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Register a new userFree-streaming and Coupled Dark Radiation Isocurvature Perturbations: Constraints and Application to the Hubble Tension
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Dark radiation (DR) appears as a new physics candidate in various scenarios beyond the Standard Model. While it is often assumed that perturbations in DR are adiabatic, they can easily have an isocurvature component if more than one field was present during inflation, and whose decay products did not all thermalize with each other. By implementing the appropriate isocurvature initial conditions (IC), we derive the constraints on both uncorrelated and correlated DR density isocurvature perturbations from the full Planck 2018 data alone, and also in combination with other cosmological data sets. Our study on free-streaming DR (FDR) updates and generalizes the existing bound on neutrino density isocurvature perturbations by including a varying number of relativistic degrees of freedom, and for coupled DR (CDR) isocurvature, we derive the first bound. We also show that for CDR qualitatively new physical effects arise compared to FDR. One such effect is that for isocurvature IC, FDR gives rise to larger CMB anisotropies compared to CDR -- contrary to the adiabatic case. More generally, we find that a blue-tilt of DR isocurvature spectrum is preferred. This gives rise to a larger value of the Hubble constant $H_0$ compared to the standard $Lambda$CDM+$Delta N_{rm eff}$ cosmology with adiabatic spectra and relaxes the $H_0$ tension.
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We study non-Gaussian properties of the isocurvature perturbations in the dark radiation, which consists of the active neutrinos and extra light species, if exist. We first derive expressions for the bispectra of primordial perturbations which are mixtures of curvature and dark radiation isocurvature perturbations. We also discuss CMB bispectra produced in our model and forecast CMB constraints on the nonlinearity parameters based on the Fisher matrix analysis. Some concrete particle physics motivated models are presented in which large isocurvature perturbations in extra light species and/or the neutrino density isocurvature perturbations as well as their non-Gaussianities may be generated. Thus detections of non-Gaussianity in the dark radiation isocurvature perturbation will give us an opportunity to identify the origin of extra light species and lepton asymmetry.
The existence of dark radiation that is completely decoupled from the standard model in the early Universe leaves open the possibility of an associated dark radiation isocurvature mode. We show that the presence of dark radiation isocurvature leads to spatial variation in the primordial abundances of helium and deuterium due to spatial variation in $N_{rm eff}$ during Big Bang nucleosynthesis. We use the result to constrain the existence of such an isocurvature mode on scales down to $sim 1$ Mpc scales. By measuring the excess variance in the primordial helium to hydrogen and deuterium to hydrogen ratio in different galaxies, we constrain the variance in average isocurvature in a galaxy to be less than $0.13/Delta bar{N}_{rm eff}$ at 95% confidence. Here $Delta bar{N}_{rm eff}$ is the spatially averaged increase in $N_{rm eff}$ due to the additional dark radiation component.
Baryon Acoustic Oscillation (BAO) surveys will be a leading method for addressing the dark energy challenge in the next decade. We explore in detail the effect of allowing for small amplitude admixtures of general isocurvature perturbations in addition to the dominant adiabatic mode. We find that non-adiabatic initial conditions leave the sound speed unchanged but instead excite different harmonics. These harmonics couple differently to Silk damping, altering the form and evolution of acoustic waves in the baryon-photon fluid prior to decoupling. This modifies not only the scale on which the sound waves imprint onto the baryon distribution, which is used as the standard ruler in BAO surveys, but also the shape, width and height of the BAO peak. We discuss these effects in detail and show how more general initial conditions impact our interpretation of cosmological data in dark energy studies. We find that the inclusion of these additional isocurvature modes leads to an increase in the Dark Energy Task Force Figure of merit by 140% and 60% for the BOSS and ADEPT experiments respectively when considered in conjunction with Planck data. We also show that the incorrect assumption of adiabaticity has the potential to bias our estimates of the dark energy parameters by $3sigma$ ($1sigma$) for a single correlated isocurvature mode, and up to $8sigma$ ($3sigma$) for three correlated isocurvature modes in the case of the BOSS (ADEPT) experiment. We find that the use of the large scale structure data in conjunction with CMB data improves our ability to measure the contributions of different modes to the initial conditions by as much as 100% for certain modes in the fully correlated case.
Standard cosmology predicts that prior to matter-radiation equality about 41% of the energy density was in free-streaming neutrinos. In many beyond Standard Model scenarios, however, the amount and free-streaming nature of this component is modified. For example, this occurs in models with new neutrino self-interactions or an additional dark sector with interacting light particles. We consider several extensions of the standard cosmology that include a non-free-streaming radiation component as motivated by such particle physics models and use the final Planck data release to constrain them. This release contains significant improvements in the polarization likelihood which plays an important role in distinguishing free-streaming from interacting radiation species. Fixing the total amount of energy in radiation to match the expectation from standard neutrino decoupling we find that the fraction of free-streaming radiation must be $f_mathrm{fs} > 0.8$ at 95% CL (combining temperature, polarization and baryon acoustic oscillation data). Allowing for arbitrary contributions of free-streaming and interacting radiation, the effective number of new non-free-streaming degrees of freedom is constrained to be $N_mathrm{fld} < 0.6$ at 95% CL. Cosmologies with additional radiation are also known to ease the discrepancy between the local measurement and CMB inference of the current expansion rate $H_0$. We show that including a non-free-streaming radiation component allows for a larger amount of total energy density in radiation, leading to a mild improvement of the fit to cosmological data compared to previously discussed models with only a free-streaming component.
A promising idea to resolve the long standing Hubble tension is to postulate a new subdominant dark-energy-like component in the pre-recombination Universe which is traditionally termed as the Early Dark Energy (EDE). However, as shown in Refs. cite{Hill:2020osr,Ivanov:2020ril} the cosmic microwave background (CMB) and large-scale structure (LSS) data impose tight constraints on this proposal. Here, we revisit these strong bounds considering the Planck CMB temperature anisotropy data at large angular scales and the SPTPol polarization and lensing measurements. As advocated in Ref. cite{Chudaykin:2020acu}, this combined data approach predicts the CMB lensing effect consistent with the $Lambda$CDM expectation and allows one to efficiently probe both large and small angular scales. Combining Planck and SPTPol CMB data with the full-shape BOSS likelihood and information from photometric LSS surveys in the EDE analysis we found for the Hubble constant $H_0=69.79pm0.99,{rm km,s^{-1}Mpc^{-1}}$ and for the EDE fraction $f_{rm EDE}<0.094,(2sigma)$. These bounds obtained without including a local distance ladder measurement of $H_0$ (SH0ES) alleviate the Hubble tension to a $2.5sigma$ level. Including further the SH0ES data we obtain $H_0=71.81pm1.19,{rm km,s^{-1}Mpc^{-1}}$ and $f_{rm EDE}=0.088pm0.034$ in full accordance with SH0ES. We also found that a higher value of $H_0$ does not significantly deteriorate the fit to the LSS data. Overall, the EDE scenario is (though weakly) favoured over $Lambda$CDM even after accounting for unconstrained directions in the cosmological parameter space. We conclude that the large-scale Planck temperature and SPTPol polarization measurements along with LSS data do not rule out the EDE model as a resolution of the Hubble tension. This paper underlines the importance of the CMB lensing effect for robust constraints on the EDE scenario.
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