Study Finds VCDM Cosmological Model Mimics Standard Lambda-CDM Framework
A new preprint analyzes the VCDM model — a modified gravity framework with a time-varying vacuum energy — using Padé cosmographic approximations and finds it behaves nearly identically to the standard Lambda-CDM model at the background level. The study used Bayesian MCMC analysis combining cosmic chronometers, DESI BAO, and multiple Type Ia supernova datasets to constrain the model parameters. The findings suggest that a previously reported distinctive 'transition feature' of VCDM may be an artifact of parametrization choice rather than a genuine observational signal.
Researchers have examined the late-time expansion history of the Universe using the VCDM model, a Type-II Minimal Modified Gravity (MMG) theory that extends beyond a constant vacuum energy without introducing new physical degrees of freedom. Employing a Padé P(2,1) approximation for the Hubble parameter and luminosity distance, the team expressed cosmographic parameters directly in terms of VCDM model parameters, enabling a model-independent reconstruction of cosmic expansion. The analysis was constrained via Bayesian MCMC methods using an affine-invariant ensemble sampler, drawing on joint datasets including cosmic chronometers, DESI BAO measurements, and three Type Ia supernova compilations (Union3, Pantheon+, and DESY5). Results show that model parameters are tightly and consistently constrained across dataset combinations, with the jerk parameter remaining close to its Lambda-CDM value of approximately 1, indicating no significant higher-order deviation. Notably, a transition feature previously attributed to VCDM was not recovered in this cosmographic reconstruction, implying it is sensitive to the choice of parametrization rather than being a robust observational requirement. The authors conclude that VCDM effectively mimics Lambda-CDM at the background level under this approach, highlighting the critical role of model-independent methods in evaluating alternative cosmological scenarios.
What's missing
The study is a preprint submitted to arXiv and has not yet undergone peer review, so its conclusions should be treated as preliminary. The analysis is limited to background-level cosmography and does not address perturbation-level observables (e.g., structure growth, CMB power spectra), which could potentially distinguish VCDM from Lambda-CDM in ways the current approach cannot.
What different sources said
- arXiv physicsCenter
Kinematic Probes of Type-II MMG: Pad\'e Cosmographic Analysis of VCDM
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