arXiv subjects

Milan M. Cirkovic

Publications and source records attributed to Milan M. Cirkovic.

51 records · Page 3Linked to original sources

arXiv2001

Backward Causation, Isolation and the Pursuit of Justice

The recent operationalization of the famous Newcomb's game by Schmidt (1998) offers an interesting and thought-provoking look at the plausibility of backward causation in a Newtonian universe. Hereby we investigate two details of the Schmidt's scenario which may, at least in principle, invalidate his conclusion in two different domains: one dealing with the issue of Newtonian predictability in specific instance of human actions, and the other stemming from a possible strategy aimed at obviating the anthropically oriented view of backward causation as applied to a judicial and ethical problem posed by a version of the scenario. We conclude that the scenario is at least to be more complex than originally presented in order to remain viable. However, it points to a very deep and delicate question of compatibility of backward causation with the conventional ethical standards.

physics.class-ph↗

arXiv2001

Cosmological constant and the fate of the DDM theory

We investigate the impact of the non-zero cosmological constant on the classical decaying dark matter theory developed by the late Dennis Sciama. In particular, we concentrate on the change in relevant values of cosmological parameters in comparison to the high-precision estimates given by Sciama (1997). It is shown that the appropriate changes in resulting parameter values are such to make the DDM concept less plausible. This is in complete agreement with recently reported observational results detrimental to this theory.

astro-ph↗

arXiv2000

QSO Absorption Lines, Gaseous Infall and Star Formation

We investigate the impact of recent observational advances in research on the gaseous content of the universe on our knowledge of star formation histories of disk galaxies. Several contemporary surveys indicate that large quantities of gas which has not been astrated or has been astrated only weakly are bound to galaxies at later epochs ($z\le 1$).

astro-ph↗

arXiv2000

On a Physical Metatheory of Consciousness

We show that the modern quantum mechanics, and particularly the theory of decoherence, allows for formulating a sort of a physical metatheory of consciousness. Particularly, the analysis of the necessary conditions for the occurrence of decoherence, along with the hypothesis that consciousness bears (more-or-less) well definable physical origin, leads to a wider physical picture naturally involving consciousness. This can be considered as a sort of a psycho-physical parallelism, but on rather wide scales bearing some cosmological relevance.

quant-ph↗

arXiv2000

On the Small-Scale Clustering of Ly-alpha Forest Clouds

Recent measurements of the autocorrelation function of the Ly-alpha clouds are analyzed from the point of view of a simple model with strong clustering on the small scales. It is shown that this toy model reproduces fairly well the important linear relation between amplitude of the absorber autocorrelation function and neutral hydrogen column density. In addition, it predicts a correct evolutionary trend of correlation amplitudes. Some possible ramifications of these results are discussed.

astro-ph↗

arXiv2000

The Anthropic Argument against Infinite Past and the Eddington-Lemaitre Universe

This study in the philosophy of cosmology is a part of an ongoing effort to investigate and reassess the importance of the anthropic (Davies-Tipler) argument against cosmologies containing the past temporal infinity. Obviously, the prime targets of this argument are cosmological models stationary on sufficiently large scale, the classical steady state model of Bondi, Gold and Hoyle being the best example. Here we investigate the extension of application of this argument to infinitely old non-stationary models and discuss additional constraints necessary to be imposed on such models for the edge of the anthropic argument to be preserved. An illustrative counterexample is the classical Eddington-Lemaitre model, in the analysis of which major such constraints are presented. Consequences of such an approach for our understanding of the nature of time are briefly discussed.

astro-ph↗

arXiv1999

Cosmological Constant and the Final Anthropic Hypothesis

The influence of recent detections of a finite vacuum energy ("cosmological constant") on our formulation of anthropic conjectures, particularly the so-called Final Anthropic Principle is investigated. It is shown that non-zero vacuum energy implies the onset of a quasi-exponential expansion of our causally connected domain ("the universe") at some point in the future, a stage similar to the inflationary expansion at the very beginning of time. The transition to this future inflationary phase of cosmological expansion will preclude indefinite survival of intelligent species in our domain, because of the rapid shrinking of particle horizons and subsequent depletion of energy necessary for information processes within the horizon of any observer. Therefore, to satisfy the Final Anthropic Hypothesis (reformulated to apply to the entire ensemble of universes), it is necessary to show that (i) chaotic inflation of Linde (or some similar model) provides a satisfactory description of reality, (ii) migration between causally connected domains within the multiverse is physically permitted, and (iii) the time interval left to the onset of the future inflationary phase is sufficient for development of the technology necessary for such inter-domain travel. These stringent requirements diminish the probability of the Final Anthropic Hypothesis being true.

gr-qc↗

arXiv1999

Microlensing Results and Removing the Baryonic Degeneracy

One of the biggest obstacles to our full understanding of the global dynamics in Milky Way and other spiral galaxies is uncertainty with respect to the form of baryonic dark matter in galactic haloes. Two basic forms discussed recently are MACHOs and various states of halo gas. We investigate constraints which could be obtained from the empirical microlensing optical depth on combined MACHO + gas models, and show that improved statistics will certainly be able to discriminate between various such models. This has profound consequences not only for galactic dynamics and baryonic budget, but for investigation of the low-redshift Ly$α$ absorption systems and general cosmological distribution of gas.

astro-ph↗

arXiv1999

Microlensing, flattening, $Ω_B$ and $h$: from assumptions, via data to tentative conclusions

Microlensing observations can be used for determining the shape of the Milky Way's halo. It can be shown that the data are best described with moderately flattened halo, 0.5 < q < 0.6. We discuss, by taking into account this result, the constraints on the baryonic mass-density parameter, $Ω_B$, and their implications on the value of the Hubble constant $H_0$, i.e. $h$. Our conclusion is that current data, in order to satisfy BBNS and Galaxy dynamics constraints, strongly suggest that $h$ should take values within the lower part of the permissible range, $h\sim 0.5$.

astro-ph↗

arXiv1999

Stability of the Protogalactic Clouds: I. Field Length in the Adiabatic Models

All gasdynamical models for the evolution of gaseous content of galaxies assume that cooling from the hot, virialized phase to the cold phase occured through some sort of thermal instability. Subsequent formation of colder clouds embedded in the hot, rarefied medium is a well-known process appearing in many astrophysical circumstances and environments. The characteristics of the condensed clouds depend on the relevant timescales for cloud formation and disruption due to either collisions or one of the operating instabilities. In this paper, the importance of the Kelvin-Helmholtz instability is investigated for the clouds forming in huge gaseous haloes of $L_\ast$ galaxies. Recent treatment of this problem by Kamaya (1997) is extended and a more realistic cooling function employed. Results show that the Kelvin-Helmholtz instability proceeds effectively on the same timescale whether we account for self-gravity or not. This has multiple significance, since these objects may have been seen as high-column density absorption line systems against the background QSOs, and probably represent the progenitors of the present-day globular clusters.

astro-ph↗

arXiv1999

Flattened Galactic Haloes and Baryonic Dark Matter

We discuss the tight interconnection between microlensing optical depths, flattening of dark haloes and low-to-intermediate redshift baryonic census. By analysing plots of the microlensing optical depth as a function of galactic coordinates for different values of axis ratio $q$ of the galactic MACHO halo, we have shown that observations are best described by a flattened halo with $q \simeq 0.6$. There is no dynamical obstacle for such a choice of global halo shape. Both extremely flattened $q \simeq 0.2$ and spherical $q \simeq 1$ haloes have several difficulties, although not of equal severity. Consequences of such flattening for the cosmological density fraction contained in MACHOs are considered and comparison with mass in low and intermediate-redshift Ly$α$ forest and other plausible reservoirs of gas is discussed in context of a unified description of the evolution of baryonic content of the universe.

astro-ph↗

arXiv1999

Massive Neutrino Decay and Shape of the Galactic Dark Halo

In this Letter we investigate the basic assumptions of the decaying dark matter (DDM) theory in the light of recent advances in observational and theoretical cosmology and physics, i.e. detection of massive astrophysical compact halo objects (MACHOs) and Super-Kamiokande results. Specifically, the consequences pertaining to the shape of the Milky Way galaxy dark halo are discussed. We find that, by taking into account the values of the main constituent of the mass in DDM theory, massive neutrino, with the mass of 30 eV, and lifetime $\sim 10^{23}$ s, the initially proposed value of extreme halo flattening $q\sim 0.2$ is no longer necessary, and that one can easily accommodate a much larger value of $q\sim 0.6$, that is in accord with all available observational data.

astro-ph↗

arXiv1999

Towards a Direct Detection of Warm Gas in Galactic Haloes at Cosmological Distances

Recent highly sensitive detections of line emission from extended gas in the local universe demonstrate the feasibility of detecting H$α$ emitting galactic halos out to $z\sim1$. We determine the form of the surface brightness vs. redshift dependence which takes into account UV background evolution. Successful detections will have a major impact on a wide range of fields, in particular, the source of ionization in QSO absorption systems.

astro-ph↗

arXiv1998

Ly$α$ forest and the total absorption cross-section of galaxies -- an example of the NTT SUSI Deep Field

By extrapolating the accumulated low-redshift data on the absorption radius of galaxies and its luminosity scaling, it is possible to predict the total absorption cross-section of the gas associated with collapsed structures in the universe at any given epoch. This prediction can be verified observationally through comparison with the well-known spatial distribution of the QSO absorption systems. In this way, it is shown that HDF, NTT SUSI Deep Field and other such data give further evidence for the plausibility of origin of the significant fraction of the Ly$α$ forest in haloes of normal galaxies.

astro-ph↗

arXiv1998

Pressure at the ISM-halo interface: a reheating frequency dependence

The dependence of the thermal pressure of hot galactic halos on a model parameter describing the frequency of major reheating episodes during galactic history is investigated. Pressure on the interface between interstellar medium and the halo gas is especially interesting, since empirical evidence here offers one of the simplest constraints on halo models. It is shown that two-phase model of Mo & Miralda-Escudé is sufficiently robust with respect to uncertainties in the average interval between reheating.

astro-ph↗