# Particle horizon and CMB

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1. May 8, 2015

### CassiopeiaA

If particle horizon is the maximum distance we can see presently in the universe, how come we are able to see CMB? CMB is radiation from surface of last scattering happened at t~380k years.

Suppose universe is expanding at a constant rate ( i.e. no acceleration), will we be able to see CMB again??

2. May 8, 2015

### Jorrie

The particle horizon radius is always larger than the CMB surface radius, because it is the hypothetical distance that a mass-less particle that originated from the 'Big Bang" itself, i.e. at t~0, could have traveled.

3. May 8, 2015

### karlzr

The last scattering surface is inside the horizon or what we call the observable universe (about 10^26 m). I am not sure what your question is.

4. May 8, 2015

### CassiopeiaA

If we see farther and farther away in universe, we can see back in time. I think this is the point at which i am getting confused. How can we see quasars and CMB, and not the intermediate things and will be able to see big bang if we again keep looking farther as the time progresses.

5. May 8, 2015

### karlzr

Before last scattering, the universe is opaque due to the very short mean free path of photons. There is no way for us to see earlier than last scattering. After last scattering, the density of electrons drops dramatically and photons can free stream almost all the way to us today. This is why we can see CMB. I am not sure how we see quasars or other galaxy stuff exactly. Perhaps through their intrinsic spectra due to different temperature? Just my guess.

6. May 8, 2015

### Jorrie

If we could "see" neutrinos, we could have looked back to around 2 seconds after the BB, the decoupling of neutrinos from baryons. Using Planck mission values, the present proper distance to the CMB origin is calculated to be around 45 billion ly, while the present proper distance to the particle horizon is about 46 billion ly. Due to the expansion, these proper distances are growing by about 3 light years per year.

7. May 8, 2015

### Chalnoth

We can see many of the intermediate things. There is a period, known as the "dark ages" between about Z = 1090 and Z = 20 or so between when the CMB was emitted and the first stars started to form. From this period we can't really see anything because all that there was at the time was a nearly-uniform and almost perfectly-transparent gas. But after Z = 20, we see more and more galaxies the closer we get to us.

8. May 10, 2015

### ChrisVer

Instead of looking it from inside out, it's better to think about it from the outside-in...
I mean think that the CMB is some surface enclosing us (in the center), and it's the surface beyond which we cannot see any photons.
Now think that everything between us and that surface is expanded. The surface itself gets more redshifted (goes further in the past) and our horizon gets expanded so we can see larger distances/further in the past-up to the point of the surface of the last scattering.

9. May 10, 2015

### Jimster41

This conversation had made me aware that I am confused about where the CMB is. I flip back and forth between "all around us", "at some distant past sphere representing a ghost of the smaller universe at the moment of last scattering", and as @ChrisVer describes it, at a distant boundary sphere hugely bigger than... any observer' light cone..., then I think it's all three, and that does not really help.

10. May 11, 2015

### ChrisVer

No I didn't mean that when I wrote "photons can't reach us". I meant that before that time, photons were trapped in the primodial plasma.

11. May 11, 2015

### Jimster41

Is was thinking of the radius of the CMB light cone. Isn't that radius going to be close to the size of the universe, since the CMB photons have been traveling at the speed of light, and have been getting a "boost" from a(t), since the primordial event of last scattering? What observers could be outside that light cone? In other words all observers are awash in CMB radiation, and no observer's past light cone extends beyond the CMB light cone?

Last edited: May 11, 2015
12. May 11, 2015

### Jorrie

For every observer, that radius is fairly close to the observable radius, but every observer in the universe has an own light cone and they do not necessarily have to overlap. Each sees different photons from different regions, precisely because every observer is "awash in CMB radiation".

BTW, the CMB does not have a light cone - it has been emitted from everywhere in the universe.

13. May 11, 2015

### marcus

Jimster, if you haven't already, check out this link in J's signature:
https://www.physicsforums.com/attachments/davisdiagramoriginal2-jpg.55869/
the top panel shows proper distance: that is actual distance at that moment, if you could pause expansion to give time to measure it.

the universe of the top panel is wide, possibly infinite, or possibly just very much wider than the observers light cone.

It helps, too, to realize that in proper distance the SHAPE of a lightcone is PEAR-SHAPE. You can see that in the top panel.

14. May 11, 2015

### Jimster41

Gotta stare at those... awhile.

What is the "co-moving" frame with respect to? I want to say "with expansion" ?

Is there a key, or write-up specifically for those?

15. May 11, 2015

### marcus

A comoving observer is at rest relative to the expansion process, i.e. relative to the CMB. the expansion is "isotropic" looks the same in all directions, doesn't have a slow spot in one direction and a fast spot in the other.
the CMB doesn't have a doppler hot spot in one direction and a cold spot in the other.

Most of the matter objects in the universe are APPROXIMATELY at rest, e.g. the solar system is only moving at about 370 km/s (it sees a CMB hot spot in that direction) but that is small by comparison with most distance expansion speeds. things have their small individual random motions.

the comoving distance to an observer at rest is simply the distance NOW, and that becomes a permanent tag or label ,
except for small individual random motion, a GAlAXY's comoving distance remains constant thru history.

So it is a way of permanently labeling things, by comoving coordinates

16. May 11, 2015

### marcus

Jimster originally before we had the CMB soup of ancient light people talked about observers and objects "comoving with respect to the Hubble flow" meaning at rest relative to the expansion process. so you are basically right in guessing as you did that comoving coordinates are "with respect to expansion". The expansion process itself is the anchor or point of reference.