732 research outputs found
The separate universe and the back reaction of long wavelength fluctuations
We investigate the backreaction of cosmological long wavelength perturbations
on the evolution of the Universe. By applying the renormalization group method
to a Friedmann-Robertson-Walker universe with long wavelength fluctuations, we
demonstrate that the renormalized solution with the backreaction effect is
equivalent to that of the separate universe. Then, using the effective
Friedmann equation, we show that only non-adiabatic mode of long wavelength
fluctuations affects the expansion law of the spatially averaged universe.Comment: 10 pages. to be published in Phys. Rev.
Advance reservation games
Advance reservation (AR) services form a pillar of several branches of the economy, including transportation,
lodging, dining, and, more recently, cloud computing. In this work, we use game theory to analyze a slotted
AR system in which customers differ in their lead times. For each given time slot, the number of customers
requesting service is a random variable following a general probability distribution. Based on statistical
information, the customers decide whether or not to make an advance reservation of server resources in
future slots for a fee. We prove that only two types of equilibria are possible: either none of the customers
makes AR or only customers with lead time greater than some threshold make AR. Our analysis further
shows that the fee that maximizes the provider’s profit may lead to other equilibria, one of which yields zero
profit. In order to prevent ending up with no profit, the provider can elect to advertise a lower fee yielding
a guaranteed but smaller profit. We refer to the ratio of the maximum possible profit to the maximum
guaranteed profit as the price of conservatism. When the number of customers is a Poisson random variable, we prove that the price of conservatism is one in the single-server case, but can be arbitrarily high in a many-server system.CNS-1117160 - National Science Foundationhttp://people.bu.edu/staro/ACM_ToMPECS_AR.pdfAccepted manuscrip
Gravitational Wave Background from Phantom Superinflation
Recently, the early superinflation driven by phantom field has been proposed
and studied. The detection of primordial gravitational wave is an important
means to know the state of very early universe. In this brief report we discuss
in detail the gravitational wave background excited during the phantom
superinflation.Comment: 3 pages, 2 eps figures, to be published in PRD, revised with
published version, refs. adde
Security assessment of audience response systems using software defined radios
Audience response systems, also known as clickers, are used at many academic institutions to offer active learning environments. Since these systems are used to administer graded assignments, and sometimes even exams, it is crucial to assess their security. Our work seeks to exploit and document potential vulnerabilities of clickers. For this purpose, we use software defined radios to perform eavesdropping attacks on an audience response system in production. The results of our study demon- strate that clickers are easily exploitable. We build a prototype and show that it is practically possible to covertly steal answers from a peer or even the entire classroom, with high levels of confidence. As a result of this study, we discourage using clickers for high-stake assessments, unless manufacturers provide proper security protection.http://people.bu.edu/staro/MIT_Conference_Khai.pdfAccepted manuscrip
Throughput Optimal On-Line Algorithms for Advanced Resource Reservation in Ultra High-Speed Networks
Advanced channel reservation is emerging as an important feature of ultra
high-speed networks requiring the transfer of large files. Applications include
scientific data transfers and database backup. In this paper, we present two
new, on-line algorithms for advanced reservation, called BatchAll and BatchLim,
that are guaranteed to achieve optimal throughput performance, based on
multi-commodity flow arguments. Both algorithms are shown to have
polynomial-time complexity and provable bounds on the maximum delay for
1+epsilon bandwidth augmented networks. The BatchLim algorithm returns the
completion time of a connection immediately as a request is placed, but at the
expense of a slightly looser competitive ratio than that of BatchAll. We also
present a simple approach that limits the number of parallel paths used by the
algorithms while provably bounding the maximum reduction factor in the
transmission throughput. We show that, although the number of different paths
can be exponentially large, the actual number of paths needed to approximate
the flow is quite small and proportional to the number of edges in the network.
Simulations for a number of topologies show that, in practice, 3 to 5 parallel
paths are sufficient to achieve close to optimal performance. The performance
of the competitive algorithms are also compared to a greedy benchmark, both
through analysis and simulation.Comment: 9 pages, 8 figure
Priority-Based Synchronization of Distributed Data
We consider the general problem of synchronizing the data on two devices using a minimum amount of communication, a core infrastructural requirement for a large variety of distributed systems. Our approach considers the interactive synchronization of prioritized data, where, for example, certain information is more time-sensitive than other information. We propose and analyze a new scheme for efficient priority-based synchronization, which promises benefits over conventional synchronization
Chlorosis - the ‘green sickness'
The literature of the nineteenth century often referred to chiorosis. Some examples, taken from French sources, are mentioned. The illness was only given its name at the beginning of the seventeenth century, but the clinical picture of febris alba virginum had been known for a long time previously. The history of chiorosis is interesting because of the variety of explanations, including the psychological, it attracted before it came to be recognized as due to hypochromic anaemia or iron deficienc
The role of language in psychiatric treatment in the French Romantic Age: A note on Dr Laurent Cerise
At the beginning of the 19th century France had many experts on the ‘moral treatment of insanity'. Very few of them, however, applied their experience and theories to the role of language in the development of behaviour from childhood on, in the pathogenesis of mental disorders, and in psychotherapy. To Dr. Louis Cerise, one of the founders of the Annales Médico-Psychologiques, belongs the great distinction of formulating a theory which tried to take account of the necessary contribution of language to individual development. In his book Des Fonctions et des Maladies Nerveuses (1842), he put forward a view of the relationship between the individual and society. His concept of ‘the goal of activity' still merits our attentio
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