The possibility of reconstructing a spherically symmetric inhomogeneous Lemaitre-Tolman-Bondi (LTB) model with $Λ$CDM observations has drawn a lot of attention. Recently, a LTB model having the same luminosity-distance and light-cone mass density as the $Λ$CDM model was reconstructed. We calculate the cosmic age at our position in this model, and get the age $t_{LTB}<11.7$Gyr in 2$σ$ confidence level from the Seven-Year WMAP observations. This result is inconsistent with some Milky Way observations.
Context. Backreactions from large-scale inhomogeneities may provide an elegant explanation for the observed accelerated expansion of the universe without the need to introduce dark energy. Aims. We propose a cosmological test for a specific model of inhomogeneous cosmology, called timescape cosmology. Using large-scale galaxy surveys such as SDSS and 2MRS, we test the variation of expansion expected in the $Λ$-CDM model versus a more generic differential expansion using our own calibrations of bounds suggested by timescape cosmology. Method. Our test measures the systematic variations of the H…
Brans-Dicke (BD) cosmology is reconsidered from an approach in which the model can be formulated in $Λ$CDM form, but at the expense of replacing the rigid cosmological constant $Λ$ with a dynamical quasivacuum component which depends on a small parameter $ε$. The product of $ε$ times the BD-parameter $ω_{\rm BD}$ becomes exactly determined in terms of the ordinary cosmological parameters. The GR limit is recovered for $ω_{\rm BD}\to\infty$ when $ε\to 0$. We solve the background cosmology of the model as well as the perturbations equations. When fitted to the cosmological data, we find that the…
我们测试了几种宇宙学模型对不同观测量的理论预测，以比较由模型拟合确定的宇宙当前膨胀率的间接估计与最近发表的基于造父变星数据的直接测量。我们进行了统计分析Ia型超新星（SN Ia），哈勃参数和重子声振荡数据。这些数据集的联合分析，使我们能够更好地约束宇宙学参数，但也打破了退化，出现在距离之间的模量定义$H_0$和绝对的B-带大小的超新星Ia，$M_0$。从理论上讲，我们考虑了空间平坦和无曲率的$Λ$ CDM，$w$ CDM和非齐次Lemaintre-Tolman-Bondi（LTB）模型。为了分析Ia超新星，我们考虑了Ia超新星内部参数的分布。对于$Λ$ CDM模型，我们发现$Ω_m=0.35\pm0.02$，$H_0=(67.8\pm1.0)\,$ km $\,$ s $^{-1}/$ Mpc，而校正后的SN绝对星等服从正态分布${\cal N}(19.13,0.11)$。$w$ CDM模型为$Ω_m$提供了相同的值，而$H_0=(66.5\pm1.8)\,$ km $\,$为$^{-1}/$ Mpc和$w=-0.93\pm0.07$。当考虑非齐次LTB模型时，组合拟合提供$H_0=(64.2\pm1.9)\,$ km $\,$ s $^{-1}/$ Mpc。赤池信息准则和贝叶斯因子分析都不能清楚地区分$Λ$ CDM和$w$ CDM宇宙学，而它们显然不支持LTB模型。对于$Λ$ C…
We examine the evolution of peculiar velocities of cold dark matter (CDM) in localized arrays of inhomogeneous cosmic structures in a $Λ$CDM background that can be identified as a frame comoving with the Cosmic Microwave (CMB). These arrays are constructed by smoothly matching to this cosmological background regions of Szekeres-II models whose source is an imperfect fluid reinterpreted as non-comoving dust, keeping only first order terms in $v/c$. Considering a single Szekeres-II region matched along two comoving interfaces to a $Λ$CDM background, the magnitudes of peculiar velocities within t…
Saulder et al [2019, A&A, 622, A83; arXiv:1811.11976] have performed a novel observational test of the local expansion of the Universe for the standard cosmology as compared to an alternative model with differential cosmic expansion. Their analysis employs mock galaxy samples from the Millennium Simulation, a Newtonian $N$-body simulation on a $Λ$CDM background. For the differential expansion case the simulation has been deformed in an attempt to incorporate features of a particular inhomogeneous cosmology: the timescape model. It is shown that key geometrical features of the timescape cosmolo…
A ready-to-fit inhomogeneous cosmological model: The axially symmetric Szekeres spacetime
Laboratoire d’étude de l’Univers et des phénomènes eXtrèmes (LUX), Observatoire de Paris-PSL, UMR 8262 CNRS, Sorbonne Université（宇宙与极端现象研究实验室（LUX），巴黎天文台-PSL，CNRS联合研究单位8262，索邦大学）
We test the possible dipole anisotropy of a Finslerian cosmological model and other three dipole-modulated cosmological models, i.e., the dipole-modulated $\rmΛ$CDM, $w$CDM and Chevallier--Polarski--Linder (CPL) model 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 $1σ$ uncertainty, the dipole directions are close to the axial …
A Case for an Inhomogeneous Einstein-de Sitter Universe
Institute of Physics and Astronomy, ELTE Eötvös Loránd University（ELTE罗兰大学物理与天文学研究所）；HUN-REN–ELTE Extragalactic Astrophysics Research Group（匈牙利研究与创新网络-ELTE河外天体物理学研究组）；Institute of Nuclear Techniques, Budapest University of Technology and Economics（布达佩斯技术与经济大学核技术研究所）；Department of Physics, University of Miami（迈阿密大学物理系）；L2IT, Laboratoire des 2 Infinis - Toulouse, Université de Toulouse, CNRS/IN2P3, UPS（图卢兹双无穷大实验室（L2IT），图卢兹大学，法国国家科学研究中心/欧洲核子研究计划国际合作小组，图卢兹第三大学）
We pursue a program to confront observations with inhomogeneous extensions of the FLRW metric. The main idea is to test the Copernican principle rather than assuming it a priori. We consider the $Λ$CDM model endowed with a spherical $Λ$LTB inhomogeneity around us, that is, we assume isotropy and test the hypothesis of homogeneity. We confront the $Λ$LTB model with the latest available data from CMB, BAO, type Ia supernovae, local $H_0$, cosmic chronometers, Compton y-distortion and kinetic Sunyaev-Zeldovich effect. We find that these data can constrain tightly this extra inhomogeneity, almost …
Our Universe may feature large-scale inhomogeneities and anisotropies which cannot be explained by the standard model of cosmology, that is, the homogeneous and isotropic FLRW metric, on which the $Λ$CDM model is built, may not describe accurately observations. Currently, there is not a satisfactory understanding of the evolution of the large-scale structure on an inhomogeneous background. We start the cosmology beyond homogeneity and isotropy (BEHOMO) project and study the inhomogeneous $Λ$LTB model with the methods of numerical cosmology. Understanding the evolution of the large-scale struct…
Discrepancies between distance measurements and $Λ$CDM predictions reveal notable features in the distance-redshift relation, possibly suggesting the presence of an evolving dark energy component. Given the central role of the Friedmann-Lemaître-Robertson-Walker (FLRW) metric in modeling cosmological distances, we investigate here whether these features instead point to a possible departure from the fundamental FLRW symmetries. Exploiting the transverse and line-of-sight distances provided by baryonic acoustic oscillations (BAO) observations, we demonstrate that observed distances hint at a sl…
Cosmic Strings as Dynamical Dark Energy: Novel Constraints
Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（李政道研究所与上海交通大学物理与天文学院）；University of Sheffield（谢菲尔德大学）；Università degli Studi di Salerno（萨莱诺大学）；Istituto Nazionale di Fisica Nucleare - Gruppo Collegato di Salerno - Sezione di Napoli（意大利国家核物理研究所-萨莱诺协作组-那不勒斯分部）
Some remarkable examples of alternative cosmological theories are reviewed here, ranging from a compilation of variations on the Standard Model through the more distant quasi-steady-state cosmology, plasma cosmology, or universe models as a hypersphere, to the most exotic cases including static models. The present-day standard model of cosmology, Lambda-CDM, gives us a representation of a cosmos whose dynamics is dominated by gravity with a finite lifetime, large scale homogeneity, expansion and a hot initial state, together with other dark elements necessary to avoid certain inconsistencies w…
In the late 1990's, observations of 93 Type Ia supernovae were analysed in the framework of the FLRW cosmology assuming these to be `standard(isable) candles'. It was thus inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant $Λ$. This is still the only direct evidence for the `dark energy' that is the dominant component of the standard $Λ$CDM cosmological model. Other data such as BAO, CMB anisotropies, stellar ages, the rate of structure growth, etc are all `concordant' with this model but do not provide independent evidence for accelerated …
The standard cosmological model is based on the fundamental assumptions of a spatially homogeneous and isotropic universe on large scales. An observational detection of a violation of these assumptions at any redshift would immediately indicate the presence of new physics. We quantify the ability of the Euclid mission, together with contemporary surveys, to improve the current sensitivity of null tests of the canonical cosmological constant $Λ$ and the cold dark matter (LCDM) model in the redshift range $0<z<1.8$. We considered both currently available data and simulated Euclid and external da…
