Quantum vacuum and virtual gravitational dipoles: the solution to the dark energy problem?

The cosmological constant problem is the principal obstacle in the attempt to interpret dark energy as the quantum vacuum energy. We suggest that the obstacle can be removed, i.e. that the cosmological constant problem can be resolved by assuming that the virtual particles and antiparticles in the quantum vacuum have the gravitational charge of the opposite sign. The corresponding estimates of the cosmological constant, dark energy density and the equation of state for dark energy are in the intriguing agreement with the observed values in the present day Universe. However, our approach and the Standard Cosmology lead to very different predictions for the future of the Universe; the exponential growth of the scale factor, predicted by the Standard Cosmology, is suppressed in our model.     

Is dark matter an illusion created by the gravitational polarization of the quantum vacuum?

Assuming that a particle and its antiparticle have the gravitational charge of the opposite sign, the physical vacuum may be considered as a fluid of virtual gravitational dipoles. Following this hypothesis, we present the first indications that dark matter may not exist and that the phenomena for which it was invoked might be explained by the gravitational polarization of the quantum vacuum by the known baryonic matter.     

A few provoking relations between dark energy, dark matter, and pions

I present three relations, striking in their simplicity and fundamental appearance. The first one connects the Compton wavelength of a pion and the dark energy density of the Universe; the second one connects the Compton wavelength of a pion and the mass distribution of non-baryonic dark matter in a galaxy; the third one relates the mass of a pion to fundamental physical constants and cosmological parameters. All these relations are in excellent numerical agreement with observations.     

Non-minimal coupling of the phantom field and cosmic acceleration

Motivated by the recent interest in phantom fields as candidates for the dark energy component, we investigate the consequences of the phantom field when is minimally coupled to gravity. In particular, the necessary (but insufficient) conditions for the acceleration and superacceleration of the universe are obtained when the non-minimal coupling term is taken into account. Furthermore, the necessary condition for the cosmic acceleration is derived when the phantom field is non-minimally coupled to gravity and baryonic matter is included.     

Possible acceleration of cosmic rays in a system of rotating gases: Uehling–Uhlenbeck model

Possible acceleration of cosmic rays passing through a kind of amplification channel (via anomalous diffusion modes of propagating plane-wave fronts) induced by a system of rotating gases (or disk-like gases) is presented. Our novel numerical results after detailed analysis were based on the quantum discrete kinetic model (considering Uehling–Uhlenbeck collision term) which has been used to study the propagation of plane (e.g., acoustic) waves propagating in composite-particle gases under uniform gravitational fields.     

Are fossil groups a challenge of the cold dark matter paradigm?

We study six groups and clusters of galaxies suggested in the literature to be 'fossil' systems (i.e. to have luminous diffuse X-ray emission and a magnitude gap of at least 2 mag R between the first and the second ranked member within half of the virial radius), each having good quality X-ray data and Sloan Digital Sky Survey (SDSS) spectroscopic or photometric coverage out to the virial radius. The poor cluster AWM 4 is clearly established as a fossil system, and we confirm the fossil nature of four other systems (RX J1331.5+1108, RX J1340.6+4018, RX J1256.0+2556 and RX J1416.4+2315), while the cluster RX J1552.2+2013 is disqualified as fossil system. For all systems, we present the luminosity functions within 0.5 and 1 virial radius that are consistent, within the uncertainties, with the universal luminosity function of clusters. For the five bona fide fossil systems, having a mass range  2 × 10¹³–3 × 10¹⁴ M_⊙  , we compute accurate cumulative substructure distribution functions (CSDFs) and compare them with the CSDFs of observed and simulated groups/clusters available in the literature. We demonstrate that the CSDFs of fossil systems are consistent with those of normal observed clusters and do not lack any substructure with respect to simulated galaxy systems in the cosmological Λ cold dark matter (ΛCDM) framework. In particular, this holds for the archetype fossil group RX J1340.6+4018 as well, contrary to earlier claims.     

Holographic dark energy with time depend gravitational constant in the non-flat Ho?ava-Lifshitz cosmology

We study the holographic dark energy on the subject of Hořava-Lifshitz gravity with a time dependent gravitational constant G(t), in the non-flat space-time. We obtain the differential equation that specify the evolution of the dark energy density parameter based on varying gravitational constant. We find out a relation for the state parameter of the dark energy equation of state to low red-shifts which containing varying G corrections in the non-flat space-time.     

On the various aspects of interacting Ricci dark energy and?tachyonic field

In this letter we considered an interaction between Ricci dark energy and tachyonic field. We investigated the equation of state parameters in presence of the interaction and revealed a quintessence like behavior. Also, we investigated the validity of the generalized second law of thermodynamics in presence of this interaction and observed that it is broken down under this interaction.     

Are gamma-ray bursts the sources of ultra-high energy cosmic rays?

We reconsider the possibility that gamma-ray bursts (GRBs) are the sources of the ultra-high energy cosmic rays (UHECRs) within the internal shock model, assuming a pure proton composition of the UHECRs. For the first time, we combine the information from gamma-rays, cosmic rays, prompt neutrinos, and cosmogenic neutrinos quantitatively in a joint cosmic ray production and propagation model, and we show that the information on the cosmic energy budget can be obtained as a consequence. In addition to the neutron model, we consider alternative scenarios for the cosmic ray escape from the GRBs, i.e., that cosmic rays can leak from the sources. We find that the dip model, which describes the ankle in UHECR observations by the pair production dip, is strongly disfavored in combination with the internal shock model because (a) unrealistically high baryonic loadings (energy in protons versus energy in electrons/gamma-rays) are needed for the individual GRBs and (b) the prompt neutrino flux easily overshoots the corresponding neutrino bound. On the other hand, GRBs may account for the UHECRs in the ankle transition model if cosmic rays leak out from the source at the highest energies. In that case, we demonstrate that future neutrino observations can efficiently test most of the parameter space – unless the baryonic loading is much larger than previously anticipated.     

The sizes of minivoids in the local Universe: an argument in favour of a warm dark matter model?

Using high-resolution simulations within the cold dark matter (CDM) and warm dark matter (WDM) models, we study the evolution of small-scale structure in the local volume, a sphere of 8-Mpc radius around the Local Group. We compare the observed spectrum of minivoids in the local volume with the spectrum of minivoids determined from the simulations. We show that the ΛWDM model can easily explain both the observed spectrum of minivoids and the presence of low-mass galaxies observed in the local volume, provided that all haloes with circular velocities greater than 20 km s⁻¹ host galaxies. On the contrary, within the ΛCDM model the distribution of the simulated minivoids reflects the observed one if haloes with maximal circular velocities larger than  35 km s⁻¹  host galaxies. This assumption is in contradiction with observations of galaxies with circular velocities as low as 20 km s⁻¹ in our local Universe. A potential problem of the ΛWDM model could be the late formation of the haloes in which the gas can be efficiently photoevaporated. Thus, star formation is suppressed and low-mass haloes might not host any galaxy at all.     

The direct detection of non-baryonic dark matter in the Galaxy?

It has been argued in a number of recent papers that dark matter is in the form of Jupiter-mass primordial black holes that betray their presence by microlensing quasars. This lensing accounts for a number of characteristic properties of quasar light curves, in both single quasars and gravitationally lensed multiple systems, that are not explained on the basis of intrinsic variation. One prediction of this idea is that Jupiter-mass bodies will be detected by the MACHO experiment as short events of about 2 d duration, although the expected frequency of detection is still very hard to estimate. However, the recent report by the MACHO group of the detection of a Jupiter-mass body in the direction of the Galactic bulge is consistent with this prediction, and is possibly the first direct detection of non-baryonic matter in the Galaxy.     

Supersonic steady accretion of a rotating cold “Iron Gas” onto a neutron star after gravitational collapse

We analytically generalize the well-known solution of steady supersonic spherically symmetric gas accretion onto a star (Bondi 1952) for an iron atmosphere with completely degenerate electrons with an arbitrary degree of relativity. This solution is used for typical physical conditions in the vicinity of protoneutron stars produced by gravitational collapse with masses M ₀=(1.4?1.8)M _⊙ and over a wide range of nonzero “iron gas” densities at infinity, ρ_∞=(10⁴?5×10⁶)g cm^?3. Under these conditions, we determine all accretion parameters, including the accretion rate, whose value is ～(10?50)M _⊙s^?1 at M ₀=1.8M _⊙ (it is a factor of 1.7 lower for M ₀=1.4M _⊙, because the accretion rate is exactly ∝M ₀ ² ). We take into account the effect of accreting-gas rotation in a quasi-one-dimensional approximation, which has generally proved to be marginal with respect to the accretion rate.     

The gravitational collapse of iron-oxygen stars with masses of 2M ⊙ and 10M ⊙ II

The collapse of iron-oxygen stars with masses of 2M has been calculated. The commencement of the collapse is due to dissociation of iron-group nuclei into free nucleons. After a while, the collapse proceeds in consequence of intensive energy losses due to neutrino volume radiation. At an intermediate stage of the collapse, the core — opaque with respect to neutrino radiation (neutrino core) — is formed inside the collapsing star. Both the gradual increase of the mass of the neutrino core and the partial absorption of neutrinos radiated from the surface of the neutrino core by the stellar envelope (deposition) were taken into account in our calculations. The kinetics of oxygen burning in the outer layers of the envelope was also allowed for. Neither the deposition, nor the oxygen burning, result in ejection of stellar envelopes.     

LRS Bianchi type-II dark energy model in a scalar–tensor theory of gravitation

A locally rotationally symmetric Bianchi type-II (LRS B-II) space-time with variable equation of state (EoS) parameter and constant deceleration parameter have been investigated in the scalar-tensor theory proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). The scalar-tensor field equations have been solved by applying variation law for generalized Hubble’s parameter given by Bermann (Nuovo Cimento 74:182, 1983). The physical and kinematical properties of the model are also discussed.