Rumraket wrote:

newolder wrote:With regard to inflationary theory and fine tuning, Andre Linde is quoted in a SymmetryMag post on The Growth of Inflation:

... The authors admit that this is only an encouraging first step. For one thing, “stringy” inflation seems to require a very complicated fine tuning, as Linde found in a paper with seven other authors. Linde says that was a record number for him. “The reason is, it took eight authors to fine-tune the parameters to get a nice inflation,” he says. ...

The (Steinhardt's) implication being that a different group of authors could fine tune another inflationary scenario to fit another data set, and so on. That is to say inflationary cosmology is no longer disprovable by any observational data.

And yet there's an author of inflation (at 49:56) in the video I linked above who gives a testable prediction that, he says, would falsify inflationary cosmology. Sounds pretty unambigous to me.

Yes, it's an unambiguous claim that positive spatial curvature falsifies inflationary cosmology. However, here are 4 authors that discuss inflationary cosmology with positive spatial curvature.
https://www.researchgate.net/publicatio ... _Curvature

newolder wrote:

Rumraket wrote:

newolder wrote:With regard to inflationary theory and fine tuning, Andre Linde is quoted in a SymmetryMag post on The Growth of Inflation:

... The authors admit that this is only an encouraging first step. For one thing, “stringy” inflation seems to require a very complicated fine tuning, as Linde found in a paper with seven other authors. Linde says that was a record number for him. “The reason is, it took eight authors to fine-tune the parameters to get a nice inflation,” he says. ...

The (Steinhardt's) implication being that a different group of authors could fine tune another inflationary scenario to fit another data set, and so on. That is to say inflationary cosmology is no longer disprovable by any observational data.

And yet there's an author of inflation (at 49:56) in the video I linked above who gives a testable prediction that, he says, would falsify inflationary cosmology. Sounds pretty unambigous to me.

Yes, it's an unambiguous claim that positive spatial curvature falsifies inflationary cosmology. However, here are 4 authors that discuss inflationary cosmology with positive spatial curvature.
https://www.researchgate.net/publicatio ... _Curvature

Yes, but are positive-curvature inflationary cosmologies also multiverse models that lead towards infinitely many bubble universes?

I suppose it is possible to propose that there was inflation in the early universe, but that it never spawned new bubble universes, nor that our own local cosmic expansion itself could be one such expanding bubble. In that sense it is possible to retain an inflationary cosmology (which is merely inflationary in the sense that it proposes early and fast inflation mechanism), yet having no multiverse implications.

Rumraket wrote:

newolder wrote:

Rumraket wrote:

newolder wrote:With regard to inflationary theory and fine tuning, Andre Linde is quoted in a SymmetryMag post on The Growth of Inflation:


The (Steinhardt's) implication being that a different group of authors could fine tune another inflationary scenario to fit another data set, and so on. That is to say inflationary cosmology is no longer disprovable by any observational data.

And yet there's an author of inflation (at 49:56) in the video I linked above who gives a testable prediction that, he says, would falsify inflationary cosmology. Sounds pretty unambigous to me.

Yes, it's an unambiguous claim that positive spatial curvature falsifies inflationary cosmology. However, here are 4 authors that discuss inflationary cosmology with positive spatial curvature.
https://www.researchgate.net/publicatio ... _Curvature

Yes, but are positive-curvature inflationary cosmologies also multiverse models that lead towards infinitely many bubble universes?

I don't know. It simply supports the idea that another group of researchers can fine tune inflationary cosmology to fit any scenario, per the original claim.

However, eternal inflation does not disallow such universes to exist for the limited number of e-foldings in the linked study. If they then fade away to nothing, another region of the eternally inflating spacetime will go on to produce another one, and so on. That's the thing about eternal inflation - it produces all possible universes an infinite number of times.

I suppose it is possible to propose that there was inflation in the early universe, but that it never spawned new bubble universes,

No, that's not possible, for the reasons given by Steinhardt et al.

nor that our own local cosmic expansion itself could be one such expanding bubble.

What? If our bubble wasn't created by inflation then what was it produced by? (Or, have I misunderstood your "nor" clause, here?)

In that sense it is possible to retain an inflationary cosmology (which is merely inflationary in the sense that it proposes early and fast inflation mechanism), yet having no multiverse implications.

No, any inflation leads to eternal inflation - it's inevitable, for the reasons given by Steinhardt. That's the reason Steinhardt et al dropped the idea.

I do see your point and I concede I don't have the insight to rebut it if that is even possible. These different cosmologists do have their own axes to grind and models to push and I can't say which of them is correct. Steinhardt and his collaboraters are of course pushing their own alternative "multiverse"-type models.
Though as far as I'm aware it is cyclical, with infinitely many different universes happening one after another for an eternity serially. As opposed to the possibly infinitely many different universes existing in "parallel" in the eternal inflation-type models.

There's also the eternally cycling model of Roger Penrose to add to the mix - it's neither inflationary nor a brane world solution to cosmology. Conformal Cyclic Cosmology seems to have added a few more supporters with their statistical analyses on the CMBR yielding interesting features and more predictions about Planck mass particles that may be contributing to the gravitational wave "noise" at LIGO, & VIRGO. Penrose penned a recent article about it that I can't find at the moment* but there is this 1-hour-ish youtube from earlier this year that helps twist a few brain cells, for a while:

The dismissal of W L Craig's objections is that he misunderstood the central idea.

* This isn't what I'm trying to remember but is the latest arXiv on the subject: https://arxiv.org/abs/1808.01740

Also, this ScienceAlert Article discusses the idea and that^ paper but it's still not what I'm looking for. Heigh ho.

Since the discussion that started this thread has not proven particularly fruitful, it might be worth contemplating (again) this sort of disappointment. Here's a (quite new) video by the redoubtable Sabine Hossenfelder, which (again) dismantles the multiverse concept not as impossible, but as unscientific:

....

Cito di Pense wrote:Since the discussion that started this thread has not proven particularly fruitful, it might be worth contemplating (again) this sort of disappointment. Here's a (quite new) video by the redoubtable Sabine Hossenfelder, which (again) dismantles the multiverse concept not as impossible, but as unscientific:

I read somewhere that the idea of a multiverse was created to solve the paradox that Schrödinger's cat presented, to solve a thought experiment. The cat can then exist in any stage of life, near death, death, and decomposition, therefore no need to look in the box.

Thomas Eshuis wrote:The absence of evidence for fine-tuning undermines fine-tuning.

The large seizure causing blood clot and bleed on my brain also undermine the evidence for fine tuning, as do, er, like, black holes?!

dogsgod wrote:

Cito di Pense wrote:Since the discussion that started this thread has not proven particularly fruitful, it might be worth contemplating (again) this sort of disappointment. Here's a (quite new) video by the redoubtable Sabine Hossenfelder, which (again) dismantles the multiverse concept not as impossible, but as unscientific:

I read somewhere that the idea of a multiverse was created to solve the paradox that Schrödinger's cat presented, to solve a thought experiment. The cat can then exist in any stage of life, near death, death, and decomposition, therefore no need to look in the box.

That's not a physics problem, but a philosophical/linguistic problem. Ginning up a multiverse to solve a linguistically-generated 'paradox' is overkill, is my guess as to what Sabine would say about all that. Hossenfelder's comment is strictly about fine tuning arguments in the sense of wibbling about the conditions of the physical constants we observe, and her argument is that fine-tuning wibbles are firstly a misuse of statistical reasoning. People who argue the cat paradox have misunderstood what physicists mean by the consequences of an observation for the system in question. Almost never do lengthy debates in environments like this one lead to correct understanding of what the uncertainty principle implies and at whose expense the joke is made in the cat paradox. Fapping about fine tuning the way philosophers do it is not about evidence, because evidence is not something that philosophical arguments are about. People who yammer about the kind of inflation that would lead to the conclusion of a multiverse are just yammering about theoretical arguments unrelated to experiment.