29
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 ...
- 39.1k
28
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 ...
- 158k
27
votes
How is the Cosmic Microwave Background so big?
The CMB isn't really an image of the universe. The universe is full of the CMB radiation, with (almost) equal numbers of photons travelling in all directions at every point in space. An analogy would ...
- 158k
23
votes
Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?
The Planck spectrum of a blackbody is given by
$$B(\nu, T) = \frac{2h\nu^3}{c^2} \left( \exp[h\nu/k_BT] - 1\right)^{-1} \ , $$
where $\nu$ is the frequency, $T$ is the temperature and $h$ and $k_B$ ...
- 158k
21
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 ...
- 127k
21
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 ...
- 158k
20
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). ...
- 158k
20
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 ...
- 158k
20
votes
Accepted
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 ...
- 127k
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/...
- 206
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 ...
- 4,914
17
votes
Accepted
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)...
- 158k
15
votes
Accepted
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 ...
- 158k
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 ...
- 158k
15
votes
Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?
According to the Hubble law all radiation is progressively red-shifted with distance traveled, with the wavelength changing from $\lambda$ to $\lambda(1+z)$ (with $z>0$). Now the formula for the ...
- 3,617
14
votes
Accepted
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 ...
- 16k
12
votes
Accepted
How is the Cosmic Microwave Background so big?
The size of the CMB sphere is not related to the size of the universe. It could be that the universe is infinite in size, and was infinite in size when the CMB was emitted. It could also be that the ...
- 4,143
11
votes
Accepted
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 ...
- 1,166
10
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 ...
- 39.1k
10
votes
Accepted
Why is the Cosmic Microwave Background evidence of 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 ...
- 158k
10
votes
Accepted
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. ...
- 127k
10
votes
Would it be possible to use existing radio-telescopes to do spot measurements of CMB?
Water vapour in the atmosphere emits microwave radiation, which interferes with the observation of the microwave background. For this reason, space telescopes such as COBE or WMAP are best placed to ...
- 127k
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 ...
- 16k
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 ...
- 2,946
7
votes
Accepted
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 ...
- 39.1k
7
votes
Is the universe older than 13.7 Billion years?
No, the oldest light we can see is as old as the Universe$^\dagger$$^\ddagger$. This is simply because light started traveling at that time, so after one year you'd se light that had been traveling ...
- 39.1k
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 ...
- 4,481
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 ...
- 158k
6
votes
Accepted
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, ...
- 39.1k
6
votes
Accepted
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 ...
- 18k
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