Accelerated expansion: is the jury still out?

[Chronos]

I thought this was interesting:

High Redshift Supernovae: Cosmological Implications
http://arxiv.org/abs/astro-ph/0502247

... it appears that the problem of whether there is an acceleration in the Universe, and if so what its strength is, is not definitely solved and that much work is still needed before reaching the final conclusion.

 

[ohwilleke]

The gist of the argument (from the abstract): "We also discuss the possibility and the consequences of the fact that SNIa may not be "perfect" standard candles, in the sense of having properties in the early Universe that are systematically different from those they have at the present times."

 

[Garth]

From the study by Mannucci et al. (2005), it appears that in the local Universe there are two distinct populations of progenitors of SNIa, each characterized by very different delay times, i.e. characteristic times between star formation and stellar explosion. About half of the SNIa originate from a relatively young stellar population, whose progenitors explode after a short delay time (say, <100 Myrs) since their formation(“prompt” SNIa), and may be the result of the merging of two degenerate stars in a binary system. The other half (“tardy” SNIa) comes from older stellar populations and their progenitors explode with delay times of 2–4 Gyrs

Nino goes on to say that as the SNIa luminosity is calibrated from local galaxies, these would tend to be the older systems, whereas the distant SNIa would tend to be the younger ones. Hence

As a result, all quantitative conclusions about the acceleration of the Universe would have to be drastically revised, if not even reversed (i.e., there might not be any acceleration...).

Remember the problems calibrating the Cepheid Variable 'standard candle'? A error of factor of about three was introduced because it was not realized that there were two types of Cepheid Variables, belonging to Population I and Population II stars respectively.

Methinks we may be making the same mistake again here!

Just a thought!

Garth

BTW Chronos thank you for that very interesting link.

 

[Chronos]

I share those sentiments, Garth. I've never been comfortable with the notion the universe suddenly changed speed. The explanations offered appear large and jagged [hard to swallow]. As long as I'm on a roll, there is room to doubt the Cepheid yardstick is fully calibrated:

Population II standard candle calibration ...
http://arxiv.org/abs/astro-ph/0303035

The Status of the Distance Scale
http://arxiv.org/abs/astro-ph/9911296

Supernova type Ia luminosities, their dependence on second parameters, and the value of H_0
http://arxiv.org/abs/astro-ph/0004063