relativity question re supernovae

[I've taken the liberty of adding a cross-post to sci.astro.research,
as lots of cosmology types hang out over there. Conflict-of-interest
disclaimer: I am the s.a.r deputy moderator.]

I'll assume a standard hot-big-bang cosmology throughout this posting,
but the details of dark matter, cosmological constant, etc, don't matter.

Cyberkatru wrote:
One of the first things that one learns from relativity is the idea that
simultaneity is not an absolute notion. *When* something happened, and
*where* it happened are dependent on the observers state of motion. Now our
state of motion is not special.

In cosmology there _is_ a special state of motion: that in which
the cosmic microwave background radiation (CMBR) is closest to
isotropic over the sky.

Correspondingly, there _is_ a special "cosmic time coordinate".
It's usually defined in terms of the CMBR temperature, but it should
also coincide with proper time for the CMBR-closest-to-isotropic
observers.

So how is it that astronomers so nonchalantly speak about a supernova
explosion that we see now, as really happening at the "same time" as some
other old event such as the earth moon system forming. (I am watching an
astronomy thing on the science channel right now).
They seems to speak as if every cosmic event was ordered into before and
after! Doesn't that violate the spirit of relativistic thinking?
Are they just picking some reference (fiducial) foliation by spacelike
3-surfaces perhaps picked out becuase of the leaves being approximately
totally geodesic or some other geometric fact coming from the presummed
solution of Einstein's equations being used?

Yes. They're assuming a cosmic time coordinate.

You should also realise that scientists almost always use time
measured in "years before today" for extragalactic/cosmological
events -- we almost always use "redshift" as a time coordinate.
This terminology is slightly confusing to the uninitiated, but
is much more convenient: Observations often tie directly into
redshift (i.e. "we observed this supernova at a redshift of z=1"
while you need a cosmological model (with its associated
uncertainties) to map that into a statement like "the light we
observed from this supernova was emitted 5.8 billion years ago".
(I'm just making up that number "5.8 billion", I haven't worked
out the actual number.)

This use of redshift as a time coordinate is only useful for a
special class of cosmological models... but since this "special class"
includes all the widely-accepted models, this usage is common.

For more details, see (at an introductory level):
Ned Wright's web pages
http://www.astro.ucla.edu/~wright/cosmolog.htm
and/or Harrison's book
@BOOK {EHarrison81a,
AUTHOR = "E. R. Harrison",
TITLE = "Cosmology, the science of the universe",
PUBLISHER = "Cambridge University Press",
YEAR = "1981",
ADDRESS = "Cambridge"}
[This is a *fantastic* elementary book (no calculus needed) with a
*lot* of physical insight. I think there's now a 2nd edition out.]

For more details at a beginning-graduate-student level, see
P. J. E. Peebles,
Principles of Physical Cosmology,
Princeton University Press, 1993

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"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 -- remove -animal to reply wrote:

You should also realise that scientists almost always use time
measured in "years before today" for extragalactic/cosmological
events -- we almost always use "redshift" as a time coordinate.

The first "always" should really be a "never," right?

-Ted

--
[E-mail me at , as opposed to .]

In article , I wrote
scientists almost always use time
measured in "years before today" for extragalactic/cosmological
events -- we almost always use "redshift" as a time coordinate.

Sorry, what I should have said was
scientists almost never use time
^^^^^
measured in "years before today" for extragalactic/cosmological
events -- we almost always use "redshift" as a time coordinate.

Thanks to Ted Bunn for pointing out this mistake!

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"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

Jonathan Thornburg wrote:
In cosmology there _is_ a special state of motion: that in which
the cosmic microwave background radiation (CMBR) is closest to
isotropic over the sky.

Correspondingly, there _is_ a special "cosmic time coordinate".
It's usually defined in terms of the CMBR temperature, but it should
also coincide with proper time for the CMBR-closest-to-isotropic
observers.

Jonathan knows this, but for the benefit of other readers it might be
worth mentioning that the CMBR reference frame is "special" only in the
sense that it is uniquely defined for all observers. The equations of
physics don't look any different (as far as is known!) if they are
written for that frame or any other inertial one. An analogy is that
we can all determine UTC from the broadcast services, but local
physical experiments could equally well use any other convenient time
reference and get the same answer.

There have been experiments -- notably the Michelson-Morley experiment
but many more recent ones as well -- to look for effects that might
depend on "preferred" reference frames, but so far all have been
negative.