Some questions about Cosmic Microwave Background radiation

[PainterGuy]

Hi,

I have some questions about cosmic microwave background radiation, CMB, and I thought it's better to ask them together. I have combined all related content for each question to make the question clearer, understandable, and to provide proper context for any person like who stumbles upon this thread. I'd appreciate it if you could help me with the questions in layman terms. Thank you, in advance!Question 1:

In cosmology, recombination refers to the epoch during which charged electrons and protons first became bound to form electrically neutral hydrogen atoms. Recombination occurred about 370,000 years[1][notes 1] after the Big Bang (at a redshift of z = 1100). The word "recombination" is misleading, since the Big Bang theory doesn't posit that protons and electrons had been combined before, but the name exists for historical reasons since it was named before the Big Bang hypothesis became the primary theory of the creation of the universe.

Immediately after the Big Bang, the universe was a hot, dense plasma of photons, leptons, and quarks: the quark epoch. At 10^−6 seconds, the Universe had expanded and cooled sufficiently to allow for the formation of protons: the hadron epoch. This plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short. This is the current state of the interior of the Sun. As the universe expanded, it also cooled. Eventually, the universe cooled to the point that the formation of neutral hydrogen was energetically favored, and the fraction of free electrons and protons as compared to neutral hydrogen decreased to a few parts in 10,000.

Recombination involves electrons binding to protons (hydrogen nuclei) to form neutral hydrogen atoms. Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient[clarification needed], these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons. Two main pathways exist: from the 2p state by emitting a Lyman-a photon - these photons will almost always be reabsorbed by another hydrogen atom in its ground state - or from the 2s state by emitting two photons, which is very slow.

This production of photons is known as decoupling, which leads to recombination sometimes being called photon decoupling, but recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter and constitute what is observed today as cosmic microwave background radiation (in that sense, the cosmic background radiation is infrared [and some red] black-body radiation emitted when the universe was at a temperature of some 3000 K, redshifted by a factor of 1100 from the visible spectrum to the microwave spectrum).

Source: https://en.wikipedia.org/wiki/Recombination_(cosmology)

In cosmology, decoupling refers to a period in the development of the universe when different types of particles fall out of thermal equilibrium with each other. This occurs as a result of the expansion of the universe, as their interaction rates decrease (and mean free paths increase) up to this critical point. The two verified instances of decoupling since the Big Bang which are most often discussed are photon decoupling and neutrino decoupling, as these led to the cosmic microwave background and cosmic neutrino background, respectively.

Photon decoupling is closely related to recombination, which occurred about 378,000 years after the Big Bang (at a redshift of z = 1100), when the universe was a hot opaque ("foggy") plasma. During recombination, free electrons became bound to protons (hydrogen nuclei) to form neutral hydrogen atoms. Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons. Because the neutral hydrogen that formed was transparent to light, those photons which were not captured by other hydrogen atoms were able, for the first time in the history of the universe, to travel long distances. They can still be detected today, although they now appear as radio waves, and form the cosmic microwave background ("CMB"). They reveal crucial clues about how the universe formed.

Source: https://en.wikipedia.org/wiki/Decoupling_(cosmology)

The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K. The spectral radiance dEν/dν peaks at 160.23 GHz, in the microwave range of frequencies, corresponding to a photon energy of about 6.626 ⋅ 10^−4 eV. Alternatively, if spectral radiance is defined as dEλ/dλ, then the peak wavelength is 1.063 mm (282 GHz, 1.168 ⋅ 10^−3 eV photons).

Source: https://en.wikipedia.org/wiki/Cosmic_microwave_backgroundQuestion statement:
The CMB correspond to a blackbody spectrum at almost 2.7 K and it has frequency range of about 3×10^8 to 3×10^11 Hz, or 0.3–300 GHz.

Temperature is an emergent or ensemble property of a system or an object. A blackbody at a temperature of almost 2.7 K radiates a range of different photons as given by the range of frequencies for CMB. The relevant formula is E=hf where E is energy of photon, h is Planck's constant, and f is frequency.

I think that the set of photons radiated by a blackbody at temperature of 2.7 K or the range photons making up the CMB could be emitted by things at temperatures higher than 2.7 K. I think even antennas can radiate those photons. In other words, you don't need something at 2.7 K to get that range of CMB photons. You can get those same photons from objects at different temperatures. The photons in themselves do not carry any information about the temperature. Assuming what I'm saying is true, how one could differentiate actual CMB photons and seemingly/fake CMB photons?Question 2:
The source both images shown below is https://www.britannica.com/science/cosmic-microwave-background .

Description for Image #1 shown below is from the source.

A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Colour differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the "seeds" that became the galaxies. WMAP's data support the big bang and inflation models.

Description for Image #2 shown below is from the source.

Image of the cosmic microwave background, taken by the Differential Microwave Radiometer on board the U.S. satellite Cosmic Background Explorer. The red features in the image show places where the universe was slightly denser, thus stimulating gravitational separation and, ultimately, the formation of galaxies.

Question statement:
In Image #1, the blue spots are parts where matter density was higher and these higher density also had higher temperature. So, in Image 1 blue represents high density and high temperature regions. Could you please confirm this?Question 3:
In Image #1 shown above, there are different colors used: dark blue -> light blue -> green -> yellow -> red.

Is dark blue being used to represent the most dense parts and red the least dense parts?Question 4:
Do both Image #1 and Image #2 shown above convey the same information?Question 5:

Neutrinos are the most abundant particles that have mass in the universe. Every time atomic nuclei come together (like in the sun) or break apart (like in a nuclear reactor), they produce neutrinos. Even a banana emits neutrinos—they come from the natural radioactivity of the potassium in the fruit.

Once produced, these ghostly particles almost never interact with other matter. Tens of trillions of neutrinos from the sun stream through your body every second, but you can’t feel them.

Source: https://www.energy.gov/science/doe-explainsneutrinos

The cosmic neutrino background (CNB or CνB) is the universe's background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos.

The CνB is a relic of the Big Bang; while the cosmic microwave background radiation (CMB) dates from when the universe was 379,000 years old, the CνB decoupled (separated) from matter when the universe was just one second old. It is estimated that today, the CνB has a temperature of roughly 1.95 K.

Source: https://en.wikipedia.org/wiki/Cosmic_neutrino_backgroundYou can see in the part of table below that the gravity separated from other three forces around 10^-43 seconds after the Big Bang.

You can see below that the neutrino decoupling occurred around 1 second after the Big Band and the radius of observable universe was 10 light-years at that time.

You can access the full table here: https://photos.app.goo.gl/jS3apBR5gQ2SuDFv9
Table source: https://en.wikipedia.org/wiki/Chronology_of_the_universe#Tabular_summary

Assuming dark energy remains constant (an unchanging cosmological constant), so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit could never reach the Earth. This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that we can ever theoretically observe in the infinite future is only larger than the number currently observable by a factor of 2.36.

Source: https://en.wikipedia.org/wiki/Observable_universe#The_universe_versus_the_observable_universe

Consider, for example, a cosmic microwave background (CMB) photon that was emitted as visible light about 379,000 years after the big bang and is just now hitting our microwave detectors (the redshift is z=1089): that photon has been traveling for 13.7 billion years so it has traveled a distance of 13.7 billion light years. So you might imagine that the current radius of the observable universe is 13.7 billion light years. However, during this time the universe has been expanding, so the current position of the matter that emitted that photon will now be 46.5 billion light years away. (By now, the little 10^−51 bumps on the CMB will have condensed into galaxies and stars at that distance.) This gives a diameter of the current observable universe of 93 billion light years. Note that as time passes, the size of the observable universe will increase. In fact it will increase by significantly more than two (to convert radius to diameter) light years per year because of the continued (accelerating) expansion of the universe. Also note that we will not be able to use photons (light) to explore the universe earlier than 379,000 years after the big bang since the universe was opaque to photons at that time. However, in the future we could conceivably use neutrinos or gravitational wave telescopes to explore the earlier universe.

Source: https://physics.stackexchange.com/a/32936/84624

Question Statement:
I have read that gravitational waves are also red-shifted and neutrinos should also red-shifted but it hasn't been confirmed experimentally.

In the source given above, you can see the text in blue suggests the use of gravitational waves in future. I understand that the cosmic neutrino background resulted from neutrino decoupling around 1 second after the Big Bang. Where did those gravitational waves which make up cosmic gravitational background come from? From inflation era?Question 6:
In the source given above, the text in purple says that the matter which emitted photons constituting cosmic microwave background is almost at the radius of 46.5 billion light-years away from us. Please note that future visibility limit has radius of 62 billion light-years. How far away will the sources of those neutrinos and gravitational waves be if one is able to experimentally detect cosmic neutrino background and cosmic gravitational waves?Note to self:
As the time passes, the cosmic microwave background radiation will get fainter and fainter as a result of red-shifting.

Helpful links:
1: https://web.archive.org/web/20190630092955/http://cmb.physics.wisc.edu/pub/tutorial/cmb.html
2: https://www.esa.int/Science_Explora...ck/Planck_and_the_cosmic_microwave_background
3: https://pages.uoregon.edu/imamura/123cs/lecture-8/wmap.html
4: http://www.sci-news.com/astronomy/s...0-million-years-later-than-thought-02469.html
5: https://physics.stackexchange.com/q...erse-expanding-during-the-inflationary-period
6: https://en.wikipedia.org/wiki/Observable_universe
7: https://www.forbes.com/sites/starts...able-universe-will-we-someday-be-able-to-see/
8: https://en.wikipedia.org/wiki/Cosmic_background_radiation
9: https://en.wikipedia.org/wiki/Cosmic_background_radiation#Timeline_of_significant_events
10: https://www.amnh.org/explore/videos/space/cosmic-microwave-background-the-new-cosmology/essay-what-is-the-cosmic-microwave-background
11: http://abyss.uoregon.edu/~js/ast123/lectures/lec23.html
12: https://www.space.com/1217-ghostly-ripples-space.html
13: https://briankoberlein.com/blog/three-peaks-big-bang/
14: https://plus.maths.org/content/what-planck-saw

 

[malawi_glenn]

how one could differentiate actual CMB photons and seemingly/fake CMB photons?

You can't. You need to have knowledge about the other sources too. It is classic "signal vs. background" scenario. What you do is to have a microwave radiometer, which you calibrate to know sources of temperature. Then the CMB will show up as a "noise" in your signal.
https://www.researchgate.net/publication/317275082_An_Amateur_Instrument_for_the_Detection_of_the_Cosmic_Microwave_Background

So, in Image 1 blue represents high density and high temperature regions. Could you please confirm this?

Perhaps you understand this picture better than your "image 1"

 

[DrClaude]

Do both Image #1 and Image #2 shown above convey the same information?

Yes. Both show temperature fluctuations. WMAP is more recent and has higher precision and resolution.

In other words, you don't need something at 2.7 K to get that range of CMB photons. You can get those same photons from objects at different temperatures.

Yes, but the agreement with blackbody radiation is too good, you couldn't get that from other sources:

 

[LastScattered1090]

@PainterGuy Yeah the short answer to Question 1 is that you need to measure the whole CMB spectrum (as the COBE telescope did) to derive the blackbody temperature.

And yeah, CMB emission is contaminated by astrophysical foregrounds (like thermal emission from dust in the Galactic ISM, for example). Foreground separation is challenging, and is an active area of research. That's especially true when it comes to observations of CMB polarization, where (as the Planck telescope discovered) diffuse polarized dust emission exists even at quite high Galactic latitudes.

So for total intensity observations, it was a bit easier to go to "clean" patches of sky away from the Galactic plane, where you could be confident that the dominant source of emission on the sky (at your observation frequencies) was CMB. Certainly this is true at common CMB telescope bands like 90 and 150 GHz. Component separation is often done by assuming that each of the sky-signal components (e.g. CMB, Galactic dust, & Galactic synchroton) have different shapes to their spectra. CMB is modeled as blackbody (one of the most perfect examples of a blackbody we have in nature), dust as a greybody with a wavelength-dependent emissivity, and synchrotron as a power-law vs wavelength. The WMAP team did a pretty bang-up job of separating signal components using this sort of approach.

In contrast to observing total intensity, when observing linear polarization (where the CMB signal is also orders of magnitude weaker), this kind of component separation is still attempted, but is much harder, posing a huge challenge for experiments looking for a B-mode of CMB polarization on large scales (to test Inflation). It may be an insurmountable challenge, actually.

 

[malawi_glenn]

you need to measure the whole CMB spectrum (as the COBE telescope did) to derive the blackbody temperature.

It is not possible to measure the entire CMB spectrum. What you do is you sample data points for certain (many) frequencies and then do a blackbody curve fit. Also isn't it a matter of detector limitation. We have no means of detecting all frequencies.

At least that is my understanding of it.

 

[LastScattered1090]

It is not possible to measure the entire CMB spectrum. What you do is you sample data points for certain (many) frequencies and then do a blackbody curve fit. Also isn't it a matter of detector limitation. We have no means of detecting all frequencies.

At least that is my understanding of it.

Yeah I know that. I was speaking loosely there. TBH, I'm a bit surprised that you felt the need to nitpick/call me out on this wording when I clearly spoke about CMB telescopes having discrete wavelength bands of observation in the rest of my answer. But anyway, I'm talking about a level of completeness comparable to that shown in Figure 3 (posted by DrClaude). You need to have enough bands spanning enough of a wavelength range to be confident in your model-parameter fit. If you look at the figure, you see that the data are an absurdly good match to the theoretical prediction at the best-fit (maximum-likelihood) temperature.

 

[malawi_glenn]

Yeah I know that. I was speaking loosely there. TBH, I'm a bit surprised that you felt the need to nitpick/call me out on this wording when I clearly spoke about CMB telescopes having discrete wavelength bands of observation in the rest of my answer.

I know that you know it, I just wanted to make sure that it clearly came across to the OP.

I did not call you out - what is wrong with this world? Everybody is such a snowflake / easily offended nowadays...

 

[LastScattered1090]

I know that you know it, I just wanted to be sure that it clearly came across to the OP.

I did not call you out - what is wrong with this world? Everybody is such a snowflake / easily offended nowadays...

Yeah actually, I realize that was unnecessary indignation on my part. I was going to edit it out, but I'll leave it in for honesty, and since you've already responded to it. It was a good point of clarification, thanks.

 

[Orodruin]

I did not call you out - what is wrong with this world? Everybody is such a snowflake / easily offended nowadays...

I don’t think it is about being a snowflake as much as the short and quickly written word being imprecise in conveying certain aspects and tone. If it is something I have learned during 8 years here it is that it is very easy to misconstrue clarifications as critisism and that it is therefore extra important to carefully word such statements. I still don’t get it right all the time. A few extra words such as ”just to clarify” or similar go a long way to avoid appearing overly critical.

 

[malawi_glenn]

A few extra words such as ”just to clarify” or similar go a long way to avoid appearing overly critical.

Thanks, I will try to remember that.

Maybe its just me, but I feel that nowadays at forums and other online social medias - the default reaction when someone is replying "short" and not stating explicity "nice done, here are a few clarifications / suggestions" it is received as harsh critiscsm/"calling out" and so on. I have for almost 20 years writing and reading at several computer game forums and I have felt a huge shift.

Yeah actually, I realize that was unnecessary indignation on my part. I was going to edit it out, but I'll leave it in for honesty, and since you've already responded to it. It was a good point of clarification, thanks.

Your original reply was a very good explanation about the matter of detecting CMB which I really enjoyed reading myself. I just wanted to clarify some things for the sake of OP's understanding, that's all.

 

[PainterGuy]

Thanks a lot, everyone!

Could you please also comment on Question 5 and Question 6?

 

[Bandersnatch]

I can only help with Q6.

In the source given above, the text in purple says that the matter which emitted photons constituting cosmic microwave background is almost at the radius of 46.5 billion light-years away from us. Please note that future visibility limit has radius of 62 billion light-years. How far away will the sources of those neutrinos and gravitational waves be if one is able to experimentally detect cosmic neutrino background and cosmic gravitational waves?

It won't be much farther.
Take a look at the graph below. It shows the expansion history according to current-best model in certain convenient coordinates. The light cone shows the path of the signals we can observe today. It doesn't matter what kind of signals - as long as they travel at the speed of light, it's the path they take.
The signals we'd receive in infinite future are shown as the line marked 'event horizon'. It shows the maximum extent of what can be ever observed.
By looking at where these paths fall on the bottom of the graph, you can read the current distance to the farthest possible emitters.
The CMB-emitting surface of last scattering cuts off the bottom part of the graph just below the level indicated by scalefactor a=0.001, as shown by the thin green line. The light signals from before that are scrambled by the early plasma.
The information that is left to be gleaned by probing signals other than electromagnetic are the bits circled in red. The one on the left - currently, the one on the right - in the far future.

It's maybe an extra billion light years in present-day distances, if that.

 

[PainterGuy]

It's maybe an extra billion light years in present-day distances, if that.

Thank you! So, it's safe to say that it's located somewhere between the present radius, 46.5 billion light-years, of the visible universe and the radius of future visibility limit which is 62 billion light-years.

Could someone please comment on Question 5?

 

[256bits]

I have read that gravitational waves are also red-shifted and neutrinos should also red-shifted but it hasn't been confirmed experimentally.

If when the neutrinos were decoupled from a thermal bath at the temperature at the 1 sec period based on the hot Big Bang model, they would have had a high energy. From the red shift of the photons CMB one can estimate the reduction in energy ( and temperature ) they have now.
Not really that much of an answer but it could kick start a few better responses..