When the world is in crisis, physics doesn’t stop, science doesn’t stop. Wars, famine, the need to rebuild – all of these are actually great motivators for researchers. Unfortunately, most of us aren’t in fields that can actually offer anything close to intellectual assistance during these times. There is a strange and sad fact about this planet and the animals that inhabit it though, and that is that there is always some part of the world that is in crisis. If we took a pause every time something unimaginable to us happened, very little would ever get done. Nevertheless, I paused.

To get back into things, here’s about a months worth of reading – there should be two other posts out later this week (back to some more serious blogging too). Astrophysics and Gravitation gives us some updates on the MOND versus dark matter debate (is MOND winning? or have we already seen dark matter?), a silly paper on stars with wormhole centres, another success for Einstein@Home, a possible explanation for the “Fermi Bubble”, a new value for the Hubble constant, a long overdue fix for the Pioneer Anomaly, and a huge explosion in space for NASA. Lots of news from CERN and Fermilab in High Energy & Particles, including the search for SUSY at the LHC, the hunt for the Higgs, a single top quark at the CMS, heavy antimatter for ALICE, the much hyped results of new physics at the Tevatron, and using a Fermi-Fermi gas to model the Big Bang. Finally in General Relativity & Quantum Gravity we get a lesson on quantum Riemann surfaces in Chern-Simons theory, a curious experiment using gravity waves to suss out dimensionality, an introduction to group field theories, BF models of gravity, and spin foams, an update on the status of Horava gravity, an absurd paper on life inside black holes, and an essay from the 1960s by George Gamow.

MOND is Winning?

Stacy S. McGaugh (2011). A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies Phys.Rev.Lett.106:121303,2011 arXiv: 1102.3913v1

No, it’s not (maybe in the Charlie Sheen sense though). I’ve honestly never been enchanted with dark matter, the fact that it’s this giant unknown bothers me, and MOND holds a lot of appeal, logically for me, but the fact is, it doesn’t work, in the original form or any current one. Yes, some observations do fit into MOND predictions – we wouldn’t be talking about it now if it was entirely unphysical – but MOND is also wrong about a lot of things (you know, like the CMB). Yes, whatever this data was collected on gas rich galaxies probably does fit with what MOND predicts, and may very well disagree with Lambda-CDM (it’s certainly not a perfect model), but if MOND is wrong about other things, it’s still wrong. If someone comes up with a Modified Modified Newtonian dynamics that explains the CMB and fits with the Bullet Cluster, then we’ll talk.

For more, see Gas Rich Galaxies Confirm Prediction of Modified Gravity Theory (PR), Is this the end of dark matter?, Alternate theory poses dark matter challenge, More Evidence Against Dark Matter?, Dark Matter: Just Fine, Thanks,

Has EDELWEISS *Seen *Dark Matter?

EDELWEISS Collaboration (2011). Final results of the EDELWEISS-II WIMP search using a 4-kg array of cryogenic germanium detectors with interleaved electrodes arXiv arXiv: 1103.4070v2

No, it hasn’t. Well, probably not, anyway. So, the EDELWEISS-II collaboration released some fairly unexciting results, that some people are jumping on as evidence for dark matter. Why? Because the results point to possible evidence for signals for a few WIMP candidates. The CDMS collaboration last year published similarly tentative claims, though the energies of the EDELWEISS and CDMS candidates are different and thus both cannot be the same WIMP (if they are actually anything at all). The noise here is very high, so high that it’s not even clear if any of these are signals at all. People on the project don’t even really believe they’ve seen anything. The spokesman for EDELWEISS, Gilles Gerbier, said, “We cannot say for sure that there is no signal. We are in the uncomfortable situation… We may have a signal but we cannot make any claim now.” So, have we seen dark matter? No, it doesn’t sound like it. It’s not 100% out of the question, however (but probably 99.9% out of it).

For more, see Have Physicists Already Glimpsed Particles of Dark Matter?, Dark matter signal sparks interest, but falls short of discovery.

V. Dzhunushaliev, V. Folomeev, B. Kleihaus, & J. Kunz (2011). A Star Harbouring a Wormhole at its Center arXiv arXiv: 1102.4454v1

I’m sad I left talking about this article for so long because now it’s old news, but it’s just silly. Hey, let’s create some nonsensey sounding “phantom” matter and then say that wormholes can connect stars because of it! Why? Because speculation is fun! If anyone is taking this seriously, I can write up a “why this isn’t based on anything” post. I don’t call this science, I call it science fiction (but that doesn’t mean we’re not working on teleporters and cloaking devices today).

For more, see Stars Could Have Wormholes at Their Cores, Say Astrophysicists (TechReview arXiv blog), Stellar Wormholes May Exist.

B. Knispel, et al., (2011). Arecibo PALFA Survey and Einstein@Home: Binary Pulsar Discovery by Volunteer Computing The Astrophysical Journal Letters, 732, L1 (2011) arXiv: 1102.5340v2

I love these citizen science accomplishments. The Einstein@Home project has made its second discovery of a radio pulsar orbiting a white dwarf star. The pulsar, J1952+2630, was discovered within data collected back in 2005. J1952+2630’s white-dwarf companion is especially massive, suggesting the pulsar belongs to this rare class of intermediate-mass binary pulsars, making it the 6th of its type known. Congrats, Einstein@Home!

For more, see Volunteers find another prize pulsar, Einstein@Home Discovers New Binary Radio Pulsar.

K. S. Cheng, D. O. Chernyshov, V. A. Dogiel, C. -M. Ko, & W. -H. Ip (2011). Origin of the Fermi Bubble arXiv arXiv: 1103.1002v1

Remember that “unknown” gamma ray “bubble” structure discovered last year? A recent paper has suggested that it might be caused by some supermassive black hole “star capture process” at the centre of our galaxy. Honestly, I’ve got no means to judge this one; they claim it’s better than any other explanation, so let’s leave it at that for now.

For more, see Star-hungry black hole could blow galactic ‘bubbles’.

Riess, A., Macri, L., Casertano, S., Lampeitl, H., Ferguson, H., Filippenko, A., Jha, S., Li, W., & Chornock, R. (2011). A 3% SOLUTION: DETERMINATION OF THE HUBBLE CONSTANT WITH THE AND WIDE FIELD CAMERA 3 The Astrophysical Journal, 730 (2) DOI: 10.1088/0004-637X/730/2/119

Hubble’s Wide Field Camera 3 has provided a new and more accurate measurement of the Hubble constant, *H*_{0} = 73.8 ± 2.4 (km/s)/Mpc based off of distance and redshift data. In 2010, gravitational lensing data helped put *H*_{0} at 72.6 ± 3.1(km/s)/Mpc, while the WMAP seven-year results arrived at *H*_{0} = 71.0 ± 2.5 (km/s)/Mpc. So, are we narrowing in on the right value? With those errors, it’s really hard to say. Does this prove anything about the existence of dark energy? No…

For more, see (in order of increasing credulity) Have scientists cracked the speed at which the universe is expanding?, The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec! So there., The Hubble telescope eliminates one possible alternative to dark energy, Dark energy is not an illusion after all.

F. Francisco, O. Bertolami, P. J. S. Gil, & J. Páramos (2011). Modelling the reflective thermal contribution to the acceleration of the Pioneer spacecraft arXiv arXiv: 1103.5222v1

This isn’t one I’ve read thoroughly, but it seems quite plausible. The Pioneer anomaly has never sat well with me, so I might be a little too eager to accept this explanation, but if it just comes down to some uneven heat radiation, I really wouldn’t be very surprised. I’d be happy not to call this an *anomaly *anymore.

For more, see Pioneer Anomaly Solved By 1970s Computer Graphics Technique (TechReview arXiv Blog). *Solved by a 1970s computer graphics technique? No, no it wasn’t, but I won’t necessarily argue with the “solved” part*.

By their powers combined, NASA’s Swift, Hubble Space Telescope and Chandra X-ray Observatory observed a massive, and beautiful, explosion (now named GRB 110328A) at the centre of a distant galaxy. What was it? The usual party line is “supermassive black hole up to no good”, but with what little we know about the centre of galaxies, it’s not very meaningful to speculate in any particular direction.

For more, see Star-Eating Black Hole May Be Producing Universe’s Biggest Blast, NASA Telescopes Join Forces to Observe Unprecedented Explosion (PR).

Philip Bechtle, Klaus Desch, Herbi K. Dreiner, Michael Krämer, Ben O’Leary, Carsten Robens, Björn Sarrazin, & Peter Wienemann (2011). What if the LHC does not find supersymmetry in the sqrt(s)=7 TeV run? arXiv arXiv: 1102.4693v1

O. Buchmueller, & et al. (2011). Implications of Initial LHC Searches for Supersymmetry arXiv arXiv: 1102.4585v1

ATLAS Collaboration (2011). Search for Supersymmetry Using Final States with One Lepton, Jets, and Missing Transverse Momentum with the ATLAS Detector in sqrt[s]=7 TeV pp Collisions Physical Review Letters, 106 (13) DOI: 10.1103/PhysRevLett.106.131802

Dine, M., & Mason, J. (2011). Supersymmetry and its dynamical breaking Reports on Progress in Physics, 74 (5) DOI: 10.1088/0034-4885/74/5/056201

Part of me hopes that “SUSY” will be the new “it” baby name for this year, seeing how often everyone is saying it these days. There is practically as much excitement for ruling out (or in) supersymmetry as there is for finding the Higgs. Has the LHC (or anyone else) seen evidence for supersymmetry? No. Should they have yet? No, but it depends on what you were looking for specifically. Like everything HEP, some models have lower energy predictions, some have higher, and experimental physics is all about ruling out segments of those predictions. What if the LHC doesn’t see supersymmetry at all? Okay, some people will move on, some people will push their theories to higher energies. Things like SUSY are almost impossible to rule out entirely because there are so many different versions out there. I realise a statement like that sounds rather unscientific, “Well you can never completely rule out the existence of [blank] because maybe [blank] is more like this instead and you couldn’t see it where you were looking before”, and, wait… Hmm, I wasn’t trying to make an anti-SUSY point when I started this, I actually kind of like SUSY. Let me come back to this. At this stage, all we can say is that the parameter space for SUSY has been further constrained, thanks to ATLAS and the CMS.

For more, see Implications of 35/pb SUSY searches on best fit parameters., Beautiful theory collides with smashing particle data, What if the LHC doesn’t see SUSY?, The Large Hadron Collider enters the race for supersymmetry, Results from the first published search for supersymmetry at ATLAS have arrived.

CMS Collaboration (2011). Measurement of WW Production and Search for the Higgs Boson in pp Collisions at sqrt(s) = 7 TeV arXiv arXiv: 1102.5429v2

ATLAS and the CMS have decreased the MSSM Higgs parameter space a little bit more, which is exactly what we’d hoped for. It seems like the LHC might be very close to the Higgs by the end of this year.

Rivalry Drives Higgs Hunt

Almost surprisingly fast, the LHC has seen evidence of single top quark production, something that took the Tevatron years to build up too (the difference in energy is what really matters here). The ease at which the data was mined for this signal should make people very optimistic that if new physics is in there, it won’t stay buried in bins for too long.

For more, see Speedy single top sighting at the LHC, Standard Model Measurements: CMS Collaboration, March 14th, 2011 [pdf].

CERN’s ALICE collaboration has seen the formation of four anti-nuclei of Helium 4, which are the heaviest kind of antimatter we can currently make in a lab. ALICE is catching up to RHIC quickly in the quest for large anti-nuclei. This is not only an impressive accomplishment, but it can help us understand the early universe better, by creating nuclei that would have existed then.

For more, see ALICE’s wonderland reveals the heaviest antimatter ever observed (CERN Bulletin).

Starting in March, gossip began coming out of Fermilab that the Tevatron had seen some “new particle” or “new physics”. A sad truth about big projects that lose their funding is that you often get a lot of over-hyped, lacklustre, results getting publicized as the money runs out. In some cases it’s a mad dash to secure more last minute funding (not likely in this case) and in others it’s more of a “you’ll miss me when I’m gone” kind of PR stunt. Given the fact that the Tevatron’s latest efforts in narrowing in on the Higgs mass range yielded no improvement, I’d say that it’s unlikely that new physics, or new particles, will be able to come out of their data. However, all of the hype neglected to mention what this “new physics” was…

For more, see the hype: Interesting effect at the Tevatron hints at new physics, At Particle Lab, a Tantalizing Glimpse Has Physicists Holding Their Breaths.

And then we got to see the results:

CDF Collaboration, & T. Aaltonen (2011). Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV arXiv arXiv: 1104.0699v1

So, it appears the CDF collaboration has seen some particles with a mass distribution that doesn’t quite fit with anything we know or would expect. They don’t have a huge number of these events, so the statistics aren’t great, and there is also some discussion that the energies (mass distributions) haven’t been measured or interpreted correctly, so unfortunately, nothing can be said for certain at this point (although when is that ever the case). There are some good remarks here A hint of something new in “W+dijets” at CDF and Fermilab: CDF “new force” seminar tonight as well as the recorded seminar: Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in proton-antiproton Collisions at sqrt(s) = 1.96 TeV. It’s really too soon to say much of anything yet on this “possible discovery”. Despite some reports, it’s not evidence for some non-standard Higgs; we don’t even know if the signal is a real one or not yet.

Trenkwalder, A., & et al. (2011). Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture Physical Review Letters, 106 (11) DOI: 10.1103/PhysRevLett.106.115304

This one was too quantum/condensed matter for me, but is of some interest to cosmologists: an ultracold Fermi-Fermi mixture provides an interesting model for very early universe conditions.

For more, see An Icy Gaze into the Big Bang, Model offers icy gaze into the Big Bang.

Tudor Dimofte (2011). Quantum Riemann Surfaces in Chern-Simons Theory arXiv arXiv: 1102.4847v1

This is a sizable volume on quantum Riemann surfaces within the Chern-Simons theory and definitely a worthwhile to read for those interested in topological quantum field theory. Dimofte ends up with a state integral model like I’ve never seen before for finding analytical solutions for the holomorphic *blocks *of Chern-Simons theory. It’s not a light read.

Mureika, J., & Stojkovic, D. (2011). Detecting Vanishing Dimensions via Primordial Gravitational Wave Astronomy Physical Review Letters, 106 (10) DOI: 10.1103/PhysRevLett.106.101101

This is a really interesting concept, but a paper I haven’t read especially well yet. The authors are suggesting that we might be able to use gravity waves to determine if, at high energies, our spacetime has a lower dimensionality. The basic idea actually seems pretty brilliant – we know we can’t have gravity waves in a (2+1) spacetime, so if we could find a situation with energies high enough that we should expect dimensional reduction, use something like LISA to look for gravity waves. If we see gravity waves, we must be looking at a (3+1), or higher dimension, spacetime. The basic concept really only hitches on us being able to see gravity waves (which we currently can’t, and may not even theoretically ever be able to) and LISA existing (and LISA has just been cancelled). Hmm… Despite the obvious huge problems, there is something I really like about this concept. This is an idea I think warrants further consideration.

For more, see Testing for Vanishing Dimensions, Physicists investigate lower dimensions of the universe.

Patrizia Vitale (2011). A field-theoretic approach to Spin Foam models in Quantum Gravity arXiv arXiv: 1103.4172v1

I’ve been seeing more and more work on BF models of gravity lately, so I think it might be time I start looking at them more seriously. This is a nice, and fairly short, introduction to some concepts of Group Field Theory as it relates to BF models.

Matt Visser (2011). Status of Horava gravity: A personal perspective arXiv arXiv: 1103.5587v2

Here is a nice, thoughtful, analysis of some of the current investigations into Horava gravity. I especially like some of his concluding remarks:

Without a deeper understanding of the fundamental framework one is operating in, detailed phenomenological studies (and in particular specific applications to cosmology and astrophysics) are simply premature. Specifically, one needs more than hand-waving “of course it runs to general relativity in the IR” arguments. There may be subtle (or even not so subtle) qualitative deviations from general relativity due to the preferred foliation, and really pinning that issue down would be a good idea before investing more time on detailed applications.

Vyacheslav I. Dokuchaev (2011). Is there life inside black holes? arXiv arXiv: 1103.6140v1

I was confused when someone first sent me this paper because it sounded alarmingly crankish. After reading briefly into it, I was convinced it was especially crankish. Could super advanced civilizations live on some weirdly “spacious” orbits “inside” a black hole? This isn’t science, this is an episode of Stargate SG-1. The language issues (and strange diagrams) make it fairly painful to read, but if pushed, I can go through it thoroughly.

Joseph Silk (2011). Feedback in Galaxy Formation arXiv arXiv: 1102.0283v1

Abstract:

I review the outstanding problems in galaxy formation theory, and the role of feedback in resolving them. I address the efficiency of star formation, the galactic star formation rate, and the roles of supernovae and supermassive black holes.

Silk’s Figure 1. “The theoretical mass function of galaxies compared to the observed luminosity function.”

So, as most of us know, there are still quite a few puzzles when it comes to how galaxies form. Joseph Silk has put together a little discussion on some of these problems, and, perhaps more interestingly, ways in which they can be rectified (or already have been). For example, cold dark matter simulations alone predicted more *halo* dwarf galaxies than were observed (called the “missing satellites” problem; see Kravtsov and pdf slides). Through both a better understanding of observation (there were in fact more dwarfs out there than we though, they were just very faint) and more sophisticated models (taking into account baryonic physics too), this problem doesn’t seem so huge anymore (it’s not 100% resolved, mind you). There are many other much *less resolved* issues when it comes to gravitation and galaxy formation that are also deserving of some serious study.

Alexandre Amblard, & et al. (2011). Sub-millimetre galaxies reside in dark matter halos with masses greater than 3×10^11 solar masses Nature arXiv: 1101.1080v1

From the press release:

ESA’s Herschel space observatory has discovered a population of dust-enshrouded galaxies that do not need as much dark matter as previously thought to collect gas and burst into star formation.

This is certainly good news for some galaxy formation theorists and another fun piece of the puzzle to think about. The latest analysis of Herschel observations suggest the existence of galaxies that are roughly 300 billion solar masses but with as many stars as expected from a galaxy of *5000* billion solar masses (ie. *that’s not got much dark matter in it*). This is quite fascinating, because most of the current theories dealing with galaxy formation require these huge amounts of dark matter to allow budding galaxies to stay together, but now there are observations that suggest otherwise. It looks like we’ll have to adjust our ideas of dark matter’s role in the galaxy (not that this should surprise anyone).

Earlier this month, there was a really excellent guest post at Cosmic Variance about the state of dark matter detection experiments by Neal Weiner (to complete the discussion of dark matter in the galaxy) so you should give that a read.

San-Jose, P., González, J., & Guinea, F. (2011). Electron-Induced Rippling in Graphene Physical Review Letters, 106 (4) DOI: 10.1103/PhysRevLett.106.045502

So this was a hot topic this month that I’m just getting around to: graphene as an analogy for the Higgs field. Now, as always with these analogy papers, I get a little nervous. When there isn’t an explicit (AdS/CFT-esque) correspondence, it’s really very difficult (and a somewhat philosophical matter) to say what we are actually able to learn from analogies. In this case, the analogy comes from the fact that the “energy landscape” of graphene moving in 2-dimensions is *similar *to that of the Higgs field in 3-dimensions, in that they are both described by a *similar *Mexican hat potential. Okay. There are other situations where we see Mexican hat potentials, like when rotating a bead on a circle, but that doesn’t mean that they would be at all useful in thinking about spontaneous symmetry breaking. Since I don’t really know anything relevant about graphene, quantum criticality, or… well, materials in general, I am completely unqualified to to judge this analogy, but it is still an analogy, not a correspondence. *Similar *and *the same *are, importantly, and fundamentally different.* *

For more, see Theorists turn to graphene for clues to Higgs.

Lloyd, S., Maccone, L., Garcia-Patron, R., Giovannetti, V., Shikano, Y., Pirandola, S., Rozema, L., Darabi, A., Soudagar, Y., Shalm, L., & Steinberg, A. (2011). Closed Timelike Curves via Postselection: Theory and Experimental Test of Consistency Physical Review Letters, 106 (4) DOI: 10.1103/PhysRevLett.106.040403

So this paper was very “win, lose, win” for me. “Closed timelike curves” (CTCs) mean general relativity, which means I’m happy. “Postselection” means quantum interpretations, which means I’m less happy. Now, this isn’t me saying that I don’t think interpretations are wonderful, in fact, they’re my favourite part of quantum mechanics, but it is a huge field, which takes a paper on time travel, away from general relativity, and thus outside of my wheelhouse.

As we know, CTCs are not forbidden by general relativity, although they are usually excluded because of the “logical” paradoxes they lead to (*if being able to kill your own grandfather really bothers you, that i*s). There are many people who are interested in universes that include CTCs however, not because of the *time travel* applications, but simply because they are not impossible, and thus might be able to be included in a self consistent model of our universe.

From the paper:

This self-consistency requirement gives rise to a theory of closed timelike curves via entanglement and postselection, P-CTCs. P-CTCs are based on the Horowitz-Maldacena ‘‘final state condition’’ for black hole evaporation, and on the suggestion of Bennett [pdf] and Schumacher that teleportation could be used to describe time travel.

This is pretty neat stuff, that has gone through a few iterations within the quantum information community, most famously led by David Deutsch in 1991 with his *Quantum Mechanics Near Closed Timelike Lines*. Given my naiveté when it comes to QI, I’m just going to have to take the authors’ word for it that their model doesn’t, in fact, agree with Deutsch’s (although it is consistent within their framework). What is really neat though is that, because of the nature of postselection, it’s possible to do (and they did) a little *grandfather paradox* experiment with their model using P-CTCs and photons. Their “grandfather paradox circuit” is worth a look, if you’re interested. The argument is technical, and I can’t personally say I followed it thoroughly, but I can appreciate their (wonderfully worded) conclusions:

[S]uicidal photons in a CTC obey the Novikov principle: they cannot kill their former selves.

Our P-CTCs always send pure states to pure states: they do not create entropy. Hence, P-CTCs provide a distinct resolution to Deutsch’s unproved theorem paradox, in which the time traveler reveals the proof of a theorem to a mathematician, who includes it in the same book from which the traveler has learned it (rather, will learn it). How did the proof come into existence? Deutsch adds an additional maximum entropy postulate to eliminate this paradox. By contrast, postselected CTCs automatically solve it through entanglement: the CTC creates a random mixture of all possible ‘‘proofs.’’

So, they have a resolution to the grandfather paradox within P-CTCs. While theoretically, their model is inequivalent to Deutsch’s, experimentally, one can not distinguish the two, unfortunately. They also state that they “cannot test whether a general relativistic CTC obeys [their] theory or not,” which confuses me a little by the terminology, because CTCs don’t make sense as a concept outside of general relativity, so what a “non-general relativistic CTC” is vs. as “general relativistic CTC” is, I can’t say (I’m assuming they just mean a CTC in a distinctly curved spacetime, which would be very hard to work into an experiment).

For more, see Time Travel Without Regrets.

Hohm, O., & Kwak, S. (2011). Frame-like geometry of double field theory Journal of Physics A: Mathematical and Theoretical, 44 (8) DOI: 10.1088/1751-8113/44/8/085404

The abstract:

We relate two formulations of the recently constructed double field theory to a frame-like geometrical formalism developed by Siegel. A self-contained presentation of this formalism is given, including a discussion of the constraints and its solutions, and of the resulting Riemann tensor, Ricci tensor and curvature scalar. This curvature scalar can be used to define an action, and it is shown that this action is equivalent to that of double field theory.

This is still in my “to read” list, but I thought I’d mention it as double field theory has a little bit of buzz right now, that makes it worth a look. I do find it curious though, that there seems to be several incredibly similar papers to the above on this topic, by the same authors, but I’m going to go with the most recent one.

Carlo Rovelli (2011). Lectures on loop gravity arXiv arXiv: 1102.3660v2

The abstract:

This is a preliminary version of the introductory lectures on loop quantum gravity that I will give at the quantum gravity school in Zakopane. The theory is presented in self-contained form, without emphasis on its derivation from classical general relativity. Dynamics is given in the covariant form. The approximations needed to compute physical quantities are discussed. Some applications are described, including the recent derivation of de Sitter cosmology from full quantum gravity.

As if this needs explanation: Carlo Rovelli, one of the physicists who impresses me the most, has a great introduction to loop quantum gravity online (that is being updated currently). For anyone interested in the topic but wondering how to start, I imagine this is *the* recommendation now.

For more, see NASA’s Hubble Finds Most Distant Galaxy Candidate Ever Seen in Universe.

M. Vardanyan, R. Trotta, & J. Silk (2011). Applications of Bayesian model averaging to the curvature and size of the Universe arXiv arXiv: 1101.5476v1

For more, see New Model Says the Cosmos Is At Least 250 Times Larger Than the Visible Universe.

John Kormendy, & Ralf Bender (2011). Supermassive black holes do not correlate with dark matter halos of galaxies Nature 469, 377 (2011) arXiv: 1101.4650v1

For more, see Dark matter does not act as a growth factor.

D. M. Webber et al. (MuLan Collaboration) (2011). Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant to Part-per-Million Precision Physical Review Letters, 106 (4) DOI: 10.1103/PhysRevLett.106.041803

For more, see How Strong Is the Weak Force? New Measurement of the Muon Lifetime.

Igor I. Smolyaninov (2011). Virtual Black Holes in Hyperbolic Metamaterials arXiv arXiv: 1101.4625v1

For more, see Physicist Discovers How To Make Quantum Foam In A Test Tube.

Kirill Krasnov (2011). Gravity as a diffeomorphism invariant gauge theory arXiv arXiv: 1101.4788v1

]]>The CMS on SUSY, Bill Unruh on simulated Hawking radiation, Ed Witten on knots, and Schenkel and Van Oystaeyen on noncommutative space(times):

CMS Collaboration (2011). Search for Supersymmetry in pp Collisions at 7 TeV in Events with Jets and Missing Transverse Energy arXiv arXiv: 1101.1628v1

The CMS Collaboration released results this month ruling out supersymmetric particles with masses of less than ~ 0.5 TeV/c^{2}. This is just one of a series of ongoing SUSY related papers analyzing last years data and spitting out constraints on models (which is hugely important). We’ll be seeing results papers for years to come, but it’s nice to see evidence of the LHC being exactly what we all hoped it would be already: the thing that tells us if we’re likely on the right track or not.

For more, see Reality check at the LHC.

Silke Weinfurtner, Edmund W. Tedford, Matthew C. J. Penrice, William G. Unruh, & Gregory A. Lawrence (2010). Measurement of stimulated Hawking emission in an analogue system Phys. Rev. Lett., 106 (2), 1302-1306 arXiv: 1008.1911v2

The abstract:

Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. To address this issue experimentally, we utilize the analogy between the propagation of fields around black holes and surface waves on moving water. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include surface wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep-water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process.

Analogues often make me a little uncomfortable in physics, for what are probably obvious reasons, but Bill Unruh has had a lot of success and acceptance with his analogue black hole/white hole models in the past. The line between *similar* and *the same*, and if it is actually telling us anything to observe properties in these analogue systems (which have some major fundamental differences) always gets to me in these matters, so I’m going to have to come back to this one to give further comments.

For more, see Wave-Generated ‘White Hole’ Boosts Hawking Radiation Theory: UBC Research.

Edward Witten (2011). Fivebranes and Knots arXiv arXiv: 1101.3216v1

The abstract:

We develop an approach to Khovanov homology of knots via gauge theory (previous physics-based approches involved other descriptions of the relevant spaces of BPS states). The starting point is a system of D3-branes ending on an NS5-brane with a nonzero theta-angle. On the one hand, this system can be related to a Chern-Simons gauge theory on the boundary of the D3-brane worldvolume; on the other hand, it can be studied by standard techniques of S-duality and T-duality. Combining the two approaches leads to a new and manifestly invariant description of the Jones polynomial of knots, and its generalizations, and to a manifestly invariant description of Khovanov homology, in terms of certain elliptic partial differential equations in four and five dimensions.

So Ed Witten is one of those few authors whose work I can feel safe about getting excited over before I’ve read it, and at 146 pages, well, it’s unlikely I’ll ever make it through all of this (although its length is only due to the fact that it very thorough, and thus imaginably very useful). I’m going to defer to the University of Toronto’s Daniel Moskovich from Low Dimensional Topology (which I can’t recommend enough) on this one, as he wrote:

Based on Witten’s record on such topics, and on a preliminary visual scan of the introduction, it would not be unreasonable to surmise that this preprint could change history. Khovanov homology will never look the same again. …This is trully a momentous occasion for knot theory!

For more, see Newsflash: Witten’s new preprint.

Alexander Schenkel (2011). Quantum Field Theory on Curved Noncommutative Spacetimes arXiv arXiv: 1101.3492v2

The basic concepts from the paper:

Noncommutative (NC) geometry provides a rich mathematical framework to modify the standard formalism of quantum field theory (QFT) in order to include quantum effects of spacetime

itself. … we collect the required tools from Drinfel’d twists and their associated NC geometry… We define an action functional for a real and free scalar field on twist-deformed curved spacetimes … and derive the corresponding deformed wave operator. … The deformed Green’s operators are constructed… to construct the space of real solutions of the deformed wave equation …The quantization is performed… “

And the important conclusions:

We have shown that the deformed symplectic R[[l ]]-module is isomorphic, via symplectic isomorphisms, to the formal power series extension of the undeformed symplectic vector space.

A direct consequence of this symplectic isomorphism for the deformed QFT is that it is ∗-algebra isomorphic to the formal power series extension of the undeformed QFT. This immediately yields isomorphisms between the corresponding groups of symplectic automorphisms and bijections between

the corresponding spaces of algebraic states.

Noncommutative spacetimes, or spaces, for that matter, are not things I have spent much time looking at, so I honestly can’t speak to the real content of this paper, other than by saying that there is growing interest in QFT on noncommutative spacetimes and those in quantum gravity should perhaps take note of it.

This isn’t about new research, per se, but Fred Van Oystaeyen has an interesting article on noncommutative space that is worth a read.

]]>Planck Collaboration (2011). Planck Early Results: The Planck mission arXiv arXiv: 1101.2022v1

The *Early Results Papers *from the Planck Collaboration are based on the data acquired by the Planck satellite between August 13th, 2009 to June 6th, 2010. This work is “an overview of the history of Planck in its first year of operations” and was released along side Planck’s Early Release Compact Source Catalogue, “the first data product based on Planck to be released publicly”. Andrew Jaffe has a great summary of the results so far.

For more, see Planck: First results.

Sukanya Chakrabarti, Frank Bigiel, Philip Chang, & Leo Blitz (2011). Finding Dark Galaxies From Their Tidal Imprints arXiv arXiv: 1101.0815v1

From the abstract:

We describe ongoing work on a new method that allows one to determine the mass and relative position (in galactocentric radius and azimuth) of galactic companions purely from analysis of observed disturbances in gas disks….This approach has broad implications for many areas of astrophysics — for the indirect detection of dark matter (or dark-matter dominated dwarf galaxies), and for galaxy evolution in its use as a decipher for the dynamical impact of satellites on galactic disks. Here, we provide a proof of principle of the method by applying it to infer and quantitatively characterize optically visible galactic companions of local spirals, from the analysis of observed disturbances in outer gas disks.”

The tl;dr version is that they have a technique for detecting companion galaxies that need not be optically visible (which is great, because sometimes we can’t see things for reasons *other *than them being made out of dark matter) and this paper acts as a proof of concept by using it to correctly infer and characterize galaxies that we already can observe. Does it say anything about having detected a dark matter galaxy? No. If such things existed it *could* be used to detect them (if they were acting as companion to regular matter galaxies), but it doesn’t say anything about their existence. I genuinely feel I read a different paper than the authors who wrote the below two articles.

For more, see Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?, The Milky Way might be surrounded by invisible dark matter galaxies.

From the NASA press release:

Astronomers have turned up the first direct proof that “standard candles” used to illuminate the size of the universe, termed Cepheids, shrink in mass, making them not quite as standard as once thought. The findings, made with NASA’s Spitzer Space Telescope, will help astronomers make even more precise measurements of the size, age and expansion rate of our universe.

Obviously, this is rather significant, but the immediate consequence of standard candles not being standard isn’t that it will allow for accurate future measurements of things, it’s that it calls into question the current measurements we have (for things like galactic distances).

From lead author of the study, Massimo Marengo*:

When using Cepheids as standard candles, we must be extra careful because, much like actual candles, they are consumed as they burn.

*He’s also an author on a wonderfully titled paper, Close Binaries with Infrared Excess: Destroyers of Worlds?.

For more, see Cosmology Standard Candle not so Standard After All.

Baldi, M., & Pettorino, V. (2011). High-z massive clusters as a test for dynamical coupled dark energy Monthly Notices of the Royal Astronomical Society: Letters DOI: 10.1111/j.1745-3933.2010.00975.x

Abstract:

The recent detection by Jee et al. of the massive cluster XMMU J2235.3−2557 at a redshift

z≈ 1.4, with an estimated massM_{324}= (6.4 ± 1.2) × 10^{14}M_{⊙}, has been claimed to be a possible challenge to the standard ΛCDM cosmological model. More specifically, the probability to detect such a cluster has been estimated to be ∼0.005 if a ΛCDM model with Gaussian initial conditions is assumed, resulting in a 3σ discrepancy from the standard cosmological model. In this Letter we propose to use high-redshift clusters as the one detected in Jee et al. to compare the cosmological constant scenario with interacting dark energy models. We show that coupled dark energy models, where an interaction is present between dark energy and cold dark matter, can significantly enhance the probability to observe very massive clusters at high redshift.

So I actually haven’t read this paper yet, but was told by a cosmologist friend that it might turn out to be significant. I think it also might be too outside my area for me to have a serious comment on, but for the astro/dark/cosmologists out there, it could be worth a read.

Turok, N. (2011). Particle physics: Beyond Feynman’s diagrams Nature, 469 (7329), 165-166 DOI: 10.1038/469165a

Abstract:

Generations of physicists have spent much of their lives using Richard Feynman’s famous diagrams to calculate how particles interact. New mathematical tools are simplifying the results and suggesting improved underlying principles.

So this was a *news* piece more than it was anything, but I thought Neil raised some interesting questions in it: Feynman diagrams aren’t *cutting edge* anymore, should we be looking for better approaches in QFT?

The formulation of quantum field theory used in Feynman’s rules emphasizes locality, the principle that particle interactions occur at specific points in space-time; and unitarity, the principle that quantum-mechanical probabilities must sum to unity. However, the price of making these features explicit is that a huge amount of redundancy (technically known as gauge freedom) is introduced at intermediate steps, only to eventually cancel out in the final, physical result.

There are non-Feynman calculus approaches to some *classical *(you know what I mean) problems, see: An Operator Product Expansion for Polygonal null Wilson Loops and The All-Loop Integrand For Scattering Amplitudes in Planar N=4 SYM (new formulations of QFT), but do these methods provide more insight than Feynman diagrams, while escaping some of the problems? Neil thinks so (and many others agree).

Quantum field theory is the most powerful mathematical formalism known to physics, successfully predicting, for example, the magnetic moment of the electron to one part in a trillion. The recent discovery of mathematical structures that are now seen to control quantum field theory is likely to be of enormous significance, allowing us not only to calculate complex physical processes relevant to real experiments, but also to tackle fundamental questions such as the quantum structure of space-time itself. The fact that the new formulations of the theory jettison much of the traditional language of quantum field theory, and yet are both simpler and more effective, suggests that an improved set of founding principles may also be at hand.

He makes a convincing argument. Maybe someday we won’t even be teaching Feynman diagrams for practical use.

]]>The dwarf galaxy Henize 2-10, seen in visible light by the Hubble Space Telescope. The central, light-pink region shows an area of radio emission, seen with the Very Large Array. This area indicates the presence of a supermassive black hole drawing in material from its surroundings. This also is indicated by strong X-ray emission from this region detected by the Chandra X-Ray Observatory.

Astronomers have identified a supermassive black hole candidate at the centre of the dwarf galaxy Henize 2-10. Amy Reines, one of the members of the discovery team, on why this is important:

This galaxy gives us important clues about a very early phase of galaxy evolution that has not been observed before.

For more, see Surprise: Dwarf Galaxy Harbors Supermassive Black Hole.

Obviously the big news of today is the rumour that the Tevatron will cease operations at the end of 2011. We’re still waiting on the official announcement though.

Andreyev, A., et al., (2010). New Type of Asymmetric Fission in Proton-Rich Nuclei Physical Review Letters, 105 (25) DOI: 10.1103/PhysRevLett.105.252502

An exotic fission process is studied and an exciting and anomalous asymmetry in the daughter masses is discussed.

As Abhishek Agarwal writes:

The ISOLDE team’s puzzling result hints that a very subtle interplay between macroscopic and microscopic interactions plays a deeper role in the fission process than expected and is likely to inspire detailed theoretical studies and further experiment.

Neat.

For more, see Unequal Parts.

Siye Wu (2010). Projective flatness in the quantisation of bosons and fermions arXiv arXiv: 1008.5333v2

The abstract:

We compare the quantisation of linear systems of bosons and fermions. We recall the appearance of projectively flat connection and results on parallel transport in the quantisation of bosons. We then discuss pre-quantisation and quantisation of fermions using the calculus of fermionic variables. We then define a natural connection on the bundle of Hilbert spaces and show that it is projectively flat. This identifies, up to a phase, equivalent spinor representations constructed by various polarisations. We introduce the concept of metaplectic correction for fermions and show that the bundle of corrected Hilbert spaces is naturally flat. We then show that the parallel transport in the bundle of Hilbert spaces along a geodesic is the rescaled projection or the Bogoliubov transformation provided that the geodesic lies within the complement of a cut locus. Finally, we study the bundle of Hilbert spaces when there is a symmetry.

So I’m a sucker for nice math, and this paper has got that in spades. If you want some beautiful geometry, and some quantum mechanics (which together, I firmly believe are critical to good quantum gravity), then this is well worth the read.

]]>Tracy Holsclaw, Ujjaini Alam, Bruno Sanso, Herbert Lee, Katrin Heitmann, Salman Habib, & David Higdon (2010). Nonparametric Dark Energy Reconstruction from Supernova Data Phys. Rev. Lett. arXiv: 1011.3079v1

The abstract:

Understanding the origin of the accelerated expansion of the Universe poses one of the greatest challenges in physics today. Lacking a compelling fundamental theory to test, observational efforts are targeted at a better characterization of the underlying cause. If a new form of mass-energy, dark energy, is driving the acceleration, the redshift evolution of the equation of state parameter w(z) will hold essential clues as to its origin. To best exploit data from observations it is necessary to develop a robust and accurate reconstruction approach, with controlled errors, for w(z). We introduce a new, nonparametric method for solving the associated statistical inverse problem based on Gaussian process modeling and Markov chain Monte Carlo sampling. Applying this method to recent supernova measurements, we reconstruct the continuous history of w out to redshift z=1.5.

As the paper says, “In order to extract useful information from cosmological data, a reliable and robust reconstruction method for w(z) [the equation of state parameter] is crucial”, and that’s what this paper aims to provide. It’s not the most exciting thing you’ll ever read (although it is short), but without work along these lines, much of cosmology and astrophysics is actually pretty meaningless, so it’s certainly worth remembering that.

For more, see Statistical modeling could help us understand cosmic acceleration.

Valeri P. Frolov, & Shinji Mukohyama (2010). Brane Holes arXiv arXiv: 1012.4541v1

The abstract:

The aim of this paper is to demonstrate that in models with large extra dimensions under special conditions one can extract information from the interior of 4D black holes. For this purpose we study an induced geometry on a test brane in the background of a higher dimensional static black string or a black brane. We show that at the intersection surface of the test brane and the bulk black string/brane the induced metric has an event horizon, so that the test brane contains a black hole. We call it a brane hole. … We discuss thermodynamic properties of brane holes and interesting questions which arise when such an extra dimensional channel for the information mining exists.

Who doesn’t love higher dimensional solutions for black holes? Honestly, I haven’t had time to give this a thorough read yet but it looks rather promising.

For more, see Cosmologists Discover How Black Holes Can Leak.

]]>Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, & Hiranya V. Peiris (2010). First Observational Tests of Eternal Inflation arXiv DOI: 1012.1995

The abstract:

The eternal inflation scenario predicts that our observable universe resides inside a single bubble embedded in a vast multiverse, the majority of which is still undergoing super-accelerated expansion. Many of the theories giving rise to eternal inflation predict that we have causal access to collisions with other bubble universes, opening up the possibility that observational cosmology can probe the dynamics of eternal inflation. We present the first observational search for the effects of bubble collisions, using cosmic microwave background data from the WMAP satellite. Using a modular algorithm that is designed to avoid a posteriori selection effects, we find four features on the CMB sky that are consistent with being bubble collisions. If this evidence is corroborated by upcoming data from the Planck satellite, we will be able to gain insight into the possible existence of the multiverse.

Interesting, but far from conclusive, evidence has been found suggesting the possible existence of the multiverse. Of course, these observations could also happen if there was no multiverse, but they allow for the possibility of its existence. Further study will say more one way or the other (although it’s likely to never be conclusive).

For more, see Cosmic Radiation Features Could Suggest Our Universe Is Not Alone.

CMS Collaboration (2010). Search for Microscopic Black Hole Signatures at the Large Hadron Collider arXiv arXiv: 1012.3375v1

So no micoscopic black holes have been produced or detected at the LHC. This is not surprising; it’s almost not even news. Outside of the tabloids, I don’t think very many people thought there was any chance of it happening, as very few versions of quantum gravity would allow for black hole production at these, relatively low, energy levels. However, ruling things out is always important (but it isn’t this grand failure of string theory as some people are claiming).

For more, see Search for microscopic black hole signatures at the Large Hadron Collider, LHC confirms a modest stringy prediction on black holes, Missing Black Holes Cause Trouble for String Theory, String Theory Fails Another Test, the “Supertest”.

Marcin Domagala, Kristina Giesel, & Wojciech Kaminski, Jerzy Lewandowski (2010). Gravity quantized arXiv DOI: 1009.2445

So I’m honestly not sure why this one is here, but io9 did a write up that seems to have a lot of people talking. Okay, so I haven’t read more than the abstract (*I’m supposed to be on vacation!), but *the abstract was fairly lackluster*.*

Here is the abstract in its entirety:

…”but we do not have quantum gravity.” This phrase is often used when analysis of a physical problem enters the regime in which quantum gravity effects should be taken into account. In fact, there are several models of the gravitational field coupled to (scalar) fields for which the quantization procedure can be completed using loop quantum gravity techniques. The model we present in this paper consist of the gravitational field coupled to a scalar field. The result has similar structure to the loop quantum cosmology models, except for that it involves all the local degrees of freedom because no symmetry reduction has been performed at the classical level.

You guys can tell me if it’s worth reading when I’m back.

For more, see Loop quantum gravity could unite physics and take us back to the Big Bang.

Slava G. Turyshev, & Viktor T. Toth (2010). The Pioneer Anomaly arXiv DOI: 1001.3686

This is actually great (although far too long to read unless you’re especially interested). I’ll direct you to the wonderful summaries below, but basically, the Pioneer anomaly, an *anomaly* some people suggest indicates problems with general relativity, is probably not an anomaly at all.

For more, see The Pioneer Anomaly, a 30-Year-Old Cosmic Mystery, May Be Resolved At Last, Farewell, Pioneer Anomaly?.

]]>I’m not going to call these the best papers, or the most cited (although some of them are), but they all contain things that were interesting or unique that encouraged further work and discussion (even if myself and others disagreed with the results) and thus, they got gingerbread cookies baked in their honour. So without further ado, these are the 10 cookies highlights from 2010 literature in general relativity, quantum gravity, and gravitation (ranked by date of e-print, so don’t read into the order):

Yun Soo Myung, & Yong-Wan Kim (2010). Thermodynamics of *Hořava*-Lifshitz black holes. Eur.Phys.J. C68 (2010) 265-270 arXiv: 0905.0179v3

We study black holes in the Ho

řava-Lifshitz gravity with a parameter λ. For 1/3≤ λ < 3, the black holes behave the Lifshitz black holes with dynamical exponent 0 < z ≤ 4, while for λ > 3, the black holes behave the Reissner-Nordstr¨om type black hole in asymptotically flat spacetimes. Hence, these all are quite different from the Schwarzschild-AdS black hole of Einstein gravity. The temperature, mass, entropy, and heat capacity are derived for investigating thermodynamic properties of these black holes.

Using this first law [of thermodynamics], we derive an entropy…

So, obviously a *Hořava*-Lifshitz gravity paper was a must for 2010, but selecting which one was difficult. While this paper was technically written in 2009, it was baked published in the *European Physical Journal* in 2010 (and it was in 2010 that it was really being discussed). Cited, approximately 95 times, it’s clearly on the more delicious side of *Hořava*-Lifshitz.

Asimina Arvanitaki, Savas Dimopoulos, Sergei Dubovsky, Nemanja Kaloper, & John March-Russell (2009). String Axiverse Phys.Rev. D, 81 arXiv: 0905.4720v2

String theory suggests the simultaneous presence of many ultralight axions possibly populating each decade of mass down to the Hubble scale 10⁻³³eV. Conversely the presence of such a plenitude of axions (an ‘axiverse’) would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. We investigate how upcoming astrophysical experiments will explore the existence of such axions over a vast mass range… The rapidly rotating black hole in the X-ray binary LMC X-1 implies an upper limit on the decay constant of the QCD axion fₐ ≤ 2 x 10¹⁷ GeV, much below the Planck mass…

Testing stringy ideas with astrophysics! At 42 pages and a respectable 35 citations, I choose this paper as one of the most enjoyable from 2010 because it presents a fairly abstract idea with a clever way to test it. One of the hardest tasks in theoretical physics, especially in quantum gravity, is to figure out recipes observations that would uniquely confirm your ideas, and that’s basically what this team has done.

Erik P. Verlinde (2010). On the Origin of Gravity and the Laws of Newton arXiv arXiv: 1001.0785v1

Starting from first principles and general assumptions Newton’s law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton’s law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

When a particle has an entropic reason to be on one side of the membrane and the membrane carries a temperature, it will experience an eeffective force equal to

This is the entropic force.

I’m sure some can guess how it pains me to bake this one, but Verlinde certainly got a lot of people talking about new ideas, and spawned a lot of publications by other researchers in 2010 (132 citations and counting). For that unsatisfied taste left in your mouth after the above, try Padmanabhan’s “Surface Density of Spacetime Degrees of Freedom from Equipartition Law in theories of Gravity” 1003.5665v2.

E. Komatsu, et al. (2010). Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation arXiv arXiv: 1001.4538v3 (see also: *There Cosmic Microwave Background Anomalies?* 1001.4758v1)

The 7-year WMAP data and improved astrophysical data rigorously test the standard cosmological model and its extensions. By combining WMAP with the latest distance measurements from BAO and H0 measurement, we determine the parameters of the simplest LCDM model. …

Now while this is certainly on the observational side of things, seeing as it was the culmination of a huge experiment, profoundly critical to cosmology, it seemed well worth to include in a 2010 list (the 635 citations this year also suggest that). The prep. time was well worth the results here.

Gary T. Horowitz (2010). Introduction to Holographic Superconductors arXiv arXiv: 1002.1722v2

These lectures give an introduction to the theory of holographic superconductors. These are superconductors that have a dual gravitational description using gauge/gravity duality. After introducing a suitable gravitational theory, we discuss its properties in various regimes: the probe limit, the effects of backreaction, the zero temperature limit, and the addition of magnetic fields. Using the gauge/gravity dictionary, these properties reproduce many of the standard features of superconductors. …

The gauge/gravity dictionary says that the retarded Green’s function (for Jx) in the dual field theory is

So this one is a little different than the above, as it doesn’t really present a new result, but it is, in fact a mini lecture series on a hot new topic. Why did I choose this instead of one of the papers it cited, perhaps? Well, because Horowitz sets out the ingredients better than almost anybody else. In terms of clear pieces of literature written on the amazing beauty that is the AdS/CFT correspondence, this has got to be one of the best from 2010 (and it has been cited 64 times by those who hungrily agree).

Antonio De Felice, & Shinji Tsujikawa (2010). f(R) theories Living Rev. Rel. 13: 3, 2010 arXiv: 1002.4928v2

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity – such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.

We start with the 4-dimensional action in f (R) gravity:

where κ² = 8πG, g is the determinant of the metric gμν, and LM is a matter Lagrangian1 that depends on gμν and matter fields ΨM.

Now this is another review paper (verging on “cook book”), but it is also a distinctly tasty one (cited 102 times so far). If you wanted *the* resource on f(R) theories of gravity, you’re in luck because it was written this year.

Carlo Rovelli, & Matteo Smerlak (2010). Thermal time and the Tolman-Ehrenfest effect: temperature as the “speed of time” arXiv arXiv: 1005.2985v3

The thermal time hypothesis has been introduced as a possible basis for a fully general-relativistic thermodynamics. Here we use the notion of thermal time to study thermal equilibrium on stationary spacetimes. Notably, we show that the Tolman-Ehrenfest effect (the variation of temperature in space so that Tgₒₒ⁻½ remains constant) can be reappraised as a manifestation of this fact: at thermal equilibrium, temperature is locally the rate of flow of thermal time with respect to proper time – pictorially, “the speed of (thermal) time”. Our derivation of the Tolman-Ehrenfest effect makes no reference to the physical mechanisms underlying thermalization, thus illustrating the import of the notion of thermal time.

Given a statistical state ρ, we define the thermal time flow α :

A→Aas the Poisson flow of (−ln ρ) inA. That is

where the r.h.s. is the Poisson bracket.

Short and sweet (and currently only cited two times, not that that stops me from including it), Rovelli and Smerlak bring thermal time to stationary spacetimes. “Thermal time” is a catchy idea that is supposed to help general relativity and quantum mechanics blend with thermodynamics. Not only could these ideas be important for the unification of general relativity, quantum effects and thermodynamics, but they also play an important role in the *nature of time* debate. This is a paper that has serious rising potential.

Steven Carlip (2010). The Small Scale Structure of Spacetime arXiv arXiv: 1009.1136v1

Several lines of evidence hint that quantum gravity at very small distances may be effectively two-dimensional. I summarize the evidence for such “spontaneous dimensional reduction,” and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests a fascinating relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity.

For a scalar field, in particular, the propagator is determined by the heat kernel, and the behavior of the spectral dimension implies a structure

This is one of my favourites from the year; there is a lot of elegant physics contained within these pages (and yet still only two citations). Understanding the small scale structure of spacetime is going to be a major part of physics for the next few decades (at least), and coming at spontaneous dimensional reduction from CDT is a decent looking approach. Some of the delicious ideas discussed by Carlip may very well prove to be the base for our future understanding of a quantum spacetime.

Eugenio Bianchi (2010). Black Hole Entropy, Loop Gravity, and Polymer Physics arXiv arXiv: 1011.5628v1

Loop Gravity provides a microscopic derivation of Black Hole entropy. In this paper, I show that the microstates counted admit a semiclassical description in terms of shapes of a tessellated horizon. The counting of microstates and the computation of the entropy can be done via a mapping to an equivalent statistical mechanical problem: the counting of conformations of a closed polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of Black Holes and the physics of polymers.

In particular, in the slowly rotating case, Smarr formula applies: the dependence of the entropy on the angular momentum is quadratic and given by

This was another paper that I really enjoyed recently; it’s hard not to find these correspondences staggeringly beautiful, honestly. While approaches combining polymer physics techniques with general relativity don’t have a big following yet, they’re definitely one of *the* growing ideas in LQG from 2010. Loop quantum gravity and polymer physics are just heating up the kitchen; we haven’t even see what they’ll really be making yet.

V. G. Gurzadyan, & R. Penrose (2010). Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity arXiv arXiv: 1011.3706

Conformal cyclic cosmology (CCC) posits the existence of an aeon preceding our Big Bang ‘B’, whose conformal infinity ‘I’ is identified, conformally, with ‘B’, now regarded as a spacelike 3-surface. Black-hole encounters, within bound galactic clusters in that previous aeon, would have the observable effect, in our CMB sky, of families of concentric circles over which the temperature variance is anomalously low, the centre of each such family representing the point of ‘I’ at which the cluster converges. These centres appear as fairly randomly distributed fixed points in our CMB sky. The analysis of Wilkinson Microwave Background Probe’s (WMAP) cosmic microwave background 7-year maps does indeed reveal such concentric circles, of up to 6 σ significance. This is confirmed when the same analysis is applied to BOOMERanG98 data, eliminating the possibility of an instrumental cause for the effects. These observational predictions of CCC would not be easily explained within standard inflationary cosmology.

It’s everyone’s favourite topic this month: Penrose and Gurzadyan’s “evidence”* *for a cyclic cosmological model. Sure, they’re probably wrong, but come on, it’s Christmas! (expect the citation count to grow on this one steadily into the new year), plus, who doesn’t still get a little excited when Roger Penrose puts that apron on (*… wait… what?*).

_________________

Overarching themes: I did intentionally choose papers that were published on the arXiv (so they could be accessed from anywhere/by anyone), but that criteria didn’t actually affect my selections (as really, what from 2010 wasn’t on the arXiv?). Yes, yes, it was a little light on string theory topics, but that could very well be because more exciting things are happening elsewhere (or because I’m not all that stringy).

Note: Citation counts are only approximate and from the time that I am writing this post, of course.

And Another Note: The cookies start changing colour part way through because my second batch of dough was made with a different colour molasses. Also, the order they were made in correlates to how nice they look, as it was tiring.

Further Note: If any of the authors want their cookies… they’ll have probably been eaten already, but if you email me quickly, I’m willing to send them to you (or if you know me, just ask and I’ll make you some for whenever we’ll be running into each other next).

The Milky Way Project aims to sort and measure our galaxy, the Milky Way. Initially we’re asking you to help us find and draw bubbles in beautiful infrared data from the Spitzer Space Telescope.

Understanding the cold, dusty material that we see in these images, helps scientists to learn how stars form and how our galaxy changes and evolves with time.

The GalaxyZoo project expands! Help astronomers out when you’re feeling in the mood to procrastinate.

GREAT10, a simulation challenge that aims to improve image analysis algorithms for cosmic gravitational lensing.

GREAT10 is a way for astronomers, astrophysicists, computer vision, and AI people to come together and find new ways of solving problems. Contest details are online.

For more, see Computer Geeks: Compete to Help NASA Explain Dark Energy.

From CERN Bulletin:

After a very fast switchover from protons to lead ions, the LHC has achieved performances that allowed the machine to exceed both peak and integrated luminosity by a factor of three. Thanks to this, experiments have been able to produce high-profile results on ion physics almost immediately, confirming that the LHC was able to keep its promises for ions as well as for protons.

Another milestone finished; it’s been a great year for the LHC.

For more, see CERN Bulletin.

GravityGeek is a cooperative project to help encourage interaction amongst physicists in gravitation/general relativity with journalists and the public.

GravityGeek, the beta collaboration/networking site for professionals in general relativity, quantum gravity, cosmology, etc., has recommendations for Christmas/other gift giving, in case you have a physicist to buy for (as well as non-technical recommendations for kids and those who just like good popular science literature).

For more, see The GravityGeek Mission.

Abhay Ashtekar, Frans Pretorius, & Fethi M. Ramazanoğlu (2010). Surprises in the Evaporation of 2-Dimensional Black Holes arXiv arXiv: 1011.6442v1

The abstract:

Quantum evaporation of Callen-Giddings-Harvey-Strominger (CGHS) black holes is analyzed in the mean field approximation. The resulting semi-classical theory incorporates back reaction. Detailed analytical and numerical calculations show that, while some of the assumptions underlying the standard evaporation paradigm are borne out, several are not. Furthermore, if the black hole is initially macroscopic, the evaporation process exhibits remarkable universal properties. Although the literature on CGHS black holes is quite rich, these features had escaped previous analyses, in part because of lack of required numerical precision, and in part because certain properties and symmetries of the model were not recognized. Finally, our results provide support for the full quantum scenario recently developed by Ashtekar, Taveras and Varadarajan.

This is fairly nice for something so dense to read (it’s a lot crammed into four pages). The key result: for 2D black holes, information in the matter profile on Ī⁻R will not all be recovered at Ī⁺R, in generality. Slight twists on our understanding of 2D black holes *might* be suggestive of solutions in 4D. Of course, the usual problems of discussing anything in 2D are still there, but still…

The big topic of the past few weeks has been Roger Penrose and V.G. Gurzadyan’s November paper, suggesting there was evidence, via circle matching in the CMB, of a cyclic cosmology. There are so many papers being discussed right now, that this requires it’s own section. Now, because Penrose being a co-author makes any paper big news, mainstream media was all over this “evidence for time before time” (and other completely offensive and nonsensical catchphrases). What Gurzadyan and Penrose believed they had shown was that patterns in the CMB could not fit with standard inflationary cosmology and were strongly suggestive of a cyclic cosmology – ie. multiple “big bangs” (so *our *big bang wasn’t the first/didn’t start the cosmic clock, so to speak). Now, many people who’ve *looked for circles* in the CMB (because it *could* be very suggestive of the topology/geometry/history of the universe) were sceptical of this, because, unfortunately, patterns in the CMB are a little like bible codes. If you’re just looking for *something*, with a data set that big, you’re bound to find it and it doesn’t make it at all meaningful. Doubters appeared quickly on the arXiv and in blogs, and Gurzadyan and Penrose quickly responded in kind (see NASA, this is how it’s supposed to work). Below are the papers in the discussion as it stands, from November 16th to today:

V. G. Gurzadyan, & R. Penrose (2010). Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity arXiv arXiv: 1011.3706v1

I. K. Wehus, & H. K. Eriksen (2010). A search for concentric circles in the 7-year WMAP temperature sky maps arXiv arXiv: 1012.1268v1

Adam Moss, Douglas Scott, & James P. Zibin (2010). No evidence for anomalously low variance circles on the sky arXiv arXiv: 1012.1305v1

V. G. Gurzadyan, & R. Penrose (2010). More on the low variance circles in CMB sky arXiv arXiv: 1012.1486v1

Amir Hajian (2010). Are There Echoes From The Pre-Big Bang Universe? A Search for Low Variance Circles in the CMB Sky arXiv arXiv: 1012.1656v1

Basically, the critiques (1 & 2) go as follows: Yes, the patterns you’re seeing are really there, but they don’t mean what you think they mean. They’re just random, it’s not significant. Response: No, you don’t understand, they are significant, we have proof here. They can’t be random, it’s not a Gaussian distribution. Critique 3: No, I just checked, my Monte Carlo simulations showed they were random, sorry.

Honestly, it’s hard to believe, based on what has been shown, that these *patterns *are anything meaningful, but the discussion is certainly not over, and I’m sure that there will be many more papers being written on this, well into the new year.

For more, see Nature News: No evidence of time before Big Bang.

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