the Universe - accelerating expansion vs. the Hubble constant

[Chronos]

Let me put it to you this way: Why would you expect to be able to see the acceleration in data from nearby galaxies?

[Asympotically_fat]

@PBear42

Asympotically fat, I’ve already acknowledged the Hubble constant isn’t a constant in time, neither in theory (the formula by which it’s derived doesn’t even have a time term) nor in fact (the 1998 observations and later corroborations demonstrate it doesn’t hold at great distances). This doesn’t explain how the rate of expansion can be currently increasing yet not show up in data going out 1.3 billion light years. Your approximation argument is basically the same as Chronos’ and ZenBeam’s, no? Are you aware of any authorities making it? Cuz I’m not. For example, this paper by Riess (one of the three Nobel laureates) doesn’t as far as I can see. Yes, it could be the answer, but that would be tantamount to using a relatively small data set to throw out a much larger one. This isn’t how science is usually done.

The only reason I bring it up again is that it seemed to me you seemed to be implying that the use of the Hubble constant implies constant expansion, which it doesn’t.

If you look at the second paragraph of 2.1 on the paper you cite it talks about previous work and a previous “modest bound” on the deceleration parameter as -0.6 >q[sub]0[/sub]<2.5. This error bar actually covers both cases of the expansion either accelerating or deccelerating (or indeed is proceeding at constant rate - which I am at pains to point out was not seriously considered to be the case, in the long term, either pre or post 1998)- i.e. previous measurements were not accurate enough to qualitatively determine if the expansion of the Universe was accelerating or not.

Moreover, please understand that I’m not trying to reinstate the view of the universe prevailing prior to 1998. As I understand it, the big sea change came from cosmic background microwave radiation data, which (i) demonstrated a flat or nearly flat universe and (ii) couldn’t be accounted for by ordinary and dark matter, from which (iii) dark energy (however defined and conceived) turned out to be the dominant component of the aggregate matter-energy density of the universe. Taking this as given, I wonder whether we can’t harmonize the Hubble data and the distant Type 1a data by assuming (a) dark energy creates a constant rate of expansion but (b) in the distant past, gravity kept galaxies from sweeping away unfettered at this rate. On this view, the old gravitationally bound conception of the universe was partly right but mostly wrong. That is, gravity had an effect, but was ultimately overcome by fundamental expansion of the universe. As my thought experiment (which no one seems to want to discuss) suggests, it’s possible to have two forces operating in opposite directions which terminate at a constant expansion, notwithstanding observing a positive change in the rate of expansion over a significant portion of the curve.

There is no disharmony between the data, which is what I am trying to say. That previous data did not demonstrate the expansion of the Universe does not mean it is conflict with current data.

Another fact to realize is that it is not believed that the Universe’s expansion has always been accelerating.

Perhaps there are good theoretical reasons why this solution doesn’t work. Goodness knows I’m no physicist, as I’ve repeatedly acknowledged. But it seems to me an intuitively plausible model (much more plausible than the view that dark energy somehow increases in effect as distance increases). And, more importantly, accounts for both data sets without discarding either.

As I’ve pointed out the big bang model of a Universe with an asymptotically constant (and asymptotically non-zero) a’(t) (in other words a Universe whose expansion decelerates to approach a constant rate of expansion) is an FLRW open Universe with zero cosmological constant (i.e. no need for dark energy). This model was one of the main models pre-1998, however it was the kind of model ruled out by the discovery of accelerated expansion.

[PBear42]

ZenBeam: Unfortunately, no. The statement quoted is based on Freedman & Kaufmann. They give a plot of recessional velocity vs. distance, but I can think of no way to share that here. Frankly, it’s a rough plot anyway, intended for illustration rather than close analysis. Doubtless there are detailed plots in the literature, but I haven’t tried to find them. That’s going on my “to do” list.

Chronos: The galaxies used to calculate the Hubble constant aren’t nearby. They’re just closer than the ones used for the observations regarded as showing an accelerating expansion.

Asympotically fat; In a sense, I guess this is the answer to my question. It seems to me obvious the two data sets conflict. You’re saying they don’t. Presumably Riess, Schmidt and Perlmutter, et al. see it the same way. This would explain why they don’t discuss the “discrepancy.” I’m not convinced, but I’m not a physicist. My lack of agreement (or failure of comprehension) matters not one whit. In any event, thank you for your efforts. If nothing else, I picked up an understanding of scale factors. That alone was worth the thread.

[Asympotically_fat]

@PBear42

**Asympotically fat; In a sense, I guess this is the answer to my question. It seems to me obvious the two data sets conflict. You’re saying they don’t. Presumably Riess, Schmidt and Perlmutter, et al. see it the same way. This would explain why they don’t discuss the “discrepancy.” I’m not convinced, but I’m not a physicist. My lack of agreement (or failure of comprehension) matters not one whit. In any event, thank you for your efforts. If nothing else, I picked up an understanding of scale factors. That alone was worth the thread.

My problem though in answering your question is that I can’t understand why you think they’re in conflcit. See the following graph:

http://snap.lbl.gov/multimedia/images/hubbleDiagwReshift.gif

If you look at the yellow data points, which are from 1996 and of nearer galaxies you can see, that when the density parameter of visible matter and dark matter is taken as ~0.28 and the Universe’s geometry is assumed to be flat (both assumptions taken from observation) they are consistent with both an accelerating Universe and a decelerating Universe (Ω[sub]Λ[/sub]=0 implies a decelerating Universe), however the red data points from Permlutter et al in 1998 of further galaxies are 99% inconsistent with a non-zero cosmological constant and suggest a value for Ω[sub]Λ[/sub] which is quite a bit bigger than the minimum value needed for the Universe to be currently accelerating.

Neither sets of data are inconsistent with each other because firstly they measurements of different things and secondly they are both consistent with a Universe whose expansion is accelerating.

[PBear42]

I’m not going to debate this. I’m still learning the subject and have the good sense not to try to advocate a model when my understanding is incomplete. Indeed, as this isn’t my field and it’s a very technical topic, I may never get there. I push on out of intellectual curiosity and nothing more. I opened this thread to learn something, and I have, not to prove anything.

As for your question, my problem is that it seems to me the curve is in the wrong place, both in the plot you link and in Figure 1 given by Riess in the survey paper linked in Post #19. If we start with constant expansion at z = 1.0 and are experiencing accelerating expansion thereafter, the right side should be straight and the left curved. Whereas curved on the right and straight on the left suggests something like the model proposed in the OP. In any event, I never assumed ΩΛ=0 (sorry, don’t know how to do the subscript thing). Rather, I stipulated quite the opposite in Post #19.

Again, I’m not going to debate this. You asked a fair question and I’ve answered as best I can. That’s as far as I’m willing to go.

[Asympotically_fat]

@PBear42

As for your question, my problem is that it seems to me the curve is in the wrong place, both in the plot you link and in Figure 1 given by Riess in the survey paper linked in Post #19. If we start with constant expansion at z = 1.0 and are experiencing accelerating expansion thereafter, the right side should be straight and the left curved. Whereas curved on the right and straight on the left suggests something like the model proposed in the OP. In any event, I never assumed ΩΛ=0 (sorry, don’t know how to do the subscript thing). Rather, I stipulated quite the opposite in Post #19.

Again, I’m not going to debate this. You asked a fair question and I’ve answered as best I can. That’s as far as I’m willing to go.

I see what your saying, though I think what you are trying to say is that the line showing a model of accelerating Universe should be steeper on the left than the right rather than more curved (because whether the Universe’s acceleration is increasing is a separate issue, plus it’s almost impossible to tell from the graph whether the acceleration is increasing, decreasing or constant).

However the x axis of the graph is redshift which is a measurement of the scale factor of the Universe (i.e. the greater the redshift, the smaller the scale factor) and the y axis is ‘effective magnitude’ which is a measurement of the time (i.e. the greater the effective magnitude the further back in time). Therefore when the line on the graph is steeper it means the Universe is expanding at a slower rate (just think of the limit of a non-expanding Universe where the cosmological redshift would be zero leading to all the results lining up as a straight vertical line along the y axis), hence as the lines which correspond to models of an accelerating universes approach the present day they should become less steep.

[PBear42]

Thanks. That was very helpful.

[Chronos]

It’s not a matter of “curved on the right and straight on the left”. According to current understanding, it’s all curved. It’s just that it’s curved a small enough amount that, with just the left portion of the data, you can’t tell the difference between the curve and straight.

[PBear42]

Thanks.

[PBear42]

On revew, I realize that sounds dismissive. Not my intention. This is a point you have made all along. As I said, I’m trying to understand, not debate. I take this input quite as seriously as AF’s.