The source count dipole from wide-area radio continuum surveys allows us to test the cosmological standard model. Many radio sources have multiple components, which can cause an overdispersion of the ...

This chapter introduces gravitational wave cosmology, focusing on the use of gravitational waves as standard sirens to probe the expansion history of the Universe. It presents and explains the methodo...

We propose that the Hubble tension arises due to an unaccounted additional component, that behaves as \emph{matter with pressure}. We demonstrate that this fluid remains subdominant compared to both dust and radiation throughout nearly the entire universe expansion history. Specifically, the additional fluid satisfies the Zel'dovic limit with a constant equation of state, $ω_s > 0$, and a quite small normalized energy density, $Ω_s$. Accordingly, this component modifies both the sound horizon and the background expansion rate, \emph{acting quite differently from early dark energy models}, without significantly affecting the other cosmological parameters. To show this, we perform a Monte Carlo Markov chain analysis of our model, hereafter dubbed $Λ_{ω_s}$CDM paradigm, using the publicly available \texttt{CLASS} Boltzmann code. Our results confirm the presence of this fluid, with properties that closely resemble those of radiation. We find best-fit values that satisfy $ω_s \lesssim ω_γ$ and a relative energy density $Ω_s / Ω_γ= 0.45$, with $ω_r$ and $Ω_r$ the equation of state and density of photons, respectively. The effective fluid may be associated with generalized K-essence models or, alternatively, with Proca-type vector fields, albeit we do not exclude \emph{a priori} more exotic possibilities, i.e., dark radiation, axions, and so on. Physical implications of our results are analyzed in detail, indicating a statistical preference for the $Λ_{ω_s}$CDM scenario over the conventional $Λ$CDM background.

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance mod...

The Cosmological Principle, which states that the Universe is homogeneous and isotropic (when averaged on large scales), is the foundational assumption of Friedmann-Lemaitre-Robertson-Walker (FLRW) co...

Everpresent $Λ$ is a cosmological scenario in which the observed cosmological "constant" $Λ$ fluctuates between positive and negative values with a vanishing mean, and with a magnitude comparable to t...

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In this work, we seek characterizations of global hyperbolicity in smooth Lorentzian manifolds that do not rely on the manifold topology and that are inspired by metric geometry. In particular, strong...

We describe an unexpected anthropic fine-tuning of gravity: human cognition arose on Earth only because the laws of gravity included gravitational waves. Their link is the heat from decays of the radi...

The length of the day varies by milliseconds over the course of weeks, years, and centuries. Conservation of angular momentum explains why.

Einstein's general theory of relativity is based on the principle of equivalence - in essence, dating back to Galileo - which asserts that, locally, the effect of a gravitational field is equivalent t...

In a recent paper [R. Dale. R. Lapiedra, and J. A. Morales-Lladosa, Phys. Rev. D {\bf 107}, 023506 (2023)] a cosmic-like Clauser-Horne-Shimony-Holt (CHSH) inequality was proved for the temperature flu...

Steven Weinberg was a giant of late 20th Century physics on whose shoulders we stand while groping for the science of the 21st Century. This article provides a too-brief summary of a selection of his ...

Emmy Noether showed that fundamental physical laws are just a consequence of simple symmetries. A century later, her insights continue to shape physics.

The study of regular black holes and black hole mimickers as alternatives to standard black holes has recently gained significant attention, driven both by the need to extend general relativity to des...

For a precise determination of cosmological parameters we need to understand the effects of the local large-scale structure of the Universe on the measurements. They include modifications of the cosmi...

The Thomas--Wigner rotation (TWR) results from the fact that a combination of boosts leads to a non-trivial rotation of a physical system. Its origin lies in the structure of the Lorentz group. In thi...

Relative motions have long been known to mislead the unsuspecting observers to false interpretations of reality. The deceptions are usually brief and unimportant, though relative motions have also led...