Can We See CMB Again If Universe Expands at a Constant Rate?

In summary: Then, 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.
  • #1
CassiopeiaA
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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??
 
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  • #2
CassiopeiaA said:
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.
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
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
karlzr said:
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.

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
CassiopeiaA said:
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.
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
CassiopeiaA said:
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.
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
CassiopeiaA said:
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.
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
I am not sure about this analogy but people can correct me if I am wrong:
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
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
Jimster41 said:
than... any observer' light cone...

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
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?
 
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  • #12
Jimster41 said:
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?

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.
 
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  • #13
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
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
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
 
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  • #16
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.
 
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What is a particle horizon?

The particle horizon is the maximum distance that light has traveled in the entire history of the universe. It marks the boundary of the observable universe, beyond which we cannot see due to the expansion of space.

What is the Cosmic Microwave Background (CMB)?

The CMB is the remnant radiation from the Big Bang that fills the entire universe. It is the oldest light in the universe, and it is nearly uniform in all directions, with a temperature of about 2.7 Kelvin.

How is the particle horizon related to the CMB?

The CMB is the farthest light that we can observe, and its distance from us is determined by the particle horizon. The CMB is essentially the "edge" of the observable universe, beyond which we cannot see.

What is the significance of the particle horizon and CMB?

The particle horizon and CMB provide important information about the age, size, and structure of the universe. They also support the Big Bang theory and offer evidence for cosmic inflation, the rapid expansion of the universe in its early stages.

How is the particle horizon and CMB measured?

The particle horizon and CMB are measured through observations of the cosmic microwave background radiation using telescopes and satellites. The CMB is also studied through data analysis, such as measuring its temperature fluctuations and polarization patterns.

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