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parallaxicality
2010-May-01, 02:23 PM
How reliable are cepheids or type 1a supernovae as standard candles? Can we be certain that their luminosities or fluctuations do not vary over the age of the universe? Are there better standard candles we might eventually be able to use?

Romanus
2010-May-02, 01:04 PM
Answer: Pretty reliable.

It's hard to say more than that, because standard candles are all interrelated in some way. For instance, Type Ia supernovae have very consistent behavior, but we know this only by comparing their brightness as determined using other standard candles for intergalactic distances (like Cepheids). Other methods, such as "tip of the red giant branch" are again dependent on other standard candle references. The overall result is a set of standard candles that are more accurate in sum than each taken alone.

What will help a great deal are missions like GAIA, that will record parallactic distances to numerous standard candles in our own Galaxy (RR Lyrae, Cepheids, red giants, planetary nebulae, and much else), and cut down on the error bars in general. The uncertainty will never go away completely though, partly because of the errors inherent in any measurement, and partly because there are other factors that influence standard candles (metallicity in RR Lyrae and Cepheid variables, age, mass uncertainties, etc.) that we'll probably never be able to fully correct for.

Jerry
2010-May-03, 05:17 AM
Type Ia supernovae are good distance indicators, but we are limited by possible evolution as you mentioned above; but also by several limitations:

1) It has recently been determined that the rise times of the most luminous events may be faster that average events while the fall times are longer. (for about two decades it has been assumed that both rise and fall times were both directly proportional to magnitude). This means the time of peak magnitude is very critical, but since observation periods are often a week apart; if the gestimate of when the peak occurs is wrong; so is the stretch factor applied to the supernovae.

2) There is some evidence that two families of events may create type Ia signatures; with the more brilliant family brighter in the UV spectra. we need more information about the UV spectra of local events before we can say with certainty the most distant events are statistically similar to local events.

3) There is tension in the various supernova studies; indicating that the various assumptions made during data reduction may contain unsuspected systemic errors.

For now, this constraints appear to only limit cosmological conclusions drawn from the most distant events; and future studies many either tighten or loosen current constraints.

Kwalish Kid
2010-May-03, 10:35 AM
Type Ia supernovae are good distance indicators, but we are limited by possible evolution as you mentioned above; but also by several limitations:

1) It has recently been determined that the rise times of the most luminous events may be faster that average events while the fall times are longer. (for about two decades it has been assumed that both rise and fall times were both directly proportional to magnitude). This means the time of peak magnitude is very critical, but since observation periods are often a week apart; if the gestimate of when the peak occurs is wrong; so is the stretch factor applied to the supernovae.
But the practice for over a decade now is to use supernovae that have been observed since peak.

2) There is some evidence that two families of events may create type Ia signatures; with the more brilliant family brighter in the UV spectra. we need more information about the UV spectra of local events before we can say with certainty the most distant events are statistically similar to local events.
But there is no such evidence.

Cougar
2010-May-03, 02:09 PM
How reliable are cepheids or type 1a supernovae as standard candles? Can we be certain that their luminosities or fluctuations do not vary over the age of the universe?

Cepheids are reliable, but not bright enough to see at cosmological distances. My 5-second search came up with M100 as the most distant galaxy in which a cepheid has been spotted - and estimated at 56 million lightyears. Even if that find is dated, I doubt any cepheid has been seen beyond one-tenth of a billion lightyears. So cepheid evolution would be unknown.

Jerry
2010-May-04, 05:11 AM
arXiv:1005.0026

The Role of Variations of Central Density Of White Dwarf Progenitors Upon Type Ia Supernovae


Recent observations of SNe Ia have indicated a significant population difference depending on the host galaxy. These observational findings are consistent with SNe Ia Ni-56 production in star-forming spiral galaxies some 0.1 solar masses higher - and therefore more luminous than in elliptical galaxies. We present recent full-star, 3D simulations of Type Ia supernovae which may help explain the nature of this systematic variation in SNe Ia luminosities, as well as the nature of the Ia explosion mechanism. These insights may in turn eventually shed light on the mystery of dark energy itself.