7 ms·

I totally mean it: Inflation never solved the flatness problem

83 points by Santosh83 9y ago

pavlov 9y ago
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.
- mannykannot 9y ago
  TL;DR: cosmology.
- deleted 9y ago
  [deleted]
- Simon_says 9y ago
  Is flatness a problem in economics? I've never heard of such a thing.
  - marblar 9y ago
    Inflation
    - Simon_says 9y ago
      What?
- nathancahill 9y ago
  Or cosmetology.
jbmorgado 9y ago
Inflation solves more than the flatness issue, namely, the uniformity of Cosmic Microwave Background.
joosters 9y ago
... 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 9y ago
  Would you please not do this here? https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
platz 9y ago
https://www.reddit.com/r/Physics/comments/7711mr/backreaction_inflation_never_solved_the_flatness/doit3ii/ https://www.reddit.com/r/Physics/comments/7711mr/backreactio... > > "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-as-evidence-against.html http://backreaction.blogspot.com/2017/01/the-bullet-cluster-... (The Bullet Cluster as Evidence against Dark Matter)
- Luc 9y ago
  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 9y ago
    the commenter on Reddit happens to be an astrophysicist
    - Luc 9y ago
      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
- Sniffnoy 9y ago
  Wrong about what, exactly? The details matter here! It seems the commenters point out that she's wrong that it's difficult to explain with particle dark matter, but if there's anything there to show that she's wrong about it being explainable with STVG I've missed it.
deleted 9y ago
[deleted]
ShabbosGoy 9y ago
I thought the flattening of spacetime has more to do with the negative pressure exerted by dark energy?
autocorr 9y ago
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.
- privong 9y ago
  > 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. 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.
- jwfxpr 9y ago
  > 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.
autokad 9y ago
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
danbruc 9y ago
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
jack9 9y ago
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/fried.html#c2 http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/fried.html#... > 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 9y ago
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).
- nonbel 9y ago
  >"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." 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.
maxpolun 9y ago
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 9y ago
  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.
simoalx 9y ago
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!

83 points by Santosh83 9y ago

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.

mannykannot 9y ago
TL;DR: cosmology.
deleted 9y ago
[deleted]
Simon_says 9y ago
Is flatness a problem in economics? I've never heard of such a thing.
- marblar 9y ago
  Inflation
  - Simon_says 9y ago
    What?
nathancahill 9y ago
Or cosmetology.

jbmorgado 9y ago

Inflation solves more than the flatness issue, namely, the uniformity of Cosmic Microwave Background.

joosters 9y ago

... 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 9y ago
Would you please not do this here? https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html

platz 9y ago

https://www.reddit.com/r/Physics/comments/7711mr/backreaction_inflation_never_solved_the_flatness/doit3ii/ https://www.reddit.com/r/Physics/comments/7711mr/backreactio... > > "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-as-evidence-against.html http://backreaction.blogspot.com/2017/01/the-bullet-cluster-... (The Bullet Cluster as Evidence against Dark Matter)

Luc 9y ago
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 9y ago
  the commenter on Reddit happens to be an astrophysicist
  - Luc 9y ago
    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
Sniffnoy 9y ago
Wrong about what, exactly? The details matter here! It seems the commenters point out that she's wrong that it's difficult to explain with particle dark matter, but if there's anything there to show that she's wrong about it being explainable with STVG I've missed it.

deleted 9y ago

[deleted]

ShabbosGoy 9y ago

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

autocorr 9y ago

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.

privong 9y ago
> 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. 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.
jwfxpr 9y ago
> 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.

autokad 9y ago

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

danbruc 9y ago

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

jack9 9y ago

http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/fried.html#c2 http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/fried.html#... > 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 9y ago

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).

nonbel 9y ago
>"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." 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.

maxpolun 9y ago

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 9y ago
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.

simoalx 9y ago

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!