According to this paper, the last scattering surface has a comoving thickness of about 19 Mpc, which corresponds to a physical thickness of (19 Mpc)/1100 ~ 17 kpc or a duration of (17 kpc)/c ~ 56000 years.

Edit: The above concerns how thick the last scattering surface is at any given point on the sky (which is connected to how long recombination -- the process by which the universe became transparent -- took, as well as how opaque the universe was before recombination). I just realized that you are instead asking how the recombination time varied between different patches of the sky. Temperature variations in the CMB are around the 10^-4 level (one part in ten thousand), which implies that the recombination time varied to a similar degree. 10^(-4) of 370000 years is 37 years, so the spatial variation in the recombination time is of order tens of years.

Right, found this myself after a quick Google search, correct me if I'm wrong. It's not the density or temperature of the plasma that really affect transparency, rather, it's the fact that this plasma is opaque only because there exist photons of high enough energy to rip apart nuclei and electrons again (the electrons can then scatter incoming photons galore). As the expansion of the universe redshifts the photons, at a certain point there will be no photons of sufficient energy to ionise atoms, hence simultaneity.

Follow up question: assuming we're talking about a Boltzmann distribution, at what timescales do we expect to go from e.g. 1% to 99% of photons falling below ionisation energy?

That's right, the scales are precisely proportional in that way.

With respect to whether such resolution is possible, I can't say much about the instrumentation side, but I can point out a major physical challenge. According to our calculations, there simply wasn't much structure on very small scales in the early universe, due to diffusion damping. Photons were able to gradually diffuse between hot and cold regions, allowing their temperatures to equalize. This effectively smoothed out the early universe; due to photon travel times, it affected small scales more than large scales.

In fact, they do. It's very useful for us, allowing us to get a lot more data from them.

https://www.newscientist.com/article/dn13792-cosmic-time-warp-revealed-in-slow-motion-supernovae/

Redshifting slows down the frequency of light. But the total amount of oscillations of the light signal isn't affected.

Suppose a far away galaxy sends a 1 second long burst at 1 GHz, 1 billion cycles. At Earth it arrives redshifted to twice the wavelength, and a frequency of 0.5 GHz. We still receive 1 billion cycles, they're just spread over 2 seconds of time now. So we effectively see the signal - and everything else from that distant galaxy - at 0.5 speed.

[removed]

I just lazily took pi/l for l=2500, the largest l that Planck papers plot. Indeed, the ground-based telescopes push to somewhat higher l.

Hmm, around 1 degree (where the fluctuation power peaks), the time scale for the CMB to change would be of order a billion years, or one part in ~10^(9) per year. I wonder how far off that kind of sensitivity is.

Remember the CMB light originated everywhere. So there will always be a distance such that light originating from that distance is just reaching us now. Cosmic expansion doesn't come into play here.

> The Andromeda galaxy is expected to collide with the Milky Way in approximately 4.5 billion years. Does this time take into account the expansion of space in between the two galaxies?

Space expanding doesn't physically do anything. It's just a convention that's useful in some contexts. (It represents a choice of coordinates on spacetime.)

Since the misconceived reification of expanding space is pretty deeply ingrained in the public consciousness, here are some articles discussing the point further.

(1) A diatribe on expanding space. This is pretty technical, but it's the most direct attack on the idea of expanding space. One key quote is that

> there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost.

For example, the Milky Way-Andromeda system is no longer expanding, so cosmic expansion is simply no longer relevant to it.

(2) The kinematic origin of the cosmological redshift. Very well written and less technical, although there are mathematical arguments. The main point of this article is that the cosmological redshift -- often framed as a consequence of space expanding -- is more precisely viewed as just a Doppler shift.

(3) On The Relativity of Redshifts: Does Space Really "Expand"? The least technical of the batch. This article is also focused on the interpretation of the cosmological redshift. It includes the choice paragraph:

> While it may seem that railing against the concept of expanding space is somewhat petty, it is actually important to set the scene straight, especially for novices in cosmology. One of the important aspects in growing as a physicist is to develop an intuition, an intuition that can guide you on what to expect from the complex equation under your fingers. But if you assuming that expanding space is something physical, something like a river carrying distant observers along as the universe expands, the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results.

Expansion is a big factor but you can think of the light of the CMB as having originated from everywhere. The part of the CMB we observe is a growing bubble. The light is traveling uninterrupted after having been emitted in the early universe.

To the second question, the expansion of space is not that significant between nearby galaxies.

On average, yes, but in reality the recession velocity is neither constant nor linear. Points at the distance of the CMB scattering surface are currently receding from us at more than three times the speed of light.

The Milky Way-Andromeda system is bound together by gravity, and so is unaffected by expansion. Newtonian mechanics is all you need to calculate the time until the two galaxies collide (notwithstanding the uncertainty in the current distance and relative velocity of Andromeda).

See this comment! About 56000 years.

What happened is that the universe cooled enough that all of the free protons and electrons condensed into neutral hydrogen. Neutral atoms interact with light much more weakly than do free electric charges, so this process made the universe transparent to light.

No, the time light took to reach us only correlates with the amount that the light's frequency is shifted and not with the light's absolute frequency. For example, the CMB is actually a whole spectrum of frequencies. Those frequencies don't take (significantly) different amounts of time to reach us; if they did, the CMB spectrum wouldn't be such a perfect blackbody spectrum.

JWST is tuned to detect infrared light. Microwaves, the 'M' in CMB, are just too long of a wavelength for JWST unfortunately. It might be able to do a similar survey and find other data but it won't see the same CMB.

JWST infrared filters can't see the CMB microwave radiation.

• JWST infrared filters range from 0.6 to 28.5 micrometers wavelength

• CMB microwave radiation = 1.9 millimeters wavelength.

• 1.9 millimeters = 1,900 micrometers

Sorry

[removed]