arXiv subjects

Fulvio Melia

Publications and source records attributed to Fulvio Melia.

At least 73 records · Page 4Linked to original sources

arXiv2018

The Apparent (Gravitational) Horizon in Cosmology

In general relativity, a gravitational horizon (more commonly known as the "apparent horizon") is an imaginary surface beyond which all null geodesics recede from the observer. The Universe has an apparent (gravitational) horizon, but unlike its counterpart in the Schwarzschild and Kerr metrics, it is not static. It may eventually turn into an event horizon---an asymptotically defined membrane that forever separates causally connected events from those that are not---depending on the equation of state of the cosmic fluid. In this paper, we examine how and why an apparent (gravitational) horizon is manifested in the Friedmann-Robertson-Walker metric, and why it is becoming so pivotal to our correct interpretation of the cosmological data. We discuss its observational signature and demonstrate how it alone defines the proper size of our visible Universe. In so doing, we affirm its physical reality and its impact on cosmological models.

gr-qc↗

arXiv2018

Model Selection with Strong-lensing Systems

In this paper, we use an unprecedentedly large sample (158) of confirmed strong lens systems for model selection, comparing five well studied Friedmann-Robertson-Walker cosmologies: LCDM, wCDM (the standard model with a variable dark-energy equation of state), the R_h=ct universe, the (empty) Milne cosmology, and the classical Einstein-de Sitter (matter dominated) universe. We first use these sources to optimize the parameters in the standard model and show that they are consistent with Planck, though the quality of the best fit is not satisfactory. We demonstrate that this is likely due to under-reported errors, or to errors yet to be included in this kind of analysis. We suggest that the missing dispersion may be due to scatter about a pure single isothermal sphere (SIS) model that is often assumed for the mass distribution in these lenses. We then use the Bayes information criterion, with the inclusion of a suggested SIS dispersion, to calculate the relative likelihoods and ranking of these models, showing that Milne and Einstein-de Sitter are completely ruled out, while R_h=ct is preferred over LCDM/wCDM with a relative probability of ~73% versus ~24%. The recently reported sample of new strong lens candidates by the Dark Energy Survey, if confirmed, may be able to demonstrate which of these two models is favoured over the other at a level exceeding 3 sigma.

astro-ph.CO↗

arXiv2018

J1342+0928 Supports the Timeline in the R_h=ct Cosmology

The discovery of quasar J1342+0928 (z=7.54) reinforces the time compression problem associated with the premature formation of structure in LCDM. Adopting the Planck parameters, we see this quasar barely 690 Myr after the big bang, no more than several hundred Myr after the transition from Pop III to Pop II star formation. Yet conventional astrophysics would tell us that a 10 solar-mass seed, created by a Pop II/III supernova, should have taken at least 820 Myr to grow via Eddington-limited accretion. This failure by LCDM constitutes one of its most serious challenges, requiring exotic `fixes', such as anomalously high accretion rates, or the creation of enormously massive (~10^5 solar-mass) seeds, neither of which is ever seen in the local Universe, or anywhere else for that matter. Indeed, to emphasize this point, J1342+0928 is seen to be accreting at about the Eddington rate, negating any attempt at explaining its unusually high mass due to such exotic means. In this paper, we aim to demonstrate that the discovery of this quasar instead strongly confirms the cosmological timeline predicted by the R_h=ct universe. We assume conventional Eddington-limited accretion and the time versus redshift relation in this model to calculate when a seed needed to start growing as a function of its mass in order to reach the observed mass of J1342+0928 at z=7.54. Contrary to the tension created in the standard model by the appearance of this massive quasar so early in its history, we find that in the R_h=ct cosmology, a 10 solar-mass seed at z~15 (the start of the Epoch of Reionization at t~878 Myr) would have easily grown into an 8 x 10^8 solar-mass black hole at z=7.54 (t~1.65 Gyr) via conventional Eddington-limited accretion.

astro-ph.CO↗

arXiv2018

A Cosmological Solution to the Impossibly Early Galaxy Problem

To understand the formation and evolution of galaxies at redshifts z < 10, one must invariably introduce specific models (e.g., for the star formation) in order to fully interpret the data. Unfortunately, this tends to render the analysis compliant to the theory and its assumptions, so consensus is still somewhat elusive. Nonetheless, the surprisingly early appearance of massive galaxies challenges the standard model, and the halo mass function estimated from galaxy surveys at z > 4 appears to be inconsistent with the predictions of LCDM, giving rise to what has been termed "The Impossibly Early Galaxy Problem" by some workers in the field. A simple resolution to this question may not be forthcoming. The situation with the halos themselves, however, is more straightforward and, in this paper, we use linear perturbation theory to derive the halo mass function over the redshift range z < 10 for the R_h=ct universe. We use this predicted halo distribution to demonstrate that both its dependence on mass and its very weak dependence on redshift are compatible with the data. The difficulties with LCDM may eventually be overcome with refinements to the underlying theory of star formation and galaxy evolution within the halos. For now, however, we demonstrate that the unexpected early formation of structure may also simply be due to an incorrect choice of the cosmology, rather than to yet unknown astrophysical issues associated with the condensation of mass fluctuations and subsequent galaxy formation.

astro-ph.CO↗

arXiv2018

Cosmological Tests With Strong Gravitational Lenses using Gaussian Processes

Strong gravitational lenses provide source/lens distance ratios D_obs useful in cosmological tests. Previously, a catalog of 69 such systems was used in a one-on-one comparison between the standard model, LCDM, and the R_h=ct universe, which has thus far been favored by the application of model selection tools to many other kinds of data. But in that work, the use of model parametric fits to the observations could not easily distinguish between these two cosmologies, in part due to the limited measurement precision. Here, we instead use recently developed methods based on Gaussian Processes (GP), in which D_obs may be reconstructed directly from the data without assuming any parametric form. This approach not only smooths out the reconstructed function representing the data, but also reduces the size of the 1-sigma confidence regions, thereby providing greater power to discern between different models. With the current sample size, we show that analyzing strong lenses with a GP approach can definitely improve the model comparisons, producing probability differences in the range ~10-30%. These results are still marginal, however, given the relatively small sample. Nonetheless, we conclude that the probability of R_h=ct being the correct cosmology is somewhat higher than that of LCDM, with a degree of significance that grows with the number of sources in the subsamples we consider. Future surveys will significantly grow the catalog of strong lenses and will therefore benefit considerably from the GP method we describe here. In addition, we point out that if the R_h=ct universe is eventually shown to be the correct cosmology, the lack of free parameters in the study of strong lenses should provide a remarkably powerful tool for uncovering the mass structure in lensing galaxies.

astro-ph.CO↗

arXiv2018

Model Selection Using Cosmic Chronometers with Gaussian Processes

The use of Gaussian Processes with a measurement of the cosmic expansion rate based solely on the observation of cosmic chronometers provides a completely cosmology-independent reconstruction of the Hubble constant H(z) suitable for testing different models. The corresponding dispersion sigma_H is smaller than ~9% over the entire redshift range (0 < z < 2) of the observations, rivaling many kinds of cosmological measurements available today. We use the reconstructed H(z) function to test six different cosmologies, and show that it favours the R_h=ct universe, which has only one free parameter (i.e., H_0) over other models, including Planck LCDM. The parameters of the standard model may be re-optimized to improve the fits to the reconstructed H(z) function, but the results have smaller p-values than one finds with R_h=ct.

astro-ph.CO↗

arXiv2017

Evidence of a truncated spectrum in the angular correlation function of the cosmic microwave background

The lack of large-angle correlations in the fluctuations of the cosmic microwave background (CMB) conflicts with predictions of slow-roll inflation. But though probabilities (< 0.24%) for the missing correlations disfavor the conventional picture at > 3 sigma, factors not associated with the model itself may be contributing to the tension. Here we aim to show that the absence of large-angle correlations is best explained with the introduction of a non-zero minimum wavenumber k_min for the fluctuation power spectrum P(k). We assume that quantum fluctuations were generated in the early Universe with a well-defined power spectrum P(k), though with a cutoff k_min not equal to 0. We then re-calculate the angular correlation function of the CMB and compare it with Planck observations. The Planck 2013 data rule out a zero k_min at a confidence level exceeding 8 sigma. Whereas purely slow-roll inflation would have stretched all fluctuations beyond the horizon, producing a P(k) with k_min=0---and therefore strong correlations at all angles---a k_min > 0 would signal the presence of a maximum wavelength at the time (t_dec) of decoupling. This argues against the basic inflationary paradigm---perhaps even suggesting non-inflationary alternatives---for the origin and growth of perturbations in the early Universe. In at least one competing cosmology, the R_h=ct universe, the inferred k_min corresponds to the gravitational radius at t_dec.

astro-ph.CO↗

arXiv2017

A Two-point Diagnostic for the HII Galaxy Hubble Diagram

A previous analysis of starburst-dominated HII Galaxies and HII regions has demonstrated a statistically significant preference for the Friedmann-Robertson-Walker cosmology with zero active mass, known as the R_h=ct universe, over LCDM and its related dark-matter parametrizations. In this paper, we employ a 2-point diagnostic with these data to present a complementary statistical comparison of R_h=ct with Planck LCDM. Our 2-point diagnostic compares---in a pairwise fashion---the difference between the distance modulus measured at two redshifts with that predicted by each cosmology. Our results support the conclusion drawn by a previous comparative analysis demonstrating that R_h=ct is statistically preferred over Planck LCDM. But we also find that the reported errors in the HII measurements may not be purely Gaussian, perhaps due to a partial contamination by non-Gaussian systematic effects. The use of HII Galaxies and HII regions as standard candles may be improved even further with a better handling of the systematics in these sources.

astro-ph.CO↗

arXiv2017

Reconstruction of the HII Galaxy Hubble Diagram using Gaussian Processes

The Hubble diagram constructed using HII galaxies (HIIGx) and Giant extragalactic HII regions (GEHR) as standard candles already extends beyond the current reach of Type Ia SNe. A sample of 156 HIIGx and GEHR sources has been used previously to compare the predictions of LCDM and R_h=ct, the results of which suggested that the HIIGx and GEHR sources strongly favour the latter over the former. But this analysis was based on the application of parametric fits to the data and the use of information criteria, which disfavour the less parsimonious models. In this paper, we advance the use of HII sources as standard candles by utilizing Gaussian processes (GP) to reconstruct the distance modulus representing these data without the need to pre-assume any particular model, none of which may in the end actually be the correct cosmology. In addition, this approach tightly constrains the 1 sigma confidence region of the reconstructed function, thus providing a better tool with which to differentiate between competing cosmologies. With this approach, we show that the Planck concordance model is in tension with the HII data at more than 2.5 sigma, while R_h=ct agrees with the GP reconstruction very well, particularly at redshifts > 10^{-3}.

astro-ph.CO↗

arXiv2017

Unseen Progenitors of Luminous High-z Quasars in the R_h=ct Universe

Quasars at high redshift provide direct information on the mass growth of supermassive black holes and, in turn, yield important clues about how the Universe evolved since the first (Pop III) stars started forming. Yet even basic questions regarding the seeds of these objects and their growth mechanism remain unanswered. The anticipated launch of eROSITA and ATHENA is expected to facilitate observations of high-redshift quasars needed to resolve these issues. In this paper, we compare accretion-based supermassive black hole growth in the concordance LCDM model with that in the alternative Friedmann-Robertson Walker cosmology known as the R_h=ct universe. Previous work has shown that the timeline predicted by the latter can account for the origin and growth of the > 10^9 M_sol highest redshift quasars better than that of the standard model. Here, we significantly advance this comparison by determining the soft X-ray flux that would be observed for Eddington-limited accretion growth as a function of redshift in both cosmologies. Our results indicate that a clear difference emerges between the two in terms of the number of detectable quasars at redshift z > 6, raising the expectation that the next decade will provide the observational data needed to discriminate between these two models based on the number of detected high-redshift quasar progenitors. For example, while the upcoming ATHENA mission is expected to detect ~0.16 (i.e., essentially zero) quasars at z ~ 7 in R_h=ct, it should detect ~160 in LCDM---a quantitatively compelling difference.

astro-ph.CO↗

arXiv2017

Analyzing H(z) Data using Two-point Diagnostics

Measurements of the Hubble constant H(z) are increasingly being used to test the expansion rate predicted by various cosmological models. But the recent application of 2-point diagnostics, such as Om(z_i,z_j) and Omh^2(z_i,z_j), has produced considerable tension between LCDM's predictions and several observations, with other models faring even worse. Part of this problem is attributable to the continued mixing of truly model-independent measurements using the cosmic-chronomter approach, and model-dependent data extracted from BAOs. In this paper, we advance the use of 2-point diagnostics beyond their current status, and introduce new variations, which we call Delta h(z_i,z_j), that are more useful for model comparisons. But we restrict our analysis exclusively to cosmic-chronometer data, which are truly model independent. Even for these measurements, however, we confirm the conclusions drawn by earlier workers that the data have strongly non-Gaussian uncertainties, requiring the use of both "median" and "mean" statistical approaches. Our results reveal that previous analyses using 2-point diagnostics greatly underestimated the errors, thereby misinterpreting the level of tension between theoretical predictions and H(z) data. Instead, we demonstrate that as of today, only Einstein-de Sitter is ruled out by the 2-point diagnostics at a level of significance exceeding ~ 3 sigma. The R_h=ct universe is slightly favoured over the remaining models, including LCDM and Chevalier-Polarski-Linder, though all of them (other than Einstein-de Sitter) are consistent to within 1 sigma with the measured mean of the Delta h(z_i,z_j) diagnostics.

astro-ph.CO↗

arXiv2016

Impact of a locally measured H_0 on the interpretation of cosmic chronometer data

Whereas many measurements in cosmology depend on the use of integrated distances or time, galaxies evolving passively on a time scale much longer than their age difference allow us to determine the expansion rate H(z) solely as a function of the redshift-time derivative dz/dt. These model-independent `cosmic chronometers' can therefore be powerful discriminators for testing different cosmologies. In previous applications, the available sources strongly disfavoured models (such as LambdaCDM) predicting a variable acceleration, preferring instead a steady expansion rate over the redshift range 0 < z < 2. A more recent catalog of 30 objects appears to suggest non-steady expansion. In this paper, we show that such a result is entirely due to the inclusion of a high, locally-inferred value of the Hubble constant H_0 as an additional datum in a set of otherwise pure cosmic-chronometer measurements. This H_0, however, is not the same as the background Hubble constant if the local expansion rate is influenced by a Hubble Bubble. Used on their own, the cosmic chronometers completely reverse this conclusion, favouring instead a constant expansion rate out to z ~ 2.

astro-ph.CO↗

arXiv2016

Cosmological Perturbations without Inflation

A particularly attractive feature of inflation is that quantum fluctuations in the inflaton field may have seeded inhomogeneities in the cosmic microwave background (CMB) and the formation of large-scale structure. In this paper, we demonstrate that a scalar field with zero active mass, i.e., with an equation of state rho+3p=0, where rho and p are its energy density and pressure, respectively, could also have produced an essentially scale-free fluctuation spectrum, though without inflation. This alternative mechanism is based on the Hollands-Wald concept of a minimum wavelength for the emergence of quantum fluctuations into the semi-classical universe. A cosmology with zero active mass does not have a horizon problem, so it does not need inflation to solve this particular (non) issue. In this picture, the 1-10 degree fluctuations in the CMB correspond almost exactly to the Planck length at the Planck time, firmly supporting the view that CMB observations may already be probing trans-Planckian physics.

astro-ph.CO↗

arXiv2016

Alcock-Paczynski Test with Model-independent BAO Data

Cosmological tests based on the statistical analysis of galaxy distributions usually depend on source evolution. An exception is the Alcock-Paczynski (AP) test, which is based on the changing ratio of angular to spatial/redshift size of (presumed) spherically-symmetric source distributions with distance. Intrinsic redshift distortions due to gravitational effects may also have an influence, but they can now be overcome with the inclusion of a sharp feature, such as the Baryonic Acoustic Oscillation (BAO) peak. Redshift distortions affect the amplitude of the peak, but impact its position only negligibly. As we shall show here, the use of this diagnostic, with new BAO peaks from SDSS-III/BOSS at average redshifts =0.38, 0.61 and 2.34, disfavors the current concordance (LCDM) model at 2.3 sigma. Within the context of expanding Friedmann-Robertson-Walker (FRW) cosmologies, these data instead favor the zero active mass equation-of-state, rho+3p=0, where rho and p are, respectively, the total density and pressure of the cosmic fluid, the basis for the R_h=ct universe.

astro-ph.CO↗

arXiv2016

The Linear Growth of Structure in the R_h=ct Universe

We use recently published redshift space distortion measurements of the cosmological growth rate, f sigma_8(z), to examine whether the linear evolution of perturbations in the R_h=ct cosmology is consistent with the observed development of large scale structure. We find that these observations favour R_h=ct over the version of LCDM optimized with the joint analysis of Planck and linear growth rate data, particularly in the redshift range 0 < z < 1, where a significant curvature in the functional form of f sigma_8(z) predicted by the standard model---but not by R_h=ct---is absent in the data. When LCDM is optimized using solely the growth rate measurements, however, the two models fit the observations equally well though, in this case, the low-redshift measurements find a lower value for the fluctuation amplitude than is expected in Planck LCDM. Our results strongly affirm the need for more precise measurements of f sigma_8(z) at all redshifts, but especially at z < 1.

astro-ph.CO↗

arXiv2016

The HII Galaxy Hubble Diagram Strongly Favors $R_{\rm h}=ct$ over $Λ$CDM

We continue to build support for the proposal to use HII galaxies (HIIGx) and giant extragalactic HII regions (GEHR) as standard candles to construct the Hubble diagram at redshifts beyond the current reach of Type Ia supernovae. Using a sample of 25 high-redshift HIIGx, 107 local HIIGx, and 24 GEHR, we confirm that the correlation between the emission-line luminosity and ionized-gas velocity dispersion is a viable luminosity indicator, and use it to test and compare the standard model $Λ$CDM and the $R_{\rm h}=ct$ Universe by optimizing the parameters in each cosmology using a maximization of the likelihood function. For the flat $Λ$CDM model, the best fit is obtained with $Ω_{\rm m}= 0.40_{-0.09}^{+0.09}$. However, statistical tools, such as the Akaike (AIC), Kullback (KIC) and Bayes (BIC) Information Criteria favor $R_{\rm h}=ct$ over the standard model with a likelihood of $\approx 94.8\%-98.8\%$ versus only $\approx 1.2\%-5.2\%$. For $w$CDM (the version of $Λ$CDM with a dark-energy equation of state $w_{\rm de}\equiv p_{\rm de}/ρ_{\rm de}$ rather than $w_{\rm de}=w_Λ=-1$), a statistically acceptable fit is realized with $Ω_{\rm m}=0.22_{-0.14}^{+0.16}$ and $w_{\rm de}= -0.51_{-0.25}^{+0.15}$ which, however, are not fully consistent with their concordance values. In this case, $w$CDM has two more free parameters than $R_{\rm h}=ct$, and is penalized more heavily by these criteria. We find that $R_{\rm h}=ct$ is strongly favored over $w$CDM with a likelihood of $\approx 92.9\%-99.6\%$ versus only $0.4\%-7.1\%$. The current HIIGx sample is already large enough for the BIC to rule out $Λ$CDM/$w$CDM in favor of $R_{\rm h}=ct$ at a confidence level approaching $3σ$.

astro-ph.CO↗

arXiv2016

Definitive Test of the R_h=ct Universe Using Redshift Drift

The redshift drift of objects moving in the Hubble flow has been proposed as a powerful model-independent probe of the underlying cosmology. A measurement of the first and second order redshift derivatives appears to be well within the reach of upcoming surveys using ELT-HIRES and the SKA Phase 2 array. Here we show that an unambiguous prediction of the R_h=ct cosmology is zero drift at all redshifts, contrasting sharply with all other models in which the expansion rate is variable. For example, multi-year monitoring of sources at redshift z=5 with the ELT-HIRES is expected to show a velocity shift Delta v = -15 cm/s/yr due to the redshift drift in Planck LCDM, while Delta v=0 cm/s/yr in R_h=ct. With an anticipated ELT-HIRES measurement error of +/-5 cm/s/yr after 5 years, these upcoming redshift drift measurements might therefore be able to differentiate between R_h=ct and Planck LCDM at ~3 sigma, assuming that any possible source evolution is well understood. Such a result would provide the strongest evidence yet in favour of the R_h=ct cosmology. With a 20-year baseline, these observations could favor one of these models over the other at better than 5 sigma.

astro-ph.CO↗

arXiv2016

The Zero Active Mass Condition in Friedmann-Robertson-Walker Cosmologies

Many cosmological measurements today suggest that the Universe is expanding at a constant rate. This is inferred from the observed age versus redshift relationship and various distance indicators, all of which point to a cosmic equation of state (EoS) p=-rho/3, where rho and p are, respectively, the total energy density and pressure of the cosmic fluid. It has recently been shown that this result is not a coincidence and simply confirms the fact that the symmetries in the Friedmann-Robertson-Walker (FRW) metric appear to be viable only for a medium with zero active mass, i.e., rho+3p=0. In their latest paper, however, Kim, Lasenby and Hobson have provided what they believe to be a counter argument to this conclusion. Here, we show that these authors are merely repeating the conventional mistake of incorrectly placing the observer simultaneously in a comoving frame, where the lapse function g_tt is coordinate dependent when rho+3p is not 0, and a supposedly different, free-falling frame, in which g_tt=1, implying no time dilation. We demonstrate that the Hubble flow is not inertial when rho+3p is not 0, so the comoving frame is generally not in free fall, even though in FRW, the comoving and free-falling frames are supposed to be identical at every spacetime point. So this confusion of frames not only constitutes an inconsistency with the fundamental tenets of general relativity but, additionally, there is no possibility of using a gauge transformation to select a set of coordinates for which g_tt=1 when rho+3p is not 0.

physics.gen-ph↗