Particle theory

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:

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 [Formula: see text]. 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'.

Abstract:

We study a set of large-N tensor field theories with a rich structure of fixed points, encompassing both the melonic and prismatic CFTs observed previously in the conformal limits of other tensor theories and in the generalised Sachdev-Ye-Kitaev (SYK) model. The tensor fields interact via an O(N) 3 -invariant generalisation of the quartic Yukawa model, ϕ 2ψψ¯ +ϕ 6 . To understand the structure of IR/UV fixed points, we perform a partial four-loop perturbative analysis in D = 3 − ϵ. We identify the flows between the melonic and prismatic fixed points in the bosonic and fermionic sectors, finding an apparent line of fixed points in both. We reproduce these fixed points non-perturbatively using the Schwinger-Dyson equations, and in addition identify the supersymmetric fixed points in general dimension. Selecting a particular fermionic fixed point, we study its conformal spectrum non-perturbatively, comparing it to the sextic prismatic model. In particular, we establish the dimensional windows in which this theory remains stable. We comment on the structure of large-N melonic CFTs across various dimensions, noting a number of features which we expect to be common to any such theory.