Dark Energy WiggleZ Survey

[sjastro]

Well done Australian Science.

http://wigglez.swin.edu.au/site/

Regards

Steven

[renormalised (Carl)]

I concur with that sentiment

Quite a few papers have come out of the survey!!.

[CraigS]

So, I've been puzzling over this one for most of the day.
I’m finding it somewhat difficult to understand what they've actually done here.

Correct me if I’ve missed the mark, but they seem to have done a bunch of accurate redshift measurements and found that the measurements fit the Lambda CDM model with a ‘flat’ matter density parameter. (This is also what the latest CMBR measurements support). They’ve then measured the cosmic growth rate, and found that it also aligns well with previous distance-redshift measurements such as Supernovae (probability of 98.8%) and ‘Standard Ruler’, Baryon Acoustic Oscillations.

They’ve also made a ~30% improvement in measurement accuracy of the ‘constant equation of state’ (little omega ?).

So, there’s nothing very new in what they’ve done .. its basically a truing-up of measurements and cross-correlating the results with other datasets from other sources.

Several articles I’ve read today are really over-hyping it all ... like this headline (from Physorg) ...
Dark Energy is real: WiggleZ galaxy project proves Einstein was right again

An Australian-based astronomy team, co-led by Professor Michael Drinkwater from the School of Mathematics and Physics (SMP) at The University of Queensland (UQ), has shown that the mysterious 'dark energy' is indeed real and not a mistake in Einstein's theory of gravity.

What a load of misrepresentative nonsense !

They haven’t proven anything .. they’ve merely reinforced that the Lambda CDM model is on track, and the presently known parameters are now more accurate !

.. continuing .. from the same article ..

“WiggleZ says dark energy is real. Einstein remains untoppled,” said Dr Chris Blake, of Swinburne University, lead author of the recent findings, which will be published in two papers in the Monthly Notices of the Royal Astronomical Society.

Not much wonder ‘Darkness’ cops such a dud wrap, with sensationalist statements and headlines like this ! Its not necessary to make the sweeping statement: 'dark energy is real'. This is not what this survey was about !

The basic research was your typical meat-and-potatoes type of stuff, which was good value research, and full compliments to the the WiggleZ team !

Why cheapen it by making statements like the one above ?

(Thanks also for your better balanced article, Steven).

Cheers

[renormalised (Carl)]

I haven't actually read the physics.org article, so I don't know if Chris Blake actually said what was reported, but you have to remember Craig that they're trying to "sell" the science that was done, so they tart it up a bit to make it sound gee whiz. That's your typical journalist twist on things. Only thing is they get it wrong when they do this and invariably misquote what people say and misrepresent what was done.

The average punter reading the article would find it rather boring if the story was reported as it should've been. Actually, they wouldn't understand the science behind it, anyway.

[CraigS]

Since writing my post #3, and reading Carl's response, I've had a few more thoughts. Putting aside the possible media misrepresentations on it all, I think my reaction was to the statements:

- mysterious 'dark energy' is indeed real

and ….

WiggleZ says dark energy is real.

So here's the rude question …. as a result of this study (and other available evidence from other sources) ..

Is Dark Energy real, (or not) ?
… and why, (or why not) ?

I'm still wrestling with my thoughts on this .. what do others think, and why?

Cheers

[sjastro]

Originally Posted by CraigS

Since writing my post #3, and reading Carl's response, I've had a few more thoughts. Putting aside the possible media misrepresentations on it all, I think my reaction was to the statements:

- mysterious 'dark energy' is indeed real

and ….

WiggleZ says dark energy is real.

So here's the rude question …. as a result of this study (and other available evidence from other sources) ..

Is Dark Energy real, (or not) ?
… and why, (or why not) ?

I'm still wrestling with my thoughts on this .. what do others think, and why?

Cheers

Baryon Acoustic Oscillation allows us to verify high-z supernova type 1 distances which until recently were only confirmed to low-z values through other standard candle methods such as Cepheid variables.

From this we can say the effects of something we term "Dark Energy" are real. It still doesn't however tell us what Dark Energy is.
As Leon Susskind points out Quantum Field Theory is ready made to explain what Dark Energy is, but predicts way too much of it when compared to what is observed in the Universe.

Hence the jury is still out.

Regards

Steven

[CraigS]

Hmm ..

Can something be real if you don't know what it is ?

See, if something can be measured .. ie: confirmed that it possesses distance, time or form, then it must exist in the physical world.
It is therefore physically real.

Mind you, I agree with Steven's take on it. But just because we don't know what it is, doesn't mean that its not real.

So, (against my better judgement), if pushed, from a purely scientific perspective, I think by this definition, I'd have to concur and say I agree with this guy, Blake.

I could also claim that it doesn't make any difference to my world whther its real or not, so the question is irrelevant.

Interesting ...

Cheers

[CraigS]

Just thought of another point …

If the effects of something exist and are real, can we infer that its causes are therefore real, also ?

Ie: the cause of comic expansion/acceleration is 'dark energy', therefore we can infer that dark energy is real, because expansion is real (measurable).

Cheers
PS: I think the BAO measurement in this case was achieved at a highest redshift of z = 0.6. I think the best achieved prior to this was z=0.2 and z=0.35 (?)

[CraigS]

Ahh .. another PhysOrg article which gives a bit more detail on what's so neat about this study …
Galaxy Evolution Explorer finds dark energy repulsive

They {the results} contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.

"The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster," said Chris Blake of the Swinburne University of Technology in Melbourne, Australia. Blake is lead author of two papers describing the results that appeared in recent issues of the Monthly Notices of the Royal Astronomical Society. "The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn't be seeing these constant effects of dark energy throughout time."

.. a case of where my ignorance about the existence of this 'theory' led to my confusion about what these guys were trying to disprove.
… Hang on a minute ...!!… Alex's (push gravity) theory may have just bitten the dust (?)

Cheers

[sjastro]

Originally Posted by CraigS

Just thought of another point …

If the effects of something exist and are real, can we infer that its causes are therefore real, also ?

Ie: the cause of comic expansion/acceleration is 'dark energy', therefore we can infer that dark energy is real, because expansion is real (measurable).

If someone knocks on my front door part of the energy is converted into sound energy. Since this is based on sound physics I can infer the effect is very real. It doesn't tell me anything about the identity of the person knocking on my door. The causes remain an open question.

The acceleration of the Universe on the basis of type 1 supernova survey isn't unanimously accepted by cosmologists. Some papers have come out (these escape me for the time being) which suggest the survey results could be explained for an inertia based dark energy free expanding Universe.
In this case the question arises is the effect real.

The WiggleZ survey makes this question less relevant.

PS: I think the BAO measurement in this case was achieved at a highest redshift of z = 0.6. I think the best achieved prior to this was z=0.2 and z=0.35 (?)

We already know what happens with Quasar surveys that are "incomplete".
Selection effects leading to perceived patterns that pseudoscience likes to exploit.

Regards

Steven

[mjc (Mark)]

Originally Posted by CraigS

Just thought of another point …

If the effects of something exist and are real, can we infer that its causes are therefore real, also ?

Ie: the cause of comic expansion/acceleration is 'dark energy', therefore we can infer that dark energy is real, because expansion is real (measurable).

That would be an invalid inference of the implication if A then B (which might be valid and true) and making the mistake of believing the converse is valid and true (if B then A) which might not be the case.

If it has just rained then my garden is wet - but just because my garden is wet does not imply that it has rained - garden hoses etc.

With regards to the WiggleZ stuff - completely over my head.

Mark C.

[CraigS]

Hi Mark;
Well … hmm … I dunno … Given that no-one understands what Dark Energy is (ie: what gives rise to it, under what conditions it creates force, its interaction to result in the effects we see, etc, etc), I think it might be still valid to say that it exists even if we don't understand it.

The angle I can see (although I am finding it hard to accept myself, and this is only for a chin-wag type discussion), is that the following alternatives to explain the accelerated expansion exist:

i) some mechanism we don't yet even know about (what we don't know);
ii) our measurements are faulty/inaccurate/imprecise so its effects aren't real;
iii) other plausible explanations that have not yet been eliminated;
iv) 'dark energy'

Now, because 'dark energy' is not yet understood (or characterised in any particular specific sense), the label can still be applied to types (i) and (iv) above.
Now, these guys are claiming that they have disproven 'reverse gravity' (which would have fallen into type (iii) above, but is now eliminated). They have also shown that their measurements are accurate and are confirmed to be so, (type (ii) above bites the dust).

Steven raises "an inertia based dark energy free expanding Universe" and Susskind's QFT explanations, which might fall into (iii) above, although it is likely to be argued that neither of these is a 'plausible' hypothesis. (I'm not too familiar with these hypotheses).

So, if it were to happen that 'dark energy', being defined as the 'catch-all' for what we don't know, and what we can't explain, it could still be argued that it is real. This would be because at the end of the day, something is causing it, so that something must also be real (because we can measure its effects) and that something is the catch-all label called 'dark energy' (the last-man-standing).

Steven;
If I don't quite know what (or the identity of who) is knocking at the door, but I can measure its action's effects (hence they, at least, are real), the only cause in my world of such an effect is real - ie: some kind of force, which I know is real … I've reproduced forces and characterised them for yonks … they are very familiar to me, and are caused ultimately by energy of some kind. So, I'm not sure that the door analogy works for me (?)

(Also guys, I'm easy on all this .. I'm not hung up on it and I'm prepared to admit defeat where a convincing counter is presented, but I think I'm also prepared to let Chris Blake off the hook. Perhaps I initially over-reacted to his claims. Carl's media conspiracy explanation still stands also, of course ! )

Have fun.

Cheers

[CraigS]

Another thought …

I think that because the term 'dark energy' is so broadly encompassing of what we are familiar with, and what we're not familiar with, you can almost use it to cover anything anyone comes up with, as a plausible explanation … and then ride off into the sunset with guns smokin' !

You can almost say anything qualitative by using the term .. and get away with it. After all, it comes from a parameter in an equation. It is crawling out of pure theory, and into the empirical 'real world' courtesy of these measurements, also.

It works well as a 'taunt' … the evidence for this is everywhere …
I think this is what scientists are doing more thesedays … look at Hawking as an example ... the world's greatest stirrer-in-science !

The downside of 'taunts' unfortunately, is the rise of pseudoscience !