Prof Subir Sarkar

Abstract:

Under the assumption that they are standard(izable) candles, the lightcurves of Type Ia supernovae have been analysed in the framework of the standard Friedmann–Lemaitre–Robertson–Walker cosmology to conclude that the expansion rate of the Universe is accelerating due to dark energy. While the original claims in the late 1990s were made using overlapping samples of less than 100 supernovae in total, catalogues of nearly 2000 supernovae are now available. In light of recent developments such as the cosmic dipole anomaly and the larger-than-expected bulk flow in the local Universe (which does not converge to the Cosmic Rest Frame), we analyse the newer datasets using a Maximum Likelihood Estimator and find that the acceleration of the expansion rate of the Universe is unequivocally anisotropic. The associated debate in the literature highlights the artifices of using supernovae as standardizable candles, while also providing deeper insights into a consistent relativistic view of peculiar motions as departures from the Hubble expansion of the Universe. The effects of our being ‘tilted observers’ embedded in a deep bulk flow may have been mistaken for cosmic acceleration. This article is part of the discussion meeting issue ‘Challenging the standard cosmological model’.

Abstract:

We outline the rationale for holding a Discussion Meeting thus titled at the Royal Society, London during 15 and 16 April 2024, and summarize what we learnt there. This article is part of the discussion meeting issue ‘Challenging the standard cosmological model’.

Abstract:

The standard cosmological model ΛCDM is described by the Friedman–Lemaitre–Robertson–Walker (FLRW) metric, which requires that the universe be isotropic and homogeneous on large scales, an assumption called the Cosmological Principle. If this assumption is accurate, then the dipole anisotropy observed in the cosmic microwave background (CMB) corresponds to our motion with respect to large-scale structure at approximately 370 km s −1 , which can be tested by measuring the corresponding dipole predicted in counts of cosmologically distant sources. This consistency test, first proposed in 1984 by Ellis & Baldwin, became possible in the twenty-first century with the advent of large catalogues of radio sources and quasars. Subsequent Ellis–Baldwin tests have consistently shown an anomalously large dipole, two to three times larger than predicted by the kinematic interpretation of the CMB dipole, which has recently reached a statistical significance of over 5 σ . In these proceedings, I review the Ellis–Baldwin test, the key results that revealed this anomaly, and comment on the status of research on this problem, which threatens a foundational assumption underpinning FLRW-based cosmologies such as ΛCDM. This article is part of the discussion meeting issue ‘Challenging the standard cosmological model’.