Cosmic Microwave Background Anisotropies Research Articles

A non-linear solution to the S8 tension?

ABSTRACT Weak galaxy lensing surveys have consistently reported a lower amplitude for the matter fluctuation spectrum, as measured by the S8 parameter, than expected in the ΛCDM cosmology favoured by Planck. However, the expansion history follows the predictions of the Planck ΛCDM cosmology to high accuracy, as do measurements of lensing of the cosmic microwave background anisotropies. Redshift space distortion measurements also appear to be consistent with Planck ΛCDM. In this paper, we argue that these observations can be reconciled with the Planck ΛCDM cosmology if the matter power spectrum is suppressed more strongly on non-linear scales than assumed in analyses of weak galaxy lensing. We demonstrate this point by fitting a one-parameter model, characterizing a suppression of the non-linear power spectrum, to the KiDS-1000 weak lensing measurements. Such a suppression could be attributed to new properties of the dark matter that affect non-linear scales, or to a response of the matter fluctuations to baryonic feedback processes that are stronger than expected from recent cosmological simulations. Our proposed explanation can be tested using measurements of the amplitude of the matter fluctuation spectrum on linear scales, in particular via high precision redshift space distortion measurements from forthcoming galaxy and quasar redshift surveys.

Investigation of CMB constraints for dark matter-helium scattering

We study dark matter-helium scattering in the early Universe and its impact on constraints from cosmic microwave background (CMB) anisotropy measurements. We describe possible theoretical frameworks for dark matter-nucleon interactions via a scalar, pseudoscalar, or vector mediator; such interactions give rise to hydrogen and helium scattering, with cross sections that have a power-law dependence on relative velocity. Within these frameworks, we consider three scenarios: dark matter coupling to only neutrons, to only protons, and to neutrons and protons with equal strength. For these various cases, we use Planck 2018 temperature, polarization, and lensing anisotropy data to place constraints on dark matter scattering with hydrogen and/or helium for dark matter masses between 10 keV and 1 TeV. For any model that permits both helium and hydrogen scattering with a non-negative power-law velocity dependence, we find that helium scattering dominates the constraint for dark matter masses well above the proton mass. Furthermore, we place the first CMB constraints on dark matter that scatters dominantly/exclusively with helium in the early Universe.

Redshift-space fluctuations in stochastic gravitational wave background

We study the redshift-space fluctuations induced by a stochastic gravitational wave background (SGWB) via the Sachs-Wolfe effect. The redshift-space fluctuations can be encapsulated in a line-of-sight integral that is useful for studying the imprint of short-wavelength gravitational waves on the cosmic microwave background (CMB) anisotropy. We thus derive constraints on the SGWB from small-scale CMB anisotropy measurements. Our results reproduce the constraint on the short-wavelength SGWB, previously derived from the Planck and BICEP/Keck array CMB data with a CMB Boltzmann numerical code. Furthermore, we improve the constraint and extend it to shorter wavelengths by using the CMB measurements made by the Atacama Cosmology Telescope and the South Pole Telescope. Also, the integral provides us with a precise redshift fluctuation correlation between a pair of pulsars in pulsar timing measurements, which conveniently incorporates the effect of the pulsar term into a small-angle correlation. We further discuss the observation of pulsar pairs in globular clusters to look for this small-angle correlation.

Living in a non-flat universe: theoretical formalism

Recent analysis of the Planck measurements opened a possibility that we live in a non-flat universe. Given the renewed interest in non-zero spatial curvature, here we re-visit the light propagation in a non-flat universe and provide the gauge-invariant expressions for the cosmological probes: the luminosity distance, galaxy clustering, weak gravitational lensing, and cosmic microwave background anisotropies. With the positional dependence of the spatial metric, the light propagation in a non-flat universe is much more complicated than in a flat universe. Accounting for all the relativistic effects and including the vector and tensor contributions, we derive the expressions for the cosmological probes and explicitly verify their gauge invariance. We compare our results to previous work in a non-flat universe, if present, but this work represents the first comprehensive investigation of the cosmological probes in a non-flat universe. Our theoretical formalism in a non-flat universe will play a crucial role in constraining the spatial curvature in the upcoming large-scale surveys.

Non-Gaussianity constraints with anisotropic μ distortion measurements from Planck

ABSTRACT Primordial non-Gaussianity can source μ-distortion anisotropies that are correlated with the large-scale temperature and polarization signals of the cosmic microwave background (CMB). A measurement of μT and μE correlations can therefore be used to constrain it on wavelengths of perturbations not directly probed by the standard CMB anisotropies. We carry out a first rigorous search for μ-distortion anisotropies with Planck data, applying the well-tested constrained ILC component-separation method combined with the needlet framework. We correlate the reconstructed μ map with the CMB anisotropies to derive constraints on the amplitude fNL of the local form bispectrum, specifically on the squeezed configurations with effective wavenumbers $k_s \simeq {740}\, \mathrm{Mpc^{-1}}$ and $k_L \simeq {0.05}\, \mathrm{Mpc^{-1}}$, improving previously estimated constraints by more than an order of magnitude. This enhancement is owing to the fact that we are able to use the full multipole information by carefully controlling biases and systematic effects in the analysis. We also for the first time incorporate constraints from measurements of μE correlations, which further tighten the limits. A combination of the derived Planck μT and μE power spectra yields |fNL| ≲ 6800 (95 per cent c.l.) on this highly squeezed bispectrum. This is only ≃3 times weaker than the anticipated constraint from Litebird. Furthermore we show that a combination of Litebird with Planck can improve the expected future constraint by ${\simeq}20{{\%}}$. These limits can be used to constrain multifield inflation models and primordial black hole formation scenarios, thus providing a promising novel avenue forward in CMB cosmology.

Weaker yet again: mass spectrum-consistent cosmological constraints on the neutrino lifetime

We consider invisible neutrino decay nu _H rightarrow nu _l + phi in the ultra-relativistic limit and compute the neutrino anisotropy loss rate relevant for the cosmic microwave background (CMB) anisotropies. Improving on our previous work which assumed masslessnu _l and phi , we reinstate in this work the daughter neutrino mass m_{nu l} in a manner consistent with the experimentally determined neutrino mass splittings. We find that a nonzero m_{nu l} introduces a new phase space factor in the loss rate varGamma _mathrm{T} proportional to (varDelta m_nu ^2/m_{nu _H}^2)^2 in the limit of a small squared mass gap between the parent and daughter neutrinos, i.e., varGamma _mathrm{T} sim (varDelta m_nu ^2/m_{nu H}^2)^2 (m_{nu H}/E_nu )^5 (1/tau _0), where tau _0 is the nu _H rest-frame lifetime. Using a general form of this result, we update the limit on tau _0 using the Planck 2018 CMB data. We find that for a parent neutrino of mass m_{nu H} lesssim 0.1 eV, the new phase space factor weakens the constraint on its lifetime by up to a factor of 50 if varDelta m_nu ^2 corresponds to the atmospheric mass gap and up to 10^{5} if the solar mass gap, in comparison with naïve estimates that assume m_{nu l}=0. The revised constraints are (i) tau ^0 > rsim (6 rightarrow 10) times 10^5 s and tau ^0 > rsim (400 rightarrow 500) s if only one neutrino decays to a daughter neutrino separated by, respectively, the atmospheric and the solar mass gap, and (ii) tau ^0 > rsim (2 rightarrow 6) times 10^7 s in the case of two decay channels with one near-common atmospheric mass gap. In contrast to previous, naïve limits which scale as m_{nu H}^5, these mass spectrum-consistent tau _0 constraints are remarkably independent of the parent mass and open up a swath of parameter space within the projected reach of IceCube and other neutrino telescopes in the next two decades.

What does cosmology tell us about the mass of thermal-relic dark matter?

The presence of light thermally coupled dark matter affects early expansion history and production of light elements during the Big Bang Nucleosynthesis. Specifically, dark matter that annihilates into Standard Model particles can modify the effective number of light species in the universe N eff, as well as the abundance of light elements created buring BBN. These quantities in turn affect the cosmic microwave background (CMB) anisotropy. We present the first joint analysis of small-scale temperature and polarization CMB anisotropy from Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT), together with Planck data and the recent primordial abundance measurements of helium and deuterium to place comprehensive bounds on the mass of light thermal-relic dark matter. We consider a range of models, including dark matter that couples to photons and Standard-Model neutrinos. We discuss the sensitivity of the inferred mass bounds on measurements of N eff, primordial element abundances and the baryon density, and quantify the sensitivity of our results to a possible existence of additional relativistic species. We find that the combination of ACT, SPT, and Planck generally leads to the most stringent mass constraint for dark matter that couples to neutrinos, improving the lower limit by 40%–80%, with respect to previous Planck analyses. On the other hand, the addition of ACT and SPT leads to a slightly weaker bound on electromagnetically coupled particles, due to a shift in the preferred values of Y p and N eff driven by the ground based experiments. In most scenarios, the combination of CMB data has a higher constraining power than the primordial abundance measurements alone, with the best results achieved when all data are combined. Combining all CMB measurements with primordial abundance measurements, we rule out masses below ∼4 MeV at 95% confidence, for all models.We show that allowing for new relativistic species can weaken the mass bounds for dark matter that couples to photons by up to an order of magnitude or more.Finally, we discuss the reach of the next generation of the CMB experiments in terms of probing the mass of the thermal relic dark matter.

The QCD phase transition behind a PBH origin of LIGO/Virgo events?

The best-motivated scenario for a sizable primordial black hole (PBH) contribution to the LIGO/Virgo binary black hole mergers invokes the QCD phase transition, which naturally enhances the probability to form PBH with masses of stellar scale. We reconsider the expected mass function associated not only to the QCD phase transition proper, but also the following particle antiparticle annihilation processes, and analyse the constraints on this scenario from a number of observations: the specific pattern in cosmic microwave background (CMB) anisotropies induced by accretion onto PBHs, CMB spectral distortions, gravitational wave searches, and direct counts of supermassive black holes (SMBHs) at high redshift. We find that the scenario is not viable, unless an ad hoc mass evolution for the PBH mass function and a cutoff in power-spectrum very close to the QCD scale are introduced by hand. Despite these negative results, we note that a future detection of coalescing binaries involving sub-solar PBHs has the potential to check the cosmological origin of SMBHs at the e ± annihilation epoch, if indeed the PBH mass function is shaped by the changes to the equation of state driven by the thermal history of the universe.

Indirect detection of Cosmological Constant from interacting open quantum system

We study the indirect detection of Cosmological Constant from an open quantum system of interacting spins, weakly interacting with a thermal bath, a massless scalar field minimally coupled with the static de Sitter background, by computing the spectroscopic shifts. By assuming pairwise interaction between spins, we construct states using a generalization of the superposition principle. The corresponding spectroscopic shifts, caused by the effective Hamiltonian of the system due to Casimir Polder interaction, are seen to play a crucial role in predicting a very tiny value of the Cosmological Constant, in the static patch of de Sitter space, which is consistent with the observed value from the Planck measurements of the cosmic microwave background (CMB) anisotropies.

Effect of the field self-interaction of General Relativity on the cosmic microwave background anisotropies

Field self-interactions are at the origin of the non-linearities inherent to General Relativity. We study their effects on the cosmic microwave background (CMB) anisotropies. We find that they can reduce or alleviate the need for dark matter and dark energy in the description of the CMB power spectrum.

Spectral distortion anisotropy from inflation for primordial black holes

Single-field inflationary models that seek to greatly enhance small-scale power in order to form primordial black holes predict both a squeezed bispectrum that is enhanced by this small-scale power and a potentially detectable enhancement of cosmic microwave background (CMB) spectral distortions. Despite this combination, spectral distortion anisotropy on CMB scales remains small since the squeezed bispectrum represents an unobservable modulation of the scale rather than local amplitude for the short-wavelength acoustic power that dissipates and forms the $\ensuremath{\mu}$ spectral distortion. The leading-order amplitude effect comes from the local modulation of acoustic dissipation at the beginning of the $\ensuremath{\mu}$ epoch at the end of thermalization by a long-wavelength mode that is correlated with CMB anisotropy itself. Compensating factors from the suppression by the square of the ratio the comoving horizon at thermalization to the smallest detectable primary CMB scales ($\ensuremath{\sim}0.0005$) and maximal allowed enhancement of $\ensuremath{\mu}$ ($\ensuremath{\sim}5000$) leaves a signal in the $\ensuremath{\mu}T$ cross spectrum that is still well beyond the capabilities of PIXIE or LiteBIRD space missions due to sensitivity and resolution while remaining much larger than in single-field slow-roll inflation and potentially observable.

Neutrino Mass Bounds in the Era of Tension Cosmology

The measurements of cosmic microwave background (CMB) anisotropies made by the Planck satellite provide extremely tight upper bounds on the total neutrino mass scale (Σm ν < 0.26 eV at 95% C.L.). However, as recently discussed in the literature, the Planck data show anomalies that could affect this result. Here we provide new constraints on neutrino masses using the recent and complementary CMB measurements from the Atacama Cosmology Telescope DR4 and the South Pole Telescope SPT-3G experiments. We found that both the ACT-DR4 and SPT-3G data, when combined with WMAP, mildly suggest a neutrino mass with Σm ν = 0.68 ± 0.31 and eV at 68% C.L., respectively. Moreover, when CMB lensing from the Planck experiment is included, the ACT-DR4 data now indicate a neutrino mass above the two standard deviations, with eV at 95% C.L., while WMAP+SPT-3G provides a weak upper limit of Σm ν < 0.37 eV at 68% C.L. Interestingly, these results are consistent with the Planck CMB+lensing constraint of eV at 68% C.L. when variations in the A lens parameter are considered. We also show that these indications are still present after the inclusion of BAO or Type Ia supernova data in extended cosmologies that are usually considered to solve the so-called Hubble tension. In this respect, we note that in these models, CMB+BAO constraints prefer a higher neutrino mass for higher values of the Hubble constant. A combination of ACT-DR4, WMAP, BAO, and constraints on the Hubble constant from the SH0ES collaboration gives eV at 68% C.L. in extended cosmologies.

Analogue cosmological particle creation in an ultracold quantum fluid of light

The rapid expansion of the early universe resulted in the spontaneous production of cosmological particles from vacuum fluctuations, some of which are observable today in the cosmic microwave background anisotropy. The analogue of cosmological particle creation in a quantum fluid was proposed, but the quantum, spontaneous effect due to vacuum fluctuations has not yet been observed. Here we report the spontaneous creation of analogue cosmological particles in the laboratory, using a quenched 3-dimensional quantum fluid of light. We observe acoustic peaks in the density power spectrum, in close quantitative agreement with the quantum-field theoretical prediction. We find that the long-wavelength particles provide a window to early times. This work introduces the quantum fluid of light, as cold as an atomic Bose-Einstein condensate.

Primordial standard clock models and CMB residual anomalies

The residuals of the power spectra of WMAP and Planck's cosmic microwave background (CMB) anisotropies data are known to exhibit a few interesting anomalies at different scales with marginal statistical significance. Combining bottom-up and top-down model-building approaches and using a pipeline that efficiently compares model predictions with data, we construct a model of primordial standard clock that is able to link and address the anomalies at both the large and small scales. This model, and its variant, provide some of the best fits to the feature anomalies in CMB. According to Bayes evidences, these models are currently statistically indistinguishable from the Standard Model. We show that the difference between them will soon become statistically significant with various higher quality data on the CMB polarization. We demonstrate that such a model-building and data-analyses process may be used to uncover a portion of detailed evolutionary history of our universe during its primordial epoch.

Decay of multiple dark matter particles to dark radiation in different epochs does not alleviate the Hubble tension

Decaying cold dark matter (CDM) has been considered as a mechanism to tackle the tensions in the Hubble expansion rate and the clustering of matter. However, polarization measurements of the cosmic microwave background (CMB) severely constrain the fraction of dark matter decaying before recombination, and lensing of the CMB anisotropies by large-scale structure set strong constraints on dark matter decaying after recombination. Together, these constraints make an explanation of the Hubble tension in terms of decaying dark matter unlikely. In response to this situation, we investigate whether a dark matter ensemble with CDM particles decaying into free streaming dark radiation in different epochs can alleviate the problem. We find that it does not.

Improved Gibbs samplers for cosmic microwave background power spectrum estimation

We study different variants of the Gibbs sampler algorithm from the perspective of their applicability to the estimation of power spectra of the cosmic microwave background (CMB) anisotropies. These include approaches studied earlier in the CMB literature as well as new ones which are proposed in this work. We demonstrate all these variants on full- and cut-sky simulations and compare their performance, assessing both their computational and statistical efficiency. For this we employ a consistent comparison metric, an effective sample size per second, commonly used in this context in the statistical literature. We show that one of the proposed approaches, referred to as Centered overrelax, which capitalizes on additional, auxiliary variables to minimize the computational time needed per sample, uses overrelaxation to decorrelate subsequent samples, and performs better than the standard Gibbs sampler by a factor between one and two orders of magnitude in the nearly full-sky, satellitelike cases. It therefore potentially provides an interesting alternative to the currently favored approaches.

Constant-roll f(R) inflation compared with cosmic microwave background anisotropies and swampland criteria

Inflationary models derived from f(R) gravity, where the scalaron rolls down with a constant rate from the top to the minimum of the effective potential, are considered. Specifically, we take into account three f(R) models, i.e. Starobinsky $$R^{2}$$ , $$R^{2p}$$ and the logarithmic corrected models. We compare the inflationary parameters derived from the models with the observational data of CMB anisotropies, i.e., the Planck and Keck/array datasets in order to find observational constraints on the parameters space. We find that although our f(R) constant-roll models for $$\gamma =0$$ show observationally acceptable values of r, they do not predict favored values of the spectral index. In particular, we have $$n_{s}>1$$ for the Starobinsky $$R^{2}$$ and $$R^{2p}$$ models and $$0.996<n_{s}<0.999$$ for logarithmic model. Finally, we study the models from the point of view of Weak Gravity Conjecture adopting the swampland criteria.

Non-adiabatic evolution of dark sector in the presence of U(1)Lμ – Lτ gauge symmetry

In secluded dark sector scenario, the connection between the visible and the dark sector can be established through a portal coupling and its presence opens up the possibility of non-adiabatic evolution of the dark sector. To study the non-adiabatic evolution of the dark sector, we have considered a U(1) Lμ – Lτ ⊗ U(1) X extension of the standard model (SM). Here the dark sector is charged only under U(1) X gauge symmetry whereas the SM fields are singlet under this symmetry. Due to the presence of tree-level kinetic mixing between U(1) X and U(1) Lμ – Lτ gauge bosons, the dark sector evolves non-adiabatically and thermal equilibrium between the visible and dark sector is governed by the portal coupling. Depending on the values of the portal coupling (ϵ), dark sector gauge coupling (gX ), mass of the dark matter (m χ) and mass of the dark vector boson (m Z'), we study the temperature evolution of the dark sector as well as the various non-equilibrium stages of the dark sector in detail. Furthermore we have also investigated the constraints on the model parameters from various laboratory and astrophysical searches. We have found that the parameter space for the non-adiabatic evolution of dark sector is significantly constrained for m Z' ≲ 100 MeV from the observations of beam dump experiments, stellar cooling etc. The relic density satisfied region of our parameter space is consistent with the bounds from direct detection, and self interaction of dark matter (SIDM) for the mass ratio r ≡ m Z'/m χ = 10-3 and these bounds will be more relaxed for larger values of r. However the constraints from measurement of diffuse γ-ray background flux and cosmic microwave background (CMB) anisotropy are strongest for r = 10-1 and for smaller values of r, they are not significant.

Eigenmode analysis of perturbations in the primordial medium at and before recombination

Context. Anisotropies of the cosmic microwave background are thought to be due to perturbations of the primordial medium, which, post recombination, lead to the formation of galaxy clusters and galaxies Aims. The perturbation wave modes of the primordial medium at and before recombination, consisting of a fully ionised baryonic plasma, a strong black body radiation field, and cold dark matter, are analysed. Methods. We use the linear perturbation theory of the relativistic equations of motion, utilising a strict thermodynamic equilibrium model that relates the radiation energy density to the plasma temperature. Results. It is shown that a wave mode corresponding to the postulated baryon acoustic waves exists with a phase velocity close to the speed of light, but the participation of the dark matter in this mode is very small. Instead, the dark matter has its own dominant mode in the form of gravitational collapse, with very little participation by the baryonic plasma. Conclusions. In view of this very weak coupling between baryons and dark matter, the initial conditions postulated for computer simulations of large-scale structure and galaxy formation – which assume that after recombination, when galaxy formation is getting underway, baryon and dark matter density perturbations are spatially coincident and are equal in terms of fractional amplitude – may be unjustified. Additionally, the possible non-coincidence of baryon and dark matter perturbations at the last scattering surface has implications for the analysis of cosmic microwave background anisotropies.

Uncovering the history of cosmic inflation from anomalies in cosmic microwave background spectra

We propose an inflationary primordial feature model that can explain both the large and small-scale anomalies in the currently measured cosmic microwave background anisotropy spectra, revealing a clip of adventurous history of the Universe during its primordial epoch. Although the model is currently statistically indistinguishable from the Standard Model, we show that planned observations such as the Simons Observatory, LiteBIRD and CMB-S4 will complement each other in distinguishing the model differences due to their accurate E-mode polarization measurements, offering very optimistic prospects for a detection or exclusion. The model predicts a signal of classical primordial standard clock, which can also be used to distinguish the inflation and alternative scenarios in a model-independent fashion.