Browsing by Author "Dinda, Bikash"

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    Improved null tests of ΛCDM and FLRW in light of DESI DR2
    (Institute of Physics, 2025) Dinda, Bikash; Maartens, Roy; Clarkson, Chris
    The DESI DR2 BAO data, in combination with CMB and different SNIa datasets, exclude the flat ΛCDM model at more than 2.5σ when analyzed through the w 0 w aCDM parametrization for evolving dark energy. This simple parametrization may not accurately capture the behavior of the entire redshift range at late times, which may introduce bias in the results. We use null tests that probe for deviations from flat ΛCDM at late times, independent of any specific dark energy parametrization. We provide several diagnostics for null tests and discuss their advantages and disadvantages. In particular, we derive diagnostics that improve on previous ones, such as the popular O m diagnostic. The diagnostics are derived from both background and perturbed quantities. Using the combination of DESI DR2 BAO and supernova data, with or without CMB data, we find that deviations from flat ΛCDM are at ∼1σ confidence level in most of the redshift range (more than 1σ for a few small redshift intervals in a few cases). When considering SDSS BAO data instead of DESI BAO data, in combination with PantheonPlus, with or without CMB data, we find even smaller deviations. Since spatial curvature can potentially modify the results, we also test for curvature in the general ΛCDM model and the general FLRW model. While there is slight evidence for nonzero cosmic curvature at lower redshifts in a general ΛCDM model, there is no statistically significant evidence in a general FLRW model.
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    Model-agnostic assessment of dark energy after DESI DR1 BAO
    (Institute of Physics, 2025) Dinda, Bikash; Maartens, Roy
    Baryon acoustic oscillation measurements by the Dark Energy Spectroscopic Instrument (Data Release 1) have revealed exciting results that show evidence for dynamical dark energy at ∼ 3σ when combined with cosmic microwave background and type Ia supernova observations. These measurements are based on the w 0 w a CDM model of dark energy. The evidence is less in other dark energy models such as the wCDM model. In order to avoid imposing a dark energy model, we reconstruct the distance measures and the equation of the state of dark energy independent of any dark energy model and driven only by observational data. Our results show that the model-agnostic (in terms of late-time models) evidence for dynamical dark energy from DESI is not significant. Our analysis also provides model-independent constraints on cosmological parameters such as the Hubble constant and the matter-energy density parameter at present. Although we used CMB distance priors (not full CMB data) from a ΛCDM early-time model, our results remain largely similar for other cosmological models, provided that these models do not differ significantly from the standard model.
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    Physical versus phantom dark energy after DESI: thawing quintessence in a curved background
    (Oxford University Press, 2025) Dinda, Bikash; Maartens, Roy
    Recent data from Dark Energy Spectroscopic Instrument, in combination with other data, provide moderate evidence of dynamical dark energy, w = −1. In the w 0 , w a parametrization of w, there is a preference for a phantom crossing, w < −1, at redshift z ∼ 0 . 5. In general relativity, the phantom equation of state is unphysical. Thus, it is important to check whether phantom crossing is present in other physically self-consistent models of dark energy that have equi v alent e vidence to the w 0 , w a parametrization. We find that thawing quintessence with non-zero cosmic curvature can fit the recent data as well as w 0 , w a in a flat background, based on both parametric and realistic scalar field evolutions. Although the realistic model does not allow w < −1, the parametrizations do allow it. However even if we allow w < −1 the data do not enforce phantom crossing. Thus, the phantom crossing is an artefact of a parametrization that is not based on a physical model.