Cosmic Anisotropy Research Articles

Testing cosmic anisotropy with Padé approximations and the latest Pantheon+ sample

Cosmography can be used to constrain the kinematics of the Universe in a model-independent way. In this work, we attempt to combine the Pad$ e $ approximations with the latest Pantheon+ sample to test the cosmological principle. Based on the Pad$ e $ approximations, we first applied cosmographic constraints to different-order polynomials including third-order (Pad$ e $), fourth-order (Pad$ e $), and fifth-order (Pad$ e $) ones. The statistical analyses show that the Pad$ e $ polynomial has the best performance. Its best fits are $H_ $ = 72.53pm 0.28 km s$^ $ Mpc$^ $, $q_ $, and $j_ $. By further fixing $j_ $ = 1.00, it can be found that the Pad$ e $ polynomial can describe the Pantheon+ sample better than the regular Pad$ e $ polynomial and the usual cosmological models (including the Lambda CDM, $w$CDM, CPL, and $R_h$ = ct models). Based on the Pad$ e $ ($j_ $ = 1) polynomial and the hemisphere comparison method, we tested the cosmological principle and found the preferred directions of cosmic anisotropy, such as (l, b) = (304.6$^ circ circ $) and (311.1$^ circ circ $) for $q_ $ and $H_ $, respectively. These two directions are consistent with each other at a $1 confidence level, but the corresponding results of statistical isotropy analyses including isotropy and isotropy with real positions are quite different. The statistical significance of $ is stronger than that of $q_ $; that is, 4.75sigma and 4.39sigma for isotropy and isotropy with real positions, respectively. Reanalysis with fixed $q_ = -0.55$ (corresponds to $ m $ = 0.30) gives similar results. Overall, our model-independent results provide clear indications of a possible cosmic anisotropy, which must be taken seriously. Further testing is needed to better understand this signal.

Constraints on Bianchi-I type universe with SH0ES anchored Pantheon+ SNIa data

We study the Bianchi-I cosmological model motivated by signals of statistical isotropy violation seen in cosmic microwave background (CMB) observations and others. To that end, we consider various kinds of anisotropic matter that source anisotropy in our model, specifically Cosmic strings, Magnetic fields, Domain walls and Lorentz violation generated magnetic fields. These anisotropic matter sources, taking one at a time, are studied for their co-evolution with standard model (isotropic) sources viz., dust-like (dark/normal) matter, and dark energy modelled as cosmological constant. We constrain the Hubble parameter, density fractions of anisotropic matter, cold dark matter (CDM), and dark energy (Λ) in a Bianchi-I universe with planar symmetry i.e., which has a global ellipsoidal geometry, and try to find signatures of a cosmic preferred axis if any.The latest compilation of Type Ia Supernova (SNIa) data from Pantheon+SH0ES collaboration is used in our analysis to obtain constraints on cosmological parameters and any preferred axis for our universe.In our analysis, we found mild evidence for a cosmic preferred axis. It is interesting to note that this preferred axis lies broadly in the vicinity of other prominent cosmic anisotropy axes reported in the literature from diverse data sets.Also we find some evidence for non-zero (negative) cosmic shear and eccentricity that characterize different expansion rates in different directions and deviation from an isotropic scale factor respectively.The energy density fractions of two of the sources considered are found to be non-zero at a2σ confidence level.To be more conclusive, we require more SNIa host galaxy data for tighter constraints on distance and absolute magnitude calibration which are expected to be available from the future JWST observations and others.

Testing the cosmological principle with the Pantheon+ sample and the region-fitting method

The cosmological principle is fundamental to the standard cosmological model. It assumes that the Universe is homogeneous and isotropic on very large scales. As the basic assumption, it must stand the test of various observations. In this work, we investigated the properties of the Pantheon+ sample, including redshift distribution and position distribution, and we give its constraint on the flat ΛCDM model: Ωm = 0.36 ± 0.02 and H0 = 72.83 ± 0.23 km s−1 Mpc−1. Then, using the region fitting (RF) method, we mapped the all-sky distribution of cosmological parameters (Ωm and H0) and find that the distribution significantly deviates from isotropy. A local matter underdensity region exists toward (308.4°−48.7+47.6, −18.2°−28.8+21.1) as well as a preferred direction of the cosmic anisotropy (313.4°−18.2+19.6, −16.8°−10.7+11.1) in galactic coordinates. Similar directions may imply that local matter density might be responsible for the anisotropy of the accelerated expansion of the Universe. Results of statistical isotropy analyses including Isotropy and Isotropy with real-data positions (RP) show high confidence levels. For the local matter underdensity, the statistical significances are 2.78σ (isotropy) and 2.34σ (isotropy RP). For the cosmic anisotropy, the statistical significances are 3.96σ (isotropy) and 3.15σ (isotropy RP). The comparison of these two kinds of statistical isotropy analyses suggests that inhomogeneous spatial distribution of real sample can increase the deviation from isotropy. The similar results and findings are also found from reanalyses of the low-redshift sample (lp+) and the lower screening angle (θmax = 60°), but with a slight decrease in statistical significance. Overall, our results provide clear indications for a possible cosmic anisotropy. This possibility must be taken seriously. Further testing is needed to better understand this signal.

Modelling the emergence of cosmic anisotropy from non-linear structures

Astronomical observations suggest that the Universe may be anisotropic on the largest scales. In order to model this situation, we develop a new approach to cosmology that allows for large-scale anisotropy to emerge from the growth of non-linear structure. This is achieved by decomposing all relevant fields with respect to a preferred space-like direction, and then averaging the resulting scalar quantities over spatial domains. Our approach allows us to derive a set of large-scale effective field equations that govern the dynamics of any emergent large-scale anisotropy, and which (up to back-reaction terms) take the form of the field equations of the locally rotationally symmetric Bianchi cosmologies. We apply our approach to the dust-filled Farnsworth solutions, which are an interesting set of exact cosmological models that allow for both anisotropic expansion and large-scale bulk flow.

Testing cosmic anisotropy with the Ep−Eiso (‘Amati’) correlation of GRBs

ABSTRACT We test the possible cosmic anisotropy in 118 long GRBs with the Ep−Eiso (‘Amati’) correlation by employing the dipole fitting (DF) and hemisphere comparison (HC) methods. The distribution of the GRB sample is nearly homogeneous in the sky. The dipole anisotropy is weak in the dipole-modulated $\rm \Lambda$CDM model and the Finslerian cosmological model. The dipole directions from the GRB sample are consistent with ones given by the Pantheon SNe-Ia sample, but with smaller uncertainties. We also investigate whether the GRB sample can reduce the anisotropic signal from inhomogeneous samples like the Pantheon one. The GRB sample is then combined with the Pantheon one, thus providing an SN-G sample. In the dipole-modulated $\rm \Lambda$CDM model, the dipole direction in the SN-G sample shows a considerable change from the one in the Pantheon sample. The angle between the two directions is 26${_{.}^{\circ}}$78. For the HC method, the result of maximum anisotropy level from the G-SN sample is 0.257 ± 0.060 at 68 per cent confidence level (CL) and the corresponding preferred direction is $(l,b)=(82{_{.}^{\circ}}97^{+52{_{.}^{\circ}}73}_{~-61{_{.}^{\circ}}88}, -15{_{.}^{\circ}}09^{+60{_{.}^{\circ}}09}_{~-13{_{.}^{\circ}}54})$. The statistical significance of the $\rm AL_{max}$ is 1.4σ. The angle between the preferred direction and the one from the Pantheon sample is 44${_{.}^{\circ}}$40. Although the amount of data in the GRB sample is about a tenth of that in the Pantheon sample, the GRB sample can considerably impact the results from the Pantheon sample. Our results show that GRBs have the potential to search for a convincing cosmic anisotropy.

Quasi-matter Bounce Cosmology in Light of Planck

Abstract We study quasi-matter bounce cosmology in light of Planck cosmic microwave background angular anisotropy measurements along with the BICEP2/Keck Array data. We propose a new primordial scalar power spectrum by considering a linear approximation of the equation of state w ≅ w 0 + κ(η − η 0) for quasi-matter field in the contracting phase of the universe. Using this new primordial scalar power spectrum, we constrain the zeroth-order approximation of the equation of state w 0 = −0.00340 ± 0.00044 and first-order correction 10 4 ζ = − 1.67 − 0.83 + 1.50 at the 1σ confidence level by Planck temperature and polarization in combination with the BICEP2/Keck Array data in which ζ = 12κ/k * with pivot scale k *. The spectral index of scalar perturbations is determined to be n Bs = 0.9623 ± 0.0055, which lies 7σ away from the scale-invariant primordial spectrum. We find scale dependency for n s at the 1σ confidence level and a tighter constraint on the running of the spectral index compared to ΛCDM+α s cosmology. The running of the spectral index in quasi-matter bounce cosmology is α Bs = π ζ/2c s = −0.0021 ± 0.0016, which is nonzero at the 1.3σ level, whereas in ΛCDM+α s it is nonzero at the 0.8σ level for Planck temperature and polarization data. The sound speed of density fluctuations of the quasi-matter field at the crossing time is c s = 0.097 − 0.023 + 0.037 , which is not a very small value in the contracting phase.

The nonlinear patterns of the cosmic anisotropy: the spatially flat perfect fluid universes

In this manuscript, we investigate the patterns of the cosmological anisotropy in the spatially flat Bianchi models filled with a perfect fluid. We analyse the factor , the ratio of the Hubble parameter in the anisotropic model over its isotropic counterpart. In general, starts to deviate significantly from zero at a specific redshift , which depends on the type of the fluid and the value of the anisotropy magnitude. We also show that the deceleration and the jerk along the principal directions of the expansion tensor are constrained by simple algebraic equations that do not depend on the type of matter present. These characteristic patterns form a valuable framework to probe the cosmological anisotropy in the late-time universe.

A tomographic test of cosmic anisotropy with the recently-released quasar sample

We test the cosmic anisotropy in the dipole-modulated Lambda CDM model and Finslerian cosmological model with the recently-released quasar sample. Based on the redshift tomographic method, the quasar sample is divided into two subsets z le z_{cut} and z > z_{cut} by different cutting redshifts. The dipole amplitudes of the two cosmological models from the subsets z le z_{cut} are very weak. We find that quasars at a higher redshift range may provide more detailed information about the dipole amplitude A_D. The dipole directions of each cosmological model from the subsets z le 1.1 and z > 1.1 are deviated by 1sigma level. The Pantheon sample is combined with the two subsets. The dipole amplitude from the two combined datasets is also very weak. In the dipole-modulated Lambda CDM model, the dipole direction from the combined dataset quasar at z le 1.1 and Pantheon sample is far away from the one given by the Pantheon sample. In the Finslerian cosmological model, the dipole directions from the two combined datasets are close to the one in the Pantheon sample.

Constraining the anisotropy of the Universe with the X-ray and UV fluxes of quasars

We test the anisotropy in the Finslerian cosmological model with the X-ray and ultraviolet (UV) fluxes of quasars. The 2015 and 2020 compilations of quasars are used in the cosmological constraints. We find that the dipole direction given by the 2015 quasar compilation is not far away from the one provided by the Pantheon sample and the angular differences are around 30^{circ }. The Pantheon sample is combined with quasars as the “standardized candles” to test the cosmic anisotropy. The results from two combined datasets are consistent. They show that the dipole anisotropy is weak in the Finslerian cosmological model. We investigate the Hubble constant H_0 in the Finslerian cosmological model. Though the central value of H_0 from the combination of six gravitationally lensed quasars, Pantheon sample, and 2020 quasar compilation decreases a little bit, it is consistent with the result from six gravitationally lensed quasars within statistical uncertainties.

Testing cosmic anisotropy with Pantheon sample and quasars at high redshifts

In this paper, we investigate the cosmic anisotropy from the SN-Q sample, consisting of the Pantheon sample and quasars, by employing the hemisphere comparison (HC) method and the dipole fitting (DF) method. Compared to the Pantheon sample, the new sample has a larger redshift range, a more homogeneous distribution, and a larger sample size. For the HC method, we find that the maximum anisotropy level is ALmax = 0.142 ± 0.026 in the direction (l, b) = (316.08°−129.48+27.41, 4.53°−64.06+26.29). The magnitude of anisotropy is A = (−8.46−5.51+4.34) × 10−4 and the corresponding preferred direction points toward (l, b) = (29.31°−30.54+30.59, 71.40°−9.72+9.79) for the quasar sample from the DF method. The combined SN and quasar sample is consistent with the isotropy hypothesis. The distribution of the dataset might impact the preferred direction from the dipole results. The result is weakly dependent on the redshift from the redshift tomography analysis. There is no evidence of cosmic anisotropy in the SN-Q sample. Though some results obtained from the quasar sample are not consistent with the standard cosmological model, we still do not find any distinct evidence of cosmic anisotropy in the SN-Q sample.

Cosmic anisotropy and fast radio bursts

In the recent years, the field of fast radio bursts (FRBs) is thriving and growing rapidly. It is of interest to study cosmology by using FRBs with known redshifts. In the present work, we try to test the possible cosmic anisotropy with the simulated FRBs. In particular, we only consider the possible dipole in FRBs, rather than the cosmic anisotropy in general, while the analysis is only concerned with finding the rough number of necessary data points to distinguish a dipole from a monopole structure through simulations. Noting that there is no a large sample of actual data of FRBs with known redshifts by now, simulations are necessary to this end. We find that at least 2800, 190, 100 FRBs are competent to find the cosmic dipole with amplitude 0.01, 0.03, 0.05, respectively. Unfortunately, even 10 000 FRBs are not competent to find the tiny cosmic dipole with amplitude of . On the other hand, at least 20 FRBs with known redshifts are competent to find the cosmic dipole with amplitude 0.1. We expect that such a big cosmic dipole could be ruled out by using only a few tens of FRBs with known redshifts in the near future.

Constraining Bianchi type I universe with type Ia supernova and H(z) data

Abstract We use recent 36 observational Hubble data (OHD) in the redshift range 0 . 07 ≤ z ≤ 2 . 36 , latest “joint light curves” (JLA) sample, comprised of 740 type Ia supernovae (SNIa) in the redshift range 0 . 01 ≤ z ≤ 1 . 30 , and their joint combination datasets to constrain anisotropic Bianchi type I (BI) dark energy (DE) model. To estimate model parameters, we apply Hamiltonian Monte Carlo technique. We also compute the covariance matrix for BI dark energy model by considering different datasets to compare the correlation between model parameters. To check the acceptability of our fittings, all results are compared with those obtained from 9 year WMAP as well as Planck (2015) collaboration. Our estimations show that at 68% confidence level the dark energy equation of state (EOS) parameter for OHD or JLA datasets alone varies between quintessence and phantom regions whereas for OHD+JLA dataset this parameter only varies in phantom region. It is also found that the current cosmic anisotropy is of order ∼ 1 0 − 3 which imply that the OHD and JLA datasets do not put tight constraint on this parameter. Therefore, to constraint anisotropy parameter, it is necessary to use high redshift dataset namely cosmic microwave background (CMB). Moreover, from the calculation of p -value associated with χ 2 statistic we observed that non of the ω BI and flat ω CDM models rule out by OHD or JLA datasets. The deceleration parameter is obtained as q = − 0 . 4 6 − 0 . 41 − 0 . 37 + 0 . 89 + 0 . 36 , q = − 0 . 61 9 − 0 . 095 − 0 . 24 + 0 . 12 + 0 . 20 , and q = − 0 . 5 2 − 0 . 046 − 0 . 15 + 0 . 080 + 0 . 014 for OHD, SNIa, and OHD+SNIa data respectively.

Constraining the anisotropy of the Universe with the Pantheon supernovae sample * * Supported by National Natural Science Foundation of China (11675182, 11690022)

We test the possible dipole anisotropy of the Finslerian cosmological model and the other three dipole-modulated cosmological models, i.e. the dipole-modulated ΛCDM, wCDM and Chevallier–Polarski–Linder (CPL) models, by using the recently released Pantheon sample of SNe Ia. The Markov chain Monte Carlo (MCMC) method is used to explore the whole parameter space. We find that the dipole anisotropy is very weak in all cosmological models used. Although the dipole amplitudes of four cosmological models are consistent with zero within the uncertainty, the dipole directions are close to the axial direction of the plane of the SDSS subsample in Pantheon. This may imply that the weak dipole anisotropy in the Pantheon sample originates from the inhomogeneous distribution of the SDSS subsample. A more homogeneous distribution of SNe Ia is necessary to constrain the cosmic anisotropy.

Probing cosmic anisotropy with GW/FRB as upgraded standard sirens

Recently it was shown that cosmic anisotropy can be well tested using either standard siren measurement of luminosity distance dL(z) from gravitational-wave (GW) observation or dispersion measure (DM(z)) from fast radio burst (FRB). It was also observed that the combined measurement of dL(z)⋅DM(z) from the GW/FRB association system as suggested in some of FRB models is more effective to constrain cosmological parameters than dL(z) or DM(z) separately. In this paper, we show that this upgraded siren from the combined GW/FRB observations could test cosmic anisotropy with a double relative sensitivity compared to the usual standard siren from GW observations only.

Anisotropy of the Universe via the Pantheon supernovae sample revisited

We employ the hemisphere comparison (HC) method and the dipole fitting (DF) method to investigate the cosmic anisotropy in the recently released Pantheon sample of type Ia supernovae (SNe Ia) and five combinations among Pantheon. For the HC method, we find the maximum anisotropy level in the full Pantheon sample is $\mathrm{AL}_{max}=0.361\pm0.070$ and corresponding direction $(l,b)=({123.05^{\circ}}^{+11.25^{\circ}}_{-4.22^{\circ}}, {4.78^{\circ}}^{+1.80^{\circ}}_{-8.36^{\circ}})$. A robust check shows the statistical significance of maximum anisotropy level is about $2.1\sigma$. We also find that the Low-$z$ and SNLS subsamples have decisive impact on the overall anisotropy while other three subsamples have little impact. Moreover, the anisotropy level map significantly rely on the inhomogeneous distribution of SNe Ia in the sky. For the DF method, we find the dipole anisotropy in the Pantheon sample is very weak. The dipole magnitude is constrained to be less than $1.16\times10^{-3}$ at $95\%$ confidence level. However, the dipole direction is well inferred by MCMC method and it points towards $(l,b)=({306.00^{\circ}}^{+82.95^{\circ}}_{-125.01^{\circ}}, {-34.20^{\circ}}^{+16.82^{\circ}}_{-54.93^{\circ}})$. This direction is very close to the axial direction to the plane of SDSS subsample. It may imply that SDSS subsample is the decisive part to the dipole anisotropy in the full Pantheon sample. All these facts imply that the cosmic anisotropy found in Pantheon sample significantly rely on the inhomogeneous distribution of SNe Ia in the sky. More homogeneous distribution of SNe Ia is necessary to search for a more convincing cosmic anisotropy.

An Outline Picture of a Growing and Rotating Planck Universe with Emerging Dark Foam

With reference to Planck scale, Mach’s relation, increasing support for large scale cosmic anisotropy and preferred directions and by introducing two new parameters Gamma and Beta, right from the beginning of Planck scale, we make an attempt to estimate ordinary matter density ratio, dark matter density ratio, mass, radius, temperature, age and expansion velocity (from and about the bay universe in all directions). We would like suggest that, from the beginning of Planck scale, 1) Dark matter can be considered as a kind of cosmic foam responsible for formation of galaxies. 2) Cosmic angular velocity is directly proportional to squared cosmic temperature. 3) Cosmic expansion velocity increases with decreasing total matter density ratio. 4) There is no need to consider dark energy for understanding cosmic acceleration.

Anisotropic cosmological reconstruction in f(R, T) gravity

Anisotropic cosmological models are constructed in f(R, T) gravity theory to investigate the dynamics of universe concerning the late time cosmic acceleration. Using a more general and simple approach, the effect of the coupling constant and anisotropy on the cosmic dynamics have been investigated. In this study, it is found that cosmic anisotropy substantially affects cosmic dynamics.

Null signal for the cosmic anisotropy in the Pantheon supernovae\xa0data

The cosmological principle assumes that the universe is homogeneous and isotropic on cosmic scales. There exist many works testing the cosmic homogeneity and/or the cosmic isotropy of the universe in the literature. In fact, some observational hints of the cosmic anisotropy have been claimed. However, we note that the paucity of the data considered in the literature might be responsible for the “found” cosmic anisotropy. So, it might disappear in a large enough sample. Very recently, the Pantheon sample consisting of 1048 type Ia supernovae (SNIa) has been released, which is the largest spectroscopically confirmed SNIa sample to date. In the present work, we test the cosmic anisotropy in the Pantheon SNIa sample by using three methods, and hence the results from different methods can be cross-checked. All the results obtained by using the hemisphere comparison (HC) method, the dipole fitting (DF) method and HEALPix suggest that no evidence for the cosmic anisotropy is found in the Pantheon SNIa sample.

Testing the cosmic anisotropy with supernovae data: Hemisphere comparison and dipole fitting

The cosmological principle is one of the cornerstones in modern cosmology. It assumes that the universe is homogeneous and isotropic on cosmic scales. Both the homogeneity and the isotropy of the universe should be tested carefully. In the present work, we are interested in probing the possible preferred direction in the distribution of type Ia supernovae (SNIa). To our best knowledge, two main methods have been used in almost all of the relevant works in the literature, namely the hemisphere comparison (HC) method and the dipole fitting (DF) method. However, the results from these two methods are not always approximately coincident with each other. In this work, we test the cosmic anisotropy by using these two methods with the Joint Light-Curve Analysis (JLA) and simulated SNIa datasets. In many cases, both methods work well, and their results are consistent with each other. However, in the cases with two (or even more) preferred directions, the DF method fails while the HC method still works well. This might shed new light on our understanding of these two methods.

Probing cosmic anisotropy with gravitational waves as standard sirens

The gravitational wave (GW) as a standard siren directly determines the luminosity distance from the gravitational waveform without reference to the specific cosmological model, of which the redshift can be obtained separately by means of the electromagnetic counterpart like GW events from binary neutron stars and massive black hole binaries (MBHBs). To see to what extent the standard siren can reproduce the presumed dipole anisotropy written in the simulated data of standard siren events from typical configurations of GW detectors, we find that (1) for the Laser Interferometer Space Antenna with different MBHB models during five-year observations, the cosmic isotropy can be ruled out at $3\sigma$ confidence level (C.L.) and the dipole direction can be constrained roughly around $20\%$ at $2\sigma$ C.L., as long as the dipole amplitude is larger than $0.03$, $0.06$ and $0.025$ for MBHB models Q3d, pop III and Q3nod with increasing constraining ability, respectively; (2) for Einstein Telescope with no less than $200$ standard siren events, the cosmic isotropy can be ruled out at $3\sigma$ C.L. if the dipole amplitude is larger than $0.06$, and the dipole direction can be constrained within $20\%$ at $3\sigma$ C.L. if the dipole amplitude is near $0.1$; (3) for the Deci-Hertz Interferometer Gravitational wave Observatory with no less than $100$ standard siren events, the cosmic isotropy can be ruled out at $3\sigma$ C.L. for dipole amplitude larger than $0.03$ , and the dipole direction can even be constrained within $10\%$ at $3\sigma$ C.L. if the dipole amplitude is larger than $0.07$. Our work manifests the promising perspective of the constraint ability on the cosmic anisotropy from the standard siren approach.