|Nov7-03, 12:59 AM||#1|
New Lineweaver paper/Superluminal expansion speeds
Davis and Lineweaver
common misconceptions of cosmological horizons
and the superluminal expansion of the universe"
Just came out 28 October
Among other things, the paper explains how light can reach us even from objects that were receding away from us faster than c at the time the light was emitted. Clearest explanation I've seen so far.
Quotes some eminent people (who should have known better) making mistakes and thereby illustrating common misconceptions about the cosmological redshift and expansion speeds. Steven Weinberg was caught in both "The First Three Minutes" and in his 1972 textbook "Gravitation and Cosmology" (!)
for Weinberg quotes see bottom of page 21 and top of 22.
|Nov11-03, 07:50 AM||#2|
Blog Entries: 4
Seems like Lineweaver has also some misconceptions, since he claims that the most distant object observed is a galaxy with z=6.68 of name STIS 123627+621755, however this galaxy is not longer believed to have such a high redshift. Even Chen, the person that claimed that redshift, has renounced to it, see
|Nov11-03, 09:03 AM||#3|
Thanks for mentioning that. the most distant I've read about
has z = 6.4 and is IIRC in UrsaMajor.
I didnt notice that Davis and Lineweaver mentioned something
further like 6.6 or 6.7
I assume there is no special reason to doubt claims to see z = 6.7, and this just a case where the guy found an error and withdrew it. Lineweaver must have missed seeing the paper you cite. Why not write him with the reference?
|Nov11-03, 09:26 AM||#4|
New Lineweaver paper/Superluminal expansion speeds
I found the passage in Davis/Lineweaver where they say 6.68 and you would, I gather, prefer they say 6.4. The point of the paragraph is that hundreds of objects have been observed with redshifts greater than 1.46 (the critical number here) and they are mentioning examples to make that point:
"Hundreds of galaxies with z > 1.46 have been observed. The highest spectroscopic redshift observed in the Hubble deep field is z = 6.68 (Chen et al., 1999) and the Sloan digital sky survey has identified four galaxies at z > 6 (Fan et al., 2003). All of these galaxies have always been receding superluminally.
The particle horizon, not the Hubble sphere, marks the size of our observable universe because we cannot have received light from, or sent light to, anything beyond the particle horizon. Our effective particle horizon is the cosmic microwave background (CMB), at redshift
approximately 1100, because we cannot see beyond the surface of last scattering. Although the last scattering surface is not at any fixed comoving coordinate, the current recession velocity of the points from which the CMB was emitted is 3.2c (Fig. 2). At the time of emission their speed was 58.1c, assuming (ΩM, ΩΛ) = (0.3, 0.7). Thus we routinely observe objects that are receding faster than the speed of light and the Hubble sphere is not a horizon."
The figure of 6.68 is immaterial to what they are trying to say. So why not write them (it could be that Tamara Davis, the junior author, was responsible for much of the article so she might be the one to contact) and suggest dropping the questionable first part of the sentence and letting it start
"The Sloan digital sky survey has identified 4 galaxies at redshift greater than 6..."
|Nov19-03, 09:12 PM||#5|
|Feb17-04, 06:31 AM||#6|
This is the oldest object yet discovered, and we are looking back to when the universe was about 750 million years old. Its light is reaching us with a red shift of about 7.0.
1. So that implies its recession velocity is substantially greater than the speed of light, is that correct?
2. Therefore, this object (long since extinct, I imagine) would exist in a part of space that is now much farther away from us than 13 billion LYs. Is that correct, per the Lineweaver Davis article?
3. And similarly, the region of space we were in at the time the light was emitted from that galaxy (long before the Milky Way existed, of course) was perhaps a few hundred million LYs away. Is that interpretation correct, per the Lineweaver Davis article?
|Feb17-04, 09:52 AM||#7|
til now I knew of redshift 6.4 max, now you have pointed us
to an example of redshift 7
general astronomy forum has a link-basket sticky ("reference shelf")
with a link to the cosmology calculators, so we can find out the speed the thing was traveling away from us when it emitted the light that is now reaching us. I will get that link.
2. correct, at present it is more than 13billion LY distant from us
3. I think you are saying something that agrees with the article, namely that in the past, when the light was emitted, our two regions of space were much closer.
I think that we can use the calculators to find that distance
(using the now-standard cosmology model) and it will come out to be, as you suggest, only a few hundred million LY. I'll get the link and try the calculation
|Feb17-04, 10:00 AM||#8|
"The Hubble data suggest the newly observed galaxy lies between redshift 6.6 and 7.1, while long exposures with the Keck telescopes narrow the value to about 7.0.
The galaxy appears to form stars at the rate of nearly three Suns per year and is just 2000 light-years' wide, about 50 times smaller than our Milky Way. Kneib his colleagues will report their observation in an upcoming issue of the Astrophysical Journal."
If we look up Jean-Paul Kneib at arxiv.org we can probably find the preprint for the technical paper that will be coming out in Astrophysical Journal.
It is interesting that the object is a galaxy, not a quasar
and also that it could only be seen because of gravitational lensing (like having an extra section of telescope)
|Feb17-04, 10:06 AM||#9|
Here is the post from Astronomy and Cosmology "reference" sticky.
it is 3 or 4 posts from the beginning of the thread:
two good online cosmology calculators:
homepage for Siobahn in case you want to see who she is
homepage for Ned in case you want to see who he is
I see that Meteor has noted the same NewScientist article you did, in the General Astronomy forum
|Feb17-04, 10:21 AM||#10|
I used Siobahn's calculator
hubble (H) 71
matter density 0.27
cosmological constant (lambda) 0.27
it found that the age of universe when the light was emitted was 770 million years
and the galaxy's distance from us THEN was 3.59 billion LY
and the speed it was moving away from us THEN was 3.07 times the speed of light
(as you point out saying the galaxy's distance from us back then is a little peculiar since although the Milkyway galaxy might have already begun forming, the solar system hadnt, so we did not exist in any recognizable way, but as you say talking about its distance from "our region of space" can help make sense of it)
EDIT: I just saw the next post by DrChinese and the way he tells the story makes sense to me---seems like a reasonably clear picture of how it is supposed to have happened. So I wont reply but just leave DrC post as a capper
EDIT: the technical article (Astrophysical Journal) by Kneib
it is by 3 guys at Caltech and one other from I-dont-know-where
they call it "exploring the dark ages" because it is seeing a galaxy from very young time when galaxies dont usually show up---they got to see it by a lucky lensing accident
|Feb17-04, 12:00 PM||#11|
Even though the universe was less than a billion years old at that time, there were already objects over 3 billion LY away from "us" at that time. This is because the total recession velocity was over 3x the speed of light. The light being emitted at that time would need about 13 billion years to catch up with us as it journeyed from one part of space to another.
As it moved towards us, the total recession velocity from us slowly diminished until it was eventually less than the speed of light. At that time it was actually starting to get closer and closer to us and eventually, at T=+13.7 billion years, we looked out and happened to see it.
What a journey!
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