Why do we observe only microwaves in the background radiation?

In summary, the cosmic microwave background (CMB) radiation, which was emitted at a temperature of 3,000 kelvin, has been stretched by the expansion of space over the last 13 billion years. This means that the radiation now falls mostly in the microwave range and is the only known source of electromagnetic radiation in the universe at 380,000 years old. Due to the broad spectrum of radiation emitted by matter, it is difficult to differentiate the CMB from equipment noise. This is why it took several years to confirm the recent report on CMB polarization. If the CMB were in the visible range, the night sky would still appear black due to its low energy density.
  • #1
genefalk
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Why aren't the other bands (visible, UV, etc.) in the electromagnetic spectrum also seen today as part of the background radiation?
 
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  • #2
When the CMB was first emitted, the plasma filling all of space was at a temperature of around 3,000 kelvin and much of the radiation was in the visible range. However, over the last 13 billion years the expansion of space has stretched out the radiation until the wavelengths now fall mostly in the microwave range.
 
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  • #3
The CMB is now stretched by a factor of 1090, as Drakkith noted. That 3000K emission is the only known EM radiation source in the universe at age 380,000 years.
 
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If you are familiar with the theory of black body radiation, you realize that at a given temperature all matter radiates a broad spectrum of emr. Since any detection device is comprised of matter, there is an intensity of emr which is simply not differentiable from equipment noise. (not to mention external sources). Take a look at the recent report from antarctica on the CMB polarization. It took them 3 years to make sure what they thought they were seeing wasn't an artifact. Basically, its the same reason as why we couldn't see the meteor that struck Russia (till it was right on top of us) ...its called "glare".
 
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If the background radiation that we see today were in the visible range (for example red), would the night sky be red instead of black?
 
  • #6
genefalk said:
If the background radiation that we see today were in the visible range (for example red), would the night sky be red instead of black?

The light would still be very faint, as there's very little energy density of the CMB now. I doubt you would be able to see it even if it was in the visible spectrum.
 

FAQ: Why do we observe only microwaves in the background radiation?

1. Why do we observe only microwaves in the background radiation?

The main reason for observing only microwaves in the background radiation is because of the expansion of the universe. This radiation, also known as the Cosmic Microwave Background (CMB), was emitted about 380,000 years after the Big Bang. As the universe expanded, the wavelengths of this radiation stretched and shifted towards longer wavelengths, which fall within the microwave range.

2. Is the CMB the same in all directions?

Yes, the CMB is the same in all directions. This is one of the key pieces of evidence for the Big Bang theory. The CMB is a remnant of the hot, dense state of the early universe, and as the universe expanded, the radiation became more uniform and isotropic. This is known as the isotropy of the CMB.

3. Can we detect any other types of radiation in the CMB?

Yes, there are small variations in the CMB that can be detected in other wavelengths, such as infrared and radio waves. These variations are important in understanding the structure and evolution of the universe, and they can also provide clues about the properties of dark matter and dark energy.

4. How does the CMB support the Big Bang theory?

The CMB is a crucial piece of evidence for the Big Bang theory. Its uniformity and isotropy support the idea that the universe was once in a hot, dense state and has been expanding ever since. The CMB also contains small variations, known as anisotropies, which can be studied to learn more about the early universe and its evolution.

5. Can we see the CMB with the naked eye?

No, we cannot see the CMB with the naked eye. The CMB is a form of electromagnetic radiation, which is invisible to the human eye. It can only be detected and studied using specialized equipment, such as telescopes and detectors designed to pick up microwaves.

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