arXiv subjects

Hongya Liu

Publications and source records attributed to Hongya Liu.

51 records · Page 3Linked to original sources

arXiv2005

Five-Dimensional Cosmological Scaling Solution

A five-dimensional Ricci-flat cosmological solution is studied by assuming that the induced 4D matter contains two components: the usual fluid for dark matter as well as baryons and a scalar field with an exponential potential for dark energy. With use of the phase-plane analysis it is shown that there exist two late-time attractors one of which corresponds to a universe dominated by the scalar field alone and the other is a scaling solution in which the energy density of the scalar field remains proportional to that of the dark matter. It is furthermore shown that for this 5D scaling solution the universe expands with the same rate as in the 4D FRW models and not relies on which 4D hypersurface the universe is located in the 5D manifold.

astro-ph↗

arXiv2005

Brane Formation and Cosmological Constraint on the Number of Extra Dimensions

Special relativity is generalized to extra dimensions and quantized energy levels of particles are obtained. By calculating the probability of particles' motion in extra dimensions at high temperature of the early universe, it is proposed that the branes may have not existed since the very beginning of the universe, but formed later. Meanwhile, before the formation, particles of the universe may have filled in the whole bulk, not just on the branes. This scenario differs from that in the standard big bang cosmology in which all particles are assumed to be in the 4D spacetime. So, in brane models, whether our universe began from a 4D big bang singularity is questionable. A cosmological constraint on the number of extra dimensions is also given which favors $N\geq 7$.

gr-qc↗

arXiv2004

Compactified Newtonian Potential and a Possible Explanation for Dark Matter

Exact Newtonian potential with an extra space dimension compactified on a circle is derived and studied. It is found that a point mass located at one side of the circle can generate almost the same strength of gravitational force at the other side. Combined with the brane-world scenarios, it means that although one can not see a particle which is confined to another brane, one can feel its gravity explicitly as if it were located on our brane. This leads to the conclusion that matter on all branes may contribute equally to the curvature of our brane. Therefore, dark matter is probably nothing unusual but a `shadow' cast on our brane by hidden ordinary matter of other branes. It is shown that the physical effect of this `shadow' could be modeled by a perfect fluid with an effective mass density and a non-zero pressure. This fluid could serve as a dark matter candidate with an equation of state p_{eff}=omega rho _{eff} where omega is a positive r-dependent function and ω\neq 0,1/3,\ implying that it is neither cold nor hot. So, if the higher-dimensional universe contains several branes, the total amount of hidden matter would be able to provide the required amount for dark matter. Meanwhile, dark matter halos of galaxies might be just halos of the effective mass distribution yielded by hidden ordinary stars, and these stars are confined on other branes but captured by our galaxies. Some more properties and implications are also discussed.

hep-ph↗

arXiv2004

Bounce Models in Brane Cosmology and a Gravitational Stability Condition

Five-dimensional cosmological models with two 3-branes and with a buck cosmological constant are studied. It is found that for all the three cases ($Λ=0$, $Λ>0$, and $Λ<0$), the conventional space-time singularity ``big bang'' could be replaced by a matter singularity ``big bounce'', at which the ``size'' of the universe and the energy density are finite while the pressure diverges, and across which the universe evolves from a pre-existing contracting phase to the present expanding phase. It is also found that for the $Λ>0$ case the brane solutions could give an oscillating universe model in which the universe oscillates with each cosmic cycle begins from a ``big bounce'' and ends to a ``big crunch'', with a distinctive characteristic that in each subsequent cycle the universe expands to a larger size and then contracts to a smaller (but non-zero) size. By studying the gravitational force acted on a test particle in the bulk, a gravitational stability condition is derived and then is used to analyze those brane models. It predicts that if dark energy takes over ordinary matter, particles on the brane may become unstable in the sense that they may escape from our 4D-world and dissolve in the bulk due to the repulsive force of dark energy.

gr-qc↗

arXiv2003

Accelerating Universe in a Big Bounce Model

Recent observations of Type Ia supernovae provide evidence for the acceleration of our universe, which leads to the possibility that the universe is entering an inflationary epoch. We simulate it under a ``big bounce'' model, which contains a time variable cosmological ``constant'' that is derived from a higher dimension and manifests itself in 4D spacetime as dark energy. By properly choosing the two arbitrary functions contained in the model, we obtain a simple exact solution in which the evolution of the universe is divided into several stages. Before the big bounce, the universe contracts from a $Λ$-dominated vacuum, and after the bounce, the universe expands. In the early time after the bounce, the expansion of the universe is decelerating. In the late time after the bounce, dark energy (i.e., the variable cosmological ``constant'') overtakes dark matter and baryons, and the expansion enters an accelerating stage. When time tends to infinity, the contribution of dark energy tends to two third of the total energy density of the universe, qualitatively in agreement with observations.

gr-qc↗

arXiv2003

On the Big Bounce Singularity of a Simple 5D Cosmological Model

The big bounce singularity of a simple 5D cosmological model is studied. Contrary to the standard big bang space-time singularity, this big bounce singularity is found to be an event horizon at which the scale factor and the mass density of the universe are finite, while the pressure undergoes a sudden transition from negative infinity to positive infinity. By using coordinate transformation it is also shown that before the bounce the universe contracts deflationary, and the universe has been existed, according to the proper-time, for an infinitely long time.

gr-qc↗

arXiv2003

Exact Global Solutions of Brane Universes and Big Bounce

Exact global solutions of five-dimensional cosmological models compactified on a $S_1/Z_2$ orbifold with two 3-branes are presented, and evolution of a simple model is studied. It is found that on all 4D spacetime hypersurfaces, except a singular one, the expanding universe was started not from a big bang but from a big bounce, and before the bounce the universe was in a deflationary contracting phase. It is also found that whether the energy density on the second brane is positive, zero, or negative depends on the size of the fifth dimension.

hep-th↗

arXiv2002

Time-Dependent Solution for a Star Immersed in a Background Radiation

We study a time-dependent and spherically-symmetric solution with a star-like source. We show that this solution can be interpreted as an exterior solution of a contracting star which has a decreasing temperature and is immersed in a homogenous and isotropic background radiation. Distribution of the temperature in the fields and close-to Schwarzschild approximation of the solution are studied. By identifying the radiation with the cosmic background one, we find that the close-to-Schwarzschild approximate solution is valid in a wide range in our solar system. Possible experimental tests of the solution are discussed briefly.

gr-qc↗

arXiv2002

Quantum Effects of an Extra Compact Dimension on the Wave Function of the Universe

We extend the direct quantum approach of the standard FRW cosmology from 4D to 5D and obtain a Hamiltonian formulation for a wave-like 5D FRW cosmology. Using a late-time approximation we isolate out a y-part from the full wave function of the 5D Universe. Then we find that the compactness of the fifth dimension y yields a quantized spectrum for the momentum $P_{5}$ along the fifth dimension, and we show that the whole space-part of the wave function of the 5D Universe satisfies a two-dimensional Schrödinger equation.

gr-qc↗

arXiv2002

Instability of the Randall-Sundrum Model and Exact Bulk Solutions

Five dimensional geodesic equation is used to study the gravitational force acted on a test particle in the bulk of the Randall-Sundrum two-brane model.This force could be interpreted as the gravitational attraction from matters on the two branes and may cause the model to be unstable. By analogy with star models in astrophysics, a fluid RS model is proposed in which the bulk is filled with a fluid and this fluid has an anisotropic pressure to balance the gravity from the two branes. Thus a class of exact bulk solutions is obtained which shows that any 4D Einstein solution with a perfect fluid source can be embedded in $y=$ constant hypersurfaces in the bulk to form an equilibrium state of the brane model. By requiring a 4D effective curvature to have a minimum, the compactification size of the extra dimension is discussed.

hep-th↗

arXiv2001

Universe Models with a Variable Cosmological "Constant" and a "Big Bounce"

We present a rich class of exact solutions which contains radiation-dominated and matter-dominated models for the early and late universe. They include a variable cosmological ``constant'' which is derived from a higher dimension and manifests itself in spacetime as an energy density for the vacuum. This is in agreement with observational data and is compatible with extensions of general relativity to string and membrane theory. Our solutions are also typified by a non-singular ``big bounce'' (as opposed to a singular big bang), where matter is created as in inflationary cosmology.

gr-qc↗

arXiv2001

Radiating sources in higher-dimensional gravity

We study a time-dependent 5D metric which contains a static 4D sub-metric whose 3D part is spherically symmetric. An expansion in the metric coefficient allow us to obtain close-to Schwarzschild approximation to a class of spherically-symmetric solutions. Using Campbell's embedding theorem and the induced-matter formalism we obtain two 4D solutions. One describes a source with the stiff equation of state believed to be applicable to dense astrophysical objects, and the other describes a spherical source with a radial heat flow.

gr-qc↗

arXiv2001

The Cosmological Constant Problem and Kaluza-Klein Theory

We present technical results which extend previous work and show that the cosmological constant of general relativity is an artefact of the reduction to 4D of 5D Kaluza-Klein theory (or 10D superstrings and 11D supergravity). We argue that the distinction between matter and vacuum is artificial in the context of ND field theory. The concept of a cosmological ``constant'' (which measures the energy density of the vacuum in 4D) should be replaced by that of a series of variable fields whose sum is determined by a solution of ND field equations in a well-defined manner.

gr-qc↗

arXiv2000

Spacetime Measurements in Kaluza-Klein Gravity

We extend the classical general relativistic theory of measurement to include the possibility of existence of higher dimensions. The intrusion of these dimensions in the spacetime interval implies that the inertial mass of a particle in general varies along its worldline if the observations are analyzed assuming the existence of only the four spacetime dimensions. The variations of mass and spin are explored in a simple 5D Kaluza-Klein model.

gr-qc↗

arXiv2000

Solar System Tests of Higher-Dimensional Gravity

The classical tests of general relativity - light deflection, time delay and perihelion shift - are applied, along with the geodetic precession test, to the five-dimensional extension of the theory known as Kaluza-Klein gravity, using an analogue of the four-dimensional Schwarzschild metric. The perihelion advance and geodetic precession calculations are generalized for the first time to situations in which the components of momentum and spin along the extra coordinate do not vanish. Existing data on light- bending around the Sun using long- baseline radio interferometry, ranging to Mars using the Viking lander, and the perihelion precession of Mercury all constrain a small parameter b associated with the extra part of the metric to be less than |b| < 0.07 in the solar system. An order-of-magnitude increase in sensitivity is possible from perihelion precession, if better limits on solar oblateness become available. Measurement of geodetic precession by the Gravity Probe B satellite will improve this significantly, probing values of b with an accuracy of one part in 10^4 or more.

gr-qc↗