This is the story of a simple observation made by Peter (and independently by Haim Harari) more than half a century ago and whose unexpectedly rich consequences have kept bugging my mind, on and off, ...till these days.
A
bstract
We reformulate in Hamiltonian language the recent proposal by Hohm and Zwiebach of an action yielding the most general
O
(
d
,
d
)-symmetric string cosmology equations, at tree-level in the ...string-loop expansion, but to all orders in the
α′
expansion. This allows us to give a simple characterization of a large class of non-singular, non-perturbative, pre-big bang scenarios smoothly interpolating between a low-energy initial accelerated (string frame) expansion and a phase of final (string and Einstein frame) decelerated expansion. Interestingly, these solutions must necessarily include, just around the bounce, a very short phase of (string-frame) contraction.
We consider a thought experiment in which an energetic massless string probes a 'stringhole' (a heavy string lying on the correspondence curve between strings and black holes) at large enough impact ...parameter for the regime to be under theoretical control. The corresponding, explicitly unitary, S-matrix turns out to be perturbatively sensitive to the microstate of the stringhole: in particular, at leading order in ls b, it depends on a projection of the stringhole's Lorentz-contracted quadrupole moment. The string-black hole correspondence is therefore violated if one assumes quantum hair to be exponentially suppressed as a function of black-hole entropy. Implications for the information paradox are briefly discussed.
We present a general gauge invariant formalism for defining cosmological averages that are relevant for observations based on light-like signals. Such averages involve either null hypersurfaces ...corresponding to a family of past light-cones or compact surfaces given by their intersection with timelike hypersurfaces. Generalized Buchert-Ehlers commutation rules for derivatives of these light-cone averages are given. After introducing some adapted "geodesic light-cone" coordinates, we give explicit expressions for averaging the redshift to luminosity-distance relation and the so-called "redshift drift" in a generic inhomogeneous Universe.
Summary
Regional anaesthesia in children has evolved rapidly in the last decade. Although it previously consisted of primarily neuraxial techniques, the practice now incorporates advanced peripheral ...nerve blocks, which were only recently described in adults. These novel blocks provide new avenues for providing opioid‐sparing analgesia while minimising invasiveness, and perhaps risk, associated with older techniques. At the same time, established methods, such as infant spinal anaesthesia, under‐utilised in the last 20 years, are experiencing a revival. The impetus has been the concern regarding the potential long‐term neurocognitive effects of general anaesthesia in the young child. These techniques have expanded from single shot spinal anaesthesia to combined spinal/epidural techniques, which can now effectively provide surgical anaesthesia for procedures below the umbilicus for a prolonged period of time, thereby avoiding the need for general anaesthesia. Continuous 2‐chloroprocaine infusions, previously only described for intra‐operative regional anaesthesia, have gained popularity as a means of providing prolonged postoperative analgesia in epidural and continuous nerve block techniques. The rapid, liver‐independent metabolism of 2‐chloroprocaine makes it ideal for prolonged local anaesthetic infusions in neonates and small infants, obviating the increased risk of local anaesthetic systemic toxicity that occurs with amide local anaesthetics. Debate continues over certain practices in paediatric regional anaesthesia. While the rarity of complications makes comparative analyses difficult, data from large prospective registries indicate that providing regional anaesthesia to children while under general anaesthesia appears to be at least as safe as in the sedated or awake patient. In addition, the estimated frequency of serious adverse events demonstrates that regional blocks in children under general anaesthesia are no less safe than in awake adults. In infants, the techniques of direct thoracic epidural placement or caudal placement with cephalad threading each have distinct advantages and disadvantages. As the data cannot support the safety of one technique over the other, the site of epidural insertion remains largely a matter of anaesthetist discretion.
The remarkable properties of the recently proposed geodesic light-cone (GLC) gauge allow to explicitly solve the geodesic-deviation equation, and thus to derive an exact expression for the Jacobi map ...J super(A) sub(B)(s, o) connecting a generic source s to a geodesic observer o in a generic space time. In this gauge J super(A) sub(B) factorizes into the product of a local quantity at s times one at o, implying similarly factorized expressions for the area and luminosity distance. In any other coordinate system J super(A) sub(B) is simply given by expressing the GLC quantities in terms of the corresponding ones in the new coordinates. This is explicitly done, at first and second order, respectively, for the synchronous and Poisson gauge-fixing of a perturbed, spatially- at cosmological background, and the consistency of the two outcomes is checked. Our results slightly amend previous calculations of the luminosity-redshift relation and suggest a possible non-perturbative way for computing the effects of inhomogeneities on observations based on light-like signals.
A
bstract
We recall and update, both theoretically and phenomenologically, our (nearly) forty-years-old proposal of a string-junction as a necessary complement to the conventional classification of ...hadrons based just on their quark-antiquark constituents. In that proposal single (though in general metastable) hadronic states are associated with “irreducible” gauge-invariant operators consisting of Wilson lines (visualized as strings of color flux tubes) that may either end on a quark or an antiquark, or annihilate in triplets at a junction
J
or an anti-junction
J
¯
. For the junction-free sector (ordinary
q
q
¯
mesons and glueballs) the picture is supported by large-
N
(number of colors) considerations as well as by a lattice strong-coupling expansion. Both imply the famous OZI rule suppressing quark-antiquark annihilation diagrams. For hadrons with
J
and/or
J
¯
constituents the same expansions support our proposal, including its generalization of the OZI rule to the suppression of
J
−
J
¯
annihilation diagrams. Such a rule implies that hadrons with junctions are “mesophobic” and thus unusually narrow if they are below threshold for decaying into as many baryons as their total number of junctions (two for a tetraquark, three for a pentaquark). Experimental support for our claim, based on the observation that narrow multiquark states typically lie below (well above) the relevant baryonic (mesonic) thresholds, will be presented.
A
bstract
Following up on recent work by Caron-Huot et al. we consider a generalization of the old Lovelace-Shapiro model as a toy model for
ππ
scattering satisfying (most of) the properties expected ...to hold in (’t Hooft’s) large-
N
limit of massless QCD. In particular, the model has asymptotically linear and parallel Regge trajectories at positive
t
, a positive leading Regge intercept
α
0
<
1, and an effective bending of the trajectories in the negative-
t
region producing a fixed branch point at
J
= 0 for
t < t
0
<
0. Fixed (physical) angle scattering can be tuned to match the power-like behavior (including logarithmic corrections) predicted by perturbative QCD:
A
(
s, t
) ∼
s
−
β
log(
s
)
−γ
F
(
θ
). Tree-level unitarity (i.e. positivity of residues for all values of
s
and
J
) imposes strong constraints on the allowed region in the α
0
-
β
-γ parameter space, which nicely includes a physically interesting region around
α
0
= 0
.
5,
β
= 2 and γ = 3. The full consistency of the model would require an extension to multi-pion processes, a program we do not undertake in this paper.
We present a new method to compute the deflection of light rays in a perturbed FLRW geometry. We exploit the properties of the Geodesic Light Cone (GLC) gauge where null rays propagate at constant ...angular coordinates irrespectively of the given (inhomogeneous and/or anisotropic) geometry. The gravitational deflection of null geodesics can then be obtained, in any other gauge, simply by expressing the angular coordinates of the given gauge in terms of the GLC angular coordinates. We apply this method to the standard Poisson gauge, including scalar perturbations, and give the full result for the deflection effect in terms of the direction of observation and observed redshift up to second order, and up to third order for the leading lensing terms. We also compare our results with those presently available in the literature and, in particular, we provide a new non trivial check of a previous result on the luminosity-redshift relation up to second order in cosmological perturbation theory.
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