154 research outputs found
Energy composition of the Universe: time-independent internal symmetry
The energy composition of the Universe, as emerged from the Type Ia supernova
observations and the WMAP data, looks preposterously complex, -- but only at
the first glance. In fact, its structure proves to be simple and regular. An
analysis in terms of the Friedmann integral enables to recognize a remarkably
simple time-independent covariant robust recipe of the cosmic mix: the
numerical values of the Friedmann integral for vacuum, dark matter, baryons and
radiation are approximately identical. The identity may be treated as a
symmetry relation that unifies cosmic energies into a regular set, a quartet,
with the Friedmann integral as its common genuine time-independent physical
parameter. Such cosmic internal (non-geometrical) symmetry exists whenever
cosmic energies themselves exist in nature. It is most natural for a finite
Universe suggested by the WMAP data. A link to fundamental theory may be found
under the assumption about a special significance of the electroweak energy
scale in both particle physics and cosmology. A freeze-out model developed on
this basis demonstrates that the physical nature of new symmetry might be due
to the interplay between electroweak physics and gravity at the cosmic age of a
few picoseconds. The big `hierarchy number' of particle physics represents the
interplay in the model. This number quantifies the Friedmann integral and gives
also a measure to some other basic cosmological figures and phenomena
associated with new symmetry. In this way, cosmic internal symmetry provides a
common ground for better understanding of old and recent problems that
otherwise seem unrelated; the coincidence of the observed cosmic densities, the
flatness of the co-moving space, the initial perturbations and their amplitude,
the cosmic entropy are among them.Comment: 32 page
Can modified gravity explain accelerated cosmic expansion?
We show that the recently suggested explanations of cosmic acceleration by
the modification of gravity at small curvature suffer violent instabilities and
strongly disagree with the known properties of gravitational interactions.Comment: 4 pages, no figure, revised version (one footnote added
Dark energy and key physical parameters of clusters of galaxies
We study physics of clusters of galaxies embedded in the cosmic dark energy
background. Under the assumption that dark energy is described by the
cosmological constant, we show that the dynamical effects of dark energy are
strong in clusters like the Virgo cluster. Specifically, the key physical
parameters of the dark mater halos in clusters are determined by dark energy:
1) the halo cut-off radius is practically, if not exactly, equal to the
zero-gravity radius at which the dark matter gravity is balanced by the dark
energy antigravity; 2) the halo averaged density is equal to two densities of
dark energy; 3) the halo edge (cut-off) density is the dark energy density with
a numerical factor of the unity order slightly depending on the halo profile.
The cluster gravitational potential well in which the particles of the dark
halo (as well as galaxies and intracluster plasma) move is strongly affected by
dark energy: the maximum of the potential is located at the zero-gravity radius
of the cluster.Comment: 8 pages, 1 figur
Current constraints on Cosmological Parameters from Microwave Background Anisotropies
We compare the latest observations of Cosmic Microwave Background (CMB)
Anisotropies with the theoretical predictions of the standard scenario of
structure formation. Assuming a primordial power spectrum of adiabatic
perturbations we found that the total energy density is constrained to be
while the energy density in baryon and Cold Dark
Matter (CDM) are and ,
(all at 68% C.L.) respectively. The primordial spectrum is consistent with
scale invariance, () and the age of the universe is
Gyrs. Adding informations from Large Scale Structure and
Supernovae, we found a strong evidence for a cosmological constant
and a value of the Hubble parameter
. Restricting this combined analysis to flat universes, we put
constraints on possible 'extensions' of the standard scenario. A gravity waves
contribution to the quadrupole anisotropy is limited to be (95%
c.l.). A constant equation of state for the dark energy component is bound to
be (95% c.l.). We constrain the effective relativistic degrees
of freedom and the neutrino chemical potential and (massless neutrinos).Comment: The status of cosmological parameters before WMAP. In press on Phys.
Rev. D., Rapid Communication, 6 pages, 5 figure
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Newtonian limit of the singular f(R) gravity in the Palatini formalism
Recently D. Vollick [Phys. Rev. D68, 063510 (2003)] has shown that the
inclusion of the 1/R curvature terms in the gravitational action and the use of
the Palatini formalism offer an alternative explanation for cosmological
acceleration. In this work we show not only that this model of Vollick does not
have a good Newtonian limit, but also that any f(R) theory with a pole of order
n in R=0 and its second derivative respect to R evaluated at Ro is not zero,
where Ro is the scalar curvature of background, does not have a good Newtonian
limit.Comment: 9 page
Inflation with a constant ratio of scalar and tensor perturbation amplitudes
The single scalar field inflationary models that lead to scalar and tensor
perturbation spectra with amplitudes varying in direct proportion to one
another are reconstructed by solving the Stewart-Lyth inverse problem to
next-to-leading order in the slow-roll approximation.
The potentials asymptote at high energies to an exponential form,
corresponding to power law inflation, but diverge from this model at low
energies, indicating that power law inflation is a repellor in this case. This
feature implies that a fine-tuning of initial conditions is required if such
models are to reproduce the observations. The required initial conditions might
be set through the eternal inflation mechanism.
If this is the case, it will imply that the spectral indices must be nearly
constant, making the underlying model observationally indistinguishable from
power law inflation.Comment: 20 pages, 7 figures. Major changes to the Introduction following
referee's comments. One figure added. Some other minor changes. No conclusion
was modifie
Toward a Possible Solution to the Cosmic Coincidence Problem
It is a mystery why the density of matter and the density of vacuum energy
are nearly equal today when they scale so differently during the expansion of
the Universe. We suggest a paradigm that might allow for a non-anthropic
solution to this cosmic coincidence problem. The fact that the half life of
Uranium 238 is very near to the age of the solar system is not considered a
coincidence since there are many nuclides with half lives ranging over a huge
range of time scales implying that there is likely to be some nuclide with a
half life near to any given time scale. Likewise it may be that the vacuum
field energy causing the universal acceleration today is just one of a large
ensemble of scalar field energies, which have dominated the Universe in the
past and then faded away. Given that in standard cosmology and particle physics
there are already several scalar fields that probably contribute to universal
vacuum energy (the Higgs field, the inflaton, and whatever quintessence/dark
energy field causes the current acceleration), the idea of a large ensemble of
fields does not seem far fetched. Predictions of the idea include: 1) The
current vacuum energy driving the acceleration is not constant and will
eventually fade away, 2) The ratio w of scalar field pressure to density is
currently changing and is not precisely negative one, 3) There were likely
periods of vacuum dominance and acceleration in the past, 4) the current phase
of acceleration will end but there may be additional periods of acceleration in
the future, 5) the ultimate fate of the Universe cannot be decided until the
nature of these fields is known, but the eventual sum of densities from all
scalar fields could be zero, as was usually assumed before the discovery of the
current universal acceleration.Comment: Version to appear in Physical Review D; corrections made to units and
numerical integration. 5 Pages, LaTeX with 2 imbedded postscript figure
Photon Spectrum Produced by the Late Decay of a Cosmic Neutrino Background
We obtain the photon spectrum induced by a cosmic background of unstable
neutrinos. We study the spectrum in a variety of cosmological scenarios and
also we allow for the neutrinos having a momentum distribution (only a critical
matter dominated universe and neutrinos at rest have been considered until
now). Our results can be helpful when extracting bounds on neutrino electric
and magnetic moments from cosmic photon background observations.Comment: RevTex, 14 pages, 3 figures; minor changes, references added. To
appear in Phys. Rev.
Dark Energy and Extending the Geodesic Equations of Motion: Connecting the Galactic and Cosmological Length Scales
Recently, an extension of the geodesic equations of motion using the Dark
Energy length scale was proposed. Here, we apply this extension to the
analyzing the motion of test particles at the galactic scale and longer. A
cosmological check of the extension is made using the observed rotational
velocity curves and core sizes of 1393 spiral galaxies. We derive the density
profile of a model galaxy using this extension, and with it, we calculate
to be ; this is within experimental error of the
WMAP value of . We then calculate to be
kpc, which is in reasonable agreement with observations.Comment: 25 pages. Accepted for publication in General Relativity and
Gravitation. Paper contains the published version of the second half of
arXiv:0711.3124v2 with corrections include
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