
Supernova’s messy birth casts doubt on reliability of astronomical yardstick - okket
http://www.nature.com/news/supernova-s-messy-birth-casts-doubt-on-reliability-of-astronomical-yardstick-1.22066
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antognini
The headline here is a bit of an overstatement. Nothing in the original paper
is casting doubt on Type Ia supernovae as reliable standard candles. It is
definitely an embarrassment that we still don't know the mechanism by which
Sne Ia occur, but empirically their luminosities can be calibrated well enough
that the accelerating expansion of the universe is not in doubt. However, to
determine the nature of dark energy (specifically, its equation of state), it
may be necessary to calibrate SN Ia luminosities better, and that will
probably require understanding exactly what they are.

There are two major SN Ia models: the double degenerate, which involves two
white dwarfs colliding, and the single degenerate, which involves one white
dwarf accreting matter from a regular star. There have been two approaches to
distinguishing between the two models. One is to look for hydrogen in the
spectrum a long time after the supernova. In the single degenerate model the
supernova shock will strip hydrogen from the companion star's atmosphere, but
in the double degenerate model, there will be no hydrogen to strip.

The other method is to look at the very early light curve of the supernova, as
this paper did. The early light is expected to rise in a straight line as the
surface area of the shock expands. A deviation from this straight line,
particularly in the blue bands, then could indicate some interaction between
the shock and a companion. This paper found such a "blue bump" in the early
light curve of a recent supernova. (There has been at least one earlier paper
that also found a blue bump in another supernova.)

I'm out of the field now, so my knowledge isn't quite up to date, but my
impression is that the observational evidence may be pointing to a mix between
the two mechanisms. For anyone interested, I wrote a little bit about this
problem here:

[https://joe-antognini.github.io/astronomy/typeia-progenitors](https://joe-
antognini.github.io/astronomy/typeia-progenitors)

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JumpCrisscross
How would you explain, to a layman, why standard candles--phenomena we must
measure from great distances--are trusted to be as consistent as well believe
them to be?

~~~
carljoseph
It's essentially built up of a chain or ladder. From parallax measurements to
cepheid variables to supernova. This might give a good introduction
[http://www.iop.org/resources/topic/archive/cosmic/](http://www.iop.org/resources/topic/archive/cosmic/)

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ohthehugemanate
I always hate it when an article drops that we've "proven the existence of
dark matter". We've done no such thing. Usually that sentence describes some
of the proof of the fact that our matter and energy predictions are off by an
order of magnitude... Proof of the problem that dark matter/energy theories
attempt to resolve. I know it's a nitpick, but it drives me nuts.

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pdonis
_> we've "proven the existence of dark matter"_

The article says we've proven the existence of dark _energy_ , not dark
matter. They're not the same thing.

That said, if it does turn out that the SN1a "standard candle" scale has to be
revised, that might impact the finding of accelerated expansion on which the
dark energy model is based. It will be interesting to see how other
cosmologists respond to this paper.

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sgt101
Do cosmologists allow for the doping of the matter involved in these
interaction due to stella aging? It seems to me that more distant supernovas
must act differently from the closer ones because they are much younger and
therefore undoped by earlier supernova debris.

~~~
pdonis
As I understand it, the assumption behind using SN1a as standard candles is
not that they all have exactly the same absolute brightness, but that there is
a reliable relationship between the light curve (the curve of apparent
brightness vs. time) and the absolute brightness. This relationship takes into
account variation in the chemical composition of the supernova, which is what
"doping" would affect.

However, what this particular paper seems to be saying is that there might not
be just one light curve-brightness relationship, but two, corresponding to two
different explosion processes. If that's true, astronomers would need to go
back and review the SN1a data with this new classification scheme in mind.

