289 research outputs found
Degravitation, Inflation and the Cosmological Constant as an Afterglow
In this report, we adopt the phenomenological approach of taking the
degravitation paradigm seriously as a consistent modification of gravity in the
IR, and investigate its consequences for various cosmological situations. We
motivate degravitation-- where Netwon's constant is promoted to a scale
dependent filter function-- as arising from either a small (resonant) mass for
the graviton, or as an effect in semi-classical gravity. After addressing how
the Bianchi identities are to be satisfied in such a set up, we turn our
attention towards the cosmological consequences of degravitation. By
considering the example filter function corresponding to a resonantly massive
graviton (with a filter scale larger than the present horizon scale), we show
that slow roll inflation, hybrid inflation and old inflation remain
quantitatively unchanged. We also find that the degravitation mechanism
inherits a memory of past energy densities in the present epoch in such a way
that is likely significant for present cosmological evolution. For example, if
the universe underwent inflation in the past due to it having tunneled out of
some false vacuum, we find that degravitation implies a remnant `afterglow'
cosmological constant, whose scale immediately afterwards is parametrically
suppressed by the filter scale () in Planck units . We discuss circumstances through which this scenario reasonably
yields the presently observed value for . We also
find that in a universe still currently trapped in some false vacuum state,
resonance graviton models of degravitation only degravitate initially Planck or
GUT scale energy densities down to the presently observed value over timescales
comparable to the filter scale.Comment: To appear in JCAP; sections discussing degravitation as a
semi-classical effect and the modified Bianchi identities adde
Polarization Effects on the process with Large Extra Dimensions
We study large extra dimension effects on the polarizations of the pair
and electron beam at the process. It is shown that the
measurements of the cross section for transversely polarized pair with the
right-handed electron beam remarkably enhance the possibilities to see the low
scale quantum gravity effects. Higher Linear Collider bounds on the string
scale in this model can be obtained by using the left-handed electron beam.Comment: Final version to appear in Phys.Lett.B. More references are adde
Dynamic Dilatonic Domain Walls
Motivated by the ``universe as a brane'' idea, we investigate the motion of a
-brane (or domain wall) that couples to bulk matter. Usually one would
expect the spacetime outside such a wall to be time dependent however we show
that in certain cases it can be static, with consistency of the Israel
equations yielding relationships between the bulk metric and matter that can be
used as ans\"atze to solve the Einstein equations. As a concrete model we study
a domain wall coupled to a bulk dilaton with Liouville potentials for the
dilaton both in the bulk and on the wall. The bulk solutions we find are all
singular but some have black hole or cosmological horizons, beyond which our
solutions describe domain walls moving in time dependent bulks. A significant
period of world volume inflation occurs if the potential on the wall is not too
steep; in some cases the bulk also inflates (with the wall comoving) while in
others the wall moves relative to a non-inflating bulk. We apply our method to
obtain cosmological solutions of Ho\v{r}ava-Witten theory compactified on a
Calabi-Yau space. tive to a non-inflating bulk. We apply our method to obtain
cosmological solutions of Ho\v{r}ava-Witten theory compactified on a Calabi-Yau
space.Comment: 32 pages LaTeX, 5 .eps figures, corrected some typo
Ghost Condensation and a Consistent Infrared Modification of Gravity
We propose a theoretically consistent modification of gravity in the
infrared, which is compatible with all current experimental observations. This
is an analog of Higgs mechanism in general relativity, and can be thought of as
arising from ghost condensation--a background where a scalar field \phi has a
constant velocity, = M^2. The ghost condensate is a new kind of
fluid that can fill the universe, which has the same equation of state, \rho =
-p, as a cosmological constant, and can hence drive de Sitter expansion of the
universe. However, unlike a cosmological constant, it is a physical fluid with
a physical scalar excitation, which can be described by a systematic effective
field theory at low energies. The excitation has an unusual low-energy
dispersion relation \omega^2 \sim k^4 / M^2. If coupled to matter directly, it
gives rise to small Lorentz-violating effects and a new long-range 1/r^2 spin
dependent force. In the ghost condensate, the energy that gravitates is not the
same as the particle physics energy, leading to the possibility of both sources
that can gravitate and antigravitate. The Newtonian potential is modified with
an oscillatory behavior starting at the distance scale M_{Pl}/M^2 and the time
scale M_{Pl}^2/M^3. This theory opens up a number of new avenues for attacking
cosmological problems, including inflation, dark matter and dark energy.Comment: 42 pages, LaTeX 2
SensiBlend: Sensing blended experiences in professional and social contexts
Unlike traditional workshops, SensiBlend is a living experiment about the future of remote, hybrid, and blended experiences within professional and other social contexts. The interplay of interpersonal relationships with tools and spaces-digital and physical-has been abruptly challenged and fundamentally altered as a result of the COVID-19 pandemic. With this meta-workshop, we seek to scrutinize and advance the role and impact of Ubiquitous Computing in the new "blended"social reality, and raise questions relating to the specific attributes of socio-Technical experiences in the future organization of interpersonal relationships. How do we better equip people to deal with blended experiences? What dimensions of socio-Technical experiences are at stake? To this end, we will utilize the occasion of a virtual UbiComp in combination with novel remote-working tools and participatory sensing with attendees to collectively examine, discuss, and elicit the potential routes of augmenting social practices in a discourse about the future of blended working, socializing, and living
Stationary Einstein-Maxwell fields in arbitrary dimensions
The Einstein-Maxwell equations in D-dimensions admitting (D-3) commuting
Killing vector fields have been investigated. The existence of the electric,
magnetic and twist potentials have been proved. The system is formulated as the
harmonic map coupled to gravity on three-dimensional base space generalizing
the Ernst system in the four-dimensional stationary Einstein-Maxwell theory.
Some classes of the new exact solutions have been provided, which include the
electro-magnetic generalization of the Myers-Perry solution, which describes
the rotating black hole immersed in a magnetic universe, and the static charged
black ring solution.Comment: 26 page
Spatial infinity in higher dimensional spacetimes
Motivated by recent studies on the uniqueness or non-uniqueness of higher
dimensional black hole spacetime, we investigate the asymptotic structure of
spatial infinity in n-dimensional spacetimes(). It turns out that the
geometry of spatial infinity does not have maximal symmetry due to the
non-trivial Weyl tensor {}^{(n-1)}C_{abcd} in general. We also address static
spacetime and its multipole moments P_{a_1 a_2 ... a_s}. Contrasting with four
dimensions, we stress that the local structure of spacetimes cannot be unique
under fixed a multipole moments in static vacuum spacetimes. For example, we
will consider the generalized Schwarzschild spacetimes which are deformed black
hole spacetimes with the same multipole moments as spherical Schwarzschild
black holes. To specify the local structure of static vacuum solution we need
some additional information, at least, the Weyl tensor {}^{(n-2)}C_{abcd} at
spatial infinity.Comment: 6 pages, accepted for publication in Physical Review D, published
versio
Constraints on Large Extra Dimensions with Bulk Neutrinos
We consider right-handed neutrinos propagating in (large) extra
dimensions, whose only coupling to Standard Model fields is the Yukawa coupling
to the left-handed neutrino and the Higgs boson. These theories are attractive
as they can explain the smallness of the neutrino mass, as has already been
shown. We show that if is bigger than two, there are strong
constraints on the radius of the extra dimensions, resulting from the
experimental limit on the probability of an active state to mix into the large
number of sterile Kaluza-Klein states of the bulk neutrino. We also calculate
the bounds on the radius resulting from requiring that perturbative unitarity
be valid in the theory, in an imagined Higgs-Higgs scattering channel.Comment: 24 pages, 4 figures, revtex4. v2: Minor typos corrected, references
adde
Scaling solution, radion stabilization, and initial condition for brane-world cosmology
We propose a new, self-consistent and dynamical scenario which gives rise to
well-defined initial conditions for five-dimensional brane-world cosmologies
with radion stabilization. At high energies, the five-dimensional effective
theory is assumed to have a scale invariance so that it admits an expanding
scaling solution as a future attractor. The system automatically approaches the
scaling solution and, hence, the initial condition for the subsequent
low-energy brane cosmology is set by the scaling solution. At low energies, the
scale invariance is broken and a radion stabilization mechanism drives the
dynamics of the brane-world system. We present an exact, analytic scaling
solution for a class of scale-invariant effective theories of five-dimensional
brane-world models which includes the five-dimensional reduction of the
Horava-Witten theory, and provide convincing evidence that the scaling solution
is a future attractor.Comment: 17 pages; version accepted for PRD, references adde
The Supersymmetric Standard Models with Decay and Stable Dark Matters
We propose two supersymmetric Standard Models (SMs) with decaying and stable
dark matter (DM) particles. To explain the SM fermion masses and mixings and
have a heavy decay DM particle S, we consider the Froggatt-Nielsen mechanism by
introducing an anomalous U(1)_X gauge symmetry. Around the string scale, the
U(1)_X gauge symmetry is broken down to a Z_2 symmetry under which S is odd
while all the SM particles are even. S obtains a vacuum expectation value
around the TeV scale, and then it can three-body decay dominantly to the
second/third family of the SM leptons in Model I and to the first family of the
SM leptons in Model II. Choosing a benchmark point in the constrained minimal
supersymmetric SM with exact R parity, we show that the lightest neutralino DM
is consistent with the CDMS II experiment. Considering S three-body decay and
choosing suitable parameters, we show that the PAMELA and Fermi-LAT experiments
and the PAMELA and ATIC experiments can be explained in Model I and Model II,
respectively.Comment: RevTex4, 26 pages, 6 figures, references added, version to appear in
EPJ
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