# Comments - Inflationary Misconceptions and the Basics of Cosmological Horizons

1. Aug 28, 2015

### bapowell

Last edited by a moderator: Oct 17, 2016
2. Aug 28, 2015

### eltodesukane

The usefulness of comoving coordinates tells me that "expanding space" may be better viewed as "contracting matter", that "receding galaxies" may be better understood as "contracting galaxies".
Those point of view are probably exactly equivalent, but maybe they are not.

3. Aug 28, 2015

### Greg Bernhardt

Fantastic work!

4. Aug 28, 2015

### marcus

Really nice work, Brian. This could be our "go to" essay for a lot of newcomer questions.

5. Aug 28, 2015

### marcus

Brian I was thinking how, when answering a question, I could direct somebody to a specific place in your tutorial---like Figure 6 which shows graphically all the events which can have influenced us by time T, and also all the events which we (or our matter starting in ancient times) can have have influenced by time T. Very interesting sets of events to focus on and think about.

There is a footnote #8 right near that figure 6. I wonder if I could use this link, to direct someone to that part of your essay:

https://www.physicsforums.com/insig...nceptions-basics-cosmological-horizons/#back8

Let me see how that works. Yes that works, it jumps right to Figure 6. So that way I wouldn't have to tell the person to read the whole essay, or to go to the start and scroll down to such and such. I could just say "look at this figure". the proximity of the footnote gives a mark to jump to. there may be other ways I don't know about to jump to a specific passage

Last edited: Aug 28, 2015
6. Aug 28, 2015

### bapowell

Thanks Marcus. This is a good idea. I can easily add linkable tags to figures and such if they end up being useful.

Last edited: Aug 29, 2015
7. Aug 29, 2015

### timmdeeg

Great article! Please allow me a remark regarding the balloon analogy. No doubt, it is a very helpful layman's guide but at the same time eventually a source of a common misunderstanding. Saying "the points separate on account of the expanding rubber" possible supports a laymen's notion to understand space as a sort of substance which expands physically. The analogy shows increasing distances perfectly, but perhaps one should clarify the role of the rubber.

Last edited: Aug 29, 2015
8. Aug 29, 2015

### Chiclayo guy

In the introduction you said, “…but my goal is to present the key ideas at a popular level, without assuming any prior understanding of cosmology.” In my view you’ve accomplished that, at least to the degree possible for someone with no physics or math background. My first read has already clarified several concepts for me. Thank you.

9. Aug 29, 2015

### bapowell

Good point timmdeeg. I've added a footnote warning against this pitfall.

10. Aug 29, 2015

### Torbjorn_L

Very useful, even though I've read D&L (a long time ago). Thanks!

Some typos, the most visible in the equation below fig. 4, and as you note by my circumstantial reference, the equation numbers are missing (re using this article as a reference).

I know this is a matter of taste and hence opinion, but the balloon analogy never did anything for me. The first time I met it it was used to discuss the then unknown topology of the universe and to make away with the question of a boundary. Very confusing at the time, which is why I prefer the 3D risin' raisin bread analogy instead even though the analogy breaks down re boundaries.

11. Aug 29, 2015

### phinds

You might find it informative to check out the link in my signature

12. Aug 29, 2015

### S Buschmann

Great article! I'm really a novice at this, forgive me. At some point, would the expansion reverse to a contraction?

13. Aug 29, 2015

### bapowell

It certainly could, but that does not appear to be on the menu. For the last 9 billion years or so, the universe has been undergoing an accelerated expansion with no change of pace in sight.

14. Aug 30, 2015

### timmdeeg

Agreed: No Stretching (!)

15. Sep 13, 2015

### slatts

Great article!...If I can just remember where I bookmarked it, it should save PF a few tortuous threads on the relative speeds of objects within colliding bubble universes. (My parenthesizing and underlining fingers thank you, too.)

16. Sep 16, 2015

### JohnnyGui

A very nice and clear article which definitely helps me understand things better.

A small thing that confused me though is the following quote:
"The result dH > c is the hallmark of decelerated expansion"

Wouldn't an increase of the Hubble radius with ANY velocity, not just > c, mean a decelerating expansion?

I'm a real novice at this.

17. Sep 16, 2015

### bapowell

Thanks for the comment JohnnyGui. If you look at the equation above Fig. 3 (yes, I know, no equation numbers!!), $\dot{d}_H = c(q+1)$, where $q$ is the deceleration parameter, you'll see that when $-1 < q < 0$ -- when the universe is accelerating -- the Hubble scale grows at a rate smaller than c (and conversely). A good way to think about decelerated expansion is that comoving lengths (the size of spacings on an expanding grid) grow more slowly than the Hubble scale (this is identical to the statement that $\dot{d}_H > c$ (since points with $r = d_H$ have $v_{rec} = c$ and the only way for the Hubble scale to overtake them is if it itself is growing at a rate greater than $c$)). On the other hand, if $\dot{d}_H < c$, that means that comoving lengths are growing more quickly than the Hubble scale: this is accelerated expansion.

18. Sep 16, 2015

### JohnnyGui

I'm probably missing something here, but how can the Hubble radius grow faster than c if its very own limit (dH) is determined by c?
How I see it, the only way to let the Hubble radius grow at a larger rate is to make the recession velocity (i.e. the growing rate of the comoving distance) of the objects behind it ≤ c but I can't see how that translates into a $\dot{d}_H > c$. Decelerating an object to ≤ c doesn't make the Hubble radius go any faster than c is what I would think.

Sorry for my misunderstanding.

Last edited: Sep 16, 2015
19. Sep 16, 2015

### bapowell

Good question. The key here is that the Hubble radius is not itself a comoving object, receding with the expansion: one does not apply Hubble's Law to the Hubble radius itself. You can think of it merely as a speed limit marker that is moving faster than the speed limit it is imposing on comoving objects.

20. Sep 16, 2015

### JohnnyGui

Is it correct if I say that relative to earth (physical distance), the Hubble radius is always traveling/expanding at c? And if the expansion of the universe is decelerating, more objects fall into the Hubble radius while with acceleration objects escape out of the Hubble radius? If so, relative to what does the Hubble radius travel faster than c in a decelerating scenario?