I totally mean it: Inflation never solved the flatness problem
backreaction.blogspot.com
backreaction.blogspot.com
> I don't get it. How does modified gravity explain displacement of visible and gravitating matter in the case of Bullet Cluster?
> The improbability of conditions that are needed to set up the collision is another question.
I didn't find the reply especially convincing, or the arguments for the likelihood of small numbers raised in the main essay. Attachments to certain "elegant solutions" by theorists have posed significant hurdles to scientific progress. Einstein removing the cosmological constant for an expanding universe is a famous example.
The author isn't trying to convince you of her counterargument to doctrine. The author is trying to convince you to be less convinced by hand-waviness in lieu of rigorous deduction. The author doesn't even believe that inflation is incorrect, nor does she claim to believe MOND over standard dark matter models; she is pointing out that taking one good, well-evidenced, well-reasoned theory and using it as a prop to chock up a piece of shaky, though attractive, supposition is bad science. She's pointing out that even highly regarded thinkers can allow simplification for the purpose of communication and impact to erase complexity in reasoning and teaching.
So yeah, her arguments may not convince you. She's not trying to convince you, she's trying to show you reasonable doubt. Different standard of evidence.
I've heard it in a few places, starting not too long after the bullet cluster lensing results were published. I thought some revised LCDM studies found the velocities were less discrepant than originally argued, but I think it's still a few sigma off.
Regarding the question of the offset in visible versus total inferred matter, I think MOND proponents would note that MOND doesn't work in clusters with only the baryonic mass we see. At those large scales MOND still requires some form of dark matter, though I believe it is argued a light non-WIMP particle can do it (neutralinos?) and is at a factor of ~2 level, not a factor of 5 or 8 like dark matter. But I have never heard if you can get that type of displacement with those types of particles.
> > "I am fascinated by this for the same reason I’m fascinated by the widely-spread and yet utterly wrong idea that the Bullet-cluster rules out modified gravity. As I explained in an earlier blogpost, it doesn’t. Never did. The Bullet-cluster can be explained just fine with modified gravity. It’s difficult to explain with particle dark matter."
> As others explained to her in the comments section of that earlier blog post [1], she was wrong. And yet here she is, saying it again, this time even more confidently.
[1] http://backreaction.blogspot.com/2017/01/the-bullet-cluster-... (The Bullet Cluster as Evidence against Dark Matter)
That's grand and all, but is anyone in that chain actually a cosmologist like Sabine Hossenfelder?
A) Very close to the critical density right now that would give us a flat universe
B) That we will rapidly diverge from having a flat curvature with a deviation from that critical density
C) Therefore at the start of the Universe we must have been even closer to 1 than we are now (and we're pretty close)
The way we measure the flatness of the universe using the CMB and Type 1a super novae is super interesting (especially for someone technically minded), and definitely worth checking out!
> What’s important is that this variable increases in value over time, meaning it must have been smaller in the past.
No? The point of the variable is to (basically, meaning it's so tightly correlated it might as well mean it can...) describe expansion rate of the universe. It must have been larger in the past given a big-bang origin model is used.
My third-year Cosmology course was the single most confusing and hand-wavey course I've ever taken during my Physics degree. It seemed to be mostly a grab-bag of "clever insights" (the cosmological constant being "wrong", but then later turning out to be "right" being the most flagrant). Maybe I just had a bad lecturer, but at the time I was working in an astrophysics research group, and I asked a few of the post-docs about cosmology and they didn't have any convincing answers either (to be fair, our research topic was asteroseismology not cosmology). It just felt as though there was a lot less rigor in cosmology than in other sciences that I was more familiar with. They definitely had some evidence for the theories presented, but most of the actual derivations felt deliberately vague and confusing.
All of that being said, it feels like the author has some axe to grind and is herself engaging in unscientific dogma. Even I can tell that some of her comments on cosmology (the point about the curvature density parameter specifically) are not quite right, and I'm a little confused about her tirade on "parameters close to 1" -- not to mention that she just shrugs it off with "I have no idea" rather than following up.
The points she made about choosing parameters are very odd, bordering on incorrect, as she appears to be trying to claim that unless you have a theory to derive initial parameters then the initial parameters are useless. But that's ridiculous, it's like saying (using her analogy) you need to have a theory for the position and speed of a particular arrow in order for Newton's laws to be correct. Fitting a model to data, by searching for an optimum parameter, is an incredibly common practice and is one of the most common ways of doing data science. However, saying that you need to know the probability distribution of some parameter is definitely true.
It also appears that the author's views were challenged by another astrophysicist in her previous blog post, and she doesn't address this at all (in fact, she doubles down).
Anyway, her argument about 1 in that particular case is afaiu simply that it's rather preposterous to assume a specific (essentially impossible to know) iv. and then massage the theory around it until the results fit the observations.
In data science it would be like determining the model parameters by gut feeling, and then start searching for some kind of model with those parameters that actually fit the data. Almost exactly the opposite of how an parameter search/optimization is usually done. I tend to start with some model and see what parameters that fit instead of randomly deciding one parameter is 1 and then go look for a model that might fit my data with that arbitrarily fixed parameter.
I'm no astrophysicist, or anything even remotely close to the authors competence, so I'll refrain from commenting on those points, but I really see no reason to critique her arguments in general because somebody pointed out some possible weakness of one a sub argument. Rather the opposite, if you would only have one counter comment on a physics related blog, then you are probably correct, nobody has read it yet, or the comment function is broken.
From my brief touches with cosmology, omega=+1 comes from measurements that the universe appears to have a flat topology (both from the size of structure in the CMBR and from a variety of other measurements). The flatness problem effectively is a fine-tuning problem that states if omega has slight deviations from +1 (of the order 1e-62) then the universe of today would not be able to sustain galaxies or other large structures. The measurement of _today's_ omega (omega_0) is also very close to +1 (which is what forces the initial condition limit). From that, it was concluded that if we accept the model of an accelerating universe, then the initial condition must be close to +1. Inflation is the proposed physical solution to explain why that is the case. To use the Newton analogy, you are backtracking the path of an arrow to find where it was fired from. To be clear, I personally don't like how hand-wavey cosmology is as a science, but pretending as though modern cosmologists just came up with a random "cult-like" belief that omega=1 is just ludicrous. [The author calls omega 'k'. While omega and k are related (k is the curvature, and can be used to deduce the critical density -- which omega is a fraction of) I'm not sure if that's quite right. But I'm speaking as a senior undergrad here, I might've missed something.]
As for the "possible weakness of a sub-argument", you can go to the reddit discussion of this particular post (others have linked it here) and see for yourself. It's quite a bit more than a "possible weakness of a sub-argument". Effectively the analysis is on her entire thesis (that the Bullet-cluster can be explained with modified gravity perfectly fine). The rest of the complains about modern cosmology are sub-arguments to justify why modified gravity should be considered.
Why do cosmologists prefer numbers close to one?
>"The points she made about choosing parameters are very odd, bordering on incorrect, as she appears to be trying to claim that unless you have a theory to derive initial parameters then the initial parameters are useless. But that's ridiculous, it's like saying (using her analogy) you need to have a theory for the position and speed of a particular arrow in order for Newton's laws to be correct."
I'm not seeing it:
>"The initial conditions are either designed by the experimenter or inferred from observation. Either way, they’re not predictions. They can not be predicted. That would be a logical absurdity. You can’t use a differential equation to predict its own initial conditions. If you want to speak about the probability of initial conditions you need another theory."
She is just saying initial conditions are not predictions of the same equation you plug them into.
Omega=+1 is preferred, because:
a) We measure Omega to be approximately 1 in the modern day. This indicates that the density of our universe is approximately the critical density (for a flat universe). Really, Omega=+1 means that our universe is flat.
b) In order for Omega to be approximately 1 _today_ in an expanding universe, it must have been even more close to 1 at t=0.
The only example of a parameter that she gave is omega. She didn't give any other examples, so it's quite a stretch to say that "cosmologists prefer numbers closer to one). Here's the wikipedia article on the problem: https://en.wikipedia.org/wiki/Flatness_problem. All of the points in that article were things we discussed in third-year cosmology as possible solutions to this fine-tuning problem.
> She is just saying initial conditions are not predictions of the same equation you plug them into.
The initial conditions are deduced by looking at modern observations and "winding back the clock" to see what set of parameters match the observed facts. I don't know what she's referring to "using a differential equation to predict its own initial conditions".
And you can create a probability distribution of possible initial conditions for omega, it's just very small (of the order 1e-62). Inflation is the proposed physical explanation for why that is the case.
So for the first issue you claim her premise that cosmologists arbitrarily prefer values "near" to 1 is incorrect. Ok, I have no idea. I guess that would make for a good review article or even just some science journalism where all the bigshots get asked.
For the second issue you seem to be in agreement with her for the most part:
>"What happens if you ignore this and go with the belief that the likely initial value for the curvature density should be about 1? Well, then you do have a problem indeed, because that’s incompatible with data to a high level of significance.
Inflation then “solves” this supposed problem by taking the initial value and shrinking it by, I dunno, 100 or so orders of magnitude. This has the consequence that if you start with something of order 1 and add inflation, the result today is compatible with observation. But of course if you start with some very large value, say 1060, then the result will still be incompatible with data. That is, you really need the assumption that the initial values are likely to be of order 1. Or, to put it differently, you are not allowed to ask why the initial value was not larger than some other number."
She is claiming that the inflation theory presupposes the initial value should be near 1, this value is not derived from anywhere.
Dr. Hossenfelder is not arguing against the mathematical theory of inflation, or about its observational evidence. It's about whether there is any metaphysical problem if a certain parameter turns out to be much smaller than one.
and then they back away slowly, trying to escape and find someone else to talk to?