Exploring Rivals for Inflation in Cosmology: Characteristics and Implications

[jake jot]

At the footnote of the chapter "Large-Scale Structure of the Universe" in Julian Babour new book The Janus Point is this passage:

"Besides a Janus-point origin, there are now several proposals—involving bouncing universes or ones that expand after a long quasi-stable period—in which the universe does not explode into existence. These have often been created with properties tailored to be rivals for inflation.".

What are the official rivals for inflation? What must be the characteristics of the rivals that can also explain the horizon problem, flatness problem, and the origin of galaxies and clusters of galaxies? (What else?)

 

[jake jot]

If inflation didnt occur. The rest still occurred like having Cosmic Microwave Background, right?

The last two paragraph about Inflation at Wiki:

"In order to work, and as pointed out by Roger Penrose from 1986 on, inflation requires extremely specific initial conditions of its own, so that the problem (or pseudo-problem) of initial conditions is not solved: "There is something fundamentally misconceived about trying to explain the uniformity of the early universe as resulting from a thermalization process. [...] For, if the thermalization is actually doing anything [...] then it represents a definite increasing of the entropy. Thus, the universe would have been even more special before the thermalization than after."[132] The problem of specific or "fine-tuned" initial conditions would not have been solved; it would have gotten worse. At a conference in 2015, Penrose said that "inflation isn't falsifiable, it's falsified. [...] BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye."[7]

A recurrent criticism of inflation is that the invoked inflaton field does not correspond to any known physical field, and that its potential energy curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. Paul Steinhardt, one of the founding fathers of inflationary cosmology, has recently become one of its sharpest critics. He calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either [...] Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation [...] Other configurations lead to a uniform, flat universe directly – without inflation. Obtaining a flat universe is unlikely overall. Penrose's shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of 10 to the googol (10 to the 100) power!"[5][114] Together with Anna Ijjas and Abraham Loeb, he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the Planck satellite.[133][134] Counter-arguments were presented by Alan Guth, David Kaiser, and Yasunori Nomura[135] and by Andrei Linde,[136] saying that "cosmic inflation is on a stronger footing than ever before".[135]"

What is your opinion? Which do you believe and why?

 

[PeterDonis]

What is your opinion? Which do you believe and why?

These are not questions on which a useful PF discussion can be based. This is an open area of research. That means there is no generally accepted mainstream answer to the questions being researched.

Asking what alternatives are being considered, as you did in your OP of this thread, is fine: but you seem to have found a Wiki (but you need to give a link to it--see below) that provides plenty of references that you can look at to answer that question.

But just asking what people believe is pointless. Science is not based on "belief" or "opinion". It's based on finding models that make accurate predictions. If an area of research is open, that means scientists are still working on that.

Inflation at Wiki

Please give a link.

 

[jake jot]

These are not questions on which a useful PF discussion can be based. This is an open area of research. That means there is no generally accepted mainstream answer to the questions being researched.

Asking what alternatives are being considered, as you did in your OP of this thread, is fine: but you seem to have found a Wiki (but you need to give a link to it--see below) that provides plenty of references that you can look at to answer that question.

But just asking what people believe is pointless. Science is not based on "belief" or "opinion". It's based on finding models that make accurate predictions. If an area of research is open, that means scientists are still working on that.
Please give a link.

https://en.m.wikipedia.org/wiki/Inflation_(cosmology)

(Note: wiki entries were just added by random members of the public, not the scientists themselves)

I thought Inflation was as solid as Big Bang. So surprised to read at Barbour book there were rivals to it. The wiki article didn't mention about the rivals, so I was asking all the alternatives to Inflation, and also for other data that may point or not point to Inflation.

 

[PeterDonis]

I thought Inflation was as solid as Big Bang.

Then you were mistaken. (I'm assuming that by "Big Bang", you mean the hot, dense, rapidly expanding state labeled as $10^{-32} \ \text{s}$ in the image you posted, which is the earliest state of the universe for which we have good evidence.) Many cosmologists who favor inflation like to talk as though it's the only viable model of what happened prior to the Big Bang, but it's not.

 

[PeterDonis]

If inflation didnt occur. The rest still occurred like having Cosmic Microwave Background, right?

Everything after the $10^{-32} \ \text{s}$ point in the image you posted is supported by good evidence.

 

[kimbyd]

https://en.m.wikipedia.org/wiki/Inflation_(cosmology)

(Note: wiki entries were just added by random members of the public, not the scientists themselves)

I thought Inflation was as solid as Big Bang. So surprised to read at Barbour book there were rivals to it. The wiki article didn't mention about the rivals, so I was asking all the alternatives to Inflation, and also for other data that may point or not point to Inflation.

No, not really.

The primary piece of evidence that we have in favor of inflation is just that the primordial power spectrum of the CMB has a slight tilt to it, which was first predicted in inflationary models.

As of the latest Planck release, a number of inflation models now seem unlikely or have been ruled out. In particular the simplest ones now seem unlikely. But there are a large number of models, so it's hard to say that inflation itself has been ruled out.

The smoking gun for inflation would be the detection of primordial B-mode polarization in the CMB. This type of polarization is a "smoking gun" because alternatives to inflation typically don't predict any of this polarization at all. But it's really, really hard to detect. So far we haven't seen it, but it's pretty easy for inflation to produce little enough that we wouldn't have seen it by now. The jury is still out on that.

I'm not entirely sure what the smoking gun would be for any of the alternatives to inflation.

Cosmologists generally run the gamut of being nearly certain inflation happened to being nearly certain it didn't. But I think they all agree that it's the evidence that needs to guide the way.

 

[lomidrevo]

The smoking gun for inflation would be the detection of primordial B-mode polarization in the CMB.

Would detection of B-modes imply existence of primordial gravitational waves, that could be created only during inflationary period?

I would like to better understand the B-modes, but actually I find it problematic to even understand the E-modes. For example here, they explain that for E-modes, the polarization is parallel or perpendicular to wave vector. If the wave vector points in the direction of wave propagation, what kind of polarization is that parallel to this direction?

 

[lomidrevo]

As of the latest Planck release, a number of inflation models now seem unlikely or have been ruled out.

Is it this paper (June 2019) you are referring to? In the end of the abstract, they write:

All these findings support the key predictions of the standard single-field inflationary models, which will be further tested by future cosmological observations.

Single-field models are still in the game?

I have searched a bit, and found a more recent article related to a Multi-field model:
From MIT news article (Oct 2019):

“While versions of inflation based on a single form of matter give a remarkably good fit to observations, Dave and his collaborators have for a number of years been studying well-motivated models that involve multiple forms of matter… also giving an excellent fit to the data,” Guth says. “Until now, however, the work has been limited to studying the early stages of the ending of inflation, where the math is relatively simple. The new work is based on a high-powered numerical lattice simulation which can probe much further into the complicated interactions at the end of inflation. The work shows more definitively than ever that a large class of models involving multiple forms of matter are in excellent agreement with observations.”

The original paper of this research, a conclusion from the abstract:

We therefore find that predictions for primordial observables in this class of models retain a close match to the latest observations.

So does it mean that at least some of the Multifield models are also still in the game?

Thanks

 

[kimbyd]

Would detection of B-modes imply existence of primordial gravitational waves, that could be created only during inflationary period?

I would like to better understand the B-modes, but actually I find it problematic to even understand the E-modes. For example here, they explain that for E-modes, the polarization is parallel or perpendicular to wave vector. If the wave vector points in the direction of wave propagation, what kind of polarization is that parallel to this direction?

Yes. Specifically, the particular gravity waves that show up as B-mode polarization in the CMB are not modified by gravitational processes that occur between the end of inflation and the emission of the CMB. And the large-scale B-mode polarization is not produced by structures like galaxy clusters. These structures do mix B-mode and E-mode polarization, so one can show up as the other, but only on small angular scales. Detecting this kind of polarization, then, would be a very direct window into the very early universe.

The frustrating thing, however, is that the amount of this kind of polarization may be too small to ever detect.

 

[kimbyd]

Is it this paper (June 2019) you are referring to? In the end of the abstract, they write:

Single-field models are still in the game?

In general, yes. A few of them appear unlikely now, such as natural inflation and $\phi^2$ inflation, which are the simplest models. But there are a number of them that are still well within the confidence contours.

So does it mean that at least some of the Multifield models are also still in the game?

Multifield models, simply by adding more parameters, are generally going to be much harder to rule out. More parameters pretty much always allow a model to fit a more diverse range of possible scenarios. This is why as long as single-field models are still in play, multi-field models are unlikely to gain much traction. And many people proposing alternatives to inflation would see the rejection of single-field models as significant support of these alternatives.

Multi-field models are still worth exploring, of course. It's just much harder to gain supporting evidence for them.

 

[lomidrevo]

This is why as long as single-field models are still in play, multi-field models are unlikely to gain much traction.

Ah, I see, Occam's razor applied

 

[lomidrevo]

Btw. I found a very helpful page, if anyone is interested in the CMB polarization (at introductory level):
http://background.uchicago.edu/~whu/intermediate/polar.html

The included visualizations were very helpful to me!

 

[windy miller]

No, not really.

The primary piece of evidence that we have in favor of inflation is just that the primordial power spectrum of the CMB has a slight tilt to it, which was first predicted in inflationary models.

As of the latest Planck release, a number of inflation models now seem unlikely or have been ruled out. In particular the simplest ones now seem unlikely. But there are a large number of models, so it's hard to say that inflation itself has been ruled out.

The smoking gun for inflation would be the detection of primordial B-mode polarization in the CMB. This type of polarization is a "smoking gun" because alternatives to inflation typically don't predict any of this polarization at all. But it's really, really hard to detect. So far we haven't seen it, but it's pretty easy for inflation to produce little enough that we wouldn't have seen it by now. The jury is still out on that.

I'm not entirely sure what the smoking gun would be for any of the alternatives to inflation.

Cosmologists generally run the gamut of being nearly certain inflation happened to being nearly certain it didn't. But I think they all agree that it's the evidence that needs to guide the way.

I think primordial gravitational waves are not quite the smoking un they are cracked up to be, we need to measure the tilt in this signal as well as there alternatives to inflation , for example strong gas cosmology, which predict them as well. https://arxiv.org/abs/1106.5059

 

[kimbyd]

I think primordial gravitational waves are not quite the smoking un they are cracked up to be, we need to measure the tilt in this signal as well as there alternatives to inflation , for example strong gas cosmology, which predict them as well. https://arxiv.org/abs/1106.5059

Interesting. My understanding has been that these alternatives predict far, far lower amplitudes of gravitational waves than inflation, which is why it's called a "smoking gun". I don't see much discussion in that paper about how one might achieve a high tensor amplitude in these other models, though. Just a statement that if the amplitude was sufficiently high, then the tilt would distinguish the models.

If measuring the tilt is indeed required, that is unfortunate, because the signal level for the tilt will always be lower than the signal level for the magnitude, and the magnitude is already undetectable with current observations.