RUTA said:
Next you have to convert Dp to luminosity distance (DL), which is a model-dep relationship, then DL gives you mu. As you can see, this fit is highly dependent on your choice of cosmology model.
But my point is that the model dependence does not affect the conclusion. You can assume that GR is wrong and then create an alternative theory of gravity by changing the parameters in the equations. GR may be wrong but we have enough observations so that we know that GR is a good enough approximation to within some error.
Once you do that you quickly figure out that the signal is strong enough so that if you put in any plausible non-GR model (i.e. one that isn't excluded by other observations), you still end up with acceleration. The only way out is if you assume that there is some effect which is not taken into account by your parameterization. If you just use a single scale factor then you aren't taking into account dark flows and voids. Once you take those into account, you still can get rid of the signal without some very tricky arguments. At that point you try to think of anything else that you may have missed, and after ten years of doing that, you start to think that you didn't miss anything.
What you can do is to make list all of the assumptions that go into the conclusion. We don't know if GR is correct, but we know that the "real model of gravity" looks like GR within certain limits. You then vary the gravity model within the known observational limits, and it turns out that it doesn't make that much difference. There are other parts of the problem that make a lot more difference than gravity model (like the assumption that SN Ia are standard candles).
If your point is that the supernova measurements cannot be interpreted without reference to other data, sure. But that's true with *any* measurement. I have a GPS device. That device gives me my position, but it turns out that there are as many if not more assumptions in the GPS result than in the supernova measurement. If I assume GR is wrong and Newtonian gravity is correct, it turns out that this doesn't change my conclusions w.r.t. supernova. However it does change my GPS results.
if the scale of the universe is larger now than at emission regardless of the relative velocites of emitter and receiver at emission or reception, i.e., this is NOT a Doppler redshift.
Then we get into another semantic argument as to "what is a Doppler redshift?" and then "what is an acceleration?". You can do a lot of cosmology with Newtonian gravity. It's wrong but it's more intuitive. There is a correspondence between the term acceleration in the Newtonian picture and the GR picture. Similarity there is a correspondence between the concept of "Doppler shift" in Newtonian cosmology and that of GR.
The reason that these correspondences are important is that it let's you take observations that are done in "Newtonian" language and then figure out what it means in GR language. Sometimes semantics are important. If you do precision measurements, then you have to very clearly define what you mean by "distance", "brightness", and "acceleration."
But in this situation it doesn't matter. You use any theory of gravity that isn't excluded by observation and any definition of acceleration that you want, and you still end up with a positive result.
And, think about it, the model that best fits this data tells us the universe was first decelerating and then changed to acceleration. If you believe that's true
That doesn't make sense to me. In order to get scientific measurements, you have to make assumptions, and it's important to know what assumptions you are making, and to minimize those assumptions.
In order to do the conversion from brightness distance to some other distance, you have to make assumptions about the theory of gravity from distance 0 to the location that you are looking at. You *don't* have to make any assumptions about anything more distant.
Now it turns out that if you assume that you have a cosmological constant, you get a nice fit, but it's really, really important point that this assumption was not used to get the to the conclusion that the universe is accelerating. This matters because in order to interpret the supernova results, you have observations that limit what you can do to gravity. Now if you go into the early universe, you can (and people do) make up all sorts of weird gravity models.
It's important to keep things straight here, to make sure that you aren't doing any sort of circular reasoning. If it turns out that assuming GR is correct was critical to getting the conclusions we are getting, then that is a problem because things get circular.
You also (erroneously) believe the data gives you time rate of change of velocity directly and independent of model
The observers were measuring q. You get q by performing mathematical operations on the data. Now what q means, is something else. Within the limit of models of the universe that are not excluded by other observations, the observed q=-0.6 means that you have an accelerating universe.
I'm asserting is that for these particular results, gravity model dependence doesn't introduce enough uncertainty to invalidate the conclusions. The model *does* have influence the numbers, but whether or not that matters is another issue. For the supernova situation, the model dependencies aren't enough to allow for non-acceleration.
What I'm asserting is that if you plot the results on a graph and then include all possible values for the acceleration/deceleration of the universe, then anything with non-acceleration is excluded.
But if that was true, we would've known about the accelerated expansion all along, the large z would only be used to find the turning point between acceleration and deceleration.
No we wouldn't because for anything out past a certain distance we don't have any good independent means of measuring distance other than redshift. For anything over z=1, all we have is z, and there isn't any independent way of turning that into a distance. We can make some guesses based on things that have nothing to do with direct measurements, but unlike the supernova measurements those are just guesses that could very easily be wrong.
Also this matters because the assertion that the universe is decelerating at early times and that this deceleration turned into an acceleration *is* heavily model dependent. If we've gotten our gravity models wrong, then most of the evidence that indicates that the universe is decelerating at early times just evaporates. Now people are extending the supernova data to regions where we should see the universe decelerating (interesting things with GRB's).
I suppose that's one more reason to make the distinction between what we "know" and what we are guessing. Up to z=1, we know. We might be wrong but we know. For z=7, we are guessing.
This is also where the gravity models come in. For z=1, you look at the list of gravity models that are not excluded by observation, and the impact of the gravity model turns out not to be important. There is an impact, but it doesn't kill your conclusions. At z=5, then it does make a huge difference.
But that's not what happened, decelerating models fit the small z fine and no one suspected accelerated expansion. It's the large z that keeps us from fitting a decelerating model.
Decelerating models fit small Z (z<0.1) fine. Accelerating models also fit small Z (z<0.1) fine. The problem is that before we had supernova, we had no way of converting between z and "distance". We do now.
I'll stop here and let you respond. I'm on the road and posting is non-trivial so I apologize for the terse nature of this response.
The problem is the dividing line between what we "know" and what we are guessing. If we were talking about using the WMAP results to infer the early expansion of the universe, then we are guessing. In order to go from WMAP to expansion rate, we have to make a lot of assumptions, and those assumptions are not constrained by data. We get nice fits if we assume GR, but GR could be wrong, and the for z=10, the possibility exists that we've gotten gravity wrong is enough so that it could totally invalidate our conclusions.
I'm trying to argue that this is not the situation with supernova data.
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