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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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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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19 votes
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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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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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18 votes
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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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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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15 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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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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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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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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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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7 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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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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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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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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5 votes
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Formation of Cosmic Microwave Background

We observe the temperature of the CMB as a ~2.7 K blackbody, but that's the redshifted version we observe. The CMB is also know as the "surface of last scattering" at the point of recombination when ...
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5 votes
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Do the E and B modes of the CMB polarization have anything to do with electric and magnetic fields?

In this context, the $\kappa$ you are referring to is called the dimensionless matter density field. It is gravitational lensing jargon, and is usually just referred to as the 'convergence' field. ...
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Temperature of the intracluster medium (ICM)

The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles". The answer to your second question is ...
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5 votes
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What is the physics of the "spinning dust" contribution to Cosmic Microwave Background measurements?

"Spinning dust" is a mechanism proposed to explain a particular feature in the foreground emission of CMB; a bump around $\nu\sim20\,\mathrm{GHz}$. Dust grains acquire charge through photoelectric ...
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5 votes
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What would the CMB look like somewhere else in the universe?

The CMB is nearly featureless, but the operative word is nearly. The tiny temperature fluctuations that we've measured should look about the same anywhere within tens to hundreds of millions of light ...
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5 votes
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Does the CMB pattern evolve in a human lifetime?

In principle yes, in practice no. As seen in the temperature power spectrum below, the Planck satellite detects power (i.e. "a signal") even on the smallest probed scales, which is a few arcminutes. ...
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What Is The Maximum Distance Our Finest Instruments Could See When They're Perfected?

The furthest we can "see" is the cosmic microwave background at a redshift of about 1100. The proper distance of the CMB-emitting gas that we see today is about 46 billion light years. If you are ...
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5 votes
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Why is the Cosmic Microwave Background at the same distance no matter the direction we look?

The CMB is visible at a distance of 13.8 billion light years in all directions from Earth, leading scientists to determine that this is the true age of the Universe. This is wrong in a few ways. ...
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5 votes
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Why does the first measurements of the thermal Sunyaev-Zel’dovich effect from ALMA show a temperature decrease and not an increase at the cluster?

Calling it a "temperature decrease" is kind of misleading. (Possibly this is a side effect of the tendency to use "brightness temperature" in radio astronomy to mean measured ...
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5 votes

Does the Cosmic microwave background (CMB) have an amplitude? Does it vary, like the 'temperature' (wavel./freq.) and the polarization?

The cosmic microwave background (CMB) does of course vary with direction, in terms of its amplitude, temperature and polarisation. It is these variations which lead to its diagnostic power in ...
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4 votes

How do we know background radiation does not come from undiscovered galaxies?

Assuming you mean the Cosmic Microwave Background ... The CMB is entirely in a very narrow range of the microwave part of the spectrum. Galaxy emissions are all over the spectrum. The CMB comes ...
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4 votes

Intuitive explanation for why the universe is flat

The CMB lets us measure how close to flat the universe is right now. On the other hand, inflation tries to explain how we got from whatever the early universe was to right now. The motivation for ...
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