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Universe, Volume 4, Issue 4 (April 2018) – 4 articles

Cover Story (view full-size image): The cosmological constant Λ ≃ 10−52 m−2 is currently loosely constrained by solar system planetary motions to the ≲10−43−10−44 m−2 level. Since Λ has never been modeled so far in data reductions, such bounds may be optimistic. A possible way to obtain independent, tighter local constraints may be offered by pulsars orbiting the supermassive black hole in the galactic center, should they finally be discovered. With an expected timing accuracy of the order of 1−10 μs, they might be able to constrain Λ more effectively. We calculate its perturbations ΔδτpΛ on the periodic variation δτp of the time of arrivals τp of the pulses. Then, we apply our results to a putative pulsar moving along an S2-type orbit inferring |Λ| ≲ 10−48 m−2 for στp = 1 μs. View the paper here.
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Article
Accurate Mass Measurements for Planetary Microlensing Events Using High Angular Resolution Observations
Universe 2018, 4(4), 61; https://doi.org/10.3390/universe4040061 - 24 Apr 2018
Cited by 6 | Viewed by 1483
Abstract
The microlensing technique is a unique method to hunt for cold planets over a range of mass and separation, orbiting all varieties of host stars in the disk of our galaxy. It provides precise mass-ratio and projected separations in units of the Einstein [...] Read more.
The microlensing technique is a unique method to hunt for cold planets over a range of mass and separation, orbiting all varieties of host stars in the disk of our galaxy. It provides precise mass-ratio and projected separations in units of the Einstein ring radius. In order to obtain the physical parameters (mass, distance, orbital separation) of the system, it is necessary to combine the result of light curve modeling with lens mass-distance relations and/or perform a Bayesian analysis with a galactic model. A first mass-distance relation could be obtained from a constraint on the Einstein ring radius if the crossing time of the source over the caustic is measured. It could then be supplemented by secondary constraints such as parallax measurements, ideally by using coinciding ground and space-born observations. These are still subject to degeneracies, like the orbital motion of the lens. A third mass-distance relation can be obtained thanks to constraints on the lens luminosity using high angular resolution observations with 8 m class telescopes or the Hubble Space Telescope. The latter route, although quite inexpensive in telescope time is very effective. If we have to rely heavily on Bayesian analysis and limited constraints on mass-distance relations, the physical parameters are determined to 30–40% typically. In a handful of cases, ground-space parallax is a powerful route to get stronger constraint on masses. High angular resolution observations will be able to constrain the luminosity of the lenses in the majority of the cases, and in favorable circumstances it is possible to derive physical parameters to 10% or better. Moreover, these constraints will be obtained in most of the planets to be discovered by the Euclid and WFIRST satellites. We describe here the state-of-the-art approaches to measure lens masses and distances with an emphasis on high angular resolution observations. We will discuss the challenges, recent results and perspectives. Full article
(This article belongs to the Special Issue Gravitational Lensing and Astrometry)
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Article
String Sigma Models on Curved Supermanifolds
Universe 2018, 4(4), 60; https://doi.org/10.3390/universe4040060 - 24 Apr 2018
Cited by 2 | Viewed by 1204
Abstract
We use the techniques of integral forms to analyze the easiest example of two-dimensional sigma models on a supermanifold. We write the action as an integral of a top integral form over a D = 2 supermanifold, and we show how to interpolate [...] Read more.
We use the techniques of integral forms to analyze the easiest example of two-dimensional sigma models on a supermanifold. We write the action as an integral of a top integral form over a D = 2 supermanifold, and we show how to interpolate between different superspace actions. Then, we consider curved supermanifolds, and we show that the definitions used for flat supermanifolds can also be used for curved supermanifolds. We prove it by first considering the case of a curved rigid supermanifold and then the case of a generic curved supermanifold described by a single superfield E. Full article
(This article belongs to the Special Issue Super Geometry for Super Strings)
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Article
Perspectives on Constraining a Cosmological Constant-Type Parameter with Pulsar Timing in the Galactic Center
Universe 2018, 4(4), 59; https://doi.org/10.3390/universe4040059 - 26 Mar 2018
Cited by 11 | Viewed by 1702
Abstract
Independent tests aiming to constrain the value of the cosmological constant Λ are usually difficult because of its extreme smallness [...] Read more.
Independent tests aiming to constrain the value of the cosmological constant Λ are usually difficult because of its extreme smallness ( Λ 1 × 10 - 52 m - 2 , or 2 . 89 × 10 - 122 in Planck units ) . Bounds on it from Solar System orbital motions determined with spacecraft tracking are currently at the 10 - 43 10 - 44 m - 2 ( 5 1 × 10 - 113 in Planck units ) level, but they may turn out to be optimistic since Λ has not yet been explicitly modeled in the planetary data reductions. Accurate ( σ τ p 1 10 μ s ) timing of expected pulsars orbiting the Black Hole at the Galactic Center, preferably along highly eccentric and wide orbits, might, at least in principle, improve the planetary constraints by several orders of magnitude. By looking at the average time shift per orbit Δ δ τ ¯ p Λ , an S2-like orbital configuration with e = 0 . 8839 , P b = 16 yr would permit a preliminarily upper bound of the order of Λ 9 × 10 - 47 m - 2 2 × 10 - 116 in Planck units if only σ τ p were to be considered. Our results can be easily extended to modified models of gravity using Λ -type parameters. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2018 - Gravitational Physics)
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Article
Scaling Properties of Spectra in New Exact Solutions of Rotating, Multi-Component Fireball Hydrodynamics
Universe 2018, 4(4), 58; https://doi.org/10.3390/universe4040058 - 26 Mar 2018
Cited by 4 | Viewed by 1228
Abstract
We describe fireballs that rehadronize from a perfect fluid of quark matter, characterized by the lattice QCD equation of state, to a chemically frozen, multi-component mixture, that contains various kinds of observable hadrons. For simplicity and clarity, we apply a non-relativistic approximation to [...] Read more.
We describe fireballs that rehadronize from a perfect fluid of quark matter, characterized by the lattice QCD equation of state, to a chemically frozen, multi-component mixture, that contains various kinds of observable hadrons. For simplicity and clarity, we apply a non-relativistic approximation to describe the kinematics of this expansion. Unexpectedly, we identify a secondary explosion that may characterize fireball hydrodynamics at the QCD critical point. After rehadronization, the multi-component mixture of hadrons keeps on rotating and expanding together, similarly to a single component fluid. After kinetic freeze-out, the effective temperature T i of the single-particle spectra of hadron type h i is found to be a sum of the kinetic freeze-out temperature T f (that is independent of the hadron type h i ) and a term proportional to the mass m i of hadron type h i . The coefficient of proportionality to m i is found to be independent of the hadron type h i but to be dependent on the radial flow and vorticity of collective dynamics. Full article
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