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Like many of you reading this book, I have been a lifelong fan of science fiction movies, books, and television shows. This is not very unusual for a math and computer geek, since there has always been a strong correlation between interest in mathematics, space travel, and science fiction. To some extent, this is inherent in the topic: after all, our space program and our knowledge of astrophysics, astronomy, and cosmology are heavily dependent on some of our most advanced mathematical techniques. But I think it goes beyond this, with some fundamental similarity between the way we stretch our minds to think about what might lie beyond us in space, and what might lie in the many mathematical possibilities beyond our mundane systems of counting and measuring.

Recently we demonstrated that mapping of monochromatic extraordinary light distribution in a hyperbolic metamaterial along some spatial direction may model the flow of time and create an experimental toy model of the big bang. Here we extend this model to emulate cosmological inflation. This idea is illustrated in experiments performed with two-dimensional plasmonic hyperbolic metamaterials. Spatial dispersion which is always present in hyperbolic metamaterials results in scale-dependent (fractal) structure of the inflationary "metamaterial spacetime". This feature of our model replicates hypothesized fractal structure of the real observable universe., Comment: 17 pages, 3 figures. This version is accepted for publication in Physics Letters A

I discuss how modern cosmology illustrates underdetermination of theoretical hypotheses by data, in ways that are different from most philosophical discussions. I confine the discussion to the history of the observable universe from about one second after the Big Bang, as described by the mainstream cosmological model: in effect, what cosmologists in the early 1970s dubbed the ‘standard model’, as elaborated since then. Or rather, the discussion is confined to a (very!) few aspects of that history. I emphasize that despite the underdetermination, a scientific realist can, and should, endorse this description.

In order to test Dicke’s idea of a clock hidden inside the Sun and determine the initial phase of the solar cycle, the epochs of the extrema of the Wolf numbers observed over the past 400 years are examined. It is shown that extrema that obey the period P W equaled 11.07(4) years retain the initial phase, which cannot be explained in terms of local physics and concepts of the past century regarding the mechanism of the solar cycle based on the theory of a magnetic dynamo and the phenomenological model of the Babcock-Leighton cycle. It is suggested that the cycle has a cosmic (cosmological) origin. This is clearly indicated by the correlation of the cycle period with a holographic time-scale of the Universe, (a 0 R 3)1/4/c ≈ 11.0(4) years, where a 0 and R are the radii of the first Bohr orbit of a hydrogen atom and the observable Universe, respectively, and c is the speed of light. It is noted that there are other strict holographic relations that include a 0, R, P W , the wavelength of the microwave background radiation (with a temperature of 2.7 K), and a period of the global solar pulsations equal to 9600.6 s. The true physical nature of the governing mechanism for the 11-year cycle can perhaps only be understood based on modern concepts about the nonlocality of our world, which follows from Bell’s theorem, which is grounded on the achievements of quantum mechanics at the turn of the 20th and 21st centuries, as well as using a model of a holographic Universe free of c.

We impose constraints on the topology of the Universe determined from a search for matched circles in the temperature anisotropy patterns of the 7-yr Wilkinson Microwave Anisotropy Probe (WMAP) data. We pay special attention to the sensitivity of the method to residual foreground contamination of the sky maps and show that for a full-sky estimate of the CMB signal (the Internal Linear Combination map) such residuals introduce a non-negligible effect on the statistics of matched circles. In order to reduce this effect, we perform the analysis on maps for which the most contaminated regions have been removed. A search for pairs of matched back-to-back circles in the higher resolution WMAP W-band map allows tighter constraints to be imposed on topology. Our results rule out universes with topologies that predict pairs of such circles with radii larger than α min ≈ 10°. This places a lower bound on the size of the fundamental domain for a flat universe of about 27.9 Gpc. This bound is close to the upper limit on the size of Universe possible to be detected by the method of matched circles, i.e. the diameter of the observable Universe is 28.3 Gpc.

It is widely believed that the Standard Model of elementary particles is a low-energy manifestation of a more complete theory that unifies gravity with the other interactions (electromagnetic, weak, and strong nuclear forces). Many such unified theories include variation of fundamental constants as a possibility or even a necessity (see the review [1]). For example, some theories contain extra spatial dimensions that would change size in an expanding universe, and this would manifest itself as variation of fundamental constants in our observable Universe. Another possibility is the existence of scalar fields that vary in space and time and could lead to a variable vacuum dielectric constant, for example. These fields may be related to the accelerated expansion of the Universe (dark energy).

Although the observable universe strictly obeys the laws of quantum mechanics, in many instances, a classical description that either ignores quantum effects entirely or accounts for them at a very crude level is sufficient to describe a wide variety of phenomena. However, when this approximation breaks down, as is often the case for processes involving light nuclei, a full quantum treatment becomes indispensable. This Special Topic in The Journal of Chemical Physics showcases recent advances in our understanding of nuclear quantum effects in condensed phases as well as novel algorithmic developments and applications that have enhanced the capability to study these effects.

The causal limit usually considered in cosmology is the particle horizon, delimiting the possibilities of causal connection in the expanding universe. However it is not a realistic indicator of the effective local limits of important interactions in spacetime. We consider here the matter horizon for the Solar System, that is,the comoving region which has contributed matter to our local physical environment. This lies inside the effective domain of dependence, which (assuming the universe is dominated by dark matter along with baryonic matter and vacuum-energy-like dark energy) consists of those regions that have had a significant active physical influence on this environment through effects such as matter accretion and acoustic waves. It is not determined by the velocity of light c, but by the flow of matter perturbations along their world lines and associated gravitational effects. We emphasize how small a region the perturbations which became our Galaxy occupied, relative to the observable universe -- even relative to the smallest-scale perturbations detectable in the cosmic microwave background radiation. Finally, looking to the future of our cosmic domain, we suggest simple dynamical criteria for determining the present domain of influence and the future matter horizon. The former is the radial distance at which our local region is just now separating from the cosmic expansion. The latter represents the limits of growth of the matter horizon in the far future., 18 pages and 2 figures

The general review on the problem of dark matter and dark energy in observable Universe is given. The problem is treated from the point of view of research fields on the boundary of which this fundamental task is situated, namely, the astrophysics and theoretical physics. We emphasize that interdisciplinary nature of “dark sector” of Universe problem can assist in its resolution. The paper does not describe the history of the question, but is centred on revealing the main trends and perspectives.

The objective of this paper is to draw attention to the close similarity between the observable universe and the photon mean free path sphere. It is hoped that by analyzing in depth this apparent connection one will be able to explain why our present epoch appears to have special properties. It is shown that some theoretical arguments point to an equality between the number of particles in the observable universe and the number of particles in the largest self-gravitating photon mean free path sphere (MxPhMFPS.) This equality, supported by observational data, leads to a series of equations that relate in simple manner characteristics of the observable universe with characteristics of the MxPhMFPS, and allows a more precise approximation of the values of the main cosmological parameters. It is also shown that by replacing the protons in the MxPhMFPS with positrons, the radiation resulted by their interaction with the existing electrons has an energy equal to the energy of the electromagnetic radiation in the observable universe.

We search for the most suitable set of cosmological parameters that describes the observable universe. The search includes the possibility of quintessential flat universes, i.e., the analysis is restricted to the determination of the dimensionless matter density and the quintessential parameters, $\Omega_{\rm M}$ and $w_{\rm Q}$, respectively. Our study is focused on comparing the position of features at large scales in the density fluctuation field at different redshifts by analysing the evolution of the quasar two-point correlation function. We trace the density field fluctuations at large scales using a large and homogeneous sample of quasars ($\sim$ 38000 objects with 0.3 $\lesssim$ z $\le$ 2.4 and a median $z=1.45$) drawn from the Sloan Digital Sky Survey Data Release Six. The analysis relies on the assumption that, in the linear regime, the length scale of a particular feature should remain fixed at different times of the universe for the proper cosmological model. Our study does not assume any particular comoving length scale at which a feature should be found, but intends to perform a comparison for a wide range of scales instead. This is done by quantifying the amount of overlap among the quasar correlation functions at different times using a cross-correlation technique. The most likely cosmological model is $\Omega_{\rm M}=0.21\pm 0.02$ and $w_{\rm Q}=-0.93\pm0.04$, in agreement with previous studies. These constraints are the result of a good overall agreement of the correlation function at different redshifts over scales $\sim 100-300\mpc$. Under the assumption of a flat cosmological model, our results indicate that we are living in a low density universe with a quintessential parameter greater than the one corresponding to a cosmological constant., Comment: 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

We investigate the full 5D dynamics of general braneworld models. Without making any further assumptions we show that cyclic behavior can arise naturally in a fraction of physically accepted solutions. The model does not require brane collisions, which in the stationary case remain fixed, and cyclicity takes place on the branes. We indicate that the cosmological constants play the central role for the realization of cyclic solutions and we show that its extremely small value on the observable universe makes the period of the cycles and the maximum scale factor astronomically large., 17 pages, 1 figure, version published in Nucl. Phys. B

Terrestrial mass extinctions have been attributed to a wide range of causes. Some of them are external to Earth, such as bolide impacts (as widely discussed for the K/T boundary) and radiation events. Among radiation events, there are possible large solar flares, nearby supernovae, gamma-ray bursts (GRBs), and others. These have variable intensity, duration, and probability of occurrence, although some generalizations are possible in understanding their effects (Ejzak et al. 2007). Here we focus on gamma-ray bursts (Thorsett 1995; Scalo and Wheeler 2002), a proposed causal agent for the end-Ordovician extinction. These are the most remote and infrequent of events, but by virtue of their power, a threat approximately competitive with, for example, that of nearby supernovae. A GRB of the most powerful type (Woosley and Bloom 2006) is thought to result from a supernova at the end of stellar evolution for very massive stars with high rotational speed. Much of their energy is channeled into beams, or jets, which include very high energy electromagnetic energy, i.e., gamma-rays and X-rays. It is a testament to the power of these events, far across the observable universe, that they were first detected in the 1969–1970 results from monitoring satellites designed to detect nuclear explosions on Earth's surface. It was not until the 1990s, when the distance to the events became known, that their power became apparent. Several such events occur every day in the observable universe. Other kinds of events are also potentially damaging, such as so-called short bursts and solar flares, but rate information is only now beginning to clarify how much threat is likely from such sources. From the rate of these events in the universe as a whole, it is possible to estimate the rate and distribution of likely distances to events that irradiate Earth (Scalo and Wheeler …

My interest in scientific visualization began when I was an undergraduate student at the University of Calgary and saw the film "Powers of 10" for the first time. The film begins and ends with the image of a man asleep at a picnic. In between the first and last scene, we zoom out from the picnic to the vast reaches of space, changing the distance scale by leaps of powers of 10. After reaching the size of the observable universe, at 1024 m, the view zooms back to the picnicker and into his hand, ultimately focusing down to the level of a single carbon nucleus, at the scale of 10−16 m.

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

In this note, we point out that in the eternal inflation driven by the metastable vacua of the landscape, it might be possible that some large and local quantum fluctuations with the null energy condition violation can stride over the barriers between different vacua and straightly create some islands with radiation and matter in new vacua. Then these thermalized islands will evolve with the standard cosmology. We show that such islands may be consistent with our observable universe, while has some distinctly observable signals, which may be tested in coming observations., 4 pages, 1 eps figure, contents added and discussions extended, publish in PLB

The finite age of the universe and the existence of cosmological horizons provides a strong argument that the observable universe represents a finite causal region with finite material and informational resources. A similar conclusion follows from the holographic principle. In this paper I address the question of whether the cosmological information bound has implications for fundamental physics. Orthodox physics is based on Platonism: the laws are treated as infinitely precise, perfect, immutable mathematical relationships that transcend the physical universe and remain totally unchanged by physical processes, however extreme. If instead the laws of physics are regarded as akin to computer software, with the physical universe as the corresponding hardware, then the finite computational capacity of the universe imposes a fundamental limit on the precision of the laws and the specifiability of physical states. That limit depends on the age of the universe. I examine how the imprecision of the laws impacts on the evolution of highly entangled states and on the problem of dark energy.

We present an axiomatic modification of quaternionic quantum mechanics with a possible-worlds semantics capable of predicting essential "nonquantum" features of an observable universe model - the dimensionality and topology of spacetime, the existence, the signature and a specific form of a metric on it, and certain naturally distinguished directions (vistas) in spacetime unrelated to its metric properties.

Matter is distributed very homogeneously and isotropically on scales larger than a few hundred Mpc. The measurements of the microwave background temperature fluctuations show that at recombination the universe was extremely homogeneous and isotropic (with accuracy ∼10 −4 ) on all scales up to the present horizon (Spergel et al 2006 Preprint astro-ph/0603449; MacTavish et al 2005 Preprint astro-ph/0507503). On the other hand, there is a large scale structure in the observable universe and one of the central issues of contemporary cosmology is the explanation of the origin of primordial inhomogeneities, which serve as the seeds for structure formation. Before the advent of inflationary cosmology the initial perturbations were postulated and their spectrum was designed to fit observational data. In this way practically any observation could be ‘explained’, or more accurately described, by arranging the appropriate initial conditions. In contrast, inflationary cosmology truly explains the origin of primordial inhomogeneities and predicts their spectrum (Mukhanov and Chibisov 1981 JETP Lett. 33 532; Mukhanov and Chibisov 1982 Sov. Phys.—JETP 56 258). Thus it becomes possible to test this theory by comparing its predictions with observations. According to cosmic inflation, primordial perturbations originated from quantum fluctuations. These fluctuations have substantial amplitudes only on scales close to the Planckian length, but during the inflationary stage they are stretched to galactic scales with nearly unchanged amplitudes. Thus, inflation links the large-scale structure of the universe to its microphysics. The resulting spectrum of inhomogeneities is not very sensitive to the details of any particular inflationary scenario and has nearly universal shape. This leads to concrete predictions for the spectrum of cosmic microwave background anisotropies.

Two different derivations of the observed vacuum energy density are presented. One is based on a class of proper and novel generalizations of the (Anti) de Sitter solutions in terms of a family of radial functions R(r) that provides an explicit formula for the cosmological constant along with a natural explanation of the ultraviolet/infrared (UV/IR) entanglement required to solve this problem. A nonvanishing value of the vacuum energy density of the order of $${10^{- 123} M_{\rm Planck}^4}$$ is derived in agreement with the experimental observations. A correct lower estimate of the mass of the observable universe related to the Dirac–Eddington–Weyl’s large number N = 1080 is also obtained. The presence of the radial function R(r) is instrumental to understand why the cosmological constant is not zero and why it is so tiny. Finally, we rigorously prove why the proper use of Weyl’s Geometry within the context of Friedman–Lemaitre–Robertson–Walker cosmological models can account for both the origins and the value of the observed vacuum energy density (dark energy). The source of dark energy is just the dilaton-like Jordan–Brans–Dicke scalar field that is required to implement Weyl invariance of the most simple of all possible actions. The full theory involving the dynamics of Weyl’s gauge field Aμ is very rich and may explain also the anomalous Pioneer acceleration and the temporal variations (over cosmological scales) of the fundamental constants resulting from the expansion of the Universe. This is consistent with Dirac’s old idea of the plausible variation of the physical constants but with the advantage that it is not necessary to invoke extra dimensions.

Short-lived gamma rays bursts have been the subject of intensive research work during the last few years. For this class of gamma ray bursts several models were proposed. These include neutron star mergers, primordial black holes, and neutron star collapse to strange-quark star. Difficulties associated with these models are discussed. A plausible model of the origin of short-lived GRBs may take into account their observed features, distances, distributions, and frequency of occurrence. It was shown that some of these requirements are in favor of an entirely new class of objects whose distribution extends to near the edge of the observable universe. It was shown recently that primordial black holes form stable gravitational bound states called Holeums. These states are created in the early stages of the big bang. We argue in this paper that annihilation of Holeums gives rise to short-lived gamma ray bursts and exhibits a large dynamical range of characteristics that can encompass a wide spectrum of the observed features of these bursts.

Hubble’s discovery of the recession of distant galaxies, the observation of the cosmic microwave background radiation, and recent observational data coupled with Einstein’s gravitational theory have established the inflationary Big Bang model as the standard scheme for determining the important milestones in the evolution of the Universe.

We try to explain the known asymmetry between baryons and antibaryons in the observable universe as the result of random fluctuations in the number of baryons and antibaryons. The establishment of this asymmetry occurs at a poorly constrained time, before the epoch of baryon/antibaryon annihilation. The low initial gravitational entropy of the universe is due to this initial matter/antimatter asymmetry. The observed ratio of cosmic microwave background photons to baryons (~ one billion) is a measure of this asymmetry. Different horizons render different probability distributions, all variants of a Gaussian centered at zero. Using the current levels of fluctuation witnessed in our particle horizon, along with the time evolution of the size and matter density of the universe, we estimate the time of baryogenesis.

Clusters of galaxies outline the network of the distribution of visible matter in the Universe, marking the highest-mass knots where filamentary structures join together. If we observe the sky in X rays, clusters of galaxies stand out as cosmic lighthouses by virtue of a thin gas trapped and heated within their gravitational potential wells. This powerful emission is directly linked to the total gravitating mass they contain, such that they can be efficiently used as tracers of the cosmic mass distribution within a sizeable fraction of the observable Universe. Recent observational campaigns have for the first time used X-ray clusters to map cosmic structures on scales approaching 3 billion light years. The emerging picture is remarkably consistent with the expectation of a low-density Universe dominated by cold dark matter., 27 pages, 7 figures, to appear as a Review in Nature, 4 January 2001 issue (higher resolution figures available at http://www.daut.univ.trieste.it/borgani/Nature/)

We exploit the parallel between dynamical black holes and cosmological spacetimes to describe the evolution of Friedmann-Lema\^itre-Robertson-Walker universes from the point of view of an observer in terms of the dynamics of the apparent horizon. Using the Hayward-Kodama formalism of dynamical black holes, we clarify the role of the Clausius relation to derive the Friedmann equations for a universe, in the spirit of Jacobson's work on the thermodynamics of spacetime. We also show how dynamics at the horizon naturally leads to the quantum-mechanical process of Hawking radiation. We comment on the connection of this work with recent ideas to consider our observable Universe as a Bose-Einstein condensate and on the corresponding role of vacuum energy., Comment: 16 pages, 1 figure

We take the postulate of Special Relativity, that the cosmic rules observable through physical phenomena, are the same for all stars in all galaxies. We have deliberately avoided using the phrase; “in all inertial frames of reference” to avoid conceptual mathematical debate in defining what such frames of references are [1-4]. Then, accepting the universal validity of light velocity defined by Maxwell’s wave equation, c 2 = 1 / (e 0 μ 0 ) ; we revive the old ether concept with physically descriptive phrase that space is a continuous Complex Tension Field (CTF). This is strengthened by the fact that all non-dissipative tension fields allow for perpetual propagation of waves when excited within its linear restoration capability. We accommodate the particles as localized self-phase-looped resonant oscillations of the same CTF; thus integrating particles as another kind of excited states of the same CTF [5]. Further, all tension fields allow co-propagation and cross-propagation of multiple waves (preservation of wave properties and the respective Poynting vectors) through the same physical volume (linear Superposition Principle) in the absence of perturbing resonant detectors within the volume of superposition. We have re-named this universal property of all waves as Non-Interaction of Waves [6,7]. Thus, Doppler shifted waves from different stars and galaxies can cross through each other unperturbed while bringing to us the signatures of the properties of their parent stars. Now, if these light signals are waving of CTF, the optical Doppler effects must also be, as for sound waves in air pressure tension field, discernable into two different frequency shifts: as due to (i) source velocity (distant stars) and (ii) detector velocity (that of the earth) [8,9]. In other words, we are proposing that CTF (modified old ether) is the stationary cosmic rest frame. Since we have been routinely assuming that quantum phenomena are same in all stars; we strengthen our position by analyzing the origin of absorption lines in distant stars as the same energy level transition phenomenon as we observe on earth and well-modeled by precision spectrometry and validated by QM formalism. The analysis also reveals that Cosmological (Hubble) Redshift cannot be due to optical Doppler Effect; since the Doppler Effect is determined by the velocities of the source-atoms within the star coronas. We have also proposed a satellite based one-way light propagation measurement; which could identify the absolute velocity of the satellite and validate that CTF is the stationary rest frame for our observable universe.

We derive a general expression for the probability of observing deviations from statistical isotropy in the cosmic microwave background (CMB) if the primordial fluctuations are non-Gaussian and extend to superhorizon scales. The primary motivation is to properly characterize the monopole and dipole modulations of the primordial power spectrum that are generated by the coupling between superhorizon and subhorizon perturbations. Unlike previous proposals for generating the hemispherical power asymmetry, we do not assume that the power asymmetry results from a single large superhorizon mode. Instead, we extrapolate the observed power spectrum to superhorizon scales and compute the power asymmetry that would result from a specific realization of non-Gaussian perturbations on scales larger than the observable universe. Our study encompasses many of the scenarios that have been put forward as possible explanations for the CMB hemispherical power asymmetry. We confirm our analytic predictions for the probability of a given power asymmetry by comparing them to numerical realizations of CMB maps. We find that non-local models of non-Gaussianity and scale-dependent local non-Gaussianity produce scale-dependent modulations of the power spectrum, thereby potentially producing both a monopolar and a dipolar power modulation on large scales. We then provide simple examples of finding the posterior distributions for the parameters of the bispectrum from the observed monopole and dipole modulations., 21 pages, 11 figures; v2: minor changes to match the PRD accepted version

Eternal inflation arising from a potential landscape predicts that our universe is one realization of many possible cosmological histories. One way to access different cosmological histories is via the nucleation of bubble universes from a metastable false vacuum. Another way to sample different cosmological histories is via classical transitions, the creation of pocket universes through the collision between bubbles. Using relativistic numerical simulations, we examine the possibility of observationally determining if our observable universe resulted from a classical transition. We find that classical transitions produce spatially infinite, approximately open Friedman-Robertson-Walker universes. The leading set of observables in the aftermath of a classical transition are negative spatial curvature and a contribution to the Cosmic Microwave Background temperature quadrupole. The level of curvature and magnitude of the quadrupole are dependent on the position of the observer, and we determine the possible range of observables for two classes of single-scalar field models. For the first class, where the inflationary phase has a lower energy than the vacuum preceding the classical transition, the magnitude of the observed quadrupole generally falls to zero with distance from the collision while the spatial curvature grows to a constant. For the second class, where the inflationary phase has a higher energy than the vacuum preceding the classical transition, the magnitude of the observed quadrupole generically falls to zero with distance from the collision while the spatial curvature grows without bound. We find that the magnitude of the quadrupole and curvature grow with increasing centre of mass energy of the collision, and explore variations of the parameters in the scalar field lagrangian., 21 pages, 15 figures

In light of the recent interest in dynamical dark energy models based on a cosmology with varying gravitational and cosmological parameters $G$ and $\varLambda$ , we present here a model of inertia in a type of Friedmann universe with $G = G_{0}(A/A_{0})^{\sigma}$ ; $A$ being the dimensionless scale factor, that was recently studied by Singh et al. (Astrophys. Space Sci. 345:213, 2013). The proposed Machian model of inertia utilizes the curved space generalization of Sciama’s law of inertial induction, which is based on the analogy between the retarded far fields of electrodynamics and those of gravitation, and expresses the total inertial force $F= -ma$ on an accelerating mass $m$ in terms of contributions from all matter in the observable Universe. We show that for a varying Friedmann model with $\sigma=-3/2$ , inertial induction alone can account for the total inertial force on the accelerating mass. We then compare this cosmological model with current observational constraints for the variation of $G$ .

According to the cosmological principle, Universal large-scale structure is homogeneous and isotropic. The observable Universe, however, shows complex structures even on very large scales. The recent discoveries of structures significantly exceeding the transition scale of 370 Mpc pose a challenge to the cosmological principle. We report here the discovery of the largest regular formation in the observable Universe; a ring with a diameter of 1720 Mpc, displayed by 9 gamma ray bursts (GRBs), exceeding by a factor of five the transition scale to the homogeneous and isotropic distribution. The ring has a major diameter of $43^o$ and a minor diameter of $30^o$ at a distance of 2770 Mpc in the 0.78, Comment: Accepted for publication in MNRAS, 13 pages, 8 figures and 4 tables

The challenging issue of determining the correct $f(R)$ among several possibilities is here revised by means of numerical reconstructions of the modified Friedmann equations around the redshift interval $z\in[0,1]$. Frequently, a severe degeneracy between $f(R)$ approaches occurs, since different paradigms correctly explain present time dynamics. To set the initial conditions on the $f(R)$ functions, we involve the use of the so called cosmography of the Universe, i.e. the technique of fixing constraints on the observable Universe by comparing expanded observables with current data. This powerful approach is essentially model independent and correspondingly we got a model independent reconstruction of $f(z)$ classes within the interval $z\in[0,1]$. To allow the Hubble rate to evolve around $z\leq1$, we considered three relevant frameworks of effective cosmological dynamics, i.e. the $\Lambda$CDM model, the CPL parametrization and a polynomial approach to dark energy. Finally cumbersome algebra permits to pass from $f(z)$ to $f(R)$ and the general outcome of our work is the determination of a viable $f(R)$ function, that effectively describes the observed Universe dynamics.

In this paper we will discuss how cosmic strings can be used to bridge the gap between the local geometry of our spacetime model and the global topology. The primary tool is the theory of foliations and surfaces, and together with observational constraints we can isolate several possibilities for the topology of the spatial section. This implies that the discovery of cosmic strings would not just be significant for an understanding of structure formation in the early universe, but also for the global properties of the spacetime model., 13 pages

The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions., Comment: Invited review article for the focus issue "Planck and the fundamentals of cosmology" in Classical and Quantum Gravity. Accepted for publication. 9 pages and 2 figures

In a flat Priedmann-Lemaitre-Robertson-Walker background, a scheme of dark matter-dark energy interaction is studied considering a holographic Ricci-like model for the dark energy. Without giving a priori some specific model for the interaction function, we show that this function can experience a change of sign during the cosmic evolution. The parameters involved in the holographic model are adjusted with Supernova data and we obtained results compatible with the observable universe.

The Great Filter interpretation of Fermi's great silence asserts thatNpqis not a very large number, whereNis the number of potentially life-supporting planets in the observable universe,pis the probability that a randomly chosen such planet develops intelligent life to the level of present-day human civilization, andqis the conditional probability that it then goes on to develop a technological supercivilization visible all over the observable universe. Evidence suggests thatNis huge, which implies thatpqis very small. Hanson (1998) and Bostrom (2008) have argued that the discovery of extraterrestrial life would point towardspnot being small and therefore a very smallq, which can be seen as bad news for humanity's prospects of colonizing the universe. Here we investigate whether a Bayesian analysis supports their argument, and the answer turns out to depend critically on the choice of prior distribution.

Power suppression of the cosmic microwave background on the largest observable scales could provide valuable clues about the particle physics underlying inflation. Here we consider the prospect of power suppression in the context of the multifield landscape. Based on the assumption that our observable universe emerges from a tunnelling event and that the relevant features originate purely from inflationary dynamics, we find that the power spectrum not only contains information on single-field dynamics, but also places strong con- straints on all scalar fields present in the theory. We find that the simplest single-field models giving rise to power suppression do not generalise to multifield models in a straightforward way, as the resulting superhorizon evolution of the curvature perturbation tends to erase any power suppression present at horizon crossing. On the other hand, multifield effects do present a means of generating power suppression which to our knowledge has so far not been considered. We propose a mechanism to illustrate this, which we dub flume inflation., Comment: 22 pages plus appendices, 8 figures. Minor corrections to match JCAP published version

We consider the propagation of electromagnetic waves through a dilaton-Maxwell domain wall of the type introduced by Gibbons and Wells [G.W. Gibbons and C.G. Wells, Class. Quant. Grav. 11, 2499-2506 (1994)]. It is found that if such a wall exists within our observable universe, it would be absurdly thick, or else have a magnetic field in its core which is much stronger than observed intergalactic fields. We conclude that it is highly improbable that any such wall is physically realized., Comment: 9 pages

We investigate the possibility of the construction of the conventional Friedmann cosmology for our observable Universe if the underlying theory is the multidimensional Kaluza–Klein model. We show that the effective Friedmann model obtained by dynamic compactification of the multidimensional model is faced with too strong variations in the fundamental constants. On the other hand, models with stable compactification of the internal space are free from this problem and also result in conventional four-dimensonal cosmological behaviour for our Universe. We prove a no-go theorem, which shows that stable compactification of the internal spaces is possible only if the equations of state in the external and internal spaces are properly adjusted to each other. With a proper choice of parameters (fine tuning), the effective cosmological constant in this model provides the late-time acceleration of the Universe.

Revision of the paper previously entitled "Learning a Machine for the Decision in a Partially Observable Markov Universe" In this paper, we are interested in optimal decisions in a partially observable universe. Our approach is to directly approximate an optimal strategic tree depending on the observation. This approximation is made by means of a parameterized probabilistic law. A particular family of hidden Markov models, with input \emph{and} output, is considered as a model of policy. A method for optimizing the parameters of these HMMs is proposed and applied. This optimization is based on the cross-entropic principle for rare events simulation developed by Rubinstein., Submitted to EJOR

We consider the scenario where our observable universe is devised as a dynamical four-dimensional hypersurface embedded in a five-dimensional bulk spacetime, with a large extra dimension, which is the {\it generalization of the flat FRW cosmological metric to five dimensions}. This scenario generates a simple analytical model where different stages of the evolution of the universe are approximated by distinct parameterizations of the {\it same} spacetime. In this model the evolution from decelerated to accelerated expansion can be interpreted as a "first-order" phase transition between two successive stages. The dominant energy condition allows different parts of the universe to evolve, from deceleration to acceleration, at different redshifts within a narrow era. This picture corresponds to the creation of bubbles of new phase, in the middle of the old one, typical of first-order phase transitions. Taking $\Omega_{m} = 0.3$ today, we find that the cross-over from deceleration to acceleration occurs at $z \sim 1-1.5 $, regardless of the equation of state in the very early universe. In the case of primordial radiation, the model predicts that the deceleration parameter "jumps" from $q \sim + 1.5$ to $q \sim - 0.4$ at $z \sim 1.17$. At the present time $q = - 0.55$ and the equation of state of the universe is $w = p/\rho \sim - 0.7 $, in agreement with observations and some theoretical predictions., Comment: The abstract and introduction are improved and the discussion section is expanded. A number of references are added

Susskind claims in his recent book The Cosmic Landscape that evidence for the existence and nature of `pocket universes' in a multiverse would be available in the detailed nature of the Cosmic Blackbody Background Radiation that constantly bathes all parts of our observable universe. I point out that acceptance of the complex chain of argument involved does not imply possible experimental verification of multiverses at the present time. Rather this claim relates only to theoretically possible observations in the very far future of the universe., Comment: Revised to clarify claims; essential substance unchanged

Cosmology has undergone a significant advance in recent years both observationally and theoretically. New high quality data from a host of Cosmic Microwave Background and high redshift surveys has provided strong support for the inflationary paradigm, which postulates that the universe expanded exponentially fast when it was a fraction of a second old. A key question that has arisen is the nature of the underlying physics that drove such a phase of accelerated expansion. An exciting theoretical development has been the realization that the standard model interactions may be confined to a four–dimensional brane, whereas gravitational interactions may also propagate through the extra dimensions. The radical proposal of the ‘braneworld scenario’ is that our observable universe is a brane embedded in a higher–dimensional bulk. We will review some of the recent developments in this field.

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

If string theory is correct, then our observable Universe may be a 3-dimensional "brane" embedded in a higher-dimensional spacetime. This theoretical scenario should be tested via the state-of-the-art in gravitational experiments -- the current and upcoming gravity-wave detectors. Indeed, the existence of extra dimensions leads to oscillations that leave a spectroscopic signature in the gravity-wave signal from black holes. The detectors that have been designed to confirm Einstein's prediction of gravity waves, can in principle also provide tests and constraints on string theory., Comment: Fourth prize in the Gravity Research Foundation 2005 essay competition

We study a closed model of the universe filled with viscous fluid and quintessence matter components in a Brans-Dicke type cosmological model. The dynamical equations imply that the universe may look like an accelerated flat Friedmann-Robertson-Walker universe at low redshift. We consider here dissipative processes which follow a causal thermodynamics. The theory is applied to viscous fluid inflation, where accepted values for the total entropy in the observable universe is obtained., 11 pages, revtex 4. For a festschrift honoring Alberto Garcia. To be publishen in Gen. Rel. Grav

We propose a simple mechanism for driving the recently detected non-decelerating expansion of the observable Universe. It is based on the assumption of the predominance of the vacuum energy over the inert matter and a fractal structuring of the latter. The Newton’s universal gravitational force ensures the attraction between celestial bodies at small and medium astronomical distances, but gives rise to the repulsive interaction between mutually very separated cosmic subsystems, like clusters and superclusters, and thus to the observed accelerating expansion and the fractal structuring.

The main task for cosmology today is the completion of the program of cosmological tests that commenced in the 1930s. I offer comments on the state of the program and the issues to be addressed to establish a convincing large-scale picture of the observable universe., Comment: Contribution to TAUP 2003, Seattle, September, 2003; five pages

Cosmic magnetism has that exotic “Je ne sais quoi”! Magnetism has been observed in various objects, located near the edge of the Universe and all the way down to the Milky Way's center. The observed magnetic field can take the cell-type shape in randomly-oriented large blobs found in intracluster gas or outside of clusters of galaxies, the helix shape in synchrotron jets, the longitudinal shape in ram-pressured shocks in radio lobes near elliptical galaxies, the spiral shape of logarithmic arms in spiral galaxies, or the egg shape of an enlarged interstellar bubble. In strength, the magnetic field varies from 0.1 nG (cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mG in the Milky Way filaments. Magnetism plays a small physical role in the formation of large structures. It acts as a tracer of the dynamical histories of cosmological and intracluster events, it guides the motion of the interstellar ionised gas, and it aligns the charged dust particles. Batteries and dynamos are often employed in models to create and amplify seed magnetic fields. Starting soon after the Big Bang (redshift z>2000), this review covers the cosmological background surface (z≈1100, distance ≈4.3 Gpc), the epoch of first stars (z≈20; distance ≈4.1 Gpc), the currently observable Universe (z≈10, distance ≈3.9 Gpc), superclusters of galaxies (size ≈50 Mpc), intracluster gas (size ≈10 Mpc), galaxies (≈30 kpc), spiral arms (≈10 kpc), interstellar superbubbles (≈100 pc), synchrotron filaments (≈10 pc), and the Milky Way's center.

Gamma-ray bursts (hereafter GRB) produce a flux of radiation detectable across the observable Universe, and at least some of them are associated with galaxies. A GRB within our own Ggalaxy could do considerable damage to the Earth's biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRB may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRB in terms of terrestrial effects is a nearly impulsive energy input of order 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB., Comment: Accepted for publication in the International Journal of Astrobiology. 24 pages, moderate revisions, including more quantitative detail on GRB rates, atmospheric effects, and more description of correlations of extinction rates with habitat and lifestyle of fossil fauna