If yous receive got 46 spare minutes, here's some other video on string cosmology together with things of this sort:
It has the same woman soul host equally a documentary on inflation together with both of them were brought to us yesteryear Peter F.
Gabriele Veneziano – the founder of the firstly "stringy" formula inward the history, the Veneziano aAmplitude (unless yous attribute the breakthrough to Leonhard Euler because it's the Euler Beta function) – sketches some history together with basics of string theory equally good equally his favorite (and much to a greater extent than controversial) topic inward the next decades, "before the large bang" cosmological models.
I plainly consider all these models together with claims most the rattling early on Universe according to string theory to hold upwards much less established than string theory's declaration most nearly apartment or AdS spacetimes (especially those amongst some unbroken supersymmetry – banknote that dramatic early on cosmology unavoidably breaks SUSY heavily). Influenza A virus subtype H5N1 few examples of the doubts volition hold upwards mentioned momentarily.
On the other hand, I was much closer to the folks who were producing some of this research. It's especially the instance of Ali Nayeri who is a fun (Persian) guy inward the exhibit above. When they were creating their paper amongst Cumrun Vafa together with Robert Brandenberger at Harvard, I was watching every mensuration together with it was amusing. You may belike regain several TRF weblog posts on that topic.
This documentary reminds me that they believe many of the same things equally Veneziano which I only don't believe. One of the key dogmas is that a geometric singularity is ever an inconsistency together with so whatever consistent description of the rattling starting fourth dimension must really supercede it amongst some geometrically large space.
You may listen many specific versions of this philosophy inward the show. One of them involves T-duality. Physics on a circle of radius \(R\) is the same equally physics on a circle of radius \(1/R\) inward some string units. So far together with so good. But both Nayeri together with Veneziano basically say that it "follows" that the rattling early on development after the starting fourth dimension amongst a growing \(R\lt 1\) should hold upwards T-dualized together with so that yous acquire a decreasing T-dual \(R'=1/R\). They believe that it's unavoidable that some rattling early on cosmology contains some shrinking Universe, a decreasing \(R\).
I just don't purchase it. The T-duality transformation also changes the value of the coupling constant \(g_s\). When yous carefully follow all these moduli together with inquire whether T-duality may ever map whatever toroidal compactification to "large tori", yous volition regain out (see Dualities vs Singularities) that the response is "Yes" for several compactified dimensions. But when yous compactify all or almost all dimensions – together with so that 1+1 or 0+1 large dimensions are left – at that spot are extreme portions of the moduli infinite where the compactified manifold is "tiny" regardless of the T- or U-dualities that yous perform. Only 1/2 of the infinite for 1+1 large dimensions; or a "future calorie-free cone" for 0+1 large dimensions may hold upwards dualized to a compactification whose all radii are large.
So I just don't recall that that a polish spacetime at the starting fourth dimension of the cosmological development is made necessary yesteryear dualities. More generally, I also don't believe that it's needed for consistency. String theory is capable of defining consistent physics fifty-fifty on some backgrounds that await classically singular (orbifolds, conifolds) or that are solely non-geometric. So the possibility that things may hold upwards made polish at the rattling starting fourth dimension is just a possibility together with I recall that a rather unlikely one.
I am fifty-fifty to a greater extent than skeptical most the attempts to brand the cosmology \(t\to -t\) time-reversal-symmetric together with usage T-duality for that. The development of the Universe involves an increasing entropy – together with it is rattling rapidly increasing at the beginning, indeed. So whatever would-be perfect symmetry is heavily broken yesteryear the cosmology. Also, they sometimes believe that the thermal circle obeys some T-duality together with so inward the string units, the inverse temperature \(\beta\) is equivalent to \(1/\beta\). I just don't recall that yous may consider a higher-than-Hagedorn temperature inward a kosher (or halal, for Ali together with Cumrun) way at all. The segmentation total is only divergent for super-Hagedorn temperatures. When yous approach the Hagedorn temperature from below, the assumptions of perturbative string theory receive got to hold upwards dropped shortly or later.
This myth is also linked to a correction to Joe Polchinski's String Theory textbook that I am most proud about. You may regain my shout Motl on that page some 128 times, adjacent to some modest mistakes inward the otherwise almost flawless textbook. But if yous search for Hagedorn, yous acquire the next hit:
p. 322 (4/18/99)*: Annoying mistake: inward the work inward a higher house eq. 9.8.18, the Hagedorn temperature should hold upwards _half_ of the self-dual value. So inward eqs. 9.8.19 together with 9.8.20, "\(T_H^2\)" becomes "\(4 T_H^2\)" everywhere. (Thanks to L. Motl)You know, this is non just an extra component of 2 or 4 that is incorrect, much similar factors of \(i\) together with \(\pi\) together with others inward many other equations. Here, the hypothesized absence of the component of \(4\) was really used to brand a rather qualitative together with would-be pretty claim inviting yous to deduce some far-reaching consequences. The self-dual value nether T-duality is precisely the circumference of the thermal circle of the thermal path integral at the Hagedorn temperature, Joe wrote earlier the correction. So if yous estrus strings upwards together with acquire to the Hagedorn temperature, yous may just cross to the other side – hotter than Hagedorn temperatures – yesteryear performing a T-duality.
Except that this declaration is wrong. The 2 temperatures differ yesteryear a component of two. If yous estrus the strings upwards to the Hagedorn temperature together with apply T-duality, the temperature jumps to 4 times the Hagedorn temperature. The interval from 1 to 4 Hagedorn temperatures cannot receive got whatever convergent segmentation sum, whether yous apply a T-duality or not. There is really no careful argue why these temperatures should hold upwards the same. And I really experience that Ali Nayeri together with mayhap fifty-fifty Gabriele Veneziano together with others even together with so assume the wrong equality betwixt these 2 temperatures because this supposition sounds "pretty". Except that it's false.
String gas cosmology inward detail – pioneered 3 decades agone yesteryear Vafa together with Brandenberger together with revived a decade agone amongst Ali Nayeri's aid – adds some other attractive "simplifying" stories. It is said to explicate why precisely 3+1 dimensions are large. Well, stringy globe sheets are 2-dimensional together with so precisely inward 4 dimensions, 2 generic globe sheets intersect at 0-dimensional points, together with this intersection is the right status needed to permit all these dimensions to grow yesteryear annihilating some injure strings.
Except that inward string theory, 1 also has branes amongst assorted dimensions, non just 1-dimensional strings. For the branes, the counting is clearly different together with the naively preferred spacetime dimension should hold upwards different, too. Nayeri together with pals receive got insisted on having some explanation that inward these brane gas cosmologies, the string's dimension remains critical together with they've been explaining the arguments to me a few times but I receive got never gotten it or something – I even together with so don't believe it tin flaming hold upwards true.
It's conceivable that some total explanation why nosotros alive inward large 3+1 dimensions – either based on the string gas or something else – exists. On the other hand, it's also possible that the only explanation is environmental or "anthropic". I don't really receive got a problem amongst Nature if it allows the environmental selection to pick the dimensionality 3+1 from eleven or together with so a priori possible choices.
There are other ideas inward the documentary. The host correctly says that no 1 really buys Lee Smolin's ideas most "the universe born from a dark hole". But she indicates that the status of this newspaper yesteryear 4 famous string theorists is the same. Well, it's a tough analogy. While the large claim of the four-author newspaper is for sure non by together with large accepted, it's a newspaper trying to practise rattling clever together with quantitative things amongst combining the AdS/CFT descriptions of an ensemble of such spaces etc., amongst some nontrivial segmentation sums that string theorists working on "related but less ambitious" papers know rattling well. Smolin's claims are just childish materials for pop books which is non backed yesteryear whatever tangible mathematics whatsoever. When an equation appears inward this Smolin's paper, it's some equation found inward 1900 that is added just to increase the lay out of equations. Or it may hold upwards a piffling equation that "obviously tin flaming ever hold upwards written" together with cannot qualitatively behavior on the pre-determined philosophical picture. Vafa together with collaborators really practise remove some mathematical properties of the objects they calculate amongst together with these properties had the potential to kill their large thought altogether. So the "field" of the 2 papers is rattling different – Smolin's is just demagogic pop philosophy spell Vafa's together with pals' is a speculative newspaper trying to usage modern calculational methods to written report much to a greater extent than ambitious questions than the tiresome technical papers inward the AdS/CFT industry.
As a guy discusses for a long fourth dimension on the show, the string gas cosmological models are also claimed to hold upwards competitors to inflation. To persuade anybody, they should reproduce the successes of inflation, together with avoid the production of novel problems. I am non certain whether I believe either. The firstly thing 1 should predict is the just about scale-invariant spectrum. Inflation does together with so for a clear argue – the expansion is exponential together with thence self-similar inward the right way for a long plenty time. My agreement is that the expansion inward the string gas cosmology is unavoidably rattling far from an exponential one. I experience that "exponential" together with "scale-invariant" are basically synonymous. Violating 1 agency similar violating the other.
So they tell us that everyone takes their mightiness to reproduce inflation's successes for granted together with nosotros should only hold upwards interested inward the differences. The top dog deviation discussed on the exhibit is that the string gas cosmology only produces primordial gravitational waves at curt wavelengths – Nayeri et al. predict or claim to predict a "blue tilt". Well, I don't convey the claim "string gas cosmology is at to the lowest degree equally skilful equally inflation" for granted. I just haven't seen explanations of this claim that would satisfy me and, to a greater extent than seriously, I experience it's straightforward to make explanations showing that string gas cosmology does not piece of work equally good equally inflation. Because the theories are meant to hold upwards genuinely inequivalent, the string gas' explanation of the inflationary successes has to hold upwards rattling different. And because I was thrilled to larn how together with why inflation works, I recall that I remove to experience a 2d similar thrill to acknowledge that a different explanation of the same facts exists.
Influenza A virus subtype H5N1 woman soul cosmologist gave an oversimplified (in my opinion) pic of how the (non)-observation of the tensor modes etc. would bear witness or disprove the diverse alternatives to inflation. OK, if yous convey their words seriously, they say that the tensor modes "discovered" yesteryear BICEP2 are skilful for string gas cosmology, meet this 2014 weblog post. Now, when the BICEP2 "discovery" looks questionable, this positive evidence has gone away together with may receive got turned into a slightly negative evidence. But equally always, the absence of novel groundbreaking evidence ever basically preserves the status quo.
In the 33rd minute, they define a much broader community of "these" cosmologists. Various loop quantum cosmologists are included together with the "big bounce" is their shared belief. Well, equally I receive got already mentioned, telephone telephone me a large bounce skeptic. I am only non a bouncing Czech. I don't believe that there's whatever evidence that the large bounce is implied either yesteryear the empirical or theoretical knowledge; together with I don't really meet whatever metaphysical wages of a model amongst a large bounce. The woman soul cosmologist justifies the bounce yesteryear a discrete spacetime. Well, the spacetime isn't discrete inward this sense. These 2 questions – bounce together with discreteness – aren't quite equivalent but they're overlapping. My skepticism most both has related reasons together with they seem to a greater extent than technically established to me than whatever positive evidence these bounce people receive got presented.
Whether the infinite inward the theory describing the rattling starting fourth dimension is "large" or "small" is unknown together with the preference for 1 response or some other is a pure prejudice. It's similar arguing whether monotheism or polytheism is better. You may pick the monotheist Judeo-Christian pic but yous should realize that it's really a religion. Your having an thought most a unmarried or triple God doesn't really disprove the overpopulation of Zeus together with his relatives.
Thankfully, Veneziano says the same thing around 35:00. You can't bear witness or disprove either of these claims. Some pre-big-bang cosmology is a possibility. But if it doesn't really explicate whatever information or relationships together with it doesn't increase the consistency inward whatever mathematically verifiable way, it may hold upwards sensible to cutting the whole pre-big-bang stage yesteryear Occam's razor. It may hold upwards said to hold upwards some unphysical fantasy added on transcend of the well-known cosmology whose consistency amongst the known physics is just a affair of legends together with prejudices.
I am careful amongst razors together with Occam's razors equally good but I practise prefer the minimal picture. The inflation seems to hold upwards the oldest era that really explains something nosotros remove inward physics (even though, obviously, I tin flaming imagine a rattling dainty storey most a previous era when the compactified dimensions got the shape they needed etc.). So it's economical to assume that there's some truthful starting fourth dimension shortly earlier the inflation together with the fundamental physical theory makes this starting fourth dimension consistent. It may ultimately hold upwards equivalent to some Hartle-Hawking initial state that basically allows yous to calculate the initial weather for a Universe of radius \(R=0\). One may invent to a greater extent than structured theories amongst some novel eras but others may sensibly inquire "Who ordered that?". The "established minimal" theory of the Universe does imply that there was a large bang singularity at the beginning, indeed.
So again, I disagree amongst the guy inward the majestic shirt who says that at that spot can't hold upwards an "edge of the globe inward time". There rattling good may hold upwards one. I believe it's the same for an infalling observer within the dark hole. The singularity represents the terminate of his time. If there's a way to speak most "the life after the dark hole singularity" at all, the continuation of the life must expire on yesteryear some non-local together with basically non-geometric mechanisms. The large bang is just an epitome of that singularity. If 1 tin flaming exist, the other may belike exist, too.
The adult woman mentions Alex Vilenkin's claims that the Universe can't receive got a truthful beginning, meet e.g. this 2012 weblog post. I handgrip amongst her that the electrical current arguments seem to imply that yous can't avoid a truthful beginning, fifty-fifty inward eternal inflation etc. So the improver of other bouncing phases inward some cyclic or ekpyrotic cosmology is just making things complicated together with non-minimal but ultimately cannot alter the pic qualitatively.
Ali Nayeri says that he prefers no clearcut beginning. Great. Even if I thought it's possible, together with I don't, it would hold upwards scientifically unavoidable to consider – for the practical predictive purposes – the theories most "what nosotros tin flaming really predict". Because nosotros couldn't receive got made observations inward some causally disconnected regions of this "very ancient Universe", nosotros remove to gauge the cognition most the "ancestor Universes" yesteryear some effective data, anyway. Call me an engineer if yous wishing but I recall that such a theory that "integrates out" all this inaccessible pre-history inward some consummate way is precisely equally scientifically consummate equally your "cosmological model without a clearcut beginning". So I believe that fifty-fifty if a pic without a clearcut starting fourth dimension existed, it wouldn't hold upwards the only scientifically consummate way to clitoris our Universe.
In the concluding minute, the host frames the programme equally a way to debunk the Šmoit-style claim that string theory can't hold upwards tested. Well, string theory for sure has physical consequences but equally I mentioned, I am non certain whether the implications for the rattling early on or pre-big-bang cosmologies may hold upwards counted equally skilful examples of these implications because a bulk of the "hot ideas" are mostly most aesthetic preferences.