Perhaps in time the so-called Dark Ages will be thought of as including our own.
-Georg Lichtenberg
We've been going through the history of the Universe -- from inflation to the present day -- and you can read parts 1, 2, 3, 4, 5, 6, 7, 8, and 9 here. The last thing that happened was we had a Universe filled with neutral atoms -- mostly hydrogen with some helium and a negligible amount of everything else -- that had begun to collapse under its own gravity.
When a few regions of space got dense enough, about 50 million years into the story, we made the first stars!
But there's a problem when you make stars surrounded by neutral atoms that you may not be aware of: neutral atoms block light! If we look around in our own galaxy, it's pretty easy to find where neutral atoms are, because the light doesn't get through!
Astronomers call this phenomenon extinction. So, what does this mean for these first stars? It means that, as long as this neutral gas is around, the Universe is opaque to starlight, and we can't see it! At least, not until this gas gets ionized. What ionizes gas?
These very same hot, young stars that form from gravitational collapse! Most of a Sun-like star's energy comes out in visible light, but a hotter, more massive star emits more Ultraviolet (UV) light. What's special about UV light? It smacks these neutral atoms and knocks electrons out, ionizing them! This means that, once enough stars form, more and more of the Universe becomes transparent to visible light!
And finally, when the Universe has had enough stars on for long enough, these so-called Dark Ages end, and you can actually observe the light!
So, again, this is a prediction of the Big Bang. Sometime, if we look back far enough, these atoms that are ionized in between galaxies today should be neutral, and should absorb the light! So what happens when we confront this prediction (made in 1985, by Jim Gunn) with the observations?
We find that once things are far enough away (and hence, young enough), we can't see any light, which is why there are no features to the left of the more distant (higher redshift) object! In fact, if we take a slew of these distant objects, you'll see very clearly that the highest redshift ones (most distant) are totally muted to the left of the big hydrogen emission line!
So making the first stars isn't enough, we need to give them time to work their nuclear fusion engines to ionize all the gas in the Universe in order to see! And this wonderful prediction was only confirmed in 2001, and has since been confirmed by many subsequent observations, including measurements of ionization by the WMAP satellite!
So this brings us all the way up to the Universe being almost one billion years old! Come back next time, and we'll find out what else has been going on, and for some gorgeous early pictures of our Universe!
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A moment, please... In part 7, you said:
I. e., there you said that once there are neutral atoms, the photons can travel on freely, the universe is transparent. But now you say here that the universe becomes *opaque* once you have neutral atoms, and the light can only travel freely when the atoms are ionized again?!? Which is it?
Bjoern,
It's dependent on wavelength. If you notice, the wavelengths that fail to get through are the ones to the left of about 8600 Angstroms, which is ultraviolet light at these high redshifts.
The light coming from the microwave background, even at the time of the microwave background, is already in the infrared part of the spectrum, so it can pass through unimpeded.
That, incidentally, is partially why the James Webb Space Telescope is going to be such a great project; its "eyes" are deep into the infrared, so perhaps it will be able to see these early stars before reionization!
So does this means we can never view the 1st stars created b/c the neutral atoms block all the light? Even the gamma and radiation?
@Ethan: Thanks. I should have thought of that... ;-)
hadn't refreshed the post so i didnt see Bjoern and Ethans comments. Question answered.
Isn't it a case of neutral H and He (and singly ionized He) blocking light? Otherwise, most ionized states of other gases block light as well (although heavier nuclei didn't exist in the beginning).
I was just thinking - a star would emit over a very broad range and is somewhat like a blackbody whereas neutral H and He will only absorb in fairly narrow regions unless they are at very high pressures - so wouldn't we be able to see anyway? (Assuming we were magically there to observe.) Now if neutral H and He were at a high enough pressure to block the light of stars, then singly ionized He would do the same - but at the high pressures required for this feat I can't imagine maintaining ionized He except on extremely short timescales.
Madscientist,
How, you ask? Hydrogen has an extraordinarily strong absorption line (and emission line, under the proper conditions) at "Lyman Alpha", the n=2 to n=1 transition, "Lyman Beta", the n=3 to n=1, etc.
If you have an object at a redshift of 10, and the Universe isn't reionized until a redshift of 6, then you get the absorption from the Lyman series at a wavelength of Ly-alpha * (1 + z) for all of the redshifts, continuously, from z = 6 through z = 10. You also get absorption from Lyman beta, gamma, etc., all the way to the Lyman limit.
It doesn't take a whole lot of gas to wipe out the entire spectrum to the left of (Ly-alpha)*(1 + z_star).
Does that help? The wikipedia page for "Gunn-Peterson trough" has some explanation for this as well, if you want something more complete.
Very nice. I'll have to reread slowly to better understand. But a few of questions.
Is the astronomical phenomena of extinction only a distant phenomena or does it happen locally in our Milky Way galaxy? In particular is that why we can't see the center of the galaxy? And are there other regions of our Milky Way where extinction occurs? Also, does extinction occur all over the great expanses of space between galaxies?
Finally, about future predictions. "James Webb Space Telescope is going to be such a great project; its "eyes" are deep into the infrared, so perhaps it will be able to see these early stars before reionization!" OK this is pretty interesting. Now my question is this: what is predicted by big bang physicists? ie many more stars, many metal free stars; what exactly is predicted?
Thanks much.
Thanks Ethan, I thought you meant would we see anything if we were standing there, not could we possibly see anything now. So effectively, the continuum of Lyman-alpha due to redshifts between us and objects far far away blocks out light so that we can't see beyond some point in the past?
@Thomas:
Well, Wikipedia has this to say:
And yes, that's why we can't see the center of the galaxy (in visible light; there is no problem in infrared light or other wavelengths).
Essentially none, since there is essentially no dust between the galaxies. In galaxy clusters, there is a lot of gas between the galaxies, but even there, as far as I know, there is virtually no dust.
I don't know exactly what the abilities of the James Webb telescope are, but I'd say we indeed expect to see many more stars (or more probably, galaxies), which should have a lot less metal than today. It could be possible to observe even metal-free stars, I don't know.
Bjoern
Thank you for those explanations...
I'm still pondering
Oh here's a link to the James Webb telescope.
http://www.jwst.nasa.gov/about.html
Wow.
First of all, I have to say, this series is AWESOME.
Second... yay reionization! During the Dark Ages and Reionization, you still have neutral hydrogen gas that gives off radio light. Radio astronomers are chasing that signal way down at low frequencies (150 MHz or so), and will probably start making detections before JWST is running.
I think I've used that "Outline of Cosmic History" in every single talk I've given (about this anyway.) It's great.
This strikes be as really weird, because everything I know about electrodynamics says that charged particles are what interact with (including absorb) EM waves. Metals are not transparent because of free electrons, water severely limits transmission distance because it's highly polarized, etc, etc. So why are NEUTRAL atoms a problem and ions not?