We study spatially flat bouncing cosmologies and models with the early-time Genesis epoch in a popular class of generalized Galileon theories. We ask whether there exist solutions of these types which are free of gradient and ghost instabilities. We find that irrespectively of the forms of the Lagrangian functions, the bouncing models either are plagued with these instabilities or have singularities. The same result holds for the original Genesis model and its variants in which the scale factor tends to a constant as t → −∞. The result remains valid in theories with additional matter that obeys the Null Energy Condition and interacts with the Galileon only gravitationally. We propose a modified Genesis model which evades our no-go argument and give an explicit example of healthy cosmology that connects the modified Genesis epoch with kination (the epoch still driven by the Galileon field, which is a conventional massless scalar field at that stage).

We study "classical" bouncing and Genesis models in beyond Horndeski theory. We give an example of spatially flat bouncing solution that is non-singular and stable throughout the whole evolution. We also provide an example of stable geodesically complete Genesis with similar features. The model is arranged in such a way that the scalar field driving the cosmological evolution initially behaves like full-fledged beyond Horndeski, whereas at late times it becomes a massless scalar field minimally coupled to gravity.

We suggest a novel version of a cosmological Genesis model within beyond Horndeski theory. It combines the initial Genesis behavior of Creminelli et al. [1,2] with complete stability property of the previous beyond Horndeski construction [3]. The specific features of the model are that space-time rapidly tends to Minkowski in asymptotic past and that both asymptotic past and future are described by General Relativity (GR). 1 sa.mironov 1@physics.msu.ru 2 rubakov@inr.ac.ru 3 volkova.viktoriya@physics.msu.ru 1 arXiv:1905.06249v1 [hep-th] 15 May 2019Horndeski theories are general scalar-tensor gravities with second order equations of motion. These have been further generalised to theories with higher order equations of motion, dubbed DHOST theories [8,9,10,11,12,13]. The constraint structure of the DHOST theories is such that they propagate only three dynamical degrees of freedom, just like Horndeski theories. Horndeski theories and their generalizations are interesting playground for studying stable NEC/NCC-violating cosmologies (for a review see, e.g., Ref. [14]), and Genesis in particular [15,16,17].One of the main reasons for going beyond Horndeski, at least in the context of the early cosmology, is to construct examples of complete spatially flat, non-singular cosmological scenarios like Genesis. Modulo options that are dangerous from the viewpoint of geodesic completeness and/or strong coupling [18,19,20] (see, however [21]), Horndeski theories are not suitable for this purpose because of inevitable development of gradient or ghost instabilities at some stage of the evolution [18,19,22,23]. However, this no-go theorem does not apply to DHOST theories, as demonstrated in Refs. [24,25,3] for a subclass usually referred to as "beyond Horndeski" (aka GLVP [9]). Indeed, this subclass has been used for constructing non-singular cosmological models of the bouncing Universe and Genesis, which are stable at the linearised level during entire evolution [3,26,27].Previous constructions of complete bouncing and Genesis models in beyond Horndeski theories were limited by overestimating the danger of a phenomenon called γ-crossing (or Θ-crossing). The discussion of this phenomenon is fairly technical, and we postpone it to Section 2. It suffices to point out here that insisting on the absence of γ-crossing prevents one from constructing bounce and Genesis models where linearized gravity agrees with GR both in asymptotic future and in asymptotic past, and, in Genesis case, whose space-time rapidly tends to Minkowski in asymptotic past. An example is a Genesis-like model of Ref. [3] where the scale factor behaves as a(t) ∝ |t| −1/3 as t → −∞.It has been shown, however, that γ-crossing is, in fact, an innocent phenomenon. Originally, this fact has been established in Newtonian gauge [28], and then confirmed in unitary gauge [27]. It opens up a possibility to construct new bouncing and Genesis models 4 . Indeed, an example of fully stable spatially flat bouncing model has been constructed in beyond Horndeski theory [27], whose ...

Experimental implementation of a quantum computing algorithm strongly relies on the ability to construct required unitary transformations applied to the input quantum states. In particular, near-term linear optical computing requires universal programmable interferometers, capable of implementing an arbitrary transformation of input optical modes. So far these devices were composed as a circuit with well defined building blocks, such as balanced beamsplitters. This approach is vulnerable to manufacturing imperfections inevitable in any realistic experimental implementation, and the larger the circuit size grows, the more strict the tolerances become. In this work we demonstrate a new methodology for the design of the high-dimensional mode transformations, which overcomes this problem, and carefully investigate its features. The circuit in our architecture is composed of interchanging mode mixing layers, which may be almost arbitrary, and layers of variable phaseshifters, allowing to program the device to approximate any desired unitary transformation. arXiv:1906.06748v1 [quant-ph]

We consider theories which explain the flatness of the power spectrum of scalar perturbations in the Universe by conformal invariance, such as conformal rolling model and Galilean Genesis. We show that to the leading non-linear order, perturbations in all models from this class behave in one and the same way, at least if the energy density of the relevant fields is small compared to the total energy density (spectator approximation). We then turn to the intrinsic non-Gaussianities in these models (as opposed to non-Gaussianities that may be generated during subsequent evolution). The intrinsic bispectrum vanishes, so we perform the complete calculation of the trispectrum and compare it with the trispecta of local forms in various limits. The most peculiar feature of our trispectrum is a (fairly mild) singularity in the limit where two momenta are equal in absolute value and opposite in direction (folded limit). Generically, the intrinsic non-Gaussianity can be of detectable size.

We study whether it is possible to design a "classical" spatially flat bouncing cosmology or a static, spherically symmetric asymptotically flat Lorentzian wormhole in cubic Galileon theories interacting with an extra scalar field. We show that bouncing models are always plagued with gradient instabilities, while there are always ghosts in wormhole backgrounds. * While, in the cosmological context, the energy density of Galileons can indeed increase in time in a healthy way, constructing a complete bouncing or Genesis cosmology (full evolution from t = −∞ to t = +∞) is a challenge. For example, one can construct a spatially flat bouncing model without pathologies at or near the bounce [21,30], yet, in known examples, the gradient instabilities occur at some later or earlier epoch [22,[24][25][26]. Although these instabilities have been argued to remain under control due to higher derivative terms [31], it would be interesting to design an example of a complete "classical" bouncing cosmological model without ghosts and gradient instabilities.Another potential application of the NEC violation is a putative construction of stable asymptotically flat Lorentzian wormholes. However, previous attempts to design a wormhole supported by Galileon have failed [32,33].It is worth noting that there exist bouncing models with nonzero spatial curvature [34,35]. Likewise, there are Lorentzian wormholes which are not asymptotically flat [35]. These solutions employ scalar fields with fairly conventional kinetic terms that do not violate the NEC. On the contrary, we are interested in spatially flat bouncing cosmologies and asymptotically flat wormholes, which necessarily require NEC violation (hence our interest in Galileons).Recently, two no-go theorems have been proven in the Galileon context [33,36,37]. Both apply to general relativity with the Galileon field and no other matter. One theorem shows that spatially flat bouncing cosmological solutions are always plagued with gradient instabilities [36,37]. The other states that, in cubic Galileon theory, static spherically symmetric Lorentzian wormholes are always plagued with ghosts [33].One might hope that these problems can be overcome by adding extra non-Galileonic matter. This matter, if it satisfies the NEC, must interact with Galileon directly; otherwise, the above theorems remain valid [33,36,37]. The simplest option is to add a scalar field with first-derivative terms in the Lagrangian. This is precisely the system studied in this paper. Somewhat surprisingly, we show that, at least for cubic Galileon, the above theorems are still at work: there are always gradient instabilities about bouncing cosmological solutions, and there always exist ghosts in backgrounds of static spherically symmetric Lorentzian wormholes.Concerning wormholes, our spherically symmetric setting is not general. That is, we do not consider a cross term in a metric characteristic of Newman-Unti-Tamburino (NUT) spacetimes. In view of recent interesting results on NUT wormholes [35,38], this generalizat...

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.

We find generalized Jack polynomials for the SU (3) group and verify that their Selberg averages for several first levels are given by Nekrasov functions. To compute the averages we derive recurrence relations for the sl3 Selberg integrals. IntroductionThe AGT relations [1] provide an extremely interesting link between the four dimensional N = 2 gauge theories of class S [2] and two dimensional conformal field theories. Moreover, these relations offer a new view on a variety of related fields, such as integrable systems The most surprising property of the AGT relations is that they give an additional and unexpected structure on the conformal field theory Hilbert space. One usually writes the states in this space as descendants of some primary field: L −Y |α , all the correlators therefore become sums over Young diagrams Y . However, the Nekrasov function is a sum over pairs of Young diagrams Y . Finding the corresponding basis | Y in CFT is an interesting problem. This basis was found explicitly in the case of c = 1 [6] and it was argued to exist in the general case [7]. Concretely, if one performs the bosonization of the Virasoro algebra, the basis vectors are expressed through the generalized Jack polynomials which are defined as the polynomial eigenfunctions of a certain differential operator 1 [8]. One can also compute the correlators in CFT using the Dotsenko-Fateev approach, in which they are given by certain multiple integrals related to the Selberg integrals [9]. In this setting the generalized Jack polynomials also play a distinguished role: their Selberg averages are factorised into a product of linear functions of momenta. More precisely, the averages are given by the Nekrasov functions.More generally, the AGT relations map the gauge theory with the SU (N ) group to the four point conformal block in the Toda field theory with two general and two degenerate fields [10]. The same factorisation of the sl N Selberg averages should happen in this case as well.In this Letter we explicitly find generalized Jack polynomials for the group SU (3) and check that their Selberg averages indeed reproduce the Nekrasov functions on the first levels. To compute the averages we derive the W -constraints for the β-deformed A 2 quiver matrix model. In section 2 we introduce the differential operator whose polynomial eigenfunctions are given by the generalized Jack polynomials, compute them explicitly and check their elementary properties. In section 3 we derive the DotsenkoFateev representation of the conformal block in Toda field theory and show that the AGT relations hold if certain Selberg averages of generalised Jack polynomials are given by the Nekrasov functions. Using the W constraints we compute the averages and check the relevant formulas for the first levels. The Nekrasov functions and AGT relations are provided in Appendix A. The W constraints are presented in Appendix B. * sa.mironov_1@physics.msu.ru † andrey.morozov@itep.ru ‡ yegor.zenkevich@gmail.com, zenkevich@ms2.inr.ac.ru 1 It is suspected that g...