106 research outputs found
Gamma-Ray Background from Structure Formation in the Intergalactic Medium
The universe is filled with a diffuse and isotropic extragalactic background
of gamma-ray radiation, containing roughly equal energy flux per decade in
photon energy between 3 MeV-100 GeV. The origin of this background is one of
the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux
can be attributed to unresolved discrete sources, but the remainder appears to
constitute a truly diffuse background whose origin has hitherto been
mysterious. Here we show that the shock waves induced by gravity during the
formation of large-scale structure in the intergalactic medium, produce a
population of highly-relativistic electrons with a maximum Lorentz factor above
10^7. These electrons scatter a small fraction of the microwave background
photons in the present-day universe up to gamma-ray energies, thereby providing
the gamma-ray background. The predicted diffuse flux agrees with the observed
background over more than four decades in photon energy, and implies a mean
cosmological density of baryons which is consistent with Big-Bang
nucleosynthesis.Comment: 7 pages, 1 figure. Accepted for publication in Nature. (Press embargo
until published.
The Cosmic Microwave Background and Particle Physics
In forthcoming years, connections between cosmology and particle physics will
be made increasingly important with the advent of a new generation of cosmic
microwave background (CMB) experiments. Here, we review a number of these
links. Our primary focus is on new CMB tests of inflation. We explain how the
inflationary predictions for the geometry of the Universe and primordial
density perturbations will be tested by CMB temperature fluctuations, and how
the gravitational waves predicted by inflation can be pursued with the CMB
polarization. The CMB signatures of topological defects and primordial magnetic
fields from cosmological phase transitions are also discussed. Furthermore, we
review current and future CMB constraints on various types of dark matter (e.g.
massive neutrinos, weakly interacting massive particles, axions, vacuum
energy), decaying particles, the baryon asymmetry of the Universe,
ultra-high-energy cosmic rays, exotic cosmological topologies, and other new
physics.Comment: 43 pages. To appear in Annual Reviews of Nuclear and Particle Scienc
Fluids in cosmology
We review the role of fluids in cosmology by first introducing them in
General Relativity and then by applying them to a FRW Universe's model. We
describe how relativistic and non-relativistic components evolve in the
background dynamics. We also introduce scalar fields to show that they are able
to yield an inflationary dynamics at very early times (inflation) and late
times (quintessence). Then, we proceed to study the thermodynamical properties
of the fluids and, lastly, its perturbed kinematics. We make emphasis in the
constrictions of parameters by recent cosmological probes.Comment: 34 pages, 4 figures, version accepted as invited review to the book
"Computational and Experimental Fluid Mechanics with Applications to Physics,
Engineering and the Environment". Version 2: typos corrected and references
expande
The dusty SF history of high-z galaxies, modelling tools and future prospects
We summarize recent advances in the determination of the cosmic history of star formation and other properties of high-z galaxies, and the relevance of this information in our understanding of the formation of structures. We emphasize the importance of dust reprocessing in the high--z universe, as demonstrated in particular by IR and sub-mm data. This demand a panchromatic approach to observations and suitable modelling tools. We spend also some words on expectations from future instruments
CMB Telescopes and Optical Systems
The cosmic microwave background radiation (CMB) is now firmly established as
a fundamental and essential probe of the geometry, constituents, and birth of
the Universe. The CMB is a potent observable because it can be measured with
precision and accuracy. Just as importantly, theoretical models of the Universe
can predict the characteristics of the CMB to high accuracy, and those
predictions can be directly compared to observations. There are multiple
aspects associated with making a precise measurement. In this review, we focus
on optical components for the instrumentation used to measure the CMB
polarization and temperature anisotropy. We begin with an overview of general
considerations for CMB observations and discuss common concepts used in the
community. We next consider a variety of alternatives available for a designer
of a CMB telescope. Our discussion is guided by the ground and balloon-based
instruments that have been implemented over the years. In the same vein, we
compare the arc-minute resolution Atacama Cosmology Telescope (ACT) and the
South Pole Telescope (SPT). CMB interferometers are presented briefly. We
conclude with a comparison of the four CMB satellites, Relikt, COBE, WMAP, and
Planck, to demonstrate a remarkable evolution in design, sensitivity,
resolution, and complexity over the past thirty years.Comment: To appear in: Planets, Stars and Stellar Systems (PSSS), Volume 1:
Telescopes and Instrumentatio
A planetary system as the origin of structure in Fomalhaut's dust belt
The Sun and >15 percent of nearby stars are surrounded by dusty debris disks
that must be collisionally replenished by asteroids and comets, as the dust
would otherwise be depleted on <10 Myr timescales (ref. 1). Theoretical studies
show that disk structure can be modified by the gravitational influence of
planets (ref. 2-4), but the observational evidence is incomplete, at least in
part because maps of the thermal infrared emission from disks have low linear
resolution (35 AU in the best case; ref. 5). Optical images provide higher
resolution, but the closest examples (AU Mic and Beta Pic) are edge-on (ref.
6,7), preventing the direct measurement of azimuthal and radial disk structure
that is required for fitting theoretical models of planetary perturbations.
Here we report the detection of optical light reflected from the dust grains
orbiting Fomalhaut (HD 216956). The system is inclined 24 degrees away from
edge-on, enabling the measurement of disk structure around its entire
circumference, at a linear resolution of 0.5 AU. The dust is distributed in a
belt 25 AU wide, with a very sharp inner edge at a radial distance of 133 AU,
and we measure an offset of 15 AU between the belt's geometric centre and
Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the
presence of planet-mass objects orbiting Fomalhaut.Comment: 8 pages, 3 figures, 1 tabl
Equilibrium configurations of two charged masses in General Relativity
An asymptotically flat static solution of Einstein-Maxwell equations which
describes the field of two non-extreme Reissner - Nordstr\"om sources in
equilibrium is presented. It is expressed in terms of physical parameters of
the sources (their masses, charges and separating distance). Very simple
analytical forms were found for the solution as well as for the equilibrium
condition which guarantees the absence of any struts on the symmetry axis. This
condition shows that the equilibrium is not possible for two black holes or for
two naked singularities. However, in the case when one of the sources is a
black hole and another one is a naked singularity, the equilibrium is possible
at some distance separating the sources. It is interesting that for
appropriately chosen parameters even a Schwarzschild black hole together with a
naked singularity can be "suspended" freely in the superposition of their
fields.Comment: 4 pages; accepted for publication in Phys. Rev.
The establishment of the Standard Cosmological Model through observations
Over the last decades, observations with increasing quality have
revolutionized our understanding of the general properties of the Universe.
Questions posed for millenia by mankind about the origin, evolution and
structure of the cosmos have found an answer. This has been possible mainly
thanks to observations of the Cosmic Microwave Background, of the large-scale
distribution of matter structure in the local Universe, and of type Ia
supernovae that have revealed the accelerated expansion of the Universe. All
these observations have successfully converged into the so-called "concordance
model". In spite of all these observational successes, there are still some
important open problems, the most obvious of which are what generated the
initial matter inhomogeneities that led to the structure observable in today's
Universe, and what is the nature of dark matter, and of the dark energy that
drives the accelerated expansion. In this chapter I will expand on the previous
aspects. I will present a general description of the Standard Cosmological
Model of the Universe, with special emphasis on the most recent observations
that have us allowed to consolidate this model. I will also discuss the
shortfalls of this model, its most pressing open questions, and will briefly
describe the observational programmes that are being planned to tackle these
issues.Comment: Accepted for publication in the book "Reviews in Frontiers of Modern
Astrophysics: From Space Debris to Cosmology" (eds Kabath, Jones and Skarka;
publisher Springer Nature) funded by the European Union Erasmus+ Strategic
Partnership grant "Per Aspera Ad Astra Simul" 2017-1-CZ01-KA203-03556
The Cosmological Constant
This is a review of the physics and cosmology of the cosmological constant.
Focusing on recent developments, I present a pedagogical overview of cosmology
in the presence of a cosmological constant, observational constraints on its
magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity
(http://www.livingreviews.org/), December 199
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