A Cosmic Controversy

This document is a review of the article: https://blogs.scientificamerican.com/observations/a-cosmic-controversy/ with the title "A Cosmic Controversy" In Scientific American by 33 physicists
The article "A Cosmic Controversy" is an reaction on the article "Pop Goes the Universe" in Scientific American of February 2017 In the last paragraph I explain my own opinion.

The article "A Cosmic Controversy" starts with the following text:
Our February 2017 article “Pop Goes the Universe” argues against the dominant idea that the early cosmos underwent an extremely rapid expansion called inflation.
Next we read:
Its authors instead advocate for another scenario - that our universe began not with a bang but with a bounce from a previously contracting cosmos
Also this is highly controversial. The concept 'our universe' is also controversial.
Next we read:
In the letter below, a group of 33 physicists (hereafter "G33") who study inflationary cosmology respond to that article.
It is followed by a reply from the authors
Please read the full article.

Letter by 33 physicists

In “Pop Goes the Universe,” by Anna Ijjas, Paul J. Steinhardt and Abraham Loeb, the authors (hereafter “IS&L”) make the case for a bouncing cosmology, as was proposed by Steinhardt and others in 2001.
The problem with the cyclic universe is that it is as difficult to test as the open or flat universe.
We disagree with a number of statements in their article, but in this letter, we will focus on our categorical disagreement with these statements about the testability of inflation.
IMO to test or to falsify the inflation theory, in short any theory that is based on different expansions speeds just after the Big Bang, is very difficult. The problem is that it is not so difficult to have a theory which describes what happened after the Big Bang, but more how and when it happened.
The biggest problem is when you observe the universe at present you see the past. The best example is the CMB radiation which gives an impression 300.000 years after the BigBang. The uniformity in the past is no guarantee for the same uniformity at present at the boundary of the universe. The fact that some people assume that the universe contains bubbles supports non-uniformity.
New paragraph
There is no disputing the fact that inflation has become the dominant paradigm in cosmology.
Correct.
In fact there are at least three theories at stake:
  1. Cosmic expansion after the Big Bang without inflation.
  2. Cosmic expansion after the Big Bang with inflation.
  3. Cosmic expansion after the Big Bang with eternal inflation.
Inflation meaning at least a period of rapid expansion above the average expansion.
The issue is which of the possible theories or flavours is correct.
By claiming that inflationary cosmology lies outside the scientific method, IS&L are dismissing the research of not only all the authors of this letter but also that of a substantial contingent of the scientific community.
That is an unlucky sentence. The issue is which of the possible flavours is correct. To do that you have to clearly specify which theory has the closests match which all the observations.
Moreover, as the work of several major, international collaborations has made clear, inflation is not only testable, but it has been subjected to a significant number of tests and so far has passed every one.
These test should at least demonstrate the difference between the three theories.
New paragraph.
Inflation is not a unique theory but rather a class of models based on similar principles.
This is not very strong science. You can have a set of models but the difference between each should be clear.
Of course, nobody believes that all these models are correct, so the relevant question is whether there exists at least one model of inflation that seems well motivated, in terms of the underlying particle physics assumptions, and that correctly describes the measurable properties of our universe.
I 100% agree.
And what are the basic parameters of that theory?
When I investigate what follows apperently no decision is made what that theory is.
New paragraph.
The standard inflationary models predict that the universe should have a critical mass density (that is, it should be geometrically flat),
That the universe is geometrical flat is a very logical assumption. It means that in the universe geometrical straight lines are straight lines. IMO it also means a standard 3D universe which is infinite in all directions.
The true issue is what has this to do with the inflation theory.

How do you measure that the mass density is equal to the critical density? .

and they also predict the statistical properties of the faint ripples that we detect in the cosmic microwave background (CMB).
Also here the issue is what does this have to do with the inflation theory.
The remarkable fact is that, starting with the results of the Cosmic Background Explorer (COBE) satellite in 1992, numerous experiments have confirmed that these predictions (along with several others too technical to discuss here) accurately describe our universe.
The COBE satelite is a wounderfull experiment, but in relation to the inflation theory it counts as one observation and by itself it is no prove that the inflation theory is correct.
New paragraph
The average mass density of the universe has now been measured to an accuracy of about half of a percent, and it agrees perfectly with the prediction of inflation.
Also here, I do not want to down grade the measurements involved, but the issue is to what extend the average mass density of the Universe has been measured. The more detailed question is: exactly what is the Universe? This has in some sense "nothing" to do what we observe, but involves everything influenced at present after the Big Bang. IMO it is impossible to measure the total mass of the Universe. This whole issue is very tricky because infact the whole concept of Inflation makes this more difficult. The more inflation the larger the Universe, the less we can observe.
The ripples of the CMB have been measured carefully by two more satellite experiments, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, as well as many ground- and balloon-based experiments—all confirming that the primordial fluctuations are indeed nearly scale-invariant and very accurately adiabatic and Gaussian, precisely as predicted (ahead of time) by standard models of inflation.
The issue is to what extend these measurements are trully caused by inflation.
While the successes of inflationary models are unmistakable, IS&L nonetheless make the claim that inflation is untestable.
In some sense IS&L should answer.
An important point is that standard inflationary models could have failed any of the empirical tests described above, but they did not.
The whole issue is that each of the observations is a trully implication that inflation happened.
The CMB radiation is an implication of the decoupling Era , but not that there was inflation. What we want to know is which processes took place until that moment.
IS&L write about how “a failing theory gets increasingly immunized against experiment by attempts to patch it,” insinuating that this has something to do with inflation.
IS&L argument is wrong.
But despite IS&L’s rhetoric, it is standard practice in empirical science to modify a theory as new data come to light, as, for example, the Standard Model has been modified to account for newly discovered quarks and leptons.
The testing of the Standard Model is performed in a laboratory. IMO this is strong science. To test the inflation theory IMO is weak science. The fact that there is no agreement of what exactly the inflation theory is makes it weak.
New paragraph.
IS&L also assert that inflation is untestable because it leads to eternal inflation and a multiverse.
IMO IS&L have a point.
Yet although the possibility of a multiverse is an active area of study, this possibility in no way interferes with the empirical testability of inflation.
The first question to answer is what is the difference between eternal inflation and that stand big bang cosmology without inflation? IMO they are the same because both are based on a continuous form of expansion. With eternal inflation there exists no burst in the expansion rate.
If the multiverse picture is valid, then the Standard Model would be properly understood as a description of the physics in our visible universe, and similarly the models of inflation that are being refined by current observations would describe the ways inflation can happen in our particular part of the universe.
This is correct except it has nothing to with the multiverse picture.
The problem is that IMO the people that are in favour of the Multiverse also support the idea that each Universe has its own Standard Model. When that is the case these supporters should describe the different processes that lead to these different Standard Models which IMO is tricky. Anyway IMO to test these Multiverses is extremely difficult because we can not observe them.
Scientists would still be able to compare newly obtained data—from astrophysical observations and particle physics experiments—with precise, quantitative predictions of specific inflationary and particle physics models.
To test different particle physics models in our Universe is simple. To test the same for different Universes is extremely difficult.
IMO at present it is very difficult to make quantitative predictions related to different inflation models. First of all you have to make a list what the different models are and clearly specify what the differences are. To test between which of these models is correct is difficult.
Note that this issue is separate from the loftier goal of developing a theoretical framework that can predict, without the use of observational data, the specific models of particle physics and inflation that should be expected to describe our visible universe.
I do not think you can do any form of science without obsservational data. I also think make a clear difference between particle physics because each require completely different tests.
Anyway the issue is not the visible universe (which is not a physical entity) but the universe from the Big Bang until the present.
New paragraph.
This, however, does not undermine the success of inflationary models.
The issue is why do inflationary models describe our observations better than the Standard Big Bang model without inflation.
The situation is similar to the standard hot big bang cosmology: the fact that it left several questions unresolved, such as the near-critical mass density and the origin of structure (which are solved elegantly by inflation), does not undermine its many successful predictions, including its prediction of the relative abundances of light chemical elements.
Also here how does a theory without inflation predict the abundance of light chemical elements versus a theory with inflation.

Reply authors.

We have great respect for the scientists who signed the rebuttal to our article, but we are disappointed by their response, which misses our key point: the differences between the inflationary theory once thought to be possible and the theory as understood today.
That is correct. First you have to describe expansion with or without inflation and secondly the differences between the different forms of inflation.
The claim that inflation has been confirmed refers to the outdated theory before we understood its fundamental problems.
The problem with the letter by "G33" is that they do not mention which inflation theory they mean. The test results can even valid for theories without inflation.
Next paragraph.
It was not understood that the outcome of inflation is highly sensitive to initial conditions.
Every astrophysical theory depents highly on the initial physical conditions. What they exactly are is often a hugh problem.
Papers claiming that inflation predicts this or that ignore these problems.
Tricky sentence. Not very clear.
Logically, if the outcome of inflation is highly sensitive to initial conditions that are not yet understood, as the respondents concede, the outcome cannot be determined.
The problem with any inflation theory is, that what happened before that inflation started, is no longer vissible. Inflation by itself is partly cause of this.
This invisibility is also true for all what happened before the decoupling era.
For more details, see our 2014 paper “Inflationary Schism” (preprint available at https://arxiv.org/abs/1402.6980 ).

Reflection 1 - Empirical Evidence versus Experiments.

It is important to define what we mean with Empirical Evidence.
Empirical Evidence comes in two flavours: By observations and by means of Experiments. With Experiments we mean laboratory experiments.
The problem is that each type of observation or measurement is also an experiment, but that is not what we mean with performing an experiment.
The measurement of the CMB radiation as such is not called an experiment but an observation. The issue is to what extend the CMB radiation can be used as evidence for the inflation theory. It is much more evidence that space is expanding.

Reflection 2 - reply G33 physicists

In order to evaluate which theory is correct, you should compare the same issues which each theory.
At stake are 4 theories: 1) a theory without inflation. 2) the standard theory of inflation as invented by Alan Guth 3) the theory of eternal inflation 4) a theory with eternal inflation and multiverses.
One issue is the critical mass density. When you measure the average mass density apparently this value is accordingly to the critical mass density. That means the parameter rho=1. Inflation claims that that is what they predict. However in order to be significant in this discussion the other theories should not make the same claim, but a different value. Unfortunate that is not mentioned.
A slightly different issue is how is this mass density measured, because it is not that simple.
When rho=1 this is equivalent to a flat geometry. Flat geometry is the simplest geometry. To predict something else is not what I should expect.
Along that same line is the question: when a theory predict that at present the geometry is flat does that theory assumes that the geometry was always flat.
This type of detail should G33 supply, but they don't.

What my critique burns down to is that it is not enough to claim that all the tests are in favour of inflation, but also what the other theories say related to these tests. In principle each theory could all agree with the same test results and claim that they each are correct.

A case in point is the power spectrum of the CMB radiation. The problem with the power spectrum is that by it self it proves nothing. it is like an immediate result. The central two questions are: first based on which input parameters the power spectrum of the CMB radiation is based and secondly which output parameters does it calculates. This output parameters can than be used to verify the model behind the power spectrum. The results should make it possible to decide which inflation theory is correct. The reply letter does not give this type of information.

Reflection 3 - reply IS&L physicists

IS&L have a point that inflation theory is difficult as not impossible to test, specific if more theories are involved.
IS&L have no point as far as their opinion that you cannot modify a specific theory.
IS&L have no point as far as their opinion that the whole concept of inflation does not belong to science.
The problem with the inflation theory is that it is already difficult to test if their exists any form of inflation compared with the standard Big Bang theory which supports no (burst of) inflation. IS&L biggest problem is that their own solution (a bouncing universe) can also not be tested.

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