Firs a clarification about the title. Most people know who Einstein or Galois are, but maybe not so many will know Margalef. He is an Spanish ecologist who begun his career as a self educated amateur and ended up with a tenure. I choose him because I needed a somewhat Spanish equivalent of the other two personalities.

All those people share a common point. They were brilliant scientifcs (Einstein and Galois simply top notch genius) who did at least part of it´s work outside academia.

The posts, as some can have supposed, is related to the famous "new Einstein" issue initiated by Lee Smollin. For those who don't know this affair simply to say that Lee Smollin published an article entitled "why no new Einstein" where he asked for somewhat who would revolutionize the nowadays physic in a similar way to what Einstein did in its time. Smollin was particularly interested in the philosophical nature of the Einstein contributions.

Well, I think that Einstein had a great intuition and that he presented his ideas in a very elegant way. But I don't think that to consider them Philosophy would make them any justice.

Possibly most interesting was his other consideration about Einstein. He did part of his work (the work in his "anni mirabilis") outside academy, you know, when he was in the patent office. In this respect Einstein was not alone. Before him many well known names had a similar role. Galois never had an academic position (possibly because he was killed before his work was broadly known). Lagrange was a self educated guy. Faraday made significant contributions to physics without a formal academic formation. The list is long and includes a large amount of well known scientifics in the ninety century. Also in the twenty century there are some names beyond Einstein (I am not totally sure but I guess that Banach was one of them).

But the truth is hat nowadays seemingly there are not too many examples (if any) of them. Maybe Perelman is the nearest example. He proved the Poincares conjecture which deserved him the corresponding Claymath prize. Also he was honoured with Fields medal, but he rejected it. He declined his academic position and now works in his home (or his mother home). But it is not the exact situation because as fr as I know he , until them, has followed a conventional way in academy.

Well, Smollin stated that he expected that the revolution of physic would come from out of the academy and was very interested in searching the "new Einstein".

I will say some comments about this particular. There is a difference between Einstein and previous times to nowadays. The scientific knowledge has growth a lot and it is necessary much more time to get the right preparation to be in conditions to publish important and revolutionary things. For example the gap between graduate studies and cutting edge physics is a lot greater now that in Einstein times (I would say that in Einstein times were around two or three years and now is 40 years). that means that an intelligent guy who has ended his undergraduate studies in physics and elaborates creatively with them has very few possibilities to create something valuable.

Another aspect is that in Einstein times there were no computers (nor financial markets). It is very easy that the "should be new Einstein" would end programming in an anonymous software company or doing "econophysic" in the financial markets.

I would add other thing. The goodbeeing state doesn't aim people to make a big effort to make big contributions that would give them fame, and, consequently money if they can live reasonably well without them. Anyone with the intelligent to be a "should be Einstein" surely knows hot to make some money without too much effort so creating something terribly special that would make him famous is not a priority.

Said that, is impossible a "new Einstein"?. I totally agree with Lubos that the most probable place to find him is inside academy. I totally recommend to anyone interested in purchasing a research position to care his expedient and all that. Certainly academy has is dark points (Lubos apparently is out of it because he doesn't like them) but it is not that bad.

Still there is a point about the new Einstein issue that is interesting. To become famous and doing great contributions being inside academy is "easy" (haha). But if you are a real genius it is possible that you would think precisely that, that is too easy and that the good point is to do your great work outside academy. Only the "minor minds" need to depend on good teachers and all the facilities that a first class university offers. Certainly it sound more "fashion", but I wouldn't recommend none to do that way.

And no, I am not at all trying to, subtlety, qualify myself as a "new Einstein" candidate. The main point of this post is to clarify th issue of the gap between undergraduate (or graduate, I always get lost with the correspondence between the Spanish word "licenciatura" and the corresponding English equivalent)and the cutting edge physic. If someone dreams about doing valuable work outside academy after finishing his studies he would be aware that he would need at least five years of intensive study to become near the frontier of knowledge. He would consider seriously if his economic/personal positions would allow him to do that, specially in this incoming years of economic crisis. I think that in that sense the claims of Smollin, and in fact not inly him, are dangerous because in a certain sense they make look glamorous a path that is mainly destined to fail.

## Sunday, April 26, 2009

## Wednesday, April 22, 2009

### Naked singularities

This month, April 2009, the Spanish edition of Scientific American(investigación y ciencia)has an article about naked singularities (the English version was dated on February).

The author, Pankaj S. Joshi, seems to be an total expert in the subject (a common issue in Scientific American)and has a recent book- from 2008- about the particular,

Gravitational Collapse and Spacetime Singularities.

I didn´t read the book, but I found the article interesting so I searched in wikipedia and some of the papers linked there. I´ll try to explain some of the aspects now.

In general relativity there are two well known places where singularities appear, black holes and cosmology. The most naive way of thinking/defining singularities is to characterise them as points where the metric (or curvature) of space-time becomes infinite. According to that the event horizon of a Schwarschild black hole would be a singularity. It was realized that that infinity was due to a bad choice of coordinates. In that solution the centrer of the black hole, the R=0 point, also get an infinity value and this can´t be overcome by any other choice of coordinates so it is a genuine singularity.

As far as we like to have coordinate free definitions there is more technical definition, a singularity is defined to be one which contains geodesics which cannot be extended in a smooth manner. The end of such a geodesic is considered to be the singularity. That definition is useful to probe theorems (as did Penrose and Hawkings in the 70´s) about how singularities can´t be avoided in classical general relativity in cosmological sceneries. It also permit to make a diferentiation about coordinate singularities, the ones I talked before, related to black holes, and what are known as conical singularities, related to things such as cosmic strings. A conical singularity occurs when there is a point where the limit of every diffeomorphism invariant quantity is finite. In which case, spacetime is not smooth at the point of the limit itself. Thus, spacetime looks like a cone around this point, where the singularity is located at the tip of the cone. The metric can be finite everywhere if a suitable coordinate system is used.

The S.A article treats about singularities associated to gravitational collapse. There exists what is known like the cosmic censorship conjecture (or hypothesis) , own to Roger Penrose, which states that there are not naked singularities. That is, every singularity must be hidden behind an event horizon and cant be seen from the outside. It has the status of conjecture because it hasn´t be proved in a rigorous way under physically reasonable assumptions. In fact it hasn´t even stated in a mathemathical rigorous way.

The article, obviously, try to answer the conjecture in the negative. Before describing it's arguments I´ll talk about an aspect closely related to the CCC. If one examines the solutions for rotating black holes (Kerr solution) one sees that if is allowed that J, the angular momentum is greater than the mass M of the black hole, that is J/M>1 a naked singularity (a ringed shaped one) appears. A similar thing goes for charged (electric of whatever associated to U(1) gauge symmetry)black hole solutions (Reissner-Nordstom black holes) where a naked singularity appears if the charge, Q, is greater than the mass of the b-h, i.e., Q/M>1. It is commonly assumed that the CCC holds and the case where the equality would arise are called extremal black holes. The possibility of a reverse, negative, sign in the above expressions is not even contemplated in most theoretical considerations.

That´s a reason why I was somewhat surprised when I read the article and saw that in fact when one makes actual calculations, in classical general relativity, of how gravitational collapse behaves when some oversimplifying assumptions about the state of the star are neglected naked singularities actually are shown to be possible.

If the star is perfectly spherically symmetric and of uniform density everything is o.k and the black hole is formed. But relaxing one of the assumptions separately(or both at once) it can be shown that naked singularities actually appear.

Intuitively the reason is, for the case of non uniform density, that it can happen that the rate of accretion into the centre is never fast enough to actually form an even horizon and a central, unique, singularity appears. In the case of non sphericity it is shown that the collapse is neither spheric so the mass is concentrated in two points that become singularities at the end of the collapse avoiding the formation of the event horizon because of the oblong shape of the infalling matter that forbids the concentration of enough mass inside the Schwarschild radius.

Once that this facts are established one can wonder about how realistic and stable are. After all they mean that a large amount of the mass of a big star is concentrated in a point. The precise nature of what a singularity actually is usually is thought to be a question related to quantum gravity. But for regions relatively close (but not too much) to the singularity classical relativity still holds and it is expected that neighbouring matter would be attracted to the singularity. The inexistence of the event horizon means that there is the possibility of going arbitrarily near the singularity and returning to the original point (well, if tide forces don´t kill you and such that). In particular light can go near the singularity and scape to a distant observer so we can see what happens there. But even thought some matter in certain trajectories could scape I think that it is reasonable that most of the matter would be trapped in the singularity. Intuitively one would think that that increases the mass of the singularity and that it sooner or later it will become a black hole. Possibly that is a too naive way of think and that is one of the particularities of the singularity (but I am not sure about it).

Anyway, if singularities re shown to be possible (at least for a certain time) one could try to consider if they are distinguishable from black holes. The answer is in the positive. Even one could try a little bit further and consider the possibility that an existing black hole could break and leave behind the singularity. The most natural case would be kerr black hole which is led to rotate faster than it´s extremal limit. Because astrophysical black holes are usually believed to be Kerr ones one immediately can answer for particular observable signatures of this breaking. ONe arxiv article where one can read the details is this: Magnification relations for Kerr lensing and testing Cosmic Censorship. There are described some mathematical details of calculations made on the pna (post Newtonian approximation)of some optical effects. The author claim that the differences in behaviour among black holes and naked singularities could be observed with the incoming new generation of available technology.

From the viewpoint of an string theorist 4 dimensions are very restrictive, what about the influence of additional dimensions? You can read a paper about the particular: Spherical gravitational collapse in N-dimensions. It is co-authored by Joshi and the answer is mildly positive. I recommend to read the considerations that he makes in the conclusions.

A later thing I am going to discuss is the role of quantum gravity. If naked singularities actually exist they are a window to do observations of quantum gravitational effects (or at least one so expects). But before going there one could answer if quantum gravity considerations modify the classical predictions of formation of naked singularities. I don´t know the "asscendence" of Joshi, tat is, if he is an string theoretic oriented or an LQG oriented researcher. Being an specialist in general relativity one, maybe, would expect him being an LQG researcher, but reading his papers I guess that a better fir would be to consider him a "naked singularity phenomenologist". Anyway, LQG is easier to learn that string theory and is accepted by a plausible quantum gravity by hundreds of people with a tenures/investigation positions in universities so it is reasonable to expect some paper using LQG to investigate the question. And, effectively, there is such paper: Quantum evaporation of a naked singularity.

This papers point in a different direction that the classical results. Using a toy model with an scalar field (in a way similar to loop quantum cosmology calculations) it is shown that near the should be singularity gravity becomes a repulsive force and the naked singularity isn't formed. I guess that one must understand that this calculations make sense in the case where the classical equations point to the formation of the singularity. That is, classically one expect the formation of the naked singularity, but looking at quantum phenomena one sees that actually the singularity is avoided. I must clarify that this calculations are not claimed to be fully quantum by the authors and they still believe that in full quantum sceneries the naked singularity would easily reappear.

Still this last scenery could have relevant observational consequences in the form of powerful gamma ray bursts that result in the evaporation of the should be singularity. The precise signature of that bursts depend in some free factors of the theory, and, in particular, the claim that can be used to estimate a value of LQG, j, the value of the representation of (complex) SU(2) used.

Well, certainly I don´t believe that string theory people would take too seriously this considerations, but claiming possible near future experimental results I believe it well deserves to say something about the subject.

In fact actually there are some results in string theory about singularities, particularly the enhanchon mechanism, but it is related to "educated" singularities inside a black hole who are prudent enough to not show themselves naked. Abut black holes in string theory I hope to write a post soon.

To end this post to leave a link to a self claimed naked singularity who is kind enough have a blog (in Spanish), that links to this: La Singularidad Desnuda.

The author, Pankaj S. Joshi, seems to be an total expert in the subject (a common issue in Scientific American)and has a recent book- from 2008- about the particular,

Gravitational Collapse and Spacetime Singularities.

I didn´t read the book, but I found the article interesting so I searched in wikipedia and some of the papers linked there. I´ll try to explain some of the aspects now.

In general relativity there are two well known places where singularities appear, black holes and cosmology. The most naive way of thinking/defining singularities is to characterise them as points where the metric (or curvature) of space-time becomes infinite. According to that the event horizon of a Schwarschild black hole would be a singularity. It was realized that that infinity was due to a bad choice of coordinates. In that solution the centrer of the black hole, the R=0 point, also get an infinity value and this can´t be overcome by any other choice of coordinates so it is a genuine singularity.

As far as we like to have coordinate free definitions there is more technical definition, a singularity is defined to be one which contains geodesics which cannot be extended in a smooth manner. The end of such a geodesic is considered to be the singularity. That definition is useful to probe theorems (as did Penrose and Hawkings in the 70´s) about how singularities can´t be avoided in classical general relativity in cosmological sceneries. It also permit to make a diferentiation about coordinate singularities, the ones I talked before, related to black holes, and what are known as conical singularities, related to things such as cosmic strings. A conical singularity occurs when there is a point where the limit of every diffeomorphism invariant quantity is finite. In which case, spacetime is not smooth at the point of the limit itself. Thus, spacetime looks like a cone around this point, where the singularity is located at the tip of the cone. The metric can be finite everywhere if a suitable coordinate system is used.

The S.A article treats about singularities associated to gravitational collapse. There exists what is known like the cosmic censorship conjecture (or hypothesis) , own to Roger Penrose, which states that there are not naked singularities. That is, every singularity must be hidden behind an event horizon and cant be seen from the outside. It has the status of conjecture because it hasn´t be proved in a rigorous way under physically reasonable assumptions. In fact it hasn´t even stated in a mathemathical rigorous way.

The article, obviously, try to answer the conjecture in the negative. Before describing it's arguments I´ll talk about an aspect closely related to the CCC. If one examines the solutions for rotating black holes (Kerr solution) one sees that if is allowed that J, the angular momentum is greater than the mass M of the black hole, that is J/M>1 a naked singularity (a ringed shaped one) appears. A similar thing goes for charged (electric of whatever associated to U(1) gauge symmetry)black hole solutions (Reissner-Nordstom black holes) where a naked singularity appears if the charge, Q, is greater than the mass of the b-h, i.e., Q/M>1. It is commonly assumed that the CCC holds and the case where the equality would arise are called extremal black holes. The possibility of a reverse, negative, sign in the above expressions is not even contemplated in most theoretical considerations.

That´s a reason why I was somewhat surprised when I read the article and saw that in fact when one makes actual calculations, in classical general relativity, of how gravitational collapse behaves when some oversimplifying assumptions about the state of the star are neglected naked singularities actually are shown to be possible.

If the star is perfectly spherically symmetric and of uniform density everything is o.k and the black hole is formed. But relaxing one of the assumptions separately(or both at once) it can be shown that naked singularities actually appear.

Intuitively the reason is, for the case of non uniform density, that it can happen that the rate of accretion into the centre is never fast enough to actually form an even horizon and a central, unique, singularity appears. In the case of non sphericity it is shown that the collapse is neither spheric so the mass is concentrated in two points that become singularities at the end of the collapse avoiding the formation of the event horizon because of the oblong shape of the infalling matter that forbids the concentration of enough mass inside the Schwarschild radius.

Once that this facts are established one can wonder about how realistic and stable are. After all they mean that a large amount of the mass of a big star is concentrated in a point. The precise nature of what a singularity actually is usually is thought to be a question related to quantum gravity. But for regions relatively close (but not too much) to the singularity classical relativity still holds and it is expected that neighbouring matter would be attracted to the singularity. The inexistence of the event horizon means that there is the possibility of going arbitrarily near the singularity and returning to the original point (well, if tide forces don´t kill you and such that). In particular light can go near the singularity and scape to a distant observer so we can see what happens there. But even thought some matter in certain trajectories could scape I think that it is reasonable that most of the matter would be trapped in the singularity. Intuitively one would think that that increases the mass of the singularity and that it sooner or later it will become a black hole. Possibly that is a too naive way of think and that is one of the particularities of the singularity (but I am not sure about it).

Anyway, if singularities re shown to be possible (at least for a certain time) one could try to consider if they are distinguishable from black holes. The answer is in the positive. Even one could try a little bit further and consider the possibility that an existing black hole could break and leave behind the singularity. The most natural case would be kerr black hole which is led to rotate faster than it´s extremal limit. Because astrophysical black holes are usually believed to be Kerr ones one immediately can answer for particular observable signatures of this breaking. ONe arxiv article where one can read the details is this: Magnification relations for Kerr lensing and testing Cosmic Censorship. There are described some mathematical details of calculations made on the pna (post Newtonian approximation)of some optical effects. The author claim that the differences in behaviour among black holes and naked singularities could be observed with the incoming new generation of available technology.

From the viewpoint of an string theorist 4 dimensions are very restrictive, what about the influence of additional dimensions? You can read a paper about the particular: Spherical gravitational collapse in N-dimensions. It is co-authored by Joshi and the answer is mildly positive. I recommend to read the considerations that he makes in the conclusions.

A later thing I am going to discuss is the role of quantum gravity. If naked singularities actually exist they are a window to do observations of quantum gravitational effects (or at least one so expects). But before going there one could answer if quantum gravity considerations modify the classical predictions of formation of naked singularities. I don´t know the "asscendence" of Joshi, tat is, if he is an string theoretic oriented or an LQG oriented researcher. Being an specialist in general relativity one, maybe, would expect him being an LQG researcher, but reading his papers I guess that a better fir would be to consider him a "naked singularity phenomenologist". Anyway, LQG is easier to learn that string theory and is accepted by a plausible quantum gravity by hundreds of people with a tenures/investigation positions in universities so it is reasonable to expect some paper using LQG to investigate the question. And, effectively, there is such paper: Quantum evaporation of a naked singularity.

This papers point in a different direction that the classical results. Using a toy model with an scalar field (in a way similar to loop quantum cosmology calculations) it is shown that near the should be singularity gravity becomes a repulsive force and the naked singularity isn't formed. I guess that one must understand that this calculations make sense in the case where the classical equations point to the formation of the singularity. That is, classically one expect the formation of the naked singularity, but looking at quantum phenomena one sees that actually the singularity is avoided. I must clarify that this calculations are not claimed to be fully quantum by the authors and they still believe that in full quantum sceneries the naked singularity would easily reappear.

Still this last scenery could have relevant observational consequences in the form of powerful gamma ray bursts that result in the evaporation of the should be singularity. The precise signature of that bursts depend in some free factors of the theory, and, in particular, the claim that can be used to estimate a value of LQG, j, the value of the representation of (complex) SU(2) used.

Well, certainly I don´t believe that string theory people would take too seriously this considerations, but claiming possible near future experimental results I believe it well deserves to say something about the subject.

In fact actually there are some results in string theory about singularities, particularly the enhanchon mechanism, but it is related to "educated" singularities inside a black hole who are prudent enough to not show themselves naked. Abut black holes in string theory I hope to write a post soon.

To end this post to leave a link to a self claimed naked singularity who is kind enough have a blog (in Spanish), that links to this: La Singularidad Desnuda.

Etiquetas:
black holes,
singularities

## Monday, April 13, 2009

### Horava´s quantum gravity

I have mentioned many approaches to quantum gravity, other than string theory, in this blog.

Besides LQG none of them has had major success in attracting people to do research in it. Now, at least it seems so, there is a new option, the so called Quantum Gravity at a Lifshitz Point initiated by Peter Horava, a well known string theorist (remember the Horawa-Witten model of heterotic string theory). I had the first new about it in the Lubos blog, but since them a few other papers have appeared. As far as another string theory minirevolution is going on (F-theory GUTs) which is leading to , seemengly, actual predictions testable in the LHC, as well as, maybe, in cosmology, I have had not time to read these articles, beyond an slight overview. I will use this entry mainly to keep track of the actual papers and also to encourage possible readers of this blog to investigate about them.

I will limit, so, to link some of the papers and paste the abstracts. Just to say that the theory will probably be of the liking of the people who likes condensed matter and critical phenomena.

This was the firs paper, Quantum Gravity at a Lifshitz Point. This is the abstract:

We present a candidate quantum field theory of gravity with dynamical critical

exponent equal to z = 3 in the UV. (As in condensed matter systems, z measures the degree

of anisotropy between space and time.) This theory, which at short distances describes

interacting nonrelativistic gravitons, is power-counting renormalizable in 3 + 1 dimensions.

When restricted to satisfy the condition of detailed balance, this theory is intimately related

to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor.

At long distances, this theory flows naturally to the relativistic value z = 1, and could

therefore serve as a possible candidate for a UV completion of Einstein’s general relativity

or an infrared modification thereof. The effective speed of light, the Newton constant and

the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic

z = 3 theory at short distances

This is the second one: Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point, and this is the abstract:

We extend the definition of “spectral dimension” (usually defined for fractal and

lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that

in quantum gravity dominated by a Lifshitz point with dynamical critical exponent z in D+1

spacetime dimensions, the spectral dimension of spacetime is equal to

ds = 1 + D/z

In the case of gravity in 3 + 1 dimensions presented in arXiv:0901.3775, which is dominated

by z = 3 in the UV and flows to z = 1 in the IR, the spectral dimension of spacetime flows

from ds = 4 at large scales, to ds = 2 at short distances. Remarkably, this is the qualitative

behavior of ds found numerically by Ambjørn, Jurkiewicz and Loll in their causal dynamical

triangulations approach to quantum gravity

The next article is not written by Horova, the authors are Tomohiro Takahashi and Jiro Soda. The paper is this:Chiral Primordial Gravitational Waves from a Lifshitz Point. This is the abstract:

We study primordial gravitational waves produced during inflation in quantum gravity at a Lifshitz

point proposed by Hoˇrava. Assuming power-counting renormalizability, foliation preserving

diffeomorphism invariance, and the condition of detailed balance, we show that primordial gravitational

waves are circularly polarized due to parity violation. The chirality of primordial gravitational

waves is a quite robust prediction of quantum gravity at a Lifshitz point which can be tested through

observations of cosmic microwave background radiation and stochastic gravitational waves.

I find this one particularly important because it claims that it has a measurable prediction that could falsify (or give credit to) the theory.

The last one is neither written by Horava, the authors are H. L¨u †⋆, Jianwei Mei † and C.N. Pope. The paper is: Solutions to Horava Gravity

And the abstract is:

Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which

reduces to Einstein’s general relativity at large distances, and that may provide a candidate

for a UV completion of Einstein’s theory. In this paper, we derive the full set of equations

of motion, and then we obtain spherically symmetric solutions and discuss their properties.

We also obtain the Friedman-Lemaitre-Robertson-Walker cosmological metric.

I would advise the readers of this blog to read the entries in the other blogs that I have linked in this page because the last month there have been many many interesting things that are worth reading. Maybe I will make a post resuming them.

Besides LQG none of them has had major success in attracting people to do research in it. Now, at least it seems so, there is a new option, the so called Quantum Gravity at a Lifshitz Point initiated by Peter Horava, a well known string theorist (remember the Horawa-Witten model of heterotic string theory). I had the first new about it in the Lubos blog, but since them a few other papers have appeared. As far as another string theory minirevolution is going on (F-theory GUTs) which is leading to , seemengly, actual predictions testable in the LHC, as well as, maybe, in cosmology, I have had not time to read these articles, beyond an slight overview. I will use this entry mainly to keep track of the actual papers and also to encourage possible readers of this blog to investigate about them.

I will limit, so, to link some of the papers and paste the abstracts. Just to say that the theory will probably be of the liking of the people who likes condensed matter and critical phenomena.

This was the firs paper, Quantum Gravity at a Lifshitz Point. This is the abstract:

We present a candidate quantum field theory of gravity with dynamical critical

exponent equal to z = 3 in the UV. (As in condensed matter systems, z measures the degree

of anisotropy between space and time.) This theory, which at short distances describes

interacting nonrelativistic gravitons, is power-counting renormalizable in 3 + 1 dimensions.

When restricted to satisfy the condition of detailed balance, this theory is intimately related

to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor.

At long distances, this theory flows naturally to the relativistic value z = 1, and could

therefore serve as a possible candidate for a UV completion of Einstein’s general relativity

or an infrared modification thereof. The effective speed of light, the Newton constant and

the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic

z = 3 theory at short distances

This is the second one: Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point, and this is the abstract:

We extend the definition of “spectral dimension” (usually defined for fractal and

lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that

in quantum gravity dominated by a Lifshitz point with dynamical critical exponent z in D+1

spacetime dimensions, the spectral dimension of spacetime is equal to

ds = 1 + D/z

In the case of gravity in 3 + 1 dimensions presented in arXiv:0901.3775, which is dominated

by z = 3 in the UV and flows to z = 1 in the IR, the spectral dimension of spacetime flows

from ds = 4 at large scales, to ds = 2 at short distances. Remarkably, this is the qualitative

behavior of ds found numerically by Ambjørn, Jurkiewicz and Loll in their causal dynamical

triangulations approach to quantum gravity

The next article is not written by Horova, the authors are Tomohiro Takahashi and Jiro Soda. The paper is this:Chiral Primordial Gravitational Waves from a Lifshitz Point. This is the abstract:

We study primordial gravitational waves produced during inflation in quantum gravity at a Lifshitz

point proposed by Hoˇrava. Assuming power-counting renormalizability, foliation preserving

diffeomorphism invariance, and the condition of detailed balance, we show that primordial gravitational

waves are circularly polarized due to parity violation. The chirality of primordial gravitational

waves is a quite robust prediction of quantum gravity at a Lifshitz point which can be tested through

observations of cosmic microwave background radiation and stochastic gravitational waves.

I find this one particularly important because it claims that it has a measurable prediction that could falsify (or give credit to) the theory.

The last one is neither written by Horava, the authors are H. L¨u †⋆, Jianwei Mei † and C.N. Pope. The paper is: Solutions to Horava Gravity

And the abstract is:

Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which

reduces to Einstein’s general relativity at large distances, and that may provide a candidate

for a UV completion of Einstein’s theory. In this paper, we derive the full set of equations

of motion, and then we obtain spherically symmetric solutions and discuss their properties.

We also obtain the Friedman-Lemaitre-Robertson-Walker cosmological metric.

I would advise the readers of this blog to read the entries in the other blogs that I have linked in this page because the last month there have been many many interesting things that are worth reading. Maybe I will make a post resuming them.

Etiquetas:
quantum gravities

Subscribe to:
Posts (Atom)