# The Length of the Universe

1. Oct 6, 2005

### The Guru Kid

I was researching about the highest possible EM wavelength and came across "the length of the universe" concept.

Is it just the length of the visible universe? or the universe possible?

thanx

2. Oct 6, 2005

### vincentm

I think it maybe stating the known universe a.k.a the observable universe. :/

3. Oct 6, 2005

### Orion1

Photon Phantom...

Longest photon wavelength?

Hubble photon wavelength:
$$r_u = \lambda_{\gamma}$$
$$\lambda_{\gamma} = \frac{c}{H_o}$$

$$H_o$$ - Hubble Constant

What is the 'wavelength' of this photon?

Last edited: Oct 7, 2005
4. Oct 6, 2005

### SpaceTiger

Staff Emeritus

5. Oct 6, 2005

### turbo

The highest possible wavelength is a standing wave (WRT to your frame of reference). It would be indistinguishable from the your local backgound, and therefor not measurable by you, regardless of how much energy the wave might contain.

Interestingly, quantum physics says that the energy of the vacuum is 120 OOM larger than we can measure. Sometimes, a UV cutoff is proposed to cut this discrepancy back by about 60 OOM (yeah!:yuck:). I find it curious that these same folks do not propose an IR (actually much lower frequency!) cutoff where huge energies might be expressed at wavelengths too long for us to detect.

6. Oct 7, 2005

### Chronos

Turbo, as you know, there is a theoretical IR cutoff, but not very firm. We run up against these kind of walls all the time - largely, IMO, because we have no viable quantum theory of gravity. Personally, I think both theories are incomplete. I hasten to add, however, I believe both theories are correct within their own domain [my inner crackpot is showing].

7. Oct 7, 2005

### dubmugga

so the lowest EM wavelength would be an open 1d string vibrating as a photon at planck level ???

8. Oct 7, 2005

### Chronos

No. Empirical tests deny the 1d result at around 4 sigma levels.

9. Oct 7, 2005

### dubmugga

I thought it would be lower than that !!!

10. Oct 7, 2005

### Orion1

A photon is a 4 dimentional entity, (3 space + 1 time) therefore no photon can exist below D = 4.

'observable' Hubble Photon wavelength:
$$r_u = \lambda_{\gamma}$$
$$\lambda_{\gamma} = \frac{3 c}{H_o}$$

What is the 'wavelength' of this photon?

Reference:
http://universe-review.ca/F02-cosmicbg.htm
http://universe-review.ca/R02-16-universe.htm

Last edited: Oct 7, 2005
11. Oct 7, 2005

### Chronos

Huh? That makes no sense at all... unless you are trying to prove 2 = 3.

Last edited: Oct 7, 2005
12. Oct 7, 2005

### Orion1

Last edited: Oct 7, 2005
13. Oct 8, 2005

### SpaceTiger

Staff Emeritus

14. Oct 8, 2005

### Chronos

Agreed, ST. I fail to see the relevance, which is the basis of my objection.

15. Oct 30, 2005

### Nereid

Staff Emeritus
One could approach the OP's question as some kind of 'what if?' thought exploration; most posts in the thread are of this kind.

Or one could approach this in terms of physical possibilities, within what we know of physics, the universe, etc.

Or one could simply go outside and count the horse's teeth

On the second approach. Leave aside how the longest wavelength EM (radio?) might be created, could they 'exist', in any meaningful sense? For starters, the propogation of long wavelength (low frequency) radio is limited by the plasma frequency, which is proportional to the square root of the electron density. So, the universe seems to set a low frequency bound on radio waves which can propogate, to the lowest density plasma. And where are these plasmas? What are their densities?

Is there an 'out'? Are the large regions of space which are NOT plasmas (or, more precisely, do not contain free electrons)?