We show that the dissipation of small-scale perturbations through diffusion damping after neutrino decoupling lowers the present-day neutrino temperature compared to the expected value of 1.96 K. This reduces the relic neutrino abundance by an amount controlled by the integral of the primordial curvature power spectrum Δ_ R^2(k). We find that a relic neutrino detection by PTOLEMY can set limits Δ_ R^2(k) ≲ O(0.1) on scales k ≲ 3 × 10^5 Mpc^-1, complementary to limits from Big Bang Nucleosynthesis, spectral distortions, pulsar timing arrays, and future dark ages 21-cm observations.

Axions and axion-like particles (ALPs) arise naturally in many extensions of the Standard Model and are among the well-motivated candidates for dark matter. In the presence of magnetic fields of galaxy clusters, Cosmic Microwave Background (CMB) photons can convert to ALPs, with the efficiency of the process governed by the cluster electron density and magnetic field profiles, the photon–ALP coupling strength (g_aγ), as well as the frequency (ν) of the photon at the redshift of the cluster. The CMB blackbody spectrum suggests that this resonant conversion also takes place at radio wavelengths, following the spectral behaviour of the ALP distortion signal. This opens a new window to search for ALPs using cosmic variance cancellation (CVC), with multi-frequency tracers of the same phenomenon in CMB photon–ALP resonant conversion. The constraints on the ALP signal ratios from different combinations of microwave and radio bands of the Simons Observatory (SO) and the Square Kilometre Array (SKA) can be significantly improved using CVC compared to the case of using auto-only spectra from the two experiments. With the large number of galaxy clusters that will be observed by SO and SKA, we will be able to obtain much more information using CVC, especially for low-mass ALPs with stronger signals. Using the auto-only spectra from galaxy clusters up to redshift z = 1 for inference of the normalized ratio parameter, we obtain a standard deviation of 5.9 × 10^-2 for an ALP mass m_a = 10^-14 eV, which improves to 1 × 10^-2 using CVC. This method provides a universal probe of the ALP distortion signal using its spectral dependence and can also invalidate false detections of the ALP signal based on its frequency behaviour in different bands.

This document summarizes talks and discussions from the workshop "deci-Hz Gravitational Wave Observations on the Moon and Beyond" that took place at Johns Hopkins University between September 1 and September 3, 2025. The workshop focused on experimental proposals to observe gravitational waves in the deci-Hz band, including lunar detectors, laser interferometers in space, and atom interferometry; gravitational wave sources in the deci-Hz frequency band; and the multi-messenger and multi-band astronomy that would be enabled by these observations.

The James Webb Space Telescope has uncovered a population of compact, high-redshift sources, the Little Red Dots (LRDs), which may host supermassive black holes (BHs) significantly heavier than their stellar content compared with local scaling relations. These objects challenge standard models of early galaxy formation and may represent an extreme class of early BH hosts. In this Letter, we investigate whether these BHs could have a primordial origin. We first show that the direct formation of these BH masses in the early Universe is excluded by stringent cosmic microwave background μ-distortion limits. We then investigate the assembly of massive BHs from lighter, observationally allowed primordial black holes (PBHs) via hierarchical mergers, finding that, although this channel can operate depending on the merger history, it faces challenges in explaining the observations due to the rarity of the required high-redshift dark matter halos. Finally, we estimate gas accretion onto intermediate-mass PBHs, while jointly tracking metallicity evolution, and identify regions of parameter space in which such growth could reproduce the observed properties of LRDs. As a special case, we focus on the strongly lensed source QSO1, whose extremely low metallicity and large mass provide a stringent test of these formation channels.

Supermassive black hole (SMBH) growth plausibly occurs via runaway astrophysical black hole mergers in nuclear star clusters that form intermediate mass black hole seeds at high redshifts. Such a model yields an order-of-magnitude higher rate of tidal disruption events than that of compact-object captures. Our prediction, normalized to our proposed resolution of SMBH seeding, yields detectable tidal disruption event rates at high redshift. The resulting dense gas cocoons generate compact galactic nuclei, each incorporating a central, massive, black hole star, with comparable masses in gas, stars, and massive black holes within a scale of around a parsec as inferred from the various Little Red Dot spectral signatures.

Little Red Dots, discovered by the James Webb Space Telescope, are hypothesized to be active galactic nuclei containing a supermassive black hole, possibly surrounded by a dense stellar cluster, large amounts of gas, and likely by a population of stellar-mass black holes. We develop a simple nuclear star cluster model to evolve the rapid mass growth of black hole seeds into the supermassive regime. The combined processes of tidal disruption events, black hole captures, and gas accretion are accounted for self-consistently in our model. Given the observed number density of Little Red Dots, and under reasonable assumptions, we predict at least a few tens of tidal disruption events and at least a few black hole captures at z=4-6, with a tidal disruption event rate an order of magnitude larger than the black hole capture rate. We also estimate the uncertainties in these estimates. Finally, we comment on the low x-ray luminosity of Little Red Dots.

With recent data from the James Webb Space Telescope (JWST), it is possible to calculate the mass of the supermassive black holes at the centre of galaxies, and the stellar mass of the host galaxies at z ≳ 5. In this work, we apply the method of extreme-value statistics to calculate the distributions of extreme black hole and stellar mass for the redshift range 3.5 ≲ z ≲ 8.5. We sample these distributions to obtain a prediction for the black hole to stellar mass ratio of ∼0.24 over this redshift range, with the median in each bin varying in the range 0.18-0.35. Our predictions are consistent with the highest observed values of the ratio from JWST observations of high-redshift galaxies.

JWST observations have revealed an overabundance of bright galaxies at z ≥ 9, creating apparent tensions with theoretical predictions within standard ΛCDM cosmology. We address this challenge using a semiempirical approach that connects dark matter halos to the observed UV luminosity through a physically motivated double power-law star formation efficiency (SFE) model as a function of the halo mass and redshift, and perform a joint Bayesian analysis of luminosity functions spanning z = 4–16 using combined Hubble Space Telescope and JWST data. Through systematic model comparison using information criteria (Akaike, Bayesian, and deviance information criteria), we identify the optimal framework requiring redshift evolution only in the low-mass slope parameter α ( z ) while maintaining other SFE parameters constant. Our best-fitting model achieves excellent agreement with observations using modest, constant UV scatter σ _UV = 0.32 dex–significantly lower than the ≳1.3 dex values suggested by previous studies for z > 13. This reduced scatter requirement is compensated by strongly evolving star formation efficiency, with α increasing toward higher redshifts, indicating enhanced star formation in low-mass halos during the cosmic dawn. The model also successfully reproduces other important observational diagnostics, such as effective galaxy bias and cosmic star formation density, consistently across the full redshift range. Furthermore, model predictions are consistent up to a redshift of z ∼ 20. Our results demonstrate that JWST’s early galaxy observations can be reconciled with standard cosmology through the interplay of modest stochasticity and evolving star formation physics, without invoking extreme burstiness or exotic mechanisms.

We demonstrate that the anomalous ionization rate observed in the Central Molecular Zone can be attributed to MeV dark matter annihilations into e^{+}e^{-} pairs for galactic dark matter profiles with slopes γ>1. The low annihilation cross sections required avoid cosmological constraints and imply no detectable inverse Compton, bremsstrahlung, or synchrotron emission in radio, x- and γ rays. The possible connection with the source of the unexplained 511 keV line emission in the Galactic Center suggests that both observations could be correlated and have a common origin.

The Moon is our future. It may seem like a chimera with a projected cost in excess of 100 billion, and counting, dispensed on ARTEMIS with little to show to date. However it is the ideal site for the largest telescopes that we can dream about, at wavelengths spanning decimetric radio through optical to terahertz FIR. And it is these future telescopes that will penetrate the fundamental mysteries of the first hydrogen clouds, the first stars, the first galaxies, the first supermassive black holes, and the nearest habitable exoplanets. Nor does it stop there. Our lunar telescopes will take us back to the first months of the Universe, and even back to the first 10^-36second after the Big Bang when inflation most likely occurred. Our lunar telescopes will provide high resolution images of exoplanets that are nearby Earth-like ’twins’ and provide an unrivalled attempt to answer the ultimate cosmic question of whether we are alone in the universe. Here I will set out my vision of the case for lunar astronomy over the next several decades.

Supermassive black holes are prevalent at the centers of massive galaxies, and their masses scale with galaxy properties, increasing evidence suggesting that these trends continue to low stellar masses. Seeds are needed for supermassive black holes, especially at the highest redshifts explored by the James Webb Space Telescope. We study the hierarchical merging of galaxies via cosmological merger trees and argue that the seeds of supermassive black holes formed in nuclear star clusters via stellar black hole mergers at early epochs. Observable tracers include intermediate-mass black holes, nuclear star clusters, and early gas accretion in host dwarf galaxies, along with a potentially detectable stochastic gravitational-wave background, ejection of intermediate and supermassive black holes, and consequences of a significant population of early tidal disruption events and extreme mass ratio inspirals.

We propose that a dark matter (DM) spike around the Galactic Center's (GC) supermassive black hole, Sgr A*, could account for most of the bulge's measured 511 keV line intensity while remaining cosmologically compatible. DM annihilation can be the primary source of the 511 keV line emission without violating constraints from disk emission observations and in-flight positron annihilation with the interstellar medium, provided the disk emission is dominated by an astrophysical source of low-energy positrons. We find that a DM mass up to approximately 20 MeV, either with a Gondolo-Silk spike or one softened by stellar heating, could explain the observed 511 keV bulge emission profile. Our proposal can be tested by future observations of the continuum diffuse emission close to the GC.

The resolution of photon rings of Sgr A^* and M87 is the next milestone of upcoming EHT-like interferometries. We extend the formalism developed in our previous work to constrain primordial black hole (PBH) dark matter using microlensing-induced distortions of black hole shadows. Building upon the theoretical framework for microlensing of photon rings, we apply this methodology to both Sgr A* and M87, considering multiple PBH populations: (i) PBH dark matter spikes around central supermassive black holes, (ii) NFW halo contributions in the Milky Way and M87 galaxies, and (iii) foreground Milky Way PBH dark matter affecting M87* observations. The microlensing signal manifests as a time-dependent asymmetry and deformation of the photon ring, providing the most sensitive observable for lensing effects. We assess the detectability of these signatures with future EHT-like interferometers. Our analysis reveals that M87* provides the strongest constraints on PBH dark matter. We show that the absence of photon-ring asymmetries in observations with angular resolution of order 0.1 μ as can constrain PBHs in the mass range 10^-5 M_⊙≲ M_ PBH≲ 10^6 M_⊙, with maximal sensitivity near M_ PBH∼10^3 M_⊙, for PBH dark matter fractions as small as f_ PBH∼10^-2.

We investigate neutrino signatures of a hadron-quark phase transition (HQPT) in neutron stars (NS) leading to quark star (QS) formation. We use representative hadronic and quark equations of state i.e. DD2 and MIT bag model along with a phenomenological neutrino emission model including the dominant leptonic and hadronic processes. Rather than aiming at a fully consistent hydrodynamical simulation, our goal is to identify generic temporal diagnostics that may arise when deconfinement occurs during the evolution of a compact star. We identify characteristic diagnostic features that may emerge in the neutrino light curve on ≃ 10-50 ms timescales. These include an enhanced peak-to-plateau ratio, a delay tracing the central density evolution, and a transient spectral hardening. After standard MSW flavor conversion, these temporal and spectral signatures remain potentially detectable for Galactic events under optimistic assumptions with detectors such as IceCube and Hyper-Kamiokande. Our results suggest possible temporal and spectral diagnostics of quark deconfinement in future Galactic neutrino bursts.

Bursts from the very early Universe may lead to a detectable signal via the production of positrons, whose annihilation gives an observable x-ray signal. Using the absorption parameters for the annihilation photons of 511 keV, it is found that observable photons would originate at a redshift around z approximate to 200-300, resulting in soft x-rays of energy similar to 2-3 keV at present. Positrons are expected to be absent at these times or redshifts in the standard picture of the early universe. Detection of the x-rays would thus provide dramatic support for the hypothesis of the bursts, explosive events at very early times. We urge the search for such a signal.

A supermassive black hole (SMBH) at the core of an active galactic nucleus (AGN) provides room for the elusive ultra-light scalar particles to be produced through a phenomenon called superradiance. This phenomenon produces a cloud of scalar particles around the black hole by draining its spin angular momentum. In this work, we present a study of the superradiant instability due to a scalar field in the vicinity of the central SMBH in an AGN. We begin by showing that the time evolution of the gravitational coupling alpha in a realistic ambiance created by the accretion disc around the SMBH in AGN leads to interesting consequences such as the amplified growth of the scalar cloud, enhancement of the gravitational wave emission rate, and appearance of higher modes of superradiance within the age of the Universe. We then explore the consequence of superradiance on the characteristics of the AGN. Using the Novikov-Thorne model for an accretion disc, we divide the full spectrum into three wavelength bands - X-ray (10(-4)-10(-2) mu m), UV (0.010-0.4 mu m), and Vis-IR (0.4-100 mu m) and observe sudden drops in the time variations of the luminosities across these bands and Eddington ratio (fEdd) with a characteristic time-scale of superradiance. Using a uniform distribution of spin and mass of the SMBHs in AGNs, we demonstrate the appearance of depleted regions and accumulations along the boundaries of these regions in the planes of different band luminosities and fEdd. Finally, we discuss some possible signatures of superradiance that can be drawn from the observed time variation of the AGN luminosities.

We show that collisions between particles free falling from infinity and a disk of material plunging off the retrograde innermost stable circular orbit of a near-extremal Kerr black hole is the unique astronomically natural way in which to create a gravitational particle accelerator with center of mass energies at the tens to hundreds of teraelectronvolt range; in other words, a supercollider.

The 21-cm brightness temperature fluctuation from the Dark Ages ($z \simeq 30-100$) will allow us to probe the inflationary epoch on very small scales ($>0.1 \, \mbox{Mpc}^{-1}$), inaccessible to cosmic microwave background experiments. Combined with the possibility to collect information from different redshift slices, the 21-cm bispectrum has the potential to significantly improve constraints on primordial non-Gaussianity. However, recent work has shown secondary effects source off-diagonal terms in the covariance matrix which can significantly affect forecasted constraints, especially in signals that peak in the squeezed configuration, such as the local bispectrum. In this paper we propose the three-point $\langle 21-21-\rm CMB \rangle$ bispectrum cross-correlation as a new independent observational channel sensitive to local primordial non-Gaussianity. We find that, contrary to the 21-cm bispectrum, secondary contributions are subdominant to the primordial signal for values $f_{\rm NL}^{\rm loc} \sim 1$, resulting in negligible effects from off-diagonal terms in the covariance matrix. We forecast that CMB $T$ and $E$ modes cross-correlated with an ideal cosmic variance-limited 21-cm experiment with a $0.1$ MHz frequency and $0.1$ arc-minute angular resolution could reach $f_{\rm NL}^{\rm loc} \sim 6 \times 10^{-3}$. This forecast suggests cross-correlation between CMB and 21-cm experiments could provide a viable alternative to 21-cm auto-spectra in reaching unprecedented constraints on primordial local non-Gaussianities.

The strongest experimental evidence for dark matter is the Galactic Center gamma-ray excess observed by the Fermi telescope and even predicted prior to discovery as a potential dark matter signature via weakly interacting massive particle dark matter self-annihilations. However, an equally compelling explanation of the excess gamma-ray flux refers to a population of old millisecond pulsars that also accounts for the observed boxy morphology inferred from the bulge old star population. We employ a set of Milky Way-like galaxies found in the hestia constrained simulations of the local universe to explore the rich morphology of the central dark matter distribution, motivated by the GAIA discovery of a vigorous early merging history of the Milky Way galaxy. We predict a significantly nonspherical gamma-ray morphology from the weakly interacting massive particle interpretation. Future experiments, such as the Cherenkov Telescope Array, that extend to higher energies, should distinguish between the competing interpretations.

We consider possible observable signals from explosive events in the very early Universe, dubbed “bursts.” These could be expected in connection with massive black hole or “baby Universe” formation. We anticipate that such major disruptions of spacetime would be associated with neutrino and perhaps other pulses. While these seem to be not detectable directly, we discuss how they could lead to potentially observable signals. We analyze how the pulses from very early times may “escape,” that is, propagate to the last scattering epoch at the time t _cmb and later, or alternatively be absorbed earlier, i.e., “contained.” The possibly detectable signals include effects on small regions of the cosmic microwave background, a soft X-ray resulting from positron production, or a nonthermal addition to the relic neutrino background.