scholarly journals The cosmological constant as a zero action boundary

2021 ◽  
Vol 502 (1) ◽  
pp. 436-444
Author(s):  
Enrique Gaztañaga
Keyword(s):  
Cosmological Constant ◽  
Modified Gravity ◽  
Cosmological Parameters ◽  
Average Density ◽  
Matter Density ◽  
Cosmic Acceleration ◽  
Perfect Fluid Solution ◽  
Cosmic Expansion ◽  
Causal Boundary ◽  
Boundary Force

ABSTRACT The cosmological constant Λ is usually interpreted as Dark Energy (DE) or modified gravity (MG). Here, we propose instead that Λ corresponds to a boundary term in the action of classical General Relativity. The action is zero for a perfect fluid solution and this fixes Λ to the average density ρ and pressure p inside a primordial causal boundary: Λ = 4πG <ρ+3p >. This explains both why the observed value of Λ is related to the matter density today and also why other contributions to Λ, such as DE or MG, do not produce cosmic expansion. Cosmic acceleration results from the repulsive boundary force that occurs when the expansion reaches the causal horizon. This universe is similar to the ΛCDM universe, except on the largest observable scales, where we expect departures from homogeneity/isotropy, such as CMB anomalies and variations in cosmological parameters indicated by recent observations.

scholarly journals The size of our causal Universe

2020 ◽  
Vol 494 (2) ◽  
pp. 2766-2772 ◽  
Author(s):  
Enrique Gaztañaga
Keyword(s):  
Boundary Condition ◽  
Cosmological Constant ◽  
Modified Gravity ◽  
Vacuum Energy ◽  
Cosmological Parameters ◽  
Cosmic Acceleration ◽  
Observable Universe ◽  
Local Measurements ◽  
Primordial Inflation ◽  
Or Modelling

ABSTRACT A Universe with finite age also has a finite causal scale. Larger scales cannot affect our local measurements or modelling, but far away locations could have different cosmological parameters. The size of our causal Universe depends on the details of inflation and is usually assumed to be larger than our observable Universe today. To account for causality, we propose a new boundary condition, that can be fulfill by fixing the cosmological constant (a free geometric parameter of gravity). This forces a cancellation of vacuum energy with the cosmological constant. As a consequence, the measured cosmic acceleration cannot be explained by a simple cosmological constant or constant vacuum energy. We need some additional odd properties such as the existence of evolving dark energy (DE) with energy-density fine tuned to be twice that of dark matter today. We show here that we can instead explain the current cosmic acceleration without DE (or modified gravity) as a the result of a primordial inflation with a causal scale smaller than the observable Universe today. Such scale corresponds to half the sky at z = 1 and 60 deg at z= 1100, which is consistent with the anomalous lack of correlations observed in the CMB.

scholarly journals Turnaround radius of galaxy clusters in N-body simulations

2020 ◽  
Vol 639 ◽  
pp. A122 ◽  
Author(s):  
Giorgos Korkidis ◽  
Vasiliki Pavlidou ◽  
Konstantinos Tassis ◽  
Evangelia Ntormousi ◽  
Theodore N. Tomaras ◽  
...  
Keyword(s):  
Dark Matter ◽  
Galaxy Clusters ◽  
Cosmological Parameters ◽  
Three Dimensional ◽  
Average Density ◽  
Matter Density ◽  
Tidal Forces ◽  
Spherical Collapse ◽  
Collapse Model ◽  
Model C

Aims. We use N-body simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a ΛCDM Universe, can be meaningfully identified for galaxy clusters in the presence of full three-dimensional effects. Methods. We use The Dark Sky Simulations and Illustris-TNG dark-matter-only cosmological runs to calculate radial velocity profiles around collapsed structures, extending out to many times the virial radius R200. There, the turnaround radius can be unambiguously identified as the largest nonexpanding scale around a center of gravity. Results. We find that: (a) a single turnaround scale can meaningfully describe strongly nonspherical structures. (b) For halos of masses M200 >  1013 M⊙, the turnaround radius Rta scales with the enclosed mass Mta as Mta1/3, as predicted by the spherical collapse model. (c) The deviation of Rta in simulated halos from the spherical collapse model prediction is relatively insensitive to halo asphericity. Rather, it is sensitive to the tidal forces due to massive neighbors when these are present. (d) Halos exhibit a characteristic average density within the turnaround scale. This characteristic density is dependent on cosmology and redshift. For the present cosmic epoch and for concordance cosmological parameters (Ωm ∼ 0.3; ΩΛ ∼ 0.7) turnaround structures exhibit a density contrast with the matter density of the background Universe of δ ∼ 11. Thus, Rta is equivalent to R11 – in a way that is analogous to defining the “virial” radius as R200 – with the advantage that R11 is shown in this work to correspond to a kinematically relevant scale in N-body simulations.

scholarly journals Turnaround density as a probe of the cosmological constant

2020 ◽  
Vol 638 ◽  
pp. L8 ◽  
Author(s):  
Vasiliki Pavlidou ◽  
Giorgos Korkidis ◽  
Theodore N. Tomaras ◽  
Dimitrios Tanoglidis
Keyword(s):  
Cosmological Constant ◽  
Galaxy Clusters ◽  
Cosmological Parameters ◽  
Matter Density ◽  
New Method ◽  
Local Constraint ◽  
Spherical Collapse ◽  
Hubble Flow

Spherical collapse predicts that a single value of the turnaround density, meaning the average matter density within the scale on which a structure detaches from the Hubble flow, characterizes all cosmic structures at the same redshift. It was recently shown by Korkidis et al. that this feature persists in complex non-spherical galaxy clusters that have been identified in N-body simulations. Here we show that the low-redshift evolution of the turnaround density constrains the cosmological parameters and it can be used to derive a local constraint on ΩΛ, 0 alone, independent of Ωm, 0. The turnaround density thus offers a promising new method for exploiting upcoming large cosmological datasets.

scholarly journals Gamma and Beta Model of Growing and Rotating Planck Ball

2019 ◽  
Author(s):  
Satya Seshavatharam U.V. ◽  
S. Lakshminarayana
Keyword(s):  
Angular Velocity ◽  
Large Scale ◽  
Density Ratio ◽  
Planck Scale ◽  
Matter Density ◽  
Cosmic Acceleration ◽  
Expansion Velocity ◽  
Cosmic Expansion ◽  
Scale Ratio ◽  
Total Matter

With reference to Planck scale, Mach’s relation, increasing support for large scale cosmic anisotropy & preferred directions and by introducing two new parameters Gamma and Beta, right from the beginning of Planck scale, we make an attempt to estimate ordinary matter density ratio, dark matter density ratio, mass, radius, temperature, age and expansion velocity (from and about the Planck mass in all directions). By considering H 0 ≅70 km/sec/Mpc, estimated current cosmic mass, radius, total matter density, expansion velocity, temperature and age are: 4.3352 × 1053 kg, 3.207 × 1026 m, 3.138 × 10−27 kg·m−3, 2.43c, 2.721 K and 19.78 Billion years  respectively. Point to be noted is that, with reference to Planck scale, ratio of Hubble parameter Ht to angular velocity ωt can be expressed with ( H t / ω t )≅ γ t ≅[ 1+ln( H pl / H t ) ]≅ 3 H t 2 c 2 / 8πG( a T t 4 ) where Hpl represents Planck scale angular velocity and ( a T t 4 ) is the thermal energy density. ( H 0 / ω 0 )≅ γ 0 ≅141.26 and ω 0 ≅1.606× 10 −20  rad/sec≅5.068× 10 −13  rad/year.  It needs further study. Proceeding further, from the beginning of Planck scale, a) With a ‘decreasing’ trend of total matter density ratio, cosmic expansion velocity can be shown to be increasing. b) With an ‘increasing’ trend of total matter density ratio, cosmic expansion velocity can be shown to be decreasing. c) With a constant trend of total matter density ratio, cosmic expansion velocity can be shown to be constant. In this model, in understanding the currently believed cosmic acceleration, there is no need to consider dark energy.

scholarly journals An Outline Picture of a Growing and Rotating Planck Universe with Emerging Dark Foam

2019 ◽  
pp. 1-13 ◽  
Author(s):  
U. V. S. Seshavatharam ◽  
S. Lakshminarayana
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Large Scale ◽  
Density Ratio ◽  
Planck Scale ◽  
Matter Density ◽  
Cosmic Acceleration ◽  
Expansion Velocity ◽  
Cosmic Expansion ◽  
Total Matter

With reference to Planck scale, Mach’s relation, increasing support for large scale cosmic anisotropy and preferred directions and by introducing two new parameters Gamma and Beta, right from the beginning of Planck scale, we make an attempt to estimate ordinary matter density ratio, dark matter density ratio, mass, radius, temperature, age and expansion velocity (from and about the bay universe in all directions). We would like suggest that, from the beginning of Planck scale, 1) Dark matter can be considered as a kind of cosmic foam responsible for formation of galaxies.  2) Cosmic angular velocity is directly proportional to squared cosmic temperature. 3) Cosmic expansion velocity increases with decreasing total matter density ratio. 4) There is no need to consider dark energy for understanding cosmic acceleration.

scholarly journals Constraining the Swiss-Cheese IR-Fixed Point Cosmology with Cosmic Expansion

Universe ◽  
2021 ◽  
Vol 7 (8) ◽  
pp. 263
Author(s):  
Ayan Mitra ◽  
Vasilios Zarikas ◽  
Alfio Bonanno ◽  
Michael Good ◽  
Ertan Güdekli
Keyword(s):  
Fixed Point ◽  
Cosmological Constant ◽  
Cosmic Acceleration ◽  
Fine Tuning ◽  
Clusters Of Galaxies ◽  
Swiss Cheese ◽  
Coincidence Problem ◽  
Cosmological Expansion ◽  
Cosmic Evolution ◽  
Previous Assumption

A recent work proposed that the recent cosmic passage to a cosmic acceleration era is the result of the existence of small anti-gravity sources in each galaxy and clusters of galaxies. In particular, a Swiss-cheese cosmology model, which relativistically integrates the contribution of all these anti-gravity sources on a galactic scale has been constructed assuming the presence of an infrared fixed point for a scale dependent cosmological constant. The derived cosmological expansion provides an explanation for both the fine tuning and the coincidence problem. The present work relaxes the previous assumption on the running of the cosmological constant and allows for a generic scaling around the infrared fixed point. Our analysis reveals that, in order to produce a cosmic evolution consistent with the best ΛCDM model, the IR-running of the cosmological constant is consistent with the presence of an IR-fixed point.

Cosmological constant Λ in f(R,T) modified gravity

2016 ◽  
Vol 13 (05) ◽  
pp. 1650058 ◽  
Author(s):  
Gyan Prakash Singh ◽  
Binaya Kumar Bishi ◽  
Pradyumn Kumar Sahoo
Keyword(s):  
Cosmological Model ◽  
Cosmological Constant ◽  
Modified Gravity ◽  
Bianchi Type ◽  
Field Equations ◽  
Type Iii ◽  
Expansion Scalar ◽  
Modified Theory Of Gravity ◽  
Shear Scalar ◽  
Modified Theory

In this paper, we have studied the Bianchi type-III cosmological model in the presence of cosmological constant in the context of [Formula: see text] modified theory of gravity. Here, we have discussed two classes of [Formula: see text] gravity, i.e. [Formula: see text] and [Formula: see text]. In both classes, the modified field equations are solved by the relation expansion scalar [Formula: see text] that is proportional to shear scalar [Formula: see text] which gives [Formula: see text], where [Formula: see text] and [Formula: see text] are metric potentials. Also we have discussed some physical and kinematical properties of the models.

scholarly journals The Large Number Hypothesis and Einstein's Theory of Gravitation

1985 ◽  
Vol 38 (4) ◽  
pp. 547 ◽  
Author(s):  
Yun-Kau Lau
Keyword(s):  
Cosmological Model ◽  
Cosmological Constant ◽  
Matter Density ◽  
Time Dependent ◽  
Field Equations ◽  
Large Number Hypothesis ◽  
Theory Of Gravitation ◽  
The Mean ◽  
Limiting Case ◽  
The Universe

In an attempt to reconcile the large number hypothesis (LNH) with Einstein's theory of gravitation, a tentative generalization of Einstein's field equations with time-dependent cosmological and gravitational constants is proposed. A cosmological model consistent with the LNH is deduced. The coupling formula of the cosmological constant with matter is found, and as a consequence, the time-dependent formulae of the cosmological constant and the mean matter density of the Universe at the present epoch are then found. Einstein's theory of gravitation, whether with a zero or nonzero cosmological constant, becomes a limiting case of the new generalized field equations after the early epoch.

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