A
bstract
We investigate axion-like particles (ALPs) in the context of asymptotically safe gravity-matter systems. The ALP-photon interaction, which facilitates experimental searches for ALPs, is a ...dimension-5-operator. Quantum fluctuations of gravity lower its scaling dimension, and the ALP-photon coupling can become asymptotically free or even asymptotically safe. However, quantum fluctuations of gravity need to be strong to overcome the canonical scaling and this strong-gravity regime is in tension with the weak-gravity bound in asymptotic safety. Thus, we tentatively conclude that fundamental ALPs can likely not be accommodated in asymptotically safe gravity-matter systems. In turn, an experimental discovery of an ALP would thus shed valuable light on the quantum nature of gravity.
Abstract
Horndeski gravity is a popular contender for a phenomenological model of dynamical dark
energy, and as such subject to observational constraints. In this work, we ask whether Horndeski
...gravity can be more than a phenomenological model and instead become a fundamental theory, which
extends towards high energy scales and includes quantum effects. We find that within the
asymptotic-safety paradigm, an ultraviolet completion of a simple class of models of Horndeski
gravity is achievable, but places strong constraints on the couplings of the theory. These
constraints are not compatible with dynamical dark energy. Further, we find a similar result in an
effective-field theory approach to this class of models of Horndeski gravity: under the assumption
that there is no new strongly-coupled physics below the Planck scale, quantum gravity fluctuations
force the Horndeski couplings to be too small to achieve an explanation of dynamical dark energy.
Ongoing and future gravitational wave collaborations explore different frequency ranges of the gravitational wave spectrum, probing different stages of the early universe and Beyond Standard Model ...physics. Due to the very high energies involved, accelerators cannot probe these earlier stages. Therefore, until some years ago, knowledge about new physics was limited and relied on bounds from CMB observations and theoretical assumptions about higher energy scales. While models could be constrained by CMB data, they were left unconstrained at shorter wavelength scales. Nonetheless, each one of these models has a gravitational wave density spectrum at these shorter wavelength scales that can ultimately be compared to data from ground-based, space-born, and pulsar timing array searches. These lecture notes review the formalism of gravitational waves in General Relativity and introduce stochastic gravitational waves, with a focus on primordial sources and commenting on detection efforts. These lecture notes were inspired by the course "Gravitational Waves from the Early Universe" given at the 27th W.E. Heraeus "Saalburg" Summer School 2021 by Valerie Domcke.
Horndeski gravity is a popular contender for a phenomenological model of dynamical dark energy, and as such subject to observational constraints. In this work, we ask whether Horndeski gravity can be ...more than a phenomenological model and instead become a fundamental theory, which extends towards high energy scales and includes quantum effects. We find that within the asymptotic-safety paradigm, an ultraviolet completion of a simple class of models of Horndeski gravity is achievable, but places strong constraints on the couplings of the theory. These constraints are not compatible with dynamical dark energy. Further, we find a similar result in an effective-field theory approach to this class of models of Horndeski gravity: under the assumption that there is no new strongly-coupled physics below the Planck scale, quantum gravity fluctuations force the Horndeski couplings to be too small to achieve an explanation of dynamical dark energy.
New physics beyond the Standard Model can give rise to stochastic gravitational-wave backgrounds, for example through cosmic strings. In this way, gravitational-wave searches with pulsar-timing ...arrays as well as existing and future laser interferometers may provide information on particle physics beyond the Standard Model. Here, we take one additional step and link particle physics beyond the Standard Model to quantum gravity. We investigate whether particle physics models that may give rise to cosmic strings can be embedded into an asymptotically safe theory of quantum gravity and matter. We focus on models where cosmic strings arise from U(1)-symmetry-breaking in an extended Yukawa-Abelian-Higgs sector that may be part of a dark sector. We find a negative answer for the simplest model that can give rise to cosmic strings and also find constraints on an extended model. We tentatively conclude that cosmic strings are difficult to accommodate in asymptotically safe models. This fits well with the latest 15-year dataset and search for new physics from the NANOGrav collaboration, which disfavors a stable-cosmic-string interpretation. In that sense, the recent data provide an indirect, albeit at present rather tentative, hint about the quantum theory of gravity.
We investigate axion-like particles (ALPs) in the context of asymptotically safe gravity-matter systems. The ALP-photon interaction, which facilitates experimental searches for ALPs, is a ...dimension-5-operator. Quantum fluctuations of gravity lower its scaling dimension, and the ALP-photon coupling can become asymptotically free or even asymptotically safe. However, quantum fluctuations of gravity need to be strong to overcome the canonical scaling and this strong-gravity regime is in tension with the weak-gravity bound in asymptotic safety. Thus, we tentatively conclude that fundamental ALPs can likely not be accommodated in asymptotically safe gravity-matter systems. In turn, an experimental discovery of an ALP would thus shed valuable light on the quantum nature of gravity.
Competition between weeds and crops results in a lower availability of some resources to crop species, which causes deficiencies, such as water and nutritional deficiencies and low light quality or ...quantity. The aims of this study were to evaluate the effects of Urochloa brizantha (Hochst. ex A. Rich.) R.D. Webster, cv. Xaraes (brachiaria) emergence times comparing with corn (Zea mays L.), in addition to the fertilization and application of different nicosulfuron and atrazine herbicides levels, on the physiological characteristics of intercropped species. The physiological characteristics Brachiaria plants that were treated with herbicides were negatively affected. The emergence time of Brachiaria plants in comparison with corn altered the physiological characteristics of both. The Brachiaria plants that emerged before corn presented higher photosynthetic and transpiration rates and reduced the intensity of these characteristics in corn. Thus, the Brachiaria plants that emerged before the corn crop presented a higher physiological efficiency, and they were more competitive than the weeds that emerged during other seasons.
The 15-year 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.
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