scholarly journals Coupled dark energy with perturbed Hubble expansion rate

2014 ◽  
Vol 90 (8) ◽  
Author(s):  
Weiqiang Yang ◽  
Lixin Xu
Keyword(s):  
Dark Energy ◽  
Expansion Rate ◽  
Hubble Expansion

scholarly journals Do supernovae indicate an accelerating universe?

2021 ◽  
Author(s):  
Roya Mohayaee ◽  
Mohamed Rameez ◽  
Subir Sarkar
Keyword(s):  
Dark Energy ◽  
Large Scale ◽  
Bulk Flow ◽  
Expansion Rate ◽  
Accelerated Expansion ◽  
Accelerating Universe ◽  
Acoustic Oscillations ◽  
Independent Evidence ◽  
Peculiar Velocity ◽  
Hubble Expansion

AbstractIn the late 1990’s, observations of two directionally-skewed samples of, in total, 93 Type Ia supernovae were analysed in the framework of the Friedmann–Lemaître–Robertson–Walker (FLRW) cosmology. Assuming these to be ‘standard(isable) candles’ it was inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant $$\varLambda $$ Λ in Einstein’s theory of gravity. This is still the only direct evidence for the ‘dark energy’ that is the dominant component of today’s standard $$\varLambda $$ Λ CDM cosmological model. Other data such as baryon acoustic oscillations (BAO) in the large-scale distribution of galaxies, temperature fluctuations in the cosmic microwave background (CMB), measurement of stellar ages, the rate of growth of structure, etc are all ‘concordant’ with this model but do not provide independent evidence for accelerated expansion. The recent discussions about whether the inferred acceleration is real rests on analysis of a larger sample of 740 SNe Ia which shows that these are not quite standard candles, and more importantly highlights the ‘corrections’ that are applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are carried out in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution which has grown under gravity from primordial density perturbations traced by the CMB fluctuations. The $$\varLambda $$ Λ CDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local ‘bulk flow’ are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover, the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at $$4.9\sigma $$ 4.9 σ . Hence the peculiar velocity corrections employed in supernova cosmology are inconsistent and discontinuous within the data. The acceleration of the Hubble expansion rate is in fact anisotropic at $$3.9\sigma $$ 3.9 σ and aligned with the bulk flow. Thus dark energy could be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses.

Ghost dark energy in Rastall theory

2021 ◽  
pp. 2150090
Author(s):  
E. E. Kangal ◽  
M. Salti ◽  
O. Aydogdu
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Text Analysis ◽  
Functional Form ◽  
Expansion Rate ◽  
Dark Energy Density ◽  
Cosmic Expansion ◽  
Astrophysical Data ◽  
Ghost Dark Energy ◽  
History Of

Making use of the generalized form of the Ghost dark energy density, which has the functional form [Formula: see text] where [Formula: see text] represents the Hubble expanding rate, the present accelerated enlargement behavior of the cosmos is investigated from the Rastall theory perspective. After finding a relation for the Hubble cosmic expansion rate, we consider recent cosmology-independent measurements calculated for the expansion history of the cosmos to fit the model via the [Formula: see text]-analysis. Moreover, we discuss the cosmographic properties of the model with the help of some cosmological quantities. We show that our model is stable and consistent with the recent astrophysical data. Also, for our model, we investigate cosmological interpretations of thermodynamics.

scholarly journals ΛCDM periodic cosmology

2020 ◽  
Vol 494 (2) ◽  
pp. 2183-2190
Author(s):  
Stéphane Fay
Keyword(s):  
Dark Energy ◽  
Cold Dark Matter ◽  
Accelerated Expansion ◽  
Density Parameter ◽  
Acoustic Oscillations ◽  
Microwave Background ◽  
Hubble Expansion ◽  
Background Data ◽  
Periodic Models ◽  
Cosmic Microwave Background Data

ABSTRACT We examine the possibility that Universe expansion be made of some Λ-cold dark matter (ΛCDM) expansions repeating periodically, separated by some inflation- and radiation-dominated phases. This so-called ΛCDM periodic cosmology is motivated by the possibility that inflation and the present phase of accelerated expansion be due to the same dark energy. Then, in a phase space showing the variation of matter density parameter Ωm with respect to this of the radiation Ωr, the curve Ωm(Ωr) looks like a closed trajectory that Universe could run through forever. In this case, the end of the expansion acceleration of the ΛCDM phase is the beginning of a new inflation phase. We show that such a scenario implies the coupling of matter and/or radiation to dark energy. We consider the simplest of these ΛCDM periodic models i.e. a vacuum energy coupled to radiation. From matter domination phase to today, it behaves like a ΛCDM model, then followed by an inflation phase. But a sudden and fast decay of the dark energy into radiation periodically ends the expansion acceleration. This leads to a radiation-dominated Universe preceding a new ΛCDM type expansion. The model is constrained with Markov Chain Monte Carlo simulations using supernovae, Hubble expansion, Baryon Acoustic Oscillations (BAO), and cosmic microwave background data and fits the data as well as the ΛCDM one.

OBSERVATIONAL COSMOLOGY 2004: WHAT THE DATA SAY ABOUT DARK ENERGY AND THE CONTENTS OF THE UNIVERSE

2005 ◽  
Vol 20 (14) ◽  
pp. 2931-2942
Author(s):  
JOSEPH FOWLER
Keyword(s):  
Dark Energy ◽  
Background Radiation ◽  
Current Data ◽  
Observational Cosmology ◽  
Microwave Background ◽  
Hubble Expansion ◽  
Microwave Background Radiation ◽  
Expansion Of The Universe ◽  
Cosmological Data ◽  
The Universe

The latest cosmological data point to a model of the universe that is self-consistent but deeply weird. It seems that most matter in our universe is non-baryonic and hidden from direct view. Meanwhile, a repulsive "dark energy" causes the expansion of the universe to proceed at an accelerating rate. Sources of current data include studies of the distribution of matter in the universe, the anisotropies of the cosmic microwave background radiation, and the Hubble expansion law as probed by distant supernovae. In the near future, we can hope that measurements like these will begin to illuminate the nature of dark energy, starting with the question of whether it behaves like a cosmological constant or shows a more complicated evolution.

scholarly journals VISCOUS MODIFIED COSMIC CHAPLYGIN GAS COSMOLOGY

2013 ◽  
Vol 22 (09) ◽  
pp. 1350061 ◽  
Author(s):  
B. POURHASSAN
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Nonlinear Equation ◽  
Observational Data ◽  
Function Method ◽  
Chaplygin Gas ◽  
Time Dependent ◽  
Dark Energy Density ◽  
Hubble Expansion ◽  
Exponential Function Method

In this paper, we construct viscous modified cosmic Chaplygin gas as a model of dark energy. We use exponential function method to solve nonlinear equation and obtain time-dependent dark energy density. Then, we discuss Hubble expansion parameter and scale factor and fix them by using observational data. Effect of viscosity to the evolution of Universe is investigated. We also investigate stability of this theory.

scholarly journals Dark Matter Around the Local Group?

1988 ◽  
Vol 130 ◽  
pp. 585-585
Author(s):  
Edmond Giraud
Keyword(s):  
Dark Matter ◽  
Velocity Field ◽  
Velocity Distribution ◽  
Local Group ◽  
Spiral Galaxies ◽  
Small Distance ◽  
Expansion Rate ◽  
Fall Velocity ◽  
Hubble Expansion ◽  
Local Value

The Hubble expansion rate measured in the short distance scale varies from 70–75 to 90 km s−1 Mpc−1 as the kinematic distance (corrected for in fall velocity toward Virgo) increases from Dv = 200–400 km s−1 to Dv ∼ 1300 km s−1. It should be observed in the long scale as well (starting from a lower value), if the same methods were used in the same way. The Malmquist bias for spiral galaxies in the range Dv ≤ 1300 km s−1 is very small or null. The velocity distribution of galaxies in the nearest groups compared with models of various rms velocity dispersions, suggests that at small distance, dispersions of 100 km s−1 or more do not fit the observations. The effect of the deceleration due to the mass of the Local Group on the very nearby velocity field is negligible beyond 2.5–3 Mpc. The low local value of Ho extends approximately over ∼ 6–7 Mpc.

Dark Matter

2021 ◽  
pp. 66-78
Author(s):  
Gianfranco Bertone
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Gravitational Waves ◽  
Expansion Rate ◽  
The Earth ◽  
Modern Cosmology ◽  
Large Effort ◽  
The Universe

I introduce here the problem of dark energy, a substance that appears to be pushing the Universe to expand ever faster and discuss the large effort currently in place to understand its origin. I describe the surprising recent discovery of a widening crack in the cathedral of modern cosmology arising from the measurement of the expansion rate of the Universe. And I argue that gravitational waves observations can help us to either repair that crack, or to bring down that magnificent building, in case it turns out to be fatally flawed. Before all women and all men. Before animals, plants, archaeans, bacteria. Before the Earth was formed and the stars were lit. Before everything we know, the Universe was immersed in an amorphous and oblivious darkness.

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