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Strange
2016-Oct-24, 09:16 PM
This paper was mentioned in passing, in another thread:
Marginal evidence for cosmic acceleration from Type Ia supernovae

The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.
http://www.nature.com/articles/srep35596

I just wondered if anyone had any thoughts / insights on this research...

Jens
2016-Oct-25, 12:18 AM
More of a question, but if this turns out to be true, that would mean that suddenly dark energy becomes unnecessary, so it's not longer that "baryonic matter makes up only 4% of the universe" (or whatever the percentage is)?

Strange
2016-Oct-25, 08:41 AM
That is my understanding ...

WaxRubiks
2016-Oct-25, 08:52 AM
More of a question, but if this turns out to be true, that would mean that suddenly dark energy becomes unnecessary, so it's not longer that "baryonic matter makes up only 4% of the universe" (or whatever the percentage is)?

I thought it was dark matter which means that 5% of the Universe's matter is baryonic matter..?

Noclevername
2016-Oct-25, 09:10 AM
I thought it was dark matter which means that 5% of the Universe's matter is baryonic matter..?

Dark matter is part of the tally too, but discovering that there's no dark energy will definitely mean revising some current estimates and assumptions about the content of the Universe, among other things.

Jens
2016-Oct-25, 10:39 AM
I thought it was dark matter which means that 5% of the Universe's matter is baryonic matter..?

I don't remember the exact figures, but it was something like 65 dark energy, 30 dark matter, 5 baryonic matter.

LaurieAG
2016-Oct-25, 11:30 AM
I don't remember the exact figures, but it was something like 65 dark energy, 30 dark matter, 5 baryonic matter.
https://en.wikipedia.org/wiki/Planck_(spacecraft)#2013_data_release

According to the team, the Universe is 13.798±0.037 billion years old, and contains 4.82±0.05% ordinary matter, 25.8±0.4% dark matter and 69±1% dark energy.[25][26][27] The Hubble constant was also measured to be 67.80±0.77 (km/s)/Mpc

Sardonicone
2016-Oct-26, 11:58 PM
What becomes of the cosmological constant then.


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ShinAce
2016-Oct-27, 01:16 AM
The paper isn't necessarily arguing against a cosmological constant, they're arguing that the expansion may not be accelerating. They find it could very well be constant, which a flat universe of matter(that's us) without some form of cosmological constant would not be able to do. So we still have a cosmological constant, simply a revised one.

Amber Robot
2016-Oct-27, 05:41 PM
Look at the paper's figure 2. Hard to interpret that plot as suggesting no acceleration, despite what the text may say. Just because some part of the 'no acceleration' line is within the 3-sigma contour does not mean they're ruling out acceleration, given that their best fit suggests that it is indeed accelerating. The language of the text is odd, but the figure tells the bulk of the story.

AFJ
2016-Oct-27, 08:42 PM
Look at the paper's figure 2. Hard to interpret that plot as suggesting no acceleration, despite what the text may say. Just because some part of the 'no acceleration' line is within the 3-sigma contour does not mean they're ruling out acceleration, given that their best fit suggests that it is indeed accelerating. The language of the text is odd, but the figure tells the bulk of the story.

Does seem that way, looks strange.

By looking at figure 3 in the article;
Figure 3: Comparison of the measured distance modulus with its expected value for the best fit accelerating universe (ΛCDM) and a universe expanding at constant velocity (Milne).
http://www.nature.com/articles/srep35596/figures/3
From Nielsen, J. T. et al. Marginal evidence for cosmic acceleration from Type Ia supernovae. Sci. Rep. 6, 35596; doi: 10.1038/srep35596 (2016).

Is it correct that the distance modulus/redshift where the two best fits (best fit for accelerating universe and the best fit for expanding at constant velocity) really start to divert is exactly where there are hardly any / no data points? And in the rest of the figure they are reasonably close-fitted?

Amber Robot
2016-Oct-27, 08:47 PM
Well, the two plots certainly seem to tell different stories, but that could be because it's easy to try to do a "chi-by-eye" on Figure 3. If they've done their analysis correctly and figure 2 accurately shows their result and sigma countours then, given the model they have chosen to fit the data, they would say there is acceleration, despite how difficult it might be to discern that from no acceleration in figure 3.

I guess the question comes down to this: do scientists believe that a 3-sigma result is "marginal"? In my experience, 3-sigma is a pretty good detection.

Tom Mazanec
2016-Oct-27, 08:51 PM
Here is SciAm's take on this: https://blogs.scientificamerican.com/guest-blog/no-astronomers-haven-t-decided-dark-energy-is-nonexistent/

Strange
2016-Oct-27, 09:38 PM
Here is SciAm's take on this: https://blogs.scientificamerican.com/guest-blog/no-astronomers-haven-t-decided-dark-energy-is-nonexistent/

Thanks for that. Very useful.

The study concluded that we’re now only 99.7 percent sure that the universe is accelerating, which is hardly the same as “it’s not accelerating.”

Grey
2016-Oct-27, 10:28 PM
And as always, I'm very frustrated that a new paper that should be given a headline like "Interesting Refinement of Analysis of Supernovae" is instead given a headline that's like "Scientists Realize They Were Completely Wrong About Everything and Totally Change Their Minds". I get that the latter type of title is much more likely to get people to read it, but I think it also contributes to an (all too prevalent) attitude that scientists don't actually know anything, that science is a useless tool for understanding the world, and that you can freely ignore scientific findings if they contradict your previously held beliefs, since all the scientists are likely to change their minds about it all tomorrow anyway.

Jens
2016-Oct-28, 12:39 AM
And as always, I'm very frustrated that a new paper that should be given a headline like "Interesting Refinement of Analysis of Supernovae" is instead given a headline that's like "Scientists Realize They Were Completely Wrong About Everything and Totally Change Their Minds". I get that the latter type of title is much more likely to get people to read it, but I think it also contributes to an (all too prevalent) attitude that scientists don't actually know anything, that science is a useless tool for understanding the world, and that you can freely ignore scientific findings if they contradict your previously held beliefs, since all the scientists are likely to change their minds about it all tomorrow anyway.

Just a couple of thoughts about this. First of all, when I first saw the link in this thread, I saw "nature" in the address and (without clicking on it) I assumed it was a Nature paper. Now I clicked on it and saw that it's actually Scientific Reports. . . Actually, this is quite significant, because (this is something I heard directly from a Nature journal editor), they have a kind of "trickle down" system where people submit to Nature and then the papers are shuffled to the most appropriate journal. If they are major breakthroughs that will interest a wide scientific audience, they go into Nature. If they are major breakthroughs but only of interest to people in a specific field, they are sent to the specialist journals like Nature Physics or Nature Genetics. If they are of interest to a wide audience and are breakthroughs but not major ones, they are sent to Nature Communications (an online journal). And finally, if they are good science but not breakthroughs, they are sent to Scientific Reports, which (this is my opinion), is a nice money-making venture because the articles are not really edited, just proofread, and the authors pay a nice fee for the publication. They literally publish dozens if not hundreds of papers every day.

The second thing is that apparently the University of Oxford put out a press release (https://www.sciencedaily.com/releases/2016/10/161021123238.htm) about the work. And the lead paragraph says:


Now, a team of scientists has cast doubt on this standard cosmological concept. The evidence for acceleration may be flimsier than previously thought, they say, with the data being consistent with a constant rate of expansion.

So the hype partly comes from journalists who didn't look at the paper as carefully as they might have, but also by the institution, with the authorization of the corresponding author, put out the press release. There's a nice comic about this issue here (http://www.sciencebrainwaves.com/understanding-scientific-claims/). And I should also admit that I've probably been guilty of the same thing to some extent myself, so I'm not trying to single out Oxford; it's a fairly common problem that PIOs talk about.

Reality Check
2016-Oct-28, 01:17 AM
More comment on the paper at Starts with A Bang: Universe Not Accelerating? New Battle Over Supernova Results (http://www.forbes.com/sites/startswithabang/2016/10/27/new-supernova-results-is-the-universe-not-accelerating/#25524f0967ca)
Ethan Siegel's opinion is that the paper by itself is not wrong. What is wrong is that the paper is too concentrated on supernova data. The universe contains matter and is measured to be almost perfectly spatially flat which places much stronger constraints on the results. He also states "all of whom don’t specialize in supernova studies" which may explain the issues pointed out in the Scientific American article.

Strange
2016-Oct-28, 08:48 AM
More comment on the paper at Starts with A Bang: Universe Not Accelerating? New Battle Over Supernova Results (http://www.forbes.com/sites/startswithabang/2016/10/27/new-supernova-results-is-the-universe-not-accelerating/#25524f0967ca)

That's a great article for explaining why all the other data is important.

Ken G
2016-Oct-28, 01:02 PM
This is how I would put it. Without supernova data, we still need some kind of dark energy, unless some key part of our understanding of how things work is quite wrong (always a possibility, of course). Our need for supernova data comes from the fact that we don't have an internal consistency check-- we simply infer the need for dark energy but haven't checked that the dark energy we have inferred is doing what it should, such that we can regard our model as self-consistent (which is essentially what science means by "being on the right track").

What dark energy should do is produce acceleration. If it isn't producing acceleration, something is wrong with our model. So that's the angle we should take when we look at the data-- not "does this data unequivocally show the presence of a cosmological constant, independently of all other observations," and not "can we squint our eyes and reverse engineer the data to be consistent with a cosmological constant even though it doesn't seem to be," but simply "does this data give us reason to feel confident that the acceleration we need for our model to work is not absent in the data, yet could have been." That is the appropriate scientific question to ask, has the model passed a test that it could have failed, not has the data forced the model onto us. The model predicts we'll see acceleration, and no competing model makes that prediction, so look at the data again and ask ourselves if the model has survived a test here. It's not the model's fault if the data has insufficient precision to yield better than 97% confidence, that's the key point the article is overlooking!

Indeed, the way they present the data shows no ability for the data to differentiate the Milne model they show, from the accelerating model they show, but this is no surprise because it is possible to find a Milne model that is really quite close to the accelerating model, if you are not required to satisfy any other constraint. Given how close the Milne and accelerating models are, we simply don't have precise enough data to distinguish them, and that is indeed all this paper is saying! This is because the Milne model they show is not consistent with other things we hold to be true, like the Big Bang nucleosynthesis of deuterium and so forth. So what they're not telling you is that the acceleration model they show is consistent with all that other data, and the Milne model is not, but it is certainly clear that this data by itself cannot distinguish those two models, because the data simply is not precise enough to be able to do that. It's just too much to ask of that dataset, but what is important is that the accelerating model is the only one that makes predictions that is consistent with all the data simultaneously.

David Shaw
2016-Oct-28, 11:40 PM
This paper was mentioned in passing, in another thread:
Marginal evidence for cosmic acceleration from Type Ia supernovae

http://www.nature.com/articles/srep35596

I just wondered if anyone had any thoughts / insights on this research...

New to this, but I was thinking, what if after the Big Bang when all the matter in the universe was closer together gravity was able to slow our acelleration down more. now we are further away form the centre and each other gravity can no longer slow our descent (for lack of better understanding) and as we move further away the effect of that gravity decreases thus causing an acelleration effect, and a flat universe is either the result of a streaching universe or a unknown pre verse that has a structure we can not yet comprehend?

ShinAce
2016-Oct-29, 12:49 AM
New to this, but I was thinking, what if after the Big Bang when all the matter in the universe was closer together gravity was able to slow our acceleration down more. now we are further away form the centre and each other gravity can no longer slow our descent (for lack of better understanding) and as we move further away the effect of that gravity decreases thus causing an acceleration effect, and a flat universe is either the result of a stretching universe or a unknown pre verse that has a structure we can not yet comprehend?

Assuming general relativity is correct, the basic model of the universe is the Friedmann metric. The other part you need to solve the whole thing is called the equation of state. The state parameter will tell you whether the universe is growing but slowing down, growing constant, or growing and accelerating. The state parameter is determined by how much matter, radiation, and dark energy there are.

It turns out that for the first 50,000 years, most of the energy was in the form of radiation. The universe was growing, but gravity was slowing it down a fair amount.

Then from 50k years to about 10 billion years, most of the energy is in the form of matter. Again, growing but slowing down. This time the pull of gravity doesn't slow it down as much as radiation did.

Since then, about 4 billion years ago, the radiation has been diluted to the point where it doesn't even show up in the calculation anymore. There's more dark energy than matter, and I'm thinking(I should check) that the repulsive effect for dark energy is stronger than the pull of matter, so it will accelerate even once the two are even. By even, I mean 50% of the energy in the universe is matter and 50% dark energy. We're already near 70% dark energy and 30% matter.\

The proportion of dark energy can only grow.

Sardonicone
2016-Oct-29, 02:39 AM
But don't the new findings point to perhaps not run away expansion after all? If there is a snapping back Big Crunch and then another Big Bang, how does that bypass entropy? Would there ever be remnants from previous versions of the universe?



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Solfe
2016-Oct-29, 03:19 AM
And as always, I'm very frustrated that a new paper that should be given a headline like "Interesting Refinement of Analysis of Supernovae" is instead given a headline that's like "Scientists Realize They Were Completely Wrong About Everything and Totally Change Their Minds". I get that the latter type of title is much more likely to get people to read it, but I think it also contributes to an (all too prevalent) attitude that scientists don't actually know anything, that science is a useless tool for understanding the world, and that you can freely ignore scientific findings if they contradict your previously held beliefs, since all the scientists are likely to change their minds about it all tomorrow anyway.

Pffft. I'm still eating oat bran muffins. They taste good. :)

Source: Swain JF, Rouse,IL, Curley CB, Sacks FM. Comparison of the effects of oat bran and low-fiber wheat on serum lipoprotein levels and blood pressure. N Engl J Med 1990; 322:147-52.

Noclevername
2016-Oct-29, 07:25 AM
But don't the new findings point to perhaps not run away expansion after all? If there is a snapping back Big Crunch and then another Big Bang, how does that bypass entropy? Would there ever be remnants from previous versions of the universe?



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IIRC, the current expansion as measured is already too great for a Big Crunch.

publiusr
2016-Oct-29, 05:27 PM
The paper isn't necessarily arguing against a cosmological constant, they're arguing that the expansion may not be accelerating.

I've never really bought into the whole standard candle thing. There are nukes of different yields after all--so perhaps there is more variability in supernovae as well.

I mean, it isn't as if we had a probe a light year out from each exploding star and could compare each--down to the last megaton. We see white blobs.

The big problem with the expansion accelerating is that we are supposed to think that the universe is acting like the Moon did in SPACE 1999's pilot-- "Breakaway"
Not only was the Moon expelled--it kept going faster--as if only it started feeling "dark energy" and took off.

The whole universe doing that?

The Hobus supernovae in the '09 Trek reboot seems less ridiculous

Cougar
2016-Oct-29, 06:58 PM
....as we move further away the effect of that gravity decreases thus causing an acelleration effect....

As distance between masses increases, yes, the "gravitational effect" is less, but that doesn't imply an acceleration to the expansion, which would have to have some other impetus.

WaxRubiks
2016-Oct-29, 07:48 PM
As distance between masses increases, yes, the "gravitational effect" is less, but that doesn't imply an acceleration to the expansion, which would have to have some other impetus.

yes, but if you assume there is some force for expansion, then the gravitational pull would drag less the more it all expanded..so leading to acceleration..

Tom Mazanec
2016-Oct-29, 08:35 PM
Expansion IS accelerating https://arxiv.org/pdf/1610.08972.pdf

Cougar
2016-Oct-30, 12:21 AM
yes, but if you assume there is some force for expansion...

Prior to 1998, the expansion was typically thought to be "free-wheeling," no continuing "force" required, but continuing to expand as the result of the early initial conditions.


....then the gravitational pull would drag less the more it all expanded..so leading to acceleration..

AFAIK, that would lead to a slowing expansion. Hence, one of the needs for dark energy.

Cougar
2016-Oct-30, 12:32 AM
Expansion IS accelerating https://arxiv.org/pdf/1610.08972.pdf

Ha, yes, I'd say 75 sigma is pretty confident! Looks like a good, recent paper!

Solfe
2016-Oct-30, 03:37 AM
Wow, 75 sigma? That is crazy!

I was painfully introduced to sig figs and sigma as a bonus round of a surveying class. The teacher was supremely cool and added in a lot of other information relevant to other fields in his class. It was a summer class so he improvised when we were idle.

What I do now has zero to do with survey or math, but I still remember a lot of it.

ShinAce
2016-Oct-30, 11:40 AM
I'd say it's incomprehensible.

7 sigma is 99.9999999995% sure. To even know what anything beyond 10 sigma is, I'd have to work it out by hand. I doubt a CPU would come back with an answer other than 100%.

Shaula
2016-Oct-30, 01:25 PM
7 sigma is 99.9999999995% sure. To even know what anything beyond 10 sigma is, I'd have to work it out by hand. I doubt a CPU would come back with an answer other than 100%.
... assuming a Gaussian distribution. You can calculate a standard deviation for any distribution you like but it only converts to probability if you also specify the distribution.

George
2016-Nov-01, 01:33 PM
Ha, yes, I'd say 75 sigma is pretty confident! Looks like a good, recent paper!Where did you see 75 sigma?

Shaula
2016-Nov-02, 03:37 AM
Where did you see 75 sigma?
From the abstract of the V1 of that paper (https://arxiv.org/pdf/1610.08972v1.pdf)

George
2016-Nov-02, 04:19 AM
From the abstract of the V1 of that paper (https://arxiv.org/pdf/1610.08972v1.pdf)
Ah, I simply missed it in my cursory glance. Saying "approximately 75 sigma" sounds oxymoronic. I just don't understand a number like that given a 4.2 sigma for SN1a and 11.2 sigma from flatness. Is it that easy to extrapolate to boost it into the sigma stratosphere?

Ken G
2016-Nov-02, 02:59 PM
Ah, I simply missed it in my cursory glance. Saying "approximately 75 sigma" sounds oxymoronic. I just don't understand a number like that given a 4.2 sigma for SN1a and 11.2 sigma from flatness. Is it that easy to extrapolate to boost it into the sigma stratosphere?
No doubt, 75 sigma is not a number that can be taken seriously. There are various ways to get that kind of number, but none would carry the formal meaning of a degree of uncertainty without assuming Gaussian distributions way beyond their regions of applicability. Still, the point is that by the time the assumptions of independent Gaussians breaks down, the probability is already too small to be of interest. The real problem to watch out for is systematic error-- the error of making a wrong assumption that you regard as above reproach. Systematic errors of a very basic nature would be the issue long before you ever get to anything like 75 sigma, even if the Gaussian assumption was not the systematic error in question.

Jean Tate
2016-Nov-02, 06:18 PM
There's a para in Nielsen+ (2016) (http://www.nature.com/articles/srep35596) that I can't quite grasp; help anyone?

It's the 4th para in the Discussion section (I've omitted the references):

Whether the expansion rate is accelerating or not is a kinematic test and it is only for ease of comparison with previous results that we have chosen to show the impact of doing the correct statistical analysis in the ΛCDM framework. In particular the ‘Milne model’ refers here to an equation of state p = −ρ/3 and should not be taken to mean an empty universe. For example the deceleration due to gravity may be countered by bulk viscosity associated with the formation of structure, resulting in expansion at approximately constant velocity even in an universe containing matter but no dark energy. Such a cosmology is not prima facie in conflict with observations of the angular scale of fluctuations in the cosmic microwave background or of baryonic acoustic oscillations, although this does require further investigation. In any case, both of these are geometric rather than dynamical measures and do not provide compelling direct evidence for a cosmological constant — rather its value is inferred from the assumed ‘cosmic sum rule’: ΩΛ = 1 − Ωm + Ωk. This would be altered if e.g. an additional term due to the ‘back reaction’ of inhomogeneities is included in the Friedmann equations(bold added)

It seems that GR is assumed, with different models compared, "the ΛCDM framework" and "the ‘Milne model’".

But what, in some detail, does "kinematic" refer to in this para?

Also, "geometric" and "dynamic" ("both of these are geometric rather than dynamical measures")?

George
2016-Nov-02, 07:02 PM
No doubt, 75 sigma is not a number that can be taken seriously. There are various ways to get that kind of number, but none would carry the formal meaning of a degree of uncertainty without assuming Gaussian distributions way beyond their regions of applicability. Still, the point is that by the time the assumptions of independent Gaussians breaks down, the probability is already too small to be of interest. The real problem to watch out for is systematic error-- the error of making a wrong assumption that you regard as above reproach. Systematic errors of a very basic nature would be the issue long before you ever get to anything like 75 sigma, even if the Gaussian assumption was not the systematic error in question.
Thanks. Given their unusual claim of 75 sigma, I would have assumed there would be some explanation for it, but I didn't see any. It's a little like the statement we hear too often, "I gotta be honest with you", to make us believe they are now being honest without realizing they have placed their honesty in question with such a statement.

CJSF
2016-Nov-02, 08:37 PM
Having quickly read through the linked PDF, the 75 sigma is only mentioned in the abstract, in the conclusions they say 5 sigma. I think it the 75 sigma might be a typo or something.

CJSF

KILOPI!!!!!

George
2016-Nov-02, 09:41 PM
Having quickly read through the linked PDF, the 75 sigma is only mentioned in the abstract, in the conclusions they say 5 sigma. I think it the 75 sigma might be a typo or something. I wondered that as well.


KILOPI!!!!!Congrats. [Don't attempt a name change. *wink*]

Solfe
2016-Nov-05, 04:31 PM
Where did you see 75 sigma?

I'd say that it is a typo. They probably typed "~7nbsp;5Alt+963" for web display and the PDF didn't like it. Someone proof reading likely chopped out all of the "funny" characters leaving the circa symbol, a 7, a 5 and the sigma. The 7 was supposed to be the & symbol.

George
2016-Nov-06, 05:41 PM
I'd say that it is a typo. They probably typed "~7nbsp;5Alt+963" for web display and the PDF didn't like it. Someone proof reading likely chopped out all of the "funny" characters leaving the circa symbol, a 7, a 5 and the sigma. The 7 was supposed to be the & symbol. That's interesting and logical. Not all computers translate Greek properly. :)

StupendousMan
2016-Nov-07, 01:13 AM
Ringermacher and Mead make a pretty good counter-argument to "evidence is not strong for acceleration" in their new preprint:

https://arxiv.org/abs/1611.00999

Solfe
2016-Nov-07, 12:03 PM
That's interesting and logical. Not all computers translate Greek properly. :)

"I said I never had much use for logic. Never said I didn't know how to use it." :)

Kwalish Kid
2016-Nov-12, 04:23 PM
There's a para in Nielsen+ (2016) (http://www.nature.com/articles/srep35596) that I can't quite grasp; help anyone?

It's the 4th para in the Discussion section (I've omitted the references):
(bold added)

It seems that GR is assumed, with different models compared, "the ΛCDM framework" and "the ‘Milne model’".

But what, in some detail, does "kinematic" refer to in this para?

Also, "geometric" and "dynamic" ("both of these are geometric rather than dynamical measures")?

There are a number of different ways to measure the spacetime properties of the universe. A kinematic investigation reports on the relative motion of objects in the universe. A geometric investigation reports on the geometry of the universe. A dynamic investigation reports on the forces at work in the universe that influence motion.

The investigation in the supernovae is essentially kinematic: it reports on the relative motion of galaxies. It makes certain assumptions about the nature of supernovae and about light; it also makes certain assumptions about dynamics in order to translate acceleration in to measurements of cosmological parameters of density.

The investigation into the background radiation can be thought of as a geometric investigation. It reports on the overall geometry of the universe by allowing us to find how big something was in the past and seeing how that image change between source and observation. It's also a dynamic investigation, since it reports on the dynamics that must have been in place in the early universe.

The baryon acoustic oscillations investigation report on the clustering of matter in the universe, which reports on the dynamics of the early universe (and the universe over time). So it reports on the overall density of matter (mostly) and dark energy/cosmological constant (somewhat) because these determine (through GR or something similar) how matter can cluster and develop over cosmological time.

So yeah, if we altered the assumptions that hold these things together, then they wouldn't all support the same results. But that's a pretty wild hypothesis.

Jean Tate
2016-Nov-15, 02:20 AM
Thanks Kwalish Kid! :clap:


There are a number of different ways to measure the spacetime properties of the universe. A kinematic investigation reports on the relative motion of objects in the universe. A geometric investigation reports on the geometry of the universe. A dynamic investigation reports on the forces at work in the universe that influence motion.

The investigation in the supernovae is essentially kinematic: it reports on the relative motion of galaxies. It makes certain assumptions about the nature of supernovae and about light; it also makes certain assumptions about dynamics in order to translate acceleration in to measurements of cosmological parameters of density.

The investigation into the background radiation can be thought of as a geometric investigation. It reports on the overall geometry of the universe by allowing us to find how big something was in the past and seeing how that image change between source and observation. It's also a dynamic investigation, since it reports on the dynamics that must have been in place in the early universe.

The baryon acoustic oscillations investigation report on the clustering of matter in the universe, which reports on the dynamics of the early universe (and the universe over time). So it reports on the overall density of matter (mostly) and dark energy/cosmological constant (somewhat) because these determine (through GR or something similar) how matter can cluster and develop over cosmological time.

The two other observational tests of cosmology that are relevant here are, I think, the ISW (Integrated Sachs-Wolfe effect) and the (Tolman) surface-brightness test. Sure, neither is anywhere near 5-sigma, but AFAIK they're consistent with the three you mention (SNe, CMB, and BAO). (I don't think BBN has anything but the most indirect relevance here). Tolman is (primarily) geometric, right? What about ISW?


So yeah, if we altered the assumptions that hold these things together, then they wouldn't all support the same results. But that's a pretty wild hypothesis.

Right. So the ΛCDM model, with ~Planck parameter values, is pretty consistent with five sets of observations. Is that something which can be said of any other (GR-based) cosmological model (Milne or otherwise)? I don't think so.

Ken G
2016-Nov-15, 02:33 AM
Worse, a Milne model is purely kinematic, meaning it is just a fit to the behavior, with no underlying dynamical theory (other than one we already know is wrong, a largely empty universe). Since when is that given equal time to a model that does invoke dynamical principles, fits the data better, and guides us toward the next set of observations we need to do to test it?