Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
209
result(s) for
"Melia, Fulvio"
Sort by:
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
Quantum fluctuations at the Planck scale
The recently measured cutoff,
k
min
=
4.34
±
0.50
/
r
cmb
(with
r
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
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,
ρ
ϕ
+
3
p
ϕ
=
0
. Quite revealingly, the observed amplitude of the temperature anisotropies requires the quantum fluctuations in
ϕ
to have classicalized at
∼
3.5
×
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,
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
=
c
t
universe, which has thus far been highly successful in resolving many other problems or inconsistencies in
Λ
CDM. 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.
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
m
c
2
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
R
h
=
c
t
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
=
c
t
profile than that implied by
Λ
CDM, for which adiabatic cooling would have established a spin temperature
T
s
(
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
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,
Λ
CDM, and the
R
h
=
c
t
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
σ
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
=
c
t
being the correct cosmology is somewhat higher than that of
Λ
CDM, 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
=
c
t
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.
Journal Article
A solution to the electroweak horizon problem in the Rh=ct universe
Particle physics suggests that the Universe may have undergone several phase transitions, including the well-known inflationary event associated with the separation of the strong and electroweak forces in grand unified theories. The accelerated cosmic expansion during this transition, at cosmic time
t
∼
10
-
36
-
10
-
33
s, is often viewed as an explanation for the uniformity of the CMB temperature,
T
, which would otherwise have required inexplicable initial conditions. With the discovery of the Higgs particle, it is now quite likely that the Universe underwent another (electroweak) phase transition, at
T
=
159.5
±
1.5
GeV – roughly
∼
10
-
11
s after the big bang. During this event, the fermions gained mass and the electric force separated from the weak force. There is currently no established explanation, however, for the apparent uniformity of the vacuum expectation value of the Higgs field which, like the uniformity in
T
, gives rise to its own horizon problem in standard
Λ
CDM cosmology. We show in this paper that a solution to the electroweak horizon problem may be found in the choice of cosmological model, and demonstrate that this issue does not exist in the alternative Friedmann–Robertson–Walker cosmology known as the
R
h
=
c
t
universe.
Journal Article
THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTER
The inner few parsecs at the Galactic Center have come under intense
scrutiny in recent years, in part due to the exciting broad-band observations
of this region, but also because of the growing interest from theorists
motivated to study the physics of black hole accretion, magnetized gas
dynamics, and unusual star formation. The Galactic Center is now known to
contain arguably the most compelling supermassive black hole candidate,
weighing in at a little over 2.6 million suns. Its interaction with the nearby
environment, comprised of clusters of evolved and young stars, a molecular
dusty ring, ionized gas streamers, diffuse hot gas, and a hypernova remnant, is
providing a wealth of accretion phenomenology and high-energy processes for
detailed modeling. In this review, we summarize the latest observational
results and focus on the physical interpretation of the most intriguing object
in this region-the compact radio source Sgr A*, thought to be the
radiative manifestation of the supermassive black hole.
Journal Article