Cosmology

2021 ◽  
pp. 319-352
Author(s):  
Andrew M. Steane
Keyword(s):  
Proper Time ◽  
Recombination Process ◽  
Distance Measures ◽  
Angular Diameter ◽  
Accurate Calculation ◽  
Saha Equation ◽  
Cosmic Expansion ◽  
The Universe

The main features of the universe and its history, and the application of GR to the universe as a whole are presented. The observed near-isotropy and homogeneity of the universe are described, along with a survey of its history. The Saha equation is applied to the recombination process. Cosmic proper time and comoving coordinates are defined, and the form of the metric (Friedmann-Lemaitre-Robertson-Walker) applicable to such a universe is obtained. The main features of the resulting geometry are discussed at length, with a view to both accurate calculation and sound intuition. Redshift and the cosmic expansion are described from several perspectives. Distance measures (luminosity, angular diameter) are defined and the main elements of the observational cosmic distance ladder are outlined.

scholarly journals Can accelerated expansion of the universe be due to spacetime vorticity?

2018 ◽  
Vol 33 (40) ◽  
pp. 1850240
Author(s):  
Babur M. Mirza
Keyword(s):  
Magnetic Field ◽  
Accelerated Expansion ◽  
Newtonian Gravity ◽  
Hubble’S Parameter ◽  
Cosmic Expansion ◽  
Zero Curvature ◽  
Expansion Of The Universe ◽  
General Relativistic ◽  
The Universe ◽  
The Magnetic Field

We present here a general relativistic mechanism for accelerated cosmic expansion and the Hubble’s parameter. It is shown that spacetime vorticity coupled to the magnetic field density in galaxies causes the galaxies to recede from one another at a rate equal to the Hubble’s constant. We therefore predict an oscillatory universe, with zero curvature, without assuming violation of Newtonian gravity at large distances or invoking dark energy/dark matter hypotheses. The value of the Hubble’s constant, along with the scale of expansion, as well as the high isotropy of CMB radiation are deduced from the model.

scholarly journals The failure of testing for cosmic opacity via the distance-duality relation

2020 ◽  
Vol 497 (1) ◽  
pp. 378-388
Author(s):  
Václav Vavryčuk ◽  
Pavel Kroupa
Keyword(s):  
Cosmological Model ◽  
Convincing Evidence ◽  
Duality Relation ◽  
Angular Diameter ◽  
Luminosity Distance ◽  
Angular Diameter Distance ◽  
Frequency Independent ◽  
Unique Interpretation ◽  
The Universe

ABSTRACT The distance-duality relation (DDR) between the luminosity distance DL and the angular diameter distance DA is viewed as a powerful tool for testing for the opacity of the Universe, being independent of any cosmological model. It was applied by many authors, who mostly confirm its validity and report a negligible opacity of the Universe. Nevertheless, a thorough analysis reveals that applying the DDR in cosmic opacity tests is tricky. Its applicability is strongly limited because of a non-unique interpretation of the DL data in terms of cosmic opacity and a rather low accuracy and deficient extent of currently available DA data. Moreover, authors usually assume that cosmic opacity is frequency independent and parametrize it in their tests by a prescribed phenomenological function. In this way, they only prove that cosmic opacity does not follow their assumptions. As a consequence, no convincing evidence of transparency of the universe using the DDR has so far been presented.

Cosmology: evidence for a ‘big bang’

1994 ◽  
Vol 2 (2) ◽  
pp. 155-164
Author(s):  
Martin J. Rees
Keyword(s):  
Present State ◽  
Background Radiation ◽  
Cosmic Background Radiation ◽  
Big Bang ◽  
Fundamental Physics ◽  
Cosmic Expansion ◽  
Cosmic Background ◽  
The Universe

During the last 25 years, evidence has accumulated that our universe has evolved, over a period of 10–15 billion years, from a hot dense fireball to its present state. Telescopes can detect objects so far away that the universe had only a tenth its present age when the light we now receive set out towards us. The cosmic background radiation, and the abundances of elements such as helium and lithium, permit quantitative inferences about what the universe was like when it had been expanding for only a few seconds. The laws of physics established in the laboratory apparently suffice for interpreting all astronomical phenomena back to that time. In the initial instants of cosmic expansion, however, the particle energies and densities were so extreme that terrestrial experiments offer no firm guidance. We will not understand why the universe contains the observed ‘mix’ of matter and radiation, nor why it is expanding in the observed fashion, without further progress in fundamental physics.

scholarly journals Comment on “Hubble flow variations as a test for inhomogeneous cosmology”

2019 ◽  
Vol 624 ◽  
pp. A12
Author(s):  
David L. Wiltshire
Keyword(s):  
Alternative Model ◽  
Inhomogeneous Cosmology ◽  
Cosmic Expansion ◽  
Standard Cosmology ◽  
Geometrical Features ◽  
Expansion Of The Universe ◽  
Expansion Model ◽  
Differential Expansion ◽  
The Universe ◽  
Hubble Flow

Saulder et al. (2019, A&A, 622, A83) have performed a novel observational test of the local expansion of the Universe for the standard cosmology as compared to an alternative model with differential cosmic expansion. Their analysis employs mock galaxy samples from the Millennium Simulation, a Newtonian N–body simulation on a ΛCDM background. For the differential expansion case the simulation has been deformed in an attempt to incorporate features of a particular inhomogeneous cosmology: the timescape model. It is shown that key geometrical features of the timescape cosmology have been omitted in this rescaling. Consequently, the differential expansion model tested by Saulder et al. (2019) cannot be considered to approximate the timescape cosmology.

scholarly journals Holographic cosmology from BIonic solutions

2017 ◽  
Vol 32 (05) ◽  
pp. 1750025
Author(s):  
Alireza Sepehri ◽  
Mir Faizal ◽  
Mohammad Reza Setare ◽  
Ahmed Farag Ali
Keyword(s):  
Holographic Principle ◽  
Recent Proposal ◽  
Cosmic Expansion ◽  
Acceleration Of The Universe ◽  
Black Strings ◽  
The Universe

In this paper, we will use a BIonic solution for analyzing the holographic cosmology. A BIonic solution is a configuration of a D3-brane and an anti-D3-brane connected by a wormhole, and holographic cosmology is a recent proposal to explain cosmic expansion by using the holographic principle. In our model, a BIonic configuration will be produced by the transition of fundamental black strings. The formation of a BIonic configuration will cause inflation. As the D3-brane moves away from the anti-D3-brane, the wormhole will get annihilated, and the inflation will end with the annihilation of this wormhole. However, it is possible for a D3-brane to collide with an anti-D3-brane. Such a collision will occur if the distance between the D3-brane and the anti-D3-brane reduces, and this will create tachyonic states. We will demonstrate that these tachyonic states will lead to the formation of a new wormhole, and this will cause acceleration of the universe before such a collision.

scholarly journals Long-term implications of observing an expanding cosmological civilization

2017 ◽  
Vol 17 (1) ◽  
pp. 87-95 ◽  
Author(s):  
S. Jay Olson
Keyword(s):  
Angular Separation ◽  
Separation Distance ◽  
Speed Limit ◽  
Single Observation ◽  
Cosmic Expansion ◽  
Cosmological Distance ◽  
First Mover Advantage ◽  
Expansion Speed ◽  
The Universe

AbstractSuppose that advanced civilizations, separated by a cosmological distance and time, wish to maximize their access to cosmic resources by rapidly expanding into the universe. How does the presence of one limit the expansionistic ambitions of another, and what sort of boundary forms between their expanding domains? We describe a general scenario for any expansion speed, separation distance and time. We then specialize to a question of particular interest: What are the future prospects for a young and ambitious civilization if they can observe the presence of another at a cosmological distance? We treat cases involving the observation of one or two expanding domains. In the single-observation case, we find that almost any plausible detection will limit one's future cosmic expansion to some extent. Also, practical technological limits to expansion speed (well below the speed of light) play an interesting role. If a domain is visible at the time one embarks on cosmic expansion, higher practical limits to expansion speed are beneficial only up to a certain point. Beyond this point, a higher speed limit means that gains in the ability to expand are more than offset by the first-mover advantage of the observed domain. In the case of two visible domains, it is possible to be ‘trapped’ by them if the practical speed limit is high enough and their angular separation in the sky is large enough, i.e. one's expansion in any direction will terminate at a boundary with the two visible civilizations. Detection at an extreme cosmological distance has surprisingly little mitigating effect on our conclusions.

scholarly journals Thoughts on the vacuum energy in the quantum N-portrait

2015 ◽  
Vol 30 (36) ◽  
pp. 1550197 ◽  
Author(s):  
Florian Kühnel
Keyword(s):  
Phase Transition ◽  
Critical Point ◽  
Quantum Phase Transition ◽  
Vacuum Energy ◽  
Einstein Condensate ◽  
Spacetime Geometry ◽  
Cosmic Expansion ◽  
Bose Einstein Condensate ◽  
Bose Einstein ◽  
The Universe

An application of the quantum N-portrait to the Universe is discussed, wherein the spacetime geometry is understood as a Bose–Einstein condensate of N soft gravitons. If near or at the critical point of a quantum phase transition, indications are found that the vacuum energy is partly suppressed by 1/N, as being due to quanta not in the condensate state. Time evolution decreases this suppression, which might have implications for cosmic expansion.

scholarly journals Study of growth matter index and cosmic implications in dynamical Chern–Simons modified gravity

2021 ◽  
Vol 81 (8) ◽  
Author(s):  
Abdul Jawad ◽  
Shamaila Rani ◽  
Saba Qummer ◽  
Amir Sharif
Keyword(s):  
Dark Energy ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Growth Index ◽  
Simons Theory ◽  
Cosmic Expansion ◽  
Acceleration Of The Universe ◽  
Expansion Of The Universe ◽  
Chern Simons ◽  
The Universe

AbstractIn the framework of dynamical Chern–Simons theory of gravity, we study the recent cosmic expansion with acceleration of the universe. We take interacting scenario of dark energy and dark matter with three cutoffs like Granda–Oliveros, higher order derivative of Hubble parameter and generalized holographic dark energy model. In the flat Friedmann–Robertson–Walker universe, well-known cosmological parameters have been calculated. We study the perturbation of matter density growth, growth factor and growth index behavior for the underlying framework. The cosmological parameters like equation of state parameter, deceleration parameter, and stability of each model are discussed. The outcomes of these models represent the cosmic expansion of the universe with acceleration.

Realistic cosmological measurement of distances in the Friedmann universe

2016 ◽  
Vol 31 (02n03) ◽  
pp. 1641013 ◽  
Author(s):  
Aleksei Nikolaev ◽  
Sergey Chervon
Keyword(s):  
Measurement Method ◽  
Small Deviation ◽  
Duality Relation ◽  
Angular Diameter ◽  
Accelerated Expansion ◽  
Friedmann Universe ◽  
Angular Diameter Distance ◽  
Light Rays ◽  
Expansion Of The Universe ◽  
The Universe

We consider application of our development of Zeldovich’s ideas, presented in Ref. 1, for measurement of the cosmological angular diameter distance (ADD) in the Friedmann Universe. To make a comparison with [Formula: see text]CDM we analyze ADD measurement in [Formula: see text]CDM model responsible for the later inflation (present accelerated expansion of the Universe). We also analyze a small deviation from equality in the distance duality relation induced by the fullness (by matter) of the cone of light rays (CLR) which is used for the ADD measurement method.

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