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"Melia, Fulvio"
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High-energy astrophysics
This textbook covers all the essentials, weaving together the latest theory with the experimental techniques, instrumentation, and observational methods astronomers use to study high-energy radiation from space.
Book
The cosmic timeline implied by the highest redshift quasars
The conventional picture of supermassive black-hole growth in the standard model had already been seriously challenged by the emergence of
∼
10
9
M
⊙
quasars at
z
∼
7.5
, conflicting with the predicted formation of structure in the early
Λ
CDM Universe. But the most recent
JWST
discovery of a
∼
10
8
M
⊙
source at
z
∼
10.1
argues even more strongly against the possibility that these black holes were created in Pop II or III supernovae, followed by Eddington-limited accretion. Attempts at resolving this anomaly have largely focused on the formation of seeds via an exotic, direct collapse of primordial gas to an initial mass
∼
10
5
M
⊙
– a process that has never been seen anywhere in the cosmos. Our goal in this
Letter
is to demonstrate that the emergence of these black holes is instead fully consistent with standard astrophysics in the context of the alternative Friedmann–Lemaître–Robertson–Walker cosmology known as the
R
h
=
c
t
universe. We show that, while the predicted evolution in the standard model is overly compressed, the creation, growth and appearance of such high-
z
quasars fall comfortably within the evolutionary history in this cosmology, thereby adding considerable observational support to the existing body of evidence favoring it over the standard scenario.
Journal Article
Quantum fluctuations at the Planck scale
The recently measured cutoff, \\[k_\\mathrm{min}=4.34\\pm 0.50/r_{\\mathrm{cmb}}\\] (with \\[r_{\\mathrm{cmb}}\\] the comoving distance to the last scattering surface), in the fluctuation spectrum of the cosmic microwave background, appears to disfavor slow-roll inflation and the associated transition of modes across the horizon. We show in this Letter that \\[k_{\\mathrm{min}}\\] instead corresponds to the first mode emerging out of the Planck domain into the semi-classical universe. The required scalar-field potential is exponential, though not inflationary, and satisfies the zero active mass condition, \\[\\rho _\\phi +3p_\\phi =0\\]. Quite revealingly, the observed amplitude of the temperature anisotropies requires the quantum fluctuations in \\[\\phi \\] to have classicalized at \\[\\sim 3.5\\times 10^{15}\\] GeV, consistent with the energy scale in grand unified theories. Such scalar-field potentials are often associated with Kaluza–Klein cosmologies, string theory and even supergravity.
Journal Article
Thermodynamics of the Rh=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, Sbulk, is combined with the entropy of the apparent (or gravitational) horizon, Sh. 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 Rh=ct universe, which has thus far been highly successful in resolving many other problems or inconsistencies in ΛCDM. We find that Sbulk 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 Sh∝t2 in this model.
Journal Article
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 mc2 when the inertia m is viewed as a surrogate for gravitational mass.
Journal Article
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 500-500+200 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–Lemaître–Robertson–Walker cosmology known as the Rh=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 Ts≈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 Rh=ct profile than that implied by ΛCDM, for which adiabatic cooling would have established a spin temperature Ts(z=17.2)∼6 K.
Journal Article
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 = -ρ/3, where ρ 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., ρ ＋ 3p -= 0. In their latest paper, however, Kim, Lasenby and Hobson （2016） 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 gtt is coordinate dependent when ρ ＋ 3p ≠ 0, and a supposedly different, free- falling frame, in which gtt = i, implying no time dilation. We demonstrate that the Hubble flow is not inertial when ρ ＋ 3p ≠ 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 gtt = 1 when ρ＋ 3p ≠ 0.
Journal Article
The energy conditions and model selection in the local Universe
The four principal energy conditions (ECs) in general relativity prohibit negative energies, repulsive gravity and superluminal energy flows. One must invoke exotic matter to violate any one of these, yet
Λ
CDM does so quite prominently during inflation and in the epoch of dark energy dominance. In this paper, we carry out model selection between the standard model and the
R
h
=
c
t
universe using a combination of H
II
galaxy and cosmic chronometer measurements in the local Universe, and directly compare the results to the constraints imposed by the ECs. We find that the latter cosmology is not only strongly favored by these data, with a likelihood of
∼
92
%
versus only
∼
8
%
for the former, but that its optimized fit is fully compliant with all four ECs, while
Λ
CDM’s best fit violates the so-called strong energy condition at
z
≲
2
.
Journal Article
A Candid Assessment of Standard Cosmology
Modern cosmology is broadly based on the Cosmological principle, which assumes homogeneity and isotropy as its foundational pillars. Thus, there is not much debate about the metric (i.e., Friedmann-Lemaître-Robertson-Walker; FLRW) one should use to describe the cosmic spacetime. But Einstein’s equations do not unilaterally constrain the constituents in the cosmic fluid, which directly determine the expansion factor appearing in the metric coefficients. As its name suggests, ΛCDM posits that the energy density is dominated by a blend of dark energy (typically a cosmological constant, Λ), cold dark matter (and a “contamination” of baryonic matter) and radiation. Many would assert that we have now reached the age of “precision” cosmology, in which measurements are made merely to refine the excessively large number of free parameters characterizing its empirical underpinnings. But this mantra glosses over a growing body of embarrassingly significant failings, not just “tension” as is sometimes described, as if to somehow imply that a resolution will eventually be found. In this paper, we take a candid look at some of the most glaring conflicts between the standard model, the observations, and several foundational principles in quantum mechanics, general relativity and particle physics. One cannot avoid the conclusion that the standard model needs a complete overhaul in order to survive.
Journal Article