# Search Results

1. Post

### B How to do the calculations showing the Universe is flat?

Yes. More generally, all length scales grow with the expansion. I understand where you're getting caught up: the horizon size at decoupling is a...
Post by: bapowell, Aug 1, 2017 in forum: Cosmology
2. Post

### B How to do the calculations showing the Universe is flat?

No, what we actually see now is the length of the sound horizon at decoupling after it has expanded along with the universe to the present time. ....
Post by: bapowell, Jul 31, 2017 in forum: Cosmology
3. Post

### B How to do the calculations showing the Universe is flat?

Right. Let's try this. The size of the sound horizon had a certain proper length at decoupling ($3c_s t_{dec}$), and this length has continued...
Post by: bapowell, Jul 31, 2017 in forum: Cosmology
4. Post

### B Why didn't the Universe end up in a huge black hole?

Are you talking about the observable universe? We have no idea whether the universe, of which our observable patch is but one part, is infinite,...
Post by: bapowell, Jul 30, 2017 in forum: Cosmology
5. Post

### B How to do the calculations showing the Universe is flat?

There are no "comoving units" and "proper distance units"; there are only measuring rods that grow with the expansion. In Fig 2, the proper...
Post by: bapowell, Jul 29, 2017 in forum: Cosmology
6. Post

### B Why didn't the Universe end up in a huge black hole?

Can you elaborate? The CMB is consistent with a highly uniform universe on cosmological scales.
Post by: bapowell, Jul 29, 2017 in forum: Cosmology
7. Post

### B How to do the calculations showing the Universe is flat?

Sure, you can use time if you want. I used temperature because I derived expressions for $T_0$ and $T_{dec}$ in an earlier article in the...
Post by: bapowell, Jul 28, 2017 in forum: Cosmology
8. Post

### B How to do the calculations showing the Universe is flat?

It's true that there are points comoving with the expansion that begin inside the event horizon that eventually exit it. A galaxy at this point is...
Post by: bapowell, Jul 27, 2017 in forum: Cosmology
9. Post

### I No detection of gravitational radiation

A useful criterion is the Lyth bound: $\Delta \phi/ M_{\rm Pl} \propto \sqrt{r/0.01}$. It tells us that, generically, in models where the...
Post by: bapowell, Jul 27, 2017 in forum: Cosmology
10. Post

### B How to do the calculations showing the Universe is flat?

Good insight. Indeed, objects do flow across the cosmological event horizon never to return... You can make $a_0$ anything you like: it never...
Post by: bapowell, Jul 26, 2017 in forum: Cosmology

### Insights A Poor Man's CMB Primer. Part 5: Quantum Seeds - Comments

bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 5: Quantum Seeds [ATTACH] Continue reading the Original PF Insights Post.
Thread by: bapowell, Jul 26, 2017, 2 replies, in forum: Cosmology
12. Post

### B How to do the calculations showing the Universe is flat?

It's a matter of whether the horizon is growing more or less quickly than the expansion rate of the universe. When the universe accelerates (as...
Post by: bapowell, Jul 26, 2017 in forum: Cosmology
13. Post

### B How to do the calculations showing the Universe is flat?

What do you get for the time derivative of the first FE? You caught a mistake, congratulations! The computation of the angular scale of the...
Post by: bapowell, Jul 23, 2017 in forum: Cosmology
14. Post

### B How to do the calculations showing the Universe is flat?

The text following the Friedmann equation, Eq. (19), reads: "which relates the expansion rate to the homogeneous and isotropic density, with...
Post by: bapowell, Jul 22, 2017 in forum: Cosmology
15. Post

### B How to do the calculations showing the Universe is flat?

Regarding the cosmological constant, even if nonzero it is irrelevant to the dynamics of the early universe because it is dwarfed by the energy...
Post by: bapowell, Jul 22, 2017 in forum: Cosmology
16. Post

### B How to do the calculations showing the Universe is flat?

The Friedmann equation does not show that $\rho \sim a^{-3}$. This is seen using the continuity equation, which follows from energy...
Post by: bapowell, Jul 22, 2017 in forum: Cosmology
17. Post

### B How to do the calculations showing the Universe is flat?

Sorry to take so long to respond to @Bandersnatch 's bat signal, but I'm available to help if there are still questions!
Post by: bapowell, Jul 22, 2017 in forum: Cosmology
18. Post

### A Calculating the power spectra of scalar perturbation

Check out that Stewart and Lyth references I posted earlier.
Post by: bapowell, May 19, 2017 in forum: Cosmology
19. Post

### A Calculating the power spectra of scalar perturbation

If you wish to find $r$ at N=60, you're done.
Post by: bapowell, May 19, 2017 in forum: Cosmology
20. Post

### A Calculating the power spectra of scalar perturbation

Yes. Refer to posts #4 and #5 where you confirm that you desire to find $P(k)$. And to be clear: your main goal here is to find $r$ on a...
Post by: bapowell, May 19, 2017 in forum: Cosmology
21. Post

### A Calculating the power spectra of scalar perturbation

Well, you originally asked for the spectrum as a function of $k$, remember?
Post by: bapowell, May 19, 2017 in forum: Cosmology
22. Post

### A Calculating the power spectra of scalar perturbation

If N = 200 is the end, then yes. FYI, conventionally the variable $N$ is defined as the number of e-folds before the end of inflation (which is...
Post by: bapowell, May 19, 2017 in forum: Cosmology
23. Post

### A Calculating the power spectra of scalar perturbation

The duration is not relevant to the observables as long as it's sufficient to solve the horizon/flatness problems. Whether I've got an inflation...
Post by: bapowell, May 19, 2017 in forum: Cosmology
24. Post

### A Calculating the power spectra of scalar perturbation

I don't know what you mean by "whatever $N$ I got..". Got from where? You have a range of N, and for each N there is a mode leaving the...
Post by: bapowell, May 19, 2017 in forum: Cosmology
25. Post

### A Calculating the power spectra of scalar perturbation

Observable scales leave the horizon 60 e-folds before the end of inflation in cold inflation. Is that no longer the case in warm inflation? I...
Post by: bapowell, May 19, 2017 in forum: Cosmology
26. Post

### A Calculating the power spectra of scalar perturbation

1) aH must be monotonically increasing as a function of N during inflation, since it is equal to 1 over the comoving horizon size (which is...
Post by: bapowell, May 19, 2017 in forum: Cosmology