1,720,972 research outputs found
Black hole echoes
No full textWe consider a very simple model for gravitational wave echoes from black hole merger ringdowns which may arise from local Lorentz symmetry violations that modify graviton dispersion relations. If the corrections are sufficiently soft so they do not remove the horizon, the reflection of the infalling waves which trigger the echoes is very weak. As an example, we look at the dispersion relation of a test scalar field corrected by rotonlike operators depending only on spatial momenta, in Gullstrand-Painlevé coordinates. The near-horizon regions of a black hole do become reflective, but only very weakly. The resulting "bounces"of infalling waves can yield repetitive gravity wave emissions but their power is very small. This implies that to see any echoes from black holes we really need an egregious departure from either standard GR or effective field theory, or both. One possibility to realize such strong echoes is the recently proposed classical firewalls which replace black hole horizons with material shells surrounding timelike singularities
Reviving QFT in (2+1)-dimensional de Sitter spacetime
No full textWe consider a conformally coupled scalar quantum field theory (QFT) on (2 + 1)-dimensional static Einstein universe R x S2 and write down the free theory Hilbert space. We explain that this theory is secretly a QFT in (2 + 1)-dimensional de Sitter space because all the quantum observables experience "quantum revivals," which naturally restricts the timelike R to the appropriate de Sitter time range. Our construction circumvents the causal obstruction to formulating QFT in de Sitter due to event horizons. There are not any in static Einstein. The "unitary gauge" description of the theory is realized by the zonal harmonics Pl(n n0). We verify that interactions with conformally invariant external sources are mediated only by these modes. Hence these modes comprise the complete basis of the "bulk" theory. When the theory is cut off in the UV, the basis dimension scales as the Bekenstein-Hawking formula
Double monodromy inflation: Gravitational wave factory for CMB-S4, LiteBIRD, and LISA
No full textWe consider a short rollercoaster cosmology based on two stages of monodromy inflation separated by a stage of matter domination, generated after the early inflaton falls out of slow roll. If the first stage is controlled by a flat potential, V∼φp with p<1 and lasts N∼30-40 e-folds, the scalar and tensor perturbations at the largest scales will fit the CMB perfectly, and produce relic gravity waves with 0.02≲r≲0.06, which can be tested by LiteBIRD and CMB-S4 experiments. If in addition the first inflaton is strongly coupled to a hidden sector U(1), there will be an enhanced production of vector fluctuations near the end of the first stage of inflation. These modes convert rapidly to tensors during the short epoch of matter domination, and then get pushed to superhorizon scales by the second stage of inflation, lasting another 20-30 e-folds. This band of gravity waves is chiral, arrives today with wavelengths in the range of 108 km, and with amplitudes greatly enhanced compared to the long wavelength CMB modes by vector sources. It is therefore accessible to LISA. Thus our model presents a rare early universe theory predicting several simultaneous signals testable by a broad range of gravity wave searches in the very near future
Rollercoaster Cosmology
No full text(Abridged) Does inflation have to happen all in one go? The answer is a
resounding no! All cosmological problems can be solved by a sequence of short
bursts of cosmic acceleration, interrupted by short epochs of decelerated
expansion. The spectrum of perturbations will still match the CMB and LSS if
the earliest stage of the last efolds is at least
efolds long. Other stages can be considerably shorter. But as
long as they add up to efolds and the stages of
decelerated expansion in between them are shorter and also overall last less,
the ensuing cosmology will pass muster. The presence of the interruptions
resets the efold clock of each accelerating stage, and changes its value at the
CMB pivot point. This change opens up the theory space, loosening the bounds.
In particular some models that seem excluded at fit very well as
shorter stages with . Interesting predictions are that both the
scalar and tensor spectra of perturbations are rapidly modified at short
wavelengths. These features could be tested with future CMB spectroscopy
searches and with short wavelength primordial gravity probes. The spatial
curvature in these models can be larger than the largest wavelength scalar
perturbations, because evolves differently than the scalar
perturbations . Finally, with many short
stages of accelerated expansion, the abundance of reheating products from
previous accelerated stages does not get completely wiped out. This implies
that the universe may contain additional populations of particles, more rare
than the visible ones, or even primordial black holes, created during a late
decelerated epoch before last reheating, which may be dark matter.Comment: 52 pages, 15 figures. v2: Minor changes and references added, version
published in JCAP. v3: Minor correction
Strongly Coupled Quintessence
No full textWe present a family of consistent quantum field theories of monodromy quintessence in strong coupling, which can serve as benchmarks in modeling dark energy different from cosmological constant. These theories have discrete gauge symmetries which can protect them from quantum field theory and quantum gravity corrections, both perturbative and nonperturbative. The strong coupling effects, at scales , flatten the potential and activate operators with higher powers of derivatives. The predicted equation of state is close to, but not exactly equal to, , thus being within reach of the (near) future programs to explore the nature of dark energy.We present a family of consistent quantum field theories of monodromy quintessence in strong coupling, which can serve as benchmarks in modeling dark energy different from cosmological constant. These theories have discrete gauge symmetries which can protect them from quantum field theory and quantum gravity corrections, both perturbative and nonperturbative. The strong coupling effects, at scales , flatten the potential and activate operators with higher powers of derivatives. The predicted equation of state is close to, but not exactly equal to , thus being within reach of the (near!) future programs to explore the nature of dark energy
Neutrino Masses from Outer Space
No full textNeutrinos can gain mass from coupling to an ultralight field in slow roll. When such a field is displaced from its minimum, its vev acts just like the Higgs vev in spontaneous symmetry breaking. Although these masses may eventually vanish, they do it over a very long time. The theory is technically natural, with the ultralight field-dependent part being the right-handed Majorana mass. The mass variation induced by the field correlates with the cosmological evolution. The change of the mass term changes the mixing matrix, and therefore suppresses the fraction of sterile neutrinos at earlier times and increases it at later times. Since the issue of quantum gravity corrections to field theories with large field variations remains open, this framework may give an observational handle on the Weak Gravity Conjecture
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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