We study the renormalized energy-momentum tensor (EMT) of cosmological scalar fluctuations during the slow-rollover regime for power-law inflation and find that it is characterized by a negative energy density at the leading order, with the same time behaviour as the background energy. The average expansion rate appears decreased by the back-reaction of the effective energy of cosmological fluctuations but this value becomes comparable with the energy of background only if inflation starts at a Planckian energy. We also find that, for this particular model, the first and second order inflaton fluctuations are decoupled and satisfy the same equation of motion. To conclude the fourth order adiabatic expansion for the inflaton scalar field is evaluated for a general potential V(\phi).
We investigate whether dark matter can be replaced by various source terms appearing in the effective Einstein equation, valid on a brane embedded into a higher dimensional space-time (the bulk). Such non-conventional source terms include a quadratic (ordinary) matter source term, a geometric source term originating in the Weyl curvature of the bulk, a source term arising from the possible asymmetric embedding, and finally the pull-back to the brane of possible non-standard model bulk fields.
We explore the possibility of utilizing the prompt muon fluxes at very high energies in order to discriminate various models/parametrizations of low-x QCD behaviour of hadronic cross-sections relevant at such energies. We find that the pair meter technique for measuring high energy prompt muons can be very efficient in such an endeavor. As a by product, it allows to cleanly probe the change in composition of the primary cosmic rays expected at high energies.
We consider the possibility that chaotic inflation is driven by a Minimal Supersymmetric Standard Model (MSSM) flat direction or a sneutrino. The discrepancy between our present study and past studies is in the mechanism for generating the curvature perturbations. We consider curvature perturbations generated by instant preheating at the end of inflation, instead of a conventional curvature perturbation generated by a fluctuation related to an inflaton field. Our simple mechanism relaxes some serious constraints that appeared in past studies, making chaotic inflation driven by a MSSM flat direction or a sneutrino more plausible.
In this further letter on the onset of classical behaviour in field theory due to a phase transition, we show that it can be phrased easily in terms of the decoherence functional, without having to use the master equation. To demonstrate this, we consider the decohering effects due to the displacement of domain boundaries, with implications for the displacement of defects, in general. We see that decoherence arises so quickly in this event, that it is negligible in comparison to decoherence due to field fluctuations in the way defined in our previous papers.
The energy shift due to the interaction of two particles in a large box is proportional to the free particle scattering phase-shift. This provides an approximation to the effective interaction referred to as the phase-shift approximation. For a many-body fermion system this effective interaction has to be corrected for the Pauli-blocking. It is used to calculate the energy of a spin 1/2 fermion system as a function of the two-body scattering length and effective range. In the unitary limit with scattering length going to infinity and effective range going to zero the energy is 0.540 (in the units of the non-interactiong fermi gas energy) with a pp-ladder summation. Including hh-ladders the energy is 0.570. A smooth crossover from the BCS to the BEC region is observed. Pairing is not included.
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Using the gradient expansion approach, we formulate a nonlinear cosmological perturbation theory on super-horizon scales valid to $O(\epsilon^2)$, where $\epsilon$ is the expansion parameter associated with a spatial derivative. For simplicity, we focus on the case of a single perfect fluid, but we take into account not only scalar but also vector and tensor modes. We derive the general solution under the uniform-Hubble time-slicing. In doing so, we identify the scalar, vector and tensor degrees of freedom contained in the solution. Then we consider the coordinate transformation to the synchronous gauge to compare our result with the previous result given in the literature.
We propose that the Universe is filled with a massive vector field, non-minimally coupled to gravitation. The field equations of the model are consistently derived and their application to cosmology is considered. The Friedmann equations acquire an extra dark-energy component, which is proportional to the mass of the vector particle. This leads to a late-time accelerated de Sitter type expansion. The free parameters of the model (gravitational coupling constants and initial value of the cosmological vector field) can be estimated by using the PPN solar system constraints. The mass of the cosmological massive vector particle, which may represent the main component of the Universe, is of the order of 10^-63 g.
We show that a recently constructed five-dimensional (5D) model with
gauge-Higgs unification and explicit Lorentz symmetry breaking in the bulk,
provides a natural dark matter candidate. This is the lightest Kaluza-Klein
particle odd under a certain discrete Z_2 symmetry, which has been introduced
to improve the naturalness of the model, and resembles KK-parity but is less
constraining.
The dark matter candidate is the first KK mode of a 5D gauge field and
electroweak bounds force its mass above the TeV scale. Its pair annihilation
rate is too small to guarantee the correct relic abundance; however
coannihilations with colored particles greatly enhance the effective
annihilation rate, leading to realistic relic densities.
We point out that the inflaton spontaneously decays into any gauge bosons and gauginos via the super-Weyl anomaly in supergravity, once the inflaton acquires a non-vanishing vacuum expectation value. In particular, in the dynamical supersymmetry breaking scenarios, the inflaton necessarily decays into the supersymmetry breaking sector, if the inflaton mass is larger than the dynamical scale. This generically causes the overproduction of the gravitinos, which severely constrains the inflation models.
In the presence of a very strong uniform magnetic field, we study the influence of space noncommutativity on the electromagnetic waves propagating through a quasi-static homogeneous plasma. In this treatment, we have adopted a physical model which considers plasma as quasi-neutral single fluid. By using noncommutative Maxwell theory, the ideal magnetohydrodynamics (MHD) equations are established, in which new equilibrium conditions are extracted. As empirical study, some attractive features of MHD waves behavior are studied. Furthermore, it is shown that the presence of space noncommutativity enhances slightly the phase velocity of the incompressive shear Alfv\'{e}n waves. In compressible plasma, the noncommutativity plays the role of an additional compression on the medium, in which the relevant modification on the fast mode occurs in highly oblique branchs, while the low modification appears in nearly parallel and anti-parallel propagation. In addition, it is turned out that the influence of space deformation on slow mode is $\sim 10^{3}$ times smaller than that on fast mode. The space noncommutativity effect on slow waves is negligible in low $\beta $ value, and could appear when $\beta $ is higher than $0.1,$ in which the extreme modification occurs in oblique slow waves with propagation angle between $30^{\circ}$ and $60^{\circ}$.
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Whenever a large group of people are engaged in an activity, communities will
form. The nature of these communities depends on the relationship considered.
In the group of people who regularly use scholarly literature, a relationship
like ``person i and person j have cited the same paper'' might reveal
communities of people working in a particular field. On this poster, we will
investigate the relationship ``person i and person j have read the same
paper''. Using the data logs of the NASA/Smithsonian Astrophysics Data System
(ADS), we first determine the population that will participate by requiring
that a user queries the ADS at a certain rate. Next, we apply the relationship
to this population. The result of this will be an abstract ``relationship
space'', which we will describe in terms of various ``representations''.
Examples of such ``representations'' are the projection of co-read vectors onto
Principal Components and the spectral density of the co-read network. We will
show that the co-read relationship results in structure, we will describe this
structure and we will provide a first attempt in the classification of this
structure in terms of astronomical communities.
The ADS is funded by NASA Grant NNG06GG68G.
It is well-entrenched folklore that torsion gravity theories predict observationally negligible torsion in the solar system, since torsion (if it exists) couples only to the intrinsic spin of elementary particles, not to rotational angular momentum. We argue that this assumption has a logical loophole which can and should be tested experimentally. In the spirit of action=reaction, if a rotating mass like a planet can generate torsion, then a gyroscope should also feel torsion. Using symmetry arguments, we show that to lowest order, the torsion field around a uniformly rotating spherical mass is determined by seven dimensionless parameters. These parameters effectively generalize the PPN formalism and provide a concrete framework for further testing GR. We construct a parametrized Lagrangian that includes both standard torsion-free GR and Hayashi- Shirafuji maximal torsion gravity as special cases. We demonstrate that classic solar system tests rule out the latter and constrain two observable parameters. We show that Gravity Probe B (GPB) is an ideal experiment for further constraining torsion theories, and work out the most general torsion-induced precession of its gyroscope in terms of our torsion parameters
In this paper, we discuss the system of massive nonlinear dark spinors coupling with the Friedman-Robertson-Walker(FRW) metric in detail, where the thermodynamic movement of spinors is also taken into account. The results reveal that, the nonlinear potential of the spinor field can provide a little negative pressure, which resists the universe to become singular. The model is oscillating in time and closed in space. The corresponding metric approximately takes the following form $g_{\mu\nu}=\bar R^2 (1-\delta\cos t)^2 \diag(1,-1,-\sin^2r,-\sin^2r \sin^2\theta), (\delta<1)$.
The accelerating expansion of the present Universe is an exciting challenge to the standard cosmology, which reflects that our current theories on matter have some incompletion. In this paper, we analyze the cosmological model with nonlinear spinor fields source in detail. The results may be able to provide a natural explanation for the puzzles of the acceleration and negative pressure.
There is an intriguing analogy between the gravitational dynamics of the horizons and thermodynamics. In case of general relativity, as well as for a wider class of Lanczos-Lovelock theories of gravity, it is possible to interpret the field equations near any spherically symmetric horizon as a thermodynamic identity TdS = dE + PdV. We study this approach further and generalize the results to two more generic cases within the context of general relativity: (i) stationary axis-symmetric horizons and (ii) time dependent evolving horizons. In both the cases, the near horizon structure of Einstein equations can be expressed as a thermodynamic identity under the virtual displacement of the horizon. This result demonstrates the fact that the thermodynamic interpretation of gravitational dynamics is not restricted to spherically symmetric or static horizons but is quite generic in nature and indicates a deeper connection between gravity and thermodynamics.
The null surfaces of a spacetime act as one-way membranes and can block information for a corresponding family of observers (time-like curves). Since lack of information can be related to entropy, this suggests the possibility of assigning an entropy to the null surfaces of a spacetime. We motivate and introduce such an entropy functional in terms of the normal to the null surface and a fourth-rank divergence free tensor $P_{ab}^{cd}$ with the algebraic symmetries of the curvature tensor. Extremising this entropy then leads to field equations for the background metric of the spacetime. When $P_{ab}^{cd}$ is constructed from the metric alone, these equations are identical to Einstein's equations with an undetermined cosmological constant (which arises as an integration constant). More generally, if $P_{ab}^{cd}$ is allowed to depend on both metric and curvature in a polynomial form, one recovers the Lanczos-Lovelock gravity. In all these cases: (a) We only need to extremise the entropy associated with the null surfaces; the metric is not a dynamical variable in this approach. (b) The extremal value of the entropy agrees with standard results, when evaluated on-shell for a solution admitting a horizon. The role of full quantum theory of gravity will be to provide the specific form of $P_{ab}^{cd}$ which should be used in the entropy functional. With such an interpretation, it seems reasonable to interpret the Lanczos-Lovelock type terms as quantum corrections to classical gravity.
In this letter we accurately measure the general relativistic gravitomagnetic Lense-Thirring effect by analyzing the post-fit residuals of the out-of-plane portion of the orbit of the Martian polar orbiter Mars Global Surveyor (MGS) over a time span of about 5 years (14 November 1999-14 January 2005). Our result is \mu= 0.9937 +/- 0.0053; general relativity predicts \mu=1. A previous test of the Lense-Thirring effect conducted in the Earth's gravitational field with the LAGEOS satellites reached an about 10% level, although it is still controversial. The expected accuracy of the GP-B mission, aimed at the detection of another gravitomagnetic effect in the terrestrial gravitational field, i.e. the Schiff precession of a gyroscope, is about 1%. The precision of our test is 0.5$%.
Weakly interacting massive particles (WIMPs) are arguably the most natural DM candidates from a particle physics point of view. After their number density has frozen out in the early universe, determining their relic density today, WIMPs are still kept very close to thermal equilibrium by scattering events with standard model particles. The complete decoupling from the thermal bath happens as late as around $\sim1-10 $MeV, thereby setting an important cosmological scale that can directly be translated into a small-scale cutoff of the spectrum of matter density fluctuations. We present here a full analytic treatment of the kinetic decoupling process from first principles. This allows an exact determination of the decoupling scale, for an arbitrary WIMP candidate and any scattering amplitude. As an application, we consider the situation of the lightest supersymmetric particle as well as the lightest Kaluza-Klein particle that arises in theories with universal extra dimensions; furthermore, we show that our formalism can also easily be applied to, e.g., the evolution of the non-relativistic electrons into the recombination regime. Finally, we comment on the impacts for the smallest gravitationally bound structures and the prospects for the indirect detection of dark matter.
The Inert Doublet Model (IDM), a two Higgs extension of the Standard Model with an unbroken $Z_2$ symmetry, is a simple and yet rich model of dark matter. We present a systematic analysis of the dark matter abundance and investigate the potentialities for direct and gamma indirect detection. We show that the model should be within the range of future experiments, like GLAST and ZEPLIN. The lightest stable scalar in the IDM is a perfect example, or archetype of a weakly interacting massive particle.
The generation of dark matter in late decays of quasi-stable massive particles has been proposed as a viable framework to address the excess of power found in numerical N-body simulations for cold dark matter cosmologies. We identify a convenient set of variable to illustrate which requirements need to be satisfied in any generic particle physics model to address the small scale problems and fulfill other astrophysical constraints. We re-examine the role of gravitinos and Kaluza-Klein gravitons in this context and find them disfavoured as a solution to the small-scale problems in case they are DM candidates generated in the decay of thermally produced WIMPs. We propose right-handed sneutrinos and right-handed Kaluza-Klein neutrinos as alternatives. We find that they are viable dark matter candidates, but that they can contribute to a solution of the small scale problems only in case the associated Dirac neutrino mass term appears as a subdominant contribution in the neutrino mass matrix.
There are reasons (which we enumerate) to think that an infinite extra dimension will harbor a black hole. In this case, brane-localized modes of gravity and gauge fields become quasilocalized, and light from a distant object can become extinct as it is lost to the black hole. In a concrete scenario, where the photon is localized by gravity, we find that the extinction rate for propagating photons is at least comparable to the correction to the real part of the frequency. That results, for example, in a stringent bound on renormalization of the speed of light.
As part of an experimental effort to demonstrate sensitivity in a large-mass detector to the ultra-low energy recoils expected from coherent neutrino-nucleus elastic scattering, we have designed and built a highly monochromatic 24 keV neutron beam at the Kansas State University Triga Mark-II reactor. The beam characteristics were chosen so as to mimic the soft recoil energies expected from reactor antineutrinos in a variety of targets, allowing to understand the response of dedicated detector technologies in this yet unexplored sub-keV recoil range. A full characterization of the beam properties (intensity, monochromaticity, contaminations, beam profile) is presented, together with first tests of the calibration setup using proton recoils in organic scintillator.
A new type of radiation detector, a p-type modified electrode germanium diode, is presented. The prototype displays, for the first time, a combination of features (mass, energy threshold and background expectation) required for a measurement of coherent neutrino-nucleus scattering in a nuclear reactor experiment. The device hybridizes the mass and energy resolution of a conventional HPGe coaxial gamma spectrometer with the low electronic noise and threshold of a small x-ray semiconductor detector, also displaying an intrinsic ability to distinguish multiple from single-site particle interactions. The present performance of the prototype and possible further improvements are discussed, as well as other applications for this new type of device in neutrino and astroparticle physics (double-beta decay, neutrino magnetic moment and WIMP searches).
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We discuss exotic singularities in the evolution of the universe motivated by the progress of observations in cosmology. Among them there are: Big-Rip (BR), Sudden Future Singularities (SFS), Generalized Sudden Future Singularities (GSFS), Finite Density Singularities (FD), type III, and type IV singularities. We relate some of these singularities with higher-order characteristics of expansion such as jerk and snap. We also discuss the behaviour of pointlike objects and classical strings on the approach to these singularities.
I discuss the dark energy characterized by the violation of the null energy condition ($\varrho + p \geq 0$), dubbed phantom. Amazingly, it is admitted by the current astronomical data from supernovae. We discuss both classical and quantum cosmological models with phantom as a source of matter and present the phenomenon called phantom duality.
We review how a high-statistics observation of the neutrino signal from a future galactic core-collapse supernova (SN) may be used to discriminate between different neutrino mixing scenarios. Most SN neutrinos are emitted in the accretion and cooling phase, during which the flavor-dependent differences of the emitted neutrino spectra are small and rather uncertain. Therefore the discrimination between neutrino mixing scenarios using these neutrinos should rely on observables independent of the SN neutrino spectra. We discuss two complementary methods that allow for the positive identification of the mass hierarchy without knowledge of the emitted neutrino fluxes, provided that the 13-mixing angle is large, $\sin^2\theta_{13}\gg 10^{-5}$. These two approaches are the observation of modulations in the neutrino spectra by Earth matter effects or by the passage of shock waves through the SN envelope. If the value of the 13-mixing angle is unknown, using additionally the information encoded in the prompt neutronization $\nu_e$ burst--a robust feature found in all modern SN simulations--can be sufficient to fix both the neutrino hierarchy and to decide whether $\theta_{13}$ is ``small'' or ``large.''
The string theory landscape consists of many metastable de Sitter vacua, populated by eternal inflation. Tunneling between these vacua gives rise to a dynamical system, which asymptotically settles down to an equilibrium state. We investigate the effects of sinks to anti-de Sitter space, and show how their existence can change probabilities in the landscape. Sinks can disturb the thermal occupation numbers that would otherwise exist in the landscape and may cause regions that were previously in thermal contact to be divided into separate, thermally isolated islands.
Cosmological scenarios built with generalized non-local String Field Theory and p-adic tachyons are examined. A general kinetic operator involving an infinite number of derivatives is studied as well as arbitrary parameter p. Late time dynamics of the tachyon only around the non-perturbative vacuum is shown to leave the cosmology trivial. A late time behavior of the tachyon and the scale factor of the FRW metric in a presence of the cosmological constant or a perfect fluid with w>-1 are constructed explicitly and a possibility of non-vanishing oscillations of the total effective state parameter around the phantom divide is proven.
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