NNEWS & VIEWS
Crack in the cosmological paradigm
Eleonora Di Valentino , Institut d’Astrophysique de Paris (UMR7095: CNRS & UPMC- SorbonneUniversities), F-75014, Paris, France; e-mail: [email protected] Sorbonne Universit´es, Institut Lagrange de Paris (ILP), F-75014, Paris,France
A time-dependent dark energy component of the Universe may beable to explain tensions between local and primordial measure-ments of cosmological parameters, shaking current confidence inthe concept of a cosmological constant.
The measurements of Cosmic Microwave Background (CMB) tem-perature and polarization anisotropies obtained by the Planck satellite have provided strong evidence for the Λ cold dark matter ( Λ CDM) cos-mological model of structure formation. The Λ CDM model is basedon many assumptions with only six free parameters, which presents arisk of oversimplifying the physics that drives the evolution of our Uni-verse. The most debatable assumption made in the Λ CDM scenariostates that the mysterious dark energy (DE) component that producesthe current accelerated cosmic expansion can be completely param-eterized by a constant-in-time energy-density term, the cosmologicalconstant Λ . However, tensions are arising between Planck and othercosmological measurements, which justify the study of possible exten-sions to Λ CDM . Writing in Nature Astronomy , Gong-Bo Zhaoand collaborators offer a way to relieve these tensions by introducingan evolving DE.The nature of Λ , which is actually in agreement with the Planckdata, is one of the most significant unsolved problems in fundamentalphysics we have today. As Λ is assumed not to change with time, whileboth matter and radiation the other components of the Universe evolverapidly, it follows that the recent appearance of Λ in the standard cos-mological model implies an extreme fine-tuning of initial conditions.This fine-tuning is known in cosmology as the coincidence problem.Although it is possible that some tensions between the different ex-periments may be due to measurement systematics, it is interesting toexplore whether alternatives to a constant Λ can explain these discrep-ancies. The current most statistically relevant and intriguing disagree-ment is the value of the Hubble constant. In fact, the value reportedby Riess et al. of H = 73 . ± . km/s/Mpc at confidencelevel, derived from local luminosity distance measurements, lies be-yond three standard deviations from the most recent Planck result of H = 66 . ± . km/s/Mpc at confidence level . After sev-eral years of improved analyses and datasets, the tension between theCMB and the direct constraints not only persists but is increasing withtime . Could the current tensions therefore be considered as a first hintfor new physics beyond Λ ?The local estimate of the Hubble constant is based on the combi-nation of different geometric distance calibrations of Cepheids, whichyield three independent constraints on H that are totally consis-tent with each other. Moreover, both the recent determinations of H ,from the H0LiCOW strong lensing survey H = 71 . +2 . − . km/s/Mpcand from the type Ia supernovae as near-infrared standard candles ( H = 72 . ± . stat ) ± . syst ) km/s/Mpc), go towards the valueobtained from the local luminosity measurements . Conversely, the Figure 1 | Time evolution of the dark energy equation of state. The cosmolog-ical constant (illustrated by the straight yellow line) is introduced to explain theaccelerated expansion of the Universe (shown as the expanding pink cone) due tothe presence of dark energy. Zhao et al. instead suggest that the contribution ofdark energy to this expansion is time-dependent (grey curve). The uncertainty ofthis time- dependency is also shown (green shaded area). Image credit: Gong-BoZhao, NAOC. constraints obtained from the CMB data are more precise but model-dependent: by assuming a specific scenario, theory and data are com-pared using a Bayesian approach. This model dependency implies thatthe constraints on a certain parameter can be significantly differentby imposing a different theoretical framework. Moreover, the CMBbounds are affected by the degeneracy between the parameters that caninduce similar effects on the observables.In general, the DE evolution is expressed in terms of its equation ofstate w , defined as the ratio between the DE pressure P DE and energydensity ρ DE , where w ( z ) = P DE /ρ DE , which will be equal to − for Λ , but will be a function of redshift z in dynamical DE models. Inthe work by Zhao and colleagues, the evolution of w is parametrizedby varying its amplitude in different redshift bins from z = 0 up to theCMB last scattering surface at z = 1100 . This model is the naturalextension of the common alternative parameterizations to Λ used in theliterature. These include, usually, a model where w is constant but dif-fers from − , or in which w is a linear function of the scale factor (theChevallierPolarski Linder parametrization). Both these models have al-ready been suggested to solve the H tension , thanks to the geomet-rical degeneracy that w introduces with H , as both parameters modifythe angular diameter distance at recombination. The importance of al-lowing an evolving DE is that it can also overcome the coincidenceproblem with a dynamic solution that triggers a recent DE-dominatedevolution of the Universe. Zhao et al. analyse a consistent compilationof cosmological probes with the Kullback Leibler (KL) divergence,which quantifies the degree of their disagreement with respect to an as-1 a r X i v : . [ phy s i c s . pop - ph ] S e p umed cosmological model. By performing a Bayesian reconstructionof a time-dependent equation of state, w ( z ) , they find that these ten-sions are relieved by an evolving DE. The KL divergence indicates howmuch two probability density functions resemble each other, in a com-parison of the overall concordance of datasets within a given model.The authors show that the w ( z ) CDM model results in an improved χ compared to the Λ CDM model. The reconstructed DE equation of statethat they obtained evolves with time (Fig. 1) crossing the − bound-ary, as in models with multiple scalar fields or in which the DE fieldmediates a new force between matter particles . Moreover, a dynam-ical energy solves another potential conflict on the value of the matterdensity derived by the density fluctuations of baryons (baryon acousticoscillations; BAO), which are traced by the large-scale structure of mat-ter in the Universe. Even if the dynamical DE model seems to providea physical explanation for the H disagreement, a model comparisonbased on Bayesian evidence is needed in order to understand whichof the models is really favoured by the data. The authors found that,whereas the dynamical DE model is preferred at a . σ significancelevel based on the improvement in the fit of the data, the Bayesian ev-idence for the dynamical DE is insufficient to significantly favour itover Λ CDM with the currently available data. But Zhao et al. concludethat such dynamics could be decisively detected by the upcoming BAOmeasurements provided by the Dark Energy Survey Instrument (DESI)at higher redshifts. Clearly, future data from CMB experiments, suchas the proposed Cosmic ORigin Explorer (CORE) satellite or ground-based telescopes such as Stage-4, and galaxy surveys, such as DESI andEuclid, will certainly clarify the issue, probably improving the determi-nation of w and H by an order of magnitude and potentially resolvingthis cosmic conundrum.
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