3,264 research outputs found
Spontaneously broken symmetry restoration of quantum fields in the vicinity of neutral and electrically charged black holes
We consider the restoration of a spontaneously broken symmetry of an
interacting quantum scalar field around neutral, i.e., Schwarzschild, and
electrically charged, i.e., Reissner-Nordstr\"om, black holes in four
dimensions. This is done through a semiclassical self-consistent procedure, by
solving the system of non-linear coupled equations describing the dynamics of
the background field and the vacuum polarization. The black hole at its own
horizon generates an indefinitely high temperature which decreases to the
Hawking temperature at infinity. Due to the high temperature in its vicinity,
there forms a bubble around the black hole in which the scalar field can only
assume a value equal to zero, a minimum of energy. Thus, in this region the
symmetry of the energy and the field is preserved. At the bubble radius, there
is a phase transition in the value of the scalar field due to a spontaneous
symmetry breaking mechanism. Indeed, outside the bubble radius the temperature
is low enough such that the scalar field settles with a nonzero value in a new
energy minimum, indicating a breaking of the symmetry in this outer region.
Conversely, there is symmetry restoration from the outer region to the inner
bubble close to the horizon. Specific properties that emerge from different
black hole electric charges are also noteworthy. It is found that colder black
holes, i.e., more charged ones, have a smaller bubble length of restored
symmetry. In the extremal case the bubble has zero length, i.e., there is no
bubble. Additionally, for colder black holes, it becomes harder to excite the
quantum field modes, so the vacuum polarization has smaller values. In the
extremal case, the black hole temperature is zero and the vacuum polarization
is never excited.Comment: 16 pages, 4 figure
Entropy of an extremal electrically charged thin shell and the extremal black hole
There is a debate as to what is the value of the the entropy of extremal
black holes. There are approaches that yield zero entropy , while there
are others that yield the Bekenstein-Hawking entropy , in Planck
units. There are still other approaches that give that is proportional to
or even that is a generic well-behaved function of . Here
is the black hole horizon radius and is its horizon area.
Using a spherically symmetric thin matter shell with extremal electric charge,
we find the entropy expression for the extremal thin shell spacetime. When the
shell's radius approaches its own gravitational radius, and thus turns into an
extremal black hole, we encounter that the entropy is , i.e., the
entropy of an extremal black hole is a function of alone. We speculate
that the range of values for an extremal black hole is .Comment: 11 pages, minor changes, added references, matches the published
versio
Qubit Picture of Virtual Particles
We show that virtual particles, despite being unobservable, can be described
by quantum operators which can be interpreted under certain conditions as valid
qubit quantum states. For a single virtual fermion, we prove that such a state
is a separable mixed 2-qubit state with a well-defined finite temperature. For
spin-1 virtual bosons, we find them to be associated to 4-qubit operators which
can be interpreted as quantum states for some gauges. We also study the
creation of virtual pairs of fermions, where the pair is shown to be associated
to an entangled 4-qubit operator, and show the corresponding quantum circuit.
Finally, we prove that renormalization does not structurally affect these
results. These findings represent new connections between quantum field theory,
quantum information and quantum thermodynamics.Comment: Published version in PRA. The paper was restructured with new
sections added. The spin 1 boson and fermionic pair creation cases were
introduce
Molecular mechanisms of salamander limb regeneration
Salamanders, like newts and axolotls, stand out among adult vertebrates for their
outstanding capacity to regenerate whole body parts and restore complex structures
upon injury. One of the best-known examples is their ability to fully regenerate a
functional limb. Despite the important progress in the field, our understanding of the
molecular cues that control limb regeneration is still limited. In this thesis, I focus on
the mechanisms by which skeletal muscle stimulates limb regeneration. Skeletal
muscle is particularly interesting because, in newts, it contributes to limb regeneration
by dedifferentiation. This unique process is characterized by fragmentation of the
multinucleated myofiber and subsequent cell cycle reentry by the derived
mononucleate progeny.
In Paper I, we sequenced and edited the ~20 Gigabases genome of the Iberian ribbed
newt Pleurodeles waltl, a commonly used species for regeneration studies in
salamanders. Using CRISPR/Cas9 technology we perturbed two key transcription
factors (Pax3 and Pax7) that are involved in skeletal muscle development and
regeneration in vertebrates. We found that contrary to mammals, in which Pax7
expression by skeletal muscle stem cells is indispensable for regeneration, muscle
regeneration was not altered when Pax7 gene was mutated in newts. Moreover, we
observed that embryonic stem cell-specific microRNAs (mir-93b and mir-427), as well
as Harbinger DNA transposons carrying the Myb-like proto-oncogene have expanded
dramatically in the Pleurodeles waltl genome and are co-expressed during limb
regeneration. This study provides a foundation for comparative genomic studies that
could improve our understanding of the uneven distribution of regenerative capacities
among vertebrates.
In Paper II, we identified a microRNA, miR-10b-5p, which is highly abundant in muscle
tissue across species and downregulated during early limb regeneration in newts. In
contrast, miR-10b-5p displayed the opposite regulation in mammalian cultured
myotubes, when these were induced to dedifferentiate. To investigate a possible
function of miR-10b-5p in newt limb regeneration, we overexpressed it by mimic
injection. We found that such manipulation of miR-10b-5p levels during the initial
stages of regeneration slowed down the regeneration process. Moreover, we
observed that overexpression of miR-10b-5p decreased the number of cycling cells
and counteracted blastema growth. The identification of miR-10b-5p targets will be
an important task for future studies.
In Paper III, we showed that blood clotting proteases cleaved and activated bloodderived
bone morphogenetic proteins (BMPs) to promote BMP signaling-dependent
cell cycle re-entry by myofiber progeny. In particular, we found that protease-activated
BMP4/7 heterodimers which were present in serum, strongly induced myotube cell
cycle re-entry, with protease cleavage yielding a 30-fold potency increase of BMP4/7
compared with canonical BMP4/7. Additionally, we observed that inhibition of BMP
signaling, via muscle-specific dominant-negative receptor expression, reduced cell
cycle re-entry in vitro and in vivo. Furthermore, in vivo inhibition of serine protease
activity depressed cell cycle re-entry, which in turn could be rescued by cleavedmimic
BMP. This work provides a new molecular mechanism for the reversal of the
differentiated state in muscle.
In Paper IV, we carried out a comparative analysis of centrosome dynamics in mouse
and newt muscle cells. We showed, through a detailed characterization of different
centrosome components, that centrosomes were gradually disassembled during
muscle differentiation in mammals. We also provided new insights into the underlying
mechanisms and variations in gene expression during that inactivation process. On
the other hand, we found that salamanders retained several centrosome components
even in mature myofibers. Moreover, we observed that not only the centrosomes were
maintained in salamander muscle, but they also appeared to be active as microtubule
organizing centers. This study has elucidated fundamental differences between
vertebrates at cellular level, which might help us to understand why species differ in
their ability to produce regenerative progenitor cells
- …