Oliver Jennrich (European Space Agency) has prepared a large review on technical aspects of LISA (space laser interferometer) mission – the project is extremely complicated for realization, many technologies are not even yet fully developed, and various prototypes will have to be launched. Yet, possible payout is so huge – even including possible detection of gravitational waves from the very early Universe – from inflationary and reheating stages (just think about it – detecting EM radiation did not really allow us to go beyond redshift so far, not including here CMB of course). Who is a sucker for space research as I am – please check out the paper. It does not discuss science related to the mission but contains tons of technical information about LISA you won’t find anywhere else.

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LISA technology and instrumentation

P.S. If you were unable to see embedded video, here is the link to the playlist I’ve created for you.

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Susskind’s lectures on cosmology

M. Hermanns. Condensing non-Abelian quasiparticles. Maria Hermanns discusses physics of fractional quantum Hall effect at filling factor where elementary excitations (anyons) seem to possess non-Abelian statistics ( with odd ). Corresponding wavefunctions are given by certain CFTs, and she is interested to understand the physics of daughter states in these CFTs.

H. Perets et al. A new type of stellar explosion. As we know, all supernovae explosions are divided into two classes: Type I (a,/b/c) and Type II (let me remind you that Type Ia are standard candles and are used to determine expansion rate of the Universe at large redshifts). This team claims to discover supernova of a new type which does not fit this classiication. Maybe, I’ll write about this result in more details later.

R. Williams et al., Skyalert: Real-time Astronomy for You and Your Robots. Skyalert.org is a web application which will be certainly useful for astronomers – professionals and amateurs. It collects data about time-critical astronomical transients and then pushes them to subscribers who, but setting their own trigger rules, can filter events according to the location, magnitude, etc. etc.

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Other interesting things in ArXiv (12 Jun 2009)

In short, the lithium problem in physics of Big Bang Nucleosynthesis is seeming underproduction of . What do we mean by that?

Well, the lithium abundance as predicted by the standard Big Bang Nucleosynthesis theory is

. (1)

On the other hand, the observed abundance seems to be about

(2),

i.e., (1) and (2) differ at least by , i.e., the deviation is statistically significant.

Before turning to possible scenarios explaining this anomaly, one has to explain in more details how exactly one estimates primordial abundance of lithium from observations.

As authors of the review explain, the value for primordial abundance of lithium is derived from absorption lines in atmospheres of low-metallicity galactic halo stars. If metallicity of such a star is sufficiently low, the abundance of turns out to be independent of the value of metallicity and temperature (in a certain range of both parameters – matallicity should remain small). This shows that the origin of lithium is cosmological: heavy elements that contribute to metallicity are themselves produced in stars.

One may think that the effect (anomalously low value of lithium abundance) might be related to erroneous atmospheric temperature determination, but this turns out to be ruled out. Two possibilities to explain the effect remain: a) lithium might be destroyed due to nuclear burning in the stellar interior (so, the underabundance of lithium is really an astrophysical problem) – this possibility is not very deeply analysed and b) underabundance of lithium means that standard BBN picture should be modified (so the problem is cosmological really). The first possibility seems to be more down-to-earth, while the second is of course more interesting physically. For example, in many models of dark matter where dark matter is made of supersymmetric partners of SM particles, underabundance of lithium-7 is naturally achieved (its presence is related to NLSP->LSP decays realized in the early Universe).

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Lithium problem

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Rocky Kolb’s lecture on Dark Universe

Umut Gursoy et al. “Thermal Transport and Drag Force in Improved Holographic QCD“. Umut with collaborators have shown that bulk viscosity of strongly coupled quark-gluon plasma does not exceed shear viscosity, although grows in the vicinity of the phase transition. Also, if you want to know what exactly people mean by “improved holographic QCD”, a good minireview of it is contained in the beginning of the paper.

Gavin Salam, “Towards Jetography” – everything you need to know about jets in QCD.

Charlotte Gils et al., “Topology driven quantum phase transitions in time-reversal invariant anyonic quantum liquids“. A new type of phase transition in anyonic quantum liquids is discovered – it is driven by quantum fluctuations of topology. I think it is amazingly cool result and the paper definitely deserves to be called the best paper on cond-mat on the day of publication.

C. Germani et al., “Relativistic Quantum Gravity at a Lifshitz Point“. I was not able to go through the paper thoroughly yet, but it seems that they were able to relate Horava gravity to relativistic vector-tensor theory in some particular gauge – in other words, one can derive a kind of full Einstein-Hilbert action from the non-relativistic Horava gravity. If this is indeed so (I still have my doubts), then result is definitely a strong one. Did anybody check out this paper? If yes, what’s your opinion?

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Other interesting things in ArXiv (11 Jun 2009)

Let me remind you that currently the redshift of the most distant galaxy we observed is about 6.96, of the most distant quasar – 6.43 (redshift values are defined with very good precision), while the redshift of the most distant gamma ray burst (GRB090423) is about 8.3.

The value of the redshift in the latter case is determined by photometry measurements (photometry is not exactly the most precise method we have at our disposal) and from the Lyman- absorption profile (which is much more precise method than photometry). Taking into account that both methods give the same value of the redshift (with corresponding error bars), we can consider the measurement as robust.

In usual Lambda CDM cosmology without any exotics redshift 8.26 corresponds to the age of the Universe 625 million years, and the very detection of the GRB shows that stars were already being born and dying at that time. It is interesting to note that parameters of GRB090423 are pretty much the same as of GRBs detected at lower redshifts – either the mechanics of GRB does not depend on the star much or there were already stars at that time that don’t differ too much from the ones we have today, at .

Via sergepolar.

Update: I realized after writing this blogpost that their paper, since it is submitted to Nature, is under press embargo, so I will remove the link to the paper. Honestly saying, I am not sure whether press embargo has any meaning in this particular case. First of all, the news was everywhere in press back in the end of April – for example, NewScientist made a video about it which is right now on Youtube and had about 65000 views already alone apart from its clones. Second, every dog in physics blogosphere (including your huble correspondent) has blogged about it.

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Into dark ages or again about GRB090423

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How to spot a black hole on the Sky

1. Quantum information

T. Tilma el al., “Is entanglement a critical resource for quantum metrology?”

Can we beat the shot-noise limit (and get to the Heisenberg limit) in quantum metrology by playing with entanglement of a quantum state? As it turns out, typically the answer is positive, but depending on what exactly you measure there might exist regimes such that entanglement actually prevents achievement of the Heisenberg limit.

2. Quantum field theory, string theory

J. Ellis et al., “The probable fate of the Standard Model“. If you perform RG analysis of the SM, three scenarios may realize depending on the value of the mass of the Higgs. First of all, if Higgs mass is very large, Higgs self-coupling blows up – and we need some non-perturbative physics (which we are currently not quite aware of) to control it. On the other hand, if Higgs mass is very small, the potential of SM is unstable. Finally, SM may survive up to the Planck scale if the mass of the Higgs boson is not too small or too large. Authors try to figure out which scenario will be most probably realized in reality using info that we currently have about constraints on Higgs mass.

Two nice string-theoretic papers, V. Kumar, W. Taylor, “String universality in six dimensions” and T. Erler, M. Schnabl “A Simple Analytic Solution for Tachyon Condensation” are well discussed in the recent Lubos Motl’s post and I don’t have anything to add to what he has said already.

3. Astrophysics and cosmology

W. Buchmueller et al., “Probing Gravitino Dark Matter with PAMELA and Fermi“. The main conclusion of the paper is that fluxes observed by PAMELA and Fermi LAT require astrophysical sources for gravitino dark matter.

QuAD collaboration. “Improved measurements of the temperature and polarization of the CMB from QuAD“. A white paper by QuAD collaboration. QuAD and ACBAR will allow to improve constraints on several cosmological parameters (such as running index) compared to WMAP alone.

D.L. Band et al., “Prospects for GRB Science with the Fermi Large Area Telescope“. Another white paper, for Fermi this time. Large Area Telescope will allow for detection of gamma ray bursts in the > 100 MeV band (the most data we had so far are for 10 keV – 1 MeV band).

EPIC collaboration. Study of the Experimental Probe of Inflationary Cosmology (EPIC)-Intemediate Mission for NASA’s Einstein Inflation Probe. This Monday is definitely the day for white papers – this one is huge, more than 150 pages. Actually, it is a EPIC mission concept report for NASA. EPIC will allow precision measurements of CMB B-modes of polarization.

Bruce Bassett et al., “Fisher Matrix Preloaded – Fisher4Cast“. Authors havedeveloped a very nice software package for working with Fisher matrices.

4. Condensed matter physics

M. Kastner, “Monte Carlo methods in statistical physics: Mathematical foundations and strategies“. Very nice pedagogical introduction into Monte Carlo.

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This and that in ArXiv on Monday

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Three last Susskind’s lectures on general relativity

Justin Khoury (whom I knew from Perimeter years and who is in Penn now) gave the first talk afternoon – titled “observational hints of IR modified gravity”. His talk followed Nima’s, and the latter almost completely blew me away, so I was somewhat unfocused during Justin’s presentation. Yet, I was able to capture that as such observational hints he wanted to present local bulk flow of matter within 50 MPc scale, excess power in Lyman (about 30%) and small scale CMB anomalies (which I would hardly call anomalies due to lack of statistics there). According to him, all this shows that gravity should be stronger at larger scales – and that’s exactly what many models of modified gravity predict.

Next, Stacy McGaugh has talked about MOND. I should really confess that my attitude towards MOND was always somewhat arrogant – I had no idea why people might be interested in such an ugly theory. This time I got it, I guess: in order to fit all galaxy rotation curves, you only need to pick one and fit the MOND parameter (acceleration ) – and you’ll automatically fit all others, that is, an almost infinite data set turns out to be one-parametric. Well, I guess, choosing the density of cold dark matter will do the job as well.

Nima Arkani-Hamed has asked him about the bullet cluster and Stacy’s answer was like “I find it somewhat fishy that all proponents of cold dark matter model always use this example to defend their models”. Well, that’s the point – they use this argument, because it works.

The first talk after lunch (by Ted Jacobson) was devoted to the discussion of Einstein aether theory. What is the theory possessing such a scary name that would probably create an itchy feeling in guts of every 1 year physics student?

Consider a theory with the action

,

where

,

is a time-like unit vector (since is really lagrangian multiplier). The name of the theory involves the word “aether”, since a “4-velocity” vector (velocity of aether) introduces a preferred direction in spacetime (Lorentz transformations along and perpendicular to it are different). It therefore belongs to the class of exciting deeply flawed theories according to Nima’s classification, and not surprisingly the theory messes with BH thermodynamics – no unique Hawking temperature or BH entropy has been found so far and generalized second law seems to be violated (that is, you can construct perpetuum mobile in the universe described by this theory).

Since Ted Jacobson has mostly discussed classical properties of the theory, Nima Arkani-Hamed immediately pointed out that if one turns to the quantum level, the theory above features a non-perturbative mass scale – since it looks similar to non-linear sigma model.

My personal comment to him: actually, this is true that lagrangian multiplier acquires a non-zero VEV due to non–perturbative effects, however, its fluctuations are only suppressed if the vector has many components (namely, inifinite number of them), while in this case , and lagrangian multiplier fluctuates strongly.

Levon Pogosyan has talked about CMB and LSS observations – are we able to determine from them whether gravity is modified or not? As it turns out, our data are particularly sensitive to scale dependent modifications of gravity, and future surveys such as LSST might give us more information that just effective EOS for dark energy .

Finally, Mark Wyman has discussed N-body numerical simulations of DGP. The result he presented is well understandable: since DGP means stronger gravity at larger distances (and therefore earlier times), in DGP large scale structure is formed faster and its features are stronger than in GR (see the Fig. below)

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Workshop on tests of gravity in Case Western – day 2: aether and modified gravity

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Susskind’s general relativity – lecture 9

Regarding analog models I don’t have too much to report – since I am located here at relatively close vicinity to Grigory Volovik (he works in Espoo, while I work in Helsinki), I think I know the agenda quite well, and my overall impression that no so many exciting things happen on the field was confirmed on the workshop. Basically, it proves to be relatively easy to construct models of relativistic chiral fermions and vectors from non-relativistic condensed matter systems (for example, He-3). However, it seems to be impossible to construct relativistic dynamical gravity (that is, effective theory with Einstein-Hilbert action) starting from these systems – recent attempt by Horava seemed to be promising, but the ultimate answer is still the same. What we can do at most is to model a “relativistic” field theory on a curved background (such as Painleve-Gullstrand BH), but this background is static and backreaction of our field theoretic degrees of freedom on it is zero. That’s what activities on the field of analog models of gravity revolve around for almost decade.

So, let me turn to modified gravity and Nima’s talk. Since nobody in the physics blogosphere seems to really discuss the content of his talk (see Mark Trodden’s report – he attended the workshop, too), let me proudly do it for you

As you may already know, the title of the talk is “Don’t modify gravity – understand it“. Nima started by saying that he spent too much time inventing models of modified gravity and now wants to officially confess his sins.

Why? First (but not the most important as you’ll see below), because modified gravity is boring – in all (or most all) it can be reduced to usual GR + scalar field. More accurately, he has introduced the following classification: all modified gravity models can be divided into two classes -

1. boring, with subclasses a) very boring (and not excluded) and b) moderately boring (and excluded by experiments) – because of the name of the class he did not want to talk about those models at all
2. exciting. This class, according to Nima, includes only deeply flawed models such as DGP (where aforementioned scalar field possesses “Galilean invariance”) and Higgs phases of gravity (where scalar fields are essentially Goldstone modes of spontaneously broken spacetime symmetries).

So, why exciting models are deeply flawed from Nima’s point of view? The reason is the fact that, according to well-known theorem quantum gravity (based on usual GR) can not have local observables. The physical reason for that is simple. Quantum mechanics in principle allows us to measure positions of quantum particles with infinite precision (not both position and momentum though). However, measuring position with infinite precision assumes that we have an infinitely heavy apparatus to measure it. Quantum gravity in turn does not allow us to have an infinitely heavy apparatus (sufficiently heavy one would trivially turn into black hole).

A correct language for describing gravitational degrees of freedom should look more like holography. Basically, holography means non-local degrees of freedom, and non-local degrees of freedom mean holography.

Yet, non-locality of gravity will only be noticeable only if we take into account non-perturbative effects, suppressed by

,

where is Newton constant, that is, gravity is non-local but in a very subtle way.

Now, why exciting models are deeply flawed according to Nima? Well, Higgs phases of gravity violate “non-local” part in the statement above – they are manifestly local. On the other hand, DGP violates “subtle” part of the statement above, since it allows for superluminal propagation.

Basically, a general effective scalar field theory featuring CP violation looks like

.

DGP is a rather special CP violating theory, where the last term before the dots is canceled due to a special symmetry, and this allows theory to feature superluminal propagation.

He concluded by explaining what questions should one study to understand non-local nature of gravity better. Basically, since non-locality is suppressed by a factor , it should become important again in situations where some kind of enhancement is present – such as questions related to BH information paradox and eternal inflation (in the latter case, enhancement comes from the fact that you are never able to measure more than models in dS universe, where is de Sitter entropy).

I’ll try to cover remaining talks of the second day tomorrow.

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Workshop on tests of gravity in Case Western – day 2 and Arkani-Hamed’s talk

Susskind continues to discuss covariant derivatives, parallel transport of vectors, Ricci and Riemann tensors. In the second part of the lecture he turns to geodesics. He is terrific lecturer but after the lecture N7 I start wondering whether it’s worth spending so much time discussing technicalities

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Susskind’s 8th lecture on general relativity

I am sorry for being rather quiet for a while. As many of you may already know, my laptop has decided to enter coma during the trip, as a result, I was left without appropriate internet access (sporadic use of Pascal Vaudrevange’s computer is not counted, thanks, Pascal!). Although the laptop tried to revive after we came back home, demonstrating some kind of “brain” activity, in a couple of days I understood that the growth of entropy is as inevitable as a finally victory of string theory over loop quantum gravity.

Apart from those continuous attempts to revive the bugger, the workshop proved to be fun. I learned tons of staff and had a chance to talk to many people. Here I’ll try to present an informal report from the workshop, while its official webpage can be found on the Case website.

The morning part of the Day 1 program featured several talks (by Blayne Heckel, Tom Bay, Neil Ashby, Ricardo Decca and Casey Wagoner) on tests of gravity at small distances and deviations from equivalence principles.

I guess, the main conclusion I took home from this part of the workshop is that inverse square law clearly holds up to distances as small as m. Although the opposite case would most probably mean existence of large extra dimensions and all of us spent years studying and constructing different models of modified gravity , I guess, this experimental conclusion is of no surprise for me. It is so unfortunate that gravitational physics at intermediate length scales (that is, between Planckian length and the size of cosmological horizon) is boring, but that’s Life.

Or is it really? Note that the energy density of dark matter is about , which corresponds to a length scale of the order m – exactly, to the scale that experiment was able to achieve so far. Can DM really mean some kind of modification of gravity at relatively short distances (say, in the sense of MOND)? If yes, future experiments might be able to find it.

Second part of the day was devoted to black holes, and Avi Loeb gave the first talk after lunch. The talk was based on their paper with Broderick and Narayan, which we have already discussed on NEQNET, so I am not going to repeat the conclusion. There were two pieces of information new for me though: one is that Sgr A* is no longer the only candidate which we can detect the angular size of, the BH in the center of M87 will also do (it is 700 times more massive than Sgr A* and 2000 further from us, i.e., its angular resolution is quite small).

The second piece of information was received during one-to-one conversation with Avi. I did not understand why the source would emit thermal radiation if it is not a BH (they strongly used this suggestion in their analysis with Broderick and Narayan). I thought that it has something to do with kinetics of relativistic plasma in a deep gravitational well, but the physics was much simpler of course (and as usual I missed the simplest explanation) – photons in the vicinity of strongly gravitating body often return back to the surface of the body and only small amount of them has a possibility to overcome the gravitational attraction and leave the well (those photons are the only ones we detect of course). Therefore, we essentially deal with a pinhole experiment familiar from the course of general physics for 1 year grades – the one with a sphere filled with black body radiation

In the second afternoon talk Frans Pretorius has explained how black holes collide with each other and showed some rather amazing videos of computer simulations of GR. Imagine that you have two boson stars – that is, gravitationally collapsed objects made of scalar field . Both objects are of pre-threshold mass (that is, they are not BHs). Now, what conditions do you need to satisfy if you want to form a black hole after collision of these two objects? As it turns out, they should have relatively large Lorentz factors w.r.t. each other ( around 3). It was very interesting to see how much radiation is emitted in the process of BH formation…

Next talk was by Samir Mathur, and not surprisingly Samir was discussing fuzzball conjecture. His presentation was almost formulae-less (that’s because he knew that you are in the room – as Tanmay Vachaspati explained to me ) I guess, you already know very well the physics behind the conjecture, thereofore, allow me to proceed to my discussions with Samir before and after his talk.

Before the talk I blatantly tried to push my own agenda in the conversation with Samir – the agenda was explanation how coherence pattern (in terms of correlation functions of phase of the modes) of Hawking radiation should change in space and time, and Samir Mathur very quickly explained to me that everything I am talking about is known to string theoretic community for years He was certainly right, Though there was though also some exchange which forced me to get alert for a moment:

- So you are saying that everything is entangled with everything else, myself, Sun, distant galaxy and CMB radiation?, – I asked. – Everything is quantum?
- Sure, – he replied, warmly smiling.
- Wait a moment, we know where quantum mechanical effects are important and where they are not – you need to compare the classical action on a given trajectory with ! For distances like this QM effects should be negligible.
- (Not so warmly) Yes, but we know what we are talking about – we are talking about decoherence. Of course, everything is decohered at macroscopic scales!

So, in the end of the day I seem to miss the fundamental difference between entanglement and quantum coherence, but I guess I am just nagging and really agree with him deep;y in my heart – everything is entangled. I guess, he wanted to say the following: in full Theory of Everything we are dealing with pure states (a quantum state of the Universe is a pure state), and string theory is a full theory. When we reduce it to GR, we introduce coarsegraining at scale of the order and neglect any transplanckian physics. Of course, coarsegraining in turn introduces entropy, and that’s the entropy we have to deal with in our sub-planckian world. Please correct me if I am wrong (Lubos?). With that only difference that he was trying to convince us that in BHs coarsegraining scale is not really , but BH radius instead.

(This strongly reminds me an old Russian anecdote stating that “during war times the value of cosine might reach 4″.)

Our second conversation, after Samir’s talk, was devoted to discussion of dS. In some sense, de Sitter is surprisingly similar to BH: it has a horizon (albeit specific for any observer) and thermal radiation coming from it, the essential difference is that we are now living inside the horizon So, how to apply Mathur’s machinery to dS – for example, what would be a counterpart of KK instantons, which are the microstates he would like to count inside BHs? The answer to that question remains unknown.

The last talk afternoon, about 5d BHs, was given by Dejan Stojkovic. Together with Frolov, Dejan has completely classified 5d BHs – I think it is quite a piece of work. Another thing he discussed is how 5d BHs interact with branes and evaporate.

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Workshop on tests of gravity at Case Western – day 1