The Universe without Cosmic Inflation?

[JMz]

I am skeptical that there are very many cosmologists who fail to question inflation on its merits.

Inflation is the best working hypothesis at the moment, and I believe most of them would (a) feel comfortable developing further theoretical insights based on it, including observable consequences (tests!) but (b) readily accept an alternative idea that agrees well with current observations -- possibly even agreeing a little less well with some, if it had other advantages (such as not requiring an entirely new "inflaton" field of unknown properties).

 

[bapowell]

I think it’s unfortunate that people view the consideration of “new fields” to be a weakness of a model or hypothesis. Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena. Indeed, failing to consider additional fields and new physics when attempting to understand the early universe, precisely when any new physics is expected to be especially relevant, is a bad way to do science .

 

[bapowell]

If you are asking for evidence of inflation I don't think there is any at the moment, but there could be.

There are several lines of evidence. See Wikipedia, e.g.

 

[JMz]

I think it’s unfortunate that people view the consideration of “new fields” to be a weakness of a model or hypothesis. Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena. Indeed, failing to consider additional fields and new physics when attempting to understand the early universe, precisely when any new physics is expected to be especially relevant, is a bad way to do science .

I won't argue the point strongly, since I tend to agree, and I think inflation is likely to be the right story. My only point was that any theory that can explain a phenomenon by relying only on known fields is stronger than than one that needs a new field: That's just parsimony. The BB provides an especially hard case because there are few feasible experiments or predictions that can nail down the attributes of the new field. Conversely, positing a newfangled electromagnetic field 150 years ago provided lots of feasible experiments and unified several physical laws at once.

 

[JMz]

Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena.

BTW, someone on WP believes that such new fields are not needed for inflation: https://en.wikipedia.org/wiki/Chronology_of_the_universe.

I can't speak to the validity of that claim. (It's only WP, after all.) But if true, then I will withdraw my earlier parenthetical remark about alternative theories having an advantage if they do not require a new inflaton field: They could have advantages, but avoiding a new field is not one of them.

 

[bapowell]

BTW, someone on WP believes that such new fields are not needed for inflation: https://en.wikipedia.org/wiki/Chronology_of_the_universe.

I can't speak to the validity of that claim. (It's only WP, after all.) But if true, then I will withdraw my earlier parenthetical remark about alternative theories having an advantage if they do not require a new inflaton field: They could have advantages, but avoiding a new field is not one of them.

Which claim? Off the top of my head, the only SM field that is still an inflaton candidate is the Higgs, but only with an arguably ad hoc and fine-tuned nonminimal coupling to curvature. To me, that's less attractive than a new field that might happen to work more naturally. A similar thing happens with quintessential inflation, in which the same field driving today's accelerated expansion was also responsible for primordial inflation: kinda neat if you can package the two phenomena into a single field, but if you have to mutilate it in the process, simply introducing more fields into a model that is known to be incomplete seems like the appropriate thing to do.

 

[AgentSmith]

I am curious what "too convenient" means. Aren't good scientific ideas supposed to agree with observations and be based on other, already established science (as inflation does)? In fact, aren't those among the key properties that make an idea "good"? The inflation idea is incomplete, but it is not pseudo-science.

Alan Guth started with a problem, then invented a theory to solve it without any proposed mechanism. Yes, it conveniently solves certain problems in cosmology, but a mechanism would be nice. I did not say,nor , I think, imply, that its pseudo-science. I think Neil Turok's take and comments on inflation are quite interesting and better expressed than mine(as they should be!).I have no link handy.

 

[bapowell]

Alan Guth started with a problem, then invented a theory to solve it without any proposed mechanism. Yes, it conveniently solves certain problems in cosmology, but a mechanism would be nice. I did not say,nor , I think, imply, that its pseudo-science. I think Neil Turok's take and comments on inflation are quite interesting and better expressed than mine(as they should be!).I have no link handy.

How do you define "mechanism"? Early attempts at inflation centered on presumed phase transitions, typically of the GUT variety, that are expected to have taken place in the early universe. The mechanism in which a scalar field becomes trapped in a false vacuum as the temperature drops below a critical value had been thoroughly studied outside the cosmological setting by Guth's time; he simply developed this same idea in the cosmological setting.

What is missing from this and more modern approaches that fail to make them "mechanisms" in your mind?

 

[JMz]

Which claim? Off the top of my head, the only SM field that is still an inflaton candidate is the Higgs, but only with an arguably ad hoc and fine-tuned nonminimal coupling to curvature. To me, that's less attractive than a new field that might happen to work more naturally. A similar thing happens with quintessential inflation, in which the same field driving today's accelerated expansion was also responsible for primordial inflation: kinda neat if you can package the two phenomena into a single field, but if you have to mutilate it in the process, simply introducing more fields into a model that is known to be incomplete seems like the appropriate thing to do.

My parenthetical was "(such as not requiring an entirely new 'inflaton' field of unknown properties)". The WP article states that "cosmic inflation ...is believed to have been triggered by the separation of the strong and electroweak interaction", thus requiring no new fields.

 

[bapowell]

My parenthetical was "(such as not requiring an entirely new 'inflaton' field of unknown properties)". The WP article states that "cosmic inflation ...is believed to have been triggered by the separation of the strong and electroweak interaction", thus requiring no new fields.

Yeah, that's not accurate. That was the original hope (nearly 40 years ago now) that the GUT phase transition was trapped in a metastable false vacuum, triggering inflation. That would have been a beautiful marriage of particle physics and cosmology, aligning definitive events in each arena; however, these early models failed for a variety of reasons. Either they didn't yield enough inflation or the density perturbations they generated were too large.

There are still models that are GUT-related, but these are supersymmetric as far as I know.

 

[JMz]

As I recall, supersymmetry was mentioned in the same article. But given the intro, from which I took that quote, it seems clear that the article is out of date in important ways. (Have you ever edited WP? Want to try? ;-)

 

[AgentSmith]

How do you define "mechanism"? Early attempts at inflation centered on presumed phase transitions, typically of the GUT variety, that are expected to have taken place in the early universe. The mechanism in which a scalar field becomes trapped in a false vacuum as the temperature drops below a critical value had been thoroughly studied outside the cosmological setting by Guth's time; he simply developed this same idea in the cosmological setting.

What is missing from this and more modern approaches that fail to make them "mechanisms" in your mind?

Mechanism is something like GR provided for gravitation. Newton provided a very good law of gravity, but was uncomfortable with simple asserting it, especially the action at-a-distance aspect. Einstein provided the mechanism of gravity, which was the geometry of spacetime, so now we have a Theory. I don't know what is missing; if I did I would publish it.

 

[bapowell]

If that’s your definition, there is certainly a mechanism: a supercooled uniform scalar field fuels exponential expansion until it decays to a true vacuum. The latent heat of the phase transition repopulates the universe with matter.

What’s missing? Compared to your example of GR, it’s rather more descriptive than simply saying that gravity is caused by spacetime curvature, which is really not explanatory at all.