# Age versus Size of the universe

1. Aug 22, 2014

### Jozsef

Are the parameters involved in the calculations to determine the age of the universe completely different from the parameters used to calculate its size? If indeed so, : "thread closed."
If on the contrary, one or several parameters have common use, it's like a snake that bites its own tail. Jozsef

2. Aug 22, 2014

### phinds

There is no known size to the universe. It might be infinite or it might be finite but unbounded. There is no snake biting its tail involved anywhere. Redshift is solidly confirmed and gives the distance to distance objects. The size of the OBSERVABLE universe is well known to about one part in a hundred or less. There is or hand-waving going on. Why do you think otherwise?

Last edited: Aug 22, 2014
3. Aug 22, 2014

### Jozsef

4. Aug 22, 2014

### phinds

Yes, that is exactly what I mean when I said
Do you understand that the observable universe is not THE universe?
I have no answer for you but I can assure you you are barking up the wrong tree. There is nothing wrong with the observations and interpretations of modern cosmology.

5. Aug 23, 2014

### Jozsef

6. Aug 23, 2014

### Jorrie

The only 'size' we can possibly observe is called the particle horizon, essentially how far light could have moved in the age of the universe. Hence, the two are related, but not trivially. They depend on observed parameters and the expansion dynamics of the universe.

The standard equations are neatly summarized in Fundamental[/PLAIN] [Broken] Aspects of the Expansion of the
Universe and Cosmic Horizons
, Appendix A.

Last edited by a moderator: May 6, 2017
7. Aug 23, 2014

### Drakkith

Staff Emeritus
The parameters are not completely separate. For one, there are several ways of dating both the age and the size of the universe. From wiki:

Since the universe must be at least as old as the oldest thing in it, there are a number of observations which put a lower limit on the age of the universe; these include the temperature of the coolest white dwarfs, which gradually cool as they age, and the dimmest turnoff point of main sequence stars in clusters (lower-mass stars spend a greater amount of time on the main sequence, so the lowest-mass stars that have evolved off of the main sequence set a minimum age).

In addition to this, you can plug several cosmological parameters into an equation to yield the age of the universe. This assumes that our knowledge of each parameter is correct, which may or may not be true. However, current measurements of these parameters yield an age that is in good agreement with other methods, such as the one above.

The size of the universe can be estimated using the cosmic distance ladder: http://en.wikipedia.org/wiki/Cosmic_distance_ladder
This consists of various measurements for objects at different distances.

From the article:

With few exceptions, distances based on direct measurements are available only out to about a thousand parsecs, which is a modest portion of our own Galaxy. For distances beyond that, measures depend upon physical assumptions, that is, the assertion that one recognizes the object in question, and the class of objects is homogeneous enough that its members can be used for meaningful estimation of distance.

It's important to understand that our estimates of both the age and the size of the universe depend upon the accuracy of our knowledge of the underlying physics. Since both the size and the age of the universe depend on the underlying physics, I don't see this as a case of "the snake biting its own tail" even if the physics underlying the age and size of the universe aren't completely separate. The basic rules are what determine both of them.

8. Aug 23, 2014

### Jozsef

this was exactly the "core" of my question.
May I read : on different and independent parameters?
T[QUOTEt]he standard equations are neatly summarized in Fundamental[/PLAIN] [Broken] Aspects of the Expansion of the
Universe and Cosmic Horizons
, Appendix A.[/QUOTE][/QUOTE]
Thank you for the suggested reference, but I have no access to the site, I keep trying.Jozsef

Last edited by a moderator: May 6, 2017
9. Aug 23, 2014

### Jozsef

10. Aug 23, 2014

### phinds

11. Aug 23, 2014

### Jorrie

Jozsef, we do not know the size of the universe and we certainly do not need it to calculate its age. The particle horizon (the 46 odd billion light years that you mentioned) is not the size, just how far a massless particle could possibly have traveled in the time since the expansion started (i.e. in the 'age' of the universe).

It is logical that the particle horizon depends on the age, plus a number of other parameters. There is no circularity in the calculations. Forget about the "size of the universe" issue; we may never know that.

12. Aug 23, 2014

### Jozsef

13. Aug 23, 2014

### Jorrie

I must just correct myself here. The particle horizon is the present proper distance to the farthest regions that we can in principle observe now. It is a bit misleading to call it the distance that particles could have traveled since the expansion started, because since the particles have left the region, the expansion has carried those regions away from us.

14. Aug 23, 2014

### phinds

I assume that what you mean here is that we do not need to calculate the age of the universe because its age is taken to be the same as that of the observable universe which we know to be ~14billion years, and which we DO calculate, based on red shift and other phenomenon, yes?

15. Aug 23, 2014

### Jorrie

Yes, but I was trying to stress that the 'size of the universe' does not feature in such calculations, except, I should perhaps have added, for the assumption that the total U is larger than our observable part.

16. Aug 23, 2014

### marcus

Hi Jorrie, I have to compliment you on clarity. Careful conscious choice of words.

"age of the universe" is just a conventional phrase people traditionally use for estimated duration of the expansion in geometry we are witnessing.

We don't know how old the universe is. Presumably there was universe before it started to expand, why not? We have no evidence that there wasn't universe back then, just doing something else before it started to expand.

So you are right to put 'age' in inverted comma.

17. Aug 24, 2014

### Jorrie

So the 'correct' answer to Joz's question should then be that we know neither the size nor the age of the universe; hence we cannot say if they are related... ;)

On a more serious note, he probably meant the size of the observable universe and hence also the 'age' since the present expansion started. It is not uncommon for popular writers to refer to a distant galaxy's distance in terms of "look-back time", but then simply saying it is located so many billion light years from us. While approximately true for smaller cosmic distances, it becomes problematic for larger ones, due to the expansion while the light was traveling to us.

However, it is also quite difficult for laypeople to comprehend "proper distance now" and "proper distance then". Not to mention the differences between the Hubble time and look-back time of the most distant objects, which are quite similar in magnitude. While the typical reader of this forum may easily understand our explanations, it is probably not true for the typical reader of popular articles or books.

So, what should the popular science writer do?

18. Aug 24, 2014

### phinds

Smile a lot and lie. That's what they do now and they get paid for it.

19. Aug 24, 2014

### Jozsef

Write in a understandable way as you do, even if the issue is complicated.

20. Aug 24, 2014

### Tanelorn

Thanks Marcus, "the estimated age of the expansion" is very helpful.