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Margarita
2015-Apr-12, 03:03 PM
Accelerating Universe? Not So Fast

http://uanews.org/story/accelerating-universe-not-so-fast

Certain types of supernovae, or exploding stars, are more diverse than previously thought, a University of Arizona-led team of astronomers has discovered. The results, reported in two papers published in the Astrophysical Journal, have implications for big cosmological questions, such as how fast the universe has been expanding since the Big Bang.

..." We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances -- and thus when the universe was younger," said Milne, an associate astronomer with the UA's Department of Astronomy and Steward Observatory. "There are different populations out there, and they have not been recognized. The big assumption has been that as you go from near to far, type Ia supernovae are the same. That doesn't appear to be the case."

...The Nobel laureates [2011, for discovery of dark energy] discovered independently that many supernovae appeared fainter than predicted because they had moved farther away from Earth than they should have done if the universe expanded at the same rate. This indicated that the rate at which stars and galaxies move away from each other is increasing; in other words, something has been pushing the universe apart faster and faster.

"The idea behind this reasoning," Milne explained, "is that type Ia supernovae happen to be the same brightness — they all end up pretty similar when they explode. Once people knew why, they started using them as mileposts for the far side of the universe.

"The faraway supernovae should be like the ones nearby because they look like them, but because they're fainter than expected, it led people to conclude they're farther away than expected, and this in turn has led to the conclusion that the universe is expanding faster than it did in the past."

..[the authors of the current paper] observed a large sample of type Ia supernovae in ultraviolet and visible light. For their study, they combined observations made by the Hubble Space Telescope with those made by NASA's Swift satellite.

The data collected with Swift were crucial because the differences between the populations -- slight shifts toward the red or the blue spectrum -- are subtle in visible light, which had been used to detect type Ia supernovae previously, but became obvious only through Swift's dedicated follow-up observations in the ultraviolet.

"These are great results," said Neil Gehrels, principal investigator of the Swift satellite, who co-authored the first paper. ...
"The realization that there were two groups of type Ia supernovae started with Swift data," Milne said. "Then we went through other datasets to see if we see the same. And we found the trend to be present in all the other datasets.

"As you're going back in time, we see a change in the supernovae population," he added. "The explosion has something different about it, something that doesn't jump out at you when you look at it in optical light, but we see it in the ultraviolet.

"Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at 10 of them nearby, those 10 are going to be redder on average than a sample of 10 faraway supernovae."

The authors conclude that some of the reported acceleration of the universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed.


Peter A. Milne, Ryan J. Foley, Peter J. Brown, Gautham Narayan.
THE CHANGING FRACTIONS OF TYPE IA SUPERNOVA NUV–OPTICAL SUBCLASSES WITH REDSHIFT.
The Astrophysical Journal, 2015; 803 (1): 20 DOI:*10.1088/0004-637X/803/1/20

http://iopscience.iop.org/0004-637X/803/1/20/

The earlier paper is here (http://iopscience.iop.org/0004-637X/779/1/23?rel=ref&relno=1)

A pre-print was posted to the arXiv in August 2014 and can be found here: http://arxiv.org/abs/1408.1706

Ken G
2015-Apr-12, 03:47 PM
These are certainly potentially quite important-- if you have two separate classes of SN that you are lumping together, and one class is more likely to be more distant, it will certainly mess up your determination of the dynamics of the universe. However, X-ray data show similar acceleration, independently of supernovae, so you have to find the corresponding explanation for that. Worse, we have quite good reason to think the acceleration is there-- which is that general relativity predicts it, if you put in the amount of matter we find evidence for. In other words, if you reduce the amount of dark energy (and lets face it, if we can't get rid of it altogether it's not much help), it means you either have to find more dark matter, or something else is wrong. If you don't add matter, and run time backward from now, you find that the universe was very nearly flat at early times, but not very very nearly flat. We have reasons to understand how it could be very very nearly flat, that's inflation, and we can understand why it might not be flat at all, but we have no reason to understand how it could be only nearly flat. Put differently, Weinberg was able to estimate the amount of dark energy we would find, simply from the constraint that the universe has the general attributes it does. So we have a weird relationship with dark energy-- we don't know what it could be, but if we don't have it, then we have other things that we don't know how they could be.

Margarita
2015-Apr-13, 08:12 AM
I've been reading up on what the non-SNe 1a evidence is for dark energy since posting the OP yesterday, and would be most grateful for links to useful sources, from you, Ken, or other forum members. Is the x-ray evidence you mentioned that relating to galaxy clusters?

So far, these are the most useful of what I've found:

Integrated Sachs-Wolfe effect (http://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect)

Chandra on galaxy clusters (http://chandra.harvard.edu/photo/2004/darkenergy)

General article on dark energy from the Daily Kos (http://www.dailykos.com/story/2012/10/16/1144870/-A-Proof-of-Dark-Energy)

Ken G
2015-Apr-14, 03:55 PM
I've been reading up on what the non-SNe 1a evidence is for dark energy since posting the OP yesterday, and would be most grateful for links to useful sources, from you, Ken, or other forum members. Is the x-ray evidence you mentioned that relating to galaxy clusters?Yes, that's right.


So far, these are the most useful of what I've found:

Integrated Sachs-Wolfe effect (http://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect)

Chandra on galaxy clusters (http://chandra.harvard.edu/photo/2004/darkenergy)

General article on dark energy from the Daily Kos (http://www.dailykos.com/story/2012/10/16/1144870/-A-Proof-of-Dark-Energy)

[/quote]I think you've already looked into it, all I hear about is secondary sources. I was more referring to the theoretical reasons to expect the universe to be flat, and how changing dark energy makes that harder to achieve.

Margarita
2015-Apr-15, 08:19 AM
I was more referring to the theoretical reasons to expect the universe to be flat, and how changing dark energy makes that harder to achieve.

I'm very new to looking at cosmological issues, so would value greatly an outline of what these theoretical reasons are. Did they predate the SNe 1a related findings by the two eventual Nobel prize winners?

When Reiss and Perlmutter published their findings back in 1997 it doesn't sound as tho they expected to find what they found. This is from
Hobby-Eberly Telescope Dark Energy Experiment (http://hetdex.org/dark_energy/discovery.php)

Adam Riess thought there must have been a mistake.
In late 1997, he was analyzing observations of exploding stars to help determine whether the universe would expand forever, or would someday reverse course and collapse in on itself. His observations seemed to show that not only would the universe continue to expand, but also that it was expanding faster as it got older — something that no one expected.

Cougar
2015-Apr-15, 01:32 PM
I was more referring to the theoretical reasons to expect the universe to be flat, and how changing dark energy makes that harder to achieve.

The overall flatness of space in the visible universe is not just theoretical. It's based on the angular size of the largest temperature fluctuations observed on the CMB. Using that fact and a little geometry and trigonometry, the space between us and the CMB must be very close to flat.

But how can it be flat when the mass of all the baryonic and dark matter is estimated to add up to only about 32% of the critical density required to make the overall curvature flat? Answer: the dark energy permeating all of space must have a mass equivalent to account for the missing 68%, bringing the critical density up to 1 for a flat universe.

Reality Check
2015-Apr-16, 12:59 AM
A good explanation from the WMAP web site (http://map.gsfc.nasa.gov/universe/uni_shape.html):

The WMAP spacecraft can measure the basic parameters of the Big Bang theory including the geometry of the universe. If the universe were flat, the brightest microwave background fluctuations (or "spots") would be about one degree across. If the universe were open, the spots would be less than one degree across. If the universe were closed, the brightest spots would be greater than one degree across.

Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe.

Margarita
2015-Apr-16, 08:21 AM
Many thanks for the helpful input. The WMAP site is very useful indeed. However, the book that it recommends here (http://map.gsfc.nasa.gov/universe/uni_accel.html)
Donald Goldsmith, "Einstein's Greatest Blunder? The Cosmological Constant and Other Fudge Factors in the Physics of the Universe", (Harvard University Press: Cambridge, Mass.) A well written, popular account of the cosmological constant and the current state of cosmology. was published in the mid to late 1990s.

Can anyone recommend a more recent book that covers this terrain in a similar style?

Jerry
2015-Apr-16, 07:39 PM
I'm very new to looking at cosmological issues, so would value greatly an outline of what these theoretical reasons are. Did they predate the SNe 1a related findings by the two eventual Nobel prize winners?

When Reiss and Perlmutter published their findings back in 1997 it doesn't sound as tho they expected to find what they found. This is from
Hobby-Eberly Telescope Dark Energy Experiment (http://hetdex.org/dark_energy/discovery.php)


Adam Riess thought there must have been a mistake.
In late 1997, he was analyzing observations of exploding stars to help determine whether the universe would expand forever, or would someday reverse course and collapse in on itself. His observations seemed to show that not only would the universe continue to expand, but also that it was expanding faster as it got older — something that no one expected.

It is now clear there was a mistake: The UV light curves are not the same. It is possible that they are now making an even larger mistake: Assuming that the UV in the most distant sample is consistent with the UV in under-luminous events, rather than best associating these most distant objects with over-luminous more local events that have similar UV light curves, too. Selection effects, the propensity to observer more luminous events with increasing distance; would normally lead to the association of the most distant events with the brightest events in the local sample - these also have an 'enhanced' UV light curve. However, making this association does not jive with the expansion velocities within the context of current basic theories about the nature of redshifts.

It is also important to understand that the close correlations between the WMAP data (especially the three-year results) and supernova data are completely out-the-window. What we have is a set of new observational data that is stubbornly refusing to agree with theoretical expectations.

Cougar
2015-Apr-16, 07:47 PM
Can anyone recommend a more recent book that covers this terrain in a similar style?

Well, there's Goldsmith's The Runaway Universe, the Race to Find the Future of the Cosmos [2000]. That style is pretty similar ;) and it covers the then-recent supernova findings of the accelerating expansion.

There are a lot more recent and very good books, one being The Perfect Theory: A Century of Geniuses and the Battle over General Relativity [2014] -- Pedro Ferreira. I'm currently really enjoying Trespassing on Einstein's Lawn [2014] by Amanda Gefter - philosophically quizzical as she tromps through the latest ideas in quantum physics and cosmology. It's very hard to explain this book. It is very unlike any other science-oriented book I have read over the past 20-30 years. First of all, you typically don't find profanities in this genre. Or questions like "What the H does that mean?" But she's persistent, reads and reads, sneaks into conferences, fakes being a journalist to "interview" and question leading scientists, becomes a science journalist working at New Scientist, hunts down and accesses Wheeler's journals... (well, I'm not done yet. :p )

Ken G
2015-Apr-16, 07:55 PM
I'm very new to looking at cosmological issues, so would value greatly an outline of what these theoretical reasons are. Did they predate the SNe 1a related findings by the two eventual Nobel prize winners?Sort of, the story there is quite convoluted! If the universe is not flat, general relativity predicts it will very rapidly evolve to being nothing like flat, so it's hard to imagine something that is close to flat but not flat. Still, when we looked at the matter (even including dark matter), we got that it only accounted for 30% of what was needed to make the universe flat, putting us smack dab in that uncomfortable place of "nearly flat but not flat." Dark energy came to the rescue-- but only if its type Ia-inferred value of 70% of the critical density holds up. Even if SN Ia data has to be reinterpreted as only providing, say, 50% of the critical density, that will still leave a missing hole of some 20% of something else that will be required to get it to be flat-- or face the "nearly flat but not flat" conundrum.


When Reiss and Perlmutter published their findings back in 1997 it doesn't sound as tho they expected to find what they found. This is from
Hobby-Eberly Telescope Dark Energy Experiment (http://hetdex.org/dark_energy/discovery.php)Yes, acceleration was a surprise. I guess it was assumed that some other form of matter would fill in the 70%-- how dark energy does it is still quite a mystery. But the worst case scenario is needing dark matter, dark energy, and something else. Darker energy?

Ken G
2015-Apr-16, 08:01 PM
The overall flatness of space in the visible universe is not just theoretical. It's based on the angular size of the largest temperature fluctuations observed on the CMB. Using that fact and a little geometry and trigonometry, the space between us and the CMB must be very close to flat.Yes, the reason we have confidence in the cosmological model is that three independent observations (as well as others) agree on the two free parameters of flatness and dark energy fraction. The three observations are the CMB (which confirms flatness), the Big Bang nucleosynthesis (which confirms that GR is working, given flatness, but we take that as an expectation given GR's other successes), and the type Ia SNe (which indicate that the flatness is comprised of 30% matter and 70% dark energy). This is like a tripod that overconstrains the model in the sense that if you significantly change any of the legs, the whole thing falls down. Hence, monkeying with type Ia SNe interpretations to reduce the amount of dark energy does not solve a problem, it creates one.

Jerry
2015-Apr-16, 08:10 PM
It is questionable as to whether or not it was a surprise. The early work on the cosmic microwave background and theory supporting inflation required an acoustic signature in the microwave background. When this signature was not found; it represented a 'cosmological crisis' or a failed theory. But the lack of the acoustic signature (basically an echo that was too weak) could be explained if there was another force that continued to pull the universe apart. Reiss and Perlmutter were well-aware of this dilemma, and it made it easy for them to conclude there were no flaws in their data analysis and that they had identified a force responsible for the very whimpy acoustic signature.

It has been known for a decade now that the UV signature of distant supernova events has a different light curve structure than less distant type Ia events; so there has been some degree of stubbornness in stepping away from prior conclusions.

Cougar
2015-Apr-16, 08:49 PM
This is like a tripod that overconstrains the model in the sense that if you significantly change any of the legs, the whole thing falls down.

I was rather surprised at the rapidity that dark energy was taken to fill ~70% of the flatness budget.

Margarita
2015-Apr-17, 09:58 AM
Thank you very much for the helpful responses to my queries.

What is of interest to me is whether, in the hypothetical case of supernovae type 1a never having existed in our universe, the evidence from other sources would have been seen to point inevitably to "dark energy". (I have a professional background in presenting evidence to a court and this influences my style of thinking)

I will certainly get the two books that you suggest, Cougar. It's helpful that they were both published in 2014.

Robert Fisher, the first named author of a recent paper (http://arxiv.org/abs/1504.00014)on superluminous SNe1a has been taking part in the discussion of the astrobite which explained his paper (http://astrobites.org/2015/04/07/super-bright-supernovae-are-single-degenerate/).

In one of his comments he said

The field is progressing so rapidly that material from 2013 and earlier can frequently be outdated already.

It certainly seems as tho this latest paper raises a great many issues. And thus is MOST interesting!

Ken G
2015-Apr-17, 08:44 PM
I was rather surprised at the rapidity that dark energy was taken to fill ~70% of the flatness budget.Yes, the data doesn't seem precise enough to say it fills in the hole exactly! I suppose it is a matter of Occam's Razor-- we weren't looking for Even Darker Energy, so it's simpler to attribute the entire hole to dark energy. If the Arizona claims prove true, then we'll need a third unknown physical term. Won't that be fun.

Jerry
2015-Apr-30, 04:38 AM
The Changing Fractions of Type Ia Supernova NUV-Optical Subclasses with Redshift http://arxiv.org/pdf/1408.1706v1.pdf This is a conclusion that myself, John Middleditch and other published more than a decade ago. Here is a sister paper:

Theoretical Clues to the Ultraviolet Diversity of Type Ia Supernovae http://arxiv.org/pdf/1408.1706v1.pdf

The authors are still working at understanding the 'normal' variety of events; and I still think this is a mistake; because the definition of 'normal' will naturally change if your cosmological model is wrong; and the cosmological model is what the researchers are trying to define in the first place!

If the skew of unusual events changes with distance something might be wrong. It is clear from these papers that the ration of "Red" to "Blue" events is 2:1 locally; but the ratio becomes progressively smaller with increasing distance. The authors admit that this is likely a selection effect; (the 'Red' events are of lower magnitude, and therefore less likely to be spotted with increasing distance), and this bias skews the distance scaling.

This is where 'normal' events no longer has any meaning; as any attempt to define 'normal' with increasing distance will focus on 'desirable' rather than 'outlier' data. Better understanding is needed of the broad spectrum of supernova events; and if there are inconsistencies in the mean ejecta velocity that are proportional to redshift (and there are); there may be systemic effects that have nothing to do with cosmic acceleration...and everything to do with a bad model theory.

Margarita
2015-May-01, 08:07 AM
Jerry, you've posted double links to the arXiv version of the Milne, Foley et al paper that is the subject of this thread and which is already in the OP.

The link to Theoretical Clues to the Ultraviolet Diversity of Type Ia Supernovae by Brown, Baron et al is http://arxiv.org/abs/1504.05237

Is this the paper by John Middleditch you were intending?

Core-collapse, GRBs, Type Ia Supernovae, and Cosmology
http://arxiv.org/abs/astro-ph/0608386

Margarita
2015-May-01, 08:45 AM
Just for interest, as I followed up my Google search results, this is a page about Middleditch's research.
http://public.lanl.gov/astroflash/

StupendousMan
2015-May-01, 01:29 PM
Many of the statements in Middleditch's summary of his research at the URL quoted above are not supported by the majority of astronomers. For example,


Ia's have been admitted by observers to be merger-core-collapse objects since Feb. 2007


there is likely no anomalous dimming of distant SNe, and thus no direct evidence for dark energy at all.


such jets from early SNe may help to seed star formation in linear structures up to around 100 kpc in length possilby[sic] assisting the formation of galaxies, with no dark matter required.

ShinAce
2015-May-04, 02:53 AM
+1 to StupendousMan's comments. Middletech's comments are dubious unless he can explain galactic rotation curves and the CMB's power spectrum.

ShinAce
2015-May-04, 03:10 AM
I'm very new to looking at cosmological issues, so would value greatly an outline of what these theoretical reasons are. Did they predate the SNe 1a related findings by the two eventual Nobel prize winners?


To my knowledge, these reasons do not predate the mid 1990's findings from type Ia supernovae. They include things like galaxy formation. Why aren't galaxies bigger, or smaller? Their size makes sense if 30% of the universe's energy budget is matter/dark matter. Also, what's up with the cold/hot spots in the microwave background? They are well explained by the 'integrated Sachs Wolfe effect', which is consistent with an early universe dominated by something other than matter. It could be from photons(radiation), or dark energy. Lastly, why is the universe flat if there isn't enough matter(dark or otherwise) to guarantee flatness? What is this thing that has such a large energy content yet negative pressure. Neither matter nor radiation can have negative pressure.

Cosmology seems to be going through a phase resembling particle physics many decades ago. There's evidence coming our of our ears these days, but the simplest theory has yet to emerge. Worse of all, if we were looking at the skies 50,000 after the big bang, we'd see that radiation dominated the energy content. If we looked 100,000 after the big bang, we'd see matter dominating the landscape. As we look today, we see the evidence of those previous phases, plus the dark energy 'universal solitude' that awaits us in the future. At that point in time, would we even be able to see the evidence we have today. Our timing is surprisingly spectacular for studying cosmology.

Jerry
2015-May-21, 05:09 AM
http://arxiv.org/abs/1505.04043 What do the cosmological supernova data really tell us?


This paper is featured in the 'Fun Papers' thread. It further explores the theme that the primary conclusions drawn from supernova light curve analysis are model dependent. It is interesting that they cannot constrain acceleration with supernova data, but they can with a combination of supernova and gamma ray data. I find this to be an unusual tack: There is far more supernova data; and we don't have solid calibration schemes for the gamma ray sample - redshifts and reddening are more uncertain.

publiusr
2015-May-23, 07:37 PM
So the standard candle ain't so standard after all.