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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’.

Abstract:

We present new exclusion bounds obtained at the European X-Ray Free Electron Laser facility (EuXFEL) on axionlike particles in the mass range $10^{-3}\,\mathrm{eV} \lesssim m_a \lesssim 10^{4}\,\mathrm{eV}$. Our experiment exploits the Primakoff effect via which photons can, in the presence of a strong external electric field, decay into axions, which then convert back into photons after passing through an opaque wall. While similar searches have been performed previously at a third-generation synchrotron [Yamaji et al., Phys.\ Lett.\ B 782, 523 (2018)], our work demonstrates improved sensitivity, exploiting the higher brightness of x-rays at EuXFEL.