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This paper introduces three cosmic expansion models with constant, decelerating and accelerating speed of expansion respectively. Then characters of these cosmic expansion models are compared. Based on these cosmic expansion models, the thresholds of observable universe are calculated via simulations, where the earliest observable cosmic radius R(tearliest) is always 0.368R (R is cosmic radius at current universe time) for any cosmic expansion models.

The direct product of all the 26 sporadic groups is a large group of order 2.333 × 10^365. This group would form a sphere with diameter of d = 7.638 × 10^121. The cube root of this group is one of Dirac's large numbers: 6.156 × 10^121, which is the mass of this group. If we take the square root of this group/set the identity of this group to 2π × Planck length, we'll get an object with diameter d = 93.76 billion ly and weight of 9.63×10^53 kg. This compares well to current estimates of the diameter of the observable universe of about 92.80 billion ly (error of about 1 %) and mass of 9.288 × 10^53 kg (estimated range 9.102 – 9.473 × 10^53 kg, dark matter + baryonic matter). The relative error for the radius is about 1 % and about 1 % (3.7 %) for mass, since the density of both objects is the same. The observable universe might be related to the direct product of the sporadic groups.

According to the standard model the CMB radiation is a relict of the Big Bang. Even if the temperature has varied over the years, we now have an accurate figure, 2,72548 K. The radiation exhibits an almost perfect black body spectrum, which causes some confusion, the number of photons required does not match the number available at a distant moment in the past. Several scattering mechanisms have been suggested to give the photon number the necessary gain. It is assumed that the last scattering at ca 380.000 yr after the big bang has left the radiation pattern we now observe. Some notable physicists have expressed doubts about the last scattering and emphasized that the photons originate from positron-electron (e+-e-) annihilations and that scattering would not preserve the black body spectrum. Based on known laws of physics a theory, CBU for the Continuously Breeding Universe, has been developed. The theory incorporates important ideas from the past. The universe is a complex emerging system, which starts from the single quantum fluctuation of a positron-electron pair. Expansion is driven by the emersion of new pairs. Typically, the gravitational parameter G is inversely proportional to the radius. The theory predicts correctly the radius of the observable universe, the Hubble parameter, the energy content and gives a plausible explanation to dark energy. The CBU theory postulates that the primordial universe undergoes a transition from a black hole to a photon filled universe. After the transition one half of the energy is bound to a great multitude of ‘small’ black holes, while the other half propagate as the CMB radiation in the free space. It is presumed that the CMB photons are due to e+-e- annihilations. The frequency-energy (hf) of the photons decreases according to ac2, where ac is the scale factor of the transition. As a characteristic feature the CMB photons are pairwise entangled and in a state of superposition. If we assume that photons in a superposition cannot give off energy, they compensate the hf loss by increasing the number of photons. As they move in all directions the gain will be 1/ac3. In addition, when the photons enter the observable universe at afl (first light), the Doppler effect lengthens the wavelength, whereby the 1-dimensional photon ray to be observed on Earth gets a number gain of 1/afl in compensation, the total gain being 1/afl ac3. The gain effect and the half-energy at the CMB transition result in an energy density of 4,173·10-14 J/m3, equal to BT04, where B is the Stefan-Boltzmann black body energy density constant.

This paper introduces a cosmic expansion model with constant speed of cosmic spatial expansion via derivation and simulations, where the speed of cosmic spatial expansion equals the speed of light c. Simulation results show that the earliest observable universe time is t = 5.084 Gyrs where the current universe time T = 13.82 Gyrs, and the furthest observable distance at the earliest observable universe time t is S = 0.632R, where R is the cosmic radius at current universe time T. The above constant cosmic expansion model does not consider the inflation period in the early universe according to the Big Bang model, nor does it considered the cosmic acceleration in recent universe time. However, this simplified cosmic expansion model could be a benchmark that will be helpful to understand the cosmic expansion and the observable universe. Based on the derivation and simulation of the constant cosmic expansion model, the threshold of observable universe for the accelerated cosmic expansion model can also be calculated similarly, as far as the speed of cosmic spatial expansion at any universe time t can be provided.

Abiotic emergence of ordered information stored in the form of RNA is an important unresolved problem concerning the origin of life. A polymer longer than 40--100 nucleotides is necessary to expect a self-replicating activity, but the formation of such a long polymer having a correct nucleotide sequence by random reactions seems statistically unlikely. However, our universe, created by a single inflation event, likely includes more than $10^{100}$ Sun-like stars. If life can emerge at least once in such a large volume, it is not in contradiction with our observations of life on Earth, even if the expected number of abiogenesis events is negligibly small within the observable universe that contains only $10^{22}$ stars. Here, a quantitative relation is derived between the minimum RNA length $l_{\min}$ required to be the first biological polymer, and the universe size necessary to expect the formation of such a long and active RNA by randomly adding monomers. It is then shown that an active RNA can indeed be produced somewhere in an inflationary universe, giving a solution to the abiotic polymerization problem. On the other hand, $l_{\min}$ must be shorter than $\sim$20 nucleotides for the abiogenesis probability close to unity on a terrestrial planet, but a self-replicating activity is not expected for such a short RNA. Therefore, if extraterrestrial organisms of a different origin from those on Earth are discovered in the future, it would imply an unknown mechanism at work to polymerize nucleotides much faster than random statistical processes., Comment: 9 pages, 1 figure. matches the published version from Scientific Reports

We review the role of chance, contingency, necessity, and providence among cosmic properties. The materialistic worldview in which the patterns of the universe result from random processes and humans exist in an “indifferent” universe is examined, as well as the current evidence for a fine-tuned universe. The multiverse is the preferred solution to explain why the properties of our observable universe appear to be bio-friendly. It reintroduces randomness and contingency into the cosmic process. However, this explanation comes at the cost of non-testability. And it is argued that there is no certainty about the evacuation of some kind of remaining fine-tuning in the (hypothetical) fundamental law that rules the multiverse.

In theories with extra dimensions, the cosmological hierarchy problem can be thought of as the unnaturally large radius of the observable universe in Kaluza-Klein units. We sketch a dynamical mechanism that relaxes this. In the early universe scenario we propose, three large spatial dimensions arise through tunneling from a 'cosmic egg', an effectively one-dimensional configuration with all spatial dimensions compact and of comparable, small size. If the string landscape is dominated by low-dimensional compactifications, cosmic eggs would be natural initial conditions for cosmology. A quantum cosmological treatment of a toy model egg predicts that, in a variant of the Hartle-Hawking state, cosmic eggs break to form higher dimensional universes with a small, but positive cosmological constant or quintessence energy. Hence cosmic egg cosmology yields a scenario in which the seemingly unnaturally small observed value of the vacuum energy can arise from natural initial conditions., 11 pages, 4 figures, 1 appendix

“Here’s to low-grade ore and plenty of it,” mining magnate George Hearst reportedly said after he bought the Homestake Gold Mine in Lead, SD, in June 1877. Although the mine contained less than an ounce of the precious metal per ton, the sheer quantity of ore meant that the operation was profitable (1). From 1876 to 2001, miners dug thousands of feet in search of gold, making Homestake the largest and deepest gold mine in North America. Now, physicists and engineers are excavating gigantic caverns inside the defunct mine to look for something even more elusive: neutrinos. New caverns being excavated inside the defunct Homestake Gold Mine in Lead, SD, will house DUNE. The world’s largest such detector, DUNE will start running in 2026. Image credit: Shutterstock/Tom Grundy. The new caverns will house the Deep Underground Neutrino Experiment (DUNE), the world’s largest such detector. DUNE will start running in 2026, studying an intense beam of neutrinos coming from Fermilab, near Batavia, IL, about 1,300 kilometers away. The experiment will focus on a profound question: what is the origin of matter? In the early universe, matter and antimatter should have formed in equal amounts and then annihilated each other to leave only radiation. But the observable universe with its galaxies, stars, and planets is made of matter, so clearly, not all of it was annihilated. “How did we get from a universe that was equal amounts of matter and antimatter to a universe where there’s a bit of matter left over?” asks neutrino physicist Deborah Harris of Fermilab. Neutrinos could be key to answering that question. Because of their special ability to transform from one type into another, they may reveal fundamental differences between matter and antimatter. Homestake has history with the neutrino, offering up the first signs of this particle’s …

Blanchard, A., et al., Today, thanks in particular to the results of the ESA Planck mission, the concordance cosmological model appears to be the most robust to describe the evolution and content of the Universe from its early to late times. It summarizes the evolution of matter, made mainly of dark matter, from the primordial fluctuations generated by inflation around 10 second after the Big Bang to galaxies and clusters of galaxies, 13.8 billion years later, and the evolution of the expansion of space, with a relative slowdown in the matter-dominated era and, since a few billion years, an acceleration powered by dark energy. But we are far from knowing the pillars of this model which are inflation, dark matter and dark energy. Comprehending these fundamental questions requires a detailed mapping of our observable Universe over the whole of cosmic time. The relic radiation provides the starting point and galaxies draw the cosmic web. JAXA’s LiteBIRD mission will map the beginning of our Universe with a crucial test for inflation (its primordial gravity waves), and the ESA Euclid mission will map the most recent half part, crucial for dark energy. The mission concept GAUSS, described in this White Paper, aims at being a mission to fully map the cosmic web up to the reionization era, linking early and late evolution, to tackle and disentangle the crucial degeneracies persisting after the Euclid era between dark matter and inflation properties, dark energy, structure growth and gravitation at large scale.

Cosmological phase transitions are thought to have taken place at the early Universe imprinting their properties on the observable Universe. There is strong evidence that, through the dynamics of a scalar field that lead a second order phase transition, inflation shaped the Universe accounting for the most conspicuous features of the observed Universe. It is argued that inflation has also striking implications for the vacuum energy. Considerations for subsequent second order phase transitions are also discussed., Comment: 9 pages. Version to match to one to be published in General Relativity and Gravitation

We propose the development of X-ray interferometry (XRI), to reveal the universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 $\mu$as resolution at 10 \AA (1.2 keV) and 20 $\mu$as at 2 \AA (6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH `shadow'; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within ~100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft `constellation' interferometer would resolve well below 1 $\mu$as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole., Comment: White Paper submitted in response to ESA's Voyage 2050 call

The origin of power asymmetry and other measures of statistical anisotropy on the largest scales of the universe, as manifested in cosmic microwave background (CMB) and large-scale structure data, is a long-standing open question in cosmology. In this paper, we analyse the Planck Legacy temperature anisotropy data and find strong evidence for a violation of the Cosmological principle of isotropy, with a probability of being a statistical fluctuation of the order of ∼10-9. The detected anisotropy is related to large-scale directional ΛCDM cosmological parameter variations across the CMB sky, which are sourced by three distinct patches in the maps with circularly averaged sizes between 40° and 70° in radius. We discuss the robustness of our findings to different foreground separation methods and analysis choices, and find consistent results from WMAP data when limiting the analysis to the same scales. We argue that these well-defined regions within the cosmological parameter maps may reflect finite and casually disjoint horizons across the observable universe. In particular, we show that the observed relation between horizon size and mean dark energy density within a given horizon is in good agreement with expectations from a recently proposed model of the universe that explains cosmic acceleration and cosmological parameter tensions between the high- and low-redshift universe from the existence of casual horizons within our universe., This work is dedicated to the memory of my father, Marcelo. PF is specially grateful to K. Benabed and O. Doré for useful comments on the draft, I. Tutusaus for advice with iMinuit⁠, and J. Guerrero for help with the Hidra computing cluster at ICE. Hidra is funded by CSIC project EQC2019-005664-P, with European FEDER funds. The development of this project required 800 000+ CPU hours at Hidra. Some of the results in this paper have been derived using the HEALPY (Zonca et al. 2019), HEALPIX (Górski et al. 2005), and CAMB (Lewis, Challinor & Lasenby 2000) packages. We acknowledge support from MINECO through grants ESP2017-89838-C3-1-R and PGC2019-102021-B-100, the H2020 European Union grants LACEGAL 734374 and EWC 776247 with ERDF funds, and Generalitat de Catalunya through CERCA to grant 2017-SGR-885 and funding to IEEC.

When the author was a Mathematics student at the University of California, Los Angeles (UCLA), there was a mosaic on the sides of the Mathematics building containing various symbols. One of these was the number 3 × 1074, which is a shorthand way of writing the enormous number 3 followed by 74 zeros. The author was told that this was the number of atoms in the observable Universe, although it has now increased over the years to roughly 1080. The qualification ‘observable’ is important here and represents a sphere of about 93 billion light-years in diameter, though the actual shape of the entire Universe is still not known.

Assuming a closed universe with slight positive curvature, cosmic expansion is modeled as a heat engine where the '"system'" is defined collectively as those regions of space within the observable universe which will later evolve into voids or empty space, and the '"surroundings'" are identified collectively as those pockets of space which will eventually develop into matter filled galaxies, clusters, super-clusters and filament walls. Using this model, we show that the energy needed for cosmic expansion can be found using basic thermodynamic principles, and that cosmic expansion had as its origin, a finite initial energy density, pressure, volume, and temperature. Inflation in the traditional sense, with the inflaton field, may also not be required as it can be argued that homogeneities and in-homogeneities in the WMAP temperature profile can be attributed to quantum mechanical fluctuations about a fixed background temperature in the initial isothermal expansion phase. Fluctuations in temperature can cause certain regions of space to lose heat to other pockets producing voids forcing, i.e., fueling expansion of the latter and creating slightly cooler temperatures in the former, where matter will later congregate. Upon freeze-out, this could produce the observed WMAP signature with its associated CBR fluctuation in magnitude. Finally, we estimate that the freeze-out temperature and time for WMAP in-homogeneities occurred at roughly 3.02 * 1027 K and 2.54 * 10-35 s, respectively, after first initiation of volume expansion, in line with current estimates for the end of the inflationary epoch. The heat absorbed in the inflationary phase is estimated to be Q = 1.81 * 1094 J, and the system volume increases by a factor of only 5.65. The bubble voids in the observable universe increase, collectively, in volume from about .046 m3 to .262 m3 within this time., 19 pages

The proposed cyclic universes model based on the split division algebras accounts for the inflation, the Big Bang, gravity, dark energy, dark matter, the standard model, and the masses of all elementary particles. The split algebras (complex quaternion and complex octonion) as the Furey model generate the fixed spacetime dimension number for the observable universe with the fixed 4-dimensional spacetime (4D) standard model particles and the oscillating spacetime dimension number for the oscillating universes (hidden or dark energy) with the oscillation between 11D and 11D through 10D and between 10D and 10D through 4D. 11D has the lowest rest mass, the highest speed of light, and the highest vacuum energy, while 4D has the highest rest mass, the lowest (observed) speed of light, and zero vacuum energy. In the cyclic universes model, the universes start with the positive-energy and the negative-energy 11D membrane-antimembrane dual universes from the zero-energy inter-universal void, and are followed by the transformation of the 11D membrane-antimembrane dual universes into the 10D string-antistring dual universes and the external dual gravities as in the Randall-Sundrum model, resulting in the four equal and separate universes consisting of the positive-energy 10D universe, the positive-energy external gravity, the negative-energy 10D universe, and the negative-energy external gravity. Under the fixed spacetime dimension number, the positive-energy 10D universe is transformed into 4D standard model particles through the inflation and the Big Bang. Dark matter is the right-handed neutrino, exactly five times of baryonic matter in total mass in the universe. Under the oscillating spacetime dimension number, the other three universes oscillate between 10D and 10D through 4D, resulting in the hidden universes when D > 4 and dark energy (the maximum dark energy = 3/4 = 75%) when D = 4. Eventually, all four universes return to the 10D universes.

Just 5 years ago, physicists opened a new window on the universe when they first detected gravitational waves, ripples in space itself set off when massive black holes or neutron stars spiral together. Even as discoveries pour in, researchers are already planning bigger, more sensitive detectors. And a Ford versus Ferrari kind of rivalry has emerged, with scientists in the United States simply proposing detectors 10 times bigger than the ones they have now, and researchers in Europe pursuing a more radical design that would combine six detectors in a single underground observatory. Researchers say detectors 10 times more sensitive than the ones they have now could detect all black hole mergers within the observable universe and spot hundreds of mergers of neutron stars, laying bare the nature of the ultradense matter in neutron stars. But, it9s early days for the U.S. project, which is called the Cosmic Explorer, and the European project, which is known as the Einstein Telescope.

To span the great distance from our solar system to the farthest clusters of galaxies in our universe, astronomers break down the distance indicators into steps. The nearest ones are measured geometrically, with what is known as parallax. Among the objects whose distances are thus directly measured are a rare type of star known as a Cepheid variable. Henrietta Leavitt about a hundred years ago discovered, by looking at a number of such stars in the Small Magellanic Cloud, that the brighter the star, the longer its period. This secondary indicator can be used in nearby galaxies to calibrate as a standard candle a specific type of supernova (Type Ia, from explosions of white-dwarf stars), whose peak intensities are so bright that they can be seen throughout the observable universe.

International audience; Since the baryonic matter of the observable Universe consists mainly of protons and neutrons, then the numerical value of its mass can be represented and calculated on the basis of an additive-multiplicative golden algebraic fractal, based on golden algebraic fractals of the masse of proton, neutron, and muon. Based on an analytical estimate of the mass of the observable Universe, using the law “Planck’s Universal Proportions”, an analytical estimate of the Hubble constant and the main spatial-energy characteristics of the observed Universe is obtained. An analytical estimate of the Hubble constant is consistent with the experimental data of Planck’s mission, SDSS-III Baryon Oscillation Spectroscopic Survey, DES Collaboration. The objectivity of the experimental estimation of the Hubble constant from the H0LiCOW, Riess et al, Hubble Space Telescope collaborations does not raise any doubts. This means that the Hubble constant describes two similar, but different physical processes and has at least two values. The value of the Hubble constant from the collaborations Planck’s mission, SDSS-III Baryon Oscillation Spectroscopic Survey, DES Collaboration describes the process of rotation of the space of the observed Universe, and the value of the Hubble constant from the collaborations H0LiCOW, Riess et al, Hubble Space Telescope describes the process of rotation of substance in the space of the observed Universe. New estimates of the values of Dirac's large energy numbers are presented. It is confirmed that all coefficients in Planck’s proportions, which are calculated on the basis of Planck’s constants — length, mass, time, are equal to similar coefficients, which are calculated on the basis of the main characteristics of the observed Universe. Estimation of the numerical values of the Schwarzschild and Hubble radii for the observable Universe suggests that the Hubble sphere is inside the Schwarzschild sphere, therefore the observable Universe is a black hole. The presence of a redshift from distant objects of the observable Universe suggests that this is a rotating black hole. It has been shown that the vacuum pressure cannot be a source of dark energy. It is shown that the force of imaginary dark energy is equal in magnitude to the force of gravitational compression of the observed Universe and is equal to the Planck force. It is shown that after the Big Bang, the space of the observable Universe made one incomplete revolution of at 345 degrees, and the substance in it made one complete revolution of approximately 379 degrees. Estimates are given for the energy of dark matter, as the kinetic energy of rotation of the space of the observable Universe, as well as the equivalent mass of dark matter. New estimates are given: of the gravitational constant, of the Planck energy, of the Planck acceleration, of the Planck force, of the gravity factor of the observable Universe, of the Planck temperature, of the angular velocity of rotation of the space of the observable Universe. Estimates of temperature and wavelength of thermal radiation of the observable Universe, as the Hubble sphere, are given.

Since the 1990s, a wealth of astrophysical measurements have demonstrated that we live in a universe dominated by dark energy and dark matter. The data, which range over several orders of magnitude in length scale and originate from across the observable universe, are well-fit by a model parametrization called ΛCDM, which describes a universe of cold dark matter and a cos- mological constant. However, while cosmological data sets agree broadly with ΛCDM, the model provides no insight about the fundamental nature of either dark energy or dark matter. During the 2020s, a raft of new astronomical surveys will produce cosmological measurements of unprece- dented precision and provide sensitive new probes of dark energy and gravitation. The new data will also yield information about the microphysics of dark matter, and allow tests of cosmological models beyond ΛCDM. In this contribution, we describe the cosmological measurements made possible with the next generation of large galaxy redshift surveys, and focus in particular on the upcoming Dark Energy Spectroscopic Instrument (DESI).

The little sibling of the big rip is a cosmological abrupt event predicted by some phantom cosmological models that could describe our Universe. When this event is approached the observable Universe and its expansion rate grow infinitely, but its cosmic derivative remains finite. In this work we have obtained the group of metric $f(R)$ theories of gravity that reproduce this classical cosmological background evolution. Furthermore, we have considered the quantization of some of the resulting models in the framework of quantum geometrodynamics, showing that the DeWitt criterion can be satisfied. Therefore, as it also happens in general relativity, this event may be avoided in $f(R)$ quantum cosmology.

An anthropic explanation for evident smallness of the value of the dark energy implies the existence of a time-dependent component of the scalar field, serving, together with a negative-valued cosmological constant, as one of two components to the overall density of the dark energy. The observers (i.e. us) might then only evolve in those regions of the universe where the sum of those two components (a positive and a negative ones) is sufficiently close to zero. However, according to Vilenkin and Garriga, the scalar field component has to slowly but surely diminish in time. In about a trillion years this process will put a cap to the now-observable accelerated expansion of the universe, leading to a subsequent phase of impending collapse. However, the vanishing scalar field might also produce some rather unexpected singularities for a finite non-zero scale factor. We analyse this possibility on a particular example of Sudden Future Singularities (SFS) and come to a startling conclusion: the time required for SFS to arise must be "comparable" to the lifetime of observable universe., 11 pages, expanded the references, accepted for publication in Gravitation and Cosmology, Vol. 25, Issue 4 (2019)

A monistic framework is set up where energy is the only fundamental substance. Different states of energy are ordered by a set of scalar fields. The dual elements of matter, mass and space, are described as volume- and gradient-energy contributions of the set of fields, respectively. Time and space are formulated as background-independent dynamic variables. The evolution equations of the body of the universe are derived from the first principles of thermodynamics. Gravitational interaction emerges from quantum fluctuations in finite space. Application to a large number of fields predicts scale separation in space and repulsive action of masses distant beyond a marginal distance. The predicted marginal distance is compared to the size of the voids in the observable universe.

General relativity (GR) and gravitation in flat space-time (GFST) are covariant theories to describe gravitation. The metric of GR is given by the form of proper-time and the metric of GFST is the flat space-time form different from that of proper-time. GR has as source the matter tensor and the Einstein tensor describes the gravitational field whereas the source of GFST is the total energy-momentum including gravitation and the field is described by a non-linear differential operator of order two in divergence form. The results of the two theories agree for weak gravitational fields to the order of measurable accuracy. It is well-known that homogeneous, isotropic, cosmological models of GR start from a point singularity of the universe, the so called big bang. The density of matter is infinite. Therefore, our observable universe implies an expansion of space, in particular an inflationary expansion in the beginning. This is the presently most accepted model of the universe although doubts exist because infinities don’t exist in physics. GFST starts in the beginning from a homogeneous, isotropic universe with uniformly distributed energy and no matter. In the course of time, matter is created out of energy where the total energy is conserved. There is no singularity. The space is flat and the space may be non-expanding.

The three cosmological conjectures to which our work refers are: the phenomenon called geodesic incompleteness, the physical gravitational $\theta_G$-term that would characterize the 1-parameter family of inequivalent vacua of quantum gravidynamics, and the hypothesis of multiversality, more specifically, a zero-energy multiverse. The known cosmological phenomenology leads under plausible assumptions to theorems which establish that the universe is past incomplete. Here, starting from Wilczek's definition of multiverse (a larger physical structure of which the universe forms part) and that spacetime is much larger than the observable universe, in a new sense suggested by these theorems, we place the observable universe, labelled by $U_{\theta_G^{(1)}}$, within a multiverse ensemble, $\{U_{\theta_G}\}$. Its topological $\theta_G^{(1)}$-term would characterize the observable universe from the Planck epoch until the present time, and it could have physical effects in, for example, black-hole physics. Our proposal is therefore a possible framework for a multiverse quantum cosmology, in which the temporal parameters (see figures in the main text) start from a "timeless multiverse big bang" (TLMBB), where all members of the multiverse ensemble, $\{U_{\theta_G}\}$, disappear, together with their corresponding classical spacetimes. Since quantum cosmology can be viewed as one attempt among many to face with the question of finding a gravitational quantum theory, if the TLMBB were the appropriate ground to define the physical or mathematical underlying structure of quantum cosmology, then multiversality could come to have a predictive power within our observable universe., Comment: 7 pages, 2 figures. Final version accepted for publication in CQG

The Advanced Telescope for High Energy Astrophysics (Athena) is a large X-ray observatory which has been selected by the European Space Agency for launch as the second large mission of its Cosmic Vision program. Athena has been designed to address the Hot and Energetic Universe science theme, which aims to reveal hot gas structures and black holes throughout cosmic time, and determine their wider importance for the appearance of the observable Universe. Athena consists of a single, large aperture X-ray telescope based on Silicon Pore Optics (SPO) technology, focussing X-rays onto one of two instruments. The X-ray Integral Field Unit (X-IFU) provides spatially-resolved high resolution spectroscopy, which can be used to map the dynamics and chemical composition of hot diffuse cluster gas in galaxy clusters, and to detect weak absorption and emission features needed to uncovercharacteristic of the hot components of the intergalactic medium. The Wide-Field Imager (WFI) provides spectrally-resolved imaging over a large field of view, as well as high count-rate capability, and will be used, for example, to perform deep and wide X-ray surveys necessary to uncover typical growing black holes in the early Universe at z>6. The mission concept is currently undergoing Phase A study for an eventual launch to the Sun-Earth L2 point using an Ariane 6 launcher. In this presentation, the status of the Athena project and and associated scientific activities will be reviewed.

12 pags., 5 figs., 2 apps., We analyze a quantized toy model of a universe undergoing eternal inflation using a quantum-field-theoretical formulation of the Wheeler–DeWitt equation. This so-called third quantization method leads to the picture that the eternally inflating universe is converted to a multiverse in which sub-universes are created and exhibit a distinctive phase in their evolution before reaching an asymptotic de Sitter phase. From the perspective of one of these sub-universes, we can thus analyze the pre-inflationary phase that arises naturally. Assuming that our observable universe is represented by one of those sub-universes, we calculate how this pre-inflationary phase influences the power spectrum of the cosmic microwave background (CMB) anisotropies and analyze whether it can explain the observed discrepancy of the power spectrum on large scales, i.e. the quadrupole issue in the CMB. While the answer to this question is negative in the specific model analyzed here, we point out a possible resolution of this issue., This article is based upon work from COST Action CA15117 “Cosmology and Astrophysics Network for Theoretical Advances and Training Actions (CANTATA)”, supported by COST (European Cooperation in Science and Technology). The research of M. B.-L. is supported by the Basque Foundation of Science Ikerbasque. She and J. M. also would like to acknowledge the partial support from the Basque government Grant No. IT956-16 (Spain) and the project FIS2017-85076-P (MINECO/AEI/FEDER, UE). The research of M. K. was financed by the Polish National Science Center Grant DEC2012/06/A/ST2/00395 as well as by a Grant for the Short Term Scientific Mission (STSM) “Multiverse impact onto the cosmic microwave background and its relation to modified gravity” (COST-STSM-CA15117- 36137) awarded by the above-mentioned COST Action. For their kind hospitality while part of this work was done, M. K. and J. M. would like to thank the Centro de Matemática e Aplicações of the Universidade da Beira Interior in Covilhã, Portugal and M. K. also thanks the Department of Theoretical Physics and History of Science of the University of the Basque Country (UPV/EHU).

This paper intends to design a payload for a Nanosat to record and analyze the Cosmic Microwave Background Radiation (CMBR). Since its discovery in 1965, CMBR has become a much-discussed topic with the speculation that it might hold the key to the birth of our Universe. The big bang theory not only scattered the matter which condensed into galaxies, but also a massive amount of radiation which can be detected by using appropriate instruments, albeit as microwaves due to massive redshift (Redshift is shifting of the wavelength of light or some other form of electromagnetic radiation to the red end of the spectrum. It occurs when a light source moves away from the observer) of our ever-expanding universe. The space missions for detection of CMBR have not been cost efficient, and the Earth-based laboratories deal with too much interference from the surrounding sources. The radiation that was first discovered was of the wavelength 10cm at a reported temperature of 3.5 ± 1 K. The data received about the cosmic microwave background radiation can provide us much information about the anisotropic nature of the observable universe, the cosmic inflation era as well as the recombination era. In the present scenario, CMBR can be detected at a wavelength of about 2 mm. The challenge lies in using a Nanosat in the low earth orbit (LEO) to study CMBR because, being a form of radiation in itself, it is very susceptible to noise from the surrounding sources. The radiation from the sun, the earth as well as the satellite itself will contribute to the noise which needs to be calibrated off since the radiation due to the Cosmic Microwave Background is faint. The interference due to the ambient plasma will also be added to the noise. The antennas are designed with reflectors and calibrated to deal away with all of the aforementioned noises. The use of appropriate rectifiers and amplifiers for selection and amplification of the desired data has also been employed. The whole setup is cooled externally and the orbit selected to minimise the interfering radiation. The spinning of the satellite is controlled to gather desired amount data from a particular region of space. A detailed analysis for noise mitigation and selection of appropriate equipment has been discussed in this paper.

There are many inflationary models that allow the formation of the large-scale structure of the observable universe. The non-gaussianity parameter $f_{NL}$ is a useful tool to discriminate among these cosmological models when comparing the theoretical predictions with the satellite survey results like those from WMAP and Planck. The goal of this proceeding contribution is to review the moment transport equations methodology and the subsequent calculation of the $f_{NL}$ parameter., LaTeX file in elsarticle style, 7 pages, 1 figure. To be published in Nuclear and Particle Physics Proceedings as the proceedings of the Tenth Latin American Symposium on High Energy Physics (Medellin - Colombia, 24th-28th November, 2014). v2: no changes in the file. Just to mention that this work is entirely based on Ref. [4] and is the result of Carlos A. Sierra's MSc thesis

One of the major challenges of modern physics is to decipher the nature of dark matter. Astrophysical observations provide ample evidence for the existence of an invisible and dominant mass component in the observable universe, from the scales of galaxies up to the largest cosmological scales. The dark matter could be made of new, yet undiscovered elementary particles, with allowed masses and interaction strengths with normal matter spanning an enormous range. Axions, produced non-thermally in the early universe, and weakly interacting massive particles (WIMPs), which froze out of thermal equilibrium with a relic density matching the observations, represent two well-motivated, generic classes of dark matter candidates. Dark matter axions could be detected by exploiting their predicted coupling to two photons, where the highest sensitivity is reached by experiments using a microwave cavity permeated by a strong magnetic field. WIMPs could be directly observed via scatters off atomic nuclei in underground, ultra low-background detectors, or indirectly, via secondary radiation produced when they pair annihilate. They could also be generated at particle colliders such as the LHC, where associated particles produced in the same process are to be detected. After a brief motivation and an introduction to the phenomenology of particle dark matter detection, I will discuss the most promising experimental techniques to search for axions and WIMPs, addressing their current and future science reach, as well as their complementarity.

We report the realization of the first cosmological simulations on the scale of the whole observable universe. These simulations have been carried out on 4752 nodes of the Curie supercomputer as a part of the Dark Energy Universe Simulation: Full Universe Runs (DEUS-FUR) project which aims at establishing new probes to put constraints on the nature of dark energy by comparing the growth of large-scale structures, the characteristics of extreme statistical events and the matter distribution in redshift space. The numerical challenge of the first DEUS-FUR simulation associated with the concordance ΛCDM (Λ Cold Dark Matter) model was already presented during the 2012 supercomputing conference (Alimi et al., 2012, in The international conference for high performance computing, networking, storage and analysis). Here we first focus on the numerical aspects of the two new simulations. In practice, each one of these simulations has evolved 550 billion dark matter particles in an adaptive mesh refinement grid, and one of the new simulations has pushed back the total number of grid points from 2000 billion for the ΛCDM model to 2200 billion due to the formation of a larger number of structures. We highlight the optimizations and adjustments required to run such a set of simulations and we then summarize some important lessons learnt for future exascale computing projects.

Dark energy and dark matter constitute 95% of the observable Universe. Yet the physical nature of these two phenomena remains a mystery. Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into light-emitting structures. However, contemporary cosmological results are derived upon the reasonable assumption that the Universe only contains positive masses. By reconsidering this assumption, I have constructed a toy model which suggests that both dark phenomena can be unified into a single negative mass fluid. The model is a modified $\Lambda$CDM cosmology, and indicates that continuously-created negative masses can resemble the cosmological constant and can flatten the rotation curves of galaxies. The model leads to a cyclic universe with a time-variable Hubble parameter, potentially providing compatibility with the current tension that is emerging in cosmological measurements. In the first three-dimensional N-body simulations of negative mass matter in the scientific literature, this exotic material naturally forms haloes around galaxies that extend to several galactic radii. These haloes are not cuspy. The proposed cosmological model is therefore able to predict the observed distribution of dark matter in galaxies from first principles. The model makes several testable predictions and seems to have the potential to be consistent with observational evidence from distant supernovae, the cosmic microwave background, and galaxy clusters. These findings may imply that negative masses are a real and physical aspect of our Universe, or alternatively may imply the existence of a superseding theory that in some limit can be modelled by effective negative masses. Both cases lead to the surprising conclusion that the compelling puzzle of the dark Universe may have been due to a simple sign error., Comment: Accepted for publication in Astronomy and Astrophysics (A&A). Videos of the simulations are available online at: https://goo.gl/rZN1Ps

Cosmic strings are topological defects possibly formed in the early Universe, which may be observable due to their gravitational effects on the cosmic microwave background radiation or gravitational wave experiments. To this effect it is important to quantitatively ascertain the network properties, including their density, velocity or the number of strings present, at the various epochs in the observable Universe. Attempts to estimate these numbers often rely on simplistic approximations for the string parameters, such as assuming that the network is scaling. However, in cosmological models containing realistic amounts of radiation, matter and dark energy a string network is never exactly scaling. Here we use the velocity-dependent one-scale model for the evolution of a string network to better quantify how these networks evolve. In particular we obtain new approximate analytic solutions for the behavior of the network during the radiation-to-matter and matter-to-acceleration transitions (assuming, in the latter case, the canonical $\Lambda$ cold dark matter model), and numerically calculate the relevant quantities for a range of possible dark energy models., Comment: 10 pages, 5 figures

Observational constraints on the abundance of primordial black holes (PBHs) constrain the allowed amplitude of the primordial power spectrum on both the smallest and the largest ranges of scales, covering over 20 decades from $1-10^{20}/ \rm{Mpc}$. Despite tight constraints on the allowed fraction of PBHs at their time of formation near horizon entry in the early universe, the corresponding constraints on the primordial power spectrum are quite weak, typically ${\cal P}_\mathcal{R}\lesssim 10^{-2}$ assuming Gaussian perturbations. Motivated by recent claims that the evaporation of just one PBH would destabilise the Higgs vacuum and collapse the universe, we calculate the constraints which follow from assuming there are zero PBHs within the observable universe. This extends the constraints right down to the horizon scale at the end of inflation, but does not significantly tighten the existing power spectrum constraints, even though the constraint on PBH abundance can decrease by up to 46 orders of magnitude. This shows that no future improvement in observational constraints can ever lead to a significant tightening in constraints on inflation (via the power spectrum amplitude). The power spectrum constraints are weak because an order unity perturbation is required in order to overcome pressure forces. We therefore consider an early matter dominated era, during which exponentially more PBHs form for the same initial conditions. We show this leads to far tighter constraints, which approach ${\cal P}_\mathcal{R}\lesssim10^{-9}$, albeit over a smaller range of scales and are very sensitive to when the early matter dominated era ends. Finally, we show that an extended early matter era is incompatible with the argument that an evaporating PBH would destroy the universe, unless the power spectrum amplitude decreases by up to ten orders of magnitude., Comment: 16 pages, 5 figures. Minor clarifications and references added, matches published version

Observations by the Wilkinson Microwave Anisotropy Probe and the Planck mission suggest a hemispherical power amplitude asymmetry in the cosmic microwave background, with a correlation length on the order of the size of the observable Universe. We find that this anomaly can be naturally explained by an axion-like particle (ALP) cosmic string formed near our visible Universe. The field variation associated to this cosmic string creates particle density fluctuations after inflation, which consequently decay into radiation before the Big Bang Nucleosynthesis (BBN) era and resulted in the observed power asymmetry. We find in this scenario that the hemispherical power amplitude asymmetry is strongly scale dependent: $A(k)\propto {\rm exp}(-kl)/k$. Admittedly, typical inflation models predict a relic number density of topological defects of order one per observable Universe and so in our model the cosmic string must be tuned to have an impact factor of order $1/H_0$. Interestingly, the constraints based on purely cosmological considerations also give rise to a Peccei-Quinn scale $F_a$ of order $10^3$ larger then the Hubble scale of inflation $H_I$. Assuming $H_I\sim 10^{13}$GeV, we then have an ALP with $F_a\sim 10^{16}$GeV, which coincides with the presumed scale of grand unification. As we require ALP decays occur before the BBN era, which implies a relatively heavy mass or strong self-coupling, and considering that the associated potential should break the shift symmetry softly in order to protect the system from radiative corrections, we also conclude that the required ALP potential should be monodromic in nature., Comment: 16 pages, 2 figures, references and comments added, published version

Introduction Science and philosophy have a strong interaction and an unquestionable complementarity but one needs to draw a clear line between what they tell on the world, on their scope and implications. Role of Cosmology Cosmology does indeed play an important role in this debate. It has long been part of mythology and philosophy and, during the twentieth century, adopted a scientific method, thanks to the observational possibilities offered by astrophysics. We can safely state that scientific cosmology was born with Einstein's general relativity, a theory of gravitation that made space and time dynamical, a physical field that needs to be determined by solving dynamical equations known as Einstein field equations. The concept of spacetime was born with this theory, and in its early years cosmology was a space of freedom to think general relativity (Eisenstaedt, 1989). It led to the formulation of a ‘standard’ model, often referred to as the big bang model, that allows us to offer a history of the matter in the universe, in particular by providing an explanation for the synthesis of each of the elements of the Mendeleev table, and a history of the formation of the large scale structure. Cosmology has also excluded many possible models and hypotheses, such as the steady state model or topological defects as the seeds of the large scale structure. The current standard cosmological model can be summarised by Figure 6.1 from which one can easily conclude that all the phenomena that can be observed in our local universe, from primordial big bang nucleosynthesis (BBN) to today, rely mostly on general relativity, electromagnetism and nuclear physics, that is on physics below 100 MeV and well under control experimentally (see Ellis et al. (2012); Mukhanov (2005); Peter and Uzan (2009) for textbooks on modern cosmology). This is an important property since the fact that we can understand our observable universe does not rely on our ability to construct e.g. a theory of quantum gravity. This is the positive aspect, the negative one being that it may be very difficult to find systems in which gravity, quantum physics and observations appear together, inflationary perturbations and black holes being probably the only ones. At later time, the dynamics of the large scale structure becomes non-linear and requires the use of numerical simulations to be fully understood.

High-resolution, low-noise observations of the cosmic microwave background (CMB) planned for the near-future will enable new cosmological probes based on re-scattered CMB photons -- the secondary CMB. At the same time, enormous galaxy surveys will map out huge volumes of the observable Universe. Using the technique of kinetic Sunyaev Zel'dovich (kSZ) tomography these new probes can be combined to reconstruct the remote dipole field, the CMB dipole as observed from different vantage points in our Universe. The volume accessible to future galaxy surveys is large enough that general relativistic corrections to the observed distribution of galaxies must be taken into account. These corrections are interesting probes of gravity in their own right, but can also obscure potential signatures of primordial non-Gaussianity. In this paper, we demonstrate that correlations between the reconstructed remote dipole field and the observed galaxy number counts can in principle be used to detect general relativistic corrections. We show that neglecting general relativistic corrections leads to an $\mathcal{O}(1)$ bias on the inferred amplitude of primordial non-Gaussianity, $f_{\rm NL}$. In addition, we demonstrate that the reconstructed remote dipole field can provide useful constraining power on various bias parameters appearing in the galaxy number counts, and can significantly mitigate the effects of alignment bias., Comment: 13 pages, 3 figures

An earlier elaborated model of the expanding universe with its contents of dark energy, dark matter and normal matter is reconsidered and extended. The model is found to be reconcilable with the observed cosmical dimensions and with the magnitude of the present accelerated expansion. It has the form of a freely expanding cloud of zero-pointenergy photons, with the inclusion of a small amount of normal matter. On a macroscopic scale, within the radius of the observable universe, the model has the character of a flat Euclidian geometry, without the need of introducing curved space effects due to General Relativity. This flat geometry is found to be stable with respect to expansive and compressive perturbations, thus suggesting the universe to possess an intrinsic mechanism which aims at flat geometry.

In this paper, we discuss that an observable-based single-system Copenhagen and entanglement-based two-system von Neumann measurement protocols in quantum theory can be made equivalent by considering the second part of the two-system scheme to be a Dirac-type negative sea filling up the first system. Based on this equivalence, and by considering the universe as a computational process, the choice of the apparatus state in the two-system protocol can be identified with the choice of the observable in the single-system scheme as negative sea filling up the observable universe. In particular, the measuring party's state is considered to be evolving backwards in time to the big bang as a nondeterministic computational process, which chooses the acceptable path as a time-reversal process of irreversible computation. The suggested model proposes that the prepared microstate of the universe, or reality, corresponds to the observer's choice, therefore, subjective reality. Thus, this effectively provides a specific description of the subjective universe model previously proposed, which is based on the symmetry breakdown between the Schrodinger and the Heisenberg pictures of quantum theory.

Cosmic inflation, a period of accelerated expansion in the early universe, can give rise to large amplitude ultra-large scale inhomogeneities on distance scales comparable to or larger than the observable universe. The cosmic microwave background (CMB) anisotropy on the largest angular scales is sensitive to such inhomogeneities and can be used to constrain the presence of ultra-large scale structure (ULSS). We numerically evolve nonlinear inhomogeneities present at the beginning of inflation in full General Relativity to assess the CMB quadrupole constraint on the amplitude of the initial fluctuations and the size of the observable universe relative to a length scale characterizing the ULSS. To obtain a statistically significant number of simulations, we adopt a toy model in which inhomogeneities are injected along a preferred direction. We compute the likelihood function for the CMB quadrupole including both ULSS and the standard quantum fluctuations produced during inflation. We compute the posterior given the observed CMB quadrupole, finding that when including gravitational nonlinearities, ULSS curvature perturbations of order unity are allowed by the data, even on length scales not too much larger than the size of the observable universe. Our results illustrate the utility and importance of numerical relativity for constraining early universe cosmology., 14 pages, 6 figures v3: Clarifications added regarding the generality of results - conclusions unchanged, version accepted for publication in PRD, v2: updated with minor clarifications, submitted

A model for gravitational collapse where the event horizon is a quantum critical phase transition is extended to provide an explanation for the origin of the observable universe, where the expanding universe that we observe today was proceeded by a flat universe with a negative cosmological constant. In principal this allows one derive all the features of our universe from a single parameter: the magnitude of the pre-big bang negative vacuum energy density. In this paper a simple model for the big bang is introduced which allows us to relate the present day energy density and temperature fluctuations of the CMB, to the present day density of dark matter. This model for the big bang also makes a dramatic prediction: dark matter mostly consists of compact objects with a masses on the order of 10^4 solar masses. Remarkably this is consistent with numerical simulations for how primordial fluctuations in the density of dark give rise to the observed inhomogeneous distribution of matter in our universe. Our model for the big bang also allows for the production of some compact objects with masses greater than 10^4 solar masses, which is consistent with numerical simulations of structure formation which require massive primordial comapact objects as the seeds for galaxies in order to explain galactic morphologies., 16 pages, 3 figures

The fine tuning of parameters required to reproduce our present day Universe suggests that our Universe may simply be a region within an eternally inflating super-region. Many other regions beyond our observable Universe would exist with each such region governed by a different set of physical parameters. Collision between these regions, if they occur, should leave signatures of anisotropy in the cosmic microwave background (CMB) but have not been seen. We analyze the spectral properties of masked, foreground-cleaned maps between 100 and 545 GHz constructed from the Planck dataset. Four distinct $\sim2-4\arcdeg$ regions associated with CMB cold spots show anomalously strong 143 GHz emission but no correspondingly strong emission at either 100 or 217 GHz. The signal to noise of this 143 GHz residual emission is at the $\gtrsim$6$\sigma$ level which reduces to $3.2-5.4\sigma$ after subtraction of remaining synchrotron/free-free foregrounds. We assess different mechanisms for this residual emission and conclude that although there is a 30\% probability that noise fluctuations may cause foregrounds to fall within 3$\sigma$ of the excess, there is less than a 0.5\% probability that foregrounds can explain all the excess. A plausible explanation is that the collision of our Universe with an alternate Universe whose baryon to photon ratio is a factor of $\sim$4500 larger than ours, could produce enhanced Hydrogen Paschen-series emission at the epoch of recombination. Future spectral mapping and deeper observations at 100 and 217 GHz are needed to mitigate systematics arising from unknown Galactic foregrounds and to confirm this unusual hypothesis., Comment: 27 pages, 9 figures (6 color, 3 B&W), ApJ, in press

The 1S–2S transition in magnetically trapped atoms of antihydrogen is observed, and its frequency is shown to be consistent with that expected for hydrogen. Testing to high precision whether matter behaves like antimatter could provide clues to one of the biggest puzzles in modern physics: why does the observable Universe consist almost entirely of matter? After all, the Standard Model predicts that after the Big Bang the Universe should have been made up of equal amounts of matter and antimatter. Antimatter is difficult to produce and characterize because it annihilates when it comes in contact with matter. But recent advances at CERN's Antiproton Decelerator have allowed researchers to trap and measure both antiprotons and antihydrogen. Now the ALPHA Collaboration at CERN reports the first spectroscopic characterization of antihydrogen, exciting the 1S to 2S transition with laser light. The transition frequency is consistent with that of hydrogen. The spectrum of ordinary hydrogen has been characterized to extremely high precision, so improvements in antihydrogen spectroscopy will yield highly sensitive tests of matter–antimatter symmetry. The spectrum of the hydrogen atom has played a central part in fundamental physics over the past 200 years. Historical examples of its importance include the wavelength measurements of absorption lines in the solar spectrum by Fraunhofer, the identification of transition lines by Balmer, Lyman and others, the empirical description of allowed wavelengths by Rydberg, the quantum model of Bohr, the capability of quantum electrodynamics to precisely predict transition frequencies, and modern measurements of the 1S–2S transition by Hansch1 to a precision of a few parts in 1015. Recent technological advances have allowed us to focus on antihydrogen—the antimatter equivalent of hydrogen2,3,4. The Standard Model predicts that there should have been equal amounts of matter and antimatter in the primordial Universe after the Big Bang, but today’s Universe is observed to consist almost entirely of ordinary matter. This motivates the study of antimatter, to see if there is a small asymmetry in the laws of physics that govern the two types of matter. In particular, the CPT (charge conjugation, parity reversal and time reversal) theorem, a cornerstone of the Standard Model, requires that hydrogen and antihydrogen have the same spectrum. Here we report the observation of the 1S–2S transition in magnetically trapped atoms of antihydrogen. We determine that the frequency of the transition, which is driven by two photons from a laser at 243 nanometres, is consistent with that expected for hydrogen in the same environment. This laser excitation of a quantum state of an atom of antimatter represents the most precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of about 2 × 10−10.

In this article, we present a bouncing cosmology inspired by a family of regular black holes. This scale-dependent cosmology deviates from the cosmological principle by means of a scale factor which depends on the time and the radial coordinate as well. The model is isotropic but not perfectly homogeneous. That is, this cosmology describes a universe almost homogeneous only for large scales, such as our observable universe., Comment: 9 pages, 1 figure. Version 3 matches the published version in General Relativity and Gravitation

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).

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

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.

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/)