Alernatives: Cosmological redshift/BBE

[Tubbythin]

Tubbythin: See the references cited in my paper.

Well I don't have a copy of the reference in question. I was hoping you could give me a direct quote from said reference to support your assertions.

 

[Skwinty]

Skwinty: There is no "Law" that light cannot exceed any specific speed. The purported immutability of the value of c is just a postulate of SR.

Can you cite an example of light travelling faster than 3e8 m/s?

 

[sol invictus]

sol invictus: They're not opinions - they are facts.

RJH REPLIES: In your opinion.

And that of all other experts on this, supported by masses of incredibly precise experimental data from the past century of research - yes. To avoid having to say all that, most people simply refer to them as "facts" for short.

SI: That's too incoherent to comment on.

RJH: Just plain English.

Just plain wrong.

SI: I can't think of any definition of "xt" which would make that a sensible equation.

RJH: See Einstein, "On the Electrodynamics of Moving Bodies".

I have. No "c=xt" to be seen.

SI: OK, "homogeneous" is evidently another term you do not know the meaning of. When people refer to a "homogeneous universe", they mean homogeneous in space - not in time. Expanding cosmologies change with time, obviously. As every grammar school child knows, 13 billion years ago the universe was extremely hot and dense. Now it isn't.

RJH: Homogeneous: The same everywhere and everywhen. See Einstein, "On the Electrodynamics of Moving Bodies".

That's a paper (the paper) on special relativity. It has nothing whatsoever to do with cosmology or expanding universes, and it was written ten years before Einstein discovered general relativity.

Here's the only quote where Einstein refers to homogeneity in this translation: "In the first place it is clear that the equations must be linear on account of the properties of homogeneity which we attribute to space and time."

Notice that he specifies "space and time" - because if he didn't, "homogeneity" could refer to space, time, or pancake batter.

Metric: a unit of measurement.

Question: since you very obviously have no clue, what's your purpose in posting here? If you want to convince anyone you know what you were talking about, you shouldn't post again (although I'm afraid it's too late).

Or do you want to learn something? If so, my advice is to be polite and ask questions, rather than making arrogant and utterly wrong pronouncements. There are many people here that actually understand physics, and might even be willing to explain some of it to you.

 

[Perpetual Student]

I recently read a brief essay by Paul Steinhardt about brane cosmology. He claims that the brane cosmology model is "capable of reproducing all the predictions of the consensus (big bang) picture with the same exquisite detail." If that is the case why is there a strong preference for the "consensus picture" -- because it was developed first?

 

[DeiRenDopa]

I recently read a brief essay by Paul Steinhardt about brane cosmology. [...]

Do you remember what it was called, or where it was published?

I'd be interested to read it myself.

 

[sol invictus]

I recently read a brief essay by Paul Steinhardt about brane cosmology. He claims that the brane cosmology model is "capable of reproducing all the predictions of the consensus (big bang) picture with the same exquisite detail." If that is the case why is there a strong preference for the "consensus picture" -- because it was developed first?

No, it's because very few people agree with him. Steinhardt's model requires a so-far unjustified assumption about what happens at the "bounce". Because of that, it is not taken seriously other than by a few adherents, who are working hard to try and justify it.

Ideas like that have been around for a very long time - far longer than inflation has, actually. So far none have succeeded, because they all suffer from the same fundamental difficulty.

 

[Perpetual Student]

Do you remember what it was called, or where it was published?

I'd be interested to read it myself.

It is included in a book called "Science at the Edge", edited by John Brockman (subtitle: Conversations with the Leading Scientific Thinkers of Today). It is from a subsection of the book which consists of a dialog that took place at an Edge Foundation event in 2002. The discussion included Seth Lloyd, Alan Guth, Marvin Minsky and Ray Kurzweil.

 

[Perpetual Student]

No, it's because very few people agree with him. Steinhardt's model requires a so-far unjustified assumption about what happens at the "bounce". Because of that, it is not taken seriously other than by a few adherents, who are working hard to try and justify it.

Why do you regard that as more difficult than the "unjustified assumption" about t = 0 in the BB model?

 

[sol invictus]

Why do you regard that as more difficult than the "unjustified assumption" about t = 0 in the BB model?

Because there is no such assumption in the BB model.

Let me remind you how this works. You have some laws of physics you've inferred and tested over the centuries. You have some observations about the universe. You apply the laws of physics to the observations, and you predict that there was a BB singularity in our past - but you haven't tested the laws under the conditions right at the singularity, so you can't be sure it's really there. In fact, you really have no idea at all what happened there.

Steinhardt's idea requires a particular, very specific assumption about what came before the singularity, and how what was there passed through the singularity to later times and us. Change that assumption in almost any way, and his theory is incompatible with data. And there's no good reason to believe that that assumption is any better than any other (unless you're Steinhardt, that is).

Standard inflationary cosmology requires no such assumption - all the physics takes place after the BB. What came before the BB isn't specified - and one of the virtues of inflation is that it doesn't matter what came before, because whatever it was, it gets inflated away (usually - there are some interesting exceptions).

 

[Perpetual Student]

Standard inflationary cosmology requires no such assumption - all the physics takes place after the BB. What came before the BB isn't specified - and one of the virtues of inflation is that it doesn't matter what came before, because whatever it was, it gets inflated away (usually - there are some interesting exceptions).

You regard "it doesn't matter what came before" as a virtue of the BB model. I see it as a serious deficiency that the BB model can say nothing about t = (<) 0 and a strength of Steinhardt's model that it does.

 

[sol invictus]

You regard "it doesn't matter what came before" as a virtue of the BB model. I see it as a serious deficiency that the BB model can say nothing about t = (<) 0 and a strength of Steinhardt's model that it does.

My point is that Steinhardt's model cannot say what came before. It's just a guess, one chosen precisely so that the predictions match experiment. In my view, it has no explanatory power at all - were the data to change, one could make a different guess and arrive at a similar model that matches the new data. To make an overly harsh comparison, you might as well say goddidit (it's not as bad as that, because hopefully someone will someday understand how to really resolve the BB singularity, and then we'll know which, if any, of those guesses were correct).

Contrast that to the standard BB. Precisely because the physics depends only weakly on what (if anything) came before the BB, one needn't make any such guess (modulo the interesting exceptions I mentioned). Instead, the theory manages to produce falsifiable predictions based solely on (almost) standard physics - physics that takes place in conditions were we are at least fairly sure we understand the laws that govern it.

 

[Perpetual Student]

My point is that Steinhardt's model cannot say what came before. It's just a guess, one chosen precisely so that the predictions match experiment. In my view, it has no explanatory power at all - were the data to change, one could make a different guess and arrive at a similar model that matches the new data. To make an overly harsh comparison, you might as well say goddidit (it's not as bad as that, because hopefully someone will someday understand how to really resolve the BB singularity, and then we'll know which, if any, of those guesses were correct).

Contrast that to the standard BB. Precisely because the physics depends only weakly on what (if anything) came before the BB, one needn't make any such guess (modulo the interesting exceptions I mentioned). Instead, the theory manages to produce falsifiable predictions based solely on (almost) standard physics - physics that takes place in conditions were we are at least fairly sure we understand the laws that govern it.

So, if we were postulate guesses about t = < 0 in the BB model, we would be in the same position as we are with Steinhardt's model. Are there such guesses of interest?
Steinhardt says that the discovery of cosmic gravitational waves would be consistent with the BB model and that the absence of such waves would support his model. Do you agree? And where do we now stand in any efforts to discover such waves?

 

[sol invictus]

So, if we were postulate guesses about t = < 0 in the BB model, we would be in the same position as we are with Steinhardt's model.

No, we wouldn't. That's the point I'm trying to explain - the predictions of inflationary cosmology do not depend on the details of the physics near the would-be singularity. That makes it a predictive model - if that weren't the case inflation would be in the same boat with Steinhardt.

Are there such guesses of interest?

Having said what I said above, it's not completely true - inflation doesn't get rid of everything that came before. But that's a long story, and far off-topic here.

Steinhardt says that the discovery of cosmic gravitational waves would be consistent with the BB model and that the absence of such waves would support his model. Do you agree? And where do we now stand in any efforts to discover such waves?

Well... no, I don't really agree. I'm sure the specific model he has in mind lacks gravity waves, and that's what he means. But on the other hand I'm nearly certain that if those waves are discovered, someone (maybe Steinhardt, maybe someone else) will invent a similar model that does have them. It's not at all hard to imagine, for precisely the reason I'm discussing - the model depends on assumptions about what happened at the singularity, and different assumptions lead to different answers.

Efforts to discover them? This is the most impressive proposal I know of (although I'm not certain it's sensitive in the right range to do what Steinhardt was talking about - I'd need to know the context).

 

[Perpetual Student]

s. i.:

Thanks for your helpful responses. If I understand you correctly, the point is that the inflation event accounts for the observations we are dealing with today; it does not matter what was before -- i.e., inflation negated (or rendered unimportant) the state of the universe before inflation. Steinhardt's model does require a specific set of circumstances for the very early universe and for t = < 0 to account for today’s observations.

 

[sol invictus]

Thanks for your helpful responses. If I understand you correctly, the point is that the inflation event accounts for the observations we are dealing with today; it does not matter what was before -- i.e., inflation negated (or rendered unimportant) the state of the universe before inflation. Steinhardt's model does require a specific set of circumstances for the very early universe and for t = < 0 to account for today’s observations.

Yes, that's right.

 

[Tim Thompson]

Gravitational Waves

Steinhardt says that the discovery of cosmic gravitational waves would be consistent with the BB model and that the absence of such waves would support his model.

I think that gravitational waves are a requirement of general relativity, so their true absence (as opposed to our technological inability to detect them) would be rather more significant than a mere failure of the standard model of cosmology. If the Big Bang fails, it's really no Big Deal, but if general relativity fails, that's a Big Deal squared.

And where do we now stand in any efforts to discover such waves?

Gravitational waves have already been indirectly observed. The discovery of the binary pulsar 1913+16 by Hulse & Taylor (Hulse & Taylor, 1975) earned the 1993 Nobel Prize in Physics. The fact that it's two pulsars in a binary system makes it possible to measure the binary orbital period with extreme precision. They are losing energy at a rate predicted by general relativity, as energy lost in the form of gravitational waves. There are now a few binary pulsars known, and so far as I know, they all behave in like manner, as predicted by energy loss as gravitational waves. Hence, the statement that gravitational waves have been indirectly detected. Lorimer, 2008 is a good recent review of the topic (Living Reviews is a family of free access online journals; just follow the HTML link from the ADS page I linked, and it will take you to the online paper).

Gravitational waves have not been directly detected, but there are numerous efforts online or in the works to do just that. The LISA mission has already been mentioned, but it has been delayed several times due to budget pressures, and is still not expected to launch until 2019 or 2020, if it is not again delayed. But the Caltech LIGO system has been online for some time, and although it has yet to detect gravitational waves, it has gathered enough data to establish some limits (i.e., Abbott, et al., 2009; Abbott, et al., 2008(a); Santostasi, 2008; Abbott, et al., 2008(b) & etc.). The LIGO Scientific Collaboration page provides links to other ground based gravitational wave detectors.

OK, after getting this far I found this paper: Are Pre-Big-Bang Models Falsifiable by Gravitational Wave Experiments?; Ungarelli & Vecchio, 2000. They explore the possibility of observing relic gravitational waves left over from the "Bang". The parameters of the signal are sensitive to the parameters of the model, so some string theory based pre- Big Bang models might be falsifiable by the absence of the required relic waves. I suspect this kind of "falsifiable" is what Steinhardt had in mind, and not that gravitational waves would be altogether absent.

And finally, one last extraordinarily pedantic point that bugs only me, but now you must suffer the consequence: a gravity wave is a buoyancy wave in a fluid, particularly important in planetary atmospheres. A gravitational wave is a wave of gravitational field. It is usually easy to tell from context who's talking about what, so it normally is not a point worth suffering over. But since most of my career has been in planetary atmospheres, I like to stick up for my "field" (besides, Barry Barish once told me it annoyed him too, so I am not totally alone).

 

[HannonRJ]

Skwinty:"Can you cite an example of light travelling faster than 3e8 m/s? "
RJH: No. Nevertheless, there is no "law" of physics that says light is restricted to one value everywhere and everywhen. We cannot measure the value of c elswhere in the universe; we simply assume it always has the same value.

 

[edd]

This is not entirely true. It is difficult to constrain c in a way that is not degenerate with other parameters so normally one constrains the fine structure constant instead, but this is done on cosmological distances and timescales with quasar spectra amongst other things.

 

[Tubbythin]

This is not entirely true. It is difficult to constrain c in a way that is not degenerate with other parameters so normally one constrains the fine structure constant instead, but this is done on cosmological distances and timescales with quasar spectra amongst other things.

We also know from Earth-based data (eg Oklo) that any change in the fine structure constant 'here' over the last few billion years must have been very small.

 

[Skwinty]

We also know from Earth-based data (eg Oklo) that any change in the fine structure constant 'here' over the last few billion years must have been very small.

If my memory serves me correctly, the Oklo fission incident occurred 2 billion years ago and Alpha has since then, only changed by 45 parts per billion.