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| Cocco Drillo... |
 Posted: Thu Aug 06, 2009 5:05 am |
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Guest
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Hi,
R = R ( t ) = the cosmic expansion factor,
function of the cosmic time t.
Observations show accelerated expansion.
What is the most probable form of R at
present epoch?
Thanks in advance
Corrado |
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| Phillip Helbig---remove CLOTHES to reply... |
| Posted: Thu Aug 06, 2009 8:07 pm |
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In article <4a799f0d$0$28515$4fafbaef at (no spam) reader2.news.tin.it>, "Cocco
Drillo" <chivapiano at (no spam) vasano.evalontano> writes:
[quote:60fd6ede0a]R = R ( t ) = the cosmic expansion factor,
function of the cosmic time t.
[/quote:60fd6ede0a]
The usual term is "scale factor", not "expansion factor".
[quote:60fd6ede0a]Observations show accelerated expansion.
What is the most probable form of R at
present epoch?
[/quote:60fd6ede0a]
I assume you mean "what is the function R(t)". In that case, you don't
need "at present epoch".
In any case, R is not a simple function of t. However, as time goes on,
assuming the present ideas about the values of the cosmological
parameters are correct (accelerated expansion etc), R(t) approaches the
exponential function. |
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| Chalky... |
| Posted: Thu Aug 06, 2009 9:06 pm |
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On Aug 6, 4:05 pm, "Cocco Drillo" <chivapi... at (no spam) vasano.evalontano>
wrote:
[quote:244d796706]Hi,
R = R ( t ) = the cosmic expansion factor,
function of the cosmic time t.
[/quote:244d796706]
(aka scale factor)
[quote:244d796706]Observations show accelerated expansion.
What is the most probable form of R at
present epoch?
[/quote:244d796706]
Since the scale factor is actually defined as 1 now, this only makes a
difference to what it was as a function of time in the past. However,
playing around with
http://www.astro.ucla.edu/~wright/DlttCalc.html or
http://www.astro.ucla.edu/~wright/CosmoCalc.html
can still tell you quite a lot, within the context of established
theory.
Note that R ( t ) = 1 / (1 + z) [I think I have that right]
Setting Omega_vac=0 and hitting "General" tells you what would happen
if you just remove the cosmological constant (the factor that produces
accelerating expansion).
Then setting z very high (eg 3 million), and hitting "General" or
"Flat", and reading off comoving radial distance, tells you the approx
projected size of the observable universe (now), under these different
scenarios.
(If you want corresponding figures for a universe with no accelerating
expansion but still spatially flat, you should set Omega_M =1 as well
as Omega_vac=0) |
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| John Bell (Change John to Liberty for email)... |
| Posted: Sat Aug 08, 2009 3:27 pm |
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On Aug 7, 7:07Â am, hel... at (no spam) astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:
[quote:9f20ad9cd8]In article <4a799f0d$0$28515$4fafb... at (no spam) reader2.news.tin.it>, "Cocco
Drillo" <chivapi... at (no spam) vasano.evalontano> writes:
R = R ( t ) = Â the cosmic expansion factor,
function of the cosmic time  t.
The usual term is "scale factor", not "expansion factor".
Observations show accelerated expansion.
What is the most probable form of  R  at
present epoch?
I assume you mean "what is the function R(t)". Â In that case, you don't
need "at present epoch".
[/quote:9f20ad9cd8]
Unless he wants to know how fast R(t) is believed to be changing with
time, now, in which case the answer is "very slowly".
Using Chalky's ref for convenience, currently preferred theory (and
model) indicate it is currently changing by about 0.007% per million
years.
This is, of course, far more slowly than the preferred theory and
preferred model of the currently preferred theory have both been
changing over the course of the last century. |
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| robert bristow-johnson... |
| Posted: Mon Sep 07, 2009 11:21 pm |
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On Aug 7, 2:07 am, hel... at (no spam) astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:
[quote:b451e62b42]In article <4a799f0d$0$28515$4fafb... at (no spam) reader2.news.tin.it>, "Cocco
Drillo" <chivapi... at (no spam) vasano.evalontano> writes:
R = R ( t ) = the cosmic expansion factor,
function of the cosmic time t.
The usual term is "scale factor", not "expansion factor".
Observations show accelerated expansion.
What is the most probable form of R at
present epoch?
I assume you mean "what is the function R(t)". In that case, you don't
need "at present epoch".
In any case, R is not a simple function of t. However, as time goes on,
assuming the present ideas about the values of the cosmological
parameters are correct (accelerated expansion etc), R(t) approaches the
exponential function.
[/quote:b451e62b42]
okay, here is my problem:
how does the accelerated expansion of the universe, that i've seen
first reported in the 90s, square with Hubble's Law which is much
older but still valid, i thought.
are not galaxies that are very far away (like 10^10 lightyears, not
only moving faster away from us, but are moving faster away from
nearby galaxies (that are approximately as young) than we are moving
away from our nearby galaxies? isn't it because they are younger,
closer in time to the big bang, and have more kinetic energy before
gravitation gets a few billion years to slow it down? or am i totally
misunderstanding the basis to Hubble's Law?
if stuff is flying apart a lot faster back then ca. 10 billion years
ago than stuff is now, isn't that observationally conflicting with the
evidence of the accelerated universe ca. 1998?
thanks for clearing up any misunderstanding. i have to admit that the
accelerated expansion of the universe and the dark energy explanation
for it is not a theory i like accepting. seems to violate a
sensibility of mine like of the conservation of energy.
r b-j |
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| robert bristow-johnson... |
| Posted: Fri Sep 11, 2009 7:00 am |
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On Sep 11, 2:24 am, "Jonathan Thornburg [remove -animal to reply]"
<jth... at (no spam) astro.indiana-zebra.edu> wrote:
[quote:7e7cc02c81]robert bristow-johnson <r... at (no spam) audioimagination.com> wrote:
how does the accelerated expansion of the universe, that i've seen
first reported in the 90s, square with Hubble's Law which is much
older but still valid, i thought.
Hubble's law remains alive and well. It's particularly important
to understand what it does -- and doesn't -- say. Notably, it's a
statement about objects which are "close" to us in a cosmological
sense, i.e., close enough that we can reasonably approximate a region
of spacetime containing both us and the object as (part of) Minkowski
spacetime.
[/quote:7e7cc02c81]
do you mean that these distant galaxies that HL governs has to be in
our light-cone? if so, doesn't *anything* we observe and make any
meaningful theorems about have to be in our light-cone? other than
saying that we cannot affect them nor they can affect us, does physics
say *anything* meaningful (or falsifiable) about what happens outside
of our light-cone?
[quote:7e7cc02c81] In practice, that means the object has to be much closer
than a Hubble distance (around 3000 Mparsec).
[/quote:7e7cc02c81]
that's about 10 billion lightyears, right? i thought that there were
some objects observed in that ballpark distance. and aren't they
redshifted to close to just under a velocity of c?
[quote:7e7cc02c81]Almost anything else I could say about this subject has already been
said better by this great paper:
http://adsabs.harvard.edu/abs/1993ApJ...403...28H
Title: The redshift-distance and velocity-distance laws
Authors: Harrison, Edward
Publication: Astrophysical Journal, Part 1 (ISSN 0004-637X),
vol. 403, no. 1, p. 28-31.
Publication Date: 01/1993
[/quote:7e7cc02c81]
i hit a couple of links to get either the high-res or low-res PDF, but
got to a broken page. i tried to get the site to email me a PDF.
we'll see if that works.
even though i am admittedly no good at tensors and mathematical GR, i
still thought that i had some decent understanding of SR. i'll try to
read this (if i can get it) and possibly pose my question again from
less ignorance. this accelerated expansion that was reported in the
late 90s still bothers the hell outa me. and, if it's true, it seems
to me that the old universe should be expanding with less velocity
than the current universe, and that seems, on the face of it, to be
opposite of Hubble's Law.
still confused,
r b-j |
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| Phillip Helbig---remove CLOTHES to reply... |
| Posted: Mon Sep 14, 2009 8:36 pm |
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Guest
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In article <h8krs9$uha$1 at (no spam) gide.ita.chalmers.se>, Ulf Torkelsson
<torkel at (no spam) physics.gu.se> writes:
[quote:d0910327de]robert bristow-johnson skrev:
On Sep 11, 2:24 am, "Jonathan Thornburg [remove -animal to reply]"
jth... at (no spam) astro.indiana-zebra.edu> wrote:
In practice, that means the object has to be much closer
than a Hubble distance (around 3000 Mparsec).
that's about 10 billion lightyears, right? i thought that there were
some objects observed in that ballpark distance. and aren't they
redshifted to close to just under a velocity of c?
This suggests to me that you have the usual misconception about the
cosmological redshift that it is a Doppler effect, which it is not
really.
[/quote:d0910327de]
Indeed. Here's an argument which should convince people who think that,
for large redshifts, one should use the relativistic Doppler formula:
That formula doesn't contain any of the cosmological parameters, so it
implies that the velocity (however it is defined) of an object at high
redshift is independent of the cosmological model. Not quite a
reductio ad absurdum, but close.
Note that the proper distance times Hubble's constant gives us the
velocity (time derivative of the proper distance); this can be larger
than c with no problems. It is not a "directly observable distance",
though.
[quote:d0910327de]All of this is extremely well presented, without more mathematics
than is absolutely necessary, in
Harrison, E. R., 2000,
Cosmology. The science of the universe,
Cambridge University Press
[/quote:d0910327de]
EVERYONE interested in cosmology should read this book at least twice. |
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