The 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) shows positive evidence for the presence of a low-frequency gravitational-wave (GW) background. In this paper, we investigate potential cosmological interpretations of this signal, specifically cosmic inflation, scalar-induced GWs, first-order phase transitions, cosmic strings, and domain walls. We find that, with the exception of stable cosmic strings of field theory origin, all these models can reproduce the observed signal. When compared to the standard interpretation in terms of inspiraling supermassive black hole binaries (SMBHBs), many cosmological models seem to provide a better fit resulting in Bayes factors in the range from 10 to 100. However, these results strongly depend on modeling assumptions about the cosmic SMBHB population and, at this stage, should not be regarded as evidence for new physics. Furthermore, we identify excluded parameter regions where the predicted GW signal from cosmological sources significantly exceeds the NANOGrav signal. These parameter constraints are independent of the origin of the NANOGrav signal and illustrate how pulsar timing data provide a new way to constrain the parameter space of these models. Finally, we search for deterministic signals produced by models of ultralight dark matter (ULDM) and dark matter substructures in the Milky Way. We find no evidence for either of these signals and thus report updated constraints on these models. In the case of ULDM, these constraints outperform torsion balance and atomic clock constraints for ULDM coupled to electrons, muons, or gluons.
We show that cosmic acceleration can arise due to very tiny corrections to the usual gravitational action of general relativity, of the form ${R}^{\ensuremath{-}n}$ with $ng0.$ This model eliminates the need for a nonzero cosmological constant or any other form of dark energy, attributing a purely gravitational origin to the acceleration of the universe.
LiteBIRD Collaboration E. Allys, K. Arnold, J. Aumont
et al.
LiteBIRD the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. The Japan Aerospace Exploration Agency (JAXA) selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with an expected launch in the late 2020s using JAXA’s H3 rocket. LiteBIRD is planned to orbit the Sun-Earth Lagrangian point L2, where it will map the cosmic microwave background (CMB) polarization over the entire sky for three years, with three telescopes in 15 frequency bands between 34 and 448 GHz, to achieve an unprecedented total sensitivity of 2.2 μK-arcmin, with a typical angular resolution of 0.5○ at 100 GHz. The primary scientific objective of LiteBIRD is to search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. We provide an overview of the LiteBIRD project, including scientific objectives, mission and system requirements, operation concept, spacecraft and payload module design, expected scientific outcomes, potential design extensions and synergies with other projects. Subject Index LiteBIRD cosmic inflation, cosmic microwave background, B-mode polarization, primordial gravitational waves, quantum gravity, space telescope
This Horizon Study describes a next-generation ground-based gravitational-wave observatory: Cosmic Explorer. With ten times the sensitivity of Advanced LIGO, Cosmic Explorer will push gravitational-wave astronomy towards the edge of the observable universe ($z \sim 100$). The goals of this Horizon Study are to describe and evaluate design concepts for Cosmic Explorer; to plan for the United States' leadership in gravitational-wave astronomy; and to envisage the role of Cosmic Explorer in the international effort to build a"Third-Generation"(3G) observatory network that will make discoveries transformative across astronomy, physics, and cosmology.
The Hubble tension has now grown to a level of significance which can no longer be ignored and calls for a solution which, despite a huge number of attempts, has so far eluded us. Significant efforts in the literature have focused on early-time modifications of ΛCDM, introducing new physics operating prior to recombination and reducing the sound horizon. In this opinion paper I argue that early-time new physics alone will always fall short of fully solving the Hubble tension. I base my arguments on seven independent hints, related to (1) the ages of the oldest astrophysical objects, (2) considerations on the sound horizon-Hubble constant degeneracy directions in cosmological data, (3) the important role of cosmic chronometers, (4) a number of “descending trends” observed in a wide variety of low-redshift datasets, (5) the early integrated Sachs-Wolfe effect as an early-time consistency test of ΛCDM, (6) early-Universe physics insensitive and uncalibrated cosmic standard constraints on the matter density, and finally (7) equality wavenumber-based constraints on the Hubble constant from galaxy power spectrum measurements. I argue that a promising way forward should ultimately involve a combination of early- and late-time (but non-local—in a cosmological sense, i.e., at high redshift) new physics, as well as local (i.e., at z∼0) new physics, and I conclude by providing reflections with regards to potentially interesting models which may also help with the S8 tension.
Pierre Testorf, Clemens Schannwell, Marie‐Luise Kapsch
et al.
Abstract Coupled climate‐ice‐sheet modeling is still in its developing stage, and feedback processes between ice sheets and climate are still not yet fully understood. Here, we use simulations with a coupled climate‐ice‐sheet model to investigate teleconnections between Northern Hemispheric ice sheets and the Antarctic ice sheet (AIS) without direct freshwater forcing. We show that ice mass removal in the Northern Hemisphere can alter AIS evolution through a series of feedbacks. Changes in surface properties and orographic effects warm the newly deglaciated areas and the North Atlantic Ocean at mid‐depth. The warmer water masses propagate to the Southern Ocean, where internal oscillations periodically deliver them to the Antarctic coast. These repeated warm water intrusions destabilize the Ross ice shelf, ultimately triggering a runaway retreat of the West Antarctic ice sheet. Our results underscore the importance of coupled bi‐hemispheric climate‐ice‐sheet modeling to capture global teleconnections between ice sheets and climate.
Using a recognition model of atmospheric gravity waves (AGWs), we identified 519 AGW events from the OH airglow images observed at the Dandong and Lhasa stations from 2015 to 2017. The 317 AGW events detected at the Dandong station have wavelengths ranging from 30 to 60 km, periods from 14 to 20 min, horizontal speeds from 30 to 60 m/s, and relative intensities from 0.4% to 0.6%, respectively. The parameters of 202 events recorded at the Lhasa station mainly vary within 15–35 km in horizontal wavelength, 4–6 min in period, 40–100 m/s in horizontal velocity, and 0.1%–0.3% in relative intensity. The occurrence rate peaks in winter and summer at Dandong and the peak in summer are absent at Lhasa because of the lack of convective weather. The seasonal propagation directions of the waves are influenced by both the wind field-filtering effect and the distribution of wave sources. In spring, because of the southeastward background wind field, fewer southeastward events are observed at the Dandong station. The situation at the Lhasa station is similar. In summer, both the Lhasa and Dandong stations are dominated by northeastward AGWs, which can be attributed to the southwestward wind. In autumn, ray-tracing results show that the events at Dandong mainly originate from wind shear, whereas the events at the Lhasa station are triggered by convective weather. The location of the wave sources determines the trend of the propagation directions at the Dandong and Lhasa stations in autumn. In winter, because of the eastward wind, more events are propagating to the southwest at the Dandong station.
It has recently been argued that the Hubble tension may call for a combination of both pre- and post-recombination new physics. Motivated by these considerations, we provide one of the first concrete case studies aimed at constructing such a viable combination. We consider models that have individually worked best on either end of recombination so far: a spatially uniform time-varying electron mass leading to earlier recombination (also adding non-zero spatial curvature), and a sign-switching cosmological constant inducing an AdS-to-dS transition within the $\Lambda_{\rm s}$CDM model. When confronted against Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations, and Type Ia Supernovae data, we show that no combination of these ingredients can successfully solve the Hubble tension. We find that the matter density parameter $\Omega_m$ plays a critical role, driving important physical scales in opposite directions: the AdS-to-dS transition requires a larger $\Omega_m$ to maintain the CMB acoustic scale fixed, whereas the varying electron mass requires a smaller $\Omega_m$ to maintain the redshift of matter-radiation equality fixed. Despite the overall failure, we use our results to draw general model-building lessons, highlighting the importance of assessing tension-solving directions in the parameter space of new physics parameters and how these correlate with shifts in other standard parameters, while underscoring the crucial role of $\Omega_m$ in this sense.
These notes are based on the lectures that one of us (HT) gave at the Summer School on the “Theory of Large Deviations and Applications,” held in July 2024 at Les Houches in France. They present the basic definitions and mathematical results that form the theory of large deviations, as well as many simple motivating examples of applications in statistical physics, which serve as a basis for the many other lectures given at the school that covered more specific applications in biophysics, random matrix theory, nonequilibrium systems, geophysics, and the simulation of rare events, among other topics. These notes extend the lectures, which can be accessed online, by presenting exercises and pointer references for further reading.
Various scenarios of cosmic inflation enhance the amplitude of the stochastic gravitational wave background (SGWB) at frequencies detectable by the LISA detector. We develop tools for a template-based analysis of the SGWB and introduce a template databank to describe well-motivated signals from inflation, prototype their template-based searches, and forecast their reconstruction with LISA. Specifically, we classify seven templates based on their signal frequency shape, and we identify representative fundamental physics models leading to them. By running a template-based analysis, we forecast the accuracy with which LISA can reconstruct the template parameters of representative benchmark signals, with and without galactic and extragalactic foregrounds. We identify the parameter regions that can be probed by LISA within each template. Finally, we investigate how our signal reconstructions shed light on fundamental physics models of inflation: we discuss their impact for measurements of e.g., the couplings of inflationary axions to gauge fields; the graviton mass during inflation; the fluctuation seeds of primordial black holes; the consequences of excited states during inflation, and the presence of small-scale spectral features.
The epoch of reionization represents a major phase transition in cosmic history, during which the first luminous sources ionized the intergalactic medium (IGM). However, the small-scale physics governing ionizing photon sinks — particularly the interplay between recombinations, photon propagation, and self-shielded regions — remains poorly understood. Accurately modeling these processes requires a framework that self-consistently links ionizing emissivity, the clumping factor, mean free path, and photoionization rate. In this work, we extend the photon-conserving semi-numerical framework, SCRIPT, by introducing a self-consistent sub-grid model that dynamically connects these quantities to the underlying density field, enabling a more realistic treatment of inhomogeneous recombinations and photon sinks. We validate our model against a comprehensive set of observational constraints, including the UV luminosity function from HST and JWST, CMB optical depth from Planck, and Lyman-α forest measurements of the IGM temperature, photoionization rate, and mean free path. Our fiducial model also successfully reproduces Lyman-α opacity fluctuations, reinforcing its ability to capture large-scale inhomogeneities in the reionization process. Notably, we demonstrate that traditionally independent parameters, such as the clumping factor and mean free path, are strongly correlated, with implications for the timing, morphology, and thermal evolution of reionization. Looking ahead, we will extend this framework to include machine learning-based parameter inference. With upcoming 21 cm experiments poised to provide unprecedented insights, SCRIPT offers a powerful computational tool for interpreting high-redshift observations and refining our understanding of the last major phase transition in the universe.
The scattering of extremely energetic cosmic rays with both cosmic microwave background and extragalactic background light, can produce 𝒪(1018 eV) neutrinos, known as cosmogenic neutrinos. These neutrinos are the only messengers from the extreme cosmic accelerators that can reveal the origin of the most energetic cosmic rays. Consequently, much effort is being devoted to achieving their detection. In particular, the GRAND project aims to observe the ντ and ν¯τ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\overline{\nu}}_{\tau } $$\end{document} components of the cosmogenic neutrino flux in the near future using radio antennas. In this work, we investigate how the detection of cosmogenic neutrinos by GRAND can be used to probe beyond the Standard Model physics. We identify three well-motivated scenarios which induce distinct features in the cosmogenic neutrino spectrum at Earth: neutrino self-interactions mediated by a light scalar (νSI), pseudo-Dirac neutrinos (PDν) and neutrinos scattering on ultra-light Dark Matter (νDM). We show these scenarios can be tested by GRAND, using 10 years of cosmogenic neutrino data, in a region of parameter space complementary to current experiments. For the νSI model„ we find that GRAND can constrain the coupling to ντ in the range [10−2, 10−1] for a scalar with mass in the range 0.1 to 1 GeV. For PDν, we find that GRAND is sensitive to sterile-active mass squared splitting in the range [10−15, 10−13] eV2. Finally, for the νDM model, assuming a heavy mediator, GRAND can do substantially better than the current limits from other available data. These results rely on the fact that the actual cosmogenic flux is around the corner, not far from the current IceCube limit.
The task of hyperspectral image completion generally involves random missing entries completion, stripes inpainting, and cloud removal, which can enhance the accuracy of subsequent image analysis. Recently, tensor completion has been presented for image recovery. Owing to the framelet basis redundancy, the tensor rank of the extended tensor via feature extraction is smaller, which can characterize the correlation between any two modes of the tensor more accurately. In this work, the fully connected tensor network decomposition has been suggested to depict the low-rankness of the extended tensor with feature extraction. The process of feature extraction via framelet transform reduces the need for fewer principal components to depict the low-rankness of the underlying tensor. Moreover, total variation is incorporated into the proposed completion model to capture the spatial smoothness of the underlying tensor via minimizing the sum of the gradients across the image. To solve the large-scale resulting model, the augmented Lagrange multiplier-based proximal alternating minimization algorithm has been proposed. To accelerate the optimization algorithm, the leverage score sampling and fast Fourier transform have been introduced. Numerical results on several types of hyperspectral image completion problem demonstrate that the proposed method performs better than the compared methods in data completion.
The intensification of the urban heat island (UHI) effect poses a serious threat to public health, particularly in cities. Effectively mitigating UHI has been a focus of national and international academic research over the last decades. However, most contemporary research has focused on land use mitigation measures within urban areas, with less emphasis on suburban land use. To address this research gap and explore spatial characteristics, we analyzed the driving mechanism of suburban land use patterns on UHI intensity (UHII) within the main urban area of Shenyang City based on high spatial resolution raster data, such as Landsat remote sensing images and land use, combined with extreme gradient boosting and SHapley Additive exPlanations models. The landscape fragmentation index of overall suburban land use provided a stronger contribution to the UHII in urban areas than the aggregation index. Increased cropland fragmentation and aggregation enhance UHII mitigation, whereas increased aggregation of impervious surfaces intensifies UHII. No significant difference was observed between the effects of various suburban gradient landscapes on UHII; however, the effects on different gradients in urban areas increased with decreasing distance from the countryside, with a minimal effect observed on the extreme center of the city (U1). The study provides a theoretical reference for mitigating land use pressure and reducing the UHI in urban areas based on suburban land use.
Full 3D modelling of time-domain electromagnetic data requires tremendous computational resources. Consequently, simplified physics models prevail in geophysics, using a much faster but approximate (1D) forward model. A method is introduced that combines the accuracy of a 1D simplified physics model with the flexibility of coarse grids to reduce modelling errors, thereby avoiding full 3D accurate simulations. The method is applied to airborne time-domain electromagnetic data, generating data with a modelling error often smaller than the standard measurement noise level. The modelling error depends on the specific subsurface model (electrical conductivity values, angle representing the deviation of the 1D assumption) and the specific (temporal) discretization. Computation time can be decreased by a factor of 27. This modelling strategy may serve as an alternative to surrogate models or statistical relations derived from large 3D datasets.
Many symmetry breaking patterns in grand unified theories (GUTs) give rise to cosmic strings that eventually decay when pairs of GUT monopoles spontaneously nucleate along the string cores. These strings are known as metastable cosmic strings and have intriguing implications for particle physics and cosmology. In this article, we discuss the current status of metastable cosmic strings, with a focus on possible GUT embeddings and connections to inflation, neutrinos, and gravitational waves (GWs). The GW signal emitted by a network of metastable cosmic strings in the early universe differs, in particular, from the signal emitted by topologically stable strings by a suppression at low frequencies. Therefore, if the underlying symmetry breaking scale is close to the GUT scale, the resulting GW spectrum can be accessible at current ground-based interferometers as well as at future space-based interferometers, such as LISA, and at the same time account for the signal in the most recent pulsar timing data sets. Metastable cosmic strings thus nourish the hope that future GW observations might shed light on fundamental physics close to the GUT scale.
We present the first analysis of cosmic shear measured in DES Y3 that employs the entire range of angular scales in the data. To achieve this, we build upon recent advances in the theoretical modelling of weak lensing provided by a combination of $N$-body simulations, physical models of baryonic processes, and neural networks. Specifically, we use BACCOemu to model the linear and nonlinear matter power spectrum including baryonic physics, allowing us to robustly exploit scales smaller than those used by the DES Collaboration. We show that the additional data produce cosmological parameters that are tighter but consistent with those obtained from larger scales, while also constraining the distribution of baryons. In particular, we measure the mass scale at which haloes have lost half of their gas, $\log\,M_{\rm c}=14.38^{+0.60}_{-0.56}\log(h^{-1}{\rm M_{ \odot}})$, and a parameter that quantifies the weighted amplitudes of the present-day matter inhomogeneities, $S_8=0.799^{+0.023}_{-0.015}$. Our constraint on $S_8$ is statistically compatible with that inferred from the Planck satellite's data at the $0.9\sigma$ level. We find instead a $1.4\sigma$ shift in comparison to that from the official DES Y3 cosmic shear, because of different choices in the modelling of intrinsic alignment, non-linearities, baryons, and lensing shear ratios. We conclude that small scales in cosmic shear data contain valuable astrophysical and cosmological information and thus should be included in standard analyses.
The recent observations by pulsar timing array (PTA) experiments suggest the existence of stochastic gravitational wave background in the nano-Hz range. It can be a hint for the new physics and cosmic string is one of the promising candidate. In this paper, we study the implication of the PTA result for cosmic strings and dark photon dark matter produced by the decay of cosmic string loops. It can simultaneously explain the PTA result and present dark matter abundance for the dark photon mass m~10^{-6}--10^{-4}eV. Implications for the gravitational wave detection with multi-frequency bands are also discussed.
Abstract While Mars lacks a global intrinsic magnetic field, it does exhibit crustal magnetic anomalies (mostly in its Southern Hemisphere). These crustal magnetic anomalies directly interact with solar wind, which forms a mini‐magnetosphere and a region denoted the mini‐magnetopause. Using magnetic field and plasma measurements from the Mars Atmosphere and Volatile Evolution, we report a novel case of magnetic reconnection at the Martian mini‐magnetopause. In this process, protons and oxygen ions from the Martian atmosphere were accelerated during reconnection and likely escaped along the outflow direction. Magnetic reconnection may occur between the interplanetary magnetic field and crustal magnetic fields at the Martian mini‐magnetopause, which contributes to planetary ion escape, solar wind entering the mini‐magnetosphere and the evolution of magnetic topology in the dayside Martian mini‐magnetosphere.