What is flat ΛCDM, and may we choose it?

Abstract

The Universe is neither homogeneous nor isotropic, but it is close enough that we can reasonably approximate it as such on suitably large scales. The inflationary-$\Lambda$-Cold Dark Matter ($\Lambda$CDM) concordance cosmology builds on these assumptions to describe the origin and evolution of fluctuations. With standard assumptions about stress-energy sources, this system is specified by just seven phenomenological parameters, whose precise relations to underlying fundamental theories are complicated and may depend on details of those fields. Nevertheless, it is common practice to set the parameter that characterizes the spatial curvature, ${\Omega }_K$, exactly to zero. This parameter-fixed $\Lambda$CDM is awarded distinguished status as separate model, "flat $\Lambda$CDM.'' Ipso facto this places the onus on proponents of "curved $\Lambda$CDM'' to present sufficient evidence that ${\Omega }_K\ne 0$, and is needed as a parameter. While certain inflationary model Lagrangians, with certain values of their parameters, and certain initial conditions, will lead to a present-day universe well-described as containing zero curvature, this does not justify distinguishing that subset of Lagrangians, parameters and initial conditions into a separate model. Absent any theoretical arguments, we cannot use observations that suggest small ${\Omega }_K$ to enforce ${\Omega }_K=0$. Our track record in picking inflationary models and their parameters a priori makes such a choice dubious, and concerns about tensions in cosmological parameters and large-angle cosmic-microwave-background anomalies strengthens arguments against this choice. We argue that ${\Omega }_K$ must not be set to zero, and that $\Lambda$CDM remains a phenomenological model with at least 7 parameters.

References in corpus (12)

• Ricci flow with surgery on three-manifolds
• Planck evidence for a closed Universe and a possible crisis for cosmology
• Eternal inflation and its implications
• Holographic probabilities in eternal inflation
• Dynamical Dark Energy or Simply Cosmic Curvature?
• Extending the WMAP Bound on the Size of the Universe
• A New Solution of The Cosmological Constant Problems
• Another look at redshift drift and the backreaction conjecture
• Stationary Measure in the Multiverse
• Backreaction in Cosmology
• Multipole decomposition of redshift drift -- model independent mapping of the expansion history of the Universe
• Understanding the Dyer-Roeder approximation as a consequence of local cancellations of projected shear and expansion rate fluctuations

Cited by in corpus (17)

• Interpreting DESI 2024 BAO: late-time dynamical dark energy or a local effect?
• Measuring cosmic curvature with non-CMB observations
• A novel model-marginalized cosmological bound on the QCD axion mass
• Determination of cosmic curvature independent of the sound horizon and $H_0$ using BOSS/eBOSS and DESI DR1 BAO observations
• Impact of the damping tail on neutrino mass constraints
• How robust are the parameter constraints extending the $\Lambda$CDM model?
• Anisotropies of cosmological gravitational wave backgrounds in non-flat spacetime
• Non-parametric reconstructions of cosmic curvature: current constraints and forecasts
• Spatial Curvature from Super-Hubble Cosmological Fluctuations
• One Extension to Explain Them All, One Scale-Invariant Spectrum to Test Them All, and in One Model Bind Them
• Barrow holographic dark energy interacting model in the presence of radiation and matter
• Cosmic topology. Part Ic. Limits on lens spaces from circle searches
• Role of Spatial Curvature in a Dark Energy Interacting Model
• Constraining cosmology with N-body simulations for future spectroscopic galaxy surveys at $2\le z\le 3$
• Spatial anisotropies from long wavelength scalar and tensor modes
• Cosmic Dynamics in Einstein-Cartan Theory: Analysing Hubble Tension through Curvature and Torsion field
• Testing a Sign-Switch Cosmological Model with Curvature through Latest Planck 2018, DESI DR2 and PantheonPlus\&SH0ES Observational Data