5,981 research outputs found
Inconsistency of an inflationary sector coupled only to Einstein gravity
From a model-building perspective, the inflationary sector might very well
have no direct couplings to other species, apart from inevitable gravitational
interactions. Within the context of General Relativity, a thermal universe can
still emerge after inflation if: some radiation sector is excited towards
the end of inflation, and the post-inflationary equation of state becomes
sufficiently stiff , with a
threshold depending on the inflationary scale and the initial
radiation-to-inflaton energy ratio . Furthermore, a stiff period in
the expansion history enhances significantly the inflationary gravitational
wave (GW) background, making this signal (potentially) observable by aLIGO,
LISA and other experiments. The very same enhancement leads however to an
inconsistency of the scenario: the energy of the GWs becomes too large compared
to the rest of the radiation sector, violating standard BBN and CMB bounds on
GW backgrounds. Except for very special scenarios where the initial radiation
sector comprises hundreds of fields with couplings tuned to specific values,
our result applies independently of , and . This suggests
that in order to reheat the universe, the inflationary sector should be coupled
directly to other particle species. Alternatively the inflationary sector could
be implemented in modified gravity theories.Comment: Comments added to match published version in JCAP, 22 pages (+
appendix + references), 4 figure
Ability of LIGO and LISA to probe the equation of state of the early Universe
The expansion history of the Universe between the end of inflation and the
onset of radiation-domination (RD) is currently unknown. If the equation of
state during this period is stiffer than that of radiation, , the
gravitational wave (GW) background from inflation acquires a blue-tilt
at
frequencies corresponding to modes re-entering the horizon
during the stiff-domination (SD), where is the frequency today of
the horizon scale at the SD-to-RD transition. We characterized in detail the
transfer function of the GW energy density spectrum, considering both 'instant'
and smooth modelings of the SD-to-RD transition. The shape of the spectrum is
controlled by , , and (the Hubble scale of
inflation). We determined the parameter space compatible with a detection of
this signal by LIGO and LISA, including possible changes in the number of
relativistic degrees of freedom, and the presence of a tensor tilt. Consistency
with upper bounds on stochastic GW backgrounds, however, rules out a
significant fraction of the observable parameter space. We find that this
renders the signal unobservable by Advanced LIGO, in all cases. The GW
background remains detectable by LISA, though only in a small island of
parameter space, corresponding to scenarios with an equation of state in the
range and a high inflationary scale , but low reheating temperature (equivalently, ). Implications for early
Universe scenarios resting upon an SD epoch are briefly discussed.Comment: Matching published version in JCAP, 32 pages, 8 figure
A geometric construction of the exceptional Lie algebras F4 and E8
We present a geometric construction of the exceptional Lie algebras F4 and E8
starting from the round 8- and 15-spheres, respectively, inspired by the
construction of the Killing superalgebra of a supersymmetric supergravity
background. (There is no supergravity in the paper.)Comment: 12 page
The uses of Connes and Kreimer's algebraic formulation of renormalization theory
We show how, modulo the distinction between the antipode and the "twisted" or
"renormalized" antipode, Connes and Kreimer's algebraic paradigm trivializes
the proofs of equivalence of the (corrected) Dyson-Salam,
Bogoliubov-Parasiuk-Hepp and Zimmermann procedures for renormalizing Feynman
amplitudes. We discuss the outlook for a parallel simplification of
computations in quantum field theory, stemming from the same algebraic
approach.Comment: 15 pages, Latex. Minor changes, typos fixed, 2 references adde
Preparation of Cu-based bulk metallic glasses by suction casting
A series of Cu-Hf-Ti alloys prepared by rapid solidification of the melt and by copper mould casting were studied in the present work. Alloy ingots were prepared by arc-melting mixtures of pure metals in an argon atmosphere. An indication of the cooling rate obtained was determined using an Al-4.5 wt%Cu alloy. Cooling rates varied from 540 K/s for the centre section of a 4 mm die to 885 K/s for the outside wall section of the 2 mm die. The glass-forming ability, structure and thermal stability of Cu-Hf-Ti glassy alloys were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). Bulk glass formation was observed for the Cu64Hf36, Cu55Hf25Ti20 and Cu56Hf25Ti19 alloys, with critical diameters dc for a fully glassy structure of 1, 4 and 5 mm, respectively. The substitution of Hf by Ti increased the glassforming ability (GFA) and the thermal stability
The geometry of null rotation identifications
The geometry of flat spacetime modded out by a null rotation (boost+rotation)
is analysed. When embedding this quotient spacetime in String/M-theory, it
still preserves one half of the original supersymmetries. Its connection with
the BTZ black hole, supersymmetric dilatonic waves and one possible resolution
of its singularity in terms of nullbranes are also discussed.Comment: 1+18 pages, 5 figures. v2 references adde
The return of the four- and five-dimensional preons
We prove the existence of 3/4-BPS preons in four- and five-dimensional gauged
supergravities by explicitly constructing them as smooth quotients of the AdS_4
and AdS_5 maximally supersymmetric backgrounds, respectively. This result
illustrates how the spacetime topology resurrects a fraction of supersymmetry
previously ruled out by the local analysis of the Killing spinor equations.Comment: 10 pages (a minor imprecision has been corrected
Sustainability Assessment of indicators for integrated water resources management
The scientific community strongly recommends the adoption of indicators for the evaluation and monitoring of progress towards sustainable development. Furthermore, international organizations consider that indicators are powerful decision-making tools. Nevertheless, the quality and reliability of the indicators depends on the application of adequate and appropriate criteria to assess them. The general objective of this study was to evaluate how indicators related to water use and management perform against a set of sustainability criteria. Our research identified 170 indicators related to water use and management. These indicators were assessed by an international panel of experts that evaluated whether they fulfil the four sustainability criteria: social, economic, environmental, and institutional. We employed an evaluation matrix that classified all indicators according to the DPSIR (Driving Forces, Pressures, States, Impacts and Responses) framework. A pilot study served to test and approve the research methodology before carrying out the full implementation. The findings of the study show that 24 indicators comply with the majority of the sustainability criteria; 59 indicators are bi-dimensional (meaning that they comply with two sustainability criteria); 86 are one-dimensional indicators (fulfilling just one of the four sustainability criteria) and one indicator do not fulfil any of the sustainability criteria.Postprint (author's final draft
Penrose limits of Lie Branes and a Nappi--Witten braneworld
Departing from the observation that the Penrose limit of AdS_3 x S^3 is a
group contraction in the sense of Inonu and Wigner, we explore the relation
between the symmetric D-branes of AdS_3 x S^3 and those of its Penrose limit, a
six-dimensional symmetric plane wave analogous to the four-dimensional
Nappi--Witten spacetime. Both backgrounds are Lie groups admitting bi-invariant
lorentzian metrics and symmetric D-branes wrap their (twisted) conjugacy
classes. We determine the (twisted and untwisted) symmetric D-branes in the
plane wave background and we prove the existence of a space-filling D5-brane
and, separately, of a foliation by D3-branes with the geometry of the
Nappi--Witten spacetime which can be understood as the Penrose limit of the
AdS_2 x S^2 D3-brane in AdS_3 x S^3. Parenthetically we also derive a simple
criterion for a symmetric plane wave to be isometric to a lorentzian Lie group.
In particular we observe that the maximally supersymmetric plane wave in IIB
string theory is isometric to a lorentzian Lie group, whereas the one in
M-theory is not.Comment: 21 pages (v2: references added
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