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 Extended gravity cosmography

2019 ◽  
Vol 28 (10) ◽  
pp. 1930016 ◽  
Author(s):  
Salvatore Capozziello ◽  
Rocco D’Agostino ◽  
Orlando Luongo
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
A Priori ◽  
Late Time ◽  
Extended Gravity ◽  
Energy Models ◽  
Novel Approach ◽  
History Of ◽  
Theories Of Gravity ◽  
Model Independent

Cosmography can be considered as a sort of a model-independent approach to tackle the dark energy/modified gravity problem. In this review, the success and the shortcomings of the [Formula: see text]CDM model, based on General Relativity (GR) and standard model of particles, are discussed in view of the most recent observational constraints. The motivations for considering extensions and modifications of GR are taken into account, with particular attention to [Formula: see text] and [Formula: see text] theories of gravity where dynamics is represented by curvature or torsion field, respectively. The features of [Formula: see text] models are explored in metric and Palatini formalisms. We discuss the connection between [Formula: see text] gravity and scalar–tensor theories highlighting the role of conformal transformations in the Einstein and Jordan frames. Cosmological dynamics of [Formula: see text] models is investigated through the corresponding viability criteria. Afterwards, the equivalent formulation of GR (Teleparallel Equivalent General Relativity (TEGR)) in terms of torsion and its extension to [Formula: see text] gravity is considered. Finally, the cosmographic method is adopted to break the degeneracy among dark energy models. A novel approach, built upon rational Padé and Chebyshev polynomials, is proposed to overcome limits of standard cosmography based on Taylor expansion. The approach provides accurate model-independent approximations of the Hubble flow. Numerical analyses, based on Monte Carlo Markov Chain integration of cosmic data, are presented to bound coefficients of the cosmographic series. These techniques are thus applied to reconstruct [Formula: see text] and [Formula: see text] functions and to frame the late-time expansion history of the universe with no a priori assumptions on its equation-of-state. A comparison between the [Formula: see text]CDM cosmological model with [Formula: see text] and [Formula: see text] models is reported.

scholarly journals Remarks on cosmology

1959 ◽  
Vol 9 ◽  
pp. 533-535
Author(s):  
G. C. Mcvittie
Keyword(s):  
A Priori ◽  
Initial Instant ◽  
De Sitter ◽  
Relativistic Model ◽  
Space Curvature ◽  
Composite Models ◽  
History Of ◽  
Sitter Model ◽  
The Universe ◽  
De Sitter Model

First, I should like to say something about the use of highly specialized models in cosmology. The Einstein—de Sitter model is a relativistic model in which the cosmical constant and the space-curvature constant are both equated to zero. Likewise, the pressure is assumed to be zero throughout the history of the universe, except perhaps at the initial instant. It is well-known that the first two constants can be determined from observation, if not at present, at any rate as the data are refined in the future. Hence, I think it is a weakness to prejudge the issue and assign a priori values. Nor is it self-evident to me that the pressure must always have been zero even if it is zero now. Composite models, with nonzero pressure at first, followed by a zero-pressure condition, need to be examined.

COSMOGRAPHY WITH THE HUBBLE PARAMETER

2011 ◽  
Vol 26 (20) ◽  
pp. 1459-1466 ◽  
Author(s):  
ORLANDO LUONGO
Keyword(s):  
Hubble Parameter ◽  
Free Parameter ◽  
A Priori ◽  
Matter Density ◽  
Cosmological Constraints ◽  
Fitting Procedure ◽  
Model Independent ◽  
The Universe ◽  
Friedmann Robertson Walker

In this paper we investigate the possibility to obtain constraints on the kinematics of the Universe in a flat Friedmann–Robertson–Walker cosmology, through the use of the so-called Cosmography. The basic idea lies directly on fitting the H(z) series, by adopting a more recent dataset of H(z), in the range z≼1.8, obtaining the limits of the kinematical quantities under exam. The advantage that we propose here is that this fitting procedure is model independent and it does not need any assumption given a priori on the cosmology of the Universe, but only its geometry and flatness. Moreover, as an example, we relate the measured cosmographic set to the free parameter of ΛCDM, being the matter density Ωm. In fact, by inverting Ωm in terms of the cosmographic set, it would be possible to infer limits on it. We find results in agreement with other kind of cosmological constraints.

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 Primordial nucleosynthesis constraints on high-z energy releases

2020 ◽  
Vol 499 (4) ◽  
pp. 5653-5655
Author(s):  
Gianfranco De Zotti ◽  
Matteo Bonato
Keyword(s):  
Energy Density ◽  
Cosmic Microwave Background ◽  
Thermal History ◽  
Microwave Background ◽  
Primordial Nucleosynthesis ◽  
Specific Entropy ◽  
History Of ◽  
Model Independent ◽  
Electron Photon ◽  
The Universe

ABSTRACT The cosmic microwave background (CMB) spectrum provides tight constraints on the thermal history of the universe up to z ∼ 2 × 106. At higher redshifts, thermalization processes become very efficient so that even large energy releases do not leave visible imprints in the CMB spectrum. In this paper, we show that the consistency between the accurate determinations of the specific entropy at primordial nucleosynthesis and at the electron–photon decoupling implies that no more than 7.8 per cent of the present-day CMB energy density could have been released in the post-nucleosynthesis era. As pointed out by previous studies, primordial nucleosynthesis complements model independent constraints provided by the CMB spectrum, extending them by two orders of magnitude in redshift.

scholarly journals Defying the laws of gravity I: model-independent reconstruction of the Universe expansion from growth data

2020 ◽  
Vol 494 (1) ◽  
pp. 819-826 ◽  
Author(s):  
Benjamin L’Huillier ◽  
Arman Shafieloo ◽  
David Polarski ◽  
Alexei A Starobinsky
Keyword(s):  
Type Ia Supernovae ◽  
Growth Data ◽  
Growth History ◽  
Type Ia ◽  
History Of ◽  
Space Distortion ◽  
Model Independent ◽  
Ia Supernovae ◽  
The Universe

ABSTRACT Using redshift space distortion data, we perform model-independent reconstructions of the growth history of matter inhomogeneity in the expanding Universe using two methods: crossing statistics and Gaussian processes. We then reconstruct the corresponding history of the Universe background expansion and fit it to Type Ia supernovae data, putting constraints on (Ωm, 0, σ8, 0). The results obtained are consistent with the concordance flat-ΛCDM model and General Relativity as the gravity theory given the current quality of the inhomogeneity growth data.

scholarly journals Model-independent reconstruction of the expansion history of the Universe from Type Ia supernovae

2011 ◽  
Vol 419 (1) ◽  
pp. 513-521 ◽  
Author(s):  
S. Benitez-Herrera ◽  
F. Röpke ◽  
W. Hillebrandt ◽  
C. Mignone ◽  
M. Bartelmann ◽  
...  
Keyword(s):  
Type Ia Supernovae ◽  
Type Ia ◽  
History Of ◽  
Model Independent ◽  
Ia Supernovae ◽  
The Universe
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