- 24,753
- 794
Jorrie, Wabbit (Wabbit glad you like the looks of the zeit version!) what we seem to be moving towards is a presentation that might at least put some ideas in the heads of Jorrie's constituency even if they don't take hold and work with it right away, and could be used in other contexts as well, that might go like this:
1. an introductory essay along the lines of what Jorrie already wrote. (but possibly even more explicit about "e-fold", for instance "e-fold is like three-fold except with e=2.718 instead of 3, the e-fold time is the time it takes something to expand by a factor of e. If f(t) is any positive increasing function with instantaneous fractional growth rate f'(t)/f(t) constant then it's a fact of calculus that the reciprocal of that rate, f(t)/f'(t), is the function's e-fold time. And the function must be of the form f(t) = ertwhere r equals the constant growth rate f'(t)/f(t).")
2. graphic introduction to the zeit version of Lightcone, particularly with a bunch of curves that illustrate the model.
3. the equations in part 1 have hand-calculable solutions that closely approximate the Lightcone tables and curves. In particular the expansion of the universe follows a hyperbolic sine curve: $$a = \frac{\sinh^{2/3}(\frac{3}{2}t)}{1.3}$$ and that relation of distance size to time can be inverted to tell us time as a function of the scale factor a. $$t = \frac{2}{3} \ln \big((1.3a)^{3/2} + \sqrt{(1.3a)^3 + 1}\big)$$
So here's a sample exercise. The present age (as it says in Jorrie's part 1.) is 0.8 zeit. You fall asleep and when you wake up you discover the cosmic microwave background is much colder. It must be some time in the distant future! The CMB is only one tenth its present temperature. what time is it?
Solution. The scale factor a has increased from 1 (present) to 10. So just put that 10 into the previous equation and calculate: 1.3a = 13
$$t = \frac{2}{3} \ln \big((13)^{3/2} + \sqrt{(13)^3 + 1}\big)$$
t will be the age, in zeits, at that future time.
1. an introductory essay along the lines of what Jorrie already wrote. (but possibly even more explicit about "e-fold", for instance "e-fold is like three-fold except with e=2.718 instead of 3, the e-fold time is the time it takes something to expand by a factor of e. If f(t) is any positive increasing function with instantaneous fractional growth rate f'(t)/f(t) constant then it's a fact of calculus that the reciprocal of that rate, f(t)/f'(t), is the function's e-fold time. And the function must be of the form f(t) = ertwhere r equals the constant growth rate f'(t)/f(t).")
2. graphic introduction to the zeit version of Lightcone, particularly with a bunch of curves that illustrate the model.
3. the equations in part 1 have hand-calculable solutions that closely approximate the Lightcone tables and curves. In particular the expansion of the universe follows a hyperbolic sine curve: $$a = \frac{\sinh^{2/3}(\frac{3}{2}t)}{1.3}$$ and that relation of distance size to time can be inverted to tell us time as a function of the scale factor a. $$t = \frac{2}{3} \ln \big((1.3a)^{3/2} + \sqrt{(1.3a)^3 + 1}\big)$$
So here's a sample exercise. The present age (as it says in Jorrie's part 1.) is 0.8 zeit. You fall asleep and when you wake up you discover the cosmic microwave background is much colder. It must be some time in the distant future! The CMB is only one tenth its present temperature. what time is it?
Solution. The scale factor a has increased from 1 (present) to 10. So just put that 10 into the previous equation and calculate: 1.3a = 13
$$t = \frac{2}{3} \ln \big((13)^{3/2} + \sqrt{(13)^3 + 1}\big)$$
t will be the age, in zeits, at that future time.
Last edited: