arXiv subjects

Fulvio Melia

Publications and source records attributed to Fulvio Melia.

At least 37 records · Page 2Linked to original sources

arXiv2023

The Cosmic Timeline Implied by the JWST High-redshift Galaxies

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 LCDM, 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 LCDM. 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 LCDM that incorporates the `zero active mass' condition from general relativity. This model has accounted for many cosmological data better than LCDM, 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 observations over the current standard model.

astro-ph.CO↗

arXiv2022

Model selection using time-delay lenses

The sample of time-delay gravitational lenses appropriate for studying the geometry of the Universe continues to grow as dedicated campaigns, such as the Dark Energy Survey, the VST ATLAS survey, and the Large Synoptic Survey Telescope, complete their census of high-redshift sources. This catalog now includes hundreds of strong lensing systems, at least 31 of which have reasonably accurate time delay measurements. In this paper, we use them to compare the predictions of two competing Friedmann-Lemaitre-Robertson-Walker models: flat LCDM, characterized by two adjustable parameters (H_0 and Omega_m), and the R_h=ct universe (with H_0 as the single free variable). Over the past decade, the latter has accounted for the data better than the standard model, most recently the emergence of well-formed galaxies discovered by JWST at cosmic dawn. Here we show that the current sample of time-delay lenses favours R_h=ct with a likelihood of ~84% versus ~16% for the standard model. This level of accuracy will greatly improve as the ongoing surveys uncover many thousands additional lens systems over the next several years.

astro-ph.CO↗

arXiv2022

A Pop III generated dust screen at z~16

The search for alternative cosmological models is largely motivated by the growing discordance between the predictions of LCDM and the ever improving observations, such as the disparity in the value of H_0 measured at low and high redshifts. One model, in particular, known as the R_h=ct universe, has been highly successful in mitigating or removing all of the inconsistencies. In this picture, however, the anisotropies in the cosmic microwave background (CMB) would have emerged at a redshift z ~ 16, rather than via fluctuations in the recombination zone at z~1080. We demonstrate here that a CMB created in the early Universe, followed by scattering through a Pop III generated dust screen, cannot yet be ruled out by the current data. Indeed, the Planck measurements provide a hint of a ~2-4% frequency dependence in the CMB power spectrum, which would be naturally explained as a variation in the optical depth through the dust, but not a Thomson scattering-dominated recombination environment. Upcoming measurements should be able to easily distinguish between these two scenarios, e.g., via the detection of recombination lines at z~1080, which would completely eliminate the dust reprocessing idea.

astro-ph.CO↗

arXiv2022

Constraining f(R) models with cosmic chronometers and the HII galaxy Hubble diagram

We consider several well-known f(R) cosmological models and constrain their parameters, namely the deviation parameter b and the cosmological parameters Ω_m and h. We first obtain analytical approximations for the Hubble rate H(z) and the luminosity distance d_L(z) in terms of these parameters, and then test these against the observational expansion rate derived from cosmic chronometers and the distance modulus in the HII galaxy Hubble diagram, obtained in a model-independent way using Gaussian Processes (GP). We first optimize the models based solely on the cosmic chronometers and then repeat this process with a joint analysis using both the cosmic chronometers and HII galaxies.

astro-ph.CO↗

arXiv2022

The Electroweak Horizon Problem

Spontaneously broken symmetries in particle physics may have produced several phase transitions in cosmology, e.g., at the GUT energy scale (~10^15 GeV), resulting in a quasi-de Sitter inflationary expansion, solving the background temperature horizon problem. This transition would have occurred at t~10^-36 to 10^-33 seconds, leading to a separation of the strong and electroweak forces. The discovery of the Higgs boson confirms that the Universe must have undergone another phase transition at the electroweak (EWPT) scale 159.5+/-1.5 GeV, about 10^-11 seconds later, when fermions and the W^+/- and Z^0 bosons gained mass, leading to the separation of the electric and weak forces. But today the vacuum expectation value (vev) of the Higgs field appears to be uniform throughout the visible Universe, a region much larger than causally-connected volumes at the EWPT. The discovery of the Higgs boson thus creates another serious horizon problem for LCDM, for which there is currently no established theoretical resolution. The EWPT was a smooth crossover, however, so previously disconnected electroweak vacuua might have homogenized as they gradually came into causal contact. But using the known Higgs potential and vev, we estimate that this process would have taken longer than the age of the Universe, so it probably could not have mitigated the emergence of different standard model parameters across the sky. The EWPT horizon problem thus argues against the expansion history of the early Universe predicted by standard cosmology.

physics.gen-ph↗

arXiv2022

Exploring the Hubble Tension and Spatial Curvature from the Ages of Old Astrophysical Objects

We use the age measurements of 114 old astrophysical objects (OAO) in the redshift range $0\lesssim z\lesssim 8$ to explore the Hubble tension. The age of the Universe at any $z$ is inversely proportional to the Hubble constant, $H_0$, so requiring the Universe to be older than the OAO it contains at any $z$ will lead to an upper limit on $H_0$. Assuming flat $Λ$CDM and setting a Gaussian prior on the matter density parameter $Ω_{\rm m}=0.315\pm0.007$ informed by {\it Planck}, we obtain a 95\% confidence-level upper limit of $H_0<70.6 \rm{~km} \rm{~s}^{-1} \rm{~Mpc}^{-1}$, representing a $2σ$ tension with the measurement using the local distance ladder. We find, however, that the inferred upper limit on $H_{0}$ depends quite sensitively on the prior for $Ω_{\rm m}$, and the Hubble tension between early-time and local measurements of $H_{0}$ may be due in part to the inference of both $Ω_{\rm m}$ and $H_0$ in {\it Planck}, while the local measurement uses only $H_{0}$. The age-redshift data may also be used for cosmological model comparisons. We find that the $R_{\rm h}=ct$ universe accounts well for the data, with a reasonable upper limit on $H_{0}$, while Einstein-de Sitter fails to pass the cosmic-age test. Finally, we present a model-independent estimate of the spatial curvature using the ages of 61 galaxies and the luminosity distances of 1,048 Pantheon Type Ia supernovae. This analysis suggests that the geometry of the Universe is marginally consistent with spatial flatness at a confidence level of $1.6σ$, characterized as $Ω_{k}=0.43^{+0.27}_{-0.27}$.

astro-ph.CO↗

arXiv2022

Initial Energy of a Spatially Flat Universe -- a Hint of its Possible Origin

The evidence for a Big Bang origin of the Universe is truly compelling, though its cause remains a complete mystery. As the cosmic spacetime is revealed to us with ever improving detail, however, we are beginning to refine the range of its possible initial conditions -- at least within the framework of current physical theories. The Universe, it seems, is spatially flat, and here we discuss in clear, straightforward terms why this trait implies a cosmos with zero kinetic plus gravitational energy, though apparently not zero total energy. Such an outcome has far reaching consequences because of the possibility that the Universe may have begun its existence as a quantum fluctuation. Was this from `nothing,' or perhaps a pre-existing vacuum? A non-zero total energy would seemingly preclude the former scenario, but not necessarily the latter, though this would then raise the question of how a fluctuation with non-zero energy could have lived long enough, or classicalized, for us to see it 13.5 billion years later. The high-precision measurement of the Universe's spatial curvature may thus constitute the first tangible piece of evidence impacting a possible quantum beginning.

astro-ph.CO↗

arXiv2021

A Measurement of the Cosmic Expansion Within our Lifetime

The most exciting future observation in cosmology will feature a monitoring of the cosmic expansion in real time, unlike anything that has ever been attempted before. This campaign will uncover crucial physical properties of the various constituents in the Universe, and perhaps answer a simpler question concerning whether or not the cosmic expansion is even accelerating. An unambiguous yes/no response to this query will significantly impact cosmology, of course, but also the standard model of particle physics. Here, we discuss -- in a straightforward way -- how to understand the so-called `redshift drift' sought by this campaign, and why its measurement will help us refine the standard-model parameters if the answer is `yes.' A `no' answer, on the other hand, could be more revolutionary, in the sense that it might provide a resolution of several long-standing problems and inconsistencies in our current cosmological models. An outcome of zero redshift drift, for example, would obviate the need for a cosmological constant and render inflation completely redundant.

gr-qc↗

arXiv2021

Hint of a truncated primordial spectrum from the CMB large-scale anomalies

Several satellite missions have uncovered a series of potential anomalies in the fluctuation spectrum of the cosmic microwave background temperature, including: (1) an unexpectedly low level of correlation at large angles, manifested via the angular correlation function, C(theta); and (2) missing power in the low multipole moments of the angular power spectrum, C_ell. Their origin is still debated, however, due to a persistent lack of clarity concerning the seeding of quantum fluctuations in the early Universe. A likely explanation for the first of these appears to be a cutoff, k_min=(3.14 +/- 0.36) x 10^{-4} Mpc^{-1}, in the primordial power spectrum, P(k). Our goal in this paper is twofold: (1) we examine whether the same k_min can also self-consistently explain the missing power at large angles, and (2) we confirm that the of this cutoff in P(k) does not adversely affect the remarkable consistency between the prediction of Planck-LCDM and the Planck measurements at ell > 30. We use the publicly available code CAMB to calculate the angular power spectrum, based on a line-of-sight approach. The code is modified slightly to include the additional parameter (i.e., k_min) characterizing the primordial power spectrum. In addition to this cutoff, the code optimizes all of the usual standard-model parameters. In fitting the angular power spectrum, we find an optimized cutoff, k_min = 2.04^{+1.4}_{-0.79} x 10^{-4} Mpc^{-1}, when using the whole range of ell's, and k_min=3.3^{+1.7}_{-1.3} x 10^{-4} Mpc^{-1}, when fitting only the range ell < 30, where the Sachs-Wolfe effect is dominant. These are fully consistent with the value inferred from C(theta), suggesting that both of these large-angle anomalies may be due to the same truncation in P(k).

astro-ph.CO↗

arXiv2021

The Origin of Rest-mass Energy

Today we have a solid, if incomplete, physical picture of how inertia is created in the standard model. We know that most of the visible baryonic `mass' in the Universe is due to gluonic back-reaction on accelerated quarks, the latter of which attribute their own inertia to a coupling with the Higgs field -- a process that elegantly and self-consistently also assigns inertia to several other particles. But we have never had a physically viable explanation for the origin of rest-mass energy, in spite of many attempts at understanding it towards the end of the nineteenth century, culminating with Einstein's own landmark contribution in his Annus Mirabilis. Here, we introduce to this discussion some of the insights we have garnered from the latest cosmological observations and theoretical modeling to calculate our gravitational binding energy with that portion of the Universe to which we are causally connected, and demonstrate that this energy is indeed equal to mc^2 when the inertia m is viewed as a surrogate for gravitational mass.

gr-qc↗

arXiv2021

Classicalization of Quantum Fluctuations at the Planck Scale in the R_h=ct Universe

The quantum to classical transition of fluctuations in the early universe is still not completely understood. Some headway has been made incorporating the effects of decoherence and the squeezing of states, though the methods and procedures continue to be challenged. But new developments in the analysis of the most recent Planck data suggest that the primordial power spectrum has a cutoff associated with the very first quantum fluctuation to have emerged into the semi-classical universe from the Planck domain at about the Planck time. In this paper, we examine the implications of this result on the question of classicalization, and demonstrate that the birth of quantum fluctuations at the Planck scale would have been a `process' supplanting the need for a `measurement' in quantum mechanics. Emerging with a single wavenumber, these fluctuations would have avoided the interference between different degrees of freedom in a superposed state. Moreover, the implied scalar-field potential had an equation-of-state consistent with the zero active mass condition in general relativity, allowing the quantum fluctuations to emerge in their ground state with a time-independent frequency. They were therefore effectively quantum harmonic oscillators with classical correlations in phase space from the very beginning.

gr-qc↗

arXiv2021

The Anomalous 21-cm Absorption at High Redshifts

The EDGES collaboration has reported the detection of a global 21-cm signal with a plateau centered at 76 MHz (i.e., redshift 17.2), with an amplitude of 500^(+200)_(-500) mK. This anomalous measurement does not comport with standard cosmology, which can only accommodate an amplitude < 230 mK. Nevertheless, the line profile's redshift range (15 < z < 20) suggests a possible link to Pop III star formation and an implied evolution out of the `dark ages.' Given this tension with the standard model, we here examine whether the observed 21-cm signal is instead consistent with the results of recent modeling based on the alternative Friedmann-Lemaitre-Robertson-Walker cosmology known as the R_h=ct universe, showing that--in this model--the CMB radiation might have been rethermalized by dust ejected into the IGM by the first-generation stars at redshift z < 16. We find that the requirements for this process to have occurred would have self-consistently established an equilibrium spin temperature T_s~3.4 K in the neutral hydrogen, via the irradiation of the IGM by deep penetrating X-rays emitted at the termination shocks of Pop III supernova remnants. Such a dust scenario has been strongly ruled out for the standard model, so the spin temperature (~3.3 K) inferred from the 21-cm absorption feature appears to be much more consistent with the R_h=ct profile than that implied by LCDM, for which adiabatic cooling would have established a spin temperature T_s(z=17.2)~6 K.

astro-ph.CO↗

arXiv2021

Thermodynamics of the R_h=ct Universe: A Simplification of Cosmic Entropy

In the standard model of cosmology, the Universe began its expansion with an anomalously low entropy, which then grew dramatically to much larger values consistent with the physical conditions at decoupling, roughly 380,000 years after the Big Bang. There does not appear to be a viable explanation for this `unnatural' history, other than via the generalized second law of thermodynamics (GSL), in which the entropy of the bulk, S_bulk, is combined with the entropy of the apparent (or gravitational) horizon, S_h. This is not completely satisfactory either, however, since this approach seems to require an inexplicable equilibrium between the bulk and horizon temperatures. In this paper, we explore the thermodynamics of an alternative cosmology known as the R_h=ct universe, which has thus far been highly successful in resolving many other problems or inconsistencies in LCDM. We find that S_bulk is constant in this model, eliminating the so-called initial entropy problem simply and elegantly. The GSL may still be relevant, however, principally in selecting the arrow of time, given that S_h ~ t^2 in this model.

astro-ph.CO↗

arXiv2021

Test of the cosmic distance duality relation for arbitrary spatial curvature

The cosmic distance duality relation (CDDR), eta(z)=(1+z)^2 d_A(z)/d_L(z)=1, is one of the most fundamental and crucial formulae in cosmology. This relation couples the luminosity and angular diameter distances, two of the most often used measures of structure in the Universe. We here propose a new model-independent method to test this relation, using strong gravitational lensing (SGL) and the high-redshift quasar Hubble diagram reconstructed with a Bezier parametric fit. We carry out this test without pre-assuming a zero spatial curvature, adopting instead the value Omega_K=0.001 +/- 0.002 optimized by Planck in order to improve the reliability of our result. We parametrize the CDDR using eta(z)=1 + eta_0 z, 1 + eta_1 z + eta_2 z^2 and 1 + eta_3 z/(1+z), and consider both the SIS and non-SIS lens models for the strong lensing. Our best fit results are: eta_0=-0.021^{+0.068}_{-0.048}, eta_1=-0.404^{+0.123}_{-0.090}, eta_2=0.106^{+0.028}_{-0.034}, and eta_3=-0.507^{+0.193}_{-0.133} for the SIS model, and eta_0=-0.109^{+0.044}_{-0.031} for the non-SIS model. The measured eta(z), based on the Planck parameter Omega_K, is essentially consistent with the value (=1) expected if the CDDR were fully respected. For the sake of comparison, we also carry out the test for other values of Omega_K, but find that deviations of spatial flatness beyond the Planck optimization are in even greater tension with the CDDR. Future measurements of SGL may improve the statistics and alter this result but, as of now, we conclude that the CDDR favours a flat Universe.

astro-ph.CO↗

arXiv2020

Structure formation and the matter power-spectrum in the R_h=ct universe

Inflation drives quantum fluctuations beyond the Hubble horizon, freezing them out before the small-scale modes re-enter during the radiation dominated epoch, and subsequently decay, while large-scale modes re-enter later during the matter dominated epoch and grow. This distinction shapes the matter power spectrum and provides observational evidence in support of the standard model. In this paper, we demonstrate that another mechanism, based on the fluctuation growth in the R_h=ct universe, itself an FLRW cosmology with the added constraint of zero active mass (i.e., rho+3p=0), also accounts very well for the observed matter power spectrum, so this feature is not unique to LambdaCDM. In R_h=ct, the shape of the matter power spectrum is set by the interplay between the more rapid decay of the gravitational potential for the smaller mode wavelengths and the longer dynamical timescale for the larger wavelengths. This combination produces a characteristic peak that grows in both amplitude and mode number as a function of time. Today, that peak lies at k approx 0.02 Mpc^-1, in agreement with the Ly-alpha and Planck data. But there is no need of an inflationary expansion, and a complicated epoch dependence as one finds in LambdaCDM.

astro-ph.CO↗

arXiv2020

A resolution of the Trans-Planckian problem in the R_h=ct universe

The recent measurement of a cutoff k_min in the fluctuation power spectrum P(k) 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 k_min may be interpreted more successfully in the R_h=ct 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.

astro-ph.CO↗

arXiv2020

Assessing Cosmic Acceleration with the Alcock-Paczynski Effect in the SDSS-IV Quasar Catalog

The geometry of the Universe may be probed using the Alcock-Paczynski (AP) effect, in which the observed redshift size of a spherical distribution of sources relative to its angular size varies according to the assumed cosmological model. Past applications of this effect have been limited, however, by a paucity of suitable sources and mitigating astrophysical factors, such as internal redshift-space distortions and poorly known source evolution. In this Letter, we introduce a new test based on the AP effect that avoids the use of spatially bound systems, relying instead on sub-samples of quasars at redshifts z < 1.5 in the Sloan Digital Sky Survey IV, with a possible extension to higher redshifts and improved precision when this catalog is expanded by upcoming surveys. We here use this method to probe the redshift-dependent expansion rate in three pertinent Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies: LCDM, which predicts a transition from deceleration to acceleration at z ~ 0.7; Einstein-de Sitter, in which the Universe is always decelerating; and the R_h=ct universe, which expands at a constant rate. LCDM is consistent with these data, but R_h=ct is favoured overall.

astro-ph.CO↗

arXiv2020

Reassessing Dust's Role in Forming the CMB

The notion that dust might have formed the cosmic microwave background (CMB) has been strongly refuted on the strength of four decades of observation and analysis, in favour of recombination at a redshift z ~ 1080. But tension with the data is growing in several other areas, including measurements of the Hubble constant H(z) and the BAO scale, which directly or indirectly impact the physics at the surface of last scattering (LSS). The R_h=ct universe resolves at least some of this tension. We show in this paper that---if the BAO scale is in fact equal to the acoustic horizon---the redshift of the LSS in this cosmology is z_cmb ~ 16, placing it within the era of Pop III star formation, prior to the epoch of reionization at 15 > z > 6. Quite remarkably, the measured values of z_cmb and H_0 = H(0) in this model are sufficient to argue that the CMB temperature today ought to be ~ 3 K, so H_0 and the baryon to photon ratio are not independent free parameters. This scenario might have resulted from rethermalization of the CMB photons by dust, presumably supplied to the interstellar medium by the ejecta of Pop III stars. Dust rethermalization may therefore yet resurface as a relevant ingredient in the R_h=ct universe. Upcoming high sensitivity instruments should be able to readily distinguish between the recombination and dust scenarios by either (i) detecting recombination lines at z ~ 1080, or (ii) establishing a robust frequency-dependent variation of the CMB power spectrum at the level of ~ 2-4% across the sampled frequency range.

astro-ph.CO↗