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SUMMARY:Beyond j=1: Observational Constraints on Almost-ΛCDM  Cosmologies
DTSTART;VALUE=DATE-TIME:20260917T125000Z
DTEND;VALUE=DATE-TIME:20260917T131500Z
DTSTAMP;VALUE=DATE-TIME:20261023T214010Z
UID:indico-contribution-31311@indico.ific.uv.es
DESCRIPTION:Speakers: Jess Worsley (1Department of Mathematics and Applied
  Mathematics\, University of Cape Town\, 7  University Ave N\, 7700\, Sout
 h Africa.)\nThe cosmographic condition $j(z)=1$ provides the kinematical s
 ignature of the spatially flat $\\Lambda$CDM model independently of any sp
 ecific dark-energy or modified-gravity theory. We investigate the extent t
 o which current observations permit departures from this condition by cons
 idering three phenomenological ``almost-$\\Lambda$CDM'' cosmographic closu
 res in which the cosmic jerk differs slightly from unity through a small d
 eformation parameter $\\epsilon$. The models are constrained using Markov 
 Chain Monte Carlo analyses of recent DESI baryon acoustic oscillation meas
 urements together with compressed Planck cosmic microwave background likel
 ihoods and the Union3\, Pantheon+\, and DESY5 Type Ia supernova compilatio
 ns.\n\nUnlike conventional analyses that infer the expansion history after
  imposing an explicit parameterization of the dark-energy equation of stat
 e\, our cosmographic framework reconstructs the expansion history directly
  from observations\, with the effective dark-energy equation of state emer
 ging as a derived quantity rather than an assumed one. We find that all th
 ree cosmographic closures are confined by current observations to a remark
 ably small neighbourhood of the $\\Lambda$CDM cosmographic fixed point\, w
 ith the inclusion of Planck data driving the preferred evolution toward $j
 _0\\simeq1$ and $w_{\\rm DE\,0}\\simeq-1$. Despite their distinct kinemati
 cal constructions\, the reconstructed dark-energy evolution consistently e
 xhibits a smooth freezing behaviour that remains close to $w=-1$ without c
 rossing the phantom divide.\n\nModel comparison using the Akaike and Bayes
 ian information criteria shows that the almost-$\\Lambda$CDM cosmographic 
 models remain statistically competitive with standard dark-energy paramete
 rizations while relying on fewer assumptions regarding the functional form
  of $w(z)$. Our analysis demonstrates that current observations constrain 
 the cosmic expansion history more robustly than the detailed evolution of 
 the dark-energy equation of state\, which remains sensitive to the reconst
 ruction methodology employed. These results highlight the importance of mo
 del-independent cosmographic approaches for interpreting current evidence 
 for dynamical dark energy and provide a natural framework for future studi
 es of cosmological perturbations and structure formation.\n\nhttps://indic
 o.ific.uv.es/event/8160/contributions/31311/
LOCATION:
URL:https://indico.ific.uv.es/event/8160/contributions/31311/
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