scholarly journals THERMODYNAMICAL LAWS IN HOŘAVA–LIFSHITZ GRAVITY

2011 ◽  
Vol 20 (07) ◽  
pp. 1191-1204 ◽  
Author(s):  
SAMARPITA BHATTACHARYA ◽  
UJJAL DEBNATH
Keyword(s):  
Black Hole ◽  
Dark Energy ◽  
Black Hole Entropy ◽  
Gravity Theory ◽  
Robust Approach ◽  
Barotropic Fluid ◽  
Extra Information ◽  
Thermodynamical Laws ◽  
The Universe

In this work, we investigate the validity of the GSL of thermodynamics in the universe (open, closed and flat) governed by Hořava–Lifshitz (HL) gravity. If the universe contains barotropic fluid, we obtain the corresponding solutions. The validity of the GSL is examined by two approaches: (i) the robust approach and (ii) the effective approach. In the robust approach, we consider that the universe contains only matter fluid. Also, the effect of the gravitational sector of HL gravity is incorporated through the modified black hole entropy on the horizon. The effective approach is that all extra information of HL gravity is cast into an effective dark energy fluid, and so we consider that the universe contains matter fluid plus this effective fluid. This approach is essentially the same as Einstein's gravity theory. The general prescription for the validity of the GSL is discussed. Graphically, we show that the GSL may be satisfied for the open, closed and flat universes on the different horizons with different conditions.

MODIFIED GRAVITY THEORY DUE TO POSITIVE AS WELL AS NEGATIVE CURVATURE INDUCED ACCELERATION

2012 ◽  
Vol 21 (02) ◽  
pp. 1250012
Author(s):  
PRIYADARSHI MAJUMDAR
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Negative Curvature ◽  
Gravity Theory ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Negative Power ◽  
Modified Gravity Theory ◽  
Phase Q ◽  
The Universe

We formulate a modified gravity theory that eliminates the need for dark energy and is stable for a Lagrangian containing R, R2 as well as 1/R terms (i.e. nonlinear contributions of the Ricci curvature with a non-analytic model of f(R) at R = 0) without considering any matter-dominated era. The terms with positive powers (1, 2) of the curvature support the inflationary epoch while the terms with negative power (-1) serves as effective dark energy, supporting current cosmic acceleration. We present a few analytical solutions of evolution equation for the deceleration parameter q as a function of Hubble parameter H and time t; specially in one solution, the universe evolves continuously from q = 1 (a radiation-dominated epoch) to q = -1/2 (dark-energy-dominated late-time accelerating phase) when the universe is sufficiently old. The solution is supported by numerical results. The transition from the decelerated (q > 0) to the accelerated phase (q < 0) of expansion takes place smoothly without having to resort to a study of asymptotic behavior.

Dark Energy

2021 ◽  
pp. 79-88
Author(s):  
Gianfranco Bertone
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Dark Energy ◽  
Gravitational Waves ◽  
Quantum Physics ◽  
New Physics ◽  
The Universe

I discuss here black holes, extreme astronomical objects that swallow all forms of matter and radiation surrounding them, and leave behind, as physicist John A. Wheeler said, only their ‘gravitational aura’. These endlessly fascinating objects are the gates where gravity meets quantum physics. Since the pioneering work of scientists like S. Hawking, black holes have become ‘theoretical laboratories’ to explore new physics theories. I discuss how the discovery of gravitational waves from black holes, and the first image of a black hole revealed in 2019, have transformed the study of black holes, and may soon lead to new ground-breaking discoveries. The Universe will disappear. Slowly, it will grow dimmer and dimmer, until it disappears completely.

scholarly journals Black hole solutions in a cosmological spacetime background

2014 ◽  
Vol 29 (28) ◽  
pp. 1450142 ◽  
Author(s):  
Metin Arık ◽  
Yorgo Şenikoğlu
Keyword(s):  
Black Hole ◽  
Dark Energy ◽  
Temporal Part ◽  
Field Equations ◽  
Isotropic Coordinates ◽  
Expansion Of The Universe ◽  
Special Solutions ◽  
The Universe ◽  
Black Hole Solutions ◽  
Cosmological Spacetime

We propose and analyze a new metric that has two conformal factors a(t) and b(t) that combine the expansion of the universe and its effects on the spatial and temporal part of the Schwarzschild metric in isotropic coordinates. We present the solutions, their descriptions and we comment on their shortcomings. In the spatially flat case of an expanding universe, we derive from the proposed metric the special solutions of the field equations for the dust approximation and the McVittie metric. We show that the presence of a black hole does not modify the a(t)αt2/3 law for dust and H = const. for dark energy.

scholarly journals Phenomenologically varying Λ and a toy model for the universe

2014 ◽  
Vol 92 (11) ◽  
pp. 1494-1500 ◽  
Author(s):  
M. Khurshudyan ◽  
J. Sadeghi ◽  
E. Chubaryan ◽  
H. Farahani
Keyword(s):  
Dark Energy ◽  
Second Law Of Thermodynamics ◽  
Chaplygin Gas ◽  
Barotropic Fluid ◽  
Physical Conditions ◽  
Ghost Dark Energy ◽  
Generalized Second Law ◽  
Varying Λ ◽  
First Time ◽  
The Universe

We consider a model of the Universe with variable G and Λ. The subject of interest is a phenomenological model for Λ proposed and considered in this article for the first time (to our knowledge), with the assumption that ghost dark energy exists and interacts with the Universe through Λ. We consider the possibility that there exist unusual connections between different components of the fluids in Universe. We would like to stress that this is simply an assumption and could be very far from reality. This model is interesting phenomenologically and mathematically but we will not discuss physical conditions or possibilities of implementing the modifications. To test our assumption and to observe the behavior of the Universe, we will consider toy models filled by a barotropic fluid and modified Chaplyagin gas. Finally, we will consider interaction between barotropic fluid or Chaplygin gas and ghost dark energy as a separate scenario. The statefinder diagnostic provided stability analysis of the models. All free parameters of the model are fixed to satisfy the generalized second law of thermodynamics.

DYNAMICAL TIME VARIABLE IN COSMOLOGY

1988 ◽  
Vol 03 (04) ◽  
pp. 333-343
Author(s):  
TAKESHI FUKUYAMA ◽  
KIYOSHI KAMIMURA
Keyword(s):  
Black Hole ◽  
Event Horizon ◽  
Time Variable ◽  
Gravity Theory ◽  
Two Dimensional ◽  
Maximum Radius ◽  
Dynamical Time ◽  
Dimensional Gravity ◽  
Time Variables ◽  
The Universe

Dynamical time variables are studied in two dimensional gravity theory. Dynamical time and space variables exchange their role at the maximum radius (amax) like a black hole at event horizon. Dynamical arrows of time are directed towards amax in both expanding and contracting phases. Both time flows cannot go beyond amax, and the universe becomes static at amax.

scholarly journals On the Fundamental Particles and Reactions of Nature

2021 ◽  
Author(s):  
Jian-Bin Bao ◽  
Nicholas P. Bao
Keyword(s):  
Black Hole ◽  
Dark Matter ◽  
Dark Energy ◽  
Dynamic Equilibrium ◽  
Big Bang ◽  
Observable Universe ◽  
White Hole ◽  
Fundamental Particles ◽  
Degeneracy Pressure ◽  
The Universe

There are unsolved problems related to inflation, gravity, dark matter, dark energy, missing antimatter, and the birth of the universe. Some of them can be better answered by assuming the existence of aether and hypoatoms. Both were created during the inflation in the very early universe. While aether forms vacuum, hypoatoms, composed of both matter and antimatter and believed to be neutrinos, form all observable matter. In vacuum, aether exists between the particle-antiparticle dark matter form and the dark energy form in a dynamic equilibrium: A + A-bar = gamma + gamma. The same reaction stabilizes hypoatoms and generates a 3-dimensional sink flow of aether that causes gravity. Based on the hypoatom structure, the singularity does not exist inside a black hole; the core of the black hole is a hypoatom star or neutrino star. By gaining enough mass, ca. 3 X 1022 Msun, to exceed neutrino degeneracy pressure, the black hole collapses or annihilates into the singularity, thus turning itself into a white hole or a Big Bang. The universe is anisotropic and nonhomogeneous. Its center, or where the Big Bang happened, is at about 0.671 times the radius of the observable universe at the Galactic coordinates (l, b) ~ (286&deg;, -42&deg;). If we look from the Earth to the center of the universe, the universe is rotating clockwise.

scholarly journals Dark Matter and Dark Energy in the Universe

2018 ◽  
Vol 14 (1) ◽  
pp. 5292-5295
Author(s):  
Yuanjie Li ◽  
Lihong Zhang ◽  
Peng Dong
Keyword(s):  
Black Hole ◽  
Dark Matter ◽  
Dark Energy ◽  
Early Universe ◽  
Left Behind ◽  
Time Quantum ◽  
The Universe

This paper points out that not only all quantum-ghost puzzles occur in the Time Quantum Worm Hole, but also the dark matter in the universe is hidden in it. Dark energy is the contribution of the Planck black hole left behind by the early universe.

Wormholes, Time Travel, and Other Exotic Theories

2020 ◽  
pp. 72-87
Author(s):  
John W. Moffat
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Gravitational Waves ◽  
Black Hole Entropy ◽  
Negative Energy ◽  
Superstring Theory ◽  
Quantum Radiation ◽  
Higher Dimensional ◽  
Mathematical Equations ◽  
The Universe

In 1935, Einstein and Rosen described what is now called the Einstein-Rosen bridge. Wheeler called this a wormhole, which could connect two distant parts of the universe. Thorne and Morris showed the wormhole cannot be traversable unless exotic matter with negative energy props it up. Using the Penrose mechanism of superradiance, one can produce rotational energy from a black hole, which could be used to detect dark matter particles. Higher dimensional objects such as branes in superstring theory have been considered as sources of gravitational waves. Black holes have even been proposed to be giant atoms, related to Hawking radiation and black hole entropy. Bekenstein and Mukhanov postulated that black holes radiated quantum radiation. Many such speculative ideas have been put forth that could potentially be verified by detecting gravitational waves. Yet, many physicists work with mathematical equations, unconcerned with whether their ideas can be verified or falsified by experiments.

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