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Old May 2nd 19, 09:36 PM posted to sci.astro.research
Phillip Helbig
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Posts: 38
Default Revise age of the universe?

In article , Thomas
'PointedEars' Lahn writes:

Thomas 'PointedEars' Lahn wrote:
root wrote:
become if the current rate of acceleration obtained over the entire
lifetime?


In that case I will leave the calculation to you, because you *can* do
it Because in that case the age of our universe is easily obtained as
the reciprocal of the Hubble constant:

t = 1/H_0.

| Moderator's note: True if the current RATE of expansion were constant,
| but not if the current ACCELERATION were constant. -P.H.


The OP was not even talking about the `current acceleration', but the
`current rate of acceleration'.


Right, but what did he mean? There might be a language problem. I took
"current rate of acceleration" to mean "current value of the
acceleration". It certainly can't mean "current rate of expansion" (at
least without even bigger language problems). Richard Nixon once said
that while he was president, the rate of increase of inflation had gone
down. A journalist quipped that this was the only occasion when a
President of the United States made use of the third derivative.

| Moderator's note: Actually, the deceleration and acceleration almost
| balance so that the age of the universe is very close to the Hubble
| time. In our universe, this happens only near the present epoch. There
| have been a couple of papers addressing this coincidence. -P.H.

For example, if you use the Planck Collaboration's 2015 value of H_0 =
67.31 (km/s)/Mpc (TT+lowP) [1], with 1/H_9 = 14.5 Ga you do NOT obtain
Planck's corresponding t_0 = 13.813 Ga but something considerably
larger.


I think I have shown here that the moderator's statement is not true.
A difference of several hundred million years is NOT "very close".


There are at least two papers on arXiv on this topic, one by Geraint
Lewis, Pim van Orschok (not sure of the spelling), and possibly more
authors, and one by Bob Kirshner and a co-author. The degree of
coincidence is independent of the Hubble constant, but of course depends
on the values of lambda and Omega used. (The second paper has an
obvious title; the first is also about something else, but the title
isn't obvious.) I can't check now but they can probably be found at
arXiv.org in less than a minute.

Note in this 2006 depiction of an inflationary LambdaCDM model (based on
WMAP data) the extreme expansion speed in the epoch of inflation (as per
the theory of cosmic inflation) in the first 10^9 years; then a
moderate, almost linear expansion until our universe was 13 * 10^9
years old, followed by an accelerated expansion due to Dark Energy
(Lambda) since about 770 * 10^6 years ago.


| but in any case the age of the universe is essentially
| independent of inflation since that lasted only a fraction of a second.
| -P.H. There was deceleration until a few billion years ago and since
| then acceleration.


So the image I referred to is imprecise in that regard?


Not imprecise; it's just that inflation is irrelevant. It happened
in much less than a second, so whether or not it happened doesn't much
affect the age of the universe today, which is based on the values of
lambda, Omega, and H measured today (from which their values at all
other times follow).