1,540 research outputs found
Bubbles of Nothing in Flux Compactifications
We construct a simple AdS_4 x S^1 flux compactification stabilized by a
complex scalar field winding the extra dimension and demonstrate an instability
via nucleation of a bubble of nothing. This occurs when the Kaluza -- Klein
dimension degenerates to a point, defining the bubble surface. Because the
extra dimension is stabilized by a flux, the bubble surface must be charged, in
this case under the axionic part of the complex scalar. This smooth geometry
can be seen as a de Sitter topological defect with asymptotic behavior
identical to the pure compactification. We discuss how a similar construction
can be implemented in more general Freund -- Rubin compactifications.Comment: 16 pages, 5 figures References adde
Cosmic string formation by flux trapping
We study the formation of cosmic strings by confining a stochastic magnetic
field into flux tubes in a numerical simulation. We use overdamped evolution in
a potential that is minimized when the flux through each face in the simulation
lattice is a multiple of the fundamental flux quantum. When the typical number
of flux quanta through a correlation-length-sized region is initially about 1,
we find a string network similar to that generated by the Kibble-Zurek
mechanism. With larger initial flux, the loop distribution and the Brownian
shape of the infinite strings remain unchanged, but the fraction of length in
infinite strings is increased. A 2D slice of the network exhibits bundles of
strings pointing in the same direction, as in earlier 2D simulations. We find,
however, that strings belonging to the same bundle do not stay together in 3D
for much longer than the correlation length. As the initial flux per
correlation length is decreased, there is a point at which infinite strings
disappear, as in the Hagedorn transition.Comment: 16 pages and 9 figures. (Minor changes and new references added
Large parallel cosmic string simulations: New results on loop production
Using a new parallel computing technique, we have run the largest cosmic
string simulations ever performed. Our results confirm the existence of a long
transient period where a non-scaling distribution of small loops is produced at
lengths depending on the initial correlation scale. As time passes, this
initial population gives way to the true scaling regime, where loops of size
approximately equal to one-twentieth the horizon distance become a significant
component. We observe similar behavior in matter and radiation eras, as well as
in flat space. In the matter era, the scaling population of large loops becomes
the dominant component; we expect this to eventually happen in the other eras
as well.Comment: 23 pages, 10 figures, 2 tables. V2: combine 3 figures, add 1 table,
better discussion + citation of prev. wor
Racetrack Potentials and the de Sitter Swampland Conjectures
We show that one can find de Sitter critical points (saddle points) in models
of flux compactification of Type IIB String Theory without any uplifting terms
and in the presence of several moduli. We demonstrate this by giving explicit
examples following some of the ideas recently presented by Conlon in [1], as
well as more generic situations where one can violate the strong form of the de
Sitter Swampland Conjecture. We stabilize the complex structure and the dilaton
with fluxes, and we introduce a racetrack potential that fixes the K\"ahler
moduli. The resultant potentials generically exhibit de Sitter critical points
and satisfy several consistency requirements such as flux quantization, large
internal volume, and weak coupling, as well as a form of the so-called Weak
Gravity Conjecture. Furthermore, we compute the form of the potential around
these de Sitter saddle points and comment on these results in connection to the
refined and more recent version of the de Sitter Swampland Conjecture.Comment: 23 pages, 4 figures; updated to reflect version accepted to JHE
Strings at the bottom of the deformed conifold
We present solutions of the equations of motion of macroscopic F and D
strings extending along the non compact 4D sections of the conifold geometry
and winding around the internal directions. The effect of the Goldstone modes
associated with the position of the strings on the internal manifold can be
seen as a current on the string that prevents it from collapsing and allows the
possibility of static 4D loops. Its relevance in recent models of brane
inflation is discussed.Comment: 9+1 page
Deciphering the biosynthetic origin of the aglycone of the aureolic acid group of anti-tumor agents
AbstractBackground: Mithramycin, chromomycin, and olivomycin belong to the aureolic acid family of clinically important anti-tumor agents. These natural products share a common aromatic aglycone. Although isotope labeling studies have firmly established the polyketide origin of this aglycone, they do not distinguish between alternative biosynthetic models in which the aglycone is derived from one, two or three distinct polyketide moieties. We set out to determine the biosynthetic origin of this moiety using a recombinant approach in which the ketosynthase and chain-length factor proteins from the antibiotic-producer strain, which determine the chain length of a polyketide, are produced in a heterologous bacterial host.Results: The ketosynthase and chain-length factor genes from the polyketide synthase gene cluster from the mithramycin producer, Streptomyces argillaceus ATCC12956, and the acyl carrier protein and ketoreductase genes from the actinorhodin polyketide synthase were expressed in Streptomyces coelicolor CH999. The recombinant strain produced a 20-carbon polyketide, comprising the complete backbone of the aglycone of mithramycin.Conclusions: The aglycone moieties of mithramycin, chromomycin, and olivomycin are derived from a single polyketide backbone. The nascent polyketide backbone must undergo a series of regiospecific cyclizations to form a tetracenomycin-like tetracyclic intermediate. The final steps in the aglycone biosynthetic pathway presumably involve decar☐ylation and oxidative cleavage between C-18 and C-19, followed by additional oxidation, reduction, and methylation reactions
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