Tag Archives: cosmology

Probing Effective Theories of Gravity in Strong Fields and Cosmology, Santa Barbara, USA

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Probing Effective Theories of Gravity in Strong Fields and Cosmology
KITP, UC Santa Barbara

Coordinators: Cora Dvorkin, Ira Rothstein, and Thomas Sotiriou
Scientific Advisors: Alessandra Buonanno, Pedro Ferreira, and Steve Giddings

There is strong evidence that General Relativity (GR) is only an effective field theory (EFT). While it is often assumed that the energy scale where the EFT breaks down is near the Planck scale and experimentally inaccessible, tantalizing arguments suggest that GR is not a canonical effective theory.

In particular, the apparent hints of non-locality in black hole evaporation and the small scale associated with the vacuum energy motivate reconsidering EFT ideas in gravity. Given the wealth of data in current and future high precision cosmological surveys, it is necessary to develop a tool kit of theoretically consistent EFTs for analyzing potential deviations from the predictions of GR. The birth of gravitational wave astronomy opens up other avenues for testing gravity in previously unimaginable ways, enabling the study of higher curvature backgrounds as well as black hole horizon physics.

This program is geared towards finding ways to push the boundaries of our knowledge of the EFT of GR by bringing together members of three distinct communities: strong gravity/relativistic astrophysics, cosmology, and EFT.

Primary Application Deadline: August 11th 2019. After that date application will continue to be considered in a rolling basis until the program is filled.

Please apply through the KITP website (link).

Iberian Cosmology Meeting, Bilbao, Spain

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Cosmology is the glue that binds the vocations of many researchers in Portugal and Spain. Women and men of different ages and backgrounds which pursue a wide range of topics: from theoretical particle physics to observational astrophysics.

The traditional Iberian Cosmology Meeting (aka IberiCOS) is the event that every year provides a warm setting to stimulate ongoing collaborations and favour the emergence of new ones. Bold and provocative ideas and presentations are most welcome; at the end of the day we can boast about this being a gathering of old friends and new to come rather than a rigid sort of conference.

The Cosmology and Gravitation Group of the University of the Basque Country/Euskal Herriko Unibertsitatea will host the event from the 15th to the 17th of April of 2019.

In the usual fashion painstaking efforts are made to allocate slots for everyone, but should it turn to be impossible to offer room to everyone then either PhD students or early career postdoctoral researchers will be given preference. We expect to be able to offer some financial support to some of the participants in those categories.

As there will be no registration fee and we need to manage our resources, we therefore beg all participants to register before the 18th of March.

Kurt Goedel’s Legacy: Does Future lie in the Past? Vienna, Austria

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KURT GOEDEL’S LEGACY: DOES FUTURE LIE IN THE PAST?

The Kurt Goedel Society celebrates the 70th anniversary of Goedel’s
seminal publication about time travel and the 100th anniversary of the
decisive experimental verification of general relativity by organizing
a conference at the University of Vienna, Austria from July 25 until
July 27, 2019. This event will bring together the most prominent
researchers from the fields of Physics, Mathematics, Philosophy and
Artificial Intelligence.

The conference will be complemented by a special exhibition about life
and work of Kurt Goedel with emphasis on the Goedel’s solution.

The program will consist of the invited talks, contributed talks and
posters. Confirmed invited speakers include:  MARKUS ASPELMEYER, JOHN
D.BARROW, CHARLES L. BENNETT, DAVID BENNETT, GEORGE F.R. ELLIS,
JULIET FLOYD, GEORG GOTTLOB, JAN VON PLATO, WOLFGANG SCHLEICH, MARIKA TAYLOR, TOBY WALSH,  RAINER WEISS AND PALLE YOURGRAU.
DANA SCOTT is the Honorary Chair of the Conference.

Important Dates (submission  and registration through the web site of the conference):

May 15 , 2019: Submission deadline for contributed talks and posters

May  25, 2019: Notification

June 1, 2019: Early registration deadline

July 25-27, 2019: Conference

Master/PhD Programs in Physics, Famagusta, Cyprus

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The objectives of the two-year MS program and four-year PhD program in physics are, within a physics curriculum, to provide the students with knowledge of the advanced methods of modern theoretical physics.

The student will be integrated from the very beginning into the physics group of Eastern Mediterranean University, and are expected to MS/PhD thesis during the second/fourth year.

All the lectures will be given in English.

A limited number of fellowships is available.

Physical Interpretations of Relativity Theory (PIRT-2019), Moscow, Russia

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International Conference “Physical Interpretations of Relativity Theory” (PIRT-2019), Bauman Moscow State Technical University, Moscow, Russia, 1 – 5 July , 2019, organized by Bauman University (Russia) and University of Liverpool (Great Britain).

The objectives of this conference are the same as those pursued in the Physical Interpretations of Relativity Theory (PIRT) conferences which have been held in Imperial College, London, every two years since 1988. These objectives are the physical, geometrical, and mathematical interpretations of the formal structure of Relativity Theory, and to examine the questions concerning the various interpretations of the accepted mathematical expression of the Relativity Principle.

The Conference Program of the Moscow PIRT-2019 will include papers dealing with the following major themes:
– Gravitation, cosmology and large-scale structure;
– Gravitational waves and experimental tests of the relativity theory;
– Space-time, topology and differential geometry;
– Relativistic electrodynamics;
– High energy astrophysics;
– Nature and models of physical vacuum.

SCIENTIFIC ORGANIZING COMMITTEE PIRT-2019
Anatoly Aleksandrov, Bauman University, Russia
Barry Barish, California Institute of Technology, the United States
Aroonkumar Beesham, University of Zululand, South Africa
David Blair, University of South Western Australia, Australia
Anatol Cherepashchuk, Sternberg Astronomical Institute Moscow University, Russia
Alexander Chernikov, Bauman University, Russia
Sergei Chervon, Ulyanovsk State Pedagogical University, Russia
Naresh Dadhich, IUCAA, PUNE, India
John Dainton, Liverpool University, Great Britain
Vladimir Gladyshev, Bauman University, Russia
Georgii Izmailov, Moscow Aviation Institute, Russia
Nikolay Kardashev, Astro Space Center of P.N. Lebedev Physical Institute of RAS, Russia
Louis Kauffman, University of Illinois at Chicago, the United States
Richard Kerner, University Pierre et Marie Curie, France
Vassily Manturov, Bauman University, Moscow State University, Russia
Bivudutta Mishra, BITS-Pilani, India
Andrey Morozov, Bauman University, Russia
Nicola Napolitano, INAF Astronomical Observatory of Capodimonte, Italy
Guido Pizzella, University of Rome, Italy
Konstantin Postnov, Sternberg State Astronomical Institute, Moscow State University, Russia
Vladislav Pustovoit, Russian Academy of Science, Bauman University, Russia
Carlos Romero, Universidade Federal da Paraiba, Departamento de Fisica, Brazil
Peter Rowlands, University of Liverpool, the United Kingdom
Valentin Rudenko, Sternberg State Astronomical Institute, Moscow State University, Russia
Mikhail Sazhin, Sternberg State Astronomical Institute, Moscow State University, Russia
Alexei Starobinsky, Landau Institute for Theoretical Physics, Russia
Sergey Sushkov, Kazan Federal University, Russia
Roland Triay, Centre de Physique Théorique CNRS – Aix- Marseille University, France
Nina Tyannikova, Bauman University, Russia
Rainer Weiss, Massachusetts Institute of Technology, the United States

Postal Communications may be sent to the Secretary of PIRT-2019 Organizing Committee preferably by e-mail to Dr. Nina D. Tyannikova (dekan-fn[AT]mail.ru)
Details will be at the website: http://www.pirt.info/index.php?lang=eng

Inhomogeneous Cosmologies III, Krakow, Poland

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Inhomogeneous Cosmologies meetings are small workshops of about 40 participants which gather experts in inhomogeneous cosmology. This year the workshop will take place during 16-21 September 2018 at the Jagiellonian University in Krakow, Poland.

Topics will include:
* exact cosmological solutions of the Einstein equations
* averaging and backreaction in cosmology
* numerical cosmological relativity
* observational tests

We will also have practical hands-on tutorials of the Einstein Toolkit and other free-licensed inhomogeneous cosmology software packages. The workshop sessions will start on the morning of Monday the 17th of September and continue to late afternoon Friday the 21st of September.

Due to the limited number of places available, registration by the early registration deadline of 15th June 2018, including a draft abstract, is strongly recommended. If places remain available, late registration will remain open until the late registration deadline of 24th August 2018 — see http://th.if.uj.edu.pl/indico/event/8/ for details.

Contact: cosmology[at]th.if.uj.edu.pl

Scientific Organizing Committee
Eloisa Bentivegna, Thomas Buchert, Mikolaj Korzynski, Hayley Macpherson, Jan Ostrowski, Boud Roukema, Sebastian Szybka, David Wiltshire

Local Organizing Committee
Krzysztof Glod, Szymon Sikora, Sebastian Szybka

2018 Rotman Summer Institute in Philosophy of Cosmology, Goderich, Canada

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Rotman Summer Institutes bring graduate students together with exceptional faculty from around the world to focus on a topic of special interest. The Rotman Summer Institute of 2018 will be aimed at promoting understanding of and research in issues in the philosophy of cosmology. It will be held from June 11 – 20, 2018, in a picturesque setting on the eastern shores of Lake Huron.

The two-week-long Summer Institute will consist in a series of lectures and seminar-style discussions, aimed both at surveying the current state of research in cosmology, and at discussing foundational issues. These lectures and discussions will be led by philosophers and cosmologists. Several roundtables and open discussion sessions will be scheduled to address issues that do not easily fit in any particular session, and to foster cross disciplinary exchanges. Graduate students will have an opportunity to present and discuss their own work at the end of the workshop.

The Institute will focus primarily on two new directions for research in philosophy of cosmology. The first regards the role of simulations in cosmology. In order to test a scientific theory, it is essential to extract from that theory precise predictions concerning what would be observed in the world if the theory were true. In many sciences, one can extract such predictions using various sorts of mathematical methods. But the systems that are of greatest interest in cosmology, and astrophysics more generally, are often simply too complex to treat using such methods. In order to extract testable predictions from the Standard Model of cosmology, cosmologists turn to computer simulations. But there are deep methodological difficulties that arise once we start relying on computer simulations, particularly given that in many cases we have no independent way of testing whether the assumptions, approximations, and numerical methods used in these simulations are reliable.

The second concerns the status of singularity theorems, and the global structure of spacetime, in light of proposals from quantum gravity theories that “resolve” the initial singularity. This is so despite the fact that there are a number of celebrated theorems in the context of classical general relativity, showing that singularities are “generic” features of general relativity. These theorems follow from basic facts concerning the conditions under which a “trapped surface” can be expected to develop in a relativistic spacetime with attractive gravitation. This situation leads to subtle issues concerning how we expect general relativity to approximate an underlying theory, and how that underlying theory can avoid the conditions assumed in the singularity theorems.

Invited speakers at the Institute include a mix of philosophers and cosmologists.

This event is made possible by a grant from the John Templeton Foundation, to Chris Smeenk and Jim Weatherall.

Collabor8.2, Lancaster, UK

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Collabor8.2 is a two-day meeting designed to promote acknowledgement and collaboration among early career researchers in the UK across the disciplinary divide of mathematics and physics. The focus is broad, encompassing theoretical problems from astronomy, cosmology, particle theory, string theory, condensed matter and quantum foundations.

Participants are invited to give short talks on specific research problems they face, seeking new perspectives and fresh approaches. Speakers will lead small team discussions aimed at overcoming the research hurdles presented. Presentations should focus on current research, rather than completed projects.

The conference will feature a keynote seminar by Professor Ruth Gregory who has written papers with more than 100 mathematicians and physicists in a distinguished 30 year career. There will be recruitment presentations from several industrial sponsors offering research positions in physics, data science and machine learning (to be confirmed).

We are delighted to offer free lunch, morning coffee and afternoon tea to all participants. We also have funding to reimburse some travel and accommodation costs for speakers. There is no conference fee. There will be a conference dinner on the first evening in a local restaurant (circa 10 GBP).

Observers in Quantum Gravity, Rome, Italy

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The notion of observer, which is quite a nuanced concept in quantum mechanics and in GR taken separately, can be expected to take a new meaning at the crossroads of those two theories, whatever the correct quantum theory of gravity will be.
We will gather a handful of experts in Quantum Foundations, Quantum Information Theory, Quantum Gravity and Philosophy of Physics, to discuss different aspects of the notion of observer, with an open mind regarding the particular framework or approach that each of us is pursuing, and trying to keep a focus on the conceptual and cross-disciplinary aspects.

Probing the dark sector and general relativity at all scales at CERN, Geneve, Switzerland

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The standard cosmological model, based on the theory of general relativity, has been very successful in explaining the observable properties of the cosmos. This success is achieved at the price of assuming that the energy content of the universe is currently dominated by dark contributions; namely, dark matter and dark energy. Only the large-scale gravitational interaction of these components has been detected so far and their properties remain largely unknown, despite great effort, both theoretical and experimental, that has been made to identify any direct interactions between the dark sector and luminous matter. At present we do not even know if the dark components really exist as a new kind of matter or represent a mirage produced by modifications of the laws of gravity.

The rapid improvement in the quality and quantity of observational data requires the development of more precise and detailed descriptions of the predictions of various models for the dark sector. The prediction of each candidate model must be confronted with data on all scales where the model makes calculable predictions that can be tested observationally or experimentally. Progress in this direction requires a strong cooperative effort from experimentalists, observers and theorists.

The purpose of this TH Institute is to bring together experts in theory, experiments and observations interested in dark matter, dark energy and tests of the laws of gravity. It will provide an opportunity to discuss new ideas to probe the dark sector and general relativity at diverse scales. The topics to discuss include the current consistency tests of the standard cosmological model, the identification of new observable signatures of dark matter and dark energy, experimental/observational methods, tests of gravity, and questions such as to what extent it is possible to discriminate among alternative models. The program will include review talks on the state-of-the art in various fields, as well as contributions on more specific topics. A lot of free time will be left for discussions.

Organisers: Diego Blas, Clare Burrage, Justin Khoury, Diana Lopez Nacir, Paolo Pani, Sergey Sibiryakov, Alfredo Urbano

Postdoc position in numerical relativity/cosmology at Center for Theoretical Physics, Polish Academy of Science

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The Director of the Center for Theoretical Physics invites applications for a postdoctoral position at the CTP PAS, financed from the project “Local relativistic perturbation theory in hydrodynamics and cosmology” No. 2016/22/E/ST9/00578 (SONATA BIS 6) supported by the National Science Center, decision No. DEC-2016/22/E/ST9/00578. The principal investigator is Prof. Mikolaj Korzynski. The position starts on September 1st, 2017.

The position requires a PhD in theoretical or computational physics and experience in numerical relativity, computational hydrodynamics, MHD or compatible field. A background in astrophysics, general relativity or cosmology and experience with the EinsteinToolkit framework would be an advantage.

The group of Mikolaj Korzynski will work on the application of numerical relativity to cosmology, especially the problems of structure formation and the light propagation through spacetime, combining numerics, stochastic and perturbative methods.

The applicants should submit the following documents:
1. cover letter including the statement „I hereby give consent for my personal data included in the job offer to be processed for the purposes of recruitment under the Data Protection Act 1997 (Dz. U. no. 101, item 926)”
2. a scientific CV, including the list of publications and major scientific achievements
3. brief description of research interests
4. copy of the PhD diplomma
5. personal questionnaire form from the CTP PAS webpage via email directly to Mikolaj Korzynski (korzynski[AT]cft.edu.pl). Additionally they should arrange for two letters of recommendation to be sent to the same email address. Applicants expecting to obtain their PhD soon should also include a statement from their supervisors about the scheduled date of their defence.

The deadline for applications has been extended to 1st August 2017. Selected applicants will be invited for an interview. Successful applicant will be employed for the trial period of 12 months, with the possibility of extension for up to 3 further years.

Special Issue “Progress in Cosmology in the Centenary of the 1917 Einstein paper”

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Dear Colleagues,

The first modern cosmological models emerged soon after the discovery of general relativity, putting the study of the Universe as a whole on the firm grounds of an empirically testable, coherent science. In the century since then, cosmology has developed into a precision discipline able to explain the evolution of the Universe in several of its aspects. The goal is under the way, but far than ended. The most stringent open questions remain the nature of dark matter (DM) and of dark energy (DE), and whether General Relativity holds on large cosmological scales.

Of course, many independent observation (anisotropies in CMB, large structure, SNIa data, gravitational lensing, galaxy rotational curves etc.) confirm the necessity of the introduction of these dark components.

However, the existence itself of the most likely DM candidates seem to have been seriously challenged by experiments and or astrophysical observations: e.g. supersymmetric DM and WIMPs by LHC; by LUX, PandaX-II and Xenon100; MACHOs by microlensing. Sterile neutrinos by IceCube and high redshift objects. The properties of the DM in galaxies are presently badly explainable by current theoretical scenarios. At present the nature of DM remains a mystery.

Understanding DE poses an even bigger challenge. Although the cosmological constant may explain the accelerated cosmic expansion, its physical interpretation (as vacuum energy) remains doubtful. Question comes what kind of fields can be responsible for the accelerated cosmic expansion. Several scalar field models of DE induce new type of space-time singularities (e.g. soft singularities). Alternative gravitational theories (e.g. scalar-tensor theories, the emergent gravity model of Verlinde) have been also proposed with the purpose to explain the dark sector.

We invite colleagues to submit papers on the topics:

1: The nature of Dark matter and DE

2: Present/future experiments and observations related to DM, DE and their gravitational effects.

3: Models on DM and DE including the alternative gravitational theories, new fields and their possible interaction with the particles of standard model.

4: Evolution of the Universe, cosmological perturbations, formation of nonlinear structures, first objects.

5: Inflation, initial structure, primordial gravitational waves.

6: Primordial black/white holes, their formation and gravitational waves, their effects on the synthesis of light elements.

7: Anisotropic cosmological models and their perturbations.

8: Exotic singularities, wormholes occurring in cosmological models and in virialized structure.

Dr. Zoltan Keresztes
Prof. Lorenzo Iorio
Prof. Paolo Salucci
Prof. Emmanuel Saridakis
Guest Editors

Inhomogeneous Cosmologies (2nd announcement), Torun, Poland

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During 2-7 July 2017 we are gathering experts in inhomogeneous cosmology for a small workshop of about 30 participants at Nicolaus Copernicus University in Torun, the town where Copernicus was born. We wish to map out the most promising directions for analytical, numerical and observational investigations aimed to take into account both structure formation and cosmological expansion within the constraints of general relativity. A key motivating theme will be to discuss the claim, already investigated in numerous peer-reviewed papers, that “dark energy” as inferred from observations is an artefact of assuming an average Friedmannian expansion. New techniques in numerical relativity are beginning to open new perspectives on these questions. We expect talks on the latest developments, vigorous, constructive debate between “one-percenters” and “order-unity” proponents, and practical hands-on tutorials of the Einstein Toolkit and other free-licensed inhomogeneous cosmology software packages. The workshop sessions will start on the morning of Mon 3 July and continue to late afternoon Fri 7 July.

Due to the limited number of places available, registration by the early registration deadline of 7 April 2017, including a draft abstract, is strongly recommended. If places remain available, late registration will remain open until the late registration deadline of 9 June 2017 – see http://cosmo.torun.pl/CosmoTorun17 for details.

Contact: cosmotorun17 at cosmo.torun.pl

Organising committee: Boud Roukema, Eloisa Bentivegna, Krzysztof Bolejko, Thomas Buchert, Mikolaj Korzynski, Hayley MacPherson, Jan Ostrowski, Sebastian Szybka, David Wiltshire

Topics will include:

* exact cosmological solutions of the Einstein equations
* averaging and backreaction in cosmology
* numerical cosmological relativity
* observational tests

Inhomogeneous Cosmologies (1st announcement), Torun, Poland

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During 2-7 July 2017 we are gathering experts in inhomogeneous cosmology for a small workshop of about 30 participants at Nicolaus Copernicus University in Torun, the town where Copernicus was born. We wish to map out the most promising directions for analytical, numerical and observational investigations aimed to take into account both structure formation and cosmological expansion within the constraints of general relativity. A key motivating theme will be to discuss the claim, already investigated in numerous peer-reviewed papers, that “dark energy” as inferred from observations is an artefact of assuming an average Friedmannian expansion. New techniques in numerical relativity are beginning to open new perspectives on these questions. We expect vigorous, constructive debate between “one-percenters” and “order-unity” proponents, and practical hands-on sessions of free-licensed inhomogeneous cosmology
software packages.

We will post a formal announcement and registration details by early 2017 at http://cosmo.torun.pl/CosmoTorun17.

Contact: cosmotorun17 at cosmo.torun.pl

Organising committee: Boud Roukema, Thomas Buchert, Krzysztof Bolejko, Mikolaj Korzynski, Jan Ostrowski, Sebastian Szybka, David Wiltshire

Special Issue on “Phenomenological Aspects of Quantum Gravity and Modified Theories of Gravity”

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Dear Colleagues,

A Special Issue on “Phenomenological Aspects of Quantum Gravity and Modified Theories of Gravity” will be published in the Journal “Advances in High Energy Physics” in September 2016.

You can find the Call for Papers for this Special Issue at

http://www.hindawi.com/journals/AHEP/si/212894/cfp/

Submission deadline is 6 May 2016.

Cordially Yours,

Lead Guest Editor: Ahmed Farag Ali (Benha University, Egypt)

Guest Editors: Giulia Gubitosi (Imperial College London, London, UK), Mir Faizal (Waterloo University, Waterloo, Canada), Barun Majumder (Montana State University, Bozeman, USA)

9th TRR33 Winter School, Passo del Tonale, Italy

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The school will cover topics relevant to the research subjects of the TRR33 network. The aim is to bring together observation and theory: “Theory for Observers and Observations for Theorists”.

Overview lectures:
David Mota, ITA – University of Oslo

In depth topics:
Observation of Large Scale Structure of the Universe: David Bacon, ICG Portsmouth
Non-linear evolution of Large Scale Structure: Diego Blas, CERN
Science Communication: Anais Rassat, EPFL Lausanne
Dark Matter: Pat Scott, Imperial College London
Beyond the LCDM model: Alessandra Silvestri, Lorentz Institute Leiden

Registration is OPEN: the deadline is 15th October 2015. To register, please visit our website:
http://darkuniverse.uni-hd.de/view/Main/WinterSchool15

Given the large number of applications we strongly suggest an early registration, well in advance of the deadline. Note that we will accept a maximum of 40 participants.

Please email us with any further questions at winter.school[AT]thphys.uni-heidelberg.de

Follow us on our Facebook page
https://www.facebook.com/trr.winter.school
or on twitter using the hashtag #trr33tonale for announcements and reminders

Please check the homepage of the school for more information: http://darkuniverse.uni-hd.de/view/Main/WinterSchool14

See you in Tonale!
the Organizing Committee

Yashar Akrami,
Matteo Costanzi,
Matteo Martinelli,
Matteo Maturi,
Valeria Pettorino,
Georg Wolschin,
Miguel Zumalacárregui

New book: “Introduction to General Relativity, Black Holes and Cosmology”, by Y. Choquet-Bruhat

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Dear Colleagues in General Relativity

To please the kind staff of Oxford University Press, and myself, I send you as propaganda for my last book “Introduction to General Relativity, Black Holes and Cosmology”, whose details can also be found at this website: http://ukcatalogue.oup.com/product/9780199666454.do

With best wishes to all

Yvonne Choquet-Bruhat

New focus issue on string cosmology free to read in CQG

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Dear Colleagues,

We are delighted to announce that the new Classical and Quantum Gravity (CQG) focus issue on string cosmology is now free to read online.

http://iopscience.iop.org/0264-9381/28/20

This issue includes 10 specially invited papers from some of the top researchers in the field.

Guest Edited by Dr Vijay Balasubramanian and Professor Paulo Moniz, the focus issue appraises recent applications of string-theoretic and string-inspired ideas to the cosmos. The articles in this issue also survey a number of potentially promising directions for the future.

With best wishes,

Adam Day
Publisher
Classical and Quantum Gravity
iopscience.org/cqg

Read the latest CQG focus section on inhomogeneous cosmological models and averaging in cosmology

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CQG’s latest focus section on inhomogeneous cosmological models and averaging in cosmology is now available to read on the CQG website:

http://iopscience.iop.org/0264-9381/28/16

The issue was edited by CQG Board Members Lars Andersson and Alan Coley.

The special section focuses on the physical state of the present universe and the problem of going beyond perturbation theory. The following topics are covered:
– a general overview and a discussion of the relevant issues;
– inhomogeneous cosmological models (including non-Copernican models);
– the current observations and physics of the universe and
– averaging and backreaction.

I would like to thank the guest editors and all of the authors and referees of the focus section for their contributions to this excellent issue of Classical and Quantum Gravity.

Yours sincerely

Adam Day
Publisher
Classical and Quantum Gravity