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## Symmetron scalar fields: Modified gravity, dark matter, or both?

Scalar fields coupled to gravity through the Ricci scalar have been considered both as dark matter candidates and as a possible modified gravity explanation for galactic dynamics. It has recently been demonstrated that the dynamics of baryonic matter in disk galaxies may be explained, in the absence of particle dark matter, by a symmetron scalar field that mediates a fifth force. The symmetron provides a concrete and archetypal field theory within which to explore how large a role modifications of gravity can play on galactic scales. In this article, we extend these previous works by asking whether the same symmetron field can explain the difference between the baryonic and lens masses of galaxies through a modification of gravity. We consider the possibilities for minimal modifications of the model and find that this difference cannot be explained entirely by the symmetron fifth force without extending the field content of the model. Instead, we are pushed toward a regime of parameter space where one scalar field both mediates a fifth force and stores enough energy density that it also contributes to the galaxy’s gravitational potential as a dark matter component, a regime which remains to be fully explored.

The conference was held in the form of lectures by leading scientists, oral and poster presentations of young scientists and students of physical specialties, as well as leaders of innovative structures for the purpose of mutual acquaintance with the new results of fundamental research on a wide range of areas in physics, the prospects and challenges in the expansion of relations between science , education and high technologies. **SECTION (heads):** I. LASERS (Fundam. Probl., Computer ...) (prof. A.A.Ionin) II. OPTICS (quant., And nano materials and new sources) (d.f.m.n.A.V.Masalov) III. Solid state physics, INCLUDING Nanostructures ELEM. BASE UNIT (Corresponding Member of RAS N.N.Sibeldin) IV. Nuclear physics, high energy physics (prof. O.D.Dalkarov, Head: Prof. V.A.Ryabov) V. PLASMA PHYSICS and particle beams (Head: Prof. A.V.Agafonov) VI. Astrophysics (Head: Prof. S.A.Bogachev) VII. PHYSICS IN THE MODERN INSTRUMENT AND TECHNOLOGY (prof. V.N.Nevolin)

Today we have the problem of big science data. The information collecting in science experiments, especially in bioinformatics and astrophysics grows in amazing rate. In this paper we consider special program techniques and computer technologies used for work with superlarge volumes of data. Also, we discuss the state of affairs with the big data in the Institute of Mathematical Problems of Biology RAS and in the Pushchino Radio Astronomy Observatory (Astro Space Center of Lebedev Physics Institute RAS).

Gaia16aye was a binary microlensing event discovered in the direction towards the northern Galactic disc and was one of the first microlensing events detected and alerted to by the Gaia space mission. Its light curve exhibited five distinct brightening episodes, reaching up to I = 12 mag, and it was covered in great detail with almost 25 000 data points gathered by a network of telescopes. We present the photometric and spectroscopic follow-up covering 500 days of the event evolution. We employed a full Keplerian binary orbit microlensing model combined with the motion of Earth and Gaia around the Sun to reproduce the complex light curve. The photometric data allowed us to solve the microlensing event entirely and to derive the complete and unique set of orbital parameters of the binary lensing system. We also report on the detection of the first-ever microlensing space-parallax between the Earth and Gaia located at L2. The properties of the binary system were derived from microlensing parameters, and we found that the system is composed of two main-sequence stars with masses 0.57 ± 0.05 M☉ and 0.36 ± 0.03 M☉ at 780 pc, with an orbital period of 2.88 years and an eccentricity of 0.30. We also predict the astrometric microlensing signal for this binary lens as it will be seen by Gaia as well as the radial velocity curve for the binary system. Events such as Gaia16aye indicate the potential for the microlensing method of probing the mass function of dark objects, including black holes, in directions other than that of the Galactic bulge. This case also emphasises the importance of long-term time-domain coordinated observations that can be made with a network of heterogeneous telescopes.

Thanks to the first mm studies on the territory of the former USSR in the early 1960s and succeeding sub-mm measurements in the 1970s - early 1980s at wavelengths up to 0.34 mm, a completely unique astroclimate was revealed in the Eastern Pamirs, only slightly inferior to the available conditions on the Chajnantor plateau in Chile and Mauna Kea. Due to its high plateau altitude (4300 - 4500 m) surrounded from all sides by big (~7000 m) air-drying icy mountains and remoteness from oceans this area has the lowest relative humidity in the former USSR and extremely high atmospheric stability. In particular, direct measurements of precipitated water vapor in the winter months showed typical pwv=0.8 - 0.9 mm with sometimes of 0.27 mm. To validate previous studies and to compare them with results for other similar regions we performed opacity calculations at mm - sub-mm wavelengths for different sites in the Eastern Pamirs, Tibet, Indian Himalayas, APEX, ALMA, JCM, LMT and many others. To do this we integrate radiative transfer equations using the output of NASA Global Modeling and Assimilation Office model GEOS-FPIT for more than 12 years. We confirm previous conclusions about exceptionally good astroclimate in the Eastern Pamirs. Due to its geographical location, small infrastructure and the absence of any interference in radio and optical bands, this makes the Eastern Pamirs the best place in the Eastern Hemisphere for both optical and sub-mm astronomy.

Cosmic strings are topologically stable, one-dimensional defects in vacuum which can appear during appropriate phase transitions in an adiabatically expanding early Universe which cools down from a very hot initial state. Their discovery would lead to advances in cosmology and fundamental physics. One of the most efficient ways to detect cosmic strings is related to their gravitational lensing signatures which appear to be different from those of standard lenses. We study a simple model of gravitational lensing by symmetric and asymmetric loops. An explicit form of the lens equation is obtained, and the relations for magnification are derived. We also discuss possible observational manifestations of cosmic strings within our model.

In Proceedings of the conference participants are presented on the following topics: 1) Lasers and Optics 2) Solid State Physics 3) Nuclear Physics 4) The generation and use of X-rays 5) Plasma Physics and particle beams 6) Astrophysics

Let k be a field of characteristic zero, let G be a connected reductive algebraic group over k and let g be its Lie algebra. Let k(G), respectively, k(g), be the field of k- rational functions on G, respectively, g. The conjugation action of G on itself induces the adjoint action of G on g. We investigate the question whether or not the field extensions k(G)/k(G)^G and k(g)/k(g)^G are purely transcendental. We show that the answer is the same for k(G)/k(G)^G and k(g)/k(g)^G, and reduce the problem to the case where G is simple. For simple groups we show that the answer is positive if G is split of type A_n or C_n, and negative for groups of other types, except possibly G_2. A key ingredient in the proof of the negative result is a recent formula for the unramified Brauer group of a homogeneous space with connected stabilizers. As a byproduct of our investigation we give an affirmative answer to a question of Grothendieck about the existence of a rational section of the categorical quotient morphism for the conjugating action of G on itself.

Let G be a connected semisimple algebraic group over an algebraically closed field k. In 1965 Steinberg proved that if G is simply connected, then in G there exists a closed irreducible cross-section of the set of closures of regular conjugacy classes. We prove that in arbitrary G such a cross-section exists if and only if the universal covering isogeny Ĝ → G is bijective; this answers Grothendieck's question cited in the epigraph. In particular, for char k = 0, the converse to Steinberg's theorem holds. The existence of a cross-section in G implies, at least for char k = 0, that the algebra k[G]G of class functions on G is generated by rk G elements. We describe, for arbitrary G, a minimal generating set of k[G]G and that of the representation ring of G and answer two Grothendieck's questions on constructing generating sets of k[G]G. We prove the existence of a rational (i.e., local) section of the quotient morphism for arbitrary G and the existence of a rational cross-section in G (for char k = 0, this has been proved earlier); this answers the other question cited in the epigraph. We also prove that the existence of a rational section is equivalent to the existence of a rational W-equivariant map T- - - >G/T where T is a maximal torus of G and W the Weyl group.