New blog

Announcing my new (tentative) blog, Toy Universes.

Universality of galactic surface densities

You will find an intriguing paper on today’s issue of Nature (subscription required):

Universality of galactic surface densities within one dark halo scale-length by Gentile et al.

Abstract:

It was recently discovered that the mean dark matter surface density within one dark halo scale-length (the radius within which the volume density profile of dark matter remains approximately flat) is constant across a wide range of galaxies. This scaling relation holds for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. Here we report that the luminous matter surface density is also constant within one scale-length of the dark halo. This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale-length it is constant. Our finding can be interpreted as a close correlation between the enclosed surface densities of luminous and dark matter in galaxies.

See also the Editor’s Summary.

As noted by the authors:

A large central luminous density thus implies a large core radius, and in turn a small central dark matter density. This precise balance must be the result of some unknown, fine-tuned process in galaxy formation, because it is a priori difficult to envisage how such relations between dark and baryonic galaxy parameters can be achieved across galaxies that have experienced significantly different evolutionary histories, including numbers of mergers, baryon cooling or feedback from supernova-driven winds.

Update: I thought it would be interesting to point to a previous work of mine and collaborators (back from 2003) which shows that the central dark matter halo densities for a large data sample ranging from dwarf ellipticals to clusters of galaxies, based on the application of the two-component virial theorem (2VT) to these systems, do not show universality. Only the abstract is available:

Title: The case against scale-invariant central halo densities: implications for the self-interacting dark matter scenarios in the context of the two-component virial theorem
Authors: Ribeiro, A. L. B.; Dantas, C. C.; Capelato, H. V.; Carvalho, R. R.
Publication: Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 163-163

I will attempt to find the poster PDF and make it opportunely available here.

More on the 2VT can be found here:

Title: The Two-Component Virial Theorem and the Physical Properties of Stellar Systems
Authors: Dantas, Christine C.; Ribeiro, André L. B.; Capelato, Hugo V.; de Carvalho, Reinaldo R.
Publication: The Astrophysical Journal, Volume 528, Issue 1, pp. L5-L8.

Update 2: Interesting discussions here.

News from LIGO

An upper limit on the stochastic gravitational-wave background of cosmological origin
The LIGO Scientific Collaboration & The Virgo Collaboration
Nature 460, 990-994 (20 August 2009)

Abstract

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations. Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 times 10^{-6} at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models. This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100 Hz.

Update: Now freely available in the arxiv. [0910.5772]

A Brief Introduction to Loop Quantum Cosmology

A Brief Introduction to Loop Quantum Cosmology [arxiv:0907.5160]
Authors: Guillermo A. Mena Marugan

Abstract: In recent years, Loop Quantum Gravity has emerged as a solid candidate for a nonperturbative quantum theory of General Relativity. It is a background independent theory based on a description of the gravitational field in terms of holonomies and fluxes. In order to discuss its physical implications, a lot of attention has been paid to the application of the quantization techniques of Loop Quantum Gravity to symmetry reduced models with cosmological solutions, a line of research that has been called Loop Quantum Cosmology. We summarize its fundamentals and the main differences with respect to the more conventional quantization approaches employed in cosmology until now. In addition, we comment on the most important results that have been obtained in Loop Quantum Cosmology by analyzing simple homogeneous and isotropic models. These results include the resolution of the classical big-bang singularity, which is replaced by a quantum bounce.

Comments: 15 pages, published in AIP Conference Proceedings, Volume 1130, Geometry and Physics: XVII International Fall Workshop on Geometry and Physics

How Far Are We from the Quantum Theory of Gravity?

How Far Are We from the Quantum Theory of Gravity? [arxiv:0907.4238]

R. P. Woodard (University of Florida)

Abstract: I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology.

Comments: 106 page review article solicited by Reports on Progress in Physics

A little bit of cosmographic sanity

Finally, an interesting paper on dark energy.

Cosmographic analysis of dark energy [http://arxiv.org/abs/0906.5407]

Authors: Matt Visser (Victoria University of Wellington), Celine Cattoen (Victoria University of Wellington)

Abstract: The Hubble relation between distance and redshift is a purely cosmographic relation that depends only on the symmetries of a FLRW spacetime, but does not intrinsically make any dynamical assumptions. This suggests that it should be possible to estimate the parameters defining the Hubble relation without making any dynamical assumptions. To test this idea, we perform a number of inter-related cosmographic fits to the legacy05 and gold06 supernova datasets, paying careful attention to the systematic uncertainties. Based on this supernova data, the “preponderance of evidence” certainly suggests an accelerating universe. However we would argue that (unless one uses additional dynamical and observational information, and makes additional theoretical assumptions) this conclusion is not currently supported “beyond reasonable doubt”. As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation — representations in which acceleration versus deceleration reduces to the question of whether the relevant graph slopes up or down.

Smolin against the timeless multiverse

There is a new article by Lee Smolin at Physicsworld.com, “The unique universe“, where he exposes his metaphysical position on the multiverse and the notion of time as fundamental, not emergent.

My thoughts are close to Smolin on those issues, see my previous post on the multiverse here:

The Universe

and, in a funny side, my cartoon here:

Universes Everywhere

Concerning the question whether time is fundamental or not, my philosophical position is that it is fundamental, although there is a facet of it which can be made artificially emergent. See my essay on concurrent time here .

News on Gravity Probe B

Interesting results coming from Gravity Probe B, including a better understanding of the discrepancies between the four gyroscopes, leading to better data on geodetic effect and frame dragging effect in all four gyroscopes.

Also, a complete document “Gravity Probe B Science Results—NASA Final Report” is now available from their site.

Solvay Physics Conference 1927

Unconventional computing

Just received.

…………………..

THE SCIENCE AND PHILOSOPHY OF UNCONVENTIONAL COMPUTING (SPUC09)

Cambridge (UK), March 23-25, 2009

SECOND CALL FOR PAPERS

We welcome submissions on topics normally classified under ‘natural computing’ or ‘unconventional computing’ or ‘hypercomputing’ including (but not restricted to) quantum computing, relativistic computing, biology-based computing, analogue computing, and also submissions on the philosophical implications of these new fields for topics including (but again not restricted to) philosophy of mind, philosophy of mathematics, the Church-Turing thesis.

Each presentation should last no more than 30 minutes; a further 10 minutes will be allowed for discussion.

Those wishing to make a presentation should submit by email a 250-word abstract of their paper to Mark Hogarth (mhogarth@cantab.net); enquiries to the same.

Registration fee (yet to be fixed) will be around £100.

Student bursaries are available.

Conference website: http://web.mac.com/mhogarth/Site/SPUC_Conference.html

ORGANISER

Mark Hogarth (Cambridge, UK)

CONFIRMED INVITED SPEAKERS

Selmer Brinsjord (New York, USA))

Jeff Barrett (Irvine, USA)

Philip Welch (Bristol, UK)

Tim Button (Harvard, USA)

Cristian Calude (Auckland, New Zealand))

István Németi (Budapest, Hungry)

Benjamin Wells (San Francisco, USA)

Hajnal Andréka (Budapest, Hungry)

Apostolos Syropoulos (Xanthi, Greece)

Susan Stepney (York, UK)

Bruce MacLennan (Tennessee, USA)

Peter Kugel (Boston, USA)

Mark Sprevak (Cambridge, UK)

Selim Akl (Kingston, Canada)

José Félix Costa (Swansea, UK)

ADVISORY PANEL

Mike Stannett (Sheffield, UK)

John Tucker (Swansea, UK)

Barry Cooper (Leeds, UK)

Sponsored by EPSRC through HyperNet (the Hypercomputation Research Network, EP/E064183/1)

Something I’d like to do if I were younger….

Just received.

…………………………………………………….

Dear Christine,

I am writing to ask for your assistance in drawing the attention of exceptional, highly motivated students to the Perimeter Scholars International (PSI) program.

PSI is an innovative, Masters level course designed to prepare students for cutting-edge research in theoretical physics. It provides a broad overview, allowing students to choose their preferred specialisation, and extensive tuition in formulating and solving interesting problems.

The due date for applications is February 1st: applications received after this date may still be considered but only as long as places remain available.

A number of outstanding lecturers have already signed up to teach, including for example Yakir Aharonov, Phil Anderson, Matt Choptuik, Nima Arkani-Hamed, John Cardy, Ruth Gregory, Michael Peskin, Sid Redner, Xiao-Gang Wen, and a number of Perimeter Institute research faculty. They will be supported by full-time tutors dedicated to the course.

All accepted students will be fully supported.

For further details, see www.perimeterscholars.org.

Thank you in advance for helping us to make this exciting opportunity known as widely as possible.

With my best wishes,

Neil Turok

Director
Perimeter Institute for Theoretical Physics
Waterloo, Ontario, Canada

Unexplained signal at 3 GHz from ARCADE 2

This appears to be an interesting news coming from the ARCADE 2 experiment. I quote from their recent paper “Interpretation of the Extragalactic Radio Background”, M. Seiffert et al. 2008, ApJ, submitted (available here):

We have presented evidence for isotropic radio emission detected by ARCADE 2 beyond what can be explained by Galactic emission and the unresolved emission from the known population of discrete sources. The excess emission is consistent with a power law, with an index of −2.56, which is significantly flatter that what might be expected from an unidentified population of faint, diffuse, steep spectrum (index ∼ −2.7) radio sources associated with star-forming galaxies. We have also examined and placed limits on two classes of spectral distortions to the CMB. Such distortions are not supported by the data and cannot explain the excess emission (…)

and

We conclude that the three most important sources of error, Galactic emission, instrumental systematic errors, and radio emission from the faint end of the distribution of known sources, are unlikely to be sufficient to explain the excess emission presented here.

Two of the authors are Brazilian colleagues working at the Astrophysics Division at INPE (National Institute for Space Research, Brazil).

On the Nature of Time — essay competition

I have submitted an essay to the FQXi competition. If you are interested in reading it, click here.

Title: On the Nature of Time – Or Why Does Nature Abhor Deadlocks?

Essay Abstract

This essay aims at introducing a novel point of view on the nature of time, inspired by a synthesis of three seemingly unrelated concepts: Bergson’s notion of duration, Dijkstra’s notion of concurrency, and Mach’s notion of inertia.

Edit (June 9th 2009): Apparently, the essays on the nature of time are no longer available at the FQXi site. I have made a very few small corrections and modifications in my essay and a new version is available here (pdf file).

News from the CDF and PAMELA experiments

[Thanks to Peter Woit]

Since I am not expert in particle/astroparticle high energy physics, I will not comment on those results, but I admit to find them quite interesting, so I will link them here for my record. You are encouraged to read Woit‘s and Dorigo‘s (CDF collaborator) posts on the first news and the Resonaances blog post on the second one. The technical papers are linked below:

Study of multi-muon events produced in p-pbar collisions at sqrt(s)=1.96 TeV [CDF homepage here]

Observation of an anomalous positron abundance in the cosmic radiation [PAMELA homepage here]

Update: Discussions also going on here.

Update: See a new post over at Cosmic Variance by John Conway, another CDF collaborator.

Update: Well-known physicist Nima Arkani-Hamed clarifies the genesis of and motivation for his recent papers (published a few weeks before the CDF result) # ## over at Dorigo’s blog. His response denies previous suggestions that he and his collaborators had access to CDF results before their publication. Dorigo writes a new post on this here.

Update: A new post by Woit here.

Final word: I will no longer add further updates. You have plenty of places over the blogosphere to follow this. I must get back to work.

The right attitude

In 1999 I finished my three-volume book on the quantum theory of fields
(cited here as “QTF”), and with unaccustomed time on my hands, I set
myself the task of learning in detail the theory underlying the great progress
in cosmology made in the previous two decades. Although I had done some
research on cosmology in the past, getting up to date now turned out to take
a fair amount of work. Review articles on cosmology gave good summaries
of the data, but they often quoted formulas without giving the derivation,
and sometimes even without giving a reference to the original derivation.
Occasionally the formulas were wrong, and therefore extremely difficult
for me to rederive. Where I could find the original references, the articles
sometimes had gaps in their arguments, or relied on hidden assumptions, or
used unexplained notation. Often massive computer programs had taken
the place of analytic studies. In many cases I found that it was easiest to
work out the relevant theory for myself.
This book is the result.

Weinberg, Cosmology (preface)

Update on Gravity Probe B

I have just received the news letter below, which I fully reproduce for your convenience. I am particularly glad to read that the program continues, and I am quite interested to learn about their peer-reviewed report, which will be released after the upcoming workshop on “The Nature of Gravity: Confronting Theory and Experiment in Space”. (I could not find the link to the workshop’s homepage).

The news and other information of interest follow below.

…………………………………………………………………………………………………..
================================
GP-B STATUS UPDATE — September 26, 2008
================================

Since our May 23rd status update, GP-B has continued to make significant progress–fiscal and scientific. NASA funding and sponsorship of the program ends on September 30, 2008, but GP-B has secured alternative funding that will enable our science team to continue working at least through December 2009 in order to complete the data analysis and bring GP-B to a proper close.
The GP-B science team is continuing to make large strides in the data analysis. On Friday, August 29, 2008, the 18th meeting of our external GP-B Science Advisory Committee was held at Stanford to report our progress since the previous SAC meeting in November 2007. The ensuing SAC report to NASA states:

“The progress reported at SAC-18 was truly extraordinary and we
commend the GPB team for this achievement. This has been a heroic
effort, and has brought the experiment from what seemed like a state
of potential failure, to a position where the SAC now believes that they
will obtain a credible test of relativity, even if the accuracy does not
meet the original goal. In the opinion of the SAC Chair, this rescue
warrants comparison with the mission to correct the flawed optics
of the Hubble Space Telescope, only here at a minuscule fraction
of the cost.” –SAC #18 Report to NASA

On October 6-10, six GP-B team members have been invited to present these dramatically improved, interim results at an International Space Science Institute (ISSI) workshop on “The Nature of Gravity: Confronting Theory and Experiment in Space” to be held in Bern, Switzerland. Following the Berne workshop, these improved interim results will undergo a thorough peer-review and vetting; then towards the end of this year, we plan to announce them publicly.

We very much appreciate your continued interest in GP-B, and we will keep you posted on our progress in future status updates.
===================
PREVIOUS GP-B UPDATES
===================
If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

=============================
OTHER LINKS THAT MAY INTEREST YOU
=============================

* Our NEW AND IMPROVED GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.

* Video and/or audio of May 18, 2006 public lecture by Principal Investigator, Professor Francis Everitt, on GP-B. You can view a Flash video of the lecture in your Web browser: http://einstein.stanford.edu/Media/Everitt_Brainstorm-flash.html You can also download either a video or audio only copy of the lecture to an iPod from the Stanford University iTunes U Web site: http://itunes.stanford.edu, This Web page automatically launches the Apple iTunes program on both Macintosh and Windows computers, with a special Stanford on iTunes U “music store,” containing free downloads of Stanford lectures, performances, and events. Francis Everitt’s “Testing Einstein in Space” lecture is located in the Faculty Lectures section. People with audio-only iPods can download the version under the Audio tab; people with 5th generation (video) iPodfs can download the version under the Video tab.

* Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html

* PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.

* NASA’s Marshall Space Flight Center also has a series of Web pages devoted to GP-B: http://www.gravityprobeb.com

* The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/. In addition, you’ll find information in the Extraordinary Technologies page-Telescope & Guide Star section on our Web site: http://einstein.stanford.edu/TECH/technology1.html#telescope

==========================
ABOUT THE GPB-UPDATE EMAIL LIST
==========================
The email distribution list for this GP-B Weekly Highlights update is maintained on the new Stanford University Mailman lists server.

To subscribe to this list, send an email message to:
gpb-update-join@lists.stanford.edu
The subject and body of the message will be ignored, so it doesn’t matter what you put there.

…………………………….

Edit: Link to Gravity Probe B Scientific Papers

WMAP – 5 years data released

You can access the WMAP updated data here.

There you will find 8 scientific papers for downloading (submitted to the ApJ Supplement series).

Also, a table summarizing the best fit parameters as well as other derived products.

Have a good time.

List of Books on Quantum Gravity and other helpful tips

I’m writting this post to keep track of books on quantum gravity (and closely related/helpful books). I own only the first one, which I recommend if you are interested in the main conceptual problems of quantum gravity. Regarding the others, I am presently considering purchasing Thiemann and Henneaux & Teitelboim’s books.

Rovelli – Quantum Gravity (errata)
Thiemann – Modern Canonical Quantum General Relativity
Kiefer – Quantum Gravity
Amelino-Camelia – Planck Scale Effects in Astrophysics and Cosmology
Gomberoff – Lectures on Quantum Gravity
Ambjørn et al. – Quantum Geometry: A Statistical Field Theory Approach
Carlip – Quantum Gravity in 2+1 Dimensions
Gambini & Pullin – Loops, Knots, Gauge Theories and Quantum Gravity
Henneaux & Teitelboim – Quantization of Gauge Systems
Rickles et al. – The Structural Foundations of Quantum Gravity
Callender – Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity

There is also Daniele Oriti’s book, which is an edition of several contributions about various approaches to quantum gravity. As it appears, the book has not been released yet. You can learn more about it over at my old blog, Background Independence (scroll down after my two book reviews to find Oriti’s invited post). At that blog you can also find my “Basic Curriculum for Quantum Gravity” (scroll down a little more, after Oriti’s post, or use this direct link to a backup copy of that post, with comments). There, I link to several other helpful books and downloadable tutorials/papers. That list have not been updated. I believe these two papers by Ashtekar are useful recent reviews:

- Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
- An Introduction to Loop Quantum Gravity Through Cosmology

There is also Smolin’s 2006 Lectures on quantum gravity. See here for more details.

Smolin on “The Problem of Time”

The first lecture by Lee Smolin on “The Problem of Time in Quantum Gravity and Cosmology” is available from the new PIRSA site.

Edit: Ah, glad to see a mention of Bergson in the scheduled lecture of March 19, “History of the problem of time”. But there is no mention of Saint Augustine.

Update – Here are the presently available presentation files:

Lecture 1.
Lecture 2A.
Lecture 2B.

Update Jan 25 2008:

Lecture 3.
Lecture 3B.

Update Jan 31 2008:

Lecture 4.
Lecture 4B. (appears to have been removed).

Update Feb 9 2008:

Lecture 5.
Lecture 5B.

Update Feb 14 2008:

Lecture 6.
Lecture 6B.

Update Feb 20 2008:

Lecture 6C.
Lecture 6D.

Update Mar 06 2008:

Lecture 7.
Lecture 7B.

Update Mar 20 2008:

Lecture 8.
Lecture 8B.
Lecture 9.
Lecture 9B.

Update Mar 31 2008:

Lecture 10.
Lecture 11.
Lecture 11B.

Huge hole in the sky — WMAP cold spot

Recent news on this interesting discovery: follow this link.

The technical paper is here.

Galaxies and Dark Matter Apart

Dark matter mystery deepens in ‘train wreck’ –”(…) a mass peak without galaxies [which] cannot be easily explained within the current collisionless dark matter paradigm” (see technical preprint here).

Update: Follow this link for an excellent description of the situation in Abell 520.