new measurement of time dialation in distant supernova

From: "Jonathan Thornburg [remove -animal to reply]"
Subject: new measurement of time dialation in distant supernova
Newsgroups: sci.astro.research
Summary: nice measurement confirming 1/(1+z) cosmological time dilation
Keywords: cosmology, supernova, time dilation

While looking for something else, I just ran across a nice paper
which I'd like to bring to the attention of other s.a.r readers:

S. Blondin et al,
"Time Dilation in Type Ia Supernova Spectra at High Redshift"
Astrophysical Journal 682, 724-736 (2008)
http://adsabs.harvard.edu/abs/2008ApJ...682..724B
http://www.iop.org/EJ/abstract/0004-637X/682/2/724
http://arxiv.org/abs/0804.3595
http://www.cfa.harvard.edu/~sblondin.../timedilation/

Abstract:
We present multiepoch spectra of 13 high-redshift Type Ia supernovae
(SNe Ia) drawn from the literature, the ESSENCE and SNLS projects,
and our own separate dedicated program on the ESO Very Large
Telescope. We use the Supernova Identification (SNID) code of
Blondin and Tonry to determine the spectral ages in the supernova
rest frame. Comparison with the observed elapsed time yields an
apparent aging rate consistent with the 1/(1+z) factor (where z
is the redshift) expected in a homogeneous, isotropic, expanding
universe. These measurements thus confirm the expansion hypothesis,
while unambiguously excluding models that predict no time dilation,
such as Zwicky's ``tired light'' hypothesis. We also test for
power-law dependencies of the aging rate on redshift. The best-fit
exponent for these models is consistent with the expected 1/(1+z)
factor.

What's going on here is that the standard Friedman-Lemaitre-Robertson-
-Walker (FLRW) cosmology predicts that an object observed at a redshift
z should also be time-dilated by a factor 1/(1+z). It would be nice
to experimentally (observationally) test this prediction.

The tricky part in trying to do this is to find a "standard clock"
observable at high redshift. Past studies (described and cited in
the Blondin et al paper) have used the light curves (== time evolution
of brightness over periods of a month or so) of type Ia supernovas
(SNe Ia) observed at high redshift. These have reported results
consistent with a 1/(1+z) time dilation. However, it's hard to
avoid systematic errors in such sgudies, because it's known (from
observations of nearby SNe) that intrinsically brighter SNe Ia tend
to have a slower time evolution of brightness than intrinsically
fainter ones, and observational slection effects (Malmquist bias et al)
may lead to an observed sample of SNe having intrinsic brightness
correlated with redshift.

What Blondin et al do to try to avoid such systematic errors is to
use time changes in the *spectra* of high-redshift SNe Ia as their
"standard clock", rather than the light curves. That is, Blondin
et al describe an analysis which relates an observed spectrum to the
supernova's "age" (== time in its rest frame since it was at peak
optical brightness). Then have done this analysis for 13 high-redshift
SNe Ia observed at multiple epochs (typically 2-4 times for each SN,
for a total of 35 spectra).

There are a number of other clever aspects of Blondin et al's data
analysis, but I won't try to summarize them here -- read the original
paper if you're interested! The results are quite clear-cut: they
confirm the 1/(1+z) time dialation, and strongly refute Zwicky's
"tired light" model (in which there would be no time dialation).

All in all, a nice piece of work, and a fun paper to read.

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

In article , Jonathan
Thornburg writes:

apparent aging rate consistent with the 1/(1+z) factor (where z
is the redshift) expected in a homogeneous, isotropic, expanding
universe. These measurements thus confirm the expansion hypothesis,

I'd just like to point out that the 1/(1+z) factor would also be
expected at least in some inhomogeneous anisotropic universes. In a
universe like ours, which is almost homogeneous and isotropic, the
expectation is certainly very close to 1/(1+z), certainly any
discrepancy would be much less than the observational error. In other
words, an observation of this factor is a robust result indicating that
it arises from the expansion.

This factor and the redshift are really the same thing. One can think
of the frequency of electromagnetic radiation as a standard clock. Thus
the original wavelength is 1/(1+z) as long as the observed wavelength by
the same argument which applies to the supernovae. This is just the
normal redshift.

The interesting thing is that alternative theories which have some other
mechanism for the normal redshift ("tired light" etc) usually don't
predict time dilation in other clocks like a supernova light curve,
whereas in the expanding-universe paradigm, one implies the other since
they are really the same effect.

It is also good to keep in mind that 1/(1+z) tells us the ratio of the
size of the universe at the time the light was emitted to the size of
the universe when the light was detected and the redshift measured.
Without additional assumptions, it tells us nothing else. This means
that this expectation holds regardless of what we assume for the values
of Omega, lambda, H etc. It would even hold in a universe which expands
neither at the time of emission nor at the time of reception, like this:

R _______________
/
/
/
____________/
t

where the light is emitted during the first phase when R is independent
of t and received in the second phase when R is again independent of t.