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Viktor Toth adds theoretical insights to the modified theories of gravity that aimed to solve the dark matter problem without necessitating the existence of hypothetic particles of nonbaryonic matter.

In this pedestrian approach I give my personal point of view on the various problems posed by dark matter in the universe. After a brief historical overview I discuss the various solutions stemming from high energy particle physics, and the current status of experimental research on candidate particles (WIMPS). In the absence of direct evidence, the theories can still be evaluated by comparing their implications for the formation of galaxies, clusters and superclusters of galaxies against astronomical observations. I conclude briefly with the attempts to circumvent the dark matter problem by modifying the laws of gravity., Comment: 11 pages

Dévoiler la nature de la matière noire est l’un des plus grands défis de l’astrophysique. Jean-Pierre Luminet fait la lumière sur cette question.

Since the late 1950s a growing number of composers have engaged with scientific research and have tried to incorporate their understanding of various models and theories into their musical works. A...

Discovered in 1949 by Kurt Gödel, the homogeneous rotating universe model contains closed timelike curves and therefore allows for travel into the past, accompanied by all its paradoxes.

Jacques Mertzeisen and Jean-Pierre Luminet shed light on Johannes Remus Quietanus, a much neglected seventeenth-century astronomer and royal physician, a friend of Kepler, and one of the very first to observe a transit of Mercury.

In the early 17th-century, the prospects for the Copernican revolution appeared uncertain. Jean-Pierre Luminet explores how the 17th-century Provençal humanist astronomers Nicolas-Claude Fabri de Peiresc and Pierre Gassendi, along with Jean-Dominique Cassini and Michel de Montaigne, continued the revolution and in the process helped shape the subsequent development of Western science.

I give an epistemological analysis of the developments of relativistic cosmology from 1917 to 1966, based on the seminal articles by Einstein, de Sitter, Friedmann, Lemaitre, Hubble, Gamow and other historical figures of the field. It appears that most of the ingredients of the present-day standard cosmological model, including the acceleration of the expansion due to a repulsive dark energy, the interpretation of the cosmological constant as vacuum energy or the possible non-trivial topology of space, had been anticipated by Georges Lemaitre, although his articles remain mostly unquoted.

Though some American astronomers were reluctant to accept that gravitational wave physics is indeed astronomy, this was not at all the case in France.

I give a critical review of the holographic hypothesis, which posits that a universe with gravity can be described by a quantum field theory in fewer dimensions. I first recall how the idea originated from considerations on black hole thermodynamics and the so-called information paradox that arises when Hawking radiation is taken into account. String Quantum Gravity tried to solve the puzzle using the AdS/CFT correspondence, according to which a black hole in a 5-D anti-de Sitter space is like a flat 4-D field of particles and radiation. Although such an interesting holographic property, also called gauge/gravity duality, has never been proved rigorously, it has impulsed a number of research programs in fields as diverse as nuclear physics, condensed matter physics, general relativity and cosmology. I finally discuss the pros and cons of the holographic conjecture, and emphasizes the key role played by black holes for understanding quantum gravity and possible dualities between distant fields of theoretical physics., 13 pages in Inference: The International Review of Science, vol.2 Issue 1 (2016)

Jean-Pierre Luminet explique le principe holographique, la conjecture de Maldacena, et la résolution du paradoxe d’information.

We tend to generally believe that art's mission is to make us touch the intangible, whereas science's mission only consists of providing a game of «questions-answers». However, there is a genuine scientific creation in which the game is inversed and is deployed as an «answers-questions». The question, which can arise as much as tne absence of a «too complete» answer, is in my opinion a common gateway towards these two areas of the human knowledge., On a tendance à croire généralement que la mission de l'art est de nous faire toucher l'impalpable, alors que celle de la science ne consisterait qu'à fournir un jeu de «questions-réponses». Or, il existe une authentique création scientifique dans laquelle le jeu s'inverse et se déploie en «réponses-questions». L'interrogation, qui peut naître autant de l'absence que d'un trop-plein de réponses, est à mon sens une porte d'accès commune à ces deux aires de la connaissance humaine., Luminet Jean-Pierre. Imaginaire et imagination. In: Raison présente, n°179, 3e trimestre 2011. Arts et sciences. pp. 15-23.

We interest in the case of a main-sequence star deeply penetrating within the tidal radius of a massive black hole. We focus on the compression phase leading to a so-called pancake configuration of the star at the instant of maximal compression. The aim is to study the tidal compression process paying particular attention to the development of shock waves;to deduce reliable estimates of the thermodynamical quantities involved in the pancake star; and to solve a controversy about whether or not thermonuclear reactions can be triggered in the core of a tidally compressed star. We have set up a one-dimensional hydrodynamical model well-adapted to the geometry of the problem. Based on the high-resolution shock-capturing Godunov-type approach, it allows to study the compression phase undergone by the star in the direction orthogonal to its orbital plane. We show the existence of two regimes depending on whether shock waves develop before or after the instant of maximal core compression. In both cases we confirm high compression and heating factors in the stellar core able to trigger a thermonuclear explosion. Moreover, we show that the shock waves carry outwards a brief but very high peak of temperature from the centre to the surface of the star. We tentatively conclude that the phenomenon could give rise to hard electromagnetic radiation, to be compared to some X-ray flares already observed in some galactic nuclei harbouring massive black holes. Finally, we estimate that the rate of pancake stars should be about $10^{-5}$ per galaxy per year. If generated in hard X- or $\gamma$-ray band, several events of this kind per year should be detectable within the full observable universe., Comment: 19 pages, 38 figures, 7 tables; v2 : corrected to match version accepted in Astron. Astrophys. Tables and references added, new simulations also performed for adiabatic index 4/3

The film Interstellar draws heavily from contemporary science. It makes reference to a range of topics, from established concepts such as fast-spinning black holes, accretion disks, tidal effects, and time dilation, to far more speculative ideas such as wormholes, time travel, additional space dimensions, and the theory of everything. Jean-Pierre Luminet examines how these topics were depicted.

What is the shape of space is a long-standing question in cosmology. In this talk I review recent advances in cosmic topology since it has entered a new era of experimental tests. High redshift surveys of astronomical sources and accurate maps of the Cosmic Microwave Background radiation (CMB) are beginning to hint at the shape of the universe, or at least to limit the wide range of possibilities. Among those possibilites are surprising "wrap around" universe models in which space, whatever its curvature, may be smaller than the observable universe and generate topological lensing effects on a detectable cosmic scale. In particular, the recent analysis of CMB data provided by the WMAP satellite suggests a finite universe with the topology of the Poincare dodecahedral spherical space. Such a model of a "small universe", the volume of which would represent only about 80% the volume of the observable universe, offers an observational signature in the form of a predictable topological lens effect on one hand, and rises new issues on the early universe physics on the other hand., Comment: 19 pages, 9 postscript figures. Invited talk at the XXV Brazilian National Meeting on Elementary Particles and Fields, 23-27 August 2004. To be published in Brazilian Journal of Physics

This article investigates the computation of the eigenmodes of the Laplacian operator in multi-connected three-dimensional spherical spaces. General mathematical results and analytical solutions for lens and prism spaces are presented. Three complementary numerical methods are developed and compared with our analytic results and previous investigations. The cosmological applications of these results are discussed, focusing on the cosmic microwave background (CMB) anisotropies. In particular, whereas in the Euclidean case too small universes are excluded by present CMB data, in the spherical case there will always exist candidate topologies even if the total energy density parameter of the universe is very close to unity.

This article gives the construction and complete classification of all three-dimensional spherical manifolds, and orders them by decreasing volume, in the context of multiconnected universe models with positive spatial curvature. It discusses which spherical topologies are likely to be detectable by crystallographic methods using three-dimensional catalogs of cosmic objects. The expected form of the pair separation histogram is predicted (including the location and height of the spikes) and is compared to computer simulations, showing that this method is stable with respect to observational uncertainties and is well suited for detecting spherical topologies., 32 pages, 26 figures

What is the shape of the Universe? Is it finite or infinite ? Is space multi-connected to create ghost images of faraway cosmic sources? After a "dark age" period, the field of cosmic topology has now become one of the major concerns in astronomy and cosmology, not only from theorists but also from observational astronomers. Here I give a personal account of the spectacular progress in the field since the beginning of the 1990's, when I started to work in it., 14 pages. Contribution to a special issue of the journal "Gravitation and Cosmology" devoted to the 75th birthday of Prof. Andrey Grib. Version 2 (some sentences have been rephrased to avoid text overlap with arXiv:1303.5086, arXiv:1303.4226, arXiv:1104.0015 by other authors). arXiv admin note: text overlap with arXiv:0802.2236

I discuss some aspects of the use of computers in Relativity, Astrophysics and Cosmology. For each section I provide two examples representative of the field, including gravitational collapse, black hole imagery, supernovae explosions, star-black hole tidal interactions, N-body cosmological simulations and detection of cosmic topology., 13 pages, 14 figures, Plenary lecture at 15th International Workshop on advanced computing and analysis techniques (ACAT 2013), 16-21 May, Beijing, China. To be published in ACAT 2013 Proceedings

This is an editorial note to accompany printing as a Golden Oldie in the Journal of General Relativity and Gravitation of the fundamental article by Georges Lema\^itre first published in French in 1927, in which the author provided the first explanation of the observations on the recession velocities of galaxies as a natural consequence of dynamical cosmological solutions of Einstein's field equations, and discovered the so-called Hubble law. We analyze in detail the scientific contents of this outstanding work, we describe how it remained unread or poorly appreciated until 1930, and we list and explain the differences between the 1927 and 1931 versions. Indeed the English translation published in 1931 in MNRAS was not perfectly faithful to the original text - it was updated. As it turned out very recently, the updates were done by Lema\^itre himself, but the discrepancies between the two texts caused a temporary stir among historians. Our new translation - given in the Appendix - follows the 1927 version exactly.

General relativity does not allow one to specify the topology of space, leaving the possibility that space is multi-- rather than simply--connected. We review the main mathematical properties of multi--connected spaces, and the different tools to classify them and to analyse their properties. Following the mathematical classification, we describe the different possible muticonnected spaces which may be used to construct universe models. We briefly discuss some implications of multi--connectedness for quantum cosmology, and its consequences concerning quantum field theory in the early universe. We consider in details the properties of the cosmological models where space is multi--connected, with emphasis towards observable effects. We then review the analyses of observational results obtained in this context, to search for a possible signature of multi--connectedness, or to constrain the models. They may concern the distribution of images of cosmic objects like galaxies, clusters, quasars,..., or more global effects, mainly those concerning the Cosmic Microwave Background, and the present limits resulting from them., 159 pages, LaTeX format, 32 figures available on request; v2 : all postscript figures added

The twin paradox is the best known thought experiment associated with Einstein's theory of relativity. An astronaut who makes a journey into space in a high-speed rocket will return home to find he has aged less than a twin who stayed on Earth. This result appears puzzling, since the situation seems symmetrical, as the homebody twin can be considered to have done the travelling with respect to the traveller. Hence it is called a "paradox". In fact, there is no contradiction and the apparent paradox has a simple resolution in Special Relativity with infinite flat space. In General Relativity (dealing with gravitational fields and curved space-time), or in a compact space such as the hypersphere or a multiply connected finite space, the paradox is more complicated, but its resolution provides new insights about the structure of spacetime and the limitations of the equivalence between inertial reference frames.

Aims: We investigate the stellar pancake mechanism during which a solar-type star is tidally flattened within its orbital plane passing close to a 10^6 solar masses black hole. We simulate the relativistic orthogonal compression process and follow the associated shock waves formation. Methods: We consider a one-dimensional hydrodynamical stellar model moving in the relativistic gravitational field of a non-rotating black hole. The model is numerically solved using a Godunov-type shock-capturing source-splitting method in order to correctly reproduce the shock waves profiles. Results: Simulations confirm that the space-time curvature can induce several successive orthogonal compressions of the star which give rise to several strong shock waves. The shock waves finally escape from the star and repeatedly heat up the stellar surface to high energy values. Such a shock-heating could interestingly provide a direct observational signature of strongly disruptive star - black hole encounters through the emission of hard X or soft gamma-ray bursts. Timescales and energies of such a process are consistent with some observed events such as GRB 970815., 8 pages, 11 figures, submitted to Astron. Astrophys

The full three-year WMAP results (WMAP3) reinforce the absence of large-angle correlations at scales greater than 60 degrees. The Poincare dodecahedral space (PDS) model model, which may naturally explain such features, thus remains a plausible cosmological model, despite recent controversy about whether matched circle searches would or would not push the topology beyond the horizon. Here, we have used new eigenmode calculations of the dodecahedral space to predict the cosmic microwave background (CMB) temperature fluctuations in such models, with an improved angular resolution. We have simulated CMB maps and confirmed the expected presence of matching circles. For a set of plausible cosmological parameters, we have derived the angular power spectrum of the CMB up to large wavenumbers. Comparison with the WMAP3 observations confirms a remarkable fit with a PDS model, for a value $\Omega_0 = 1.018$ of the average total energy density., Comment: Accepted by A&A, 6 pages, 4 figures ; revised version according to referee's comments