Rivals for Inflation

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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?)
 

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1280px-History_of_the_Universe.svg.png


If inflation didnt occur. The rest still occured 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?
 
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PeterDonis
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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.
 
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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.
 
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PeterDonis
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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.
 
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PeterDonis
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If inflation didnt occur. The rest still occured like having Cosmic Microwave Background, right?
Everything after the ##10^{-32} \ \text{s}## point in the image you posted is supported by good evidence.
 
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kimbyd
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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.
 
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