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Λs\Lambda_{\rm s}CDM model: A promising scenario for alleviation of cosmological tensions

Özgür Akarsu akarsuo@itu.edu.tr Department of Physics, Istanbul Technical University, Maslak 34469 Istanbul, Turkey    Eleonora Di Valentino e.divalentino@sheffield.ac.uk School of Mathematics and Statistics, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, United Kingdom    Suresh Kumar suresh.math@igu.ac.in Department of Mathematics, Indira Gandhi University, Meerpur, Haryana 122502, India    Rafael C. Nunes rafadcnunes@gmail.com Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre RS, Brazil Divisão de Astrofísica, Instituto Nacional de Pesquisas Espaciais, Avenida dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil    J. Alberto Vazquez javazquez@icf.unam.mx Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, 62210, México    Anita Yadav anita.math.rs@igu.ac.in Department of Mathematics, Indira Gandhi University, Meerpur, Haryana 122502, India
Abstract

We present a comprehensive analysis of the Λs\Lambda_{\rm s}CDM model, which explores the recent conjecture suggesting a rapid transition of the Universe from anti-de Sitter vacua to de Sitter vacua (viz., the cosmological constant switches sign from negative to positive) at redshift z2{z_{\dagger}\sim 2}, inspired by the graduated dark energy (gDE) model. Our analysis shows that, predicting z1.7z_{\dagger}\approx 1.7, Λs\Lambda_{\rm s}CDM simultaneously addresses the major cosmological tensions of the standard Λ\LambdaCDM model, viz., the Hubble constant H0H_{0}, the Type Ia Supernovae absolute magnitude MBM_{\rm B}, and the growth parameter S8S_{8} tensions, along with other less significant tensions such as the BAO Lyman-α\alpha discrepancy.

Introduction – The standard Lambda Cold Dark Matter (Λ\LambdaCDM) scenario provides a wonderful fit for the majority of astrophysical and cosmological observations carried out over the past decades. However up to recently, in the new era of high-precision cosmology, some discrepancies became statistically significant while analyzing different data sets, placing it in a crossroad. This pivotal situation has compelled the scientific community to embark on a quest for alternative explanations, either rooted in novel physics or by the identification of potential systematic errors in the data. See [1, 2, 3, 4] for recent reviews.

The most statistically significant disagreement is in the value of the Hubble constant, H0H_{0}, between the Planck-Cosmic Microwave Background (CMB) [5] estimate, assuming the standard Λ\LambdaCDM model, and the direct local distance ladder measurements conducted by the SH0ES team [6, 7, 8], reaching a significance of more than 5σ\sigma. Nevertheless the lower value of H0H_{0} derived from the Planck-CMB data is in agreement with Baryon Acoustic Oscillations (BAO)+Big Bang Nucleosynthesis (BBN) [9, 10], and also with other CMB experiments such as ACT-DR4 and SPT-3G [11, 12]. Conversely, the higher value of H0H_{0} found by SH0ES based on the Supernovae calibrated by Cepheids is in agreement with all the local H0H_{0} measurements [13, 14, 15, 16, 17, 18, 19]. Recent reanalysis of the Tip of the Red Giant Branch (TRGB) sample also shows that these local distance indicators are consistent with values of H0H_{0} from Cepheids [20, 21]. Also, it has been argued that the H0H_{0} tension is in fact a tension on the Type Ia Supernovae (SNIa) absolute magnitude MBM_{\rm B} [22, 23, 24], because the SH0ES H0H_{0} measurement comes directly from MBM_{\rm B} estimates.

On the other hand, within the framework of Λ\LambdaCDM, the CMB measurements from Planck and ACT-DR4 [25] indicate values of S8=σ8Ωm/0.3S_{8}=\sigma_{8}\sqrt{\Omega_{\rm m}/0.3} that exhibit a 1.7σ\sigma–3σ\sigma tension with those inferred from various weak lensing [26, 27, 28, 29, 30, 31, 32], galaxy clustering, and redshift-space distortion measurements (see [33, 34, 3, 35, 2, 36]). While the status of the S8S_{8} discrepancy may be somewhat less definitive compared to the H0H_{0} tension, there is a clear trend of lower structure growth from low redshift Large Scale Structure probes compared to CMB. Several other tensions and anomalies have recently emerged in the literature, e.g., BAO-Ly-α\alpha discrepancies, physical baryon density anomalies, age of the Universe issues, among others [2, 3]. Although individually these tensions may not be very significant, when considered collectively, they could point to the existence of missing components in the current standard cosmological model. The search for a satisfactory explanation for these discrepancies, tensions and anomalies, either through systematic effects in data or new physics, has been a central theme in cosmology over the past few years.

In this Letter, we present an observational investigation of the sign-switching Lambda Cold Dark Matter (Λs\Lambda_{\rm s}CDM) model, which takes into account the possibility that the Universe has recently, at redshift z2{z\sim 2}, undergone a phase of rapid transition from anti-de Sitter (AdS) vacua to de Sitter (dS) vacua (viz., the cosmological constant switches sign) [37, 38, 39]. We show that this theory-framework can simultaneously resolve the major cosmological tensions, viz., the H0H_{0}, MBM_{\rm B}, and S8S_{8} tensions, currently present in Λ\LambdaCDM. If these tensions grow in significance with new and future data, and in the absence of clear systematic effects explaining their origins, we should look at Λs\Lambda_{\rm s}CDM as a candidate that can direct us towards a new standard model of cosmology.

Λs\Lambda_{\rm s}CDM model – The Λs\Lambda_{\rm s}CDM model is inspired by the recent conjecture that the universe went through a spontaneous AdS-dS transition characterized by a sign-switching cosmological constant (Λs\Lambda_{\rm s}) at z2{z\sim 2} [37, 38, 39]. This conjecture was proposed following the promising observational findings in the graduated dark energy (gDE) model, which showed that its density smoothly transitioning from negative to positive values rapidly enough at z2{z\sim 2} can simultaneously address the H0H_{0} and BAO-Ly-α\alpha discrepancies [37]. The theoretical advantages of Λs\Lambda_{\rm s} over gDE further bolstered this conjecture [38, 39]. The simplest Λs\Lambda_{\rm s}CDM model was constructed phenomenologically by replacing the cosmological constant (Λ\Lambda) of the standard Λ\LambdaCDM model with an abrupt sign-switching cosmological constant (Λs\Lambda_{\rm s}) at a redshift zz_{\dagger}, which comes as the only additional free parameter. The present-day value of Λs\Lambda_{\rm s} is denoted as Λs0\Lambda_{\rm s0}, and the replacement is defined as:

ΛΛsΛs0sgn[zz],\Lambda\quad\rightarrow\quad\Lambda_{\rm s}\equiv\Lambda_{\rm s0}\,{\rm sgn}[z_{\dagger}-z], (1)

where the sign-switching transition of Λs\Lambda_{\rm s} is implemented by the signum function (sgn), which should be taken as an idealized description of a rapid transition phenomenon, such as a phase transition, from AdS vacua provided by Λs0-\Lambda_{\rm s0} to dS vacua provided by Λs0\Lambda_{\rm s0}, or DE models such as gDE that can mimic this behavior [39].

The Λs\Lambda_{\rm s}CDM model was first analysed using the Planck CMB data, followed by the inclusion of the full BAO data up to z=2.36z=2.36 (viz., Ly-α\alpha DR14, BAO-Galaxy consensus, MGS and 6dFGS) in [38]. It was shown that its consistency with CMB provides H0H_{0} and MBM_{\rm B} values that are inversely correlated with zz_{\dagger}. Specifically, the analysis found that for z1.6z_{\dagger}\sim 1.6, the model predicts values of H073.4kms1Mpc1H_{0}\approx 73.4~{\rm km\,s^{-1}\,Mpc^{-1}} and MB19.25magM_{B}\approx-19.25\,{\rm mag}, in excellent agreement with the SH0ES measurements [6, 40, Camarena:w2023rsd]. Additionally, provided that z2.3z_{\dagger}\lesssim 2.3, it achieves an excellent fit to the Ly-α\alpha data. This was explained because Λs\Lambda_{\rm s}CDM is exactly Λ\LambdaCDM, except having Λs=Λs0<0\Lambda_{\rm s}=-\Lambda_{\rm s0}<0 for z>zz>z_{\dagger} (thereby respects the internal consistency of the SH0ES H0H_{0} estimates utilizing MBM_{\rm B} and leaves the standard pre-recombination universe untouched), and the fact that the comoving angular diameter distance to last scattering DM(z)=c0zH1dzD_{M}(z_{*})=c\int_{0}^{z_{*}}H^{-1}{\rm d}{z} (z1090z_{*}\approx 1090) is strictly fixed by the CMB power spectra, a reduction in H(z)H(z) for z>zz>z_{\dagger} compared to Λ\LambdaCDM must be compensated by an enhancement in H(z)H(z) for z<zz<z_{\dagger} resulting in higher H0H_{0} and fainter MBM_{\rm B}. It turns out that, being consistent with Planck CMB for z1.5z_{\dagger}\gtrsim 1.5, Λs\Lambda_{\rm s}CDM simultaneously ameliorates the H0H_{0}, MBM_{\rm B}, and S8S_{8} tensions, though the inclusion of full BAO data in the analysis compromises the model’s success by moving zz_{\dagger} to 2.4\sim 2.4—in this case the negative cosmological constant, Λs(z>z)=Λs0\Lambda_{\rm s}(z>z_{\dagger})=-\Lambda_{\rm s0}, does not have enough time to significantly influence the evolution of the universe against the dust still dominating the universe.

This analysis was elaborated by using the Pantheon SNIa data [41] (to break the degeneracy between H0H_{0} and zz_{\dagger}, without using BAO), both with and without the SH0ES MBM_{\rm B} prior [23], and/or the completed BAO data [42] along with Planck CMB in [39]. It was shown that when the MBM_{\rm B} prior is utilized without the full BAO data, predicting z1.8z_{\dagger}\sim 1.8, Λs\Lambda_{\rm s}CDM reduces all the major discrepancies (related to H0H_{0}, MBM_{\rm B}, and S8S_{8}) that prevail within Λ\LambdaCDM (and its canonical extensions) below 1σ\sim 1\sigma, and is very strongly favored over Λ\LambdaCDM in terms of Bayesian evidence. It is worth noting that the presence of sign-switching at z2z_{\dagger}\sim 2 was originally motivated by BAO Ly-α\alpha (which favors negative/zero DE densities for z2z\gtrsim 2) in [37] and then this was further supported in [38] by showing that while the BAO Ly-α\alpha insists on z2.3z_{\dagger}\lesssim 2.3, the galaxy BAO from zeff=0.38z_{\rm eff}=0.38 is in opposition to this pushing zz_{\dagger} to values larger than 2. Pleasantly, the results in [39] show that the presence of the MBM_{\rm B} prior finds excellent constraints of z2z_{\dagger}\sim 2 (z1.8z_{\dagger}\sim 1.8 when full BAO data is not included) even when BAO Ly-α\alpha is not used. A close inspection of the fact that using full BAO data hinders the success of Λs\Lambda_{\rm s}CDM in [39] shows that Λs\Lambda_{\rm s}CDM is discrepant with the galaxy BAO data from zeff=0.15z_{\rm eff}=0.15 and 0.380.38. Accordingly, to reconcile Λs\Lambda_{\rm s}CDM with these two galaxy BAO data as well, adding correction to it might be an option (e.g., by introducing wavelet-type corrections to the Hubble radius of Λs\Lambda_{\rm s}CDM at low redshifts, as suggested in [43]), which, however, would come at the cost of introducing additional free parameters. Alternatively, this can be related to the discordance between low- and high-redshift BAO measurements in Λs\Lambda_{\rm s}CDM, which is also present in Λ\LambdaCDM [44, 45].

In this paper, we further investigate the Λs\Lambda_{\rm s}CDM model by using updated and extended data. To achieve this, along with Planck CMB data [46], we utilize the recent Pantheon+ SNe Ia sample [47], and additionally the 2D BAO data [48, 49], as a less model dependent alternative to 3D BAO data used in previous analyses of Λs\Lambda_{\rm s}CDM. Furthermore, for a robust assessment of the resolution of the S8S_{8} tension within Λs\Lambda_{\rm s}CDM, we use cosmic shear measurements obtained from the latest public data release of the Kilo-Degree Survey (KiDS-1000) [29]. This inclusion allows us to robustly determine its consistency with regards to amplitude and growth of structures. By combining these diverse data sets, we aim to provide a comprehensive analysis that further establishes the viability and robustness of Λs\Lambda_{\rm s}CDM in addressing various cosmological tensions and discrepancies.

Data sets and Methodology – We describe below the observational data sets and the statistical methods used to explore the parameter space. CMB: The full Planck 2018 temperature and polarization likelihood [46] in combination with the Planck 2018 lensing likelihood [50]. We refer to this data set as Planck. Transversal BAO: Measurements of 2D BAO, θBAO(z)\theta_{\text{BAO}}(z), obtained in a weakly model-dependent approach, compiled in Table I in [48, 49]. These measurements were obtained using public data releases (DR) of the Sloan Digital Sky Survey (SDSS), namely: DR7, DR10, DR11, DR12, DR12Q (quasars), and following the same methodology in all measurements. The main differences (methodology and sample) between the 3D and 2D BAO measurements are discussed in [51, 52] and references therein. We refer to this data set as BAOtr. Type Ia supernovae and Cepheids: We use the SNe Ia distance moduli measurements from the Pantheon+ sample [47], which consists of 1701 light curves of 1550 distinct SNe Ia ranging in the redshift interval z[0.001,2.26]z\in[0.001,2.26]. We refer to this dataset as PantheonPlus. We also consider the SH0ES Cepheid host distance anchors, which facilitate constraints on both MBM_{\rm B} and H0H_{0}. When utilizing SH0ES Cepheid host distances, the SNe Ia distance residuals are modified following the relationship Eq.(14) of [47]. We refer to this dataset as PantheonPlus&SH0ES. Cosmic Shear: We use KiDS-1000 data [53, 54]. This includes the weak lensing two-point statistics data for both the auto and cross-correlations across five tomographic redshift bins [55]. We employ the public likelihood in 111KiDS-1000 Montepython likelihood. We follow the KiDS team analysis and adopt the COSEBIs (Complete Orthogonal Sets of E/B-Integrals) likelihood in our results [29]. For the prediction of the matter power spectrum, we use the augmented halo model code, HMcode [57]. We highlight that at level of the linear perturbations theory and Boltzmann equations, Λs\Lambda_{\rm s}CDM have exactly the same shape as predicted by Λ\LambdaCDM. The only effect on the matter power spectrum comes from the H(z)H(z) behavior at late times. As HMcode is robustly tested at percent level for variation on H(z)H(z) functions beyond Λ\LambdaCDM, we conclude that no further change on the HMcode is necessary to apply cosmic shear measurements on Λs\Lambda_{\rm s}CDM. We refer to this data set as KiDS-1000.

We explore the full parameter space of the Λs\Lambda_{\rm s}CDM model and, for comparison, that of Λ\LambdaCDM. The baseline seven free parameters of Λs\Lambda_{\rm s}CDM are given by 𝒫={ωb,ωc,θs,As,ns,τreio,z}\mathcal{P}=\left\{\omega_{\rm b},\,\omega_{\rm c},\,\theta_{s},\,A_{\rm s},\,n_{s},\,\tau_{\rm reio},\,z_{\dagger}\right\}, where the first six are the common ones with Λ\LambdaCDM. We use CLASS++MontePython code [58, 59, 60] with Metropolis-Hastings mode to derive constraints on cosmological parameters for Λs\Lambda_{\rm s}CDM baseline from several combinations of the data sets defined above, ensuring a Gelman-Rubin convergence criterion [61] of R1<102{R-1<10^{-2}} in all the runs. For the model comparison, we compute the relative log-Bayesian evidence lnij\ln\mathcal{B}_{ij} to estimate the Evidence of Λs\Lambda_{\rm s}CDM with respect to Λ\LambdaCDM, through the publicly available package MCEvidence 222github.com/yabebalFantaye/MCEvidence [63, 64]. We use the convention of a negative value if Λs\Lambda_{\rm s}CDM is preferred against Λ\LambdaCDM, or vice versa, and we refer to the revised Jeffreys’ scale by Trotta [65, 66], to interpret the results. We will say that the evidence is inconclusive if 0|lnij|<10\leq|\ln\mathcal{B}_{ij}|<1, weak if 1|lnij|<2.51\leq|\ln\mathcal{B}_{ij}|<2.5, moderate if 2.5|lnij|<52.5\leq|\ln\mathcal{B}_{ij}|<5, strong if 5|lnij|<105\leq|\ln\mathcal{B}_{ij}|<10, and very strong if |lnij|10|\ln\mathcal{B}_{ij}|\geq 10.

Table 1: Marginalized constraints, mean values with 68% CL (bestfit value), on the free and some derived parameters of the Λs\Lambda_{\rm s}CDM and standard Λ\LambdaCDM models for different data set combinations. Bayes factors ij\mathcal{B}_{ij} given by lnij=ln𝒵ΛCDMln𝒵ΛsCDM\ln\mathcal{B}_{ij}=\ln\mathcal{Z}_{\Lambda\rm CDM}-\ln\mathcal{Z}_{\Lambda_{\rm s}\rm CDM} are also displayed for the different analyses so that a negative value indicates a preference for the Λs\Lambda_{\rm s}CDM model against the Λ\LambdaCDM scenario.
Data set Planck Planck+BAOtr Planck+BAOtr Planck+BAOtr Planck+BAOtr
+PP +PP&SH0ES +PP&SH0ES+KiDS-1000
Model 𝚲s\bm{\Lambda}_{\textbf{s}}CDM 𝚲s\bm{\Lambda}_{\textbf{s}}CDM 𝚲s\bm{\Lambda}_{\textbf{s}}CDM 𝚲s\bm{\Lambda}_{\textbf{s}}CDM 𝚲s\bm{\Lambda}_{\textbf{s}}CDM
𝚲\bm{\Lambda}CDM 𝚲\bm{\Lambda}CDM 𝚲\bm{\Lambda}CDM 𝚲\bm{\Lambda}CDM 𝚲\bm{\Lambda}CDM
𝒛z_{\dagger} unconstrained 1.700.19+0.09(1.65)1.70^{+0.09}_{-0.19}(1.65) 1.870.21+0.13(1.75)1.87^{+0.13}_{-0.21}(1.75) 1.700.13+0.10(1.67)1.70^{+0.10}_{-0.13}(1.67) 1.720.12+0.09(1.70)1.72^{+0.09}_{-0.12}(1.70)
-- -- -- -- --
𝑴𝑩[𝐦𝐚𝐠]M_{B}{\rm[mag]} -- -- 19.3170.025+0.021(19.311)-19.317^{+0.021}_{-0.025}(-19.311) 19.290±0.017(19.278)-19.290\pm 0.017(-19.278) 19.282±0.017(19.280)-19.282\pm 0.017(-19.280)
-- -- 19.407±0.013(19.411)-19.407\pm 0.013(-19.411) 19.379±0.012(19.373)-19.379\pm 0.012(-19.373) 19.372±0.011(19.369)-19.372\pm 0.011(-19.369)
𝑯𝟎[𝐤𝐦/𝐬/𝐌𝐩𝐜]H_{0}{\rm[km/s/Mpc]} 70.772.70+0.79(71.22)70.77^{+0.79}_{-2.70}(71.22) 73.301.00+1.20(73.59)73.30^{+1.20}_{-1.00}(73.59) 71.720.92+0.73(71.97)71.72^{+0.73}_{-0.92}(71.97) 72.82±0.65(73.20)72.82\pm 0.65(73.20) 73.16±0.64(73.36)73.16\pm 0.64(73.36)
67.39±0.55(67.28)67.39\pm 0.55(67.28) 68.84±0.48(68.61)68.84\pm 0.48(68.61) 68.55±0.44(68.54)68.55\pm 0.44(68.54) 69.57±0.42(69.73)69.57\pm 0.42(69.73) 69.83±0.37(69.96)69.83\pm 0.37(69.96)
𝛀𝐦\Omega_{\rm m} 0.28600.0099+0.0230(0.2796)0.2860^{+0.0230}_{-0.0099}(0.2796) 0.26430.0090+0.0072(0.2618)0.2643^{+0.0072}_{-0.0090}(0.2618) 0.27680.0063+0.0072(0.2759)0.2768^{+0.0072}_{-0.0063}(0.2759) 0.2683±0.0052(0.2646)0.2683\pm 0.0052(0.2646) 0.2646±0.0052(0.2622)0.2646\pm 0.0052(0.2622)
0.3151±0.0075(0.3163)0.3151\pm 0.0075(0.3163) 0.2958±0.0061(0.2984)0.2958\pm 0.0061(0.2984) 0.2995±0.0056(0.2992)0.2995\pm 0.0056(0.2992) 0.2869±0.0051(0.2849)0.2869\pm 0.0051(0.2849) 0.2837±0.0045(0.2816)0.2837\pm 0.0045(0.2816)
𝑺𝟖S_{8} 0.8010.016+0.026(0.791)0.801^{+0.026}_{-0.016}(0.791) 0.777±0.011(0.772)0.777\pm 0.011(0.772) 0.791±0.011(0.794)0.791\pm 0.011(0.794) 0.783±0.010(0.777)0.783\pm 0.010(0.777) 0.774±0.009(0.773)0.774\pm 0.009(0.773)
0.832±0.013(0.835)0.832\pm 0.013(0.835) 0.802±0.011(0.804)0.802\pm 0.011(0.804) 0.808±0.010(0.804)0.808\pm 0.010(0.804) 0.788±0.010(0.784)0.788\pm 0.010(0.784) 0.781±0.008(0.782)0.781\pm 0.008(0.782)
𝝌𝐦𝐢𝐧𝟐\chi^{2}_{\rm min} 2778.062778.06 2793.382793.38 4219.684219.68 4097.324097.32 4185.344185.34
2780.522780.52 2820.302820.30 4235.184235.18 4138.264138.26 4226.504226.50
𝐥𝐧𝓑𝒊𝒋{\rm ln}\mathcal{B}_{ij} 1.28-1.28 12.65-12.65 7.52-7.52 19.47-19.47 19.77-19.77
Refer to caption
Figure 1: 2D contours at 68% and 95% CLs in the H0H_{0}-Ωm\Omega_{\rm m} plane for the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models from the Planck and/or BAOtr data. It deserves mention that, in case of Λs\Lambda_{\rm s}CDM, the Planck and BAOtr contours intersect right on the vertical band of SH0ES measurement.

Results – We present, in Table 1, the 68% CL constraints on the main cosmological parameters of interest of the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models obtained in our analyses by using different combinations of data sets, while we provide the complete table for the entire parameter space of the two models in the Supplemental Material. When we consider only Planck data, we notice that the characteristic parameter of the Λs\Lambda_{\rm s}CDM model, zz_{\dagger}, remains unconstrained, and we find strong degeneracy with other derived parameters, especially with H0H_{0} and Ωm\Omega_{\rm m}, increasing values in H0H_{0} and decreasing the total matter density parameter today. To break the degeneracy, we include the BAOtr data in our analysis with Planck data, which enables the constraint: z=1.700.19+0.09z_{\dagger}=1.70^{+0.09}_{-0.19}. Interestingly, this inclusion of BAOtr data leads to a higher value of the Hubble constant, viz., H0=73.31.0+1.2kms1Mpc1H_{0}=73.3^{+1.2}_{-1.0}{\rm\,km\,s^{-1}\,Mpc^{-1}} which is perfectly consistent with the SH0ES measurement H0=73.04±1.04kms1Mpc1H_{0}=73.04\pm 1.04~{\rm km\,s^{-1}\,Mpc^{-1}} [6]. In particular, it is noteworthy to observe in Fig. 1 that the two models yield almost the identical contours for the BAOtr data (along with BBN prior 102ωbLUNA=2.233±0.03610^{2}\omega_{\rm b}^{\rm LUNA}=2.233\pm 0.036 [67]), while the BAOtr and Planck contours disagree in the case of Λ\LambdaCDM; however, when considering Λs\Lambda_{\rm s}CDM, it is striking that the BAOtr and Planck contours precisely intersect at the vertical band of SH0ES H0H_{0} measurement. Considering this remarkable success of Λs\Lambda_{\rm s}CDM in addressing the H0H_{0} tension, we proceed to incorporate the new PantheonPlus (PP) sample into our analysis, both with and without the Cepheids calibration provided by SH0ES. From the combination of the Planck, BAOtr, and PantheonPlus data sets, we find the constraints: z=1.870.21+0.13z_{\dagger}=1.87^{+0.13}_{-0.21} and H0=71.720.92+0.73kms1Mpc1H_{0}=71.72^{+0.73}_{-0.92}{\rm\,km\,s^{-1}\,Mpc^{-1}}. This constraint on H0H_{0} is again consistent with the SH0ES measurement. Based on this finding, we confidently include the calibration provided by SH0ES, leaving our conclusions unchanged. We also note that the discrepancy in MBM_{\rm B} between the SH0ES data (MB=19.244±0.037magM_{B}=-19.244\pm 0.037\,{\rm mag}) and the base Λ\LambdaCDM cosmology inferred from Planck (MB=19.401±0.027magM_{B}=-19.401\pm 0.027\,{\rm mag}) is here resolved within the framework of Λs\Lambda_{\rm s}CDM model. Thus, the Λs\Lambda_{\rm s}CDM model also provides a robust solution to the MBM_{\rm B} tension.

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Figure 2: 2D contours at 68% and 95% CLs in the Ωm\Omega_{\rm m}-S8S_{8} plane for the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models. S8=0.8010.016+0.026S_{8}=0.801^{+0.026}_{-0.016} (Λs\Lambda_{\rm s}CDM: Planck), S8=0.7460.021+0.026S_{8}=0.746^{+0.026}_{-0.021} (Λs\Lambda_{\rm s}CDM: KiDS-1000), S8=0.832±0.013S_{8}=0.832\pm 0.013 (Λ\LambdaCDM: Planck), S8=0.7490.020+0.027S_{8}=0.749^{+0.027}_{-0.020} (Λ\LambdaCDM: KiDS-1000) at 68% CL.

From the results of these analyses, we further notice that S8S_{8} and Ωm\Omega_{\rm m} get lower values in Λs\Lambda_{\rm s}CDM compared to Λ\LambdaCDM. In order to see whether S8S_{8} tension is resolved in our model, we separately analyse the models with KiDS-1000 only data. The upper panels of Fig. 2 show 2D contours at 68%, and 95% CLs in the Ωm\Omega_{\rm m}-S8S_{8} plane for the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models. We note that the two models yield very similar contours for the KiDS data (as both models being the same at redshifts relevant to KiDS data). However, while the Planck and KiDS contours disagree in Λ\LambdaCDM, they do agree in Λs\Lambda_{\rm s}CDM as the Planck contour extends directly into the KiDS contour. This occurs because the smaller zz_{\dagger} values allowed by Planck data lead to smaller values of S8S_{8} and Ωm\Omega_{\rm m}. For Λs\Lambda_{\rm s}CDM, the observational constraints on S8S_{8}, viz., S8=0.8010.016+0.026S_{8}=0.801^{+0.026}_{-0.016} from Planck and S8=0.7460.021+0.026S_{8}=0.746^{+0.026}_{-0.021} from KiDS-1000, are compatible with each other, and thus the S8S_{8} tension does not exist in Λs\Lambda_{\rm s}CDM while it prevails in Λ\LambdaCDM. The contour plots for Λs\Lambda_{\rm s}CDM in the lower left panel of Fig. 2 (see the lower right panel for Λ\LambdaCDM) further show the robustness of the constraints on S8S_{8} in the presence of other data sets under consideration.

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Figure 3: The mean values with 68% CL on H0H_{0} for the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models from various data combinations. The left vertical band stands for Planck-Λ\LambdaCDM constraint: H0=67.36±0.54kms1Mpc1H_{0}=67.36\pm 0.54{\rm\,km\,s^{-1}\,Mpc^{-1}}, and the right one is for the latest SH0ES measurement [6]: H0=73.04±1.04kms1Mpc1H_{0}=73.04\pm 1.04{\rm\,km\,s^{-1}\,Mpc^{-1}}. We see that there is no H0H_{0} tension in most data combinations for Λs\Lambda_{\rm s}CDM, in particular, when we use the less model-dependent BAOtr data. The only exception is in the cases explored in previous works [38, 39] that employ the 3D BAO data (BAO in the plot), among which the two galaxy BAO data from zeff=0.15z_{\rm eff}=0.15 and 0.380.38 preventing the model from successfully resolving the tension.

Finally, from the analysis of the models using the combination of all the data sets under consideration, viz., Planck+BAOtr+PP&SH0ES+KiDS-1000, we obtain the most robust constraints on the model parameters as shown in the last column of Table 1Fig. 3 shows the whisker plot displaying 68% CL constraints on H0H_{0} for the Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM models from various data combinations. We see that there is no H0H_{0} tension in the present analyses of Λs\Lambda_{\rm s}CDM with all data combinations including the BAOtr data. We emphasize that all data sets under consideration are compatible within the framework of Λs\Lambda_{\rm s}CDM. Note that there is no H0H_{0} tension in our results using the BAOtr data, while using 3D BAO data Λs\Lambda_{\rm s}CDM reduces the H0H_{0} tension but fails to fully resolve it in previous works [38, 39]. We attribute this to the possible model dependence of the 3D BAO reconstruction, which is (mostly) absent for the BAOtr data. In particular, [39] finds that the two galaxy 3D BAO data—viz., SDSS main galaxy sample (MGS) from zeff=0.15z_{\rm eff}=0.15 and BOSS galaxy from zeff=0.38z_{\rm eff}=0.38—are responsible for this reduced concordance under Λs\Lambda_{\rm s}CDM.

In addition, as can be seen in Supplemental Material, Λs\Lambda_{\rm s}CDM poses no problems with any of the well-known parameters of the Universe based on observations and, on theoretical side, standard physics. Instead, the constraints on its six baseline parameters that are common with Λ\LambdaCDM, show unprecedented stability in the face of different data sets. Moreover, Λs\Lambda_{\rm s}CDM is consistent with BAO Ly-α\alpha data. It predicts the age of the universe, t0=13.522±0.027t_{0}=13.522\pm 0.027 Gyr, which is consistent with estimations utilizing the oldest globular clusters, e.g., tu=13.50±0.15(stat.)t_{\rm u}=13.50\pm 0.15({\rm stat.}) Gyr [68]. Furthermore, it maintains the physics and dynamics of the pre-recombination universe as they are in the standard model. For example, the constraints on the drag redshift and the sound horizon at this epoch (rdr_{\rm d} and zdz_{\rm d}) remain unaltered compared to those obtained within Λ\LambdaCDM. Additionally, the constraints on YPY_{\rm P} (the primordial mass fraction of He4{}^{4}\rm He) and ωb\omega_{\rm b} (the present-day physical density parameter of baryons) are consistent with the standard BBN.

To provide a conclusive assessment of the robustness of our results, we conduct a Bayesian model comparison to evaluate the relative performance of Λs\Lambda_{\rm s}CDM and Λ\LambdaCDM in terms of their statistical fit to the data. The results of the relative Bayesian evidence are presented in the lower section of Table 1. According to the revised Jeffreys’ scale, the evidence in favor of the Λs\Lambda_{\rm s}CDM scenario is found weak when considering the Planck data alone. However, it strengthens to a strong level in the analysis with Planck+BAOtr+PP data. Remarkably, for all the other combinations of data-sets considered here, the evidence in favor of the Λs\Lambda_{\rm s}CDM scenario turns out be very strong. Thus, Λs\Lambda_{\rm s}CDM finds by far better statistical fit to the data compared to Λ\LambdaCDM.

The findings in this study indicate that the Λs\Lambda_{\rm s}CDM model consistently outperforms the standard Λ\LambdaCDM model not only in resolving the prominent cosmological tensions, but also in terms of the statistical fit to the data across various data-set combinations, providing very strong support for its validity and effectiveness in explaining the observed cosmological phenomena. In addition, Λs\Lambda_{\rm s}CDM mitigates several other tensions of lower statistical significance, as illustrated in Supplemental Material, where we also provide additional information about the main results discussed here.

Final remarks – Using the state-of-the-art methodology for the observational constraints and recent data available in the literature, we show that a simple model, viz., Λs\Lambda_{\rm s}CDM [37, 38, 39], which experiences a rapid transition of the Universe from anti-de Sitter vacua to de Sitter vacua (namely, the cosmological constant switches sign from negative to positive) at late times (z1.7z_{\dagger}\approx 1.7), can address the major cosmological tensions (H0H_{0}, MBM_{\rm B}, and S8S_{8} tensions) simultaneously; in particular, when we use BAOtr data, which are less model-dependent, unlike the previous works on Λs\Lambda_{\rm s}CDM that used 3D BAO data. Our proposal consists of the most economical cosmological model available in the literature with that ability, because it does not involve any extra physical parameters beyond Λ\LambdaCDM, but only a cosmic time transition which needs to be fixed by data. The abrupt/rapid nature of the Λs\Lambda_{\rm s}, or a dark energy mimicking it, and also the fact that it shifts from negative to positive value, may render finding a concrete physical mechanism underlying this model challenging. However, the phenomenological success of Λs\Lambda_{\rm s}CDM despite its simplicity, is highly encouraging to look for possible underlying physical mechanisms; see [37, 38, 39] for further discussions and [69] realizing abrupt sign-switching cosmological constant via a classical metric signature change across boundaries with a degenerate metric in different formulations of general relativity. On the other hand, our findings may have far-reaching implications in theoretical physics as negative cosmological constant is a theoretical sweet spot–viz., AdS space/vacuum is welcome due to the AdS/CFT correspondence [70] and is preferred by string theory and string theory motivated supergravities [71]. And thereby, it would be natural to associate this phenomenon to a possible (phase) transition from AdS to dS that is derived in such fundamental theories of physics, and the theories that find motivation from them. Thus, it is essential to conduct further research on refinement at both theoretical and observational level, particularly on identifying observables unique to this model and searching for their traces in the sky, for establishing the Λs\Lambda_{\rm s}CDM model as a promising candidate or guide for a new concordance cosmological model of the Universe.

Acknowledgements – This work is dedicated to the memory of Professor John David Barrow. We gratefully acknowledge Marika Asgari for the valuable discussions, inputs in the paper, and help with the KiDS data analysis. We thank Joseph Silk for useful discussions. Ö.A. acknowledges the support by the Turkish Academy of Sciences in scheme of the Outstanding Young Scientist Award (TÜBA-GEBİP). Ö.A. is supported in part by TUBITAK grant 122F124. E.D.V. is supported by a Royal Society Dorothy Hodgkin Research Fellowship. S.K. gratefully acknowledges support from the Science and Engineering Research Board (SERB), Govt. of India (File No. CRG/2021/004658). R.C.N. thanks the CNPq for partial financial support under the project No. 304306/2022-3. J.A.V. acknowledges the support provided by FOSEC SEP-CONACYT Investigación Básica A1-S-21925, Ciencias de Frontera CONACYT-PRONACES/304001/202 and UNAM-DGAPA-PAPIIT IN117723. A.Y. is supported by Junior Research Fellowship (CSIR/UGC Ref. No. 201610145543) from University Grants Commission, Govt. of India. This article is based upon work from COST Action CA21136 Addressing observational tensions in cosmology with systematics and fundamental physics (CosmoVerse) supported by COST (European Cooperation in Science and Technology).

References