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>"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 followin

by 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.

cyphar 9y ago
> Why do cosmologists prefer numbers close to one? 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 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.
- nonbel 9y ago
  Thanks, 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.

by nonbel 9y ago

cyphar 9y ago

> Why do cosmologists prefer numbers close to one? 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 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.

nonbel 9y ago
Thanks, 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.