Cosmological Frontiers in Fundamental Physics 2026

As dark matter direct detection experiments push toward the neutrino fog, the daily modulation of the dark matter signal remains the final frontier needed to evade ubiquitous backgrounds. Directional detectors are required to observe this signal, and perhaps surprisingly, experiments sensitive to sub-GeV dark matter can achieve this capability without sacrificing the overall event rate. I will describe recent theoretical and experimental progress towards the first directional detector for dark matter and the opportunities for Canada to play a leading role in dark matter science.

It has recently been argued that the merger phase of gravitational waves from binary black hole coalescences contains not only quasinormal modes but also a newly identified component, the so-called “direct wave”. As the “last scream” of two merging black holes before they are silenced by the remnant, this component exhibits a characteristic oscillation and decay pattern controlled by two fundamental properties of the final horizon: its rotation frequency and surface gravity. In this talk, I will present a general physical picture of the direct wave: what it is, how it arises, and how it can be extracted from numerical-relativity simulations and from the loudest observed event, GW250114. I will further discuss how this component can serve as a new probe of horizon dynamics.

I will discuss some ideas from causal set theory that have bearing on open questions in cosmology. My primary focus will be dark energy and the early universe. I will also speculate about new ways these ideas may fit together.

The average large-scale velocity of matter in the universe, known as **bulk flow**, is a fundamental test of the Cosmological Principle. Traditionally, this has been measured only out to $R\lesssim 100$ megaparsecs (Mpc). We present an application of **kinetic Sunyaev-Zel'dovich (kSZ) velocity reconstruction** to constrain bulk flow on cosmological scales more than $10\times$ larger, extending out to $R\sim 2000$ Mpc. kSZ velocity reconstruction isolates the Doppler shifting of CMB photons scattered by the electron plasma in galaxies to reconstruct the underlying velocity field. We use galaxy data from two catalogs (unWISE and WISExSCOS) combined with CMB maps (from Planck) to reconstruct velocities in six redshift bins ranging from $0.1\lesssim z \lesssim 1.5$. We place some of the tightest constraints on bulk velocity at $500 \lesssim R\ [{\rm Mpc}] \lesssim 2000$, finding results fully consistent with the standard cosmological model, $\Lambda$CDM. Furthermore, our constraints are relevant for the **cosmic dipole anomaly**, a persistent tension where measurements of galaxy number counts imply a bulk flow $\gtrsim 5\times$ larger than the standard theoretical expectation. Our constraints are in $\sim 2\sigma$ tension with the leading number-count dipole measurement from CatWISE, challenging their interpretation of the dipole anomaly as an excess coherent bulk flow, and reinforcing the standard cosmological model.

I will discuss a new kinetic code for simulating core collapse in self-interacting dark matter halos, KiSS-SIDM. Using this code, we find that in the intermediate stages of the collapse the dark matter is not well-described as an ideal fluid. A more general point is that studying structure formation in hidden sector models is challenging, and will require new tools and techniques.

The Jeans instability is a fundamental mechanism that explains how the initially almost uniform density of the early universe gave rise to collapsed structures—from molecular clouds and stars to galaxies and clusters. It also plays a key role in interpreting astrophysical probes such as the Lyman‑alpha forest seen in quasar spectra. In this talk, I will discuss how the classic Jeans instability is modified in the presence of dark matter (DM). I will show that when DM and baryons interact only through gravity and their relative drift is subsonic, a new resonant gravitational instability emerges—the Shalaby‑Broderick effect. I will outline efforts to identify observable signatures of this effect across a wide range of astrophysical systems, from planets and stars to galaxy clusters. On cosmological scales, probing this instability offers a direct test of the gravitational interaction between dark matter and baryons, and provides a unique window into DM perturbations on scales smaller than the baryonic Jeans scale—scales previously thought to be observationally inaccessible.

In this talk we describe how future cosmological observation can constrain long-range dark matter forces scenarios in which the new interaction acts only in the Dark Sector. We briefly summarize the effect of the new force on the CMB and galaxy surveys, and how these observables constrain long range forces (with interaction lengths up to 100 kpc ) to be weaker than few percent the strength of gravity. We conclude by mentioning an unintuitive result regarding slower structure formation in presence of this extra attractive force, and its consequences for lensing observables.

In this talk, I will present our study of primordial black holes (PBHs) as a candidate for dark matter in the dense cores of dwarf galaxies. Assuming an initially monochromatic PBH population, we investigate their dynamical evolution and hierarchical merger history within a Hubble time. We show that repeated mergers can efficiently build higher-generation BHs. I will then discuss the stochastic gravitational wave background (SGWB) produced through both hierarchical binary BH mergers and close hyperbolic encounters. While binary mergers dominate the overall emission, hyperbolic encounters generate earlier signals and contribute a continuous subdominant background. Finally, I will compare the predicted spectra with the sensitivities of current and future GW observatories, highlighting the potential of these signals to probe PBH dark matter in dwarf galaxies.

The tidal deformability of compact objects, parameterized by their Love numbers, provides a crucial window into their internal structure and fundamental physics. In this talk, I will demonstrate that foundational principles impose strict constraints on these observables. Specifically, by exploiting the analytic properties of the tidal response function via a Kramers-Kronig dispersion relation, I will show that in the regime of weak gravity (small compactness), all static electric Love numbers must be strictly positive. I will outline the derivation of this bound, with a particular focus on the necessary theoretical assumptions regarding stability and high-frequency behavior. Finally, I will illustrate the scope and physical meaning of these assumptions by exploring several concrete examples.

Cosmic relics such as the cosmic neutrino background (CνB) are among the most compelling predictions of modern cosmology, yet they remain undetected because their interactions with ordinary matter are extraordinarily weak — a 10-cm detector would expect far less than one event over the age of the universe. I will describe a new approach in which macroscopic targets, such as spin ensembles familiar from NMR, prepared in easy to achieve quantum states can respond collectively to these tiny signals. In this regime, interactions are superradiantly enhanced, with rates scaling as the square of the number of particles in the target, boosting relic neutrino signals to the Hz level. I will discuss the broader implications for detecting relic neutrinos, axion and dark-photon dark matter, and explain why neutron beams incident on tabletop systems based on NMR techniques provide a natural testing ground. Finally, I will argue that these interactions appear not as conventional energy deposits but as correlated noise, pointing toward new detection strategies rooted in quantum measurement science.