Cold Dark Matter Isocurvature Research Articles

Primordial black holes from CDM isocurvature perturbations

We show that primordial black holes can be produced from the collapse of large isocurvature perturbations of the cold dark matter. We develop a novel procedure to compute the resulting black hole abundance by studying matched perturbations of matter-only universes, and we use our procedure to translate observational constraints on black hole abundances into model-independent constraints on cold dark matter isocurvature perturbations over a wide range of scales. The constraint on the typical amplitude of the primordial perturbations weakens slightly slower than linearly on small scales.

Gravitational waves from dark matter isocurvature

The primordial fluctuations on large scales are adiabatic, but on smaller scales this need not be the case. Here we derive the general analytical framework to compute the stochastic gravitational wave background induced by primordial cold dark matter isocurvature fluctuations on small scales. We find that large isocurvature fluctuations can yield an observable gravitational wave signal, with a spectrum distinct from the one induced by adiabatic perturbations, and we provide for the first time the exact analytic expression of the kernel necessary to compute this signal. We then forecast the constraining power of future gravitational wave detectors on dark matter isocurvature on small scales and find they will dramatically improve on existing constraints.

Probing small-scale baryon and dark matter isocurvature perturbations with cosmic microwave background anisotropies

The Universe's initial conditions, in particular baryon and cold dark matter (CDM) isocurvature perturbations, are poorly constrained on sub-Mpc scales. In this paper, we develop a new formalism to compute the effect of small-scale baryon perturbations on the mean free-electron abundance, thus on cosmic microwave background (CMB) anisotropies. Our framework can accommodate perturbations with arbitrary time and scale dependence. We apply this formalism to four different combinations of baryon and CDM isocurvature modes, and use Planck CMB-anisotropy data to probe their initial amplitude. We find that Planck data is consistent with no small-scale isocurvature perturbations, and that this additional ingredient does not help alleviate the Hubble tension. We set upper bounds to the dimensionless initial power spectrum $\Delta_{\mathcal{I}}^2(k)$ of these isocurvature modes at comoving wavenumbers $1~\textrm{Mpc}^{-1} \le k \le 10^3$ Mpc$^{-1}$, for several parameterizations. For a scale-invariant power spectrum, our 95% confidence-level limits on $\Delta_{\mathcal{I}}^2$ are 0.023 for pure baryon isocurvature, 0.099 for pure CDM isocurvature, 0.026 for compensated baryon-CDM perturbations, and 0.009 for joint baryon-CDM isocurvature perturbations. Using a Fisher analysis generalized to non-analytic parameter dependence, we forecast that a CMB Stage-4 experiment would be able to probe small-scale isocurvature perturbations with initial power 3 to 10 times smaller than Planck limits. The formalism introduced in this work is very general and can be used more widely to probe any physical processes or initial conditions sourcing small-scale baryon perturbations.

Isocurvature modes: joint analysis of the CMB power spectrum and bispectrum

We perform a joint analysis of the power spectrum and the bispectrum of the CMB temperature and polarization anisotropies to improve the constraints on isocurvature modes. We construct joint likelihoods, both for the existing Planck data, and to make forecasts for the future LiteBIRD and CMB-S4 experiments. We assume a general two-field inflation model with five free parameters, leading to one isocurvature mode (which can be CDM density, neutrino density or neutrino velocity) arbitrarily correlated with the adiabatic mode. We theoretically assess in which cases (of detecting and/or fixing parameters) improvements can be expected, to guide our subsequent numerical analyses. We find that for Planck, which detected neither isocurvature modes nor primordial non-Gaussianity, the joint analysis does not improve the constraints in the general case. However, if we fix additional parameters in the model, the improvements can be highly significant depending on the chosen parameter values. For LiteBIRD+CMB-S4 we study in which regions of parameter space compatible with the Planck results the joint analysis will improve the constraints or the significance of a detection. We find that, while for CDM isocurvature this region is very small, for the neutrino isocurvature modes it is much larger. In particular for neutrino velocity it can be about half of the Planck-allowed region, where the joint analysis reduces the isocurvature error bars by up to 70%. In addition the joint analysis can also improve the error bars of some of the standard cosmological parameters, by up to 30% for θMC for example, by breaking the degeneracies with the correlation parameter between adiabatic and isocurvature modes.

Warm Little Inflaton Becomes Cold Dark Matter.

We present a model where the inflaton can naturally account for all the dark matter in the Universe within the warm inflation paradigm. In particular, we show that the symmetries and particle content of the warm little inflaton scenario (i)avoid large thermal and radiative corrections to the scalar potential, (ii)allow for sufficiently strong dissipative effects to sustain a radiation bath during inflation that becomes dominant at the end of the slow-roll regime, and (iii)enable a stable inflaton remnant in the postinflationary epochs. The latter behaves as dark radiation during nucleosynthesis, leading to a non-negligible contribution to the effective number of relativistic degrees of freedom, and becomes the dominant cold dark matter component in the Universe shortly before matter-radiation equality for inflaton masses in the 10^{-4}-10^{-1} eV range. Cold dark matter isocurvature perturbations, anticorrelated with the main adiabatic component, provide a smoking gun for this scenario that can be tested in the near future.

Axion isocurvature perturbations in low-scale models of hybrid inflation

The quantum chromodynamics (QCD) axion solves the strong $CP$ problem and represents an attractive particle candidate for cold dark matter (CDM). However, quantum fluctuations of the axion field during inflation easily result in large CDM isocurvature perturbations that are in conflict with observations of the cosmic microwave background. In this paper, we demonstrate how this problem can be solved in low-scale models of hybrid inflation that may emerge from supersymmetric grand unified theories. We consider both F-term hybrid inflation and D-term hybrid inflation in supergravity, explicitly taking into account the effect of hidden-sector supersymmetry breaking. We discuss the production of cosmic strings and show how the soft terms in the scalar potential readily allow us to achieve the correct scalar spectral index. In both cases, we are able to identify large regions in parameter space that are consistent with all constraints. In particular, we find that evading the CDM isocurvature constraint always requires a small Yukawa or gauge coupling of $\mathcal{O}({10}^{\ensuremath{-}3})$ or smaller. This translates into upper bounds on the gravitino mass of $\mathcal{O}({10}^{5})\text{ }\text{ }\mathrm{GeV}$ in F-term hybrid inflation and $\mathcal{O}({10}^{9})\text{ }\text{ }\mathrm{GeV}$ in D-term hybrid inflation. Our results point to interesting scenarios in well-motivated parameter regions that will be tested in future axion and cosmic microwave background experiments.

Curvaton as dark matter with secondary inflation

We consider a novel cosmological scenario in which a curvaton is long-lived and plays the role of cold dark matter (CDM) in the presence of a short, secondary inflation. Non-trivial evolution of the large scale cosmological perturbation in the curvaton scenario can affect the duration of the short term inflation, resulting in the inhomogeneous end of inflation. Non-linear parameters of the curvature perturbation are predicted to be fNL ≈ 5/4 and gNL ≈ 0. The curvaton abundance can be well diluted by the short-term inflation and accordingly, it does not have to decay into the Standard Model particles. Then the curvaton can account for the present CDM with the isocurvature perturbation being sufficiently suppressed because both the adiabatic and CDM isocurvature perturbations have the same origin. As an explicit example, we consider the thermal inflation scenario and a string axion as a candidate for this curvaton-dark matter. We further discuss possibilities to identify the curvaton-dark matter with the QCD axion.

Cosmological aspects of spontaneous baryogenesis

We investigate cosmological aspects of spontaneous baryogenesis driven by a scalar field, and present general constraints that are independent of the particle physics model. The relevant constraints are obtained by studying the backreaction of the produced baryons on the scalar field, the cosmological expansion history after baryogenesis, and the baryon isocurvature perturbations. We show that cosmological considerations alone provide powerful constraints, especially for the minimal scenario with a quadratic scalar potential. Intriguingly, we find that for a given inflation scale, the other parameters including the reheat temperature, decoupling temperature of the baryon violating interactions, and the mass and decay constant of the scalar are restricted to lie within ranges of at most a few orders of magnitude. We also discuss possible extensions to the minimal setup, and propose two ideas for evading constraints on isocurvature perturbations: one is to suppress the baryon isocurvature with nonquadratic scalar potentials, another is to compensate the baryon isocurvature with cold dark matter isocurvature by making the scalar survive until the present.

Compensated isocurvature perturbations in the curvaton model

Primordial fluctuations in the relative number densities of particles, or isocurvature perturbations, are generally well constrained by cosmic microwave background (CMB) data. A less probed mode is the compensated isocurvature perturbation (CIP), a fluctuation in the relative number densities of cold dark matter and baryons. In the curvaton model, a subdominant field during inflation later sets the primordial curvature fluctuation $\zeta$. In some curvaton-decay scenarios, the baryon and cold dark matter isocurvature fluctuations nearly cancel, leaving a large CIP correlated with $\zeta$. This correlation can be used to probe these CIPs more sensitively than the uncorrelated CIPs considered in past work, essentially by measuring the squeezed bispectrum of the CMB for triangles whose shortest side is limited by the sound horizon. Here, the sensitivity of existing and future CMB experiments to correlated CIPs is assessed, with an eye towards testing specific curvaton-decay scenarios. The planned CMB Stage 4 experiment could detect the largest CIPs attainable in curvaton scenarios with more than 3$\sigma$ significance. The significance could improve if small-scale CMB polarization foregrounds can be effectively subtracted. As a result, future CMB observations could discriminate between some curvaton-decay scenarios in which baryon number and dark matter are produced during different epochs relative to curvaton decay. Independent of the specific motivation for the origin of a correlated CIP perturbation, cross-correlation of CIP reconstructions with the primary CMB can improve the signal-to-noise ratio of a CIP detection. For fully correlated CIPs the improvement is a factor of $\sim$2$-$3.

Bayesian evidence of nonstandard inflation: Isocurvature perturbations and running spectral index

Bayesian model comparison penalizes models with more free parameters that are allowed to vary over a wide range, and thus offers the most robust method to decide whether some given data require new parameters. In this paper, we ask a simple question: do current cosmological data require extensions of the simplest single-field inflation models? Specifically, we calculate the Bayesian evidence of a totally anti-correlated isocurvature perturbation and a running spectral index of the scalar curvature perturbation. These parameters are motivated by recent claims that the observed temperature anisotropy of the cosmic microwave background on large angular scales is too low to be compatible with the simplest inflation models. Both a subdominant, anti-correlated cold dark matter isocurvature component and a negative running index succeed in lowering the large-scale temperature power spectrum. We show that the introduction of isocurvature perturbations is disfavored, whereas that of the running spectral index is only moderately favored, even when the BICEP2 data are included in the analysis without any foreground subtraction.

Resonant conversions of QCD axions into hidden axions and suppressed isocurvature perturbations

We study in detail MSW-like resonant conversions of QCD axions into hidden axions, including cases where the adiabaticity condition is only marginally satisfied, and where anharmonic effects are non-negligible. When the resonant conversion is efficient, the QCD axion abundance is suppressed by the hidden and QCD axion mass ratio. We find that, when the resonant conversion is incomplete due to a weak violation of the adiabaticity, the CDM isocurvature perturbations can be significantly suppressed, while non-Gaussianity of the isocurvature perturbations generically remain unsuppressed. The isocurvature bounds on the inflation scale can therefore be relaxed by the partial resonant conversion of the QCD axions into hidden axions.

Planck2013 results. XXII. Constraints on inflation

We analyse the implications of the Planck data for cosmic inflation. The Planck nominal mission temperature anisotropy measurements, combined with the WMAP large-angle polarization, constrain the scalar spectral index to $n_s = 0.9603 \pm 0.0073$, ruling out exact scale invariance at over 5 $\sigma$. Planck establishes an upper bound on the tensor-to-scalar ratio of r < 0.11 (95% CL). The Planck data thus shrink the space of allowed standard inflationary models, preferring potentials with V" < 0. Exponential potential models, the simplest hybrid inflationary models, and monomial potential models of degree n > 2 do not provide a good fit to the data. Planck does not find statistically significant running of the scalar spectral index, obtaining $d n_s/d ln k = -0.0134 \pm 0.0090$. Several analyses dropping the slow-roll approximation are carried out, including detailed model comparison and inflationary potential reconstruction. We also investigate whether the primordial power spectrum contains any features. We find that models with a parameterized oscillatory feature improve the fit $\chi^2$ by ~ 10; however, Bayesian evidence does not prefer these models. We constrain several single-field inflation models with generalized Lagrangians by combining power spectrum data with bounds on $f_\mathrm{NL}$ measured by Planck. The fractional primordial contribution of CDM isocurvature modes in the curvaton and axion scenarios has upper bounds of 0.25% or 3.9% (95% CL), respectively. In models with arbitrarily correlated CDM or neutrino isocurvature modes, an anticorrelation can improve $\chi^2$ by approximatively 4 as a result of slightly lowering the theoretical prediction for the $\ell<40$ multipoles relative to the higher multipoles. Nonetheless, the data are consistent with adiabatic initial conditions.

Relaxing isocurvature bounds on string axion dark matter

If inflation scale is high, light scalars acquire large quantum fluctuations during inflation. If sufficiently long-lived, they will give rise to CDM isocurvature perturbations, which are highly constrained by the Planck data. Focusing on string axions as such light scalars, we show that thermal inflation can provide a sufficiently large entropy production to dilute the CDM isocurvature perturbations. Importantly, efficient dilution is possible for the string axions, because effectively no secondary coherent oscillations are induced at the end of thermal inflation, in contrast to the moduli fields. We also study the viability of the axion dark matter with mass of about 7 keV as the origin of the 3.5 keV X-ray line excess, in the presence of large entropy production.

CMB constraint on non-Gaussianity in isocurvature perturbations

We study the CMB constraints on non-Gaussianity in CDM isocurvature perturbations.Non-Gaussian isocurvature perturbations can be produced in various models at the very early stage of the Universe. Since the isocurvature perturbations little affect thestructure formation at late times, CMB is the best probe of isocurvature non-Gaussianity at least in the near future. In this paper, we focus on non-Gaussian curvature and isocurvature perturbations of the local-type, which are uncorrelated and in the formζ = ζG+(3/5)fNL(ζG2−⟨ζG2⟩)and S = SG+fNL(ISO)(SG−⟨SG2⟩), and constrain the non-linearity parameter of isocurvature perturbations, fNL(ISO), as well as the curvature one fNL.For this purpose, we employ several state-of-art techniques for the analysis of CMB data and simulation. Assuming that isocurvature perturbations are subdominant, we apply our method to the WMAP 7-year data of temperature anisotropy and obtain constraints on a combination α2fNL(ISO),where α is the ratio of the power spectrum of isocurvature perturbationsto that of the adiabatic ones.When the adiabatic perturbations are assumed to be Gaussian, we obtained a constraint α2fNL(ISO) = 40±66assuming the power spectrum of isocurvature perturbations is scale-invariant.When we assume that the adiabatic perturbations can also be non-Gaussian, we obtain fNL = 38±24 and α2fNL(ISO) = −8±72. We also discuss implications of our results for the axion CDM isocurvature model.

Modulated preheating and isocurvature perturbations

We consider a model of preheating where the coupling of the inflaton to the preheat field is modulated by an additional scalar field which is light during inflation. We establish that such a model produces the observed curvature perturbation analogously to the modulated reheating scenario. The contribution of modulated preheating to the power spectrum and to non-Gaussianity can however be significantly larger compared to modulated perturbative reheating. We also consider the implications of the current constraints on isocurvature perturbations in case where the modulating field is responsible for cold dark matter. We find that existing bounds on CDM isocurvature perturbations imply that modulated preheating is unlikely to give a dominant contribution to the curvature perturbation and that the same bounds suggest important constraints on non-Gaussianity and the amount of primordial gravitational waves.

The sensitivity of BAO dark energy constraints to general isocurvature perturbations

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 a decrease in the Dark Energy Task Force figure of merit (FoM) by 46% i.e., FoMISO = 0.54 × FoMAD and 53% for the BOSS and ADEPT experiments respectively when considered in conjunction with PLANK data. We also show that the incorrect assumption of adiabaticity has the potential to bias our estimates of the dark energy parameters by 2.7σ (2.2σ) for a single correlated isocurvature mode (CDM isocurvature), and up to 4.9σ (5.7σ) 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 95% for certain modes in the fully correlated case.

CONSTRAINTS ON SCALAR AND TENSOR PERTURBATIONS IN PHENOMENOLOGICAL AND TWO-FIELD INFLATION MODELS: BAYESIAN EVIDENCES FOR PRIMORDIAL ISOCURVATURE AND TENSOR MODES

We constrain cosmological models where the primordial perturbations have both an adiabatic and a (possibly correlated) cold dark matter (CDM) or baryon isocurvature component. We use both a phenomenological approach, where the primordial power spectra are parametrized with amplitudes and spectral indices, and a slow-roll two-field inflation approach where slow-roll parameters are used as primary parameters. In the phenomenological case, with CMB data, the upper limit to the CDM isocurvature fraction is \alpha<6.4% at k=0.002Mpc^{-1} and 15.4% at k=0.01Mpc^{-1}. The median 95% range for the non-adiabatic contribution to the CMB temperature variance is -0.030<\alpha_T<0.049. Including the supernova (or large-scale structure, LSS) data, these limits become: \alpha<7.0%, 13.7%, and -0.048<\alpha_T< 0.042 (or \alpha<10.2%, 16.0%, and -0.071<\alpha_T<0.024). The CMB constraint on the tensor-to-scalar ratio, r<0.26 at k=0.01Mpc^{-1}, is not affected by the nonadiabatic modes. In the slow-roll two-field inflation approach, the spectral indices are constrained close to 1. This leads to tighter limits on the isocurvature fraction, with the CMB data \alpha<2.6% at k=0.01Mpc^{-1}, but the constraint on \alpha_T is not much affected, -0.058<\alpha_T<0.045. Including SN (or LSS) data, these limits become: \alpha< 3.2% and -0.056<\alpha_T<0.030 (or \alpha<3.4% and -0.063<\alpha_T<-0.008). When all spectral indices are close to each other the isocurvature fraction is somewhat degenerate with the tensor-to-scalar ratio. In addition to the generally correlated models, we study also special cases where the perturbation modes are uncorrelated or fully (anti)correlated. We calculate Bayesian evidences (model probabilities) in 21 different cases for our nonadiabatic models and for the corresponding adiabatic models, and find that in all cases the data support the pure adiabatic model.

Differentiating CDM and baryon isocurvature models with 21 cm fluctuations

We discuss how one can discriminate models with cold dark matter (CDM) and baryon isocurvature fluctuations. Although current observations such as cosmic microwave background (CMB) can severely constrain the fraction of such isocurvature modes in the total density fluctuations, CMB cannot differentiate CDM and baryon ones by the shapes of their power spectra. However, the evolution of CDM and baryon density fluctuations are different for each model, thus it would be possible to discriminate those isocurvature modes by extracting information on the fluctuations of CDM/baryon itself. We discuss that observations of 21 cm fluctuations can in principle differentiate these modes and demonstrate to what extent we can distinguish them with future 21 cm surveys. We show that, when the isocurvature mode has a large blue-tilted initial spectrum, 21 cm surveys can clearly probe the difference.

Cold dark matter isocurvature perturbations: Cosmological constraints and applications

In this paper we present the constraints on cold dark matter (CDM) isocurvature contributions to the cosmological perturbations. By employing Markov chain Monte Carlo method, we perform a global analysis for cosmological parameters using the latest astronomical data, such as 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) observations, matter power spectrum from the Sloan Digital Sky Survey, luminous red galaxies, and the Union2 type Ia supernovae sample. We find that the correlated mixture of adiabatic and isocurvature modes is slightly favored by the current observational data. We also obtain a tight limit on the fraction of the CDM isocurvature contributions, which should be less than 14.6% at 95% confidence level. With the presence of the isocurvature modes, the adiabatic spectral index becomes slightly bigger, ${n}_{s}^{\mathrm{adi}}=0.972\ifmmode\pm\else\textpm\fi{}0.014$ ($1\ensuremath{\sigma}$), and the tilt for isocurvature spectrum could be large, namely, the best fit value is ${n}_{s}^{\mathrm{iso}}=3.020$. Finally, we discuss the effect on WMAP normalization priors, shift parameter $R$, acoustic scale ${l}_{A}$, and ${z}_{*}$, from the CDM isocurvature perturbation. By fitting the mixed initial condition to the combined data, we find the mean values of $R$, ${l}_{A}$, and ${z}_{*}$ can be changed about $2.9\ensuremath{\sigma}$, $2.8\ensuremath{\sigma}$, and $1.5\ensuremath{\sigma}$ respectively, comparing with those obtained in the pure adiabatic condition.

Constraints on primordial isocurvature perturbations and spatial curvature by Bayesian model selection

We present posterior likelihoods and Bayesian model selection analysis for generalized cosmological models where the primordial perturbations include correlated adiabatic and cold dark matter isocurvature components. We perform nested sampling with flat and, for the first time, curved spatial geometries of the Universe, using data from the CMB anisotropies, the Union supernovae (SN) sample, and a combined measurement of the integrated Sachs-Wolfe effect. The CMB alone favors a 3% (positively correlated) isocurvature contribution in both the flat and curved cases. The nonadiabatic contribution to the observed CMB temperature variance is $0l{\ensuremath{\alpha}}_{T}l7%$ at 98% C.L. in the curved case. In the flat case, combining the CMB with SN data artificially biases the result towards the pure adiabatic $\ensuremath{\Lambda}\mathrm{CDM}$ concordance model, whereas in the curved case the favored level of nonadiabaticity stays at the 3% level with all combinations of data. However, the ratio of Bayes factors, or $\ensuremath{\Delta}\mathrm{ln}$ (evidence), is more than 5 points in favor of the flat adiabatic $\ensuremath{\Lambda}\mathrm{CDM}$ model, which suggests that the inclusion of the 5 extra parameters of the curved isocurvature model is not supported by the current data. The results are very sensitive to the second and third acoustic peak regions in the CMB temperature angular power: therefore a careful calibration of these data will be required before drawing decisive conclusions on the nature of primordial perturbations. Finally, we point out that the odds for the flat nonadiabatic model are 1:3 compared to the curved adiabatic model. This may suggest that it is not much less motivated to extend the concordance model with 4 isocurvature degrees of freedom than it is to study the spatially curved adiabatic model, though at the moment the model selection disfavors both of these models.