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Colliding Branes Cosmic Model

Posted January 26, 2011 10:00 PM by Jorrie

The idea of 'branes' (or membranes) originated in quantum physics, more specifically as an extension of superstring theory. Superstrings are one dimensional entities that operate in a ten dimensional spacetime (three space, one time and six compacted extra dimensions). They avoid singularities, because their minimum size is the Planck length (~10-35 m), not point particles as in 'standard' particle physics. However, there were a few problems.

From http://en.wikipedia.org/wiki/Superstring_theory: "Theoretical physicists were troubled by the existence of five separate string theories. A possible solution for this dilemma was suggested at the beginning of what is called the second superstring revolution in the 1990s, which suggests that the five string theories might be different limits of a single underlying theory, called M-theory." It is today more or less accepted that the "M" stands for "membrane", although it was never explicitly stated by its originator, Edward Witten of the Institute for Advanced Study. It essentially boils down to adding an extra (eleventh) dimension to superstring theory, giving a string a two dimensional character, like a membrane or simple surface in a three dimensional world. The five different superstring theories are thought to be just different aspects or views of the same thing, which lives on a brane.

The Ekpyrotic Cosmic Model

This rather weird name came out of "work by Neil Turok and Paul Steinhardt and maintains that the universe did not start in a singularity, but came about from the collision of two branes. The name comes from a Stoic term "ekpyrosis", meaning conflagration or "conversion into fire" (http://en.wikipedia.org/wiki/Ekpyrotic)(a). Weird as it may sound, the model essentially makes the same predictions as the standard ΛCDM cosmic model (without inflation(b)) and as a bonus, it offers some degree of explanation for dark matter and dark energy. The squares represent two parallel branes in normal three dimensional space (x,y,z) with one dimension (x) suppressed. The normal x-direction is replaced by the fourth space dimension (w), also call 'the bulk'. The branes are embedded into the bulk and can move in that fourth dimension.

Of even more importance, this 'conversion-into-fire' model predicts some potentially observable effects that differ from the standard cosmic model and hence it is potentially falsifiable by observation, something considered a plus point for any good scientific model or theory. The best news is that both predictions have to do with the Cosmic Microwave Background (CMB), which is currently under renewed scrutiny by the ESA Planck Mission.

Observations that may distinguish between the ekpyrotic and inflationary models include polarization of the CMB and the frequency distribution and strength of the CMB gravitational wave spectrum. The inflationary scenario of the standard BB theory makes specific predictions about these observables and all one needs is a detailed enough map of the CMB to verify it. By detailed enough is meant that very small temperature fluctuations must be detectable so that effects like polarization and gravitational wave signatures can be extracted. The ekpyrotic model does not predict the same level of gravitational waves as the inflationary model; in fact the amplitude may be extremely low. More about that in the next section.

Since the very first Planck data have just been released, we may have to wait a number of years for the finer details to be analyzed, but this is surely exiting stuff. It is not often that a new cosmological model arrives that satisfies all the present observations and makes predictions about relatively easily measurable parameters that differ from the accepted theory.

How does 'Conversion-into-Fire' differ from the 'Big Bang'?

The Big Bang is usually associated with a theorized cosmic epoch from about t=10-35 to 10-32 seconds, also simply called cosmic inflation, during which the universe expanded by a factor of at least 1026. This means a region of space the size of an atomic nucleus (about 10-14 m) 'blew up' to at least 1012 m, about the distance of Jupiter from the Sun. The temperature immediately after inflation was immense, about 1027 K. The colliding brane scenario does not produce that high a temperature, but 'only' about 1020 K, hot enough to make all the particles predicted by the inflationary cosmos. Still a very hot start, but not from the near-infinite energy density required when inflation started.

Because branes must have quantum fluctuations in energy, a collision between two of them should produce the pattern of cold and hot spots that is observed in the CMB today. However, the colliding branes should produce a different polarization and a much lower level of gravitational waves than inflation predicts. An 'explosive' inflation must produces a large amount of energetic gravitational waves, but a brane collision should happen over million or even billions of years and will produce much less, if any, gravitational waves. The Planck mission should be able to detect the level of gravitational waves (as 'ripples' in the CMB) that standard inflation should have produced. If not observed, it could perhaps have implications for the inflationary epoch and the 'shares' of theories like brane cosmology will improve.

No Beginning of Time?

Colliding brane cosmology is actually a 'cyclic model', i.e., it does not require a beginning of time, because the two (or more) branes must have existed before our current cosmic expansion started. In fact the ekpyrotic model says that it could be an 'eternal' process of collisions and 'heat death' due to expansion, because the two branes would move only a short distance apart before approaching again for another collision. The 'short distance' could however take billions of years to accomplish, by which time each brane has expanded a significant amount. Author John Gribbin describes(c) the force between the branes as a "spring-like force", i.e., when they get too far apart, they are pulled together and if they are getting very close, they are pushed apart again - an oscillation in the fourth space dimension.

The 'collision' between two branes does not have to proceed all the way - the spring-like force should stop them from actually colliding, similar to 'bounce cosmology' in ordinary three dimensional space. Hence, no singularities, even in the fourth space dimension. It is however not completely clear if there must still be a beginning of time, when the oscillations started. If so, how did they get started?

Dark Stuff explained?

The oscillations have the same effect on the expansion of each brane as dark energy in the standard ΛCDM model. It causes the branes to expand at an accelerating rate after an initial deceleration. Before the next collision, the density of our universe thins out to be virtually zero and it is near absolute zero in temperature. Then a lot of positive energy is dumped into both branes by the collision and the cycle repeats. Because of the negative value of resulting gravitational energy, there is no net energy gain - the total energy remains precisely zero, just like in the standard flat cosmic model, where it starts with zero net energy.

Dark matter of the ΛCDM model is explained as the effect that each brane has on its next-door neighbors. Unlike other particles, gravitons (the particles carrying the gravitational effect) are not constrained to just the surface of each brane. Gravitons can move between branes and so one brane can enhance the gravitational effects in another brane. This is possibly what we observe as the 'missing matter' that prevents galaxies from disintegrating under their own (too-fast) rotation. It is also exactly what is required to make the universe spatially 'flat'.

Another claim of the theory is an explanation for the extreme weakness of gravity. It is only 10-38 times as strong as the strong nuclear force, making it somewhat of a puzzle. The other force carrying particles move in the brane only, but gravitons cannot move there. They must first go into the bulk and then, according to the theory, only some of them ever return to the brane, to some other spot.(d) It may be this "leaking" of gravitons out of the brane and into the bulk that makes gravity such a weak force.

Conclusions

Is this all too good to be true? Maybe it is. The main criticism against the ekpyrotic model is the speculative nature of inter-brane forces in the fourth space dimension. There is no good mechanism in string theory (or any other solid theory) for the 'spring-like force', so it may be just another disguise for mysterious dark energy and dark matter. Another (weaker) criticism is that it still needs a beginning somewhere in the past, where the branes were created.

Nevertheless, it would be very interesting if Planck does not detect the gravitational wave signatures in the CMB that are predicted by inflation. It may obviously then mean that they are just below the threshold detectable by Planck and will be detected by the next space observatory, but still...

Jorrie

Notes

(a) According to Paul J. Steinhardt of Princeton University, their theory is based on a five-dimensional (four space plus one time) variant of M-theory, called heterotic M-theory, which was formulated by Andre Lukas (Sussex), Ovrut and Dan Waldram (Queen Mary Westerfield College). In the words of Steinhardt:

"According to Horava-Witten and heterotic M-theory, particles are constrained to move on one of the three-dimensional boundaries on either side of the extra dimensional interval. Our visible universe would be one of these boundaries; the other boundary and the intervening space would be hidden because particles and light cannot not travel across the intervening space. Only gravity is able to couple matter on one boundary to the other. In addition, there can exist other three-dimensional hypersurfaces in the interval, which lie parallel to the outer boundaries and which can carry energy. These intervening planes are called "branes", short for membranes. The collision that ignites the hot big bang phase of the ekpyrotic model occurs when a three-dimensional brane is attracted to and collides into the boundary corresponding to our visible universe".

(b) Inflation is today taken as part of the Big Bang theory, but it is not part of the ΛCDM cosmic model, which describes the expansion dynamics 'backwards in time' from the present observables till some very dense earlier state, possibly just after inflation ended (or after the last brane collision).

(c) 'In search of the MULTIVERSE' by John Gribbin (Penguin, 2010). The cyclic effect makes the ekpyrotic model a 'multiverse' theory, because 'multiple universes' are created one after the other, one per full cycle. There may also be parallel universes located in the fifth dimension.

(d) An intriguing thought occurred to me: could this be why gravity appears to act instantly over long distances? Do gravitons disappear into the bulk and then pop up some distance away with no (or negligible) travel time? General relativity solves the 'action at a distance' problem by setting up a gravitational field (in the form of curved spacetime) which is static relative to the gravitating body. Only changes to the field move at the speed of light. [Edit: also see this reply below.]

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#1

Re: Colliding Branes Cosmic Model

01/27/2011 3:04 PM

Hi Jorrie, extremely informative as always. I wish I understood string theory better. I was able to follow your post but my understanding is superficial at best.

You wrote: "could this be why gravity appears to act instantly over long distances". I know what you meant but am worried about your phrasing here. I know you don't mean to say that if the sun were to disappear this minute the Earth would instantly change orbit. It would in fact take roughly 8 seconds right? Maybe a better way to say it is it would explain "action at a distance"? It's nitpicky, I know, and if I'm misunderstanding please let me know.

My thoughts as of today on Cosmology:

Essentially the big bang model assumes a homogeneous universe with homogeneous expansion. The problem I have with that is that yes the universe in the largest scale is homogeneous and so yes the expansion is homogeneous when viewed from the largest scale, but on the smaller scale, mass distribution isn't close to homogeneous and I believe this must have some sort of measurable inhomogeneous effect on spacial expansion, if only locally within the scale examined.

Consider the hydrogen atom. The diameter of a hydrogen atom is ~.5 angstroms (10^-10) yet the vast majority of the mass of a hydrogen atom is in the nucleus, which is ~2 femtometers (10^-15) in diameter. That means the vast amount of the energy in a hydrogen atom resides in a volume approximately 4 cubic femtometers out of the possible 65 x10^12 (65 trillion) cubic femtometers that makes up the hydrogen atom. An atom is very inhomogeneous. I feel this must matter somehow but can find nothing in the literature either way on it. It's not even calculated anywhere as far as I know.

I know from past discussions that you believe the contribution would be dwarfed compared to the overall expansion. An understanding generally accepted in the literature (please don't let me put words in your mouth, but I think this is your position, and it is a logical position held by pretty much all of the scientific community).

I personally believe that an inhomogeneous relativistic cosmic model may account for some of the mysteries we find in the universe such as dark energy, dark matter, dark flow, and everything else they can't figure out that they throw a "dark" in front of. As far as I know String Theory has not yet prodicted one measurable result. Quantum Mechnanics and Relativity how produced countless. So I'm favoring the existing cosmological model but urging an investigation into highly inhomogeneous effects.

Sorry about the rant Jorrie.

Roger

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#5
In reply to #1

Re: Colliding Branes Cosmic Model

01/28/2011 10:01 AM

Hi Roger, no problem :)

Einstein's 'spooky action at a distance' is probably worth a new topic (Blog entry), because it can be a long story. Briefly, here is my view.

If we do away with Einstein's gravitational field and replace it with a force carried by string theory's gravitons, then the force must be applied instantaneously over distance, otherwise gravitational orbits cannot be stable. I've written on that in my eBook under orbital equations.(a) It does not mean that gravitons must travel at faster than c, but the effect must be instantaneous, just like entanglement experiments proved for other quantum effects. How it actually works, remains mysterious.

You wrote: "So I'm favoring the existing cosmological model but urging an investigation into highly inhomogeneous effects."

Inhomogeneities on small to medium scales should have little or zero effect on the large scale expansion, because they average out. Very large inhomogeneities are somewhat different, especially if you sit inside a concentration, or inside a void, for that matter. Large void studies have been done, specifically in an attempt to show effects similar to dark energy, but they have largely been refuted. Not many cosmologist pay much attention to them lately, probably for the good reason that the evidence for dark matter and -energy (or something like it) is overwhelming.

For our own Galaxy and Super-cluster, the gravitational effects of the 'nearby' concentrations of mass are taken into account when measurements are interpreted.

-J

Notes

(a) See pdf available from page: Orbital Equations on my site Relativity-4-Engineers. It is on page 7 in the loose pdf, or page 108 in the eBook.

PS: looking forward to your next episode of the 'dialoque'...

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#11
In reply to #5

Re: Colliding Branes Cosmic Model

01/28/2011 10:50 AM

Hi Jorrie, I could be wrong, but I think even with gravitons it isn't instantaneous. To use the sun example again, I think in either theory if the sun were to just vanish it would take ~8 seconds for the Earth to "feel" it. Otherwise gravitational waves would exist in neither theory. To get back to my inhomogeneity argument, as I pointed out the inhomogeneity of mass within an atom is massive (the majority of the mass/energy resides in 4 cubic femtometers out of a possible 65 trillion femto meters. Surely such a inhomogeneous distribution of matter must effect the local (microscopic) expansion rate? How can we ignore 10^-12? Roger

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#12
In reply to #11

Re: Colliding Branes Cosmic Model

01/28/2011 11:06 AM

Why do you refer to ~8 seconds--do you mean the approximately 8 or 9 minutes it takes for light to travel from the sun to the earth?

As Jori says (and I've seen such statements by very renowned physicists (I think even Einstein)) that recognized that (and I won't say this exactly correct), but: If gravity in all respects could travel at only the speed of light, planetary orbits would be unstable. I'm trying to remember whether their calculations (which I'm not sure I saw, and more sure that if I saw them, I wouldn't be able to confirm them myself) indicate that orbits would continually increase or continually decrease (if gravitational effects could not be effected at faster than the speed of light).

It is my gut feeling / understanding that curved space was Einstein's way to resolve this anomaly, with the added idea (that I'm sure he intended, but I haven't really seen written) that this curved space moves with the object and is in some sense joined to it. Imagine if the sun (or earth) does not just consist of the visible matter of the sun (or the earth) but also includes the curved space.

So, when the sun moves, instantaneously, the curved space created or associated with the sun also moves. In this way, orbits can be stable.

If I come across a reference, I'll try to provide it.

Maybe the most graphic way it was described in something I read was that in order for orbits to be stable, the gravitational force must act on (in the direction of) the current location of the planetary bodies, not where they were (in the case of the sun and earth) 8-9 minutes ago (which would be the case if the gravitational force could travel (or transition) only at the speed of light.

Another way I look at it (rather cynically, but maybe not so much): Einstein figured out that light could not exceed a certain speed, and came to the conclusion that nothing could exceed that speed. Then, when he found that there was a problem with gravitational speed / planetary orbits, he came up with the curved space explanation which allowed him to keep the speed of light as a maximum speed but still allowed stable orbits (and sort of finessed the question of the speed of gravity).

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#20
In reply to #12

Re: Colliding Branes Cosmic Model

01/28/2011 4:46 PM

Yes I meant ~8 minutes, not ~8 seconds. Sorry about that. Please see my response to Jorrie with regards to the rest of your comments.

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#23
In reply to #12

Re: Colliding Branes Cosmic Model

01/29/2011 8:39 AM

Just trying to reinforce some learning for myself:

"I'm trying to remember whether their calculations (which I'm not sure I saw, and more sure that if I saw them, I wouldn't be able to confirm them myself) indicate that orbits would continually increase or continually decrease (if gravitational effects could not be effected at faster than the speed of light)."

Having refreshed my memory by various means (including by reading Jori's comments), I should remember that the orbits would continually increase, because the angular momentum of the system would be increasing.

As far as providing a reference, one good one is the one Jori mentions, the Orbital Equations page in (or on the website) of his book.

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#15
In reply to #11

Re: Colliding Branes Cosmic Model

01/28/2011 1:06 PM

Hi Roger, on gravitons:

As I said last time, if we drop Einstein's gravitational field (curved spacetime), which follows any inertially moving massive body, you must have a force acting on the other body in orbit. In superstring theory, that force is created by hypothetical gravitons(a) that are absorbed by the orbiting body. You'll have a very hard time showing how gravitons moving at c can create stable orbits. They must exhibit "action at a distance" (instantaneous force) to achieve that. As you know well, the moment we go into quantum mechanics, 'ordinary' views simply do not work.

On inhomogeneity: from a specified distance, a 'point mass' and a homogeneous spherical distribution of matter (with that same mass) yield exactly the same gravitational field (or curvature), as long as you are outside of the sphere.

Also, I do not quite follow what you mean by "the local (microscopic) expansion rate". There are effectively zero expansion at local level. The nuclear forces and local gravitational binding overrides cosmic expansion completely.

Notes:

(a) I think you implied that, but for others, note that gravitational waves and gravitons are not equivalent to each other. Gravitational waves are classical spacetime 'ripples' that establishes the gravitational field and changes to the field, while gravitons are quantum-mechanical particles that are continuously present, even in a static gravity scenario.

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#19
In reply to #15

Re: Colliding Branes Cosmic Model

01/28/2011 4:44 PM

Hi Jorrie,

I mentioned gravity waves because I believe they are a direct consequence of the "speed limit" on the gravitational force. To use my earlier example, if the Sun were to suddenly disappear, then the massive depression in space that was it's gravity well would disappear as well at the location of the sun and the subsequent correction to surrounding space would travel outward (as a gravity wave). That change in space time travels through space at the speed of light affecting earth 8 minutes later (yes I meant ~8 minutes last time, sorry). So from the Earth's point of view, it doesn't "feel" the sun gone until 8 minutes later. It's orbit is exactly the same until 8 minutes later. The gravitational force between the earth and the sun is not instantaneous.

As for gravitons, they are simply conjectured (or predicted if you believe string theory) to be the force carriers of the gravitational force. Just as photons are for the Electromagnetic Force and Gluons are for the strong force. As gravitons are hypothesized to be massless (like the photon) they are limited to the speed of light. Now I know what you are talking about, there are virtual particles with regards to the exchange of force, which in fact do instantaneously appear out of the vacuum and appear to circumvent the speed of light. But it is my understanding that the macroscopic effects would still not occur instantly but rather appear to occur at the speed of light (my sun disappearing example).

From Wikipedia: "For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass"

http://en.wikipedia.org/wiki/Graviton

You Wrote:"On inhomogeneity: from a specified distance, a 'point mass' and a homogeneous spherical distribution of matter (with that same mass) yield exactly the same gravitational field (or curvature), as long as you are outside of the sphere. Also, I do not quite follow what you mean by "the local (microscopic) expansion rate". There are effectively zero expansion at local level. The nuclear forces and local gravitational binding overrides cosmic expansion completely."

I meant in the sphere. That is after all where the electron is. I've mentioned to you that the core of quantum mechanics is an uncertainty of position and momentum in microscopic particles (uncertainty relation) and conjectured that maybe that uncertainty was due to the expansion of space around them (this was a long while back). You pointed out (quite correctly) that the expansion of space was much too small to account for that sort of uncertainty and I agreed. But the expansion of space you referenced is the one assuming a homogeneous universe with a homogeneous expansion rate. Couldn't the expansion rate locally (withing the sphere) be much stronger due to local inhomogeneity? Now before you accuse of me of pseudoscience (which is what this is) let me point out you started it with string theory. ;)

Anyway that's what I meant.

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#22
In reply to #19

Re: Colliding Branes Cosmic Model

01/29/2011 2:00 AM

Hi Roger, on the graviton: I think we are mis-communicating somewhat. What you describe is perfectly valid in Einstein's gravity, but it is not true when you take away Einstein's curvature field, which I think string theory does. Then gravitons from the Sun must continuously 'pull' on the planet, in real time, otherwise its orbit will be unstable (it will spiral outwards much faster than can be accounted for by tidal effects). Since we have stable orbits, it means that either string theory gravitons propagate at infinite speed; or somehow they do the same 'trick' as quantum entanglement (and other quantum effects) - they travel at c, but the effect is instantaneous.

To open another can of worms, since no gravitational waves have ever been directly detected, there is also no experimental evidence that the waves propagate at c. Planck will hopefully settle this one soon, if it detects CMB gravitational waves. :)

On inhomogeneity: I still fail to understand why you mix atomic scales with cosmic scales when considering expansion.

It is normally sufficient to take a fair sized inhomogeneity, a lot of atoms where gravity rules (e.g. Earth, Solar system, Galaxy, etc.) and show that it has no effect on the large scale expansion rate, other than playing its part in the average energy density that governs the dynamics of overall expansion. Earth, our Solar system, our Galaxy do not expand, so how can atomic levels expand?

What we have in brane-collision theory is a lot of energy pumped into an empty space of virtually infinite size, creating particles that drifted apart very rapidly (distances between them increased, which can be likened to spatial expansion). Thus the cosmos acquired an overall expansion rate, despite certain denser spaces that did not locally expand as fast and later (when the overall rate dropped) contracted under mutual gravity to form stars, galaxies, clusters, super-clusters (anything gravitationally bound). In-between the bound structures there are vast voids which 'expands', in the sense that the proper distance between them increases over time. Exactly as per standard ΛCDM cosmology...

How do you reckon very localized inhomogeneities may affect this overall expansion (or drifting apart)?

-J

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#24
In reply to #22

Re: Colliding Branes Cosmic Model

01/29/2011 11:01 AM

Hi Jorrie,

It's possible we are miscommunicating. What I'm trying to say is that just as General Relativity reproduces Newtonian Gravity (they are equivalent in certain situations) so too does string theory reduce to General Relativity (and thus Newtonian Gravity as well). String Theory is meant to replace General Relativity in the same way that General Relativity replaced Newtonian Gravity. Take this passage from wikipedia on gravitons:

"In the classical limit, the theory would reduce to general relativity and conform to Newton's law of gravitation in the weak-field limit."

Here's another passage from the wiki on Quantum Gravity:

"This problem must be put in the proper context, however. In particular, contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called gravitons)".

Essentially Gravitons are the force carriers for the Gravitational Interaction. This is accomplished via virtual particles that pop out of the vacuum based on probability amplitudes. The way they do this reproduces the effects of General Relativity including the limitation of information to travel no faster than the speed of light. The mechanism by which it adheres to that particular aspect (or better to call it "recreates in it's own framework that particular aspect) I'm not sure of. I'll look around and see if I can put together a coherent explanation.

As for the inhomogeneous discussion, as that is rank speculation on my part, lets set it aside for now till we resolve this current issue above.

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#25
In reply to #22

Re: Colliding Branes Cosmic Model

01/29/2011 11:13 AM

I wrote in my prior post: "... it means that either string theory gravitons propagate at infinite speed; or somehow they do the same 'trick' as quantum entanglement (and other quantum effects) - they travel at c, but the effect is instantaneous."

In order to prevent misunderstanding, I should have added: the 'trick' could obviously also be that gravitons set up some sort of field that behaves just like Einstein's curved spacetime. Then the force carried by the gravitons will appear as if it suffers no propagation delay between the source and the receiver, as long as both are moving inertially. A delay will be apparent when the transmitter suffers non-inertial acceleration, like "if the Sun were to suddenly disappear", or something very massive crashes into it. Any non-inertial changes to the field should propagate at c.

The problem is that there is no fully detailed quantum theory of gravity yet, so we do not know the details as far as gravitons are concerned.

Edit: I did not notice your last post before I submitted this one - they crossed in the post... :)

You wrote: "...so too does string theory reduce to General Relativity (and thus Newtonian Gravity as well)."

Not yet! It's only the aim...

-J

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#26
In reply to #25

Re: Colliding Branes Cosmic Model

01/29/2011 12:13 PM

Hi Jorrie,

Here is a link that I think is pretty straight forward regarding this. The original question is dumb but the answers are pretty good:

http://www.physlink.com/education/askexperts/ae658.cfm

So to be clear - Gravitons can't travel faster than the speed of light and there is no "effect" that is instantaneous. I think the simplest way to think about it is that quantum gravity just means the field is not continuous but rather made up of discrete energy quanta called gravitons with properties that produce what we perceive as macroscopic gravity.

To put it quantum mechanically: Virtual Particles can't be measured. Spooky action at a distance only occurs when a measurement occurs (as in entanglement), thus spooky action at a distance involving virtual particles cannot occur. Now I know that answer seems bizarre, but believe it or not, it's probably the most accurate one I've offered you on this thread from a quantum mechanics point of view.

All of that said, I still don't know if we're trying to say the same thing or not or if there is a fundamental difference in our understanding.

When you wrote: "there is no fully detailed quantum theory of gravity yet"

That's true but they have a good idea of what they are looking for, they just don't know how to make it come out of a mathematical framework that also produces the standard model. String theory comes close as I understand it but has some trouble producing the detailed predictions of standard model (which is based on gauge transformations).

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#28
In reply to #26

Re: Colliding Branes Cosmic Model

01/30/2011 12:42 AM

Hi Roger, I read that link and it also does not quite cut to the bone. A somewhat 'deeper cut' is this one of Steve Carlip: Does Gravity Travel at the Speed of Light?

"Strictly speaking, gravity is not a "force" in general relativity, and a description in terms of speed and direction can be tricky. For weak fields, though, one can describe the theory in a sort of newtonian language. In that case, one finds that the "force" in GR is not quite central--it does not point directly towards the source of the gravitational field--and that it depends on velocity as well as position. The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the newtonian result".

Together with your: "Virtual Particles can't be measured. Spooky action at a distance only occurs when a measurement occurs (as in entanglement), thus spooky action at a distance involving virtual particles cannot occur", it seems that the best we can say is that gravity may be seen as a 'virtual force', mediated by virtual particles, which do not follow the classical paths from source to receiver.

In the case of binary pulsars, the 'cancellation' that Carlip refers to is slightly "over-applied" and the pulsars spiral inward instead of outward (as classical forces predict). The in-spiraling is normally attributed to the radiation of gravitational waves (and consequent loss of orbital energy), but the mechanism seems to be the mystical "virtual force, mediated by virtual particles".

The lines (A-B) show the classical 'retarded force vectors', which will cause the binaries to spiral outward. The 'virtual force vectors' (dotted arrows) actually go almost through the gravitational center, yet very slightly to the opposite side of the classical vectors. This causes the binaries to slowly spiral in.

Thanks for your inputs. I think we can let this rest for now and concentrate on the cosmology... :)

-J

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#29
In reply to #28

Re: Colliding Branes Cosmic Model

01/30/2011 1:23 AM

That's a great link, thanks Jorrie.

Hey, I was reading Physics Today and they have a story about the new stuff being learned about Mercury due to the Messenger probe. In the article it mentioned that 3:2 spin-orbit resonance. Have you written anything on spin-orbit resonance and how it occurs in general relativity? I'd be interested in reading it.

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#30
In reply to #29

Re: Colliding Branes Cosmic Model

01/30/2011 2:03 AM

I have only written on the 1:1 spin-orbit ratio of our moon, but the tidal effect mechanism is similar for Mercury. The stable 3:2 ratio is apparently due to the spin rate/orbit eccentricity evolution of the orbit. Good work done by Correia and Laskar:

http://astronomy.nju.edu.cn/~zly/chinese/cm/Nature02609.pdf

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#32
In reply to #30

Re: Colliding Branes Cosmic Model

01/30/2011 10:26 AM

Thanks Jorrie.

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#2

Re: Colliding Branes Cosmic Model

01/27/2011 11:31 PM

Very interesting, and I find your explanation of the Ekpyrotic Cosmic Model very helpful. I also find this model more attractive than the conventional Big Bang theory, for reasons we have discussed before- especially the fact that we don't have to fall back on some fantastic "aether" to fill in the gaps. It will be interesting to see what results can be gleaned from Planck....

As always, a most interesting post.

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#3

Re: Colliding Branes Cosmic Model

01/28/2011 6:44 AM

I'm not a wizard on string theory, but I wonder why the extra dimensions must be compacted. Why can't they (or some of them) surround our 3D world like we surround above & below the 2D world of a sheet of paper? I can also see Time as a linear dimension intersecting our 3D world, much like a pencil being poked thru a sheet of 2D paper.. The past is the linear part that already passed thru the intersection point, and we remember it as history. The present is the intersection point, and the future hasn't gotten here yet.

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#4
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Re: Colliding Branes Cosmic Model

01/28/2011 8:33 AM

Hey...uh...I just stumbled in here look'n for the bathroom and the bar. Any-o body-o speak-e da English-eo in Here-eo?

I suppose not. I'll show my self out. Good day!

Actually, I really did stumble in here. I understand what your subject matter is about but haven't the foggiest understanding of any of it. But I would like to. It sounds extremely intriguing.

So, what literature would anyone suggest I start with that could explain the very most basic understanding of this subject matter? I mean VERY basic!

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#6
In reply to #4

Re: Colliding Branes Cosmic Model

01/28/2011 10:18 AM

Well, you're in the subject world of advanced theoretical physics.

This is the thing that first brought branes to my attention in an understandable (for me) way (before I watched this, I had no idea what they were talking about).

This is a video. There's a chance this is the wrong video--if not, there is another TED talk by Neil Turok that is the one I'm referring to (I'm listening to it now to confirm it is the right one)--even if it is the wrong one, it is worth viewing / listening--the first 5 minutes will seem off topics, but around 5:30 when he asks the question "what banged?" gets into the brane theory. (I'm sure this is the right video, now--at 7:33 I'm sure.)

http://www.youtube.com/watch?v=UNbP7O6jasw

Neil Turok: 2008 TED Prize wish: An African Einstein

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#7
In reply to #6

Re: Colliding Branes Cosmic Model

01/28/2011 10:22 AM

Thanks!

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#8
In reply to #7

Re: Colliding Branes Cosmic Model

01/28/2011 10:30 AM

Oops, sorry, I gave you the wrong video. (That is, to me, an inspiring video, so I encourage you to watch it anyway.)

Here is the one I meant to refer you to (and it is Garret Lisi as opposed to Neil Turok):

Garrett Lisi: A beautiful new theory of everything

Here's another reference that helped me:

A layman's explanation of the String Theory and D-branes (M-Theory multiverse)?

Sorry about that!

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#10
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Re: Colliding Branes Cosmic Model

01/28/2011 10:42 AM

Thanks again. I appreciate your help and patience.

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#14
In reply to #10

Re: Colliding Branes Cosmic Model

01/28/2011 12:33 PM

Oops, sorry again--although both videos are interesting, it really was the first one (the Neil Turok video) that gave me my first (glimmer of) understanding of brane theory. It is between about the 5 and 10 minute marks of the video, and it was the cartoon like picture that helped me (along with Neil's explanation).

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#9
In reply to #3

Re: Colliding Branes Cosmic Model

01/28/2011 10:38 AM

Hi wcfloyd, in standard 11D M-theory the math apparently does not work unless those extra 7 dimensions are compacted to be almost, but not quite, negligible. However, in the M-variant we are working with here, "heterotic M-theory", there is only one extra space dimension of interest, the non-compacted brane dimension, giving four space and one time. Whether the other six space dimensions are simply ignored as insignificant, I do not know. The quantum world is not quite my forte...

-J

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#16
In reply to #9

Re: Colliding Branes Cosmic Model

01/28/2011 1:44 PM

Do these compacted dimensions have any physical significance, or are they just mathematical constructs to make the equations balance? Would Time be the same dimension in 3-space as 4 -space? Could they share the same time axis?Would 3-space be contained inside 4-space?

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#17
In reply to #16

Re: Colliding Branes Cosmic Model

01/28/2011 2:39 PM

"Would Time be the same dimension in 3-space as 4 -space?

Yes, I would think so.

I also think that there may be physical significance to the extra space dimension(s). It may simply be a case of we cannot observe them directly, but only the effects they have on our normal 3-space. The 'brane dimension', if it exists, may be a case in point.

-J

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#18
In reply to #17

Re: Colliding Branes Cosmic Model

01/28/2011 3:25 PM

So, you think our 3-space may be contained within a 4-space, much like a 2-space sheet -of-paper world would be contained within our 3 space. SUPPOSE we drew an animated figure on the 2-space sheet-of-paper world. He could only "see" that which is on the 2D sheet of paper with him, but we in our 3-space could see him. Do think it is stretching things to suppose that an entity in 4 space could "see" us, but our senses could not directly detect the entity? Just wondering the possibilities. I heard someone say, "If it's not forbidden, then it is possible."

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#21
In reply to #18

Re: Colliding Branes Cosmic Model

01/29/2011 12:00 AM

"Do think it is stretching things to suppose that an entity in 4 space could "see" us, but our senses could not directly detect the entity?"

Not 'stretching' it too much. :) After all, we can see 2D things, but presumably they can only see a 'cut' or projection of our 3D world on their 2D world.

-J

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#43
In reply to #21

Re: Colliding Branes Cosmic Model

02/02/2011 9:02 PM

Pardon my rambling as I contemplate the bottom of a glass of Merlot, but thoughts come where I find them. As I was pondering a 2-D world enveloped by our 3-D world with the Time axis intersecting our realm in the present, with all this enveloped in Jorrie's 4-D space, Dark Matter/Dark Energy came to mind. Hubble has photographed objects over 12 billion light years away. Chandra has seen X-ray sources. We've scanned the universe at every wavelength available. I have to think that if "something" was there in our realm, we would see it at some wavelength. Dark Matter suggests its existence by exerting gravitational effects far greater than can be accounted for by visible matter. I'm wondering if Dark Matter/Energy could be the substance of Jorrie's 4-D space, with gravity transcending the dimensions. I have no idea how the math would work out, or how this could be tested. If Dark Matter is really real, maybe this is why we don't see it. Just wondering...

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#44
In reply to #43

Re: Colliding Branes Cosmic Model

02/04/2011 12:39 AM

Hi wcfloyd, yes, this 'parallel brane' world model seems to suggest that both dark matter and dark energy come from some interaction between parallel 3-D branes. This may mean that we won't ever detect them directly. Sad!

I do not put too much faith into this colliding brane proposal (not from me, but from P. J. Steinhardt and N. Turok). The Planck mission analysis should answer some of the questions that this model pose during the next few years, but I doubt if it will be conclusive in any way.

-J

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#45
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Re: Colliding Branes Cosmic Model

02/04/2011 7:22 AM

How do you distinguish between two similar 3-D branes interacting and a 4-D space enveloping our 3-D space? It seems like 2 branes would interact at their boundaries, while 3-D would be contained within 4-D. And it would seem that gravity can transcend, or "leak" from one realm to the other?

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#46
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Re: Colliding Branes Cosmic Model

02/04/2011 8:33 AM

Hi wcfloyd, but branes sit in 4-D space, in the 4th dimension, so it also 'surrounds' our 3-D space completely. I think the only difference with the colliding brane cosmic model is that the extra 3-D brane of interest is only a very, very short distance from our 3-D brane, in the 4th space dimension, of course.

It is postulated that it is the gravity of the 'other brane' that leaks into ours with a dark matter signature. So far no different from other quantum cosmologies, I think. Dark energy in this model is postulated to come from the 'springy' force that keeps the branes slightly apart and make them oscillate into collisions.

-J

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#40
In reply to #3

Re: Colliding Branes Cosmic Model

02/01/2011 2:32 PM

I think one of the reasons is the 1/r^2 dependence of the gravitational and electromagnetic forces. That is characteristic of three dimensional space. 9 spatial dimensions, if none were contracted, would lead to a 1/r^8 dependence of the gravitational and electromagnetic force. The more spatial dimensions you add, the more diluted our three dimensional forces get.

Since we observe a 1/r^2 dependence, yet string theory needs 9 spatial dimensions to work mathematically, string theorists said "what if we just make 6 of the dimensions so contracted that they produced no noticeable effect on the 1/r^2 dependence?" So that's what they did. That particular aspect of the theory has always been a bit hard to swallow. Some have suggested the extra dimensions are not necessarily all spatial but maybe some are timelike:

http://www.physorg.com/news96027669.html

The point is you have raised an excellent question and one that should give us all pause when examining string theory. Until string theory makes a prediction that is verified empirically, something as far as I know it has yet to do, I would stick to the standard model and general relativity.

The descent into metaphysics and pseudoscience is subtle and it happens when we discard empirical data as too "messy" for our theories. An excellent blog on this subject which criticizes the seductiveness of purely mathematical theories created ex post facto is called "Not Even Wrong" and is written by Peter Woit.

Hope that helps. To be clear I'm not saying String Theory is wrong or right, I'm saying it's not useful until in predicts something we can actually measure.

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#13

Re: Colliding Branes Cosmic Model

01/28/2011 12:01 PM

By 'chance' ... does anyone believe in 4 dimensional creatures or 5 or 6 or ... or is that just stringing you along?

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#27

Re: Colliding Branes Cosmic Model

01/29/2011 11:44 PM

Hi Jorrie,

Very interesting blog. You've been doing some reading. This makes a lot more sense than a recent speculative thread on gravity.

"The Planck mission should be able to detect the level of gravitational waves (as 'ripples' in the CMB) that standard inflation should have produced. If not observed, it could perhaps have implications for the inflationary epoch and the 'shares' of theories like brane cosmology will improve."

Even if the waves are not observed, the standard BB could still be valid. However this is exciting. Explaining dark matter and the weakness of gravity could be progress.

" 'In search of the MULTIVERSE' by John Gribbin (Penguin, 2010). The cyclic effect makes the ekpyrotic model a 'multiverse' theory, because 'multiple universes' are created one after the other, one per full cycle. There may also be parallel universes located in the fifth dimension."

Wait, do we a multitude of Multiverses? If only the UFOs would land and tell us where they come from.

I find the 'spring action' highly speculative.

-S

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#31
In reply to #27

Re: Colliding Branes Cosmic Model

01/30/2011 4:52 AM

Hi S.

"Wait, do we [have] a multitude of Multiverses? If only the UFOs would land and tell us where they come from."

I suppose it simply means we have a 'multitude of Universes'.

Personally, I do not see sequential "bangs", where new 'universes' arise out of the ashes of previous ones as Multiverses, but that's just an opinion.

If parallel universes exist, I suppose that should qualify as a Multiverse.

-J

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#33

Re: Colliding Branes Cosmic Model

01/30/2011 5:51 PM

From your link about The Ekpyrotic Cosmic Model: "The ekpyrotic model of the universe is an alternative to the standard cosmic inflation model for the very early universe; both models accommodate the standard big bang Lambda-CDM model of our universe. The ekpyrotic model is a precursor to, and part of the cyclic model."

Since the Lambda-CDM model predicts everlasting expansion, the cyclic activity appears to be a contradiction. Can you explain?

"The squares represent two parallel branes in normal three dimensional space (x,y,z) with one dimension (x) suppressed. The normal x-direction is replaced by the fourth space dimension (w), also call 'the bulk'. The branes are embedded into the bulk and can move in that fourth dimension."

Why is the 4th dimension called 'the bulk'? The term 'membrane' conjures up a visualisation of two dimensions (as your diagram shows). How then can they only be in the 4th dimension? Please help my confusion.

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#34
In reply to #33

Re: Colliding Branes Cosmic Model

01/30/2011 11:36 PM

Hi Jorrie,

Interesting post. Will first read up on the links.

Tx

SL

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#35
In reply to #33

Re: Colliding Branes Cosmic Model

01/31/2011 12:15 AM

Hi S, this is Jorrie, replying form my son's computer - mine is broken... :(

"Since the Lambda-CDM model predicts everlasting expansion, the cyclic activity appears to be a contradiction. Can you explain?"

Modern 'cyclic models' (ekpyrotic and others) postulate a new BB that occurs inside the very expanded, flat, virtually empty, existing space - so far in the future that a region as big as our observable universe contains no matter, but only the vacuum and its energy. In some 'standard' models, a minute area or areas undergo a renewed BB with inflation following. AFAIK, the ekpyrotic model postulates a complete brane to 'collide' with another brane, creating a new, 'low temperature', expanding universe.

"The term 'membrane' conjures up a visualisation of two dimensions (as your diagram shows)."

The 2-d is only an aid to visualization, because we cannot draw in four dimensions. The brane is actually a flat 3-d normal space, sitting embedded in a 4-d space (the 'bulk'). Then there is also the standard 5th dimension for time, of course. The diagrams ignore (or suppress) the 4th and 5th dimensions.

This is similar to the cosmic balloon model, where we represent 3-d space with the surface of the balloon only, with the expansion happening into the 4th dimension. I think it is possible to represent the ekpyrotic model as a double skinned cosmic balloon, but I'm still thinking about the pitfalls. :)

-J

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#36
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Re: Colliding Branes Cosmic Model

01/31/2011 7:58 AM

Re: The 2-d is only an aid to visualization, because we cannot draw in four dimensions. The brane is actually a flat 3-d normal space, sitting embedded in a 4-d space (the 'bulk'). Then there is also the standard 5th dimension for time, of course. The diagrams ignore (or suppress) the 4th and 5th dimensions.

I know I'm mixing theories, but does the ekpyrotic model (if that's the correct name for the brane theory) also envision the multiple dimensions (e.g., 11) that multi-dimensional string theories consider? (Or, to ask it explicitly: does the ekpyrotic theory provide an alternate to those multiple dimensions? (i.e., just needing one extra dimension, the 4th (bulk)?)

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#37
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Re: Colliding Branes Cosmic Model

01/31/2011 11:26 AM

Hi rhk, got the old computer going again, but I suspect it has a PSU problem, so must get a new PSU; fortunately quite cheap these days :)

The 'ekpyrotic model' is not quite 'brane theory', just a 'colliding branes' cosmic model. As such, I understand that it only requires 5 of the 11 dimensions of brane theory (4 space plus time). The other 6 are required to get all the required particles to make sense, but I think they play no role in the ekpyrotic model.

-J

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#38
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Re: Colliding Branes Cosmic Model

01/31/2011 4:34 PM

Jorrie,

Thanks!

Good luck with the PS.

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#39
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Re: Colliding Branes Cosmic Model

01/31/2011 8:05 PM

Thanks Jorrie for the explanation. I will be interesting to see what plays out in the next few years.

Good luck with your computer. Three of four years ago I bought a whole computer at a 'garage sale' for $4. It was very slow until I un-installed what I didn't want and got rid of all the viruses. I used it for a year or two, but have a better one now. I am using the CD writer from it after my other one died. My son & I have 6-8 computer power supplies lying around. Too bad I can't email you one.

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#41

Re: Colliding Branes Cosmic Model

02/02/2011 10:54 AM

Fascinating subject, if there is any validity to it at all.

I did, in fact, read B. Greenes The Fabric of the Cosmos (some sections a couple of times thru), and I can wrap my head around a good bit of String / Superstring Theory.

[From that reading alone, it seems to make "E=M" (and vice-versa) so poignantly clear as to be a 'done-deal / closed-topic'.]

BUT , I cannot for the life of me come to grips with this brane concept. I read, and I re-read, but when I finish, I come back to asking myself:

How big is a brane and what does it consist of?

Sorry for the cynicism, but the texts I have seen don't seem to do it proper justice (if such justice is in fact deserved). I feel as though I have simply delved into somebody's drug-induced dream, wherein they are imagining stuff in an unknown dimension, and trying to pawn it off in the blog-o-sphere.

Can anybody add some clarification in plain old "lay-person-speak"...?

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#42
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Re: Colliding Branes Cosmic Model

02/02/2011 11:51 AM

While not dealing specifically with branes and higher dimensions, probably the best perspective on perceiving dimensions (Isaac Asimov's opinion as well as mine) is the Victorian novel, Flatland, by Edwin Abbott, available here on line. While clearly demonstrating the difficulty a two-dimensional entity would have understanding a three dimensional universe, the two dimensional entity can observe and understand a single-dimensional entity in all its limited glory. Expand this to our four dimensional universe (including, of course, time as the fourth dimension), and we can study and comprehend lower dimensional constructs, but getting one's head around higher dimensions most definitely requires flights of fancy.

Of course, just because we can not experience or fully comprehend higher dimensions does not mean they do not exist...

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