Reionization and the free-flow of light

[Ranku]

Why does neutral gas before reionization block the flow of light, and how does ionization help the flow of light?

 

[mathman]

Both phenomena are the result of photon interactions with matter. If the matter is cold (neutral gas), the photons are absorbed. If the gas is hot (ionized), photon emission takes place.

The above may be an oversimplification. These processes are part of the general description of photon matter interactions. The astronomy is is incidental.

 

[mfb]

and how does ionization help the flow of light?

It doesn't help - but at the time of re-ionization the matter was spread out so much that most light wasn't stopped any more.

 

[lomidrevo]

Why does neutral gas before reionization block the flow of light

The period before reionization is commonly referred as Dark Ages. The neutral hydrogen from this period doesn't block any light (ignoring the absorption lines). It is dark just because there are no light sources, except CMBR, which is transparently propagating since epoch of recombination.

 

[Ranku]

The period before reionization is commonly referred as Dark Ages. The neutral hydrogen from this period doesn't block any light (ignoring the absorption lines). It is dark just because there are no light sources, except CMBR, which is transparently propagating since epoch of recombination.

I meant it in the sense of free flow of radiation, rather than the visibility of it.

 

[lomidrevo]

What exactly do you mean by ?

free flow of radiation

If you mean, that light (radiation) can propagate unaffected, then as I said above, the universe was pretty transparent in this period, so there occurred no "blocking of the flow of light" as you assumed in your original question:

Why does neutral gas before reionization block the flow of light,...

If the universe were not transparent during that period, we couldn't observe the CMBR these days.

I suggest you to read this article, I think it provides very nice brief explanation of these concepts. Quoting some of the most relevant parts:

"The era of recombination" is the earliest point in our cosmic history to which we can look back with any form of light. This is what we see as the Cosmic Microwave Background today
...
Following this are the cosmic dark ages - a period of time after the Universe became transparent but before the first stars formed. When the first stars formed, it ended the dark ages, and started the next epoch in our universe.
...
Another change occurred after the first stars started to form.
...
This era, from the end of the dark ages to when the universe was around a billion years old, is known as "the epoch of reionization." It refers to the point when most of the neutral hydrogen was reionized by the increasing radiation from the first massive stars.
...
The emergence of these first stars marks the end of the "Dark Ages" in cosmic history, a period characterized by the absence of discrete sources of light.

 

[Ranku]

What exactly do you mean by ?

If you mean, that light (radiation) can propagate unaffected, then as I said above, the universe was pretty transparent in this period, so there occurred no "blocking of the flow of light" as you assumed in your original question:

If the universe were not transparent during that period, we couldn't observe the CMBR these days.

I suggest you to read this article, I think it provides very nice brief explanation of these concepts. Quoting some of the most relevant parts:

I was actually trying to clarify what Ethan Siegel meant in his blog Starts With A Bang, when he wrote "Before stars or galaxies formed, the Universe was full of light-blocking, neutral atoms."

 

[lomidrevo]

From this article:

While the electrons of neutral hydrogen can absorb photons of some wavelengths by rising to an excited state, a universe full of neutral hydrogen will be relatively opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum.

Maybe the blog is referring to Lyman alpha absorption line, when UV photons at wavelength 121.567 nm are absorbed by the hydrogen atom. As these photons travels through multiple gas clouds with various redshifts, a series of multiple absorption lines is formed, called Lyman-alpha forest, see this article. But these absorption lines affects only small part of the spectrum. I still think it is appropriate to say that universe was transparent at that time.