Fulvio Melia - Profile on Academia.edu (original) (raw)

Papers by Fulvio Melia

Zeitschrift für Naturforschung A

Momentum is characterized in terms of inertial mass for particles moving at less than the speed o... more Momentum is characterized in terms of inertial mass for particles moving at less than the speed of light, but entirely in terms of their energy for those lacking inertia. Does this difference suggest a physically distinct origin of momentum in the two cases and, if so, what is actually being conserved in interactions involving both types of particle? In this paper, we consider a recently proposed gravitational origin for rest-mass energy to demonstrate that a single definition of momentum applies to all particles, massless or otherwise. When introduced into this description, inertial mass is merely a surrogate for the particle’s ‘free’ energy, but does not imply an origin of momentum different from that of particles without mass.

Social Science Research Network, 2020

The recent measurement of a cutoff min in the fluctuation power spectrum ( ) of the cosmic microw... more The recent measurement of a cutoff min in the fluctuation power spectrum ( ) of the cosmic microwave background may vitiate the possibility that slow-roll inflation can simultaneously solve the horizon problem and account for the formation of structure via the growth of quantum fluctuations in the inflaton field. Instead, we show that min may be interpreted more successfully in the h = cosmology, as the first mode exiting from the Planck scale into the semi-classical Universe shortly after the Big Bang. In so doing, we demonstrate that such a scenario completely avoids the well-known trans-Planckian problem plaguing standard inflationary cosmology.

Wiley-VCH Verlag GmbH eBooks, Dec 31, 2007

Images of the central arcsec of the Galaxy at near-infrared(IR) wavelengths reveal a tight stella... more Images of the central arcsec of the Galaxy at near-infrared(IR) wavelengths reveal a tight stellar cluster, however, no point source exactly coincident with the radio source Sgr A* has been clearly detected in the near-IR. "Contaminating" emission due to overlapping point spread functions (PSFs) from this stellar cluster as well as from the bright IRS16 sources 1-3 away from Sgr A* makes it difficult to assess directly the near-IR characteristics of Sgr A*. Taking advantage of the stability of the PSF achieved by HST/NICMOS observations, we employ PSF subtraction techniques in order to investigate the true nature of the near-IR emission from Sgr A*. New limits on the maximum possible flux emanating from a point source coincident with Sgr A* are presented at 1.1, 1.45, 1.6, 1.9, and 2.2 µm based on NICMOS data taken at several epochs. These are the faintest reported near-IR flux limits for wavelengths shorter than 2.2 µm . The resulting extinction-corrected fluxes are compared to theoretical models (including both quiescent and flare models) for the spectrum of Sgr A*. We also present NICMOS narrow-band emission line images in Paα where significant stellar PSF residuals have been removed to reveal new diffuse structures in the ionized gas near Sgr A* at two epochs.

The Astrophysical Journal, 1992

We present in detail the results of our two-dimensional, fully self-consistent radiative-hydrodyn... more We present in detail the results of our two-dimensional, fully self-consistent radiative-hydrodynamical simulation of an accretion disk corona in low-mass X-ray binaries radiating at half the Eddington limit. The corona is comprised of two main regions, separated roughly by the radius at which the evaporated plasma first attains escape velocity. The inner portion is highly variable, and oscillates quasi-periodically on a dynamic time scale. whereas the outer corona is more stable with the gas rising to form a density sheath that merges into a wind at larger radii. Observationally. the corona can signficantly alter the appearance of the central source since it enhances the flux by as much as 100% when the neutron star is seen directly, while it suppresses the observed flux by an order of magnitude deep with in its shadow.

International Journal of Modern Physics D, Apr 1, 2016

HD) has recently been constructed in the redshift range 0 z 6.5 using a non-linear relation betwe... more HD) has recently been constructed in the redshift range 0 z 6.5 using a non-linear relation between the ultraviolet and X-ray luminosities of QSOs. The Type Ia SN HD has already provided a high-precision test of cosmological models, but the fact that the QSO distribution extends well beyond the supernova range (z 1.8), in principle provides us with an important complementary diagnostic whose significantly greater leverage in z can impose tighter constraints on the distance versus redshift relationship. In this paper, we therefore perform an independent test of nine different cosmological models, among which six are expanding, while three are static. Many of these are disfavoured by other kinds of observations (including the aforementioned Type Ia SNe). We wish to examine whether the QSO HD confirms or rejects these earlier conclusions. We find that four of these models (Einstein-de Sitter, the Milne universe, the Static Universe with simple tired light and the Static universe with plasma tired light) are excluded at the > 99% C.L. The Quasi-Steady State Model is excluded at > 95% C.L. The remaining four models (ΛCDM/wCDM, the R h = ct Universe, the Friedmann open universe and a Static universe with a linear Hubble law) all pass the test. However, only ΛCDM/wCDM and R h = ct also pass the Alcock-Paczyński (AP) test. The optimized parameters in ΛCDM/wCDM are Ωm = 0.20 +0.24 -0.20 and w de = -1.2 +1.6 -∞ (the dark-energy equation-of-state). Combined with the AP test, these values become Ωm = 0.38 +0.20 -0.19 and w de = -0.28 +0.52 -0.40 . But whereas this optimization of parameters in ΛCDM/wCDM creates some tension with their concordance values, the R h = ct Universe has the advantage of fitting the QSO and AP data without any free parameters.

arXiv (Cornell University), Feb 20, 2023

The so-called 'impossibly early galaxy' problem, first identified via the Hubble Space Telescope'... more The so-called 'impossibly early galaxy' problem, first identified via the Hubble Space Telescope's observation of galaxies at redshifts z > 10, appears to have been exacerbated by the more recent James Webb Space Telescope (JWST) discovery of galaxy candidates at even higher redshifts (z ∼ 17) which, however, are yet to be confirmed spectroscopically. These candidates would have emerged only ∼ 230 million years after the big bang in the context of ΛCDM, requiring a more rapid star formation in the earliest galaxies than appears to be permitted by simulations adopting the concordance model parameters. This time-compression problem would therefore be inconsistent with the age-redshift relation predicted by ΛCDM. Instead, the sequence of star formation and galaxy assembly would confirm the timeline predicted by the R h = ct universe, a theoretically advanced version of ΛCDM that incorporates the 'zero active mass' condition from general relativity. This model has accounted for many cosmological data better than ΛCDM, and eliminates all of its inconsistencies, including the horizon and initial entropy problems. The latest JWST discoveries at z 14, if confirmed, would add further support to the idea that the R h = ct universe is favored by the observation over the current standard model.

arXiv (Cornell University), May 11, 2012

Cosmology today is confronted with several seemingly insoluble puzzles and strange, inexplicable ... more Cosmology today is confronted with several seemingly insoluble puzzles and strange, inexplicable coincidences. But a careful re-examination of the Cosmological principle and the Weyl postulate, foundational elements in this subject, suggests that we may be missing the point. The observations actually reveal a simpler and more elegant Universe than anyone could have imagined. The polish priest Nicolaus Copernicus (1473-1543) started a revolution with his heliocentric cosmology that displaced the Earth from the center of the Universe. His remarkable shift in paradigm continues to this day, the cornerstone of a concept we now call the Cosmological Principle, in which the Universe is assumed to be homogeneous and isotropic, without a center or boundary. But few realize that even this high degree of symmetry is insufficient for cosmologists to build a practical model of the Universe from the equations of General Relativity.

Transverse conductivity of a relativistic plasma in oblique electric and magnetic fields

Physical review, Jan 15, 1991

Resistive tearing is a primary candidate for flares occurring in stressed magnetic fields. Its po... more Resistive tearing is a primary candidate for flares occurring in stressed magnetic fields. Its possible application to the strongly magnetized environments ({ital H}{sub {ital z}}{similar to}10Â¹Â² G) near the surfaces of neutron stars motivates a quantum treatment of this process, which requires knowledge of the electrical conductivity Ï of a relativistic gas in a new domain, i.e., that of a low-density ({ital n}{sub {ital e}}) plasma in oblique electric ({bold E}=(0,{ital E}{sub {ital y}},{ital E}{sub {ital z}})) and magnetic fields. We derive the mathematical formalism for calculating Ï and present numerical results for the range of parameter values 10â¹â¤{ital H}{sub {ital z}}â¤10Â¹Â² G, {ital E}{sub {ital z}}/{ital H}{sub {ital z}}{approx lt}10â»â´, {ital E}{sub {ital y}}{approx lt}10â»â´{ital H}{sub {ital z}}Â²/{ital E}{sub {ital z}}, and 10Â²Â°â¤{ital n}{sub {ital e}}â¤10Â²âµ cmâ»Â³. We find that Ï{similar to}{ital E}{sub {ital y}}Â²{ital E}{sub {ital z}n{ital e}}Â²/{ital H}{sub {ital z}}Â² over this range.

It has been known for over three decades that the monochromatic X-ray and UV luminosities in quas... more It has been known for over three decades that the monochromatic X-ray and UV luminosities in quasars are correlated, though non-linearly. This offers the possibility of using high-z quasars as standard candles for cosmological testing. In this paper, we use a recently assembled, high-quality catalog of 1598 quasars extending all the way to redshift ∼ 6, to compare the predictions of the R h = ct and ΛCDM cosmologies. In so doing, we affirm that the parameters characterizing the correlation depend only weakly on the chosen cosmology, and that both models account very well for the data. Unlike ΛCDM, however, the R h = ct model has no free parameters for this work, so the Bayesian Information Criterion favours it over ΛCDM with a relative likelihood of ∼ 88% versus ∼ 10%. This result is consistent with the outcome of other comparative tests, many of which have shown that R h = ct is favoured over the standard model based on a diverse range of observations.

Gamma-Rays from the Galactic Center: A Point-Like or Diffuse Source?

arXiv (Cornell University), Dec 17, 1993

We present the results of a λ20 cm VLA 2 observation of the compact Galactic center radio source ... more We present the results of a λ20 cm VLA 2 observation of the compact Galactic center radio source Sgr A * . The scatter-broadened image is elongated in the East-West direction, with an axial ratio of 0.6±0.05 and a position angle of 87 0 ±3 0 . A similar shape and orientation has been found previously at shorter wavelengths using VLBI and VLBA. Both the major and minor axes follow the λ 2 law appropriate for scattering by turbulence in the intervening medium. Assuming that the anisotropy is caused by a magnetic field permeating the scattering medium, we argue that the scattering occurs within extended HII regions lying in the central 100 pc of the Galaxy. The magnetic field in this region must be poloidal, organized and is estimated to have a strength of at least 30 to 100 µGauss.

arXiv (Cornell University), Oct 21, 1997

We here examine the conditions of the two-phase disk model under which magnetic flares arise abov... more We here examine the conditions of the two-phase disk model under which magnetic flares arise above the cold accretion disk due to magnetic buoyancy and produce Xrays via Comptonization of the disk's soft radiation. We find that the disk's ability to produce strong magnetic flares is substantially diminished in its radiation dominated regions due to the diffusion of radiation into the magnetic flux tubes. Using a simplified, yet physically self-consistent, model that takes this effect into account, we show that the hard X-ray spectrum of some GBHCs can be explained as the X-ray emission by magnetic flares only when the disk's bolometric luminosity is a relatively small fraction (∼ 5%) of the Eddington value, L Edd . Further, we compute the hard (20 -200 keV) and soft (1 -20 keV) X-ray power as a function of the disk's luminosity, and find an excellent agreement with the available data for GBHC transient and persistent sources. We conclude that the observed high-energy spectrum of stellar-sized accretion disk systems can be explained by Comptonization of the disk's soft radiation by the hot gas trapped inside the magnetic flares when the luminosity falls in the range ∼ 10 -3 -10 -1 × L Edd . For higher luminosities, another emission mechanism must be at work. For lower luminosities, the X-ray emissivity may still be dominated by magnetic flares, but this process is more likely to be thermal or non-thermal bremstrahlung, so that the X-ray spectrum below ∼ 10 -3 L Edd may be quite distinct from the typical hard spectrum for higher luminosities.

arXiv (Cornell University), May 10, 2007

arXiv (Cornell University), Jun 25, 2008

Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Su... more Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares. Methods. In Spring 2007, we observed Sgr A* with XMM-Newton with a total exposure of ∼230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of Sgr A* for comparison. Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (∼3 ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N H = 12.3 +2.1 -1.8 × 10 22 cm -2 and Γ = 2.3± 0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower (N H = 8.8 +4.4 -3.2 × 10 22 cm -2 and Γ = 1.7 +0.7 -0.6 ), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward Sgr A* (A V ∼ 25 mag), i.e., 4.5 × 10 22 cm -2 . However, our fitting of the Sgr A* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra.

Inferring Spherical Mass Distributions Using the Projected Mass Estimator

The Astrophysical Journal, Jun 1, 1996

... The precise term that gives rise to M(r) in the case of spherical sampling cancels out when t... more ... The precise term that gives rise to M(r) in the case of spherical sampling cancels out when the tracer is sampled in concentric annuli. What ... Fig. 2). The term <M is equal to M when the sampling volume is spherical. Yet in Fiu. ...

SGR A West: Its Kinematics and Its Role as a Possible Source of Scattering SGR A(*) at the Galactic Center

AAS, May 1, 1993

ABSTRACT

Accretion onto the Supermassive Black Hole at the Centre of Our Galaxy

Cosmic High-Energy Physics. (Book Reviews: High Energy Phenomena around Collapsed Stars)

Science, Nov 1, 1987

Theoretical Basis for Zero Active Mass

The Cosmic Spacetime, 2020