60 research outputs found
An Underlying Theory for Gravity
A new direction to understand gravity has recently been explored by
considering classical gravity to be a derived interaction from an underlying
theory. This underlying theory would involve new degrees of freedom at a deeper
level and it would be structurally different from classical gravitation. It may
conceivably be a quantum theory or a non-quantum theory. The relation between
this underlying theory and Einstein's gravity is similar to the connection
between statistical mechanics and thermodynamics. We discuss the apparent lack
of evidence of any quantum nature of gravity in this context.Comment: Contributed paper to VIIth International Conference on Gravitation
and Cosmology, 14 - 19 December, 2011 GOA, INDIA. 4 page
Weak-field limit of Kaluza-Klein models with spherical compactification: experimental constraints
We investigate the classical gravitational tests for the six-dimensional
Kaluza-Klein model with spherical (of a radius ) compactification of the
internal space. The model contains also a bare multidimensional cosmological
constant . The matter, which corresponds to this ansatz, can be
simulated by a perfect fluid with the vacuum equation of state in the external
space and an arbitrary equation of state with the parameter in the
internal space. For example, and correspond to the
monopole two-forms and the Casimir effect, respectively. In the particular case
, the parameter is also absent: . In the
weak-field approximation, we perturb the background ansatz by a point-like
mass. We demonstrate that in the case the perturbed metric
coefficients have the Yukawa type corrections with respect to the usual
Newtonian gravitational potential. The inverse square law experiments restrict
the parameters of the model: $a/\sqrt{\omega_1}\lesssim 6\times10^{-3}\
{{cm}}\gamma\omega_1>0\omega_1=0\gamma=1/3$,
which strongly contradicts the observations.Comment: 8 pages, no figures, revised version, equations and references added,
accepted for publication in Phys. Rev. D. arXiv admin note: significant text
overlap with arXiv:1107.338
Ultraslow Wave Nuclear Burning of Uranium-Plutonium Fissile Medium on Epithermal Neutrons
For a fissile medium, originally consisting of uranium-238, the investigation
of fulfillment of the wave burning criterion in a wide range of neutron
energies is conducted for the first time, and a possibility of wave nuclear
burning not only in the region of fast neutrons, but also for cold, epithermal
and resonance ones is discovered for the first time.
For the first time the results of the investigation of the Feoktistov
criterion fulfillment for a fissile medium, originally consisting of
uranium-238 dioxide with enrichments 4.38%, 2.00%, 1.00%, 0.71% and 0.50% with
respect to uranium-235, in the region of neutron energies 0.015-10.0eV are
presented. These results indicate a possibility of ultraslow wave
neutron-nuclear burning mode realization in the uranium-plutonium media,
originally (before the wave initiation by external neutron source) having
enrichments with respect to uranium-235, corresponding to the subcritical
state, in the regions of cold, thermal, epithermal and resonance neutrons.
In order to validate the conclusions, based on the slow wave neutron-nuclear
burning criterion fulfillment depending on the neutron energy, the numerical
modeling of ultraslow wave neutron-nuclear burning of a natural uranium in the
epithermal region of neutron energies (0.1-7.0eV) was conducted for the first
time. The presented simulated results indicate the realization of the ultraslow
wave neutron-nuclear burning of the natural uranium for the epithermal
neutrons.Comment: 35 pages, 19 figures (v2: Fig12 and some misprints in the text are
fixed
Kaluza-Klein models: can we construct a viable example?
In Kaluza-Klein models, we investigate soliton solutions of Einstein
equation. We obtain the formulas for perihelion shift, deflection of light,
time delay of radar echoes and PPN parameters. We find that the solitonic
parameter k should be very big: |k|\geq 2.3\times10^4. We define a soliton
solution which corresponds to a point-like mass source. In this case the
soliton parameter k=2, which is clearly contrary to this restriction. Similar
problem with the observations takes place for static spherically symmetric
perfect fluid with the dust-like equation of state in all dimensions. The
common for both of these models is the same equations of state in our three
dimensions and in the extra dimensions. All dimensions are treated at equal
footing. To be in agreement with observations, it is necessary to break the
symmetry between the external/our and internal spaces. It takes place for black
strings which are particular examples of solitons with k\to \infty. For such k,
black strings are in concordance with the observations. Moreover, we show that
they are the only solitons which are at the same level of agreement with the
observations as in general relativity. Black strings can be treated as perfect
fluid with dust-like equation of state p_0=0 in the external/our space and very
specific equation of state p_1=-(1/2)\epsilon in the internal space. The latter
equation is due to negative tension in the extra dimension. We also demonstrate
that dimension 3 for the external space is a special one. Only in this case we
get the latter equation of state. We show that the black string equations of
state satisfy the necessary condition of the internal space stabilization.
Therefore, black strings are good candidates for a viable model of
astrophysical objects (e.g., Sun) if we can provide a satisfactory explanation
of negative tension for particles constituting these objects.Comment: 11 pages, Revtex4, no figures, appendix and references adde
Latent solitons, black strings, black branes, and equations of state in Kaluza-Klein models
In Kaluza-Klein models with an arbitrary number of toroidal internal spaces,
we investigate soliton solutions which describe the gravitational field of a
massive compact object. We single out the physically interesting solution
corresponding to a point-like mass. For the general solution we obtain
equations of state in the external and internal spaces. These equations
demonstrate that the point-like mass soliton has dust-like equations of state
in all spaces. We also obtain the PPN parameters, which give the possibility to
obtain the formulas for perihelion shift, deflection of light and time delay of
radar echoes. Additionally, the gravitational experiments lead to a strong
restriction on the parameter of the model: . The point-like mass solution contradicts this restriction. The
condition satisfies the experimental limitation and defines a new
class of solutions which are indistinguishable from general relativity. We call
such solutions latent solitons. Black strings and black branes belong to this
class. Moreover, the condition of stability of the internal spaces singles out
black strings/branes from the latent solitons and leads uniquely to the black
string/brane equations of state , in the internal spaces and
to the number of the external dimensions . The investigation of
multidimensional static spherically symmetric perfect fluid with dust-like
equation of state in the external space confirms the above results.Comment: 8 pages, Revtex4, no figures, minor changes adde
Hubble flows and gravitational potentials in observable Universe
In this paper, we consider the Universe deep inside of the cell of
uniformity. At these scales, the Universe is filled with inhomogeneously
distributed discrete structures (galaxies, groups and clusters of galaxies),
which disturb the background Friedmann model. We propose mathematical models
with conformally flat, hyperbolic and spherical spaces. For these models, we
obtain the gravitational potential for an arbitrary number of randomly
distributed inhomogeneities. In the cases of flat and hyperbolic spaces, the
potential is finite at any point, including spatial infinity, and valid for an
arbitrary number of gravitating sources. For both of these models, we
investigate the motion of test masses (e.g., dwarf galaxies) in the vicinity of
one of the inhomogeneities. We show that there is a distance from the
inhomogeneity, at which the cosmological expansion prevails over the
gravitational attraction and where test masses form the Hubble flow. For our
group of galaxies, it happens at a few Mpc and the radius of the
zero-acceleration sphere is of the order of 1 Mpc, which is very close to
observations. Outside of this sphere, the dragging effect of the gravitational
attraction goes very fast to zero.Comment: 21 pages, 5 figure
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