that the Λ CDM model is favoured over our negative cosmological constant model. While our results are inconclusive, should low-redshift tensions persist with future data, it would be worth reconsidering and further refining our toy negative cosmological constant model by considering realistic string constructions.

Early Dark Energy can Resolve the Hubble Tension

Early dark energy (EDE) that behaves like a cosmological constant at early times (redshifts z≳3000) and then dilutes away like radiation or faster at later times can solve the Hubble tension. In these models, the sound horizon at decoupling is reduced resulting in a larger value of the Hubble parameter H_{0} inferred from the cosmic microwave background (CMB). We consider two physical models for this EDE, one involving an oscillating scalar field and another a slowly rolling field. We perform a detailed calculation of the evolution of perturbations in these models. A Markov Chain Monte Carlo search of the parameter space for the EDE parameters, in conjunction with the standard cosmological parameters, identifies regions in which H_{0} inferred from Planck CMB data agrees with the SH0ES local measurement. In these cosmologies, current baryon acoustic oscillation and supernova data are described as successfully as in the cold dark matter model with a cosmological constant, while the fit to Planck data is slightly improved. Future CMB and large-scale-structure surveys will further probe this scenario.