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Old 06-15-2017, 11:26 AM   #21
Anthony
 
Join Date: Feb 2005
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Default Re: Effects of a different CMBR temperature?

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Originally Posted by Wavefunction View Post
From my understanding between 10K and 20K is the temperature when stars begin to form.
It's when current generation stars form. First generation stars formed at higher temperatures, but you're not going to get planetary systems around first generation stars.
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Old 06-15-2017, 11:42 AM   #22
Fred Brackin
 
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Default Re: Effects of a different CMBR temperature?

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Originally Posted by Anthony View Post
It's when current generation stars form. First generation stars formed at higher temperatures, but you're not going to get planetary systems around first generation stars.
In addition, planets around first generation stars would be hydrogen and helium only.
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Old 06-15-2017, 12:58 PM   #23
whswhs
 
Join Date: Jun 2005
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Default Re: Effects of a different CMBR temperature?

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Also, it wouldn't change how we calculate blackbodies. The CMB may be our most perfect example of a blackbody, but the constants used (k and h) are measured in other places. I have a vague memory of the ultimate calibration of blackbodies coming from an experiment done at some observatory with molten platinum....
It wasn't molten platinum, if I remember my freshman physics correctly. It was a hollow cavity inside a solid block of platinum. You need it to be a hollow cavity to make the effects of albedo irrelevant. Checking Halliday and Resnick, I see they say it also makes the size and shape of the cavity irrelevant.
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Old 06-16-2017, 11:47 AM   #24
whswhs
 
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Default Re: Effects of a different CMBR temperature?

So, okay, on one hand, mean energy per unit volume varies as L^(-4). On the other hand, minimum mass for gravitational collapse to form a solar system varies as T^(3/2). To link the two, we need a conversion between energy and temperature.

* In basic thermo, temperature was defined as mean molecular kinetic energy, which would suggest that T is proportional to E, probably using Boltzmann's constant or the ideal gas constant.

* In blackbody radiation, energy radiated per unit time is proportional to T^4.

* However, the universe doesn't seem to be radiating into anything other than itself. I believe the proportionality for energy *content* in a medium is that internal radiation is proportional to T^3.

Which of these gives the right scaling relationship?
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Old 06-16-2017, 12:21 PM   #25
Anders
 
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Default Re: Effects of a different CMBR temperature?

The first stars formed when the universe was about 200 million years after the Big Bang. How hot would the CMB be then.
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