2,902 research outputs found
Gravitational Excitons as Dark Matter
In earlier work it was pointed out that for warped product spacetimes the
conformal (geometrical moduli) excitations of the internal compactified factor
spaces should be observable as massive scalar fields in the external spacetime.
Here we show that these scalar fields (gravitational excitons) describe weakly
interacting particles and can be considered as dark matter component. Masses of
the gravexcitons are defined by the form of the effective potential of the
theory and the stabilization scales of the internal space. This implies that
different stabilization scales result in different types of DM. An essential
role is played by the effective potential. On the one hand, its minima fix
possible stabilization scales of the internal spaces; on the other hand, they
provide possible values for the effective cosmological constant.Comment: CAPP 2000 talk, 5 pages, LaTeX2
Stable compactification and gravitational excitons from extra dimensions
We study inhomogeneous multidimensional cosmological models with a higher
dimensional space-time manifold under dimensional reduction. Stability due to
different types of effective potentials is analyzed for specific configurations
of internal spaces. Necessary restrictions on the parameters of the models are
found and masses of gravitational excitons (small inhomogeneous excitations of
the scale factors of the internal spaces near minima of effective potentials)
are calculated.Comment: 42 pages, Latex. Extended version of a lecture given at the
conference "Modern Modified Theories of Gravitation and Cosmology", Ben
Gurion University, Beer Sheva, Israel, June 29-30 199
A paradigmatic model of Earth's magnetic field reversals
The irregular polarity reversals of the Earth's magnetic field have attracted
much interest during the last decades. Despite the fact that recent numerical
simulations of the geodynamo have shown nice polarity transitions, the very
reason and the basic mechanism of reversals are far from being understood.
Using a paradigmatic mean-field dynamo model with a spherically symmetric
helical turbulence parameter alpha we attribute the essential features of
reversals to the magnetic field dynamics in the vicinity of an exceptional
point of the spectrum of the non-selfadjoint dynamo operator. At such
exceptional (branch) points of square root type two real eigenvalues coalesce
and continue as a complex conjugated pair of eigenvalues. Special focus is laid
on the comparison of numerically computed time series with paleomagnetic
observations. It is shown that the considered dynamo model with high
supercriticality can explain the observed time scale and the asymmetric shape
of reversals with a slow decay and a fast field recovery.Comment: 7 pages, 6 figures, submitted to Magnetohydrodynamic
Comment on stability of atoms and nuclei in multidimensional TeV scale gravity
We discuss the stability of atoms and nucleons in the presence of
multidimensional gravity characterized by the unified energy scale 1TeV. We
point out that the multidimensional gravitational attraction can create
additional bound states deep inside of atomic and nucleon constituents. These
states would be characterized by sizes comparable to the TeV scale distance. We
show that shallow gravity induced bound states between an electron and a quark
are ruled out, because collapse processes in atoms to such bound states would
occur on time scales which are in contradiction to current data on the
stability of ordinary matter. The gravitational attraction may also produce
diquark states, which should be considered in addition to those discussed in
the literature. The estimates indicate that, for TeV scale gravity, the problem
of UV divergencies and UV renormalization is crucial. Some universal
renormalization mechanism should exist, which stabilizes atoms in the presence
of multidimensional gravity.Comment: 5 pages, Revte
On stable compactification with Casimir-like potential
Multidimensional cosmological models with a higher dimensional space-time
manifold are investigated under dimensional reduction. In the Einstein
conformal frame, the effective potential for the internal scale factors is
obtained. The stable compactification of the internal spaces is achieved due to
the Casimir effect. In the case of more than one internal space a Casimir-like
ansatz for the energy density of the massless scalar field fluctuations is
proposed. Stable configurations with respect to the internal scale factor
excitations are found in the cases of one and two internal spaces.Comment: 17 pages, Latex2e, submitted to Gravitation and Cosmology, slightly
revised Appendix
Asymptotical AdS from non-linear gravitational models with stabilized extra dimensions
We consider non-linear gravitational models with a multidimensional warped
product geometry. Particular attention is payed to models with quadratic scalar
curvature terms. It is shown that for certain parameter ranges, the extra
dimensions are stabilized if the internal spaces have negative constant
curvature. In this case, the 4-dimensional effective cosmological constant as
well as the bulk cosmological constant become negative. As a consequence, the
homogeneous and isotropic external space is asymptotically AdS. The connection
between the D-dimensional and the 4-dimensional fundamental mass scales sets a
restriction on the parameters of the considered non-linear models.Comment: 12 pages, LaTeX2e, minor changes, improved references, to appear in
PR
Multidimensional cosmology and asymptotical AdS
A non-linear gravitational model with a multidimensional geometry and
quadratic scalar curvature is considered. For certain parameter ranges, the
extra dimensions are stabilized if the internal spaces have negative curvature.
As a consequence, the 4-dimensional effective cosmological constant as well as
the bulk cosmological constant become negative. The homogeneous and isotropic
external space is asymptotically AdS. The connection between the D-dimensional
and the 4-dimensional fundamental mass scales sets an additional restriction on
the parameters of the considered non-linear models.Comment: 6 pages, kluwer.sty; Conf. Proc., 11th EAS Meeting: "JENAM 2002: The
Unsolved Universe", Porto, Portuga
MN Lup: X-rays from a weakly accreting T Tauri star
Young T Tauri stars (TTS) are surrounded by an accretion disk, which over
time disperses due to photoevaporation, accretion, and possibly planet
formation. The accretion shock on the central star produces an UV/optical
veiling continuum, line emission, and X-ray signatures. As the accretion rate
decreases, the impact on the central star must change. In this article we study
MN Lup, a young star where no indications of a disk are seen in IR
observations. We present XMM-Newton and VLT/UVES observations, some of them
taken simultaneously. The X-ray data show that MN Lup is an active star with
L_X/L_bol close to the saturation limit. However, we find high densities (n_e >
3e10 /cm^3) in the X-ray grating spectrum. This can be well fitted using an
accretion shock model with an accretion rate of 2e-11 M_sun/yr. Despite the
simple Halpha line profile which has a broad component, but no absorption
signatures as typically seen on accreting TTS, we find rotational modulation in
Ca II K and in photospheric absorption lines. In the Halpha line we see a
prominence in absorption about 2R_* above the stellar surface - the first of
its kind on a TTS. MN Lup is also the only TTS where accretion is seen, but no
dust disk is detected that could fuel it. We suggest that MN Lup presents a
unique and short-lived state in the disk evolution. It may have lost its dust
disk only recently and is now accreting the remaining gas at a very low rate.Comment: 13 pages, 7 figures, accepted by Ap
A non-Hermitian symmetric Bose-Hubbard model: eigenvalue rings from unfolding higher-order exceptional points
We study a non-Hermitian symmetric generalization of an -particle,
two-mode Bose-Hubbard system, modeling for example a Bose-Einstein condensate
in a double well potential coupled to a continuum via a sink in one of the
wells and a source in the other. The effect of the interplay between the
particle interaction and the non-Hermiticity on characteristic features of the
spectrum is analyzed drawing special attention to the occurrence and unfolding
of exceptional points (EPs). We find that for vanishing particle interaction
there are only two EPs of order which under perturbation unfold either
into eigenvalue pairs (and in case of odd, into an additional
zero-eigenvalue) or into eigenvalue triplets (third-order eigenvalue rings) and
single eigenvalues, depending on the direction of the
perturbation in parameter space. This behavior is described analytically using
perturbational techniques. More general EP unfoldings into eigenvalue rings up
to th order are indicated.Comment: minor change
NAHUAL: A cool spectrograph for planets of ultra-cool objects
We present the status of an ongoing study to built a a high resolution near
infrared Echelle spectrograph (NAHUAL) for the 10.4-m-Gran Telescopio Canarias
(GTC) which will be especially optimised for planet searches by means of high
precision radial velocity measurements. We show that infrared radial velocity
programs are particularly suitable to search for planets very low mass stars
and brown dwarfs, as well as active stars. The goal of NAHUAL is to reach an
accuracy of the radial velocity measurement of a few m/s, which would allow the
detection of planets with a few earth-masses orbiting low-mass stars and brown
dwarfs. It is planed that NAHUAL covers simultaneously the full wavelength
range in the J, H, and K-band, and will also serve as a general purpose high
resolution near infrared spectrograph of the GTC. The planed instrument will
have a resolution of R=50,000 with a 0.175 arcsec slit, and an AO-system. An
absorption cell will serve as a simultaneous wavelength reference.Comment: 8 pages, 3 figures, conferenc
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