Concordance ΛCDM Model Research Articles

Quasars as standard candles

A sample of quasars has been recently assembled to investigate the non-linear relation between their monochromatic luminosities at 2500 Å and 2 keV and to exploit quasars as a new class of ‘standardized candles’. The use of this technique for cosmological purposes relies on the non-evolution with redshift of the UV-optical spectral properties of quasars, as well as on the absence of possible contaminants such as dust extinction and host galaxy contribution. We address these possible issues by analysing the spectral properties of our cosmological quasar sample. We produced composite spectra in different bins of redshift and accretion parameters (black hole mass, bolometric luminosity), to investigate any possible evolution of the spectral properties of the continuum of the composites with these parameters. We found a remarkable similarity amongst the various stacked spectra. Apart from the well known evolution of the emission lines with luminosity (i.e. the Baldwin effect) and black hole mass (i.e. the virial relation), the overall shape of the continuum, produced by the accretion disc, does not show any statistically significant trend with black-hole mass (MBH), bolometric luminosity (Lbol), or redshift (z). The composite spectrum of our quasar sample is consistent with negligible levels of both intrinsic reddening (with a colour excess E(B − V)≲0.01) and host galaxy emission (less than 10%) in the optical. We tested whether unaccounted dust extinction could explain the discrepancy between our cosmographic fit of the Hubble–Lemaître diagram and the concordance ΛCDM model. The average colour excess required to solve the tension should increase with redshift up to unphysically high values (E(B − V)≃0.1 at z > 3) that would imply that the intrinsic emission of quasars is much bluer and more luminous than ever reported in observed spectra. The similarity of quasar spectra across the parameter space excludes a significant evolution of the average continuum properties with any of the explored parameters, confirming the reliability of our sample for cosmological applications. Lastly, dust reddening cannot account for the observed tension between the Hubble–Lemaître diagram of quasars and the ΛCDM model.

Betti Functionals as Probes for Cosmic Topology

The question of the global topology of the Universe (cosmic topology) is still open. In the ΛCDM concordance model, it is assumed that the space of the Universe possesses the trivial topology of R3, and thus that the Universe has an infinite volume. As an alternative, in this paper, we study one of the simplest non-trivial topologies given by a cubic 3-torus describing a universe with a finite volume. To probe cosmic topology, we analyze certain structure properties in the cosmic microwave background (CMB) using Betti functionals and the Euler characteristic evaluated on excursions sets, which possess a simple geometrical interpretation. Since the CMB temperature fluctuations δT are observed on the sphere S2 surrounding the observer, there are only three Betti functionals βk(ν), k=0,1,2. Here, ν=δT/σ0 denotes the temperature threshold normalized by the standard deviation σ0 of δT. The analytic approximations of the Gaussian expectations for the Betti functionals and an exact formula for the Euler characteristic are given. It is shown that the amplitudes of β0(ν) and β1(ν) decrease with an increasing volume V=L3 of the cubic 3-torus universe. Since the computation of the βk’s from observational sky maps is hindered due to the presence of masks, we suggest a method that yields lower and upper bounds for them and apply it to four Planck 2018 sky maps. It is found that the βk’s of the Planck maps lie between those of the torus universes with side-lengths L=2.0 and L=3.0 in units of the Hubble length and above the infinite ΛCDM case. These results give a further hint that the Universe has a non-trivial topology.

Observational constraints on extended Proca-Nuevo gravity and cosmology

Abstract We confront massive Proca-Nuevo gravity with cosmological observations. The former is a non-linear theory involving a massive spin-1 field, that can be extended incorporating operators of the Generalized Proca class, and when coupled to gravity it can be covariantized in a way that exhibits consistent and ghost-free cosmological solutions, without experiencing instabilities and superluminalities at the perturbative level. When applied at a cosmological framework it induces extra terms in the Friedmann equations, however due to the special non-linear construction the field is eliminated in favor of the Hubble function. Thus, the resulting effective dark energy sector is dynamical, however it contains the same number of free parameters with the ΛCDM concordance model. We use data from Supernovae Ia (SNIa) and Cosmic Chronometers (CC) observations and we construct the corresponding likelihood-contours for the free parameters. Interestingly enough, application of various information criteria, such as AIC, BIC and DIC, shows that the scenario of massive Proca-Nuevo gravity, although having exactly the same number of free parameters with ΛCDM paradigm, it is more efficient in fitting the data. Finally, the reconstructed dark-energy equation-of-state parameter shows statistical compatibility with the model-independent, data-driven reconstructed one.

Growth rate of spherical voids with non-comoving dark matter and baryons

We present numerical solutions to Einstein’s equations describing large spherical cosmic voids constituted by two components: dark matter and baryons, with a non-vanishing initial relative velocity, in an asymptotically homogeneous background compatible with the ΛCDM concordance model. We compute numerically the evolution of such configurations in the dark matter frame, with a hypothetical homogeneous distribution of baryons, but respecting the values dictated by the concordance model for the average baryon-to-dark matter density ratio. We reproduce the well-known formation of overdensities at the edge of the void and recover the Lemaître–Tolman–Bondi solutions in the comoving limit of our simulations. We compute the average growth factor of matter fluctuations and find that it departs significantly from the linear perturbative prescription even in the comoving case, where the non-linearity of inhomogeneities has an impact.

Running Vacuum in the Universe: Phenomenological Status in Light of the Latest Observations, and Its Impact on the σ8 and H0 Tensions

A substantial body of phenomenological and theoretical work over the last few years strengthens the possibility that the vacuum energy density (VED) of the universe is dynamical, and in particular that it adopts the ‘running vacuum model’ (RVM) form, in which the VED evolves mildly as δρvac(H)∼νeffmPl2OH2, where H is the Hubble rate and νeff is a (small) free parameter. This dynamical scenario is grounded on recent studies of quantum field theory (QFT) in curved spacetime and also on string theory. It turns out that what we call the ‘cosmological constant’, Λ, is no longer a rigid parameter but the nearly sustained value of 8πG(H)ρvac(H) around any given epoch H(t), where G(H) is the gravitational coupling, which can also be very mildly running (logarithmically). Of particular interest is the possibility suggested in past works that such a running may help to cure the cosmological tensions afflicting the ΛCDM. In the current study, we reanalyze the RVM in full and we find it becomes further buttressed. Using modern cosmological data, namely a compilation of the latest SNIa+BAO+H(z)+LSS+CMB observations, we probe to what extent the RVM provides a quality fit better than the concordance ΛCDM model, with particular emphasis on its impact on the σ8 and H0 tensions. We utilize the Einstein–Boltzmann system solver CLASS and the Monte Carlo sampler MontePython for the statistical analysis, as well as the statistical DIC criterion to compare the running vacuum against the rigid vacuum (νeff=0). On fundamental grounds, νeff receives contributions from all the quantized matter fields in FLRW spacetime. We show that with a tiny amount of vacuum dynamics (νeff≪1) the global fit can improve significantly with respect to the ΛCDM and the mentioned tensions may subside to inconspicuous levels.

Primordial non-gaussianities of inflationary step-like models

We use Minkowski Functionals (MF) to explore the possible presence of non-Gaussian signatures in the recently released cosmic microwave background (CMB) Planck maps. In particular, we investigate MF outcomes for two sets of simulated CMB maps: one seeded by the ΛCDM concordance model, another seeded by the ‘best-fit’ CMB angular power spectrum. By ‘best-fit’ we mean a simulated CMB spectrum seeded by an inflationary model that well reproduce the observed low-ℓ features of the CMB temperature power spectrum. In this way, we obtain the spurious signal introduced by the power spectra with different shapes, signal to be taken into account in the corresponding MF analyses. Interestingly, we investigate the response of the four MF (i.e., Area, Perimeter, Genus, Nclusters ) for several sets of simulated maps. Our analyses confirm that, as already reported in the literature, the Perimeter exhibits the best performance. Moreover, our results show that the Perimeter obtained from the Planck maps (2015) is better fitted by the simulated CMB maps seeded by the angular power spectrum with low-ℓ features with respect to the curves obtained from the CMB maps seeded by the ΛCDM model.

High-precision Measurements of Cosmic Curvature from Gravitational Wave and Cosmic Chronometer Observations

Although the spatial curvature has been measured with very high precision, it still suffers from the well-known cosmic curvature tension. In this paper, we use an improved method to determine the cosmic curvature, by using the simulated data of binary neutron star mergers observed by the second generation space-based DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO). By applying the Hubble parameter observations of cosmic chronometers to the DECIGO standard sirens, we explore different possibilities of making measurements of the cosmic curvature referring to a distant past: one is to reconstruct the Hubble parameters through the Gaussian process without the influence of hypothetical models, and the other is deriving constraints on Ω K in the framework of the non-flat Λ cold dark matter model. It is shown that in the improved method DECIGO could provide a reliable and stringent constraint on the cosmic curvature (Ω K = −0.007 ± 0.016), while we could only expect the zero cosmic curvature to be established at the precision of ΔΩ K = 0.11 in the second model-dependent method. Therefore, our results indicate that in the framework of methodology proposed in this paper, the increasing number of well-measured standard sirens in DECIGO could significantly reduce the bias of estimations for cosmic curvature. Such a constraint is also comparable to the precision of Planck 2018 results with the newest cosmic microwave background (CMB) observations (ΔΩ K ≈ 0.018), based on the concordance ΛCDM model.

Particle production during inflation: a Bayesian analysis with CMB data from Planck 2018

A class of inflationary models that involve rapid bursts of particle productions predict observational signatures, such as bump-like features in the primordial scalar power spectrum. In this work, we analyze such models by comparing their predictions with the latest CMB data from Planck 2018. We consider two scenarios of particle production. The first one is a simple scenario consisting of a single burst of particle production during observable inflation. The second one consists of multiple bursts of particle production that lead to a series of bump-like features in the primordial power spectrum. We find that the second scenario of the multi-bump model gives better fit to the CMB data compared to the concordance ΛCDM model. We carried out model comparisons using Bayesian evidences. From the observational constraints on the amplitude of primordial features of the multi-bump model, we find that the dimensionless coupling parameter g responsible for particle production is bound to be g < 0.05.

Comparing the scalar-field dark energy models with recent observations

We focus on a class of scalar-field dark energy models (i.e., ϕCDM models) which behave like cosmological trackers at early times, and investigate the general properties of them. Particularly, we choose three ϕCDM models with typical potentials, i.e., V(ϕ)∝ϕ−α (inverse power-law (IPL) model), V(ϕ)∝cothαϕ (L-model) and V(ϕ)∝cosh(αϕ) (Oscillatory tracker model), where the latter two models are based on the α-attractors originated from the study of inflation. These models, which reduce to the ΛCDM model with α→0, are studied and compared with the recent observations, including the Pantheon sample of type Ia supernovae (SNe Ia), baryon acoustic oscillations (BAO) measurements extracted from 6dFGS, BOSS and eBOSS, as well as the temperature and polarization anisotropy power spectra data of cosmic microwave background radiation (CMB) from Planck 2018 results. The observational constraints from the combining sample (SNe Ia + BAO + CMB) indicate that none of the three ϕCDM models exclude the ΛCDM model at 68.3% confidence level. We find that the CMB anisotropy data have obvious advantages in constraining the dark energy models compared with other cosmological probes, which is particularly evident in the L-model. Furthermore, we apply the Bayesian evidence to compare the ϕCDM models and the ΛCDM model with the analysis of the combining sample. The concordance ΛCDM model is still the most supported one. In addition, among the three ϕCDM models, the IPL model is the most competitive one, while the L-model/Oscillatory tacker model is moderately/strongly disfavored.

On the maximum volume of collapsing structures

In many cosmological models, including the ΛCDM concordance model, there exist theoretical upper bounds on the size of collapsing structures. The most common formulations in the literature refer to a turnaround radius in spherical symmetry or a turnaround surface, defined as the zero-expansion boundary separating the outer Hubble flow from the inner flow of a collapsing fluid. In order to access a generic scenario, we propose an improvement of this cosmological test in terms of the maximum volume of the cosmological structures, which is equivalent to a zero-averaged expansion — instead of the zero-local expansion. By combining the Lagrangian perturbations method and the scalar averaging of Einstein's equations, we obtain a maximum volume for a collapse model without any restricting symmetries.We compare this result with some exact, inhomogeneous solutions and discuss further potential developments.

Forecasts on interacting dark energy from the 21-cm angular power spectrum with BINGO and SKA observations

ABSTRACT Neutral hydrogen (H i) intensity mapping is a promising technique to probe the large-scale structure of the Universe, improving our understanding of the late-time accelerated expansion. In this work, we first scrutinize how an alternative cosmology, interacting dark energy (IDE), can affect the 21-cm angular power spectrum relative to the concordance ΛCDM model. We re-derive the 21-cm brightness temperature fluctuation in the context of such an interaction and uncover an extra new contribution. Then we estimate the noise level of three upcoming H i intensity mapping surveys, namely Baryon Acoustic Oscillations from Integrated Neutral Gas Observations (BINGO), Square Kilometre Array (SKA1)-MID Band 1 and Band 2, and employ a Fisher matrix approach to forecast their constraints on the IDE model. We find that while Planck 2018 maintains its dominion over early-Universe parameter constraints, BINGO and SKA1-MID Band 2 provide complementary bounding to the latest cosmic microwave background measurements on the dark energy equation of state w, the interacting strength λi and the reduced Hubble constant h, and that SKA1-MID Band 1 even outperforms Planck 2018 in these late-Universe parameter constraints. The expected minimum uncertainties are given by SKA1-MID Band 1 + Planck: $\sim 0.34{{\ \rm per\ cent}}$ on w, $\sim 0.22{{\ \rm per\ cent}}$ on h, $\sim 0.64{{\ \rm per\ cent}}$ on H i bias bHi, and an absolute uncertainty of about 3 × 10−4 (7 × 10−4) on λ1 (λ2). Moreover, we quantify the effects from systematics of the redshift bin number, redshift-space distortions, foreground residuals and uncertainties on the measured H i fraction, ΩHi(z). Our results indicate a bright prospect for H i intensity mapping surveys in constraining IDE, whether on their own or by synergies with other measurements.

Cosmographic approach to Running Vacuum dark energy models: New constraints using BAOs and Hubble diagrams at higher redshifts

Abstract In this work we study different types of dark energy (DE) models in the framework of the cosmographic approach, with emphasis on the Running Vacuum models (RVMs). We assess their viability using different information criteria and compare them with the so-called Ghost DE models (GDEs) as well as with the concordance ΛCDM model. We use the Hubble diagrams for Pantheon SnIa, quasars (QSOs), gamma-ray bursts (GRBs) as well as the data on baryonic acoustic oscillations (BAOs) in four different combinations. Upon minimizing the χ2 function of the distance modulus in the context of the Markov Chain Monte Carlo method (MCMC), we put constraints on the current values of the standard cosmographic parameters in a model-independent way. It turns out that, in the absence of BAOs data, the various DE models generally exhibit cosmographic tensions with the observations at the highest redshifts (namely with the QSOs and GRBs data). However, if we include the robust observations from BAOs to our cosmographic sample, the ΛCDM and RVMs are clearly favored against the GDEs. Finally, judging from the perspective of the deviance information criterion (DIC), which enables us to compare models making use of the Markov chains of the MCMC method, we conclude that the RVMs are the preferred kind of DE models. We find it remarkable that these models, which had been previously shown to be capable of alleviating the σ8 and H0 tensions, appear now also as the most successful ones at the level of the cosmographic analysis.

Using our newest VLT-KMOS HII galaxies and other cosmic tracers to test the Lambda cold dark matter tension

ABSTRACT We place novel constraints on the cosmokinetic parameters by using a joint analysis of the newest Very Large Telescope (VLT)-K-band Multi Object Spectrograph (KMOS) HII galaxies (HIIG) with the Type Ia Supernovae (SNIa) Pantheon sample. We combine the latter data sets in order to reconstruct, in a model-independent way, the Hubble diagram to as high redshifts as possible. Using a Gaussian process, we derive the basic cosmokinetic parameters and compare them with those of Lambda cold dark matter (ΛCDM). In the case of SNIa, we find that the extracted values of the cosmokinetic parameters are in agreement with the predictions of ΛCDM model. Combining SNIa with high-redshift tracers of the Hubble relation, namely HIIG data, we obtain consistent results with those based on ΛCDM as far as the present values of the cosmokinetic parameters are concerned, but find significant deviations in the evolution of the cosmokinetic parameters with respect to the expectations of the concordance ΛCDM model.

The growth rate of cosmic structures in the local Universe with the ALFALFA survey

ABSTRACT We investigate the growth rate of structures in the local Universe. For this, we use as a cosmological tracer the HI line extra-galactic sources from the Arecibo Legacy Fast ALFA (ALFALFA) survey to obtain a measurement of the normalized growth rate parameter, fσ8, considered a powerful tool to constrain alternative models of gravity. For these analyses, we calculate the Local Group velocity due to the matter structures distribution in the ALFALFA catalogue and compare it with the Local Group velocity relative to the Cosmic Microwave Background frame to obtain the velocity scale parameter, β. Using Monte Carlo realizations and lognormal simulations, our methodology quantifies the errors introduced by shot-noise and partial sky coverage of the analysed data. The measurement of the velocity scale parameter β and the calculation of the matter fluctuation of the cosmological tracer, $\sigma _{8}^{\text{tr}}$, lead us to fσ8 = 0.46 ± 0.06 at $\bar{z} = 0.013$, in good agreement (at 1σ level) with the value expected in the ΛCDM concordance model. In addition, our analyses of the ALFALFA sample also provide a measurement of the growth rate of structures $f = 0.56 \pm 0.07$, at $\bar{z} = 0.013$.

Modeling of Accelerating Universe with Bulk Viscous Fluid in Bianchi V Space‐Time

AbstractIn this paper, we have investigated a bulk viscous anisotropic Universe and constrained its model parameters with recent and Pantheon compilation data. Using cosmic chronometric technique, we estimate the present value of Hubble's constant as , and by bounding our derived model with recent data, Pantheon and joint and Pantheon data respectively. The present age of the Universe is specified as Gyrs. The model favours a transitioning Universe with the transition red‐shift as . We have reconstructed the jerk parameter using the observational data sets. From the analysis of the jerk parameter, it is observed that, our derived model shows a marginal departure from the concordance ΛCDM model.

A Test of the Cosmological Principle with Quasars

Abstract We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 μm from the cryogenic, post-cryogenic, and reactivation phases of the WISE mission. While the direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large as expected, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a p-value of 5 × 10−7 (4.9σ for a normal distribution, one-sided), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance ΛCDM model.

Hints for possible low redshift oscillation around the best fit ΛCDM model in the expansion history of the universe

Abstract We search for possible deviations from the expectations of the concordance ΛCDM model in the expansion history of the Universe by analysing the Pantheon Type Ia Supernovae (SnIa) compilation along with its Monte Carlo simulations using redshift binning. We demonstrate that the redshift binned best fit ΛCDM matter density parameter Ω0m and the best fit effective absolute magnitude $\cal M$ oscillate about their full dataset best fit values with considerably large amplitudes. Using the full covariance matrix of the data taking into account systematic and statistical errors, we show that at the redshifts below z ≈ 0.5 such oscillations can only occur in 4 to 5% of the Monte Carlo simulations. While statistical fluctuations can be responsible for this apparent oscillation, we might have observed a hint for some behaviour beyond the expectations of the concordance model or a possible additional systematic in the data. If this apparent oscillation is not due to statistical or systematic effects, it could be due to either the presence of coherent inhomogeneities at low z or due to oscillations of a quintessence scalar field.

A Cosmography Approach to Dark Energy Cosmologies: New Constraints Using the Hubble Diagrams of Supernovae, Quasars, and Gamma-Ray Bursts

Abstract In the context of a cosmography approach to using the data of the Hubble diagram for supernovae, quasars, and gamma-ray bursts, we study dark energy (DE) parameterizations and the concordance cold dark matter (ΛCDM) universe. Using different combinations of data samples including (i) supernovae (Pantheon), (ii) Pantheon + quasars. and (iii) Pantheon + quasars + gamma-ray bursts, and applying the minimization of χ 2 function of the distance modulus of data samples in the context of the Markov Chain Monte Carlo method, we obtain constrained values of cosmographic parameters in a model-independent cosmography scenario. We then investigate our analysis, for different concordance ΛCDM cosmology, wCDM, Chevallier–Polarski–Linder, and Pade parameterizations. Comparing the numerical values of the cosmographic parameters obtained for DE scenarios with those of the model-independent method, we show that the concordance ΛCDM model has serious issues when we involve quasar and gamma-ray burst data in our analysis. While high-redshift quasars and gamma-ray bursts can falsify the concordance model, our results using a cosmography approach indicate that the other DE parameterizations are still consistent with these observations.

The growth of DM and DE perturbations in DBI non-canonical scalar field scenario

We study the effect of varying sound speed on clustering dark energy in the Dirac–Born–Infeld (DBI) scenario. The DBI action is included in the class of k-essence models, and it has an important role in describing the effective degrees of freedom of D-branes in the string theory. In the DBI setup, we take the anti-de Sitter (AdS) warp factor f(ϕ)=f0ϕ−4, and investigate the self-interacting quartic potential V(ϕ)=λϕ4∕4. We calculate the full expression of the effective sound speed for our model, and show that it can evolve with time during the cosmological evolution. Besides, the adiabatic sound speed evolves with time here, and this influences the background dynamics to some extent. We show that the effective sound speed is very close to the adiabatic sound speed. We examine the effect of the variable sound speed on growth of the perturbations in both the linear and non-linear regimes. In the linear regime, we apply the Pseudo-Newtonian formalism, and show that dark energy suppresses the growth of perturbations at low redshifts. From study of the Integrated Sachs–Wolfe (ISW) effect in our setup, we see that the model manifests some deviation from the concordance ΛCDM model. In the non-linear regime, we follow the approach of spherical collapse model, and calculate the linear overdensity δc(zc), the virial overdensity Δvir(zc), overdensity at the turn around ζ(zc) and the rate of expansion of collapsed region hta(z). Our results imply that the provided values of δc(zc), Δvir(zc), ζ(zc) and hta(z) in our clustering DBI dark energy model approach the fiducial value in the EdS universe at high enough redshifts. We further compute relative number density of halo objects above a given mass in our setting, and show that the number of structures with respect to the ΛCDM model is reduced more in the high mass tail at high redshifts.

Combined analysis of Planck and SPTPol data favors the early dark energy models

We study the implications of the Planck temperature power spectrum at low multipoles, 0ℓ<100, and SPTPol data. We show that this combination predicts consistent lensing-induced smoothing of acoustic peaks within ΛCDM cosmology and yields the robust predictions of the cosmological parameters. Combining only the Planck large-scale temperature data and the SPTPol polarization and lensing measurements within ΛCDM model we found substantially lower values of linear matter density perturbation σ8 which bring the late-time parameter S8=σ8√Ωm/0.3=0.763±0.022 into accordance with galaxy clustering and weak lensing measurements. It also raises up the Hubble constant H0=69.68±1.00kms that reduces the Hubble tension to the 2.5σ level. We examine the residual tension in the Early Dark Energy (EDE) model which produces the brief energy injection prior to recombination. We implement both the background and perturbation evolutions of the scalar field which potential scales as V(ϕ)∝ ϕ2n. Including cosmic shear measurements (KiDS, VIKING-450, DES) and local distance-ladder data (SH0ES) to the combined fit we found that EDE completely alleviates the Hubble tension while not degradating the fit to large-scale structure data. The EDE scenario significantly improves the goodness-of-fit by 2.9σ in comparison with the concordance ΛCDM model. The account for the intermediate-redshift data (the supernova dataset and baryon acoustic oscillation data) fits perfectly to our parameter predictions and indicates the preference of EDE over ΛCDM at 3σ.