scholarly journals Torsion in String-Inspired Cosmologies and the Universe Dark Sector

Universe ◽  
2021 ◽  
Vol 7 (12) ◽  
pp. 480
Author(s):  
Nick E. Mavromatos
Keyword(s):  
String Model ◽  
Dark Sector ◽  
Vacuum Model ◽  
Anomaly Cancellation ◽  
Fundamental String ◽  
Primordial Gravitational Waves ◽  
Cosmological Data ◽  
Model Independent ◽  
The Universe

Several aspects of torsion in string-inspired cosmologies are reviewed. In particular, its connection with fundamental, string-model independent, axion fields associated with the massless gravitational multiplet of the string are discussed. It is argued in favour of the role of primordial gravitational anomalies coupled to such axions in inducing inflation of a type encountered in the “Running-Vacuum-Model (RVM)” cosmological framework, without fundamental inflaton fields. The gravitational-anomaly terms owe their existence to the Green–Schwarz mechanism for the (extra-dimensional) anomaly cancellation, and may be non-trivial in such theories in the presence of (primordial) gravitational waves at early stages of the four-dimensional string universe (after compactification). The paper also discusses how the torsion-induced stringy axions can acquire a mass in the post inflationary era, due to non-perturbative effects, thus having the potential to play the role of (a component of) dark matter in such models. Finally, the current-era phenomenology of this model is briefly described with emphasis placed on the possibility of alleviating tensions observed in the current-era cosmological data. A brief phenomenological comparison with other cosmological models in contorted geometries is also made.

scholarly journals On the theory and applications of modern cosmography

2016 ◽  
Vol 13 (03) ◽  
pp. 1630002 ◽  
Author(s):  
Peter K. S. Dunsby ◽  
Orlando Luongo
Keyword(s):  
A Priori ◽  
Energy Models ◽  
Cosmological Expansion ◽  
History Of ◽  
Independent Technique ◽  
Cosmological Data ◽  
Model Independent ◽  
The Universe ◽  
Important Branch ◽  
Quantities Of Interest

Cosmography represents an important branch of cosmology which aims to describe the universe without the need of postulating a priori any particular cosmological model. All quantities of interest are expanded as a Taylor series around here and now, providing in principle, a way of directly matching with cosmological data. In this way, cosmography can be regarded a model-independent technique, able to fix cosmic bounds, although several issues limit its use in various model reconstructions. The main purpose of this review is to focus on the key features of cosmography, emphasizing both the strategy for obtaining the observable cosmographic series and pointing out any drawbacks which might plague the standard cosmographic treatment. In doing so, we relate cosmography to the most relevant cosmological quantities and to several dark energy models. We also investigate whether cosmography is able to provide information about the form of the cosmological expansion history, discussing how to reproduce the dark fluid from the cosmographic sound speed. Following this, we discuss limits on cosmographic priors and focus on how to experimentally treat cosmographic expansions. Finally, we present some of the latest developments of the cosmographic method, reviewing the use of rational approximations, based on cosmographic Padé polynomials. Future prospects leading to more accurate cosmographic results, able to better reproduce the expansion history of the universe, are also discussed in detail.

scholarly journals THE UNIVERSE DYNAMICS IN THE TACHYON COSMOLOGY WITH NON-MINIMAL COUPLING TO MATTER

2011 ◽  
Vol 26 (15) ◽  
pp. 1125-1135 ◽  
Author(s):  
H. FARAJOLLAHI ◽  
A. RAVANPAK ◽  
G. F. FADAKAR
Keyword(s):  
Distance Modulus ◽  
Tachyon Field ◽  
Cosmological Redshift ◽  
Redshift Drift ◽  
Phantom Crossing ◽  
Acceleration Of The Universe ◽  
Model Independent ◽  
The Difference ◽  
The Universe

Recently, the tachyon cosmology has been represented as dark energy model to support the current acceleration of the universe without phantom crossing. In this paper, we study the dynamics of the tachyon cosmology in which the field plays the role of tachyon field and also non-minimally coupled to the matter Lagrangian. The model shows current universe acceleration and also phantom crossing in the future. Two cosmological tests are also performed to validate the model; the difference in the distance modulus and the model independent Cosmological Redshift Drift (CRD) test.

scholarly journals Exposing dark sector with future Z-factories

2019 ◽  
Vol 34 (13n14) ◽  
pp. 1940010 ◽  
Author(s):  
Jia Liu ◽  
Lian-Tao Wang ◽  
Xiao-Ping Wang ◽  
Wei Xue
Keyword(s):  
Dark Matter ◽  
Independent Study ◽  
Dark Sector ◽  
Dark Matter Search ◽  
Hidden Sector ◽  
Leading Role ◽  
Model Independent ◽  
Higgs Portal ◽  
Inelastic Dark Matter ◽  
The Universe

We investigate the prospects of searching dark sector models via exotic [Formula: see text]-boson decay at future [Formula: see text] colliders with Giga [Formula: see text] and Tera [Formula: see text] options. Four general categories of dark sector models: Higgs portal dark matter, vector portal dark matter, inelastic dark matter and axion-like particles, are considered. Focusing on channels motivated by the dark sector models, we carry out a model independent study of the sensitivities of [Formula: see text]-factories in probing exotic decays. The limits on branching ratios of the exotic [Formula: see text] decay are typically [Formula: see text] for the Giga [Formula: see text] and [Formula: see text] for the Tera [Formula: see text], and they are compared with the projection for the high luminosity LHC. We demonstrate that future [Formula: see text]-factories can provide its unique and leading sensitivity, and highlight the complementarity with other experiments, including the indirect and direct dark matter search limits, and the existing collider limits. Future [Formula: see text] factories will play a leading role to uncover the hidden sector of the universe in the future.

scholarly journals Stringy-running-vacuum-model inflation: from primordial gravitational waves and stiff axion matter to dynamical dark energy

2021 ◽  
Author(s):  
Nick E. Mavromatos ◽  
Joan Solà Peracaula
Keyword(s):  
Dark Energy ◽  
Degrees Of Freedom ◽  
Domain Walls ◽  
Basic Structure ◽  
Vacuum Model ◽  
Living Fossils ◽  
Primordial Gravitational Waves ◽  
The Universe ◽  
Dynamical Dark Energy ◽  
Effective Description

AbstractIn previous works, we have derived a Running Vacuum Model (RVM) for a string Universe, which provides an effective description of the evolution of 4-dimensional string-inspired cosmologies from inflation till the present epoch. In the context of this “stringy RVM” version, it is assumed that the early Universe is characterised by purely gravitational degrees of freedom, from the massless gravitational string multiplet, including the antisymmetric tensor field. The latter plays an important role, since its dual gives rise to a ‘stiff’ gravitational axion “matter”, which in turn couples to the gravitational anomaly terms, assumed to be non-trivial at early epochs. In the presence of primordial gravitational wave (GW) perturbations, such anomalous couplings lead to an RVM-like dynamical inflation, without external inflatons. We review here this framework and discuss potential scenarios for the generation of such primordial GW, among which the formation of unstable domain walls, which eventually collapse in a non-spherical-symmetric manner, giving rise to GW. We also remark that the same type of “stiff” axionic matter could provide, upon the generation of appropriate potentials during the post-inflationary eras, (part of) the Dark Matter (DM) in the Universe, which could well be ultralight, depending on the parameters of the string-inspired model. All in all, the new (stringy) mechanism for RVM inflation preserves the basic structure of the original (and more phenomenological) RVM, as well as its main advantages: namely, a mechanism for graceful exit and for generating a huge amount of entropy capable of explaining the horizon problem. It also predicts axionic DM and the existence of mild dynamical Dark Energy (DE) of quintessence type in the present universe, both being “living fossils” of the inflationary stages of the cosmic evolution. Altogether the modern RVM appears to be a theoretically sound (string-based) approach to cosmology with a variety of phenomenologically testable consequences.

scholarly journals Cosmographic Parameters in Model-independent Approaches

2021 ◽  
Vol 923 (2) ◽  
pp. 274
Author(s):  
Ahmad Mehrabi ◽  
Mehdi Rezaei
Keyword(s):  
High Redshift ◽  
Data Sets ◽  
Ongoing Debate ◽  
Technical Problems ◽  
Advantages And Disadvantages ◽  
Type Ia ◽  
Cosmological Data ◽  
Model Independent ◽  
Ia Supernovae ◽  
The Universe

Abstract The cosmographic approach, a Taylor expansion of the Hubble function, has been used as a model-independent method to investigate the evolution of the universe in the presence of cosmological data. Apart from possible technical problems like the radius of convergence, there is an ongoing debate about the tensions that appear when one investigates some high-redshift cosmological data. In this work, we consider two common data sets, namely, Type Ia supernovae (Pantheon sample) and the Hubble data, to investigate advantages and disadvantages of the cosmographic approach. To do this, we obtain the evolution of cosmographic functions using the cosmographic method, as well as two other well-known model-independent approaches, namely, the Gaussian process and the genetic algorithm. We also assume a ΛCDM model as the concordance model to compare the results of mentioned approaches. Our results indicate that the results of cosmography compared with the other approaches are not exact enough. Considering the Hubble data, which are less certain, the results of q 0 and j 0 obtained in cosmography provide a tension at more than 3σ away from the best result of ΛCDM. Assuming both of the data samples in different approaches, we show that the cosmographic approach, because it provides some biased results, is not the best approach for reconstruction of cosmographic functions, especially at higher redshifts.

Quantitative Conditions and Convergent Operations That Are Compatible With the Universe as a Persistent Generator of Successive Simulations

2016 ◽  
Vol 12 (1) ◽  
pp. 4178-4187
Author(s):  
Michael A Persinger ◽  
Stanley A Koren
Keyword(s):  
Structural Organization ◽  
Total Duration ◽  
Human Cognition ◽  
Intrinsic Properties ◽  
Basic Assumptions ◽  
Electron Orbit ◽  
Quantitative Analyses ◽  
Massive Information ◽  
The Universe

                The capacity for computer-like simulations to be generated by massive information processing from electron-spin potentials supports Bostrom’s hypothesis that matter and human cognition might reflect simulations. Quantitative analyses of the basic assumptions indicate the universe may display properties of a simulation where photons behave as pixels and gravitons control the structural organization. The Lorentz solution for the square of the light and entanglement velocities converges with the duration of a single electron orbit that ultimately defines properties of matter. The approximately one trillion potential states within the same space with respect to the final epoch of the universe indicate that a different simulation, each with intrinsic properties, has been and will be generated as a type of tractrix defined by ±2 to 3 days (total duration 5 to 6 days). It may define the causal limits within a simulation. Because of the intrinsic role of photons as the pixel unit, phenomena within which flux densities are enhanced, such as human cognition (particularly dreaming) and the cerebral regions associated with those functions, create the conditions for entanglement or excess correlations between contiguous simulations. The consistent quantitative convergence of operations indicates potential validity for this approach. The emergent solutions offer alternative explanations for the limits of predictions for multivariate phenomena that could be coupled to more distal simulations.

Laws of Nature

2018 ◽  
Keyword(s):  
Philosophy Of Science ◽  
Laws Of Nature ◽  
Current Debate ◽  
Scientific Theories ◽  
Law Of Nature ◽  
First Time ◽  
The Universe

The concept of a law of nature, while familiar, is deeply puzzling. Theorists such as Descartes think a divine being governs the universe according to the laws which follow from that being’s own nature. Newton detaches the concept from theology and is agnostic about the ontology underlying the laws of nature. Some later philosophers treat laws as summaries of events or tools for understanding and explanation, or identify the laws with principles and equations fundamental to scientific theories. In the first part of this volume, essays from leading historians of philosophy identify central questions: are laws independent of the things they govern, or do they emanate from the powers of bodies? Are the laws responsible for the patterns we see in nature, or should they be collapsed into those patterns? In the second part, contributors at the forefront of current debate evaluate the role of laws in contemporary Best System, perspectival, Kantian, and powers- or mechanisms-based approaches. These essays take up pressing questions about whether the laws of nature can be consistent with contingency, whether laws are based on the invariants of scientific theories, and how to deal with exceptions to laws. These twelve essays, published here for the first time, will be required reading for anyone interested in metaphysics, philosophy of science, and the histories of these disciplines.

scholarly journals Dust Production around Carbon-Rich Stars: The Role of Metallicity

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 233
Author(s):  
Ambra Nanni ◽  
Sergio Cristallo ◽  
Jacco Th. van Loon ◽  
Martin A. T. Groenewegen
Keyword(s):  
Wind Speed ◽  
Galactic Halo ◽  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Chemical Enrichment ◽  
Wide Range ◽  
The Universe

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates.

scholarly journals Omega vs. pi, and 6d anomaly cancellation

2021 ◽  
Vol 2021 (5) ◽  
Author(s):  
Joe Davighi ◽  
Nakarin Lohitsiri
Keyword(s):  
Gauge Theories ◽  
Necessary Conditions ◽  
Homotopy Groups ◽  
Mapping Torus ◽  
Anomaly Cancellation ◽  
Global Anomaly ◽  
Gauge Anomalies ◽  
Local Anomalies ◽  
Dimensional Mapping

Abstract In this note we review the role of homotopy groups in determining non-perturbative (henceforth ‘global’) gauge anomalies, in light of recent progress understanding global anomalies using bordism. We explain why non-vanishing of πd(G) is neither a necessary nor a sufficient condition for there being a possible global anomaly in a d-dimensional chiral gauge theory with gauge group G. To showcase the failure of sufficiency, we revisit ‘global anomalies’ that have been previously studied in 6d gauge theories with G = SU(2), SU(3), or G2. Even though π6(G) ≠ 0, the bordism groups $$ {\Omega}_7^{\mathrm{Spin}}(BG) $$ Ω 7 Spin BG vanish in all three cases, implying there are no global anomalies. In the case of G = SU(2) we carefully scrutinize the role of homotopy, and explain why any 7-dimensional mapping torus must be trivial from the bordism perspective. In all these 6d examples, the conditions previously thought to be necessary for global anomaly cancellation are in fact necessary conditions for the local anomalies to vanish.

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