Hasil untuk "astro-ph"

A. Unzicker

Abstract In the past years, variable speed of light (VSL) theories have been of growing interest but also a sub-ject of controversial discussion. They have been accused both for tautologies and for violating specialrelativity, and concerns have been expressed about the validity of such approaches in general (e.g. Ellis,astro-ph/0703751). Without trying completeness on the issue, the example of Einstein’s VSL attempts(1911) and Dicke’s ‘electromagnetic’ theory (1957) are urges to give some comments on the above criticism. Introduction Exotic theories. Of course, ‘variability’ can en-compass a lot of aspects. One may introduce dis-persion, considering a dependency on λ, or on v, vi-olating Lorentz-invariance. Most of these proposalsdo not have sufficient experimental support at themoment, though many of them are interesting andseem as good as inflation for resolving the flatnessand horizon problems in cosmology; this however isnot the focus of interest here, since comments on[1] with respect to modern VSL theories [2] havealready be given [3]. There, appropriate referenceand a clear discussion of older attempts are how-ever missing. These so-called [2]Conservative theories suffered an even harsher‘Not even wrong’- criticism of being tautological.The argument is the following: ‘One assumes a goodclockcanbe constructed,andthen usesthe timing ofreflected electromagnetic radiation to determine thedistance. But then the (physical) speed of light ofnecessity has to be unity, precisely because all elec-tromagnetic radiation travels at the speed of light,and distances are being determined by use of suchradiation.’ ([1], sec. 2). One may wonder what factshould be proven by that statement. All that followsindeed from the definition of SI units, but in myhumble opinion something can either be measuredor defined, not both. Thus c = 1 is not a physical ne-cessity but at best a mathematical convention; onemay further ask if it is a possible, reasonable or eventhe only practical one. At the very end, this is not ascientific question; to illuminate the practical valueof c = 1, we investigate the following toy theory:

M. Stiavelli, S. Lilly, J. Gardner et al.

N. Degenaar, R. Wijnands

B. Parma, M. F. Brambillasca, F. Meroni et al.

A. Mitra

In a recent preprint (astro-ph/9905144) Tereno has tried to find the physical 3-velocity (V) at the Event Horizon of a Kruskal Black Hole. This has been done in the backdrop of the recent work by Mitra (astro-ph/9904162) where it was shown that the radial geodesic of material particle, unphysically, becomes null at the Event Horizon. Although Tereno stops short of finding the precise value of V at r=2M, he concludes that V <1. It is pointed out with reference to Eq.(13) of Tereno's work that since his delta = (r -2M)/r tends to 0 as r tends to 2M, V indeed tends to 1. It appears that Tereno's conclusion is driven by his inability to conceive proper limiting value of fractions. Similarly, his idea that, the velocity addition formula of Sp. Theory of Relativity breaks down when both the velocities approach unity is due his same inability. In particular, our central result that Schwarschild BHs must have M=0 has been obtained independently from three different considerations (gr-qc/9810038, astro-ph/9904162, 163). And we also offer here the essential physical reason why the speed of free fall at the Event Horizon must be equal to the speed of light in coordinate system. Yet we are thankful to Tereno for making the first attempt to scientifically criticize our relevant work.

R. Croft, D. Weinberg, N. Katz et al.

A convincing physical picture for the Lyman-alpha forest has emerged from simulations and related semi-analytic studies of structure formation models. Observations can be be used in the context of this picture to study cosmology using the structure of the forest. With the availability of well motivated predictions, not only has it become possible to test models directly, but the physical processes involved appear to be simple enough that we can attempt to reconstruct aspects of the underlying cosmology from observations. We briefly summarise the method of Croft et al (1997) (astro-ph/9708018) for recovering the primordial mass power spectrum from Lyman-alpha forest data, emphasising the physical reasons that the derived P(k) is independent of unknown "bias factors". We present an illustrative application of the method to four quasar "spectra" reconstructed from published line lists.

E. R. Prakasan, Anil Sagar, V. L. Kalyane et al.

V. Gurzadyan, S. Xue

The formula for the dark energy, derived by Padmanabhan in a recent Letter to Editor (Class.Quantum Grav. September 2005, the formula given in its Abstract), was actually derived 4 years earlier ourselves in astro-ph/0105245; Mod.Phys.Lett. A18, 561, 2003. Among dozens of references in that Letter, no quotation to our paper. Based on the same Zeldovich idea on vacuum fluctuations, Padmanabhan derives it from scaling considerations, while we had gone into more details and shown that the formula fits the observed value of the cosmological constant if l=0 modes are relevant.

Hongsheng Zhao, Lanlan Tian

R. G. Vishwakarma

A recent paper by Ahluwalia-Khalilova (astro-ph/0601489) is examined where he claims that the standard FRW cosmology with a $\Lambda$ predicts existence of dark matter without invoking the data on galactic rotation curves and gravitational lensing. However, we find that his claims are not correct. He has already assumed (without realizing) in the very outset what he wants to prove.

S. Bayin

In this paper we present an alternative explanation to the acceleration of the universe and the dark energy problems in terms of the Friedmann Thermodynamics. This model has the capability of making definite predictions in-line with the current observations of the universe. According to this model, cosmos was expanding slower at the beginning. During the galaxy formation era (with redshifts [0.54-0.91]), due to a change in the global equation of state it accelerates for a brief period of time. We expect to see this as a discontinuity in the Hubble diagram. Recent data about the galaxies with redshifts 0.5 1 is gathered. These galaxies will be among the very first galaxies formed in the universe, thus still showing the kinematics of the pre-galaxy formation era. This point is now clearly evidenced in the recent data by Riess et al. on Type Ia supernovae with redshifts z>1.25 (2004 astro-ph/0402512). In our model, galaxies with redshifts 0<z<0.5 should reflect the kinematics of the universe after the transition is completed. These galaxies are now receding from each other faster. However, for redshifts towards the upper end of this range we still expect to see deceleration. This is in contrast with the predictions of the dark energy models.

A. Kusenko, G. Segré

We have recently proposed an explanation for the birth velocities of pulsars based on neutrino oscillations (hep-ph/9606428). One of the quantities, dN/dT, was evaluated in the approximation of constant chemical potential for the electrons. An alternative approximation based on the assumption Ye=const, used by Qian (astro-ph/9705055), yields a somewhat higher prediction for the magnetic field inside a neutron star. If the same input parameters are used, the two approximations are in reasonable agreement, given the uncertainty in the geometry of the magnetic field and the simplified picture of neutrino emission that comes with the notion of a neutrinosphere.

H. Vega

The physics during the inflationary stage of the universe is of quantum nature involving extremely high energy densities. Moreover, it is out of equilibrium on a fastly expanding dynamical geometry. We complement here the 1999 Chalonge Lectures on out of equilibrium fields in self-consistent inflationary dynamics [astro-ph/0006446] investigating inflation driven by the evolution of highly excited quantum states. These states are characterized by a non-perturbatively large number of quanta in a band of momenta and with zero or nonzero expectation value of the inflaton scalar field. They represent the situation in which initially a non-perturbatively large energy density is localized in a band of high energy quantum modes and are coined tsunami-waves. The self-consistent evolution of this quantum state and the scale factor is studied analytically and numerically. It is shown that the time evolution of these quantum states lead to two consecutive stages of inflation under conditions that are the quantum analogue of slow-roll. The evolution of the scale factor during the first stage has new features that are characteristic of the quantum state. During this initial stage the quantum fluctuations in the highly excited band build up an effective homogeneous condensate with a non-perturbatively large amplitude as a consequence of the large number of quanta. The second stage of inflation is similar to the usual classical chaotic scenario but driven by this effective condensate. The excited quantum modes are already superhorizon in the first stage and do not affect the power spectrum of scalar perturbations. Thus, this tsunami quantum state provides a field theoretical justification for chaotic scenarios driven by a classical homogeneous scalar field of large amplitude.

T. Rauscher

The work of M\"uller et al. [Phys. Rev. C 73, 025804 (2006); astro-ph/0512603] provides interesting experimental data on neutron emission by photodisintegration of 187Re. However, the comparison to theory and the discussed implications for the Re/Os clock require considerable amendment.

R. Srikanth

In response to the comments of astro-ph/0006426 on Srikanth (Icarus 1999), we clarify, and thereby justify, the definition of Poynting-Robertson drag adopted in the latter. We confirm that dust absorption is a necessary condition while re-emission is neither necessary nor sufficient for the inspiralling of a rest-isotropically emitting dust.

M. Ahlers

Abstract The origin and chemical composition of ultrahigh energy cosmic rays is still an open question in astroparticle physics. The observed large-scale isotropy and also direct composition measurements can be interpreted as an extragalactic proton dominance above the ankle at about 10 10  GeV. Photopion production of extragalactic protons in the cosmic microwave background predicts a cut-off at about 5×10 10 GeV in conflict with excesses reported by some experiments. In this report we will outline a recent statistical analysis [M. Ahlers, A. Ringwald, H. Tu, Astropart. Phys. (in press). Preprint astro-ph/0506698 ] of cosmic ray data using strongly interacting neutrinos as primaries for these excesses.

V. Lyashuk

D. Roscoe

D. Roscoe

In previous work (astro-ph/0306228) we have established that a particular quasi-classical theoretical viewpoint about the nature of gravitation in galaxy discs provides extremely high concordance (comparable with MOND) with the observed dynamics in optical discs. In this paper, we consider the possibility that the optical disc and the flat radio disc represent distinct dynamical regions which are inherently described by distinct phases of the same over-arching theory of which the quasi-classical theory is but one phase. This is a defining moment for the theory under discussion since the only natural generalization available is that from the existing quasi-classical (one-clock) model into a quasi-relativistic (two-clock) model. The quasi-relativistic disc turns out to be a stationary hyperbolic disc within which information propagates along characteristics and within which the rotation curve is necessarily exactly flat. The transition between the two components of the disc is managed by physically determined jump conditions which are represented by a set of non-linear algebraic equations. These equations predict the existence of a forbidden region within the parameter space which coincides exactly with that part of the parameter space in which there is an already observed highly significant underpopulation of galaxy discs (astro-ph/0107300).

S. Sahijpal, P. Soni