Cosmic Microwave Background Observations Research Articles

Measuring the primordial gravitational waves from cosmic microwave background and stochastic gravitational wave background observations

In this paper, we constrain the primordial gravitational waves from cosmic microwave background (CMB) and stochastic gravitational wave background (SGWB) observations. SGWB provides the latest way to explore the early universe and the cosmological evolution which can be reflected by primordial gravitational waves. We not only combine LIGO observations with CMB to measure primordial gravitational waves, but also forecast the potential abilities of the LISA detector and PTA projects. In the [Formula: see text] model, the standard six parameters change slightly from SGWB observations. While the constraints on tensor-to-scalar ratio and tensor spectral index are improved obviously from SGWB observations. FAST projects have a significant impact on tensor-to-scalar ratio and tensor spectral index, namely [Formula: see text] and [Formula: see text] at [Formula: see text] confidence level.

An 8 per cent determination of the Hubble constant from localized fast radio bursts

ABSTRACT The cosmological-constant (Λ) cold dark matter (CDM) model is challenged by the Hubble tension, a remarkable difference of Hubble constant H0 between measurements from local probes and the prediction from Planck cosmic microwave background observations under ΛCDM model. So one urgently needs new distance indicators to test the Hubble tension. Fast radio bursts (FRBs) are millisecond-duration pulses occurring at cosmological distances, which are attractive cosmological probes. Here, we report a measurement of ${H_0} = 68.81^{+4.99}_{-4.33} {\rm \ km \ s^{-1} \ Mpc^{-1}}$ using eighteen localized FRBs, with an uncertainty of 8 per cent at 68.3 per cent confidence. Using a simulation of 100 localized FRBs, we find that error of H0 can be reduced to 2.6 per cent at 1σ uncertainty. Thanks to the high event rate of FRBs and localization capability of radio telescopes (i.e. Australian Square Kilometre Array Pathfinder and Very Large Array), future observations of a reasonably sized sample will provide a new way of measuring H0 with a high precision to test the Hubble tension.

Cosmic birefrigence: cross-spectra and cross-bispectra with CMB anisotropies

Parity-violating extensions of Maxwell electromagnetism induce a rotation of the linear polarization plane of photons during propagation. This effect, known as cosmic birefringence, impacts on the Cosmic Microwave Background (CMB) observations producing a mixing of E and B polarization modes which is otherwise null in the standard scenario. Such an effect is naturally parametrized by a rotation angle which can be written as the sum of an isotropic component α 0 and an anisotropic one δα(n̂). In this paper we compute angular power spectra and bispectra involving δα and the CMB temperature and polarization maps. In particular, contrarily to what happens for the cross-spectra, we show that even in absence of primordial cross-correlations between the anisotropic birefringence angle and the CMB maps, there exist non-vanishing three-point correlation functions carrying signatures of parity-breaking physics. Furthermore, we find that such angular bispectra still survive in a regime of purely anisotropic cosmic birefringence, which corresponds to the conservative case of having α o = 0. These bispectra represent an additional observable aimed at studying cosmic birefringence and its parity-violating nature beyond power spectrum analyses. They provide also a way to perform consistency checks for specific models of cosmic birefringence. Moreover, we estimate that among all the possible birefringent bispectra,〈δαTB〉and〈δαEB〉are the ones which contain the largest signal-to-noise ratio. Once the cosmic birefringence signal is taken to be at the level of current constraints, we show that these bispectra are within reach of future CMB experiments, as LiteBIRD.

Constraints on small-scale primordial density fluctuation from cosmic microwave background through dark matter annihilation

The cosmic microwave background (CMB) observation by the Planck satellite precisely determines primordial curvature fluctuations on larger scales than \U0001d4aa(1) Mpc, while the small-scale curvature fluctuation is still less constrained.The constraint on small-scale fluctuations is highly improved if we assume the standard thermal relic dark matter scenario.When small-scale fluctuations are large enough, dense regions collapse to form small halos even in a redshift z ≳ 103, which is called “ultracompact minihalos”.These minihalos enhance the annihilation of the dark matter and it is constrained by observations such as extragalactic gamma rays and the CMB. We revisit the effect of minihalos formed by the small-scale density fluctuations and calculate the ionization history modified by the dark matter annihilation assuming the thermally produced WIMP accounting for all dark matter with e + e −-channel annihilation.We perform the Markov Chain Monte Carlo method to constrain the size of small-scale curvature fluctuations by the CMB power spectrum.It is found that the constraint from the CMB power spectrum is comparable to that from the extragalactic gamma rays.Assuming a pair-annihilation cross section 〈σ v〉 = 3 × 10-26cm3/s and a mass of dark matter 1 TeV,we constrain the curvature power spectrum \U0001d4abℛ(k) < \U0001d4aa(10-6) over 1 Mpc-1 < k < 106 Mpc-1.We confirm that our constraint mainly comes from the energy injection in early time (z ≳ 100) and hence it is independent of the uncertainty of minihalo properties in the late time.

The Atacama Cosmology Telescope: Modeling bulk atmospheric motion

Fluctuating atmospheric emission is a dominant source of noise for ground-based millimeter-wave observations of the cosmic microwave background (CMB) temperature anisotropy at angular scales $\ensuremath{\gtrsim}0.5\ifmmode^\circ\else\textdegree\fi{}$. We present a model of the atmosphere as a discrete set of emissive turbulent layers that move with respect to the observer with a horizontal wind velocity. After introducing a statistic derived from the time-lag dependent correlation function for detector pairs in an array, referred to as the pair-lag, we use this model to estimate the aggregate angular motion of the atmosphere derived from time-ordered data from the Atacama Cosmology Telescope (ACT). We find that estimates derived from ACT's CMB observations alone agree with those derived from satellite weather data that additionally include a height-dependent horizontal wind velocity and water vapor density. We also explore the dependence of the measured atmospheric noise spectrum on the relative angle between the wind velocity and the telescope scan direction. In particular, we find that varying the scan velocity changes the noise spectrum in a predictable way. Computing the pair-lag statistic opens up new avenues for understanding how atmospheric fluctuations impact measurements of the CMB anisotropy.

Primordial gravitational waves from NANOGrav: A broken power-law approach

We revisit the possibility that the stochastic common-spectrum process recently detected by the NANOGrav pulsar timing array experiment could be due to primordial gravitational waves (GWs). A na\"{i}ve extrapolation down to interferometer scales of the blue GW spectrum required to explain NANOGrav consistently with Cosmic Microwave Background (CMB) observations would strongly violate upper limits on the stochastic GW background (SGWB) amplitude from LIGO/Virgo. In combination with the fact that there are over 19 decades in frequency between CMB and interferometer scales, this motivates us to move beyond the commonly adopted approximation of a pure power-law GW spectrum. We consider a broken power-law parametrization for the SGWB spectrum, which turns from blue to red above the break frequency: while phenomenological, this choice maps to various well-motivated early-Universe models, including scenarios featuring non-instantaneous reheating or a non-standard background expansion following reheating. After a detailed discussion of the contribution of the resulting SGWB to the early-Universe radiation energy density, we constrain the broken power-law model against a wide variety of multi-frequency cosmological and GW observations. We find that this phenomenological model is able to explain the NANOGrav signal while remaining in agreement with upper limits on the tensor-to-scalar ratio on CMB scales, Big Bang Nucleosynthesis constraints on the early-Universe radiation energy density, and upper limits on the SGWB amplitude on interferometer scales. We briefly discuss the very bright prospects for testing this model with next-generation probes across the GW frequency landscape, which motivate further exploring connections to specific well-motivated early-Universe models.

What can CMB observations tell us about the neutrino distribution function?

Cosmic Microwave Background (CMB) observations have been used extensively to constrain key properties of neutrinos, such as their mass. However, these inferences are typically dependent on assumptions about the cosmological model, and in particular upon the distribution function of neutrinos in the early Universe. In this paper, we aim to assess the full extent to which CMB experiments are sensitive to the shape of the neutrino distribution. We demonstrate that Planck and CMB-S4-like experiments have no prospects for detecting particular features in the distribution function. Consequently, we take a general approach and marginalise completely over the form of the neutrino distribution to derive constraints on the relativistic and non-relativistic neutrino energy densities, characterised by N eff = 3.0 ± 0.4 and ρν,0 NR < 14 eV cm-3 at 95% CL, respectively. The fact that these are the only neutrino properties that CMB data can constrain has important implications for neutrino mass limits from cosmology. Specifically, in contrast to the ΛCDM case where CMB and BAO data tightly constrain the sum of neutrinos masses to be ∑m ν < 0.12 eV, we explicitly show that neutrino masses as large as ∑ m ν∼ 3 eV are perfectly consistent with this data. Importantly, for this to be the case, the neutrino number density should be suitably small such that the bound on ρν,0 NR = ∑ m ν n ν,0 is still satisfied. We conclude by giving an outlook on the opportunities that may arise from other complementary experimental probes, such as galaxy surveys, neutrino mass experiments and facilities designed to directly detect the cosmic neutrino background. GitHub: Parameter files for MCMC analysis and code to reproduce all plots can be found here.

Search for a Scalar Induced Stochastic Gravitational Wave Background in the Third LIGO-Virgo Observing Run.

The formation of primordial black holes from inflationary fluctuations is accompanied by a scalar induced gravitational wave background. We perform a Bayesian search of such background in the data from Advanced LIGO and Virgo's first, second, and third observing runs, parametrizing the peak in the curvature power spectrum by a log-normal distribution. The search shows no evidence for such a background. We place 95%confidence level upper limits on the integrated power of the curvature power spectrum peak which, for a narrow width, reaches down to 0.02 at 10^{17} Mpc^{-1}. The resulting constraints are stronger than those arising from big bang nucleosynthesis or cosmic microwave background observations. In addition, we find that the constraints from LIGO and Virgo, at their design sensitivity, and from the Einstein Telescope can compete with those related to the abundance of the formed primordial black holes.

GW200105 and GW200115 are compatible with a scenario of primordial black hole binary coalescences

Two gravitational wave events, i.e. GW200105 and GW200115, were observed by the Advanced LIGO and Virgo detectors recently. In this work, we show that they can be explained by a scenario of primordial black hole binaries that are formed in the early Universe. The merger rate predicted by such a scenario could be consistent with the one estimated from LIGO and Virgo, even if primordial black holes constitute a fraction of cold dark matter. The required abundance of primordial black holes is compatible with the existing upper limits from microlensing, caustic crossing and cosmic microwave background observations.

Large density perturbations from reheating to standard model particles due to the dynamics of the Higgs boson during inflation

Cosmic Microwave Background (CMB) observations are used to constrain reheating to Standard Model (SM) particles after a period of inflation. As a light spectator field, the SM Higgs boson acquires large field values from its quantum fluctuations during inflation, gives masses to SM particles that vary from one Hubble patch to another, and thereby produces large density fluctuations. We consider both perturbative and resonant decay of the inflaton to SM particles. For the case of perturbative decay from coherent oscillations of the inflaton after high scale inflation, we find strong constraints on the reheat temperature for the inflaton decay into heavy SM particles. For the case of resonant particle production (preheating) to (Higgsed) SM gauge bosons, we find temperature fluctuations larger than observed in the CMB for a range of gauge coupling that includes those found in the SM and conclude that such preheating cannot be the main source of reheating the Universe after inflation.

Development of cloud monitoring system for cosmic microwave background observations

Development of cloud monitoring system for cosmic microwave background observations

Linear cosmological constraints on two-body decaying dark matter scenarios and the S8 tension

The '$S_8$ tension' is a longstanding discrepancy between the cosmic microwave background (CMB) and weak gravitational lensing determination of the amplitude of matter fluctuations, parametrized as $S_8\equiv\sigma_8(\Omega_m/0.3)^{0.5}$, where $\sigma_8$ is the root mean square of matter fluctuations on a 8 $h^{-1}$Mpc scale, and $\Omega_m$ is the total matter abundance. It was recently shown that dark matter (DM) decaying into a massless (dark radiation) and a massive (warm DM) species, with a lifetime $\Gamma^{-1} \simeq 55~(\varepsilon/0.007)^{1.4}$ Gyrs -- where $\varepsilon$ represent the mass-energy fraction transferred to the massless component -- can ease the tension. Thanks to a fast and accurate fluid approximation scheme for the warm species, we perform a comprehensive study of this 2-body decaying DM scenario, discussing in detail its dynamics and its impact on the CMB and linear matter power spectra. We then investigate the implications for the '$S_8$ tension' against a number of changes in the analysis: different $S_8$ priors, marginalization over the lensing information in Planck data, trading Planck high$-\ell$ polarization data for those from the SPTpol and ACTPol surveys, and the inclusion of the recent results from the Xenon1T collaboration. We conclude that the preference for decaying DM, apparent only when the $S_8$ value determined from weak lensing data is added to the analysis, does not sensibly degrade the fit to any of the cosmological data-sets considered, and that the model could potentially explain the anomalous electron recoil excess reported by the Xenon1T collaboration. Furthermore, we explictly show that while current CMB data alone are not sensitive enough to distinguish between standard CDM and decaying DM, next-generation CMB observations (CMB-S4) can unambiguously detect its signature.

Dynamical attractors in contracting spacetimes dominated by kinetically coupled scalar fields

We present non-perturbative numerical relativity simulations of slowly contracting spacetimes in which the scalar field driving slow contraction is coupled to a second scalar field through an exponential non-linear σ model-type kinetic interaction. These models are important because they can generate a nearly scale-invariant spectrum of super-Hubble density fluctuations fully consistent with cosmic microwave background observations. We show that the non-linear evolution rapidly approaches a homogeneous, isotropic and flat Friedmann-Robertson-Walker (FRW) geometry for a wide range of inhomogeneous and anisotropic initial conditions. Ultimately, we find, the kinetic coupling causes the evolution to deflect away from flat FRW and towards a novel Kasner-like stationary point, but in general this occurs on time scales that are too long to be observationally relevant.

Cosmological constraints on heavy sterile neutrinos

We consider heavy sterile neutrinos νs , with mass in the range 10 MeV ∼ ms ∼ mπ ∼ 135 MeV, thermally produced in the early universe and freezing out after the QCD phase transition. The existence of these neutrinos may alter the value of the effective number of neutrino species N eff, measured by the cosmic microwave background (CMB), and the 4He production during the BBN. We provide a detailed account of the solution of the relevant Boltzmann equations and we identify the parameter space constrained by current Planck satellite data and forecast the parameter space probed by future Stage-4 ground-based CMB (CMB-S4) observations.

Warm inflation, neutrinos and dark matter: a minimal extension of the Standard Model

We show that warm inflation can be realized within a minimal extension of the Standard Model with three right-handed neutrinos, three complex scalars and a gauged lepton/B-L U(1) symmetry. This simple model can address all the shortcomings of the Standard Model that are not related to fine-tuning, within general relativity, with distinctive experimental signatures that can be probed in the near future. The inflaton field emerges from the collective breaking of the U(1) symmetry, and interacts with two of the right-handed neutrinos, sustaining a high-temperature radiation bath during inflation. The discrete interchange symmetry of the model protects the scalar potential against large thermal corrections and leads to a stable inflaton remnant at late times which can account for dark matter. Consistency of the model and agreement with Cosmic Microwave Background observations naturally yield light neutrino masses below 0.1 eV, while thermal leptogenesis occurs naturally after a smooth exit from inflation into the radiation era.

Reheating in inflation models with the inflaton decaying into radiation

The physics of reheating can provide additional predictions for inflationary models and thus has spurred an increasing deal of interest. In this paper, by considering a special function for the decay of the inflaton into radiation, which renders the effective equation of state (EoS) parameter to be time varying during reheating, we investigate the constraints on reheating from the cosmic microwave background observations and the effect of reheating on the predictions for inflationary models. Our work is different from previous ones where a constant EoS parameter is assumed. Taking the power-law inflationary models as concrete examples, we find that the time-varying EoS parameter during reheating entails a non-negligible effect on the reheating predictions.

Gaussian Process Reconstruction of Reionization History

Abstract We reconstruct the history of reionization using Gaussian process regression. Using the UV luminosity data compilation from Hubble Frontiers Fields we reconstruct the redshift evolution of UV luminosity density and thereby the evolution of the source term in the ionization equation. This model-independent reconstruction rules out single power-law evolution of the luminosity density but supports the logarithmic double power-law parameterization. We obtain reionization history by integrating ionization equations with the reconstructed source term. Using the optical depth constraint from Planck cosmic microwave background observation, measurement of UV luminosity function integrated until truncation magnitude of −17 and −15, and derived ionization fraction from high redshift quasar, galaxies, and gamma-ray burst observations, we constrain the history of reionization. In the conservative case we find the constraint on the optical depth as τ = 0.052 ± 0.001 ± 0.002 at 68% and 95% confidence intervals. We find the redshift duration between 10% and 90% ionization to be 2.05 − 0.21 − 0.30 + 0.11 + 0.37 . Longer duration of reionization is supported if UV luminosity density data with truncation magnitude of −15 is used in the joint analysis. Our results point out that even in a conservative reconstruction, a combination of cosmological and astrophysical observations can provide stringent constraints on the epoch of reionization.

Minimal theory of massive gravity in the light of CMB data and the S8 tension

We investigate the minimal theory of massive gravity (MTMG) in the light of different observational datasets that are in tension within the $\mathrm{\ensuremath{\Lambda}}$ cold dark matter cosmology. In particular, we analyze the MTMG model, for the first time, with the Planck--cosmic microwave background (CMB) data, and how these precise measurements affect the free parameters of the theory. The MTMG model can affect the CMB power spectrum at large angular scales and cause a suppression on the amplitude of the matter power spectrum. We find that on adding Planck-CMB data, the graviton has a small, positive, but nonzero mass at 68% confidence level, and from this perspective, we show that the tension between redshift space distortions measurements and Planck-CMB data in the parametric space ${S}_{8}\ensuremath{-}{\mathrm{\ensuremath{\Omega}}}_{m}$ can be resolved within the MTMG scenario. Through a robust and accurate analysis, we find that the ${H}_{0}$ tension between the CMB and the local distance ladder measurements still remains but can be reduced to $\ensuremath{\sim}3.5\ensuremath{\sigma}$ within the MTMG theory. The MTMG is very well consistent with the CMB observations, and undoubtedly, it can serve as a viable candidate among other modified gravity theories.

Small-scale clumping at recombination and the Hubble tension

Despite the success of the standard $\Lambda$CDM model of cosmology, recent data improvements have made tensions emerge between low- and high-redshift observables, most importantly in determinations of the Hubble constant $H_0$ and the (rescaled) clustering amplitude $S_8$. The high-redshift data, from the cosmic microwave background (CMB), crucially relies on recombination physics for its interpretation. Here we study how small-scale baryon inhomogeneities (i.e., clumping) can affect recombination and consider whether they can relieve both the $H_0$ and $S_8$ tensions. Such small-scale clumping, which may be caused by primordial magnetic fields or baryon isocurvature below kpc scales, enhances the recombination rate even when averaged over larger scales, shifting recombination to earlier times. We introduce a flexible clumping model, parametrized via three spatial zones with free densities and volume fractions, and use it to study the impact of clumping on CMB observables. We find that increasing $H_0$ decreases both $\Omega_m$ and $S_8$, which alleviates the $S_8$ tension. On the other hand, the shift in $\Omega_m$ is disfavored by the low-$z$ baryon-acoustic-oscillations measurements. We find that the clumping parameters that can change the CMB sound horizon enough to explain the $H_0$ tension also alter the damping tail, so they are disfavored by current Planck 2018 data. We test how the CMB damping-tail information rules out changes to recombination by first removing $\ell>1000$ multipoles in Planck data, where we find that clumping could resolve the $H_0$ tension. Furthermore, we make predictions for future CMB experiments, as their improved damping-tail precision can better constrain departures from standard recombination. Both the Simons Observatory and CMB-S4 will provide decisive evidence for or against clumping as a resolution to the $H_0$ tension.

Limits on Polarized Dust Spectral Index Variations for CMB Foreground Analysis

Abstract Using Planck polarization data, we search for and constrain spatial variations of the polarized dust foreground for cosmic microwave background (CMB) observations, specifically in its spectral index, β d . Failure to account for such variations will cause errors in the foreground cleaning that propagate into errors on cosmological parameter recovery from the cleaned CMB map. It is unclear how robust prior studies of the Planck data that constrained β d variations are due to challenges with noise modeling, residual systematics, and priors. To clarify constraints on β d and its variation, we employ two pixel space analyses of the polarized dust foreground at &gt;3.°7 scales on ≈60% of the sky at high Galactic latitudes. A template fitting method, which measures β d over three regions of ≈20% of the sky, does not find significant deviations from a uniform β d = 1.55, consistent with prior Planck determinations. An additional analysis in these regions, based on multifrequency fits to a dust and CMB model per pixel, puts limits on σ β d , the Gaussian spatial variation in β d . The data support σ β d up to 0.45 at the highest latitudes, 0.30 at midlatitudes, and 0.15 at low latitudes. We also demonstrate that care must be taken when interpreting the current Planck constraints, β d maps, and noise simulations. Due to residual systematics and low dust signal-to-noise ratios at high latitudes, forecasts for ongoing and future missions should include the possibility of large values of σ β d as estimated in this paper, based on current polarization data.