# Why is the CMBR visible to us?

1. Aug 9, 2014

### connorp

Since the observable universe is expanding, over time, more galaxies should become visible on Earth as their light has had more time to reach us. But galaxies couldn't form until after recombination. So why is the CMB visible if light sources younger than it are not visible yet?

2. Aug 9, 2014

### phinds

There were no galaxies, planets, or ANY large objects up to the time of the surface of last scattering (which is now the CMB), so nothing big is older than the CMB and there is no electromagnetic radiation from before the CMB that can ever reach us.

EDIT: OOPS ... I see I answered a different question than what you actually asked, and you already know about what I said in my answer. Careless of me.

Last edited: Aug 9, 2014
3. Aug 9, 2014

### MrAnchovy

Because the regions of space where the currently visible CMB originated are closer to Earth than the regions of space where the light sources you are referring to are located.

4. Aug 9, 2014

### marcus

From the way you put the question, Connor, I think you understand that the observable portion of the universe is growing in TWO senses---over time it takes in more and more matter (as light from more matter has time to arrive) and also because geometry itself is dynamic and distances are increasing (without anybody really going anywhere).

When light from more remote matter arrives, we see that matter AS IT WAS back at recombination time when space became sufficiently transparent for light to travel freely. So we see the additional matter as it was around year 380,000, namely as glowing hot gas (like the surface of an orange-ish star) except with the wavelengths in the mix of thermal glow light stretched by a factor of about a thousand.

So seeing more GALAXIES is not exactly the issue. The observable portion of matter includes more matter, but that additional matter (as we see it) is still in the form of hot gas.

5. Aug 10, 2014

### connorp

So the space where the CMB was emitted is further back in time but closer in space than these old, distant galaxies since the universe was much smaller back then, which makes the CMB visible to us know?

6. Aug 10, 2014

### Chronos

No. The CMB is not 'closer' than any 'old distant galaxy'. All known galaxies are in the foreground of the CMB for reasons already mentioned - unless of course you know of a galaxy with a redshift greater than 1090.

7. Aug 10, 2014

### bapowell

The CMB was emitted everywhere 380,000 years after the big bang. The radiation has always been visible everywhere. The CMB photons we see here on Earth today appear to have been emitted from the inside surface of a great sphere with Earth at the center (called the surface of last scattering). It is the farthest light we can see. Tomorrow, we will still be receiving CMB photons from slightly farther out.

8. Aug 10, 2014

### Chalnoth

If we were to examine the matter that emitted the CMB we currently see right now over a period of billions of years, this is what we would see:

1. That matter would condense from a plasma into a gas and become transparent.
2. Some time later, gas clouds within that matter would collapse to form stars, and the light from those stars would re-ionize the gas into a plasma.
3. Those collections of stars and gas would collapse to form new galaxies.

This is all another way of saying that the "new" galaxies that we could see would be galaxies that we could, in principle, observe being formed.

That said, the accelerated expansion of our universe throws a big wrinkle into this, effectively making it so that even if we had an infinite amount of time to wait we couldn't see the entire history of the galaxies around us. Instead, depending upon how far away they are, we would only be able to see them age up to a certain point, depending upon when they crossed our horizon. We would essentially see the time on the far-away galaxy appear to slow to a crawl but never quite stop, and never get past the time they crossed the horizon.

9. Aug 11, 2014

### Damo ET

To answer the OP (if I understand it correctly), the last scattering is the earliest point after the BB that photons could be seen in transparent space. Galaxies did not form until much later as time was needed for the formation to take place, so there is a region between the CMB and the oldest (youngest) galaxies that we can see literally nothing but a void. I would hazard a guess that there would be a distinct distance/time between the earliest galaxies we can possibly see and the CMB. So as time goes on, we should see (in theory) the oldest galaxies forming.

Damo

10. Aug 11, 2014

### marcus

I think protogalaxies, small blobs forming their first stars, ARE visible. They are difficult to see because the light has been stretched (by a factor of 10 or so) into infrared and they are small and dim with just a few stars (fewer than more mature galaxies have). I've heard about this but I don't know much about it.

Maybe googling "protogalaxy" or "early stars" or "first stars" would get some info.

I understand that galaxies as we know them formed by the gathering together of these small shapeless protogalaxy blobs of stars. I'm not sure how well the process of spiral structure formation is understood. Structure would have taken a while to form, I imagine.

There is an interesting point here. The past lightcone in terms of real spatial distance, is TEARDROP shape! The base instead of being spread out like a cone is drawn together because distances were smaller.

The matter that was the hot gas radiating what became the cmb we now see WAS comparatively close! (only 42 million LY) because everything was closer together. Expansion (more rapid back then) delayed the light reaching us until now. Now that same matter is more distant by a factor of more than 1000. It is around 45 BILLION LY. That factor (about 1090) is also by how much the wavelengths of the ancient light have been stretched.

You can get a rough idea from this diagram ("caltech…figure 1" link in my signature) and also from distance tables made by Jorrie's "Lightcone" calculator ("LightCone" link in sig)
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
the top figure is in real or "proper" distance, as it was at the time if you could have stopped expansion long enough to measure---it shows the teardrop shape! Do you see it?
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html.
The Lightcone table shows that distance to cmb-emitting matter was about 42 million years
(see 0.042 billion years in top row of table)
whereas distance to first star forming matter was more like 3 billion years (see the S = 10 row of the table, we receive protogalaxy light wavelengths stretched by a S=z+1 factor of about 10)
$${\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}$$ $${\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&0.0004&0.0006&45.332&0.042&0.057&3.15&66.18\\ \hline 0.003&339.773&0.0025&0.0040&44.184&0.130&0.179&3.07&32.87\\ \hline 0.009&105.913&0.0153&0.0235&42.012&0.397&0.552&2.92&16.90\\ \hline 0.030&33.015&0.0902&0.1363&38.052&1.153&1.652&2.64&8.45\\ \hline 0.097&10.291&0.5223&0.7851&30.918&3.004&4.606&2.15&3.83\\ \hline 0.312&3.208&2.9777&4.3736&18.248&5.688&10.827&1.27&1.30\\ \hline 1.000&1.000&13.7872&14.3999&0.000&0.000&16.472&0.00&0.00\\ \hline 3.208&0.312&32.8849&17.1849&11.118&35.666&17.225&0.77&2.08\\ \hline 7.580&0.132&47.7251&17.2911&14.219&107.786&17.291&0.99&6.23\\ \hline 17.911&0.056&62.5981&17.2993&15.536&278.256&17.299&1.08&16.08\\ \hline 42.321&0.024&77.4737&17.2998&16.093&681.061&17.300&1.12&39.37\\ \hline 100.000&0.010&92.3494&17.2999&16.328&1632.838&17.300&1.13&94.38\\ \hline \end{array}}$$

Last edited: Aug 11, 2014
11. Aug 12, 2014

### connorp

Thanks all. I mostly understand now. Just one more question now. So as I understand (or at least think I do), if the space between us and a galaxy is expanding faster than c, those photons will never be able to reach us and the galaxy will effectively "wink out" and disappear. And space expands faster as you go out over larger distances, correct? So if distant galaxies can disappear due to space expanding faster than c, why won't the CMB since its even farther out? If expansion truly is unbounded, the CMB will eventually be redshifted beyond detection. But why won't it "wink out" well before that?

12. Aug 12, 2014

### Chronos

Again, no. See Davis and Lineweaver for details - http://arxiv.org/abs/astro-ph/0310808, Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe.

13. Aug 12, 2014

### phinds

As Chronos said, no. There are galaxies within the observable universe that are receding from us at about 3c and we can see them.

It is not helpful to think of space expanding. Space is not a "thing" that can stretch or bend or expand. Space-time is spoken of as bending or being warped, but that's an analogy to help see the effects of large masses on things. It is much better to realize that it is just distances getting larger, not space expanding. Google "metric expansion" for more information.

See above.

14. Aug 12, 2014

### bapowell

Space expands at a rate given by the Hubble parameter. In a homogeneous universe, it expands at this same rate everywhere. Via Hubble's Law, v = Hr, the recession velocity of an object depends on both this expansion rate and its separation, r, from us. So given an expansion rate that is constant in space, the recession velocities vary linearly with distance.

Indeed, once an object reaches a distance r = c/H, it begins to recede at superluminal speeds (it's never correct to say that "space expands faster than the speed of light" -- the expansion of space is not measured in terms of speed and it is the objects within space that attain superluminal speeds on account of the expansion). However, these objects are still visible, since the light they emit towards Earth still has no problem reaching us. Only under very special circumstances (so-called de Sitter inflation), is the rate of spatial expansion so great that the photons from superluminally receding objects never reach Earth.

15. Aug 12, 2014

### marcus

Connor, at any given time there is a "safe range" within which distances are NOT growing faster than light.

There are plenty of of galaxies which are today OUTSIDE that range but which have already sent us thousands of years worth of photons which are already WITHIN safe range and which will be able to eventually make it here to us.

I wouldn't expect any galaxy that is visible today to abruptly "wink out". But you are right that the light from objects can "eventually be redshifted beyond detection". You mention this in connection with the CMB but it is a more general expectation. The light we will be receiving from the galaxies (outside our immediate group) will be taking longer and longer to get here and will arrive more and more wave-stretched until the once-visible light is all radio waves and until the waves are so long that no practical-size antenna can pick them up. the galaxies we can now see (outside our local group) will very slowly, over billions of years, RED OUT, but they will not wink out.

How are you with numbers? I simplified the earlier table by eliminating unnecessary columns and increasing the number of steps and narrowing the time-span covered. I want to illustrate how the "safe range" changes over time. If you go to the "Lightcone" link you will see how you can change the number of steps or rows in the table, eliminate columns etc.

R is the "safe range" within which photons can make forward progress because distances grow < c
T is the year, the table starts with year 545 million, goes to present around year 13.8 billion, and on into future.
a is the scale of distances with present distance called "1". So back in year 545 million at the start of the table distances were ONE TENTH what they are today. and by the end of the table, the last row, those same distances will be ten times what they are today.

$${\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&T (Gy)&R (Gly) \\ \hline 0.100&0.545&0.820\\ \hline 0.126&0.771&1.157\\ \hline 0.158&1.089&1.631\\ \hline 0.200&1.536&2.294\\ \hline 0.251&2.165&3.213\\ \hline 0.316&3.041&4.463\\ \hline 0.398&4.250&6.105\\ \hline 0.501&5.883&8.135\\ \hline 0.631&8.015&10.403\\ \hline 0.794&10.669&12.602\\ \hline 1.000&13.787&14.400\\ \hline 1.259&17.257&15.649\\ \hline 1.585&20.956&16.410\\ \hline 1.995&24.789&16.836\\ \hline 2.512&28.694&17.063\\ \hline 3.162&32.638&17.180\\ \hline 3.981&36.601&17.240\\ \hline 5.012&40.575&17.270\\ \hline 6.310&44.553&17.285\\ \hline 7.943&48.534&17.292\\ \hline 10.000&52.516&17.296\\ \hline \end{array}}$$

The technical name for what I called the "safe range" is actually "Hubble radius". at any given time in history, the Hubble radius is the size of distances which are growing exactly at speed c. the others grow proportionately. So if a distance is half R then it is growing at half c speed.
Any distance less than R is growing at less than c speed. So a photon coming towards us which has made it within that range is safe, it will not be swept back.
There is even an additional help to photons which comes from R gradually increasing so that it reaches out to some ones that were getting swept back and takes them in too. But that is a fine point. the real "safe" zone is actually slightly larger than the radius R indicates, because R is growing over time.

Last edited: Aug 12, 2014
16. Aug 12, 2014

### Tanelorn

Marcus, regarding winking out presumably the galaxy photon RF frequencies would drop to KHz, Hz and then milli Hz first, before vanishing completely. I wonder if these frequencies can be detected with an extremely sensitive and selective radio receiver? I am guessing way too much interference!

17. Aug 12, 2014

### Chalnoth

They would never vanish "completely". The wavelengths would approach infinity asymptotically as time approaches infinity. But the progression would be very, very slow.

For instance, at the current expansion rate, it will take close to 10 billion years for the scale factor to increase by a factor of two. But for a galaxy that we currently see in the visible range (hundreds of THz), we would need the scale factor to increase by a factor of around $10^{14}$ to get into the single Hz frequency range. That won't happen for something like $10^{24}$ years.

18. Aug 13, 2014

### Ich

Actually, the redshift increases exponentially. So, to reach $z=10^{14}$, we're talking about "only" some 500 billion years.

19. Aug 13, 2014

### Chalnoth

Ahh, right. It was a bad copy and paste job between calculations. My bad.