Cosmic Microwave Background Origin

In summary: CMB's temperature is uniform across the sky.In summary, the conversation discusses the possibility of the CMB (Cosmic Microwave Background) being composed of light from extremely distant galaxies and the implications of an infinitely large and old universe. However, the concept of an expanding universe and the effects of redshift on light prevent the night sky from being uniformly bright as predicted by Olbers' Paradox. The uniformity of the CMB temperature also suggests that it cannot simply be made up of redshifted galaxies.
  • #1
really
12
0
If the light from a distant galaxy was red shifted far enough wouldn't it appear as microwave? Is it possible that the CMB is just more galaxies beyond what we consider to be the "observeable universe?"
 
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  • #2
If that was the case, there would be no way to explain why the CMB is (almost) a perfect black body.
 
  • #3
nicksauce said:
If that was the case, there would be no way to explain why the CMB is (almost) a perfect black body.
That and its extreme uniformity: the CMB is uniform to one part in 100,000.
 
  • #4
If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
 
  • #5
really said:
If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?

But no one really expects the universe to be infinitely large anymore.
 
  • #6
really said:
If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
Not remotely. Basically, if you were to propose a steady-state model where this sort of thing sort of makes sense, then the far-away universe would be every bit as inhomogeneous as the nearby universe. So you wouldn't be looking out to the same distance in every direction: every place you look on the sky, you'd see a galaxy at a different distance. The temperatures would vary hugely.

Nicksauce also has a very good point about the spectrum: galaxies are very poor black bodies. Their spectra are chock full of all sorts of interesting features. By contrast, the CMB is almost a perfect black-body. So the CMB cannot simply be made up of lots of redshifted galaxies anyway.
 
  • #7
If you start stacking multiple layers of random features, what do you get? I bet it starts to look pretty uniform at some point. If we pick one point in space at our best resolution, in an infinite (or vastly larger than currently projected) universe, that point could still have trillions of galaxies. Even if they all have random and interesting features, they would blend together and look like static to us. If we get better resolution maybe we start to see through the static but no matter how good our instruments are at some point it will just look like static again.
 
  • #8
Chalnoth said:
Nicksauce also has a very good point about the spectrum: galaxies are very poor black bodies. Their spectra are chock full of all sorts of interesting features. By contrast, the CMB is almost a perfect black-body. So the CMB cannot simply be made up of lots of redshifted galaxies anyway.
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.

Although if the universe was infinite, you wouldn't have 3K temperature for the dust.
 
  • #9
What temperature would you expect?
 
  • #10
really said:
What temperature would you expect?

Either 0 or the average temperature of the surface of a star!
That's the problem with infinity, it causes a few problems!
 
  • #11
mgb_phys said:
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.

Although if the universe was infinite, you wouldn't have 3K temperature for the dust.

No it would not. No time for that.
 
  • #12
mgb_phys said:
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.
Well, no, because it would still have to interact with one last galaxy along the line of sight, which would always produce some combination of emission and absorption lines depending on said galaxy's chemistry.
 
  • #13
That's also true of the CMB - you have to remove the effects of foreground galaxies.

I'm not convinced that you can't produce a uniform thermal spectrum by putting any source through an infinite column on absorbing material.
 
  • #14
mgb_phys said:
That's also true of the CMB - you have to remove the effects of foreground galaxies.
That effect is extremely small at the frequencies where the CMB is strongest. Even our own galaxy isn't that bright at around 90-100GHz.

mgb_phys said:
I'm not convinced that you can't produce a uniform thermal spectrum by putting any source through an infinite column on absorbing material.
If the column were uniform, sure, that would work. But the problem is that the universe is expanding and sparsely-populated, which means that the radiation won't get a chance to thermalize before it redshifts away on its trip between galaxies.
 
  • #15
See Olber's paradox. If the universe was infinitely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
 
  • #16
Chronos said:
See Olber's paradox. If the universe was infinitely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
Expansion counters this. I was assuming an expanding universe, along the lines of a steady state universe idea, where you have an exponential expansion where hydrogen is continually produced out of the vacuum (completely unrealistic, but at least it isn't quite as obviously false as a non-expanding eternal universe).
 
  • #17
Chronos said:
See Olber's paradox. If the universe was infinitely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.

How would it be as bright as the surface of a star if the light redshifts out of the visible spectrum?
 
  • #18
really said:
How would it be as bright as the surface of a star if the light redshifts out of the visible spectrum?
The Olbers' Paradox assumes a static universe, with no expansion.
 
  • #19
Chalnoth said:
The Olbers' Paradox assumes a static universe, with no expansion.

So do I, a static universe with redshift caused by something other than expansion.
 
  • #20
In an infinitely spacious, ancient and populated universe, how does expansion prevent the night sky from being lit up like a birthday cake? Bear in mind every photon ever emitted in the universe had an infinite amount of time to reach us [and energize every molecule blocking its path]. Something is very wrong with this picture.
 
  • #21
really said:
So do I, a static universe with redshift caused by something other than expansion.
That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.

Furthermore, a static universe is unstable: any slight perturbation will cause either collapse or eternal expansion.
 
  • #22
Chalnoth said:
That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.

Furthermore, a static universe is unstable: any slight perturbation will cause either collapse or eternal expansion.

Photonic clocks? Time running slower in the past. I know its been refuted by gravity probe B, but nevertheless it is interesting. At least it is in the good agreement with pioneer anomaly.
 
  • #23
Calimero said:
Photonic clocks? Time running slower in the past. I know its been refuted by gravity probe B, but nevertheless it is interesting. At least it is in the good agreement with pioneer anomaly.
As near as I can tell, there is no evidence that the Pioneer anomaly has anything whatsoever to do with basic physics. It's more likely something to do with the spacecraft itself.

As for time running slower in the past, this is contraindicated by a wide variety of studies of the variation of physical constants with time, all of which, upon detailed analysis, show zero variation to within experimental errors.
 

1. What is the Cosmic Microwave Background (CMB)?

The Cosmic Microwave Background (CMB) is a form of electromagnetic radiation that permeates the entire universe. It is the remnants of the radiation that was present just 380,000 years after the Big Bang, and can be observed in all directions in the sky.

2. How was the CMB formed?

The CMB was formed when the universe was just 380,000 years old and had cooled enough for neutral atoms to form. This allowed photons (light particles) to travel freely without being constantly scattered by electrons. The photons we observe today as the CMB were released during this time.

3. What does the CMB tell us about the early universe?

Studying the CMB allows us to gain insight into the early universe, as it is the oldest light in the universe. By analyzing the temperature and polarization patterns in the CMB, we can learn about the composition, age, and expansion of the universe, as well as the formation of structures such as galaxies and galaxy clusters.

4. What is the significance of the CMB in modern cosmology?

The CMB is a crucial piece of evidence for the Big Bang theory, which is the most widely accepted explanation for the origin and evolution of the universe. It also provides a snapshot of the universe when it was only 380,000 years old, allowing us to study the universe in its infancy and make predictions about its future.

5. How is the CMB observed and measured?

The CMB is observed using specialized telescopes and instruments that can detect microwave radiation. Scientists use precise measurements of the CMB's temperature and polarization to create maps of the radiation across the sky. These maps provide valuable data for cosmological studies and can be compared to theoretical models to better understand the origin and evolution of the universe.

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