Question about the expansion of the universe.

Ok, right now the data we have tells us that the universe is expanding. We
can tell due to the redshift. But I was wondering since the light from these
distant objects is many billions of years old, wouldn't this be telling us
about the expansion x number of years ago?

Dear Fitzdraco:

"Fitzdraco" wrote in message
...
Ok, right now the data we have tells us that the universe is expanding.
We
can tell due to the redshift. But I was wondering since the light from
these
distant objects is many billions of years old, wouldn't this be telling
us
about the expansion x number of years ago?

It tells us that the expansion is greatest for the most distant objects,
yes. But redshift occurs in decreasing amounts up until you get into our
supercluster. There is no evidence that anything has slowed the expansion.
So far, it has been determined that expansion is accelerating.

David A. Smith

"Fitzdraco" wrote in message ...
Ok, right now the data we have tells us that the universe is expanding. We
can tell due to the redshift. But I was wondering since the light from these
distant objects is many billions of years old, wouldn't this be telling us
about the expansion x number of years ago?

Its much more that the expansion x number of years ago.

Edwin Hubble discovered that the universe was expanding in the lat 1920's.
Astronomers spend another 80 years pinning down the constant of expansion.
Current data indicates that not only is the universe expanding, but it is
accelerating.

The light from a distant galaxy has to travel vast distances to reach a
telescope on earth. The star/quasar/galaxy that emitted that light
emits a characteristic signature (its spetrum). Now over the vast
distance from there to our telescopes here, the light may encounter
various clouds of gas and dust that imprint their own spectral features
on the light that arrives at our telescopes. By analysing the light,
astronomers see not only the characteristics of the star/quasar/galaxy
that emitted the light, but also those of all the interveining clouds
of gas. Careful analysis using powerful spectrographs can separate the
features and allow each object to be studied directly.

This tells up of A) distance to the object, B) composition of the
boject, C) distance to gas glouds, D composition of the gas clouds.

A) and B) tell us of what the universe was "like" way back when the
light was emitted. C) and D) tell us of what the universe was like
when the light passed through the gas.

By looking at vast numbers of light emitters and carefully piecing
together a consistent story, the history of the universe unfolds.

Mitch

"Mitch Alsup" wrote in message
om...
"Fitzdraco" wrote in message
...
Ok, right now the data we have tells us that the universe is expanding.
We
can tell due to the redshift. But I was wondering since the light from
these
distant objects is many billions of years old, wouldn't this be telling
us
about the expansion x number of years ago?

Its much more that the expansion x number of years ago.

Edwin Hubble discovered that the universe was expanding in the lat 1920's.
Astronomers spend another 80 years pinning down the constant of expansion.
Current data indicates that not only is the universe expanding, but it is
accelerating.
Right, the super nova study seems to confirm that.

SNIP
Excelent explanation but it wasn't quite what I was asking.
This tells up of A) distance to the object, B) composition of the
boject, C) distance to gas glouds, D composition of the gas clouds.

A) and B) tell us of what the universe was "like" way back when the
light was emitted. C) and D) tell us of what the universe was like
when the light passed through the gas.
I never considered about the expansion rate at the gas cloud. I read a lot
of laymans books and that has never been mentioned. I guess I need to move
up to higher learning.
By looking at vast numbers of light emitters and carefully piecing
together a consistent story, the history of the universe unfolds.

Mitch
Ok a better way to ask the question would be this. If in say the recent
history of the univers, say last million years the expansion slowed
considerably. Would we have to wait 10 billion years or so to regester that
change or would we see it now?

I just had a new thought. If the expansion changed the light would in a
sense relax( I know your not supposed to use human feelings for inanimate
objects but I'm tired and that's the best I can think of) and be redshifted
less.

In article ,
"Fitzdraco" writes:
Ok, right now the data we have tells us that the universe is expanding. We
can tell due to the redshift. But I was wondering since the light from these
distant objects is many billions of years old, wouldn't this be telling us
about the expansion x number of years ago?

Yes. The redshift of a distant object tells us about the expansion
along the entire path that the light has taken to reach us.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

In article ,
"Fitzdraco" writes:
If in say the recent
history of the univers, say last million years the expansion slowed
considerably. Would we have to wait 10 billion years or so to regester that
change or would we see it now?

Someone in another newsgroup just posted a simpler way to look at
this question. Cosmologists define a "scale length," which can be
thought of as the distance between two points that participate in the
expansion but are not affected by any local forces. In the simplest
cosmology (not consistent with observations but easy to think about),
the scale length increases linearly with time.

For any object and any observer, the observed redshift is the ratio
of the scale lengths when the light was observed and when it was
emitted. What the expansion rate did in between -- sped up, slowed
down, went backwards :-) -- doesn't matter. All you need to know to
calculate redshift is the ratio of scale lengths.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote
In article ,
"Fitzdraco" writes:
If in say the recent
history of the univers, say last million years the expansion slowed
considerably. Would we have to wait 10 billion years or so to regester that
change or would we see it now?

Someone in another newsgroup just posted a simpler way to look at
this question. Cosmologists define a "scale length," which can be
thought of as the distance between two points that participate in the
expansion but are not affected by any local forces. In the simplest
cosmology (not consistent with observations but easy to think about),
the scale length increases linearly with time.

For any object and any observer, the observed redshift is the ratio
of the scale lengths when the light was observed and when it was
emitted. What the expansion rate did in between -- sped up, slowed
down, went backwards :-) -- doesn't matter. All you need to know to
calculate redshift is the ratio of scale lengths.


Forgive me if this is a stupid question, but how do you find the scale
length when the light was emitted?

Denis
--
DT
Replace nospam with the antithesis of hills

SWFor any object and any observer, the observed redshift is the ratio
SWof the scale lengths when the light was observed and when it was
SWemitted.

In article ,
DT writes:
Forgive me if this is a stupid question, but how do you find the scale
length when the light was emitted?

If you want to compare a model to observations, your model has to
predict the scale factor as a function of time (and location, if your
model is inhomogeneous). In practice, the redshift (and hence the
scale factor) is the easily-measured quantity, so model predictions
are usually given in the form of age and distances (yes, plural) as a
function of redshift (or scale factor) rather than the other way
around.

To see how this works, play around a bit with Ned Wright's cosmology
calculator:
http://www.astro.ucla.edu/~wright/CosmoCalc.html

For example, in the now-standard FRW, Omega_matter=0.27, H_0=71 flat
cosmology, redshift 1 corresponds to a luminosity distance of
6.6 Gpc. In a similar cosmology with cosmological constant = 0, the
distance would be 5.8 Gpc. This is how supernovae suggest a non-zero
cosmological constant: their brightnesses are consistent with the
larger distance, not the smaller one.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote
If you want to compare a model to observations, your model has to
predict the scale factor as a function of time (and location, if your
model is inhomogeneous). In practice, the redshift (and hence the
scale factor) is the easily-measured quantity, so model predictions
are usually given in the form of age and distances (yes, plural) as a
function of redshift (or scale factor) rather than the other way
around.

To see how this works, play around a bit with Ned Wright's cosmology
calculator:
http://www.astro.ucla.edu/~wright/CosmoCalc.html

For example, in the now-standard FRW, Omega_matter=0.27, H_0=71 flat
cosmology, redshift 1 corresponds to a luminosity distance of
6.6 Gpc. In a similar cosmology with cosmological constant = 0, the
distance would be 5.8 Gpc. This is how supernovae suggest a non-zero
cosmological constant: their brightnesses are consistent with the
larger distance, not the smaller one.

Thanks, I'll take a look.

Denis
--
DT
Replace nospam with the antithesis of hills

(Steve Willner) wrote in message ...
In article ,
"Fitzdraco" writes:
Ok, right now the data we have tells us that the universe is expanding. We
can tell due to the redshift. But I was wondering since the light from these
distant objects is many billions of years old, wouldn't this be telling us
about the expansion x number of years ago?

Yes. The redshift of a distant object tells us about the expansion
along the entire path that the light has taken to reach us.

Think again!
A photon only imparts to our eye the information percieved by the eye
in the last (very small) distance it travels before becoming an image
on the retina.
If the direction or speed of the light (photon) has been altered by
gravity, reflection, refraction, interaction with other matter or
other photons ANYWHERE along its historical path (from source to us),
then the information it carries has been ALTERED. The object viwed
will not be in its percieved position, be of its observed form, or be
in its percieved LOCATION IN TIME. Beware of ALL ASSUMPTIONS therefore
depending on what we "see".
(Google "Observations and the bent stick")

Jim G