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Cheap Astronomy
2010-Mar-20, 07:50 AM
As I understand it, one of the key findings of WMAP (http://en.wikipedia.org/wiki/Wmap) is that the universe is flat.

But as far as I can tell, flatness is a concept drawn from the Friedmann equations (http://en.wikipedia.org/wiki/Friedmann_equations) - where the universe is either contracting (big crunch), flat or hyperbolically expanding (big stretch).

So in this context 'flat' means a universe expanding at a positive but linear rate.

So isn't WMAP in conflict with the type 1a supernova data showing the universe has an accelerating expansion. Doesn't this suggest a hyperbolic rather than flat universe?

And if that all makes sense why are we doing calculations requiring dark stuff to explain why the universe is flat - when it isn't flat at all?

Not meaning to be a dark energy denialist, but I wonder if we are over-compensating. If the Planck spacecraft (http://en.wikipedia.org/wiki/Planck_(spacecraft)) comes up with higher resolution data suggesting the universe is a bit hyperbolic in shape, we should be able to revise the dark energy requirement down a bit, shouldn't we??

Thanks,

Steve

noncryptic
2010-Mar-20, 01:46 PM
from NASA:
http://map.gsfc.nasa.gov/universe/uni_shape.html

"If the density of the universe is less than the "critical density" which is proportional to the square of the Hubble constant, then the universe will expand forever."

"If the density of the universe is less than the critical density, then the geometry of space is open, negatively curved like the surface of a saddle."

So "the universe will expand forever" corresponds to "the curvature of the universe is not flat"

It sure seems that with accelerated expansion, expansion will continue forever, iow: the universe is negatively curved (thus not flat).

And yet WMAP observations say the universe is flat.

So either NASA got their explanation wrong, or WMAP data contradicts the observations that say the universe is undergoing accelerated expansion, or it means that in the future the acceleration of expansion will decrease to zero and expansion will decrease to zero, or some people (including myself) are misunderstanding the whole thing.

ngc3314
2010-Mar-20, 03:12 PM
The connection "geometry is destiny" for cosmic expansion holds only if the cosmological constant is zero (equivalently, nothing that does what dark energy is surmised to do). "Flatness" is a geometrical measure which can be defined in terms of radiation propagation and its intensity after a certain travel distance. If only gravity acts, there is a necessary connection between geometry (flat, "hyperbolic") and expansion history, which is broken if other things matter on cosmological scales.

Krauss and Turner (1999) stated in one abstract (http://www.springerlink.com/content/h925275822n54121/), "The recognition that the cosmological constant may be non-zero forces us to re-evaluate standard notions about the connection between geometry and the fate of our Universe. An open Universe can recollapse, and a closed Universe can expand forever. As a corollary, we point out that there is no set of cosmological observations we can perform that will unambiguously allow us to determine what the ultimate destiny of the Universe will be."
Here's the whole paper on arXiv (http://arxiv.org/abs/astro-ph/9904020).

noncryptic
2010-Mar-21, 02:17 PM
The connection "geometry is destiny" for cosmic expansion holds only if the cosmological constant is zero (equivalently, nothing that does what dark energy is surmised to do).

If i understand this correctly, NASA got their explanation wrong; current consensus has it there is such a thing as dark energy, so geometry is not destiny, so

"If the density of the universe is less than the "critical density" which is proportional to the square of the Hubble constant, then the universe will expand forever."

and

"If the density of the universe is less than the critical density, then the geometry of space is open, negatively curved like the surface of a saddle."

are not both true.

korjik
2010-Mar-21, 09:26 PM
One thing to remember: This is all brand freaking new physics. Dark Energy is a concept that came out of nowhere only a decade ago. It is going to take another decade or two before everything shakes itself out again.

korjik
2010-Mar-21, 09:28 PM
If i understand this correctly, NASA got their explanation wrong; current consensus has it there is such a thing as dark energy, so geometry is not destiny, so

"If the density of the universe is less than the "critical density" which is proportional to the square of the Hubble constant, then the universe will expand forever."

and

"If the density of the universe is less than the critical density, then the geometry of space is open, negatively curved like the surface of a saddle."

are not both true.

Actually, he is saying that neither statement makes any sense. The underlying understanding of cosmology has been upset, and the assumptions those statements are based on are no longer true.

forrest noble
2010-Mar-21, 10:17 PM
Cheap Astronomy,


So isn't WMAP in conflict with the type 1a supernova data showing the universe has an accelerating expansion. Doesn't this suggest a hyperbolic rather than flat universe?



And if that all makes sense why are we doing calculations requiring dark stuff to explain why the universe is flat - when it isn't flat at all?

Not meaning to be a dark energy denialist, but I wonder if we are over-compensating. If the Planck spacecraft comes up with higher resolution data suggesting the universe is a bit hyperbolic in shape, we should be able to revise the dark energy requirement down a bit, shouldn't we??


I don't think that WMAP data interpretations and type 1a supernova data interpretations are mutually exclusive of the other. WMAP data might turn out to be different upon a larger scale or analytic interpretation. Presently unknown factors could change formulations or interpretations of data from either or both groups of data that could enable future interpretations to be compatible for both groups of observations.

My own expectation is that data from WMAP will be confirmed over time and that type 1a supernovae data will be altered in time, not by observation but by data re-interpretation and re-assessment of distances based upon the Hubble formula. I think such speculation is presently up for grabs.

ngc3314
2010-Mar-22, 02:16 AM
Keep in mind that NASA is a very large group of people, and that, these days the public-outreach stuff is seldom written directly by the project scientists. I can see a distinct possibility that the "geometry=destiny" bits were left over from boilerplate written before WMAP launched, perhaps before the SN Ia results filtered through. The paragraph above that, in fact, refers to cosmic acceleration. For anything authoritative, I wouldn't take the outreach web sites as the final word, if for no other reason than the wide range of audience they hope to make some sense for and the possibility that some bits are not kept updated once some starting text fills each "box".

(This is not meant to belittle the outreach efforts, but as an observation of where I've seen people overinterpreting some paragraph on a NASA website as overturning the extensive technical literature from the same project. I've been known to try outreach material, not always making it on the web because of the mandated level of the material).

noncryptic
2010-Mar-22, 09:55 PM
The underlying understanding of cosmology has been upset, and the assumptions those statements are based on are no longer true.

It seems that's what caused my confusion, which has now been cleared up (in part thanks to reading some of the Krauss and Turner paper), thank you all very much.

Cheap Astronomy
2010-Mar-23, 11:26 AM
I don't think this an issue of reading an old NASA outreach webpage.

The Friedmann equations have dominated cosmological thought in recent decades. They were developed to pose three scenarios and then challenge observational astronomers to go out and get the data.

The three scenarios are that the destiny of the universe either is big crunch - stays flat and static - or expands forever.

The astronomers did go out and get the data - and WMAP delivered the answer that the universe is flat. But within a couple of years the supernova data came in saying there's an accelerating expansion.

As I understand it, the theorists responded by saying - phew, because the only way they could explain the 'flatness' in the first place was to invoke a critical density value that was way bigger than what our observational data tells us the universe's critical density should be.

So I see a logical fallacy in this line of reasoning. If the supernova data reflects reality, then the universe isn't flat - so we shouldn't be running with the flat universe Freidmann equation to estimate how much dark energy we need. We should be using the 'universe expands forever' Friedmann equation - or some other equation from the legion of possible equations available under general relativity field theory.

Cougar
2010-Mar-23, 12:38 PM
If the supernova data reflects reality, then the universe isn't flat...

Flatness is observed. The supernova Ia data is observed. What're you gonna do?

noncryptic
2010-Mar-23, 03:00 PM
I don't think this an issue of reading an old NASA outreach webpage.

That's not what's pointed out as the cause of confusion, the issue is broader than that:

"The underlying understanding of cosmology has been upset" (korjik)


The astronomers did go out and get the data - and WMAP delivered the answer that the universe is flat.
But within a couple of years the supernova data came in saying there's an accelerating expansion.

It looks like the order in which those results were found is the other way around:

"In 1998 observations of Type Ia supernovae suggested that the expansion of the universe is accelerating"
http://en.wikipedia.org/wiki/Accelerating_universe

"WMAP One-year data release"
11 February 2003
http://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe



WMAP delivered the answer that the universe is flat

That's "flat geometry of space".


the supernova data came in saying there's an accelerating expansion

That's about expansion (crunch, rip or something in between), not geometry.


Also if i understand it correctly, the supernova data is in fact combined with WMAP data to show that the geometry of space is flat:

(about 48 miutes into the lecture)

Segre Lecture: How Did The Universe Begin
UC Berkeley
by Andrew Lange
http://www.youtube.com/watch?gl=US&v=e_4bMIqmV9U

trinitree88
2010-Mar-23, 09:55 PM
Flatness is observed. The supernova Ia data is observed. What're you gonna do?

I think the recent paper indicating that the standard candle model for type 1a supernovae is incorrect and that they are due to the mergers of white dwarfs with variable masses of up to 8 stellar masses, leading to luminosities that vary by a factor of ~ 20 is going to have some impact on the supernovae data set. This is not a homogeneous population of progenitors, nor a methodology to claim standard candles until better spectroscopy can confirm all of the putative mergers. pete

see:http://www.universetoday.com/2010/02/17/merging-white-dwarfs-set-off-supernovae/

Cheap Astronomy
2010-Mar-24, 09:12 AM
Re 1998 and 2003. Yes, sorry my bad.

Quote:
the supernova data came in saying there's an accelerating expansion
That's about expansion (crunch, rip or something in between), not geometry.

No, you can't distinguish between the geometry and the expansion - the expansion is the geometry (or vice versa).

It's space-time - you are suggesting we can take a photo of it and say that's the geometry. It's space-time, you can't separate one from the other. Otherwise, we might as well say the universe is a singularity - because it used to be 13.7 B.y. ago.

Kwalish Kid
2010-Mar-24, 12:20 PM
No, you can't distinguish between the geometry and the expansion - the expansion is the geometry (or vice versa).

This is a thorny problem that I come upon when explaining cosmology. The expansion is an aspect of the geometry, a property that holds everywhere, but it is not the overall geometry in the sense of what the curvature of space is in the cosmological rest frame.

Cougar
2010-Mar-24, 02:18 PM
I think the recent paper indicating that the standard candle model for type 1a supernovae is incorrect and that they are due to the mergers of white dwarfs with variable masses of up to 8 stellar masses, leading to luminosities that vary by a factor of ~ 20 is going to have some impact on the supernovae data set. This is not a homogeneous population of progenitors, nor a methodology to claim standard candles until better spectroscopy can confirm all of the putative mergers. pete

see:http://www.universetoday.com/2010/02/17/merging-white-dwarfs-set-off-supernovae/

Universe Today? You believe everything you read on the web?

OK, just joking. Thanks for the alert. (Why doesn't this have a thread of its own?) I note that, since this new finding sampled only elliptical galaxies....



"An open question remains whether these white dwarf mergers are the primary catalyst for Type Ia supernovae in spiral galaxies."

I imagine there is a lot of modeling going on for white dwarf mergers...

noncryptic
2010-Mar-24, 02:18 PM
you can't distinguish between the geometry and the expansion - the expansion is the geometry (or vice versa).

As some in this thread have pointed out, that was the consensus until the new data came in, and it would be the case if gravity was the only force involved re curvature and expansion.
But apparently there's at least one other force at work that affects expansion but not geometry: dark energy.

As far as i can tell, the wiki article on dark energy is up to speed with recent findings:

http://en.wikipedia.org/wiki/Dark_energy
"The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies, most recently by the WMAP spacecraft, indicate that the universe is very close to flat. For the shape of the universe to be flat, the mass/energy density of the universe must be equal to a certain critical density. The total amount of matter in the universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%.[7] The most recent WMAP observations are consistent with a universe made up of 74% dark energy, 22% dark matter, and 4% ordinary matter.[2]"

Kwalish Kid
2010-Mar-25, 02:21 AM
I think the recent paper indicating that the standard candle model for type 1a supernovae is incorrect and that they are due to the mergers of white dwarfs with variable masses of up to 8 stellar masses, leading to luminosities that vary by a factor of ~ 20 is going to have some impact on the supernovae data set. This is not a homogeneous population of progenitors, nor a methodology to claim standard candles until better spectroscopy can confirm all of the putative mergers. pete

see:http://www.universetoday.com/2010/02/17/merging-white-dwarfs-set-off-supernovae/
This press statement has absolutely nothing to do with the use of type Ia supernovae as standard candles.

Cheap Astronomy
2010-Mar-27, 07:49 AM
This is a thorny problem that I come upon when explaining cosmology. The expansion is an aspect of the geometry, a property that holds everywhere, but it is not the overall geometry in the sense of what the curvature of space is in the cosmological rest frame.

OK, I am feeling a bit side-tracked on this one. My hypothesis is 'if the universe is expanding in an accelerated fashion then it's not flat'. Surely the two concepts are mutually exclusive.

I then question the logic of saying that since WMAP found the universe is flat, we have to calculate a huge critical density - and the only way to make that work is to add a truckload of dark (i.e. unknown) stuff.

But if the universe isn't flat in the first place why are we doing this?

StupendousMan
2010-Mar-27, 02:46 PM
OK, I am feeling a bit side-tracked on this one. My hypothesis is 'if the universe is expanding in an accelerated fashion then it's not flat'. Surely the two concepts are mutually exclusive.


If ordinary matter (and dark matter) were all that existed in the universe, then your hypothesis would be correct.

If "dark energy" = non-zero cosmological constant = lambda were to exist as well, then your hypothesis would not be correct.

Current evidence from WMAP (and other CMB experiments) is that the geometry of spacetime is flat on very large scales. Current evidence from SNe Ia (and other experiments) is that the universe's expansion is accelerating.

Hence, astronomers choose to concentrate on models with a non-zero cosmological constant, and discard your hypothesis.

noncryptic
2010-Mar-27, 02:54 PM
OK, I am feeling a bit side-tracked on this one. My hypothesis is 'if the universe is expanding in an accelerated fashion then it's not flat'. Surely the two concepts are mutually exclusive.

To the best of my understanding,
That used to be everyone's hypothesis/theory, until the supernova data showed that wrt expansion there is another force at work besides gravity (dark energy) - thus explaining non-flatness of expansion, while the wmap data showed space is geometrically flat.
Since then the geometry of space is not considered to be linked to expansion.

Cheap Astronomy
2010-Mar-28, 06:06 AM
If ordinary matter (and dark matter) were all that existed in the universe, then your hypothesis would be correct.

If "dark energy" = non-zero cosmological constant = lambda were to exist as well, then your hypothesis would not be correct.

Current evidence from WMAP (and other CMB experiments) is that the geometry of spacetime is flat on very large scales. Current evidence from SNe Ia (and other experiments) is that the universe's expansion is accelerating.

Hence, astronomers choose to concentrate on models with a non-zero cosmological constant, and discard your hypothesis.

Well hang on - I understood that Einstein set the cosmo constant (lambda) to zero to have a steady state universe (and subsequently went Doh! when the Hubble data came out). The fact that we are even talking about expansion means we all agree lambda is not zero.

Going by the Wikipedia article - where its says WMAP found the universe is flat, I assume this is meant in the context of the flat universe predicted by the Freidmann equation which has a positive lambda exactly balanced by a critical density of mass/energy in the universe (over 90% of which is dark stuff).





Thanks.

ngc3314
2010-Mar-28, 12:26 PM
Well hang on - I understood that Einstein set the cosmo constant (lambda) to zero to have a steady state universe (and subsequently went Doh! when the Hubble data came out). The fact that we are even talking about expansion means we all agree lambda is not zero.



Not quite - Einstein's original concept was for a nonzero lambda being required to have a static Universe (which is what astronomers thought they were dealing with until Slipher and Hubble). Lambda=0 will give an expanding or contracting Universe per the Friedman model, but positive lambda is needed within the Einstein equations to have an accelerating expansion.

Cheap Astronomy
2010-Apr-08, 03:48 AM
Sorry - had to go consult the manual. Yes, again sorry - my mistake. Einstein introduced a non-zero CC.

I was stuck on this idea that dark energy was introduced to balance an equation that gives you a flat universe since that is what the observations tell us (which is reiterated in the Andrew Lange lecture - thanks for that). There then seems an intellectual leap over to ah-ha this explains why the expansion is accelerating.

However, I think I misinterpreted the meaning of flat to be about space-time, when it's really just a spatial term. I think an accelerating universe is a hyperboloid space-time, but at a single point in time it could still be observed as flat. So my apologies to WMAP.

Jerry
2010-Oct-24, 07:29 PM
ON THE ORIGINS OF THE HIGH-LATITUDE H(Alpha) BACKGROUND

http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.4361v1.pdf


We find that the removal of the scattered H(Alpha) contribution leads to a narrowing of the frequency distribution of the residual H(Alpha) intensities attributable to in-situ emission, implying smaller amplitude in the spatial structure of the in-situ emission component than previously thought..., and an overall reduction in average intensity by about a factor of two. This implies that the free-free emission template for such high-latitude regions used in the WMAP reductions (Bennett et al. 2003; Gold et al. 2010) has been overestimated in both strength and spatial structure. We suggest that the standard assumption in the reduction of CMB observations, namely that the observed H(Alpha) all-sky map of Finkbeiner (2003) is a suitable template for free-free emission in the Galactic foreground, must be reexamined, especially as it applies to high-latitude parts of the sky...

The estimate by Wood & Reynolds (1999), suggesting that between 5% and 20% of the all-sky H(Alpha) background may be attributable to scattering by dust appears to be in good agreement with our estimates for about half of the high-latitude sky, especially if lower latitudes are included (Fig. 4a), but the authors failed to account for the highly structured distribution in the ratio of H(Alpha) scattering to H(Alpha) in-situ emission in their model.

It is precisely this structure that is important in attempts to determine the small-scale anisotropy of the cosmic microwave background at angular scales of about 1 degree.

Opps.

Jerry
2010-Nov-04, 03:30 PM
arXiv:1011.0722

Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem


We explore a nuclear physics resolution to the discrepancy between the predicted standard BBN abundance of 7Li and its observational determination in metal-poor stars. The theoretical 7Li abundance is 3-4 times greater than the observational values, assuming the baryon-to-photon ratio, eta_{wmap}, determined by WMAP.

Creative; but reasonable? Why start with the assumption WMAP is correct? Time to apply the Jerry test: If an ATM theorist invoked a hypothetical nuclear resonance to explain why the observational data is at odds with a pet ATM theory; would anyone give this solution a second thought? If the theory is at odds with hard data, look hard at the theory.

antoniseb
2010-Nov-05, 01:10 PM
arXiv:1011.0722
Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem
Creative; but reasonable? Why start with the assumption WMAP is correct? ...

I didn't get the impression that the resonance thing was being given as the one main-stream answer for the Lithium issue. It's being tossed out as an anecdotal example of something we may not know about yet. The fact is that the WMAP team have gotten good agreement between observation and model on a huge fraction of the measurement space, and you need more than just a small unexplained detail to say that it must *all* be wrong.

Nereid
2010-Nov-05, 02:42 PM
arXiv:1011.0722

Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem



Creative; but reasonable? Why start with the assumption WMAP is correct? Time to apply the Jerry test: If an ATM theorist invoked a hypothetical nuclear resonance to explain why the observational data is at odds with a pet ATM theory; would anyone give this solution a second thought? If the theory is at odds with hard data, look hard at the theory.
If the theory is at odds with hard data, look hard at the theory.

I'm confused; what "hard data" are you referring to Jerry? And what "theory"?

noncryptic
2010-Nov-05, 08:54 PM
good agreement between observation and model

http://www.astro.ucla.edu/~wright/CMB-LCDM-5yWMAP-03Mar08-75.gif
http://www.astro.ucla.edu/~wright/CMB-DT.html


Universe to WMAP: ΛCDM Rules, OK? (http://www.universetoday.com/55011/universe-to-wmap-cdm-rules-ok/)

Jerry
2010-Nov-12, 05:52 AM
Lithium is not the only metal showing up where alpha theories do not expect it:

Anisotropic AGN Outflows and Enrichment of the Intergalactic Medium. II. Metallicity

http://arxiv.org/abs/1011.2509


We investigate the large-scale influence of outflows from AGNs in enriching the IGM with metals in a cosmological context. We combine cosmological simulations of large scale structure formation with a detailed model of metal enrichment, in which outflows expand anisotropically along the direction of least resistance, distributing metals into the IGM... Our simulations produced an average IGM metallicity of [O/H] = -5 at z = 5.5, which then rises gradually, and remains relatively flat at a value [O/H] = -2.8 between z = 2 and z = 0. The ejection of metals from AGN host galaxies by AGN-driven outflows is found to enrich the IGM to > 10 - 20% of the observed values, the number dependent on redshift. The enriched IGM volume fractions are small at z > 3, then rise rapidly to the following values at z = 0: 6 - 10% of the volume enriched to [O/H] > -2.5, 14 - 24% volume to [O/H] > -3, and 34 - 45% volume to [O/H] > -4. At z > 2, there is a gradient of the induced enrichment, the metallicity decreasing with increasing IGM density, enriching the underdense IGM to higher metallicities, a trend more prominent with increasing anisotropy of the outflows. This can explain observations of metal-enriched low-density IGM at z = 3 - 4.

So the simlulation creates the observed local flatness in metallicity by blowing bubbles full of metal at an accelerated rate by differentiating metal content in the IGM. It is another case of high metallicity where alpha cosmology would not predict it. I get to ask again, that if I proposed a galactic scale cyclotron sucked in and reduced the apparent metal content so that some hairball theory could find agreement with observations, should anyone buy into it?

The observed metallicity is too high to meet the predictions of WMAP scenarios, no matter how many curves have been retrofitted in new releases of WMAP 7 data.

Jerry
2010-Nov-15, 02:20 AM
...
Universe to WMAP: ΛCDM Rules, OK? (http://www.universetoday.com/55011/universe-to-wmap-cdm-rules-ok/)

We are close to a PLANCK release...next month? Bet you five-to-one the first PLANCK release is at least five months late.

Jerry
2011-Jun-14, 02:52 AM
The Planck Collaboration measured the Sunyaev-Zel'dovich (SZ) decrement of optically selected
clusters from the Sloan Digital Sky Survey, nding that it falls signicantly below expectations
based on existing mass calibration of the maxBCG galaxy clusters. Resolving this tension requires
either the data to go up, or the theoretical expectations to come down. Here, we use data from
the Wilkinson Microwave Anisotropy Probe (WMAP) to perform an independent estimate of the
SZ decrement of maxBCG clusters. The recovered signal is consistent with that obtained using
Planck, though with larger error bars due to WMAP's larger beam size and smaller frequency
range. Nevertheless, this detection serves as an independent conrmation of the magnitude of the
eect, and demonstrates that the observed discrepancy must be theoretical in origin.

http://arxiv.org/abs/1106.2185

The Sunyaev-Zeldovich Signal of the maxBCG SDSS Galaxy Clusters in WMAP


Our measurements are in excellent agreement with those of the Planck Collaboration, and are clearly lower than the theoretical predictions. Given this independent conrmation of the tension first observed in, it is clear that either the intra-cluster properties of galaxy clusters are not adequately understood, or that the optical cluster selection suers from a systematic that has gone undetected. We do not currently know the solution to this problem, and indeed, the discrepancy is likely to remain an active eld of research in the immediate future. We note that a similar discrepancy appears to arise for the SZ signal about Luminous Red Galaxies (LRGs)

Tensor
2011-Jun-14, 03:29 PM
http://arxiv.org/abs/1106.2185

The Sunyaev-Zeldovich Signal of the maxBCG SDSS Galaxy Clusters in WMAP

And that's news, why (http://arxiv.org/pdf/1001.4538v3)?