Cosmological Physics Research Articles

The Poem as Icon: A Study in Aesthetic Cognition

The Poem as Icon: A Study in Aesthetic Cognition

Seeking new physics in cosmology with Bayesian neural networks: Dark energy and modified gravity

We study the potential of Bayesian neural networks (BNNs) to detect new physics in the dark matter power spectrum, concentrating here on evolving dark energy and modifications to general relativity. After introducing a new technique to quantify classification uncertainty in BNNs, we train two BNNs on mock matter power spectra produced using the publicly available code react in the $k$ range $(0.01\ensuremath{-}2.5)\text{ }\text{ }h{\mathrm{Mpc}}^{\ensuremath{-}1}$ and redshift bins (0.1, 0.478, 0.783, 1.5) with Euclid-like noise. The first network classifies spectra into five labels including $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$, $f(R)$, $w\mathrm{CDM}$, Dvali-Gabadadze-Porrati gravity and a ``random'' class, whereas the second is trained to distinguish $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ from non-$\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$. Both networks achieve a comparable training, validation and test accuracy of $\ensuremath{\sim}95%$. Each network is also capable of correctly classifying spectra with deviations from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ that were not included in the training set, demonstrated with spectra generated using the growth index $\ensuremath{\gamma}$. To obtain an indication of the BNNs classification capability, we compute the smallest deviation from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ such that the noise-averaged non-$\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ classification probability is at least 95% according to our estimated error quantification, finding these bounds to be ${f}_{R0}\ensuremath{\lesssim}{10}^{\ensuremath{-}7}$, ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{rc}}\ensuremath{\lesssim}{10}^{\ensuremath{-}2}$, $\ensuremath{-}1.05\ensuremath{\lesssim}{w}_{0}\ensuremath{\lesssim}0.95$, $\ensuremath{-}0.2\ensuremath{\lesssim}{w}_{a}\ensuremath{\lesssim}0.2$, and $0.52\ensuremath{\lesssim}\ensuremath{\gamma}\ensuremath{\lesssim}0.59$. The bounds on $f(R)$ can be improved by training a specialist network to distinguish solely between $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ and $f(R)$ power spectra which can detect a nonzero ${f}_{R0}$ at $\mathcal{O}({10}^{\ensuremath{-}8})$. We expect that further developments, such as the inclusion of smaller length scales or additional extensions to $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$, will only improve the potential of BNNs to indicate the presence of new physics in cosmological datasets, regardless of the underlying theory.

How cosmological merger histories shape the diversity of stellar haloes

ABSTRACT We introduce and apply a new approach to probe the response of galactic stellar haloes to the interplay between cosmological merger histories and galaxy formation physics. We perform dark matter-only, zoomed simulations of two Milky Way-mass hosts and make targeted, controlled changes to their cosmological histories using the genetic modification technique. Populating each history’s stellar halo with a semi-empirical, particle tagging approach then enables a controlled study, with all instances converging to the same large-scale structure, dynamical and stellar mass at z = 0 as their reference. These related merger scenarios alone generate an extended spread in stellar halo mass fractions (1.5 dex) comparable to the observed population, with the largest scatter achieved by growing late (z ≤ 1) major mergers that spread out existing stars to create massive, in-situ dominated stellar haloes. Increasing a last major merger at z ∼ 2 brings more accreted stars into the inner regions, resulting in smaller scatter in the outskirts which are predominantly built by subsequent minor events. Exploiting the flexibility of our semi-empirical approach, we show that the diversity of stellar halo masses across scenarios is reduced by allowing shallower slopes in the stellar mass–halo mass relation for dwarf galaxies, while it remains conserved when central stars are born with hotter kinematics across cosmic time. The merger-dependent diversity of stellar haloes thus responds distinctly to assumptions in modelling the central and dwarf galaxies respectively, opening exciting prospects to constrain star formation and feedback at different galactic mass-scales with the coming generation of deep, photometric observatories.

Comments on wormholes, ensembles, and cosmology

Certain closed-universe big-bang/big-crunch cosmological spacetimes may be obtained by analytic continuation from asymptotically AdS Euclidean wormholes, as emphasized by Maldacena and Maoz. We investigate how these Euclidean wormhole spacetimes and their associated cosmological physics might be described within the context of AdS/CFT. We point out that a holographic model for cosmology proposed recently in arXiv:1810.10601 can be understood as a specific example of this picture. Based on this example, we suggest key features that should be present in more general examples of this approach to cosmology. The basic picture is that we start with two non-interacting copies of a Euclidean holographic CFT associated with the asymptotic regions of the Euclidean wormhole and couple these to auxiliary degrees of freedom such that the original theories interact strongly in the IR but softly in the UV. The partition function for the full theory with the auxiliary degrees of freedom can be viewed as a product of partition functions for the original theories averaged over an ensemble of possible sources. The Lorentzian cosmological spacetime is encoded in a wavefunction of the universe that lives in the Hilbert space of the auxiliary degrees of freedom.

On cosmological expansion and local physics

We find an exact convergence in the local dynamics described by two supposedly antagonistic approaches applied at the local, solar system scale: one starting from an expanding universe perspective such as FLRW, the other based on a local model ignoring any notion of expansion, such as static Schwarzschild dS. Both models are in complete agreement when the local effects of the expansion are circumscribed to the presence of the cosmological constant. We elaborate on the relevant role of static backgrounds like the Schwarzschild-dS metric in standard form as the most proper coordinatizations to describe physics at the local scale. We also elaborate on the popular expanding 3-space picture—to be distinguished from that of the expanding universe—and point out the confusion of scales which is typically associated with it. Finally, making use of an old and too often forgotten relativistic kinematical invariant, we address some remaining misunderstandings on space expansion, cosmological and gravitational redshifts. As a byproduct we propose a unique and unambiguous prescription to match the local and cosmological expression of a specific observable.

Cutting cosmological correlators

The initial conditions of our universe appear to us in the form of a classical probability distribution that we probe with cosmological observations. In the current leading paradigm, this probability distribution arises from a quantum mechanical wavefunction of the universe. Here we ask what the imprint of quantum mechanics is on the late time observables. We show that the requirement of unitary time evolution, colloquially the conservation of probabilities, fixes the analytic structure of the wavefunction and of all the cosmological correlators it encodes. In particular, we derive in perturbation theory an infinite set of single-cut rules that generalize the Cosmological Optical Theorem and relate a certain discontinuity of any tree-level n-point function to that of lower-point functions. Our rules are closely related to, but distinct from the recently derived Cosmological Cutting Rules. They follow from the choice of the Bunch-Davies vacuum and a simple property of the (bulk-to-bulk) propagator and are astoundingly general: we prove that they are valid for fields with a linear dispersion relation and any mass, any integer spin and arbitrary local interactions with any number of derivatives. They also apply to general FLRW spacetimes admitting a Bunch-Davies vacuum, including de Sitter, slow-roll inflation, power-law cosmologies and even resonant oscillations in axion monodromy. We verify the single-cut rules in a number of non-trivial examples, including four massless scalars exchanging a massive scalar, as relevant for cosmological collider physics, four gravitons exchanging a graviton, and a scalar five-point function.

Assessing the potential of cluster edges as a standard ruler on constraining dark energy models

We assess comprehensively the potential of galaxy cluster edges as a standard ruler in measuring cosmological distances and probing exotic physics. Confronting five alternative cosmological models with cluster edges from the near future Dark Energy Spectroscopic Instrument survey, we conclude that cluster edges can serve as a promising probe to constrain models beyond $\Lambda$CDM. Especially, the constraining precision of equation of state of dark energy from cluster edges is just about two times larger than that from the Pantheon Type Ia supernovae sample. We find that the constraining power of cluster edges can be exhibited better by combining it with other probes. Combining cluster edges with cosmic microwave background, baryon acoustic oscillations, Type Ia supernovae, cosmic chronometers and simulated gravitational-wave events from the space-based Einstein Telescope, respectively, we constrain $\Lambda$CDM and find that the data combination of cosmic microwave background and cluster edges gives the best constraint on the Hubble constant $H_0$ with a $0.5\%$ precision and the matter density ratio $\Omega_m$ with a $1.6\%$ precision among these five pair datasets, and that the data combination of gravitational waves and cluster edges almost shares the same constraining power as that of Type Ia supernovae and cluster edges. We also give the most stringent constraint on $\Lambda$CDM by combining cluster edges with available cosmological data. With the help of other probes, galaxy cluster edges can give new insights on exotic physics better at cosmological scales.

Photon–Photon Interactions and the Opacity of the Universe in Gamma Rays

We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of physical science. Extensive analysis of the currently available information appears to us as revealing a global coherence among the astrophysical, cosmological, and fundamental physics data, or, at least, no evident need so far of substantial modification of our present understanding. Deeper data from future experiments will verify to what extent and in which directions this conclusion should be modified.

The scalar chemical potential in cosmological collider physics

Non-analyticity in co-moving momenta within the non-Gaussian bispectrum is a distinctive sign of on-shell particle production during inflation, presenting a unique opportunity for the “direct detection” of particles with masses as large as the inflationary Hubble scale (H). However, the strength of such non-analyticity ordinarily drops exponentially by a Boltzmann-like factor as masses exceed H. In this paper, we study an exception provided by a dimension-5 derivative coupling of the inflaton to heavy-particle currents, applying it specifically to the case of two real scalars. The operator has a “chemical potential” form, which harnesses the large kinetic energy scale of the inflaton, {overset{cdot }{phi}}_0^{1/2}approx 60H , to act as an efficient source of scalar particle production. Derivative couplings of inflaton ensure radiative stability of the slow-roll potential, which in turn maintains (approximate) scale-invariance of the inflationary correlations. We show that a signal not suffering Boltzmann suppression can be obtained in the bispectrum with strength fNL ∼ mathcal{O} (0.01–10) for an extended range of scalar masses lesssim {overset{cdot }{phi}}_0^{1/2} , potentially as high as 1015 GeV, within the sensitivity of upcoming LSS and more futuristic 21-cm experiments. The mechanism does not invoke any particular fine-tuning of parameters or breakdown of perturbation-theoretic control. The leading contribution appears at tree-level, which makes the calculation analytically tractable and removes the loop-suppression as compared to earlier chemical potential studies of non-zero spins. The steady particle production allows us to infer the effective mass of the heavy particles and the chemical potential from the variation in bispectrum oscillations as a function of co-moving momenta. Our analysis sets the stage for generalization to heavy bosons with non-zero spin.

A new determination of the primordial helium abundance using the analyses of H ii region spectra from SDSS

ABSTRACT The precision measurement of the primordial helium abundance Yp is a powerful probe of the early Universe. The most common way to determine Yp is the analyses of observations of metal-poor H ii regions found in blue compact dwarf galaxies. We present the spectroscopic sample of 100 H ii regions collected from the Sloan Digital Sky Survey. The final analysed sample consists of our sample and HeBCD data base from Izotov et al. (2007). We use a self-consistent procedure to determine physical conditions, current helium abundances, and metallicities of the H ii regions. From a regression to zero metallicity, we have obtained Yp = 0.2462 ± 0.0022, which is one of the most stringent constraints obtained with such methods up to date and is in a good agreement with the Planck result $Y_{\rm p}^{\it {\mathrm{ Planck}}} = 0.2471 \pm 0.0003$. Using the determined value of Yp and the primordial deuterium abundance taken from Particle Data Group (Zyla et al. 2020) we put a constraint on the effective number of neutrino species Neff = 2.95 ± 0.16, which is consistent with the Planck one Neff = 2.99 ± 0.17. Further increase of statistics potentially allows us to achieve Planck accuracy, which in turn will become a powerful tool for studying the self-consistency of the standard cosmological model and/or physics beyond.

Cosmological lithium problem

<p indent=0mm>Firstly proposed in 1946 by George Gamow, the hot big bang theory is now the most widely accepted cosmological model of the Universe, where the Universe expanded from a very high-density state dominated by radiation. The theory has been vindicated by the observation of the cosmic microwave background, our emerging knowledge of the large-scale structure of the Universe, and the rough consistency between calculations and observations of primordial abundances of the lightest elements in nature: Hydrogen, helium, and lithium. Primordial big-bang nucleosynthesis (BBN) began when the Universe was <sc>3 minutes</sc> old and ended less than half an hour later when nuclear reactions were quenched by the low temperature (about <sc>10⁹ K,</sc> i.e., ~1 GK, or <italic>kT</italic><sc>~100 keV)</sc> and density conditions in the expanding Universe. Only the lightest nuclides (²H, ³He, ⁴He, and ⁷Li) were synthesized in appreciable quantities through BBN, and these relics provide us a unique window on the early Universe. Constrained by the baryon density determined by the Wilkinson Microwave Anisotropy Probe (WMAP) and PLANCK satellites, the primordial abundances of ²H (referred to as D hereafter) and ⁴He inferred from observational data are in good general agreement with predictions; however, the standard BBN theory overestimates the primordial ⁷Li abundance by about a factor of three to four. This significant deviation is the so-called “cosmological lithium problem.” Over about two decades, many attempts to resolve this discrepancy using conventional nuclear physics have been unsuccessful yet. The possible solutions to this unresolved problem have been suggested, which can be mainly categorized into three groups, according to which part of the preceding analysis is called into question: (1) Astrophysical solutions revise the measured primordial lithium abundance; (2) solutions beyond the standard model invoke new particle physics or nonstandard cosmological physics; (3) nuclear physics solutions alter the reaction flow into and out of mass-7. This paper systematically reviews three major aspects of this long-pending cosmological lithium problem. It includes the scientific background, motivation, possible solutions and attempts made in the past. Here, we mainly focus on the nuclear physics solutions, which might solve this problem in an elegant way. In this review, the goals of different research directions are summarized and evaluated, and the promising direction is guided for future investigations.

Platonic Golden Network Theory for High Energy Physics and Quantum Cosmology

In the spirit of Pythagorean cosmic mathematical music of numbers and following the fundamental tenets of platonic transfinite set theory and the methodology of E-infinity Cantorian spacetime, we present a golden section based network with deep physical meaning confirming experimental results in high energy physics as well as cosmological measurements and observations related to the phenomenon of cosmic dark energy and dark matter.

Cosmology at the end of the world

In the last two decades the Anti-de Sitter/Conformal Field Theory correspondence (AdS/CFT) has emerged as focal point of many research interests. In particular, it functions as a stepping stone to a still missing full quantum theory of gravity. In this context, a pivotal question is if and how cosmological physics can be studied using AdS/CFT. Motivated by string theory, braneworld cosmologies propose that our universe is a four-dimensional membrane embedded in a bulk five-dimensional AdS spacetime. We show how such a scenario can be microscopically realized in AdS/CFT using special field theory states dual to an "end-of-the-world brane" moving in a charged black hole spacetime. Observers on the brane experience cosmological physics and approximately four-dimensional gravity, at least locally in spacetime. This result opens a new path towards a description of quantum cosmology and the simulation of cosmology on quantum machines.

Hyperbolic symmetries, inflaton–phantom cosmology, and inflation

Using a hyperbolic complex plane, we study the realization of the underlying hyperbolic symmetry as an internal symmetry that enables the unification of scalar fields of cosmological and particle physics interest. Such an unification is achieved along the universal prescriptions used in physics, avoiding the use of concepts as Euclideanization, non-canonical Lagrangians and hidden structures, that have appeared in other approaches. The scalar potentials constructed within the present scheme are bounded from below, and the realization of the spontaneous symmetry breaking of the aforementioned noncompact symmetry is studied. The profiles of these potentials with exact/broken hyperbolic symmetry replicate qualitative aspects of those ones used in inflationary models, and then a detailed com\-pa\-ri\-son is made. Moreover, the homotopy constraints of the topology induced on the corresponding vacuum manifolds, restricts the existence of topological defects associated with continuous symmetries, allowing only those defects associated with discrete symmetries; the consistency of these results is contrasted with current observational tests from the LIGO/Virgo collaboration, and terrestrial experiments based on a synchronized network of atomic magnetometers. At the end, the nonre\-la\-tivistic limit of the model is identified with a hyperbolic version of the nonlinear Schr\"odinger equation.

Cosmological Model Insensitivity of Local H0 from the Cepheid Distance Ladder

Abstract The observed tension (∼9% difference) between the local distance ladder measurement of the Hubble constant, H 0, and its value inferred from the cosmic microwave background could hint at new, exotic, cosmological physics. We test the impact of the assumption about the expansion history of the universe ( ) on the local distance ladder estimate of H 0. In the fiducial analysis, the Hubble flow Type Ia supernova (SN Ia) sample is truncated to z &lt; 0.15, and the deceleration parameter (q 0) is fixed to −0.55. We create realistic simulations of the calibrator and Pantheon samples, and account for a full systematics covariance between these two sets. We fit several physically motivated dark-energy models, and derive combined constraints from calibrator and Pantheon SNe Ia and simultaneously infer H 0 and dark-energy properties. We find that the assumption on the dark-energy model does not significantly change the local distance ladder value of H 0, with a maximum difference (ΔH 0) between the inferred value for different models of 0.47 km , i.e., a 0.6% shift in H 0, significantly smaller than the observed tension. Additional freedom in the dark-energy models does not increase the error in the inferred value of H 0. Including systematics covariance between the calibrators, low-redshift SNe, and high-redshift SNe can induce small shifts in the inferred value for H 0. The SN Ia systematics in this study contribute ≲0.8% to the total uncertainty of H 0.

Constraints on the Velocity Dispersion of Dark Matter from Cosmology and New Bounds on Scattering from the Cosmic Dawn

Abstract The observational value of the velocity dispersion, , is missing in the dark matter (DM) puzzle. Nonzero or nonthermal DM velocities can drastically influence large-scale structure and the 21 cm temperature at the epoch of the cosmic dawn, as well as the estimation of DM physical parameters, such as mass and interaction couplings. To study the phenomenology of , we model the evolution of DM in terms of a simplistic and generic Boltzmann-like momentum distribution. Using cosmological data from the cosmic microwave background, baryonic acoustic oscillations, and red luminous galaxies, we constrain the DM velocity dispersion for a broad range of masses, , finding (99% CL). Including the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) T 21 measurements, we extend our study to constrain the baryon–DM interaction in the range of DM velocities allowed by our analysis. As a consequence, we present new bounds on two electromagnetic models of DM, namely minicharged particles (MCPs) and the electric dipole moment (EDM). For MCPs, the parameter region that is consistent with EDGES and independent bounds on cosmological and stellar physics is very small, pointing to the subelectronvolt mass regime of DM. A window in the MeV–GeV may still be compatible with these bounds for MCP models without a hidden photon. But the EDM parameter region consistent with EDGES is excluded by Big Bang nucleosynthesis and collider physics.

Flag-dipole spinors: On the dual structure derivation and {mathcal {C}}, {mathcal {P}} and {mathcal {T}} symmetries

In this manuscript we report the flag-dipole spinors dual structure direct definition and analyze the properties behind the corresponding operator which generates such structure. This particular construction may be interesting for cosmological, phenomenological and mathematical physics applications. In addition, we analyse the behaviour of the flag-dipole spinors under action of discrete symmetries, facing an unconventional property encoded on (mathcal {CPT})^2.

Cosmological collider signatures of massive vectors from non-Gaussian gravitational waves

The cosmological collider provides a model-independent probe of particle physics during inflation. We extend the study of cosmological collider physics to much smaller scales through gravitational wave (GW) probes. With a Chern-Simons interaction, a massive vector field can obtain a chemical potential and its particle production can cause significant non-Gaussian GW signals. We calculate the mass and spin dependences of the induced GW 3-point correlation function in the squeezed limit, and estimate its amplitude. Such signals may be detectable in the current and upcoming GW interferometer experiments.

Cosmological collider physics and the curvaton

Primordial non-Gaussianity signatures of extremely heavy particles are re- examined within a simple alternative to the standard inflationary paradigm, in which the primordial fluctuations and the inflationary spacetime expansion are sourced by two different fields. The curvaton scenario provides an example of this in which the distinct roles are played by the curvaton and the inflaton fields, respectively. We study couplings of the curvaton to heavy particles with masses of order the inflationary Hubble scale, and show that they can lead to non-Gaussian signals orders of magnitude larger than those in standard inflation, consistent with explicit effective field theory control of inflationary dynamics. This brings various motivated particle physics signatures, such as loops of heavy gauge-charged scalars and fermions, within future observational reach.

Gravitational-lensing measurements push Hubble-constant discrepancy past 5σ

If the tension can’t be attributed to systematic errors, it could be a sign of new cosmological physics.