27 votes
Accepted

How fast are we moving relative to the CMB?

Yes, our (i.e. the Sun's) motion in the "global", or comoving, reference frame can be measured accurately from the dipole of the cosmic microwave background. The latest results from the Planck ...
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  • 33.8k
25 votes
Accepted

Why was the Cosmic Microwave Background (CMB) released at a blackbody temperature of 3000K rather than 30,000K?

The CMB is produced as the ionisation fraction of hydrogen falls from a high value to a very small value. Contrary to what is written in the Quora answer you may have been misled by, this happens at a ...
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  • 118k
24 votes
Accepted

Why can we observe the Cosmic Microwave Background no matter the direction we look?

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and ...
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  • 33.8k
21 votes

Why does the Fourier transform of this CMB image have a hole in it?

Having now looked at the paper by Aiola et al. (2020), it emerges that for that map, they filtered the data to exclude low frequency multipoles with $|l|<150$, corresponding to about 1 degree. This ...
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  • 118k
20 votes
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Where's the matter that created the CMB?

It is all around you. It was gravitationally attracted to clumps of dark matter where it reached sufficient density to begin to interact with itself (via the electromagnetic force), break up into ...
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  • 91k
19 votes
Accepted

Why does the Fourier transform of this CMB image have a hole in it?

For that specific E-mode map we have applied a Wiener filter to highlight the high SN modes (those "rings"). I also further apply the following filter: $((1 + (kx/5)^{-4})^{-1}) * ((1 + (k/...
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19 votes

Is the universe macroscopically transparent to CMB? Is the fraction intercepted by stars and dust so tiny that it doesn't have a correction factor?

For the most part, the CMB photons travel directly to our telescopes from the surface of last scattering. Some corrections need to be made to determine the blackbody nature of the spectrum, but they ...
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  • 4,744
19 votes

Why isn't the CMB blurred by the blackbodies emitted in the time after atoms first formed?

It is. They're called galaxies and stars and we see them against the cosmic microwave background. However, they aren't (all) cooling because of the energy sources they contain. But perhaps that isn't ...
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  • 118k
18 votes
Accepted

How do CMB photons 'gain energy when they pass through normal regions of space with matter' and 'lose energy when they pass through voids'?

It is the late time integrated Sachs Wolfe effect. As they travel towards us, apart from the general expansion, photons from the CMB gain energy when they fall into potential wells (where matter is). ...
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  • 118k
17 votes
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How long after the Big Bang would CMB radiation have been between 273 and 373 K?

The temperature of the cosmic microwave background scales as the inverse of the cosmic scale factor $a$. i.e. When everything was at half the separation it is now, then the CMB was twice the (absolute)...
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  • 118k
16 votes

Why can we still 'see' the CMB

Firstly, there is no centre to the universe and the universe seems to continue indefinitely in all directions. It is best to imagine the universe as infinite in size. Now we need to explain why the ...
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  • 91k
15 votes
Accepted

What happened to the reemitted photons during recombination?

There should indeed be emission lines at the appropriately redshifted frequencies. However, they are going to be incredibly faint and diluted because the ratio of photons to baryons at the epoch of ...
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  • 118k
14 votes
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Does the cosmic microwave background change over time?

The CMB patterns do indeed change over time, although statistically they remain the same, and although it will not be noticeable on human timescales. The CMB we observe now comes from a thin shell ...
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  • 33.8k
14 votes
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Scientific evidence against CMB originating from “Oort Soup”?

The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed the contribution of such dust has to be ...
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  • 118k
14 votes
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Can we track matter through time by looking at different depths in space?

Would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? No, that's not possible. If we could ...
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11 votes
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How did astronomers get rid of microwave radiation from stars and galaxies in picures of the CMB?

The raw data does show that. However, the sky is surveyed in multiple frequency bands, and the foreground and background are visible to varying degrees in each. WMAP, for example, has 5 bands, here's ...
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10 votes

Why is the Boomerang Nebula colder than the CMB?

The Boomerang Nebula (or Bow Tie Nebula) is a cloud of gas being expelled from a dying low-mass star, at $164~\mathrm{km}~\mathrm{s}^{-1}$ (cf. Raghvendra Sahai and Lars-Åke Nyman: The Boomerang ...
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  • 33.8k
10 votes
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Confused about rubber sheet analogy!

The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime. You can't get any more than that from the rubber sheet. ...
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  • 91k
9 votes

Would a future hole in the Cosmic Microwave Background show us the edge of the universe?

First, let me clear up a misunderstanding: Particle horizon The "edge" of the observable Universe is called the particle horizon, and lies roughly 47 Gly (billion lightyears) away. It is ...
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9 votes

Why do scientists assume they can measure the shape of the universe if it is also widely believed to be infinite?

Yes, the universe is believed to be infinite in size. That's what you get if the curvature is zero or negative, assuming a simple topology. The curvature has to be positive for a finite universe, once ...
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  • 10.4k
8 votes

Did cosmological inflation occur at speeds greater than $c$?

The simple answer to your question is "yes" - the universe expanded at much greater speeds than $c$ during the inflationary epoch. This period of time was very quick but very dramatic, lasting from ...
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8 votes

Can the diminishing energy of the CMBR be the source of dark energy?

No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to $a^{-4}$ is the contribution of radiation energy ...
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  • 2,856
8 votes
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Why is the Cosmic Microwave Background evidence of for a hotter, denser early Universe?

By request: Beyond the fact that the cosmic microwave background (CMB) is a direct prediction of the big bang model, there is the question of how you would produce it in any other way. It is ...
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  • 118k
7 votes
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How thick is the cosmic microwave background, including the part we cannot see within the observable universe?

If I understand you correctly, you want to know the distance from the point from which we observe the CMB, to the edge of the observable Universe. During inflation, the observable Universe expanded ...
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7 votes

Since the Hubble Constant changes over time (it's a variable parameter), why can't the conflicting values of 67.4 and 73 both be right?

You are confusing the Hubble parameter $H(t)$, which is a function of time, with the Hubble constant $H_0=H(t=\text{today})$ which is the value of the Hubble parameter today, and so of course is a ...
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  • 3,740
6 votes

Does the cosmic microwave background change over time?

When we observe the CMBR we are observing the surface of last scattering, however the comoving points that make up the surface of last scattering (which infact will actually have a comparitvely very ...
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6 votes
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Was the cosmic microwave background (CMB) radiation originally gamma rays?

The cosmic microwave background is a result of an almost perfect blackbody emitter. That means the spectrum covers a broad wavelength range with a peak that is given by Wien's law: $$\lambda_{peak} = \...
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  • 118k
6 votes

How much radiation would something need to output to sufficiently affect the CMB?

It is not very clear what you mean. There are many cosmic sources of microwave radiation that emit so much energy and/or are so close to us that they confuse measurements of the cosmic microwave ...
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  • 118k
6 votes
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Understanding The Fluctuations In The CMB Maps

The anisotropies in the CMB are caused by four effects; three at the surface of last scattering (SoLS), and one after: Temperature differences Denser regions will be more compressed and thus hotter, ...
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  • 33.8k
6 votes
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Is the universe macroscopically transparent to CMB? Is the fraction intercepted by stars and dust so tiny that it doesn't have a correction factor?

Let me add a minor addendum to Eric's excellent answer. The primary way in which CMB photons interact with matter is via scattering off of electrons in plasmas. After recombination (redshift $\sim ...
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