[HN Gopher] Scientists make further step towards understanding d...
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Scientists make further step towards understanding dark energy
Author : dnetesn
Score : 51 points
Date : 2021-04-25 10:25 UTC (1 days ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| ncmncm wrote:
| This seems to be saying they measured Dark Energy acceleration in
| galaxies only a billion years old and light years away, not 13
| billion light years old and correspondingly distant.
| hvna wrote:
| Correct, the data is regarding an age/area of the universe that
| has not been probed prior. Also, light years are a measure of
| distance, not time.
| garmaine wrote:
| It's a measure of both, since you're looking back in time.
| danparsonson wrote:
| In this specific case perhaps but not in general - consider
| for example a circle with a radius of one light year
| centered on the Earth.
| garmaine wrote:
| In the context of astronomy generally then ;)
| danparsonson wrote:
| Not sure what you mean - distance directly away from an
| observer correlates with time (things further away look
| younger due to the finite speed of light), but in general
| it's just a measure of distance. It's right there in the
| units - light (speed) years are m/s * s; that is, metres.
| I'm not aware that astronomers use ly as a unit of time,
| unless that's changed since I studied it for my
| undergrad, although that was admittedly some time ago.
| garmaine wrote:
| While astronomers are obviously very much aware that
| light-years is a measure of distance, "seeing further
| away" is a stand-in for "seeing further back in time."
| New telescopes are rated by how much further away they
| can resolve in terms of light-years because that is a
| direct translation into how close to the Big Bang they
| can resolve.
|
| What I'm saying is that it is extremely useful that being
| able to resolve objects 13.2 billion light-years away
| means being able to see back 13.2 billion years in time.
| The units are 1:1 convertible, and astronomers therefore
| frequently interchange them in casual conversation.
| ncmncm wrote:
| Typo. "...13 billion years old and correspondingly distant."
|
| This might resolve my doubts that the phenomenon could be an
| artifact of different conditions in the early universe.
| bigbillheck wrote:
| It looks like the major contribution of this paper is in a new
| signal processing technique. They have something they call the
| 'chained power spectrum' which gets used to boost a 4 sigma
| signal into an 11 sigma one.
|
| I don't have all morning to dig into their math but the results
| are going to live and die on their modeling assumptions. (However
| I do note that in 4.2 they admit that the choice of "PPP" (Eq 22,
| section 3.5, all references to the arxiv version) fails in
| simulated data and of the alternatives that don't fail that
| particular test, they went with "PQP" instead of the more
| conservative "QQP" for reasons that I have trouble as
| interpreting as 'it made the results flashier'. But I'm just some
| weirdo on the internet, and it's not my field, and I've got to
| get back to work).
| guerrilla wrote:
| 11 sigma sounds pretty high to me (non-physicist), is that kind
| of standard?
| obedm wrote:
| That's insanely accurate. For comparison most discoveries are
| usually only considered proven when it's 5 sigma.
|
| For example they waited years to announce the Higgs bosson
| because they wanted to reach 5 sigma.
| guerrilla wrote:
| Thanks, that's what I thought. I was doubting it was even the
| same metric :)
| jjoonathan wrote:
| You can trivially get to 11 sigma by making your null
| hypothesis sufficiently garbage.
|
| That's called "p-hacking" and is unfortunately very common.
| It is encouraged whenever someone innocently reads meaning
| into a sigma value without evaluating whether or not the null
| hypothesis is any good.
|
| If I saw someone going around saying "The sky is reddish
| green, 11 sigma result!" my first suspicion would not be that
| their numbers were incorrect but rather that their null
| hypothesis was something silly like "the sky is entirely the
| solid sRGB color #0000FF." Obviously this garbage null
| hypothesis can be rejected with enormous confidence, that's
| where the 11 sigma came from, but the confident rejection of
| a garbage null hypothesis should not be taken to support
| their proposed alternative theory unless you have reason to
| believe the null hypothesis was well modeled, either because
| you checked it or because someone else did. It all comes back
| to trust, in other words.
| gbrown wrote:
| > That's insanely accurate.
|
| This is misleading, and it's a misconception arising from the
| way that even physicists sometimes misspeak when talking
| about statistics. The "sigma" is a reference to a standard
| deviation, and we use it to measure evidence against a null
| hypothesis (which is generally defined relative to a model of
| some type). An 11 sigma result means that there's strong
| evidence against (hypothesis + model), not anything in
| absolute terms about "accuracy".
|
| The meaning of such a result depends on the model being used;
| sometimes it's as simple and easy to defend as there being
| some sort of distribution with a mean which exists, and other
| times there's a complicated model which may itself be
| incorrect (producing significant results).
| bigbillheck wrote:
| Arxiv link (presumably not final version):
| https://arxiv.org/abs/2007.09011
| l33tman wrote:
| Youtube visualization of the additional new data the study
| contributed (eBOSS):
|
| https://www.youtube.com/watch?v=UTlYUxucEZA
|
| The linked phys.org article btw didn't say very much. Would be
| nice with a summary of the actual scientific impact.
|
| As far as I could tell, for models of cosmological growth, it
| tightened the limits on dark energy to match the standard GR type
| even more but no new odd things appeared.
| ArtWomb wrote:
| I read these findings rather differently. The "expanding
| balloon" spacetime predicted from GR may absolutely be in
| question. It appears every type of measurement confirms a
| discrepancy in the Hubble Constant with respect to distance.
| Though none of the results from different spectra agree with
| each other. As new observations from next gen wide array
| sources such as the Vera Rubin Observatory and James Webb Space
| Telescope come online, the risk is that the overall picture
| becomes less clear with higher accuracy ;)
|
| New Distance Measurements Bolster Challenge to Basic Model of
| Universe
|
| https://public.nrao.edu/news/challenge-model-of-universe/
| supernovae wrote:
| so cool
| yummypaint wrote:
| _" The eBOSS observations are consistent with the dynamical dark
| energy probed by our team using the BOSS survey four years ago."
| _
|
| What they are referring to here is a model which is an
| alternative to having a constant cosmological constant. The
| takeaway is that they are very confident that dark energy is
| something whose density can vary in time and space. There has
| been mounting evidence for this for some time, but it's a
| departure from the original formulation of general relativity.
| Here is some more information (2012)
| https://phys.org/news/2012-11-dark-energy-static-dynamic.htm...
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