APOD: Cosmic Latte: The Average Color of... (2020 Dec 27)

[johnnydeep]

What is the context of the question? Is it asking the color from the surface of Earth? If so then day or night? If not the surface of Earth then where? If daytime on the surface of Earth then why is it not Blue? If nighttime on the surface of Earth then why is it not nearly Black? I think the question needs clarification. And saying it is the colors excluding Black space contradicts the entire sky were smeared out.

Yes, it's rather silly. Color is a physiological thing, not a physical thing. This image is not "the color of the Universe". In a limited way, it might represent the hue, but intensity is part of color, and clearly the intensity of this image does not match the average intensity of the Universe. To our eyes, the "average color" of the Universe would be very close to black.

Hmm. What would you call the average frequency of all the photons hitting a detector located outside of earth’s atmosphere? Where that is calculated at the sum of the frequencies of all such photons, divided by the total number of photons, during some time interval starting at time T? It’s not a “color” I guess, but it is something.

[Chris Peterson]

What is the context of the question? Is it asking the color from the surface of Earth? If so then day or night? If not the surface of Earth then where? If daytime on the surface of Earth then why is it not Blue? If nighttime on the surface of Earth then why is it not nearly Black? I think the question needs clarification. And saying it is the colors excluding Black space contradicts the entire sky were smeared out.

Yes, it's rather silly. Color is a physiological thing, not a physical thing. This image is not "the color of the Universe". In a limited way, it might represent the hue, but intensity is part of color, and clearly the intensity of this image does not match the average intensity of the Universe. To our eyes, the "average color" of the Universe would be very close to black.

Hmm. What would you call the average frequency of all the photons hitting a detector located outside of earth’s atmosphere? Where that is calculated at the sum of the frequencies of all such photons, divided by the total number of photons? It’s not a “color” I guess, but it is something. :ssmile:

Well... the "average frequency" of a broad spectrum of photons is not the same as the color that our eye would perceive. What you're talking about is a bit like spectral irradiance, which is the power per area per hertz.

(In fact, if you think about it, the "average frequency" would yield a single pure spectral wavelength, whose apparent color would depend on its intensity.)

[Sa Ji Tario]

(Right after Christmas), is it that you are looking at the color of classic beer?

[neufer]

Color is a physiological thing, not a physical thing. This image is not "the color of the Universe". In a limited way, it might represent the hue, but intensity is part of color, and clearly the intensity of this image does not match the average intensity of the Universe. To our eyes, the "average color" of the Universe would be very close to black.

Hmm. What would you call the average frequency of all the photons hitting a detector located outside of earth’s atmosphere? Where that is calculated at the sum of the frequencies of all such photons, divided by the total number of photons? It’s not a “color” I guess, but it is something.

Well... the "average frequency" of a broad spectrum of photons is not the same as the color that our eye would perceive. What you're talking about is a bit like spectral irradiance, which is the power per area per hertz. (In fact, if you think about it, the "average frequency" would yield a single pure spectral wavelength, whose apparent color would depend on its intensity.)

One should note that only 0.002 % of all photons are non-Cosmic microwave background photons

The observable universe contains:

• ~ 2 × 10⁸⁹ Cosmic microwave background photons (~500 photons/cm³).
  ~ 4 x 10⁸⁴ diffuse extragalactic background light photons (~0.01 photons/cm³).

<<The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers almost all wavelengths of the electromagnetic spectrum, except the microwave, which is dominated by the primordial Cosmic Microwave Background. The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the entire electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe.

The understanding of the EBL is also fundamental for extragalactic very-high-energy (VHE, 30 GeV-30 TeV) astronomy. VHE photons coming from cosmological distances are attenuated by pair production with EBL photons. This interaction is dependent on the spectral energy distribution (SED) of the EBL. Therefore, it is necessary to know the SED of the EBL in order to study intrinsic properties of the emission in the VHE sources.

The direct measurement of the EBL is a difficult task mainly due to the contribution of zodiacal light that is orders of magnitude higher than the EBL. Different groups have claimed the detection of the EBL in the optical and near-infrared. However, it has been proposed that these analyses have been contaminated by zodiacal light. Recently, two independent groups using different technique have claimed the detection of the EBL in the optical with no contamination from zodiacal light.

There are also other techniques that set limits to the background. It is possible to set lower limits from deep galaxy surveys. On the other hand, VHE observations of extragalactic sources set upper limits to the EBL.

In November 2018, astronomers reported that the EBL amounted to 4 x 10⁸⁴ photons.>>

[Chris Peterson]

Hmm. What would you call the average frequency of all the photons hitting a detector located outside of earth’s atmosphere? Where that is calculated at the sum of the frequencies of all such photons, divided by the total number of photons? It’s not a “color” I guess, but it is something. :ssmile:

Well... the "average frequency" of a broad spectrum of photons is not the same as the color that our eye would perceive. What you're talking about is a bit like spectral irradiance, which is the power per area per hertz. (In fact, if you think about it, the "average frequency" would yield a single pure spectral wavelength, whose apparent color would depend on its intensity.)

One should note that only 0.002 % of all photons are non-Cosmic microwave background photons

The observable universe contains:

• ~ 2 × 10⁸⁹ Cosmic microwave background photons (~500 photons/cm³).
  ~ 4 x 10⁸⁴ diffuse extragalactic background light photons (~0.01 photons/cm³).

<<The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers almost all wavelengths of the electromagnetic spectrum, except the microwave, which is dominated by the primordial Cosmic Microwave Background. The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the entire electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe.

The understanding of the EBL is also fundamental for extragalactic very-high-energy (VHE, 30 GeV-30 TeV) astronomy. VHE photons coming from cosmological distances are attenuated by pair production with EBL photons. This interaction is dependent on the spectral energy distribution (SED) of the EBL. Therefore, it is necessary to know the SED of the EBL in order to study intrinsic properties of the emission in the VHE sources.

The direct measurement of the EBL is a difficult task mainly due to the contribution of zodiacal light that is orders of magnitude higher than the EBL. Different groups have claimed the detection of the EBL in the optical and near-infrared. However, it has been proposed that these analyses have been contaminated by zodiacal light. Recently, two independent groups using different technique have claimed the detection of the EBL in the optical with no contamination from zodiacal light.

There are also other techniques that set limits to the background. It is possible to set lower limits from deep galaxy surveys. On the other hand, VHE observations of extragalactic sources set upper limits to the EBL.

In November 2018, astronomers reported that the EBL amounted to 4 x 10⁸⁴ photons.>>

Interesting... but for the purposes of this discussion, I assume we're only discussing photons in the range of visible light.

[VictorBorun]

Do I get it right?

1. Here on Earth we are biased. Let's take an observer in the middle of nowhere, in the center of a typical void. With a telescope, he'll get to see clusters of galaxies all around, say, 250 million ly away, or 100 times the distance of Andromeda galaxy. What mean spectrum that observer will see? Will it be like our Sun's, or will it be tainted with a color? What color that will be?
If we try to calculate the answer looking in our sky, we have to correct what we see. A distant galaxy looks smaller in proportion with the distance at the time of emitting the light, fainter and redder (z+1) times, and at a younger age of its evolution. After corrections, we can simulate a mean color of today.

2. Is it darker or lighter, we can not reason. It won't be a full blown color, it will be just hue plus saturation. We can place it near white, or near black, or near 50% gray, it does not matter. If near white, it's latte. If near black, it's raven black horse.