We propose a new avenue for understanding the cosmological singularity. The standard cosmological model contains a generic initial singularity usually referred to as the big bang. Herein, we present a novel idea to extend the description of our Universe beyond this limit. The proposal relies on rewriting physics in a purely Riemannian, i.e. locally Euclidean, 4-dimensional space and the emergence of Lorentzian patches owing to the interaction of all matter fields to a clock field that is responsible for a signature change. If our universe is contained within one of these patches, the initial singularity is replaced by a smooth boundary on which the signature of the physical metric flips. In this talk, we first define the model and draw the necessary conditions on its arbitrary functions for solutions to exist. Next, we prove the existence of solutions that lead to an emergent universe with a primordial (almost) de Sitter phase. To finish, we discuss the consequences of this construction for the universe on scales much larger than our observable universe: a large “Euclidean sea” in which Lorentzian islands locally emerge and host an expanding universe potentially similar to ours. While speculative, this scenario has specific features that can be tested, and this talk sets the basis for further phenomenological investigations.

Superconducting sensors are a cornerstone of modern frontier physics, driving discoveries from the early universe to the quantum world. In this talk, I will discuss the diverse applications and synergistic development of superconducting detector technologies, highlighting how advances in one discipline frequently drive progress in others. The extensive R&D and industry partnerships established for CMB-S4 detector fabrication have opened new pathways for fabricating superconducting sensors, which are now enabling highly sensitive detector arrays for line intensity mapping, dark matter detection and quantum computing. This talk will illustrate the feedback loop between astrophysics, particle physics, and quantum science, showing how detectors built to observe the sky simultaneously advance quantum technology.

An interesting proposal suggests that a QCD axion (a)coupling to the up quark, down quark, and electron remains viable for an axion mass near 10~MeV. In this paper, this possibility is reexamined by deriving new bounds from kaon decays. In particular, we perform a detailed analysis of the KL -> pi0 pi0 e+ e- measurement reported by the KTeV experiment, and reinterpret K+ decay measurements at the E949 and NA62 experiments to constrain both the diphoton decay and effectively invisible decay modes of the axion. We find that, combined with the previously known bounds, the viable window for the MeV-scale QCD axion is excluded, primarily due to the KTeV bound. Uncertainties associated with the chiral Lagrangian are further examined, and the scenario remains excluded even after accounting for these uncertainties, except for a tiny region of parameter space where higher-order corrections must finely cancel the leading-order contribution, suppressing the branching ratio of KL->pi0 pi0 a by three orders of magnitude.

https://www-conf.kek.jp/kincha/

3-form dark energy shows a similar behavior to but distinguishable one from that of a scalar field in an expanding universe, and its tensorial structure allows for unique couplings that cannot be experienced by canonical scalars. We here explore the possibility that 3-form dark energy can explain the cosmic birefringence angle β~0.3°, as suggested by observations of the CMB polarization. We consider two EFT-inspired couplings between the 3-form and the photon, finding a dimension-4 operator is a candidate explanation, moreover predicting a photon mass around the Hubble scale. The profiles for β(z) from the model can mimic those from ALP dark energy or ALP dark matter, or be distinguishable, depending on the field configuration in the early universe, which may be tested by precise measurements of CMB polarization or radio galaxies.

We first show how spatially all to all interaction can be induced out of the local action quantum mechanically in the presence of the disorder.Then we show that if we consider disorder in Lorentzian signature, there is a tunneling effect given by a time-non-local interaction, leading to the wormhole picture in disorder field theory. We suggest that the two point function of the random coupling play the role of the wormhole. In large N limit, for a given diagram in quenched picture with field theory wormhole, there is a corresponding unique Feynman diagram in the effective theory of the annealed avearage whose basic time-non local vertex creates entangled pair of particle sets. We call this as disorder field theory version of ER=EPR.

https://www-conf.kek.jp/kincha/

Inflationary observables are directly related to the first-passage-time statistics of the stochastic number of e-folds. In some cases, however, these statistics retain an undesirable dependence on the regularising boundary introduced to set the cutoff scale of the effective stochastic description. Although boundary-independent results can be obtained order by order in a small-noise regime, the associated perturbative series are divergent. It is shown that the well-behaved counterparts can nevertheless be extracted from these series through Borel resummation, which also indicates a motivated prescription for introducing the regularising boundary. For a quadratic model, the resummation is directly performed at the level of the full first-passage-time distribution, thereby encompassing all first-passage-time statistics. The asymptotic tail of the distribution is then determined from the analytic structure of its characteristic function.

Understanding how to gauge-fix open quantum field theories is essential for building consistent open frameworks for cosmology and gravity, where gauge symmetry must coexist with dissipation and noise and decoherence. I will present our recent work developing explicit top-down constructions of open effective field theories (EFTs) for gauge degrees of freedom, with particular emphasis on the role of gauge fixing. We implement BRST quantisation on the Schwinger-Keldysh contour and show that the in-in boundary conditions reduce the doubled global BRST symmetry to a single diagonal copy. This diagonal BRST symmetry is nevertheless sufficient to guarantee that the influence functional remains gauge invariant under two independent gauge transformations, retarded and advanced, independently of the choice of initial state, the presence of symmetry breaking phases, and whether the gauge theory is Abelian or non-Abelian. I will also remark that once a consistent quantisation scheme is chosen, the Schwinger-Keldysh path integral is uniquely determined, and with it the rules that constrain the construction of open EFTs. I will conclude by outlining bottom-up implications, and how these principles provide a systematic route to causal, gauge-invariant open EFTs suitable for cosmological and gravitational applications.
Refs:
Gauging Open EFTs from the top down https://arxiv.org/abs/2512.17089
Schwinger-Keldysh path integral for gauge theories
https://arxiv.org/abs/2604.26941

A stochastic gravitational-wave background can inevitably generate second-order scalar perturbations, which may collapse into primordial black holes (PBHs) above a calculable GW amplitude threshold. In this talk, I will present this new GW-induced PBH formation mechanism and apply it to primordial GWs, first-order phase transitions, and domain-wall annihilation. I will discuss the implications for the NANOGrav/EPTA nanohertz signal, including the predicted PBH mass range and dark-matter abundance, and highlight the complementarity between current and future GW observatories and PBH searches.