The cosmic acceleration is one of the most significant cosmological discoveries over the last century. The two categories of explanation are exotic component (dark energy) and modified gravity. We constrain the two types of model by a joint analysis with perturbation growth and direct $H(z)$ data. Though the minimal $χ^2$ of the $Λ$CDM is almost the same as that of DGP, in the sense of consistency we find that the dark energy ($Λ$CDM) model is more favored through a detailed comparison with the corresponding parameters fitted by expansion data.
We present a model where a non-conventional scalar field may act like dark energy leading to cosmic acceleration. The latter is driven by an appropriate field configuration, which result in an effective cosmological constant. The potential role of such a scalar in the cosmological constant problem is also discussed.
We review the notion of holographic dark energy and assess its significance in the light of the well documented cosmic acceleration at the present time. We next propose a model of holographic dark energy in which the infrared cutoff is set by the Hubble scale. The model accounts for the aforesaid acceleration and, by construction, is free of the cosmic coincidence problem.
The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. The braneworld theory of Dvali, Gabadadze, and Porrati (DGP) provides a context where late-time acceleration is driven not by some energy-momentum component (dark energy), but rather is the manifestation of the excruciatingly slow leakage of gravity off our four-dimensional world into an extra dimension. At the same time, DGP gravity alters the gravitational force law in a specific and dramatic way at cosmologically accessible scales. We derive the DGP gravitational force law in a cosmological se…
Brane cosmology presents many interesting possibilities including: phantom acceleration (w<-1), self-acceleration, unification of dark energy with inflation, transient acceleration, loitering cosmology, new singularities at which the Hubble parameter remains finite, cosmic mimicry, etc. The existence of a time-like extra dimension can result in a singularity-free cyclic cosmology.
The acceleration of the expansion of the universe arises from unknown physical processes involving either new fields in high energy physics or modifications of gravitation theory. It is crucial for our understanding to characterize the properties of the dark energy or gravity through cosmological observations and compare and distinguish between them. In fact, close consistencies exist between a dark energy equation of state function w(z) and changes to the framework of the Friedmann cosmological equations as well as direct spacetime geometry quantities involving the acceleration, such as ``geo…
We analyze the prospects for testing the cosmic neutrino background and its interpretation as source of Neutrino Dark Energy with the radio telescope LOFAR.
This is a short overview of spatially flat (or open) four-dimensional accelerating cosmologies for some simple exponential potentials obtained by string or M theory compactification on some non-trivial curved spaces, which may lead to some striking results, e.g., the observed cosmic acceleration and the scale of the dark energy from first principles.
A modified model of gravity with additional positive and negative powers of the scalar curvature, $R$, in the gravitational action is studied. This is done using the Palatini variational principle. It is demonstrated that using such a model might prove useful to explain both the early time inflation and the late time cosmic acceleration without the need for any form of dark energy.
We show that positive averaged acceleration obtained in LTB models through spatial averaging can require integration over a region beyond the event horizon of the central observer. We provide an example of a LTB model with positive averaged acceleration in which the luminosity distance does not contain information about the entire spatially averaged region, making the averaged acceleration unobservable. Since the cosmic acceleration is obtained from fitting the observed luminosity distance to a FRW model we conclude that in general a positive averaged acceleration in LTB models does not imply …
There is now overwhelming observational evidence that our Universe is accelerating in its expansion. I discuss how modified gravitational models can provide an explanation for this observed late-time cosmic acceleration. We consider specific low-curvature corrections to the Einstein-Hilbert action. Many of these models generically contain unstable de Sitter solutions and, depending on the parameters of the theory, late-time accelerating attractor solutions.
The study of the interactions of Cosmic Rays (CR's) with universal diffuse background radiation can provide very stringent tests of the validity of Special Relativity. The interactions we consider are the ones characterized by well defined energy thresholds whose energy position can be predicted on the basis of Special Relativity. We argue that the experimental confirmation of the existence of these thresholds can in principle put very stringent limits on the scale where Special Relativity and/or continuity of space-time may possibly break down.
We put forward a multi-time theory, in frame of which the cosmic acceleration is a natural phenomenon without cosmological constant or anything like that. The main point of this theory is that each of the gravity interaction and electromagnetic interaction has its own time, respectively. Also we give a concrete model of this theory which can exactly simulate $Λ$CDM. Further we discuss the possible observations which may improve this theory in the future.
The modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of the curvature support the inflationary epoch while the terms with negative powers of the curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar system experiments.
In this work, I develop an alternative explanation for the acceleration of the cosmic expansion, which seems to be a result of recent high redshift Supernova data. In the current interpretation, this cosmic acceleration is explained by including a positive cosmological constant term (or vacuum energy), in the standard Friedmann models. Instead, I will consider a Locally Rotationally Symmetric (LRS) and spherically symmetric (SS), but inhomogeneous spacetime, with a barotropic perfect fluid equation of state for the cosmic matter. The congruence of matter has acceleration, shear and expansion. …
In this essay, I present an alternative explanation for the cosmic acceleration which appears as a consequence of recent high redshift Supernova data. In the usual interpretation, this cosmic acceleration is explained by the presence of a positive cosmological constant or vacuum energy, in the background of Friedmann models. Instead, I will consider a Local Rotational Symmetric (LRS) inhomogeneous spacetime, with a barotropic equation of state for the cosmic matter. Within this framework the kinematical acceleration of the cosmic fluid or, equivalently, the inhomogeneity of matter, is just the…
We show that the late time rolling of the Cubic Superstring Field Theory (CSSFT) non-local tachyon in the FRW Universe leads to a cosmic acceleration with a periodic crossing of the w=-1 barrier. An asymptotic solution for the tachyon and Hubble parameter by linearizing the non-local equations of motion is constructed explicitly. For a small Hubble parameter the period of oscillations is a number entirely defined by the parameters of the CSSFT action.
It has been recently argued that higher dimensional gravity theories may manifest themselves not only at short microscopic distances but also at large cosmological scales. We study the constraints that cosmic microwave background measurements set on such large distance modifications of the gravitational potential.
We analyze WMAP 3 year data using the one-point distribution functions to probe the non-Gaussianity in the Cosmic Microwave Background (CMB) Anisotropy data. Computer simulations are performed to determine the uncertainties of the results. We report the non-Gaussianity parameter f_NL is constrained to 26<f_NL<82 for Q-band, 12<f_NL<67 for V-band, 7<f_NL<64 for W-band and 23<f_NL<75 for Q+V+W combined data at 95% confidence level (CL).
We investigate the possible occurrence of a positive cosmic acceleration in a spatially averaged, expanding, unbound Lemaitre-Tolman-Bondi cosmology. By studying an approximation in which the contribution of three-curvature dominates over the matter density, we construct numerical models which exhibit acceleration.
It is argued that the Brans-Dicke theory may explain the present accelerated expansion of the universe without resorting to a cosmological constant or quintessence matter
We investigate in detail the possibility of constraining neutrino decays with data from the cosmic microwave background radiation (CMBR). Two generic decays are considered ν_H -> ν_L ϕand ν_H -> ν_L ν_L_bar ν_L. We have solved the momentum dependent Boltzmann equation in order to account for possible relativistic decays. Doing this we estimate that any neutrino with mass m > 1 eV decaying before the present should be detectable with future CMBR data. Combining this result with other results on stable neutrinos, any neutrino mass of the order 1 eV should be detectable.
Cosmic ray acceleration in turbulent interstellar medium is considered. Turbulence is treated as ensemble of moving magnetic traps. We derive equations for particle momentum distribution function that describes acceleration of particles in this case. Rate of acceleration calculated is estimated for our Galaxy and compared with ones given by other acceleration mechanisms.
Current observations of Type Ia supernovae provide evidence for cosmic acceleration out to a redshift of $z \lsim 1$, leading to the possibility that the universe is entering an inflationary epoch. However, inflation can take place only if vacuum-energy (or other sufficiently slowly redshifting source of energy density) dominates the energy density of a region of physical radius 1/H. We argue that for the best-fit values of $Ω_Λ$ and $Ω_m$ inferred from the supernovae data, one must confirm cosmic acceleration out to at least $z \simeq 1.8$ to infer that the universe is inflating.
A recently proposed mechanism of acceleration of highest energy cosmic rays by polarization electric fields arising in plasmoids injected into neutron star magnetospheres is discussed.
I give a synopsis of two aspects of the Galactic Cosmic Ray (GCR) acceleration problem: the importance of the medium energy gamma-ray window, and several specific astrophysical sources which merit further investigation.
In this paper it has been shown that the neutrino bulk viscous stresses can give rise to the late time acceleration of the universe. It is found that a number of spatially flat FRW models with a negative deceleration parameter can be constructed using neutrino viscosity and one of them mimics a $Λ$CDM model. This does not require any exotic dark energy component or any modification of gravity.
We calculate event rate and demonstrate the observational feasibility of very high energy muons (1-1000 TeV) in a large mass underground detector operating as a pair-meter. This energy range corresponds to surface muon energies of $\sim$(5 TeV - 5000 TeV) and primary cosmic ray energies of $\sim$ (50 TeV - 5 $\times 10^4$ TeV). Such measurements would significantly assist in an improved understanding of the prompt contribution to $ν_e, ν_μ$ and $μ$ fluxes in present and future ultra-high energy neutrino detectors. In addition, they would shed light on the origin of the observed 'knee' in the c…
It is shown here that a dynamical Planck mass can drive the scale factor of the universe to accelerate. The negative pressure which drives the cosmic acceleration is identified with the unusual kinetic energy density of the Planck field. No potential nor cosmological constant is required. This suggests a purely gravity driven, kinetic inflation. Although the possibility is not ruled out, the burst of acceleration is often too weak to address the initial condition problems of cosmology. To illustrate the kinetic acceleration, three different cosmologies are presented. One such example, that of …
In cosmic holography, the fundamental quantity is the degrees of freedom on a horizon surface rather than the material contents within the volume. That is, the horizon area and hence cosmological expansion rate H is related to the entropy. Using this as a guide, we examine the consequences of an effective dark {\it entropy} on the past and future dynamics of the universe. The results, including nonmonotonic expansion behavior, are interesting apart from the theoretical motivation.
The energy spectrum of ultra-high energy cosmic rays (UHECR) is usually calculated for sources with identical properties. Assuming that all sources can accelerate UHECR protons to the same extremely high maximal energy E_max > 10^{20} eV and have the steeply falling injection spectrum 1/E^{2.7}, one can reproduce the measured cosmic ray flux above E > 10^{18} eV. We show that relaxing the assumption of identical sources and using a power-law distribution of their maximal energy allows one to explain the observed UHECR spectrum with the injection 1/E^2 predicted by Fermi shock acceleration.
Accelerated expansion of the Universe may result from an anti-frictional force that is self-consistently exerted on cold dark matter (CDM). Cosmic anti-friction is shown to give rise to an effective negative pressure of the cosmic medium. While other models introduce a component of dark energy besides ``standard'' CDM, we resort to a phenomenological one-component model of CDM with internal self-interactions. We demonstrate how the dynamics of the LambdaCDM model may be recovered as a special case of cosmic anti-friction.
This paper summarizes highlights of the OG1 session of the 30th International Cosmic Ray Conference, held in Merida (Yucatan, Mexico). The subsessions (OG1.1, OG1.2, OG1.3, OG1.4 and OG1.5) summarized here were mainly devoted to direct measurements, acceleration and propagation of cosmic rays.
We explore scenarios where the highest energy cosmic rays are produced by new particle physics near the grand unification scale. Using detailed numerical simulations of extragalactic nucleon, gamma-ray, and neutrino propagation, we show the existence of an interesting parameter range for which such scenarios may explain part of the data and are consistent with all observational constraints. A combination of proposed observatories for ultra-high energy cosmic rays, neutrino telescopes of a few kilometer scale, and gamma-ray astrophysics instruments should be able to test these scenarios. In par…
Theory of the first-order Fermi acceleration of cosmic ray particles at relativistic shock waves is reviewed. We consider shocks with parallel and oblique, sub- and super-luminal magnetic field configurations and with finite-amplitude magnetic field perturbations. A role of oblique field configurations and field perturbations in forming the cosmic ray energy spectrum and changing the acceleration time scale is discussed.
Theoretical concepts ofn cosmic ray particle acceleration at relativistic plasma flows -- shocks and shear layers -- are reviewed. We begin with a discussion of mildly relativistic shock waves. The role of oblique field configurations and field perturbations in forming the particle energy spectrum and changing the acceleration time scale is considered. Then, we report on two interesting attempts to consider particle acceleration at ultra-relativistic shocks. Finally, in contrast to the compressive shock discontinuities, we discuss the acceleration processes acting in the boundary layer at the …
Using the Monte Carlo simulations we apply a method of discrete small amplitude particle momentum scattering to reproduce highly anisotropic conditions at relativistic shocks. We discusse acceleration times scales in relativistic shocks. Next, we describe newly found acceleration mechanism at ultrarelativistic shocks. The universal spectral index and the acceleration time for the shocks are given. In the end reflections from ultrarelativistic shocks are considered.
This paper summarizes highlights of the OG3.1, 3.2 and 3.3 sessions of the XXVIth International Cosmic Ray Conference in Salt Lake City, which were devoted to issues of origin/composition, acceleration and propagation.
We assess the accuracy with which Omega and Lambda can be measured by combining various types of upcoming experiments. Useful expressions for the Fisher information matrix are derived for classical cosmological tests involving luminosity (eg, SN Ia), angular size, age and number counts. These geometric probes are found to be quite complementary both to each other and to inferences from cluster abundance and the cosmic microwave background (CMB). For instance, a joint analysis of SN Ia and CMB reduces the error bars by about an order of magnitude compared to a separate analysis of either data s…
SNAP is a candidate for the Joint Dark Energy Mission (JDEM) that seeks to place constraints on the dark energy using two distinct methods. The first, Type Ia SN, is discussed in a companion white paper. The second method is weak gravitational lensing, which relies on the coherent distortions in the shapes of background galaxies by foreground mass structures. The excellent spatial resolution and photometric accuracy afforded by a 2-meter space-based observatory are crucial for achieving the high surface density of resolved galaxies, the tight control of systematic errors in the telescope's Poi…
We present the model of cosmic rays acceleration at ultrarelativistic subshocks and confront it with the observations of gamma-ray bursts and blazars. We investigate cosmic rays acceleration in shocks with Lorentz factors in the range 3 - 40. We show that fluctuations of the magnetic field downstream of the shock do not play an important role in the acceleration process. Results of numerical simulations for shocks with considered Lorentz factors and perpendicular magnetic field inclinations are presented. We fit the derived particle energy spectral index dependence on fluctuations of the magne…
It is shown how laboratory experiments performed with high intensity femtosecond lasers can probe the physics of black holes in the near-horizon regime. The acceleration generated by the high intensity laser ranging from $10^{13}$g to more than $10^{18}$g is identified with the gravitational acceleration at stretched horizons. In the black-hole's asymptotic region, the stretched-horizon-reflected light shows a measurable universal phase acceleration of $c^4/4GM$. PACS numbers: 04.70.Bw, 97.60.Lf, 52.40.Nk, 52.50.Jm
In this lecture I outline some of the underlying physics issues associated with accelerators plausibly capable of explaining the UHECRs up to ZeV energies. I concentrate on the concentrate on mechanisms and their constraints, but provide a brief background on on observations and the constraints they supply, as well.
An overview is given on the present status of the understanding of the origin of galactic cosmic rays. Recent measurements of charged cosmic rays and photons are reviewed. Their impact on the contemporary knowledge about the sources and acceleration mechanisms of cosmic rays and their propagation through the Galaxy is discussed. Possible reasons for the knee in the energy spectrum and scenarios for the end of the galactic cosmic-ray component are described.
Cosmic-rays are ubiquitous, but their origins are surprisingly difficult to understand. A review is presented of some of the basic issues common to cosmic particle accelerators and arguments leading to the likely importance of diffusive shock acceleration as a general explanation. The basic theory of diffusive shock acceleration is outlined, followed by a discussion of some of the key issues that still prevent us from a full understanding of its outcomes. Some recent insights are mentioned at the end that may help direct ultimate resolution of our uncertainties.
Recent discovery of the year-scale variability in the synchrotron X-ray emission of the supernova remnant (SNR) RX J1713.7-3946 has initiated our study of multi-epoch X-ray images and spectra of the young SNR Cassiopeia A based on the Chandra archive data taken in 2000, 2002, and 2004. We have found year-scale time variations in the X-ray intensity for a number of X-ray filaments or knots associated with the reverse-shocked regions. The X-ray spectra of the variable filaments are characterized by a featureless continuum, and described by a power law with a photon index within 1.9-2.3. The uppe…
We derive the maximum accelerated particle energy from first-order and second-order Fermi acceleration at nonrelativistic and relativistic shocks for explosions taking place in a uniform surrounding medium. Second-order stochastic processes in relativistic flows are shown to be capable of accelerating cosmic rays to ultra-high energies. Cosmic rays above the knee of the cosmic ray spectrum can be accelerated by the second-order Fermi mechanism in relativistic flows, such as those occuring in gamma-ray bursts and unusual supernovae like SN 1998bw.
The study of Galactic diffuse $γ$ radiation combined with the knowledge of the distribution of the molecular hydrogen in the Galaxy offers a unique tool to probe the cosmic ray flux in the Galaxy. A methodology to study the level of the cosmic ray "sea" and to unveil target-accelerator systems in the Galaxy, which makes use of the data from the high resolution survey of the Galactic molecular clouds performed with the NANTEN telescope and of the data from gamma-ray instruments, has been developed. Some predictions concerning the level of the cosmic ray "sea" and the $γ$-ray emission close to c…
A mechanism of ultra-high energy (UHE) cosmic ray acceleration in extragalactic radio sources at the interface between the relativistic jet and the ambient medium is discussed as a supplement to the shock acceleration in `hot spots'. Particles accelerated at the jet side boundary are expected to dominate at highest energies. The spectrum formation near the cut-off energy is modeled using the Monte Carlo particle simulations.
This report covers the studies of energetic particle propagation and acceleration presented during the 27th ICRC in sessions OG 1.3 and 1.4, plus a few selected papers from neighbouring OG sections. We list problems discussed within these sections and we shortly describe selected interesting achievements and/or novel approaches.
Young supernova remnants are thought to be the sites where cosmic ray acceleration occurs by the mechanism of diffusive shock acceleration. The maximum energy gained in this process is conventionally extimated to have a value close to, but distinctly below, the ``knee'' ($\sim 10^{15}$ eV) of the cosmic-ray spectrum. Bell & Lucek (2001) have suggested that the generated cosmic rays simultaneously amplify the magnetic field around the supernova remnant shock to many times its pre-shock value. In this case the acceleration rate may be significantly increased and protons can easily reach energies…
Essentially all of our knowledge of the acceleration history of the Universe - including the acceleration itself - is predicated upon the validity of general relativity. Without recourse to this assumption, we use SNeIa to analyze the expansion history and find (i) very strong (5 sigma) evidence for a period of acceleration, (ii) strong evidence that the acceleration has not been constant, (iii) evidence for an earlier period of deceleration and (iv) only weak evidence that the Universe has not been decelerating since z~0.3.
The paper describes the goals and first results of the ROSAT ESO Flux-Limited X-ray (REFLEX) Cluster Survey, a redshift survey of the X-ray brightest galaxy clusters in the ROSAT All-Sky Survey in the southern hemisphere conducted in the frame of an ESO key programme.
Our purpose is to evaluate the rate of the maximum energy and the acceleration rate that cosmic rays acquire in the non-relativistic diffusive shock acceleration as it could apply during their lifetime in various astrophysical sites, where highly oblique shocks exist. We examine numerically (using Monte Carlo simulations) the effect of the diffusion coefficients on the energy gain and the acceleration rate, by testing the role between the obliquity of the magnetic field to the shock normal, and the significance of both perpendicular cross-field diffusion and parallel diffusion coefficients to …
In this paper we present the fundamentals of the cosmological special relativity (CSR) by discussing the dynamical concepts of velocity, acceleration and cosmic distances in spacevelocity. These concepts occur in CSR just as those of mass, linear momentum and energy appear in Einstein's special relativity (ESR) in spacetime.
We review the evidence for cosmic ray acceleration in the superbubble/hot phase of the interstellar medium, and discuss the implications for the composition of cosmic rays and the structure and evolution of the interstellar medium. We show that the bulk of the galactic supernovae, their expanding remnants, together with their metal-rich grain and gas ejecta, and their cosmic ray accelerating shocks, are all confined within the interiors of superbubbles, generated by the multiple supernova explosions of massive stars formed in giant OB associations. This superbubble/hot phase of the ISM provide…
Conditions for accelerated expansion of Friedmann-Robertson-Walker space-time are analyzed. Connection of this scenario with present-day observations are reviewed. It is explained how a scalar field could be responsible for cosmic acceleration observed in present times and predicted for the very early Universe. Ideas aimed at answering whether is that the actual case for our Universe are described.
An overview is given of multiwavelength observations of young supernova remnants, with a focus on the observational signatures of efficient cosmic ray acceleration. Some of the effects that may be attributed to efficient cosmic ray acceleration are the radial magnetic fields in young supernova remnants, magnetic field amplification as determined with X-ray imaging spectroscopy, evidence for large post-shock compression factors, and low plasma temperatures, as measured with high resolution optical/UV/X-ray spectroscopy. Special emphasis is given to spectroscopy of post-shock plasma's, which off…
Magnetowave induced plasma wakefield acceleration (MPWA) in a relativistic astrophysical outflow has been proposed as a viable mechanism for the acceleration of cosmic particles to ultra high energies. Here we present simulation results that clearly demonstrate the viability of this mechanism for the first time. We invoke the high frequency and high speed whistler mode for the driving pulse. The plasma wakefield so induced validates precisely the theoretical prediction. We show that under appropriate conditions, the plasma wakefield maintains very high coherence and can sustain high-gradient a…
With a model independent method the expansion history $H(z)$, the deceleration parameter $q(z)$ of the universe and the equation of state $w(z)$ for the dark energy are reconstructed directly from the 192 Sne Ia data points, which contain the new ESSENCE Sne Ia data and the high redshift Sne Ia data. We find that the evolving properties of $q(z)$ and $w(z)$ reconstructed from the 192 Sne Ia data seem to be weaker than that obtained from the Gold set, but stronger than that from the SNLS set. With a combination of the 192 Sne Ia and BAO data, a tight constraint on $Ω_{m0}$ is obtained. At the $…
