
I totally mean it: Inflation never solved the flatness problem - Santosh83
http://backreaction.blogspot.com/2017/10/i-totally-mean-it-inflation-never.html
======
danbruc
There is a 23 lectures course »The early universe« [1] from MIT by Alan Guth,
inventor of cosmological inflation. I am only half way through and inflation
is only mentioned in the introductory lectures and will not be discussed in
more detail before the last third of the course so I will not dare to voice
any opinion on the blog post. At least up to the point where I am now, the
course is very accessible and I would certainly recommend it for everyone
interested in the topic.

[1]
[https://youtube.com/watch?v=ANCN7vr9FVk](https://youtube.com/watch?v=ANCN7vr9FVk)

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autocorr
I find this essay very curious, especially the strong view against the Bullet
Cluster supporting the model of Dark Matter. This is the first time in
professional astronomy I've seen this brought up. The details in the linked
essay say as much as a few studies find that it's improbable that the two
clusters collided at the speed that would be required. It was funny to see my
same concern raised in the second comment on the blog:

> 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.

~~~
jwfxpr
> I didn't find the reply especially convincing, or the arguments for the
> likelihood of small numbers raised in the main essay.

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.

------
platz
[https://www.reddit.com/r/Physics/comments/7711mr/backreactio...](https://www.reddit.com/r/Physics/comments/7711mr/backreaction_inflation_never_solved_the_flatness/doit3ii/)

> > "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-...](http://backreaction.blogspot.com/2017/01/the-bullet-cluster-as-
evidence-against.html) (The Bullet Cluster as Evidence against Dark Matter)

~~~
Luc
So a commenter on HN quotes a commenter on Reddit who said that some
commenters on BackReaction 'explained to her she was wrong'.

That's grand and all, but is anyone in that chain actually a cosmologist like
Sabine Hossenfelder?

~~~
platz
the commenter on Reddit happens to be an astrophysicist

~~~
Luc
Well that's one hurdle cleared. Here's their comment history if anyone is
interested:
[https://www.reddit.com/user/ididnoteatyourcat](https://www.reddit.com/user/ididnoteatyourcat)

------
simoalx
This seems like a Bad Article. It's pretty apparent from the maths that we
are:

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!

------
jack9
[http://hyperphysics.phy-
astr.gsu.edu/hbase/Astro/fried.html#...](http://hyperphysics.phy-
astr.gsu.edu/hbase/Astro/fried.html#c2)

> 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.

------
cyphar
A bit of a tangent, but I'll try to make it relevant to the topic.

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).

~~~
mickronome
Nah, I don't think she has an axe to grind, except maybe to chop down bad
arguments in general.

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.

~~~
cyphar
Her tirade about omega=+1 is odd because she talks about how it's incredibly
anti-science, but doesn't even try to explain what the opposing view is (going
so far as to say that she hasn't the foggiest). The first question I would
want to know when evaluating the claim that omega=+1 is bad science is to hear
the argument _for_ it.

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.

------
maxpolun
I don't tend to believe people who say that everyone working in a particular
field are crazy and stupid. Wrong maybe. Whole fields are wrong all the time.
But not because they're all crazy and stupid.

~~~
adrianratnapala
Then be comforted that the argument here is not actually about the science,
but rather about its interpretation or about the philosophical assumptions
underlying the interpretation.

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.

------
ShabbosGoy
I thought the flattening of spacetime has more to do with the negative
pressure exerted by dark energy?

------
pavlov
I confess that, looking at the headline, I have no idea whether this article
is about economics or cosmology. I'm trying to decide which I'm hoping for.

~~~
Simon_says
Is flatness a problem in economics? I've never heard of such a thing.

~~~
marblar
Inflation

~~~
Simon_says
What?

------
joosters
... _Those who I personally speak with pretty quickly agree that what I say is
correct._ ...

and then they back away slowly, trying to escape and find someone else to talk
to?

~~~
dang
Would you please not do this here?

[https://news.ycombinator.com/newsguidelines.html](https://news.ycombinator.com/newsguidelines.html)

------
autokad
the problem with these subjects its so far over my head its hard to know if
what he is saying is reasonable, but interesting read

