PS. In this situation, it seems like either temperature or density are monotonic decreasing with time, so either could serve as an effective time axis. --Brad On Sat, Oct 10, 2020 at 9:50 PM Brad Klee <bradklee@gmail.com> wrote:
Hi Fred,
You can think of the CMB as a curtain or a wall that exists at a very early time in the history of the universe, see for example this (nice) piece of propaganda from NASA:
https://www.jpl.nasa.gov/infographics/infographic.view.php?id=10824
At times earlier than the CMB, the universe is said to be radiation dominated, and the photons are scrambled by interactions with charged particles, as described here: https://en.wikipedia.org/wiki/Recombination_(cosmology)
How can we figure out the magic number 380 kiloyears?
http://articles.adsabs.harvard.edu/pdf/1968ApJ...153....1P Table 1 says Temp. 3000K is 99% recombined, and this happens at approx z=1100 giving the approximate time as long as the redshift ---> year calculator works.
Compare: http://www.astro.ucla.edu/~wright/CosmoCalc.html z=1100 --> 370Kyr https://home.fnal.gov/~gnedin/cc/, z=1100 --> 475Kyr https://www.kempner.net/cosmic.php z=1100 --> 471Kyr
I also looked around and found some more interesting timeline graphs:
https://pages.uoregon.edu/jimbrau/astr123-2015/Notes/Chapter27.html
https://pages.uoregon.edu/jimbrau/BrauImNew/Chap27/7th/AT_7e_Figure_27_01.jp...
https://pages.uoregon.edu/jimbrau/BrauImNew/Chap27/7th/AT_7e_Figure_27_04.jp...
Here we can see what we need to do is solve for the intersection of two curves, then look ahead to get from the crossing point to the time of atom formation.
So why should this problem be irrelevant? Isn't always relevant to wonder: Where did we come from? How did we get here?
--Brad
On Sat, Oct 10, 2020 at 7:32 PM Fred Lunnon <fred.lunnon@gmail.com> wrote:
Whenever it surfaces, I do wonder whether theoretical physics is "math-fun"; --- anyway, till we get slapped down, here's my (totally amateur) two-penn'orth ... I've been fascinated by physics since childhood, but somehow never managed to focus motivation sufficiently to learn to _do_ any: so I can usually understand the questions well enough, even if the answers often go over my head.
Not this time round. "behind (or before) the CMB" puts the problem in a nutshell. Both general relativity and quantum theory deal with observations made by an observer in a specific frame of reference located in (Minkwski) spacetime. Penrose in the video kicks off by admitting this, then cheerfully proceeds to speculate about some enveloping notional spacetime without any attempt at defining what behind (or before) might mean. It's turtles all the way down ...
I sniff a fundamental cognitive trap lurking. Our notion of time is inextricably connected with our consciousness, which in turn we remain unable to relate to any physical framework --- despite strenuous efforts (by Penrose and others) over many millenia. The possibility of progress seems to demand that such questions must be stripped of their dependence on a nonexistent frame of reference even to make sense.
Fred Lunnon
On 10/9/20, Brad Klee <bradklee@gmail.com> wrote:
I was wondering about this question recently, so I was happy to get a dissenting opinion via Forbes:
https://www.forbes.com/sites/startswithabang/2020/10/08/no-roger-penrose-we-...
But the tone of the Forbes article is too businesslike for my taste, and does it mention that Penrose himself describes the idea as "crazy"? See also:
https://www.youtube.com/watch?v=YTttUigXulk
When you watch this video, how can you help but wonder about the incredible mystery of what exactly is going on in the background out there? Maybe universal reincarnation is a plausible cosmogony? We don't know.
Fifty or one hundred years from now we could have better gravity detection, and a better idea of what is going on behind (or before) the CMB. Until then...
Anyways, congratulations Penrose! More and more cheers and congratulations for unexcelled creativity!
--Brad _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
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