# Tag Info

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
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

### 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

### 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

### 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

### 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
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

### 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
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