arXiv subjects

Reuven Opher

Publications and source records attributed to Reuven Opher.

At least 19 recordsLinked to original sources

arXiv2013

Challenges of Relativistic Astrophysics

I discuss some of the most outstanding challenges in relativistic astrophysics in the subjects of: compact objects (Black Holes and Neutron Stars); dark sector (Dark Matter and Dark Energy); plasma astrophysics (Origin of Jets, Cosmic Rays and Magnetic Fields) and the primordial universe (Physics at the beginning of the Universe). In these four subjects, I discuss twelve of the most important challenges. These challenges give us insight into new physics that can only be studied in the large scale Universe. The near future possibilities, in observations and theory, for addressing these challenges, are also discussed.

astro-ph.CO↗

arXiv2011

Origin of Intense Magnetic Fields Near Black Holes Due to Non-Minimal Gravitational-Electromagnetic Coupling

The origin of magnetic fields in astrophysical objects is a challenging problem in astrophysics. Throughout the years, many scientists have suggested that non-minimal gravitational-electromagnetic coupling (NMGEC) could be the origin of the ubiquitous astrophysical magnetic fields. We investigate the possible origin of intense magnetic fields $\sim 10^{15}-10^{16}$ by NMGEC near rotating neutron stars and black holes, connected with magnetars, quasars, and gamma-ray bursts. Whereas these intense magnetic fields are difficult to explain astrophysically, we find that they are easily explained by NMGEC.

astro-ph↗

arXiv2011

The Effect of a Single Supernova Explosion on the Cuspy Density Profile of a Small-Mass Dark Matter Halo

Some observations of galaxies, and in particular dwarf galaxies, indicate a presence of cored density profiles in apparent contradiction with cusp profiles predicted by dark matter N-body simulations. We constructed an analytical model, using particle distribution functions (DFs), to show how a supernova (SN) explosion can transform a cusp density profile in a small-mass dark matter halo into a cored one. Considering the fact that a SN efficiently removes matter from the centre of the first haloes, we study the effect of mass removal through a SN perturbation in the DFs. We found that the transformation from a cusp into a cored profile is present even for changes as small as 0.5% of the total energy of the halo, that can be produced by the expulsion of matter caused by a single SN explosion.

astro-ph.CO↗

arXiv2010

Random Primordial Magnetic Fields and the Gas Content of Dark Matter Halos

We recently predicted the existence of random primordial magnetic fields (RPMF) in the form of randomly oriented cells with dipole-like structure with a cell size $L_0$ and an average magnetic field $B_0$. Here we investigate models for primordial magnetic field with a similar web-like structure, and other geometries, differing perhaps in $L_0$ and $B_0$. The effect of RPMF on the formation of the first galaxies is investigated. The filtering mass, $M_F$, is the halo mass below which baryon accretion is severely depressed. We show that these RPMF could influence the formation of galaxies by altering the filtering mass and the baryon gas fraction of a halo, $f_g$. The effect is particularly strong in small galaxies. We find, for example, for a comoving $B_0=0.1\muG$, and a reionization epoch that starts at $z_s=11$ and ends at $z_e=8$, for $L_0=100\,\text{pc}$ at $z=12$, the $f_g$ becomes severely depressed for $M<10^7\msun$, whereas for $B_0=0$ the $f_g$ becomes severely depressed only for much smaller masses, $M<10^5\msun$. We suggest that the observation of $M_F$ and $f_g$ at high redshifts can give information on the intensity and structure of primordial magnetic fields.

astro-ph.CO↗

arXiv2010

A large change in the predicted number of small halos due to a small amplitude oscillating inflaton potential

A smooth inflaton potential is generally assumed when calculating the primordial power spectrum, implicitly assuming that a very small oscillation in the inflaton potential creates a negligible change in the predicted halo mass function. We show that this is not true. We find that a small oscillating perturbation in the inflaton potential in the slow-roll regime can alter significantly the predicted number of small halos. A class of models derived from supergravity theories gives rise to inflaton potentials with a large number of steps and many transplanckian effects may generate oscillations in the primordial power spectrum. The potentials we study are the simple quadratic (chaotic inflation) potential with superimposed small oscillations for small field values. Without leaving the slow-roll regime, we find that for a wide choice of parameters, the predicted number of halos change appreciably. For the oscillations beginning in the 10^7-10^8 solar masses range, for example, we find that only a 5% change in the amplitude of the chaotic potential causes a 50% suppression of the number of halos for masses between 10^7-10^8 solar masses and an increase in the number of halos for masses <10^6 solar masses by factors ~15-50. We suggest that this might be a solution to the problem of the lack of observed dwarf galaxies in the range 10^7-10^8 solar masses. This might also be a solution to the reionization problem where a very large number of Population III stars in low mass halos are required.

astro-ph.CO↗

arXiv2010

Are the magnetic fields of millisecond pulsars ~ 10^8 G?

It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are $\sim 10^{8}$G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The $\sim 10^{8}$ G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have $< 10^{11}$ G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit $10^{11}$ G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict $\sim 10^{8}$ G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted $\sim 10^{8}$ G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields $\gg 10^{8}$ G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields $> 10^{8}$ G with their magnetic dipole emission being balanced by low level accretion from their companions.

astro-ph.SR↗

arXiv2010

Suppression of small baryonic structures due to a primordial magnetic field

We investigate the impact of the existence of a primordial magnetic field on the filter mass, characterizing the minimum baryonic mass that can form in dark matter (DM) haloes. For masses below the filter mass, the baryon content of DM haloes are severely depressed. The filter mass is the mass when the baryon to DM mass ratio in a halo is equal to half the baryon to DM ratio of the Universe. The filter mass has previously been used in semianalytic calculations of galaxy formation, without taking into account the possible existence of a primordial magnetic field. We examine here its effect on the filter mass. For homogeneous comoving primordial magnetic fields of $B_0 \sim 1$ or 2 nG and a reionization epoch that starts at a redshift $z_s=11$ and is completed at $z_r=8$, the filter mass is increased at redshift 8, for example, by factors 4.1 and 19.8, respectively. The dependence of the filter mass on the parameters describing the reionization epoch is investigated. Our results are particularly important for the formation of low mass galaxies in the presence of a homogeneous primordial magnetic field. For example, for $B_0\sim 1\nG$ and a reionization epoch of $z_s\sim 11$ and $z_r\sim7$, our results indicate that galaxies of total mass $M\sim5 \times 10^8\msun$ need to form at redshifts $z_F\gtrsim 2.0$, and galaxies of total mass $M\sim10^8\msun$ at redshifts $z_F\gtrsim 7.7$.

astro-ph.CO↗

arXiv2009

Effective growth of matter density fluctuations in the running LCDM and LXCDM models

We investigate the matter density fluctuations δρ/ρfor two dark energy (DE) models in the literature in which the cosmological term Λis a running parameter. In the first model, the running LCDM model, matter and DE exchange energy, whereas in the second model, the LXCDM model, the total DE and matter components are conserved separately. The LXCDM model was proposed as an interesting solution to the cosmic coincidence problem. It includes an extra dynamical component, the "cosmon" X, which interacts with the running Λ, but not with matter. In our analysis we make use of the current value of the linear bias parameter, b^2(0)= P_{GG}/P_{MM}, where P_{MM} ~ (δρ/ρ)^2 is the present matter power spectrum and P_{GG} is the galaxy fluctuation power spectrum. The former can be computed within a given model, and the latter is found from the observed LSS data (at small z) obtained by the 2dF galaxy redshift survey. It is found that b^2(0)=1 within a 10% accuracy for the standard LCDM model. Adopting this limit for any DE model and using a method based on the effective equation of state for the DE, we can set a limit on the growth of matter density perturbations for the running LCDM model, the solution of which is known. This provides a good test of the procedure, which we then apply to the LXCDM model in order to determine the physical region of parameter space, compatible with the LSS data. In this region, the LXCDM model is consistent with known observations and provides at the same time a viable solution to the cosmic coincidence problem.

gr-qc↗

arXiv2008

The Origin of Primordial Magnetic Fields

(abridged) We suggest here that the large scale fields $\sim μ$G, observed in galaxies at both high and low redshifts by Faraday Rotation Measurements (FRMs), have their origin in the electromagnetic fluctuations that naturally occurred in the dense hot plasma that existed just after the QHPT. We evolve the predicted fields to the present time. The size of the region containing a coherent magnetic field increased due to the fusion (polymerization) of smaller regions. Magnetic fields (MFs) $\sim 10 μG$ over a comoving $\sim 1$ pc region are predicted at redshift z $\sim 10$. These fields are orders of magnitude greater than those predicted in previous scenarios for creating primordial magnetic fields. Line-of-sight average magnetic fields (MFs) $\sim$ $10^{-2}$ $μ$G, valid for FRMs, are obtained over a 1 Mpc comoving region at the redshift z $\sim$ 10. In the collapse to a galaxy (comoving size $\sim$ 30 kpc) at z $\sim$ 10, the fields are amplified to $\sim 10 μ$G. This indicates that the MFs created immediately after the QHPT, predicted by the Fluctuation-Dissipation Theorem, could be the origin of the $\sim μG$ fields observed by FRMs in galaxies at both high and low redshifts.

astro-ph↗

arXiv2006

Inflation without inflatons

(abridged)We present a model which predicts inflation without the presence of inflaton fields, based on the εR^2 and Starobinsky models. It links the above models to the observable universe, in particular, to the ratio r of tensor to scalar fluctuations. In our model, we assume the existence of particles with the mass M that have a long decay time. These particles which were gravitationally produced \sim 60e-folds before the end of inflation produced the nearly scale invariant scalar density fluctuations which are observed. Gravitational waves (tensor fluctuations) were also produced at this epoch. The ratio of tensor to scalar fluctuations r (which are to be measured in the near future to good accuracy) determines M, which together with H_0, determine the time at the end of inflation, t_end. At t_end, the Hubble parameter begins to oscillate rapidly, gravitationally producing the bulk of the M particles, which we identify with the matter content of the universe today. The time required for the universe to dissipate its vacuum energy into M particles is found to be t_dis \simeq 6M_Pl^2/M^3. We assume that the time t_RH, (called the reheating time) needed for the M particles to decay into relativistic particles, is very much greater than that necessary to create the M particles, t_dis. From the ratio f\equiv t_dis/t_RH and g_\ast (the total number of degrees of freedom of the relativistic particles) we can, then, evaluate the maximum temperature of the universe, T_max, and the reheat temperature, T_RH, at t_RH. Our model, thus, predicts M, t_dis, t_end, T_max, T_RH, t_max, and t_RH as a function of r, f, and g_\ast (and to a weaker extent the particle content of the vacuum near the Planck epoch).

astro-ph↗

arXiv2006

A new inflaton model beginning near the Planck epoch

The Starobinsky model predicts a primordial inflation period without the presence of an inflaton field. The modified version of this model predicts a simple time dependence for the Hubble parameter $H(t)$, which decreases slowly between the Planck epoch and the end of the inflation, $H(t)=M_{\rm Pl}-βM_{\rm Pl}^2 t$, where $β$ is a dimensionless constant to be adjusted from observations. We investigate an inflaton model which has the same time dependence for $H(t)$. A reverse engineered inflaton potential for the time dependence of $H$ is derived. Normalization of the derived inflaton potential is determined by the condition that the observed density fluctuations, $δρ/ρ\approx 10^{-5}$, are created at $\sim 60 e$-folds before the end of inflation. The derived potential indicates an energy (mass) scale, $M_{\rm end}\sim 10^{13} {\rm GeV}$, at the end of inflation. Using the slow roll parameters, which are obtained from this potential, we calculate the spectral index for the scalar modes $n_S$ and the relative amplitude of the tensor to scalar modes $r$. A tensor contribution, $r\simeq 0.13$, and an approximately Harrison-Zeldovich density perturbation spectrum, $n_S \simeq 0.95$, are predicted.

astro-ph↗

arXiv2005

Strong limits on the possible decay of the vacuum energy into CDM or CMB photons

We investigate models that suggest that the vacuum energy decays into cold dark matter (CDM) and into a homogeneous distribution of a thermalized cosmic microwave background (CMB), which is characteristic of an adiabatic vacuum energy decay into photons. We show that the density fluctuation spectrum obtained from the CMB and galaxy distribution data agreement put strong limits on the rate of decay of the vacuum energy. A vacuum energy decaying into CDM increases its total density ρ, diluting (δρ/ρ)^2. The observed temperature fluctuations of the CMB photons (δT/T)^2 are approximately proportional to CDM density fluctuations (δρ/ρ)^2. In both case, when evaluating (δρ/ρ)^2 at the recombination era, its present measured value must be increased by a factor F. Since the (δρ/ρ)^2 derived from the CMB and galaxy distribution data agree to \sim 10%, the maximum value for F is F_{\rm max}\aprox 1.1. Our results indicate that the rate of the decay of the vacuum energy into CDM and CMB photons are extremely small.

astro-ph↗

arXiv2005

What is the SUSY mass scale?

The energy, or mass scale M_SUSY, of the supersymmetry (SUSY) phase transition is, as yet, unknown. If it is very high (i.e., ~ 10^3 GeV), terrestrial accelerators will not be able to measure it. We determine M_SUSY here by combining theory with the cosmic microwave background (CMB) data. Starobinsky suggested an inflationary cosmological scenario in which inflation is driven by quantum corrections to the vacuum Einstein's equation. The modified Starobinsky model (MSM) is a natural extension of this. In the MSM, the quantum corrections are the quantum fluctuations of the supersymetric (SUSY) particles, whose particle content creates inflation and whose masses terminate it. Since the MSM is difficult to solve until the end of the inflation period, we assume here that an effective inflaton potential (EIP) that reproduces the time dependence of the cosmological scale factor of the MSM can be used to make predictions for the MSM. We predict the SUSY mass scale to be M_SUSY ~ 10^15 GeV, thus satisfying the requirement that the predicted density fluctuations of the MSM be in agreement with the observed CMB data.

astro-ph↗

arXiv2005

Decay of the vacuum energy into CMB photons

We examine the possibility of the decay of the vacuum energy into cosmic microwave background (CMB) photons. It is shown that observations of the primordial density fluctuation spectrum put strong limits on the possible decay rate. When photon creation due to the vacuum energy decay takes place, the standard linear temperature dependence, T(z)=T_0(1+z), where T_0 is the present CMB temperature, is modified. The decay is described by a generic temperature dependence, T(z)=T_0(1+z)^{1-β}, of the CMB photons. A strong limit on the maximum value of the decay rate is obtained by placing a maximum value on the βparameter: β_{max}\approx 3\times 10^{-3}.

astro-ph↗

arXiv2005

Constraints on dark energy from the observed density fluctuations spectrum and supernova data

One of the greatest challenges in cosmology today is to determine the nature of dark energy, the source of the observed present acceleration of the universe. High precision experiments are being developed to reduce the uncertainties in the observations. Recently, we showed that the agreement to an accuracy of 10% of measurements of the present density fluctuations (δρ/ρ)^2, derived from galaxy distribution (GD) data and cosmic microwave background (CMB) anisotropies in the \LambdaCDM model, puts very strong limits on the possible decay of the vacuum energy into cold dark matter. Using this agreement, combined with the evidence that the matter density Ω_M^0=0.28\pm 0.02 and that the universe is approximately flat, we show that the vacuum metamorphosis model (VMM) and the popular brane-world model (BWM), both used to explain dark energy, can be discarded. When we relax the Ω_M^0 requirement, we find that an agreement within 10% can be obtained only with Ω_M^0\simeq 0.36 for the VMM and Ω_M^0\simeq 0.73 for the BWM, both of which are not consistent with observations. The agreement of the CMB and GD data and previous constraints from SNIa data exclude, or put strong limits on, other dark energy models, which have been suggested, that can be described by the parametrized equation of state (EOS) w=p/ρ= w_0 + w_a(1-a), where w_0 and w_a are constants, a is the cosmological scale factor and p (ρ) is the pressure (energy density) of the dark energy. We find that the supergravity (SUGRA) model with w_0=-0.82 and w_a=0.58 can be discarded. In general, we find best values -1.86<w_0<-1.72 with 1.53<w_a<2.0. For redshifts z\sim 0.5-1, where the supernova data is sensitive, w\sim -1 for this parametrized EOS.

astro-ph↗

arXiv2004

Studying the decay of the vacuum energy with the observed density fluctuation spectrum

One of the major problems in cosmology today is to explain the observed acceleration of the universe. The present observational data are consistent with the simplest solution to this problem, which is based on the vacuum energy. A decaying vacuum energy is a very attractive solution. It could link the value of the present vacuum energy, that is accelerating the universe today, with the value that created the inflationary epoch in the past. We investigate here models that suggest that the vacuum energy decays into cold dark matter (CDM) and show that the density fluctuation spectrum obtained from the cosmic microwave background (CMB) data together with large galaxy surveys (e.g., the Sloan Digital Sky Survey), puts strong limits on the rate of decay of the vacuum energy. CDM produced by a decaying vacuum energy would dilute the density fluctuation spectrum, created in the primordial universe and observed with large galaxy surveys at low redshifts. Our results indicate that the decay rate of the vacuum energy into CDM is extremely small.

astro-ph↗

arXiv2004

A New Method for Studying the Topology of the Universe from the Cosmic Microwave Background

The standard method for observationally confirming the existence of a predicted finite topology of the universe involves searching for the repetition of the same finite or extended source in different directions. However, serious problems are encountered when studying both types of sources, finite and extended. In studying a finite source, such as a star, galaxy, or quasar, the problem of its evolution must be dealt with. The same source seen in different directions is observed at different distances (i.e., ages), making its identification extremely difficult. Studying extended sources, such as circles-in-the-sky (CIS) within the cosmic microwave background (CMB), is also problematic since it requires an unrealistic zero width last scattering surface (LSS) to produce the predicted identical temperatures on the circles. It is shown here that these temperatures are no longer identical when a realistic finite width LSS is taken into account. A new method for studying the topology of the universe which avoids the above mentioned problems in studying both finite and extended sources is suggested here. It consists of searching for increased temperature fluctuations in the regions of the intersections of the LSS with the faces of the fundamental polyhedron of the finite topology. Appreciably greater fluctuations are predicted in these regions. A worked-out example is given.

astro-ph↗

arXiv2004

Summary Talk of the X Latin American Workshop on Plasma Physics

Of the many important topics that were discussed at the workshop, I summarize and comment on 25 presentations, which I found to be particularly interesting. They fall into all of the areas covered in the conference: basic plasma phenomena, space and astrophysical plasmas, technological applications of plasma, and thermonuclear fusion.

astro-ph↗