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Type Ia Supernovae As Not-Quite-So-Standard Cosmological Candles

Soulskill posted about 5 months ago | from the exploding-suns dept.

Space 33

Shag writes "Type Ia supernovae are used as cosmological 'standard candles' to measure distance because of their strong similarity to one another. This has made possible, for example, the research into universal expansion that led to the Nobel-winning discovery of 'dark energy.' For years, astrophysicists believed white dwarves exploded when they accreted enough mass from companion stars to reach a limit of 1.38 times the mass of our Sun. A decade ago, the 'Champagne supernova' (SN 2003fg) was so bright astrophysicists concluded the limit had been exceeded by two white dwarves colliding. Now a new paper (PDF) from the Nearby Supernova Factory collaboration suggests that type Ia supernovae occur at a wider range of stellar masses. Fortunately, there appears to be a calculable correlation between mass and light-curve width, so they can still fill the 'standard candle' role, and research based on them is probably still valid. (I took data for the paper, but am not an author.)"

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33 comments

calculations still probably valid (0)

Anonymous Coward | about 5 months ago | (#46412541)

So how far is it to the center of the galaxy this year, given the DOW/NASDAQ are what they Are:)

Reassuring (1)

Dan East (318230) | about 5 months ago | (#46412557)

research based on them is probably still valid

That's probably reassuring.

Re:Reassuring (2)

i kan reed (749298) | about 5 months ago | (#46412681)

If you don't end your paper with "Further research is needed" a secret cabal of other researchers also applying for grants in your field come into your bedroom at night, drag you off, and tie you to a post in the beam of the LHC.

Re:Reassuring (1)

Shag (3737) | about 5 months ago | (#46413235)

I thought "Further research is needed" was code for "we have seventeen more papers in the works, all of which will cite this one and each of the others, thus inflating our g-index and h-index numbers."

Re:Reassuring (2)

Ol Olsoc (1175323) | about 5 months ago | (#46413969)

I thought "Further research is needed" was code for "we have seventeen more papers in the works, all of which will cite this one and each of the others, thus inflating our g-index and h-index numbers."

No, tha's the difference between researchers and teh political types. Research only raises more questions, and politicians know all the answers before any research is done.

Re:Reassuring (1)

sFurbo (1361249) | about 5 months ago | (#46416777)

Doesn't (at least) the h-index exclude you own citations? So you have to get somebody to cite your paper in a paper you are not coauthoring.

Re:Reassuring (1)

K. S. Kyosuke (729550) | about 5 months ago | (#46413299)

come into your bedroom at night, drag you off, and tie you to a post in the beam of the LHC

They would do anything for their large hardons, wouldn't they?

God created the heavens. (0)

Anonymous Coward | about 5 months ago | (#46412573)

It's not for us to question it? Learn your bible boy! Just kidding, folks.

Re: God created the heavens. (0)

Anonymous Coward | about 5 months ago | (#46412609)

Oh, and thanks for posting something scientific on slashdot. This is why I come here.

Re:God created the heavens. (1)

maxwell demon (590494) | about 5 months ago | (#46413027)

That's why astrophysicists concentrate on the sky instead.

so how far off is this? (2)

Charliemopps (1157495) | about 5 months ago | (#46412831)

Our entire understanding of how the universe works is largly based on these measurements... so this is potentially a very big deal. Any Astrophysicists around that can give us an idea of how big a difference this will make?

Re:so how far off is this? (4, Informative)

Shag (3737) | about 5 months ago | (#46413215)

I don't think it's going to make a difference. In fact, I'm not quite sure whether the dark energy research that got the Nobel was strictly limited to type Ia supernovae - it was before my time, and since they were using high-Z (very distant) supernovae, they might have wanted more massive type II ones, or something.

For about a decade, people have accepted that some SNe Ia are "over the limit" (under arrest!) and have developed "double-degenerate" models of colliding white dwarf stars. As sky surveys discover more and more, it's started to become apparent that there are also some "under the limit." This project has studied hundreds of supernovae over the last decade, and looked pretty closely at how they evolve over time. The reassuring part of the paper is that even though these supernovae are nowhere near all the same mass blowing up every time, they're still within a reasonably sensible range (0.9 to 1.4 solar masses) and that by watching what brightness they reach at their peaks, and how quickly they decline in brightness, and looking at their spectral curves (all of which are among the things that this particular collaboration looks at), astrophysicists can calculate their masses, and thus make any necessary adjustments to compensate for that. And by the standards of astrophysicists and cosmologists, the math required to "standardize" progenitors of different masses is probably considered "easy." Of course, these are the same people who think "nearby" means 0.4-1.0 billion light years away...

Disclaimer: I am not an astrophysicist of any kind. I got involved in astronomy a decade ago, and took a few classes 5 years ago, but my roles are overwhelmingly technical or operations, and when it comes to science, I am always the "village idiot" surrounded by PhD's. I'm not the guy who'll give a lecture about what the telescope's pointing at - I'm the guy who'll fix the telescope so it points at it in the first place. I'll take data - in this particular case, over a 10-year project, I'll probably rank #1 or #2 in terms of amount of time spent taking data - but I don't do the analysis or write the papers. My background was in things like systems administration, spamfighting, web development, etc., as one would expect of someone with my user number here.

Re:so how far off is this? (5, Informative)

boristhespider (1678416) | about 5 months ago | (#46413391)

" In fact, I'm not quite sure whether the dark energy research that got the Nobel was strictly limited to type Ia supernovae..."

No, they were definitely intended to be SN1a.

Riess et al.: http://arxiv.org/abs/astro-ph/... [arxiv.org]
"We present observations of 10 type Ia supernovae (SNe Ia)..."

Perlmutter et al.: http://uk.arxiv.org/abs/astro-... [arxiv.org]
"...All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation..."

The reason is that SN1a can be standardised -- although that's an empirical (i.e. phenomenological) relationship rather than a theoretical one, it seems to be basically robust, as this paper has demonstrated -- and therefore used as standard candles. Other types of supernovae can not be used in the same way; one cannot necessarily correlate a (corrected) brightness against a (corrected) redshift.

This doesn't say that samples aren't contaminated by supernovae that aren't actually Type 1a (and a few years back an explanation for tension between the so-called "Gold sample" and other datasets was that it may have been more contaminated), but the intention is to only look at Type 1as.

I'd also argue that they weren't particularly high redshift, but then for me a redshift of 3 or 4 is very much low redshift. Come to that, redshifts of 300 are low redshift.

Re:so how far off is this? (1)

Shag (3737) | about 5 months ago | (#46414089)

for me a redshift of 3 or 4 is very much low redshift. Come to that, redshifts of 300 are low redshift.

Just as long as you don't claim things are "nearby." Even at the redshifts we deal with (0.03-0.08), you can't get a pizza delivered in 30 minutes or less.

Re:so how far off is this? (1)

boristhespider (1678416) | about 5 months ago | (#46414137)

At redshifts of 0.03-0.08 you can hardly see the Hubble flow, man!

Re:so how far off is this? (1)

Anonymous Coward | about 5 months ago | (#46417645)

Actually, you can also use type IIP supernovae (the explosions of stars 8-15 times the mass of our Sun, when their cores collapse). It's part of what Brian Schmidt did his Ph.D. thesis on at Harvard!

Schmidt et al.: http://adsabs.harvard.edu/abs/1994ApJ...432...42S
"We have used observations gathered at Cerro Tololo Inter-American Observatory (CTIO) to measure distances by the expanding photosphere method (EPM) to five Type II supernovae."

For more current work see Nugent et al.: http://adsabs.harvard.edu/abs/2006ApJ...645..841N
"We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based on five events at redshift up to z~0.3."

But we haven't yet figured out how to standardize them *quite* as well as type Ia supernovae. And even if we succeeded, they are on average substantially fainter than type Ia supernovae, so much harder to get a good lever arm in redshift. Still, they provide a useful cross-check with a different method that the SN Ia stuff is right, and SN IIP cosmology may have its day at some point if there's a place SNe Ia can't reach for whatever reason.

Re:so how far off is this? (1)

boristhespider (1678416) | about 5 months ago | (#46423613)

Mod points anyone still reading this thread please - this is very interesting stuff.

I must admit to my shame that using supernovae type II for distance measurements hasn't really been on my radar at all, although I must have been in quite a few talks discussing it. Anything that can add redundant checks to the distance ladder absolutely has to be pursued.

Re:so how far off is this? (0)

Anonymous Coward | about 5 months ago | (#46413629)

I'd like to know more on the physical-mechanism end. I can easily understand a white dwarf slowly accumulating mass from a large companion star, then going KA-BLOOEY when a mass limit is exceeded. How then does this recent research propose to expand the cases in which these things gain mass?

Re:so how far off is this? (1)

buchner.johannes (1139593) | about 5 months ago | (#46413851)

SN1a are only one of the tools astronomers use: https://en.wikipedia.org/wiki/... [wikipedia.org]
The small distance measures have to match with the medium ones and those again with the largest distance measuring tools. Also on the same level, they should agree.

The benefit of SN1a is that they are abundant, and their method seems to have particularly small systematic uncertainties. Other methods for computing distances are for instance Baryonic acoustic oscillations, which also provide a scale.

supernovae measurements not the only reason for da (0)

Anonymous Coward | about 5 months ago | (#46416843)

The measurement of an acceleration in the expansion of the universe in '98, actually solved a big puzzle for cosmologist trying to match the growth of structure with the cosmological parameters. In hindsight the clues for a universe with dark energy were already there. The combined measurements from the cosmic background radiation, large scale clustering of galaxies, and SNe distances agree on our current model of understanding the universe. Any combination of two of those experiments is enough to constrain the parameters that go into this model. That all three overlap is why we can be confident that the SNe experiment is at the very least reasonably accurate.
- anonymous coward astrophysicist

This is the Phillips relation - known for 20 years (5, Informative)

Anonymous Coward | about 5 months ago | (#46412967)

Mmm... press release spin.

There is good science here, but it is being heavily spun. The relation between light curve width and how bright SN are has been known since at least 1993 (Phillips, M., 1993, ApJ 413, L105). This was corrected for even in the original work that won the Nobel prize. So, the 'they aren't quite so standard candles' has been known for 20 years - what they are is 'standardizable' candles.

What is interesting about this work is that the SNFactory has tried to find a link between the Phillips relation and physical properties of the explosion - in this case the ejected Nickel 56 mass. And that is very interesting from a SN physics perspective, if not really so much from the 'doing cosmology with Type Ia Supernovae' one.

Re:This is the Phillips relation - known for 20 ye (1)

neurostar (578917) | about 5 months ago | (#46413367)

And to add, this is why people often refer to Ia's as "Standardizable Candles". :)

Re:This is the Phillips relation - known for 20 ye (3, Informative)

Anonymous Coward | about 5 months ago | (#46414045)

Hello! I read the same press release, but I don't see the SuperNova Factory taking credit for discovering the relationship between light curve width and luminosity (the Phillips relation, which is indeed well-known, and made the discovery of Dark Energy with Type Ia supernovae possible). So if there is any spin here, it's not on that axis. The press release even acknowledges standardization to about 10% in distance.

Rather, the paper is about explaining this relationship in terms of the underlying physics as you point out. Interestingly, instead of the driver being the amount of radioactive nickel synthesized during the explosion (which is a good candidate for your main lever arm if you believe that total mass of Type Ia progenitors was fixed at/near the Chandrasekhar mass), it is the total mass itself that drives the relationship. It means that the conventional wisdom that the Chandrasekhar mass is somehow important needs another look.

And indeed, this paper *is* interesting also from the "doing cosmology with Type Ia supernovae" perspective. We don't know very well what kinds of white dwarfs and star systems can give us Type Ia supernovae. If there are many different channels, or a continuum of channels, or the distribution of masses of white dwarfs that make Type Ia supernovae somehow evolves with redshift, we need to understand that.

Re:This is the Phillips relation - known for 20 ye (3, Informative)

Shag (3737) | about 5 months ago | (#46414317)

AC1:

The relation between light curve width and how bright SN are has been known since at least 1993 (Phillips, M., 1993, ApJ 413, L105). This was corrected for even in the original work that won the Nobel prize. So, the 'they aren't quite so standard candles' has been known for 20 years - what they are is 'standardizable' candles.

AC2:

I don't see the SuperNova Factory taking credit for discovering the relationship between light curve width and luminosity (the Phillips relation, which is indeed well-known, and made the discovery of Dark Energy with Type Ia supernovae possible).

Well... the Phillips Relationship is "well-known" in much the same way that these supernovae are "nearby" - to the people in that specific very narrow field of expertise. Yes, Wikipedia has an article on it, but I'd expect it to be unknown to the average adult walking down the street, the average amateur astronomer, the average Jeopardy contestant, the average undergraduate or first-year graduate astronomy student, or even the average science popularizer who isn't specifically dealing with supernovae. Just last month, I overheard one long-time amateur astronomer still telling tourists at the Mauna Kea Visitor Information Station that all SNe Ia are the same mass and brightness!

But anyway, as the 2nd AC said, the newer/more interesting bit is the relationship to progenitor mass, and the continued trend toward SNe Ia coming from diverse progenitors - i.e. the more we look, the more "exceptions to the rule" we find. We're already to the point where it looks like most SNe Ia aren't from single, Chandrasekhar-mass progenitors as was long thought to be the "norm," and the paper discusses some models for progenitors of varying masses that meet with varying degrees of success in attempting to match the observational results. I suspect the computational / theoretical / modeling folks will also have fun with it all.

Re:This is the Phillips relation - known for 20 ye (2, Informative)

Anonymous Coward | about 5 months ago | (#46416871)

Hi all, I'm the lead author on the paper who got linked here by a colleague. I agree the Phillips relation is very well-known and well-established *empirically*, and that it is not endangered, nor is my boss going to have to give back his Nobel Prize because we found this out. But understanding this well-known empirical relation theoretically, from first principles, is a real challenge, and it's part of what's preventing us from further improving their performance as "standard candles". We need to continue to refine our understanding of SNe Ia to go to the *next* level in SN Ia cosmology, where we try to figure out what particle or field is responsible for causing the acceleration in the Universe's expansion. So understanding the physics of the supernova explosions is absolutely on the critical path to doing cosmology. The alternative approach is to just accumulate ten million SNe and self-calibrate, which is what LSST is plannign to do. But that will certainly take a lot of time on a half-billion-dollar telescope, to the extent that it's possible without taking spectra of each SN.

There are two main results here. One is the discovery that the ejected mass may be what's driving the Phillips relation, which was suggested before by theorists but never conclusively shown (what about asymmetries? 56Ni mass? weird ejecta density profiles?). The other is simply the fact that a large fraction of normal, non-peculiar SNe Ia may well not be Chandrasekhar-mass, thereby giving us a *lot* of information about how they blew up, and possibly how they evolved to the point of blowing up as well. So it touches on cosmology, but also on stellar evolution, chemical evolution of galaxies, basically every area of astrophysics.

Part of why the Chandrasekhar-mass picture lasted this long is because people didn't think you could standardize non-Chandrasekhar-mass explosions. But you can, and the differences in ejected mass may in fact be what allow us to do that.

(Ah, I see another anon has posted the same thing below. +2!)

Best,
RS

Mod parent up (0)

Anonymous Coward | about 5 months ago | (#46417409)

Right from the horse's mouth, folks.

Talks at Google (1)

mikesum (840054) | about 5 months ago | (#46413515)

Talks at Google has a great lecture by Alex Filippenko, called "Dark Energy and the Runaway Universe." There is a wonderful section talking about supernovae as a standard candle from about 17 minutes to 36 minutes, but I recommend the whole thing, as I find it fascinating. http://youtu.be/Guvv5olLxCQ [youtu.be]

Best guess (1)

Anonymous Coward | about 5 months ago | (#46413877)

Most astronomy and astrophysics is best guess under ideal circumstances... Some of the guesses are very good indeed though. Dark energy is just an "x" variable in our mathematical understanding of gravity and space-time. Calling it (dark stuff) a 'discovery' is a bit misleading, as it hasn't actually been, it's only been added as a placeholder! I believe it might be the result of a combination of error terms in the measurements that aren't yet known and a time-geometry, space and time might knot be flat folks.

And there could be several explainations. (1)

mmell (832646) | about 5 months ago | (#46414879)

Perhaps Gravity is not constant throughout the Universe. Brane theory suggests that the scalar field called gravity may be subject to local variations in strength (which are pretty hard to measure when your species and their technologies are planet-bound). Perhaps some older theories of galaxy formation have merit, and gravity waves compress different stars differently (because they're in different places) - causing galaxies to have spiral arms and type 1a supernovae to display some variability in total energy released. Perhaps there's dark matter (the real kind, not the made-up kind) which is also feeding into 1a supernovae, causing some variability. Perhaps somebody was shining a green laser-pointer at the star first.

Okay, strike the last . . . but I can sure come up with more to add. Scientists with the correct equipment and training are going to have to make more observations, propose a hypothesis, test it, fix the details of their theory, test it . . . you know the drill.

Re:And there could be several explainations. (0)

Anonymous Coward | about 5 months ago | (#46417603)

Perhaps Gravity is not constant throughout the Universe.

That is an interesting theory, on the same level as "I don't know why things are this way, probably because God made it so."
The measurements doesn't fit the model, one of them is wrong. Gravity isn't constant essentially means "Claiming that the standard model is wrong is possibly career ending and I don't want to say that my measurements are useless so let's just say that the model is right but not over there."

Ouch! (1)

cyn1c77 (928549) | about 5 months ago | (#46416203)

(I took data for the paper, but am not an author.)

It's pretty sad when the 35 authors can take paper space acknowledge the culturally significant role that the observatory site has for the indigenous Hawaiians, but can't specifically acknowledge the people who took the data.
 

Re:Ouch! (1)

Shag (3737) | about 5 months ago | (#46416485)

It's pretty sad when the 35 authors can take paper space acknowledge the culturally significant role that the observatory site has for the indigenous Hawaiians, but can't specifically acknowledge the people who took the data.

Uh, they can, and they did, but you'd have to read the remainder of the paragraph to discover that - and for some reason, they don't refer to me by my Slashdot username. ;) Occasionally I'll also proofread papers (I know more about English than astrophysics or cosmology) and get thanked for that, too.

The author list for peer-reviewed stuff is mostly full members of the collaboration - folks with Ph.D's, folks doing their Ph.D's on collaboration stuff, folks who wrote the custom data warehouse software, and of course Saul Perlmutter. (It never hurts to have someone with a Nobel in your author list, right?) I'm not a Ph.D, and probably never will be, although I'd like to finish my MSc someday. I am on the broader authors list for occasional "Astronomer's Telegram" announcements we send out after taking spectra of a newly discovered thing and figuring out what type of SN it is, how far pre- or post-maximum it is, and how far away it is.

ain't science fun (0)

Anonymous Coward | about 5 months ago | (#46420509)

If you claim to know all the answers, then I can safely say that definitely you are NOT a scientist

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