Astronomers have found when and how the cosmic fog was lifted

By Phil Plait | October 13, 2011 10:13 am

Take a look at the image displayed here [click to redshiftenate]. Every object you see there is a galaxy, a collection of billions of stars. See that one smack dab in the middle, the little red dot? The light we see from that galaxy traveled for 12.9 billion years before reaching the ESO’s Very Large Telescope in Chile. And when astronomers analyzed the light from it, and from a handful of other, similarly distant galaxies, they were able to pin down the timing of a pivotal event in the early Universe: when the cosmic fog cleared, and the Universe became transparent.

This event is called reionization, when radiation pouring out of very young galaxies flooded the Universe and stripped electrons off of their parent hydrogen atoms. An atom like this is said to be ionized. Before this time, the hydrogen gas was neutral: every proton had an electron around it. After this: zap. Ionized. This moment for the Universe was important because it changed how light flowed through space, which affects how we see it. The critical finding here is that reionization happened about 13 billion years ago, and took less time than previously thought, about 200 million years. Not only that, the culprit behind reionization may have been found: massive stars.

OK, those are the bullet points. Now let me explain in a little more detail.

Young, hot, dense, and chaotic

Imagine the Universe as it was 13.7 billion years ago. A thick, dense soup of matter permeates space, formed in the first three minutes after the Big Bang. The Universe was expanding, too, and cooling: as it got bigger, it got less dense, so the temperature dropped. During this time, electrons and protons were whizzing around on their own. Any time an electron would try to bond with a proton to form a neutral hydrogen atom, a high-energy photon (a particle of light) would come along and knock it loose again.

During this period, the Universe was opaque. Electrons are really good at absorbing photons, so light wouldn’t get far before being sucked up by an electron. But over time, things changed. All those photons lost energy as things cooled. Eventually, they didn’t have enough energy to prevent electrons combining with protons, so once an electron got together with a proton they stuck together. Neutral hydrogen became stable. This happened all over the Universe pretty much at the same time, and is called recombination. It occurred about 376,000 years after the Big Bang.

When this happened, the Universe became transparent to visible light because neutral hydrogen is really bad at absorbing the kind of light we see. However, it’s really good at absorbing ultraviolet light, and that’s the key to our story. Up until this point, there were no stars, no galaxies. But over time, hundreds of millions of years, the gas and dark matter in the Universe clumped up, attracted by their mutual gravity, and started forming galaxies and stars. Some of these stars were massive, hot, and bright, and flooded the sky with ultraviolet light.

This UV was then promptly absorbed by the neutral hydrogen out in space. If the UV photons had enough energy, kablam! They’d blow an electron right off its hydrogen atom, ionizing it. For hundreds of millions of years, the universe was neutral, but then those pesky stars fired up, and started ionizing it again. That’s why we call this reionization.

Not only that, but all this time the Universe was still expanding. As it did, it got less dense, the matter spreading out more thinly over space. Once the stars started ionizing the hydrogen, the average distance between the electrons and protons was getting big enough that it was tough for them to recombine (and if they did, along came another UV photon, pinging off the electron again). Between the flood of UV light and the cosmic expansion, the Universe stayed ionized. It was such an efficient process that today, 13 billion years later, the Universe is still mostly ionized. Neutral hydrogen is pretty rare compared to its ionized brethren.

A long time ago, in a bunch of galaxies far, far away…

And that’s where these new results (PDF) come in. The astronomers studied a handful of galaxies at different distances. They are so far away that the light we see from them was emitted around the time of reionization. By looking at the amount of ultraviolet we see from these galaxies, we can determine how much neutral hydrogen it passed through (since that gas absorbs the light, making the galaxy appear fainter) versus ionized gas. These galaxies were all very far away, but not exactly the same distance. Remember, it takes time for light to reach us, so we may be seeing one galaxy as it was 13 billion years ago, and another as it was 12.9 billion years. That makes a big difference! Because these galaxies were at different distances, it allowed the astronomers to see what reionization was like at different times.

Sure enough, the most distant galaxies had more of their UV light absorbed than the ones that were closer. What the astronomers found was that 780 million years after the Big Bang the Universe was mostly neutral, but only 200 million years later was mostly ionized. In other words, the flood of UV radiation managed to ionize essentially the entire Universe in only 200 million years, faster than what had previously been supposed!

Ah, now it’s all clear to me

Think about that for a moment. We are looking at objects that are so far away it takes huge telescopes just to see them at all, despite the fact that they are blasting out UV radiation at a rate that makes them billions of times brighter than the Sun. We are peering across the entire Universe to see what it was like when it was young, very young, and we’re able to actually see what it was doing, and understand it.

That’s so cool.

Not only that, but another team of astronomers independently added to this finding by solving another riddle about it. I wrote above that it was stars that reionized the Universe, but in fact that’s not the entire story. Giant black holes gobbling down matter are sloppy eaters, and as material falls in it blasts out high-energy radiation, including UV, as well. How much of that reionizing UV light was from stars, and how much from those big black holes?

[Click to galactinate.]

The other study (PDF) looked at nearby galaxies that are emitting lots of UV light, more than usual for normal galaxies, and were probably more common in the early Universe. The image above is one such galaxy, NGC 5253, as seen by the Magellan Baade 6.5 meter telescope. They found that these galaxies are undergoing bursts of star formation, and that means lots of massive, hot stars that can flood space with UV. Calculating how many stars are formed, how much UV is emitted, and extrapolating that back to the early Universe, they find that stars were the main culprit in reionizing the Universe 13 billion years ago.

That’s amazing. Stars were so plentiful and so energetic even so long ago that they were capable of ionizing the entire Universe!

The proper study of the Universe is the Universe

One thing that may be confusing (OK, a lot of this is, but one thing that stands out) is that if the Universe is currently ionized, and free electrons are so good at absorbing light, why isn’t the Universe opaque today? It’s because the Universe is so thinly spread out! Sure, electrons absorb light, but there are simply so few of them in space that your random photon from a distant galaxy has a very good chance of traveling billions of light years without getting close enough to one to impact it and get swallowed up. That’s why the Universe is transparent, and allows us to see nearly all the way across it.

So you have to consider not just that neutral hydrogen is good at absorbing UV and bad at visible light (and the opposite when it’s ionized) but also how dense it is. Way back in the olden days it was thick enough to absorb light, but now, even though it’s ionized, it’s too thin to absorb light efficiently. That happened around the same time as reionization, so once the hydrogen got zapped, it stayed zapped.

I know it’s a little confusing, but the universe is a fairly complicated place. That’s why we’re still trying to figure it out! We think the rules it obeys, the laws of nature, are actually relatively simple and elegant. But there are a lot of them, and they interact in complex ways. If they didn’t, we wouldn’t be here to study them! So really, if you think about it, we are the result of the Universe’s laws made incarnate, evolved to the point where we can study ourselves.

Image credits: ESO/ L. Pentericci; NASA/ESA/Hubble; Jordan Zastrow

Related posts:

The Universe is 13.73 +/- .12 billion years old!
Record-breaking galaxy found at the edge of the Universe
Galaxy cluster at the edge of the Universe
Hubble digs deep to see baby galaxies

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (65)

  1. Chris

    During this period, the Universe was opaque. Electrons are really good at absorbing photons, so light wouldn’t get far before being sucked up by an electron. … free electrons are so good at absorbing light

    Hate to be a nitpicker here, but free electrons can’t absorb photons. Violate conservation of energy and momentum. That’s not like you Phil, violating the laws of the universe. Free electrons scatter light very efficiently (Compton scattering). You should change absorb to scatter.

  2. James

    Imagine the Universe as it was 13.7 billion years ago. A thick, dense soup of matter permeates space, formed in the first three minutes after the Big Bang. The Universe was expanding, too, and cooling:

    What was the rate of expansion?

  3. Cheyenne

    This post should be nominated for one of those science blog winner contests. Just awesome.

  4. Chris Winter

    “It was such an efficient process that today, 13 billion years later, the Universe is still mostly ionized. Neutral hydrogen is pretty rare compared to its ionized brethren.”

    So most of the [ordinary] matter in the universe is hydrogen, and most of the hydrogen is ionized. Then those ions must tend to flow along magnetic lines of flux.

    The late Isaac Asimov wrote The Currents of Space, but IIRC he got it wrong; in that novel he was referring to streams of carbon that tended to push certain stars toward nova stage. Instead we have literal currents of electrons and protons.

  5. D Hunt

    I might be wrong, but I’m pretty sure free electrons can absorb light. Doing so would cause them to speed up, effectively getting hotter. Unlike an electron orbiting a nucleus the light it could absorb wouldn’t be limited to specific wavelengths as its speed isn’t quantized (or at least its much more finely quantized).

  6. David Conrad

    “Young, hot, dense, and chaotic” — That is a terrible thing to say about Michele Bachmann! For shame! Oh, wait….

  7. Jess Tauber

    This is way too complicated- can’t you refrase it so us antiscience folks can unnerstand it? Gotta go I’m late for the next debate and my campaign advisors keep texting me.

  8. Jens

    BA, this was great. I simply enjoyed it with a smile on my face.

  9. Chris

    @5 D Hunt
    Let’s assume we have a photon which has momentum (h/lambda) and energy (hc/lambda). (Lambda is the wavelength) Our electron (or could be a proton for that matter) is at rest (energy and momentum=0). Because of relativity we can always find a reference frame where the electron is at rest. Because of energy and momentum conservation, after the absorption, the electron would have the photon’s same energy and momentum so

    mv = h/lambda
    (1/2)mv^2 = hc/lambda

    We can equate those to

    mv = (mv^2)/(2c)

    Which after cancelling leaves us with


    Even if you include the relativity, it still doesn’t work out. and you get v=c

    Obviously impossible. For an electron to absorb a photon it needs to be bound to an atom which can take some of the momentum away and keep our conservation laws intact.

  10. We can be such clever apes when we want to be. It’s really humbling and amazing to see the things we are learning and understanding as we dare explore these questions. Great post.

  11. D Hunt

    I don’t think you can’t assume an at rest reference without negating your equations. If you do so then the velocity and energy are 0. If it has energy, it’s velocity can’t be 0. It also looks like you’re leaving out the Lorentz factor in the de Broglie equations based on your at rest assumption.

  12. Chris A.

    “That’s amazing. Stars were so plentiful and so energetic even so long ago that they were capable of ionizing the entire Universe!”

    Well, yeah, but the universe was smaller back then, and thus more crowded than today.

  13. D Hunt

    It looks like you’re leaving out the Lorentz factor in the de Broglie equations based on your at rest assumption. Is there a reason why?
    From what I can find, a free electron cannot absorb a photon, but the explanation I found suggests it can take away some of its energy, which causes it to accelerate. Due to that acceleration it will emit another (lower energy) photon. This may be a result of a bad analogy though.

  14. D Hunt

    Not sure how I got a double post out of an edit…

  15. Chris

    @11 D Hunt
    Remember no matter what reference frame you are in the photon will always be traveling at the speed of light. Since we are going to the electron’s ref frame (and let’s face it, that’s what the electron will see), the electron will think it is at rest and get wacked by a photon traveling at the speed of light.

    For an electron
    Using relativity E = gmc^2 (Here g is the Lorentz factor gamma)
    p = gmv

    Now for the photon before the collision There is no gamma factor here for photons
    E = hc/lambda
    p = h/lambda

    E(photon) + E(electron_before) = E(electron_after)
    p(photon) + p(electron_before) = p(electron_after)

    hc/lambda + 0 = gmc^2
    h/lambda + 0 = gmv

    Simplifies to
    h/lambda = gmc = gmv

    So if an electron absorbed a photon, the only way momentum and energy could be conserved is if final electron velocity equals c. Not possible, can’t happen.

    BTW: I’m not crazy, phys physics profs told us this many years ago.

    @13 Hunt. Yes that is Compton scattering which is completely fine. The error in Phil’s way is saying the free electron absorbs the photon, which it clearly cannot. Compton scattering happens for high energy photons (or electrons traveling really fast against the microwave background, again in the electron’s frame, the microwaves would be blueshifted to look like x-rays or higher)

  16. Skeptical

    So let me get this straight….

    All the energy and matter that is now our known universe was, for some unknown reason, compressed into a tiny amazingly powerful uber-speck that, for some unknown reason, exploded and spewed all the energy and matter that is now our known universe in all directions.

    And over an incredibly long period of time, some of that matter and energy that was flying outward from the explosion at incredible speeds, for some unknown reason (usually referred to as random chance) managed to come together and form the basic building blocks of life.

    And then over another incredibly long period of time, those basic building blocks of life somehow, again for some unknown reason (again usually referred to random chance) accumulated and grew in both size and complexity.

    And then over yet another incredibly long period of time, those basic life forms adapted to changes and ultimately evolved into me sitting here typing this message.

    And this sounds completely plausible and irrefutable? Hmm. Interesting.

  17. Greg in Austin

    If we had a large enough telescope, would we be able to see the TARDIS and the Pandorica combined in process of rebooting the universe?


  18. CJSF

    I’m a little confused. If most of the hydrogen in the universe is ionized, because the electrons got “knocked out,” isn’t that the same as saying there’s a bunch of protons everywhere?


  19. Jess Tauber

    Re 17, Nah that was a parallel universe- besides there’s that whole opacity issue. Anything at all could have happened in there and nobody would see it, perhaps not even whoever was doing it. Lots of room for mischief. The right hand wouldn’t know what the left was up to.

  20. @Skeptical

    Much more believable than a bearded sky fairy having anything to do with the process that you (with surprising clarity) summarized.

  21. @Skeptical (#16), your “straight” is a totally and wholly innaccurate depiction of what science says. Please troll somewhere else if you are unwilling to actually learn and understand what the evidence says.


  22. Pete Jackson

    As if this isn’t complicated enough to explain to the layman, and added wrinkle is that beyond redshifts of about 10, neutral hydrogen in the early universe looks opaque to our earthbound telescopes which receive visible and near-infrared light. That is because the light was originally emitted in the far ultraviolet where neutral hydrogen is opaque!

    To see what was originally emitted in the visible and near ultraviolet at such remote distances, we need to go to space and use the infrared detectors at the wavelengths that cannot get through our atmosphere. That’s the job for James Webb Space Telescope (and no, I’m not a shill for JWST).

    The other approach is to use microwave and far infrared detectors, which is the job of the ALMA array which has just started firing up, and deals with the earth’s atmosphere from the lofty elevation of 5000 meters in Chile. It will be able to see what was originally emitted as visible light and near infrared at redshifts greater than about 200. The question is whether any such radiation was being emitted at that time. We’ll find out! ALMA will also be able to see middle infrared, emitted by collapsing dust clouds, from regions between 10 and 200.

  23. Jeff

    “I know it’s a little confusing, but the universe is a fairly complicated place. That’s why we’re still trying to figure it out! We think the rules it obeys, the laws of nature, are actually relatively simple and elegant. But there are a lot of them, and they interact in complex ways. If they didn’t, we wouldn’t be here to study them! So really, if you think about it, we are the result of the Universe’s laws made incarnate, evolved to the point where we can study ourselves. ”

    that is an excellent theme, one well worth being a guiding philosophy for study of the universe.
    I myself believe there is much we don’t know, simply because we are part of the universe and all our concepts/theories are filtered through the lens of our observations. But our very observations are not “objective”, but interact with and mess up what we are observing. And why is the universe probabilistic? It just is.

    Does time exist? not in the sense we think. Time is nothing but a crude concept intended to categorize the amount of interactions that are occuring.

  24. OtherRob

    I have the same question as CJSF, #18.

  25. Another Chris

    @Chris and @D Hunt

    I think Phil meant to say that the ionized universe was a plasma. The electrons were not infinitely separated from the hydrogen, but there was a mean separation, called the Debye length. You’ve got positive hydrogen in a soup of electrons. It is a constantly changing array of electric dipoles and therefore it can absorb photons over a very wide range of photon energy.

  26. Chris

    @18 & 24
    Yep, as far as we know the # of proton = # electrons in the universe, so for every free electron out there, there must also be a free proton also floating out there.

  27. Chris

    @ 25 AnotherChris
    Don’t put words in Phil’s mouth :-)

    Recombination is also known as the “time of last scattering”, not time of last absorbing. This is when our cosmic microwave background was formed.

  28. Bjoern


    What was the rate of expansion?

    At what time, exactly? 3 Minutes?

  29. Joseph G

    Not to derail all the fascinating science content (and it is fascinating, I’m not being sarcastic here), but holy fark 12.9 billion years.
    When our ancestors were still walking on all fours, that light was 99.9% of the way here. When multicellular life appeared on the scene, 94% of that light’s journey was complete. When our solar system was just a dense cloud of gas and dust and our sun had yet to be born, that light was already well over half way here.

    I mean… It’s… agh…
    *throws hands up in the air*
    I mean…. you know!?!?

  30. Joseph G

    @Pete Jackson: Thanks for clarifying to laypeeps such as myself why such projects (ALMA and JWST) are so important (and exciting!)

  31. Brian Powell

    @skeptical: you have an antiquated view of the big bang — it was not a localized cosmic explosion of the sort you envision. If you want to discuss science, educate yourself first rather than inventing and refuting strawmen that have no relevance to our modern understanding of the universe. Don’t you have a tea party protest to attend or something?

  32. Ron1

    @ 16.

    I must not feed the troll.
    I must not feed the troll.
    I must not feed the troll … Darn, I fed the troll.

  33. Joseph G

    Also, pardon the n00bish question, but Phil wrote:

    It was such an efficient process that today, 13 billion years later, the Universe is still mostly ionized. Neutral hydrogen is pretty rare compared to its ionized brethren.

    Am I wrong, or is there essentially no difference between a proton and a hydrogen ion? If you find a proton just floating around in space, how can you tell it was ever hydrogen that was ionized (as opposed to a proton emitted by a star, for instance)? Or is hydrogen just so common that it’s pretty much assumed that all protons were once paired with an electron at some point? Come to think of it, I guess a proton emitted by a star was probably a hydrogen atom at one point…

  34. An atom is defined by the number of protons in the nucleus (and neutrons too I guess when talking isotopes). By definition, a solitary proton is hydrogen.

  35. John

    @Skeptical the plausibility of your string of statements is not generated by the logical flow of your statements. You are correct in observing that you have not made the case for each of them, nor have you said anything irrefutable.

    Your self criticism is admirable.

    However, the evidence does point to us being here by random chance. I am glad that some random sperm and random egg from your parents came together (and that they met, and that their parents met, and that their ancestors were good at not getting eaten before procreating and that the late heavy bombardment didn’t destroy all life, or the chance of new life (it is not 100% clear if life, or organic precursors occurred before or after) and before that, that just enough hydrogen formed into our sun for it to be a G2 star instead of an O or B that would have blown up billions of years ago…), and through heredity and the way you were raised, you are here to give us your unique and irreplaceable contribution.

    We however disagree.

    In your particular ontology where everything is designed, we don’t really fit. But in ours, where everything is a happy accident, completely irreplaceable and important and unique, you fit in. So, while we disagree about skycake, I am glad we both share the universe, and in particular this planet.

    I hope you have a wonderful day. Remember, you are completely improbable, and not designed by anyone. Your life is not predetermined! Therefore you can live as you choose to see fit, create works of art… or literature, or travel, or become a doctor and cure the sick…

    We look forward to your contribution :)

  36. OtherRob

    @Chris, #26 and @Larian, #34:

    Thanks for the explanations, particularly about a single proton being considered hydrogen. That’s what was confusing me and, I suspect, CJSF.

  37. Phil, thanks for the best explaination I’ve ever read of the recombination and reionization events. I’ve read countless cosmology articles that get this completely wrong, claiming, for example, that it was reionization that made the universe transparent. I’ve never been able to square the contradictory stories I’ve read on this. Thank you, thank you, thank you.

    One question. I assume that since the light from these stars is massively red shifted, the ultraviolet light that reaches us from so far away is now deep into the infrared. Will the JWST help with this sort of thing?

  38. I will have to admit I love to learn but when it comes to science I am the dumb blonde. I grew up in Los Alamos and became a fan as it were on what people see for this theory. I thank you for the insight.

  39. Joseph G

    D’oh. *facepalm*
    Somehow I missed comment #18.

    So does this mean that solar proton events and coronal mass ejections are made of the same stuff, just traveling at different speeds?

  40. ex0du5

    A photon has spin 1. A free electron has spin 1/2. A free electron cannot absorb a single photon due to conservation of angular momentum. However, it is certainly possible to absorb 2 photons or more. Bound electrons, of course, can be in a variety of relationships with their binder in terms of angular momentum, many of which have allowed absorption transitions. There are many ways to cause temporary binding for opaqueness considerations as well, including 3-body recombination.

  41. Brian Too

    I read ” A thick, dense soup of matter…” and immediately the theme from Big Bang Theory started playing in my mind.

  42. James H. (south of Dallas)

    @Bjoern: yes the first three minutes.

  43. I think that what fascinates me more than anything else is that we cannot “see” the other side of the universe, simply because the universe expanded in all directions after the B2 at faster than the speed of light. What that implies to me is, since we can’t see it all, we may NEVER know how many galaxies, black holes, and stars there are, no matter how well we count the ones we can see. If life is sparse throughout, which is a possibility, even if there was another civilization out there, we may never find it! (That would be a tragedy, so I chose not to believe it.)


  44. Srikar

    Loved this article and all the comments. It was certainly ‘consciousness raising’ ( stealing from Dawkins here) for a petty science illiterate like me. These sort of articles feed my hunger to learn more. Thanks Phil :)

  45. complex field

    @Chris – It has been a very long time since I have dealt with the subject, but off-hand it looks as if you are tacitly assuming v_electron = c. Also the frame of reference is important as relativistic effects will be significant and no matter what v_electron is, it will never become = c because that would require infinite energy, even for only one electron. Moreover, plasmas are not conservative systems so conservation of energy does not necessarily apply.

  46. Chris

    @45 complex field
    Not assuming v_electron=c, but saying that if a photon hits an electron, the only way both energy and momentum could simultaneously be conserved is if the electron’s velocity was c. Since it is clearly an absurd result, a free electron cannot absorb a photon.

    Conservation of energy always applied, check the 1st law of thermodynamics. Unless we are dealing with times less than h/dE (from Heisenberg Uncertainty principle) which we aren’t.

  47. RwFlynn

    This was super interesting and I learned a lot about the formation of the universe from this, not having a traditional education in Astronomy — yet — but one thing I must also mention is that I love how you described the formation of the universe as “way back in the olden days.” 😀

  48. Dragonchild

    @33. Joseph G
    You’re basically 99.8% correct, but to be exact, there is a difference:

    A proton is a hydrogen ion.
    A hydrogen ion isn’t necessarily a proton.

    In a small minority of cases a proton can combine with a neutron or two, making deuterium or tritium, respectively. Sans electrons they’d be hydrogen ions, but a proton stuck to a neutron isn’t a free proton.

  49. icemith

    @ 43. The Barber of Civility …..

    I was thinking along similar lines. That we assert we can “see” (almost) all the way back to the “Big Bang”, that means we have determined the position of the actual Event. Or have we?

    What are the co-ordinates? Or are we just asserting that because we have determined the oldest found galaxy, by definition, it must be close to that event.

    I need to understand that if that event WAS the, shall we say, the center of the Universe as we know it, and it wasn’t hard up against a wall, (Dark Wall?), then the spread would have been in all directions. Yet I have seen diagrams where it is depicted as cone shaped. Is it to do with the spin rates of the photons/protons/electrons ….. or even the elements after they were formed?

    My other observation relates to the fact then, that there is another 13.7bn years, approx., of expansion straight past that Universe center out to the far edge. And also in other directions as well. That assumes that WE are at the edge, which obviously we are not. So can we flip our ‘scope around and find that there are still another 13.7bn years of light incoming! Or does it?

    At this point I tend to give up. I really don’t want to go back to the much simpler world where the Earth was the center of the Universe!


  50. Joseph G

    @Dragonchild: Ah, I see. Thanks. I always forget about those pesky isotopes 😉

    @icemith: Yep, it’s really hard to picture in the mind’s eye. Because of early inflation, and the dark energy expansion since, to a lesser extent, it appears that the universe stretches far beyond what we an see. In facy, everything seems pretty much homogenous in all directions (wiki WMAP for moar… everything). Basically, if the universe does have an edge, or even some “suburbs” that are more sparse or somehow different then the rest, we can’t see ’em.

    As far as a “center,” scientists all pretty much agree that there isn’t one. To people in that galaxy, the Milky Way would appear to be on the “edge” of the visible universe, along with other galaxies 12.9 billion lightyears away in all directions. They can see galaxies that we can’t see (those further from us than they are) and we can see galaxies they can’t see. The expansion of the universe isn’t caused by galaxies moving out from some central point, it’s caused by the space between the galaxies expanding. One metaphor I like is ants on the surface of an expanding balloon. The distance between them on the surface of the balloon is increasing, but none of them can be said to be “at the center” of the expansion, and the ants themselves aren’t moving across the surface of the balloon (assuming they’re just stopped dead in their tracks wondering where the heck their buddies are going).

  51. Hmmm … conditions in the first half-billion or so years of the early Universe were very different from the conditions of today.

    I wonder if those conditions would have permitted some kind of exotic “life”, in the sense of self-replicating units, to exist in the (much thicker than today’s) interstellar gas clouds at some point. And if so, if intelligence and self-awareness could evolve. And if so, if those life forms could ponder their ultimate long-term fate and discover that the universe would soon become too thin for them to exist anywhere.

    And now, here we are, self-aware intelligent organisms, in a different age of the universe, wondering how long WE will be able to continue to exist before (say) the Great Rip.

    After billions or trillions of years, when the universe has evolved to the point where carb0n-based life is no longer possible, will the conditions permit yet ANOTHER exotic kind of self-replicating units to evolve intelligence and self-awareness, and ask the same question about their OWN fates?

  52. complex field

    @Chris 48 – that is what i was actually trying to get at. electrons don’t travel with v = c. therefore both energy and momentum will not be preserved. energy will not be conserved because the plasma does not present a central field (eg, a nucleus for an electron or a planet for an asteroid), however momentum can be when scattering is involved. but when an electron absorbs a photon, neither will be conserved.

  53. complex field

    of course the first LoT only applies for a closed system, which a plasma rather definitively is not…

  54. icemith

    @52. Joseph G …..

    “As far as a “center,” scientists all pretty much agree that there isn’t one. To people in that galaxy, the Milky Way would appear to be on the “edge” of the visible universe, along with other galaxies 12.9 billion lightyears away in all directions. ……”

    There you go, I still cannot grasp that concept, nor the explanation that the Universe is akin to a forever (?) inflating balloon. (I don’t want to be around when that balloon gets pricked). I remember an analogy used years ago that there were spots on the surface of that balloon, and as it inflated, the spots would move further apart, relative to each other, and usually not mentioned, the fact that the spots themselves, (galaxies), would also expand. I can now appreciate that the solid matter in those galaxies cannot themselves expand, due to the otherwise rigid physical laws, without invoking the usual conservation of energy arguments among others. Shades of super-sized elements come to mind. Or Super Ants as per Joseph’s updated analogy.

    Now I know somebody out there will tell me that that is exactly what happens. I cannot grasp that. Nor can I accept that Space is “curved”. Light may curve through space, due to some influence such as gravity from its passing some relatively large body. If the “curved space” idea is taken to a logical end, or at least a good way towards it, then I would have to expect Parallel Universes, or at least “Worm-hole” short cuts, due to the curling-up of the fabric of space wrapping around itself, not unlike some leaves when they dry out, and, familiar to Australians, some Eucalyptus tree bark, as it naturally peals off, curls around itself. I know as a kid it was not a very good idea to light and smoke it. It tasted awful!

    Getting back on topic, I also have trouble trying to incorporate the “Nowness” of Now. Yes, I know that the Sunlight that bathes us, that generates the weather, gives life to vegetation etc., is already 8.25 seconds old. The conditions on the Sun’s surface, or even the rest of the Sun, for that matter, haven’t changed for the WHOLE of man’s existence. The gravity it exerts on our planet has been a determinate of us, and we have seen that the Astronauts have worked up means to cope without it.

    I can conceive of a “Time”, that accepts ALL things happening now, regardless of the consequences, but as it does not rely on the the velocity of light limitation, can this be related to the “Dark Matter” question, (does light even travel through it, or does something else travel much, much faster through it? – What’s the velocity of Dark?). I wonder what may be happening to those outlying galaxies at this moment, that we can see 13bn years ago. Maybe they have dissipated, as we have seen much closer and younger galaxies change and absorb/get absorbed and generate new galaxies. With dissipation out there, there may not be any more galaxies formed, only slightly foggy space, near the limit as we think we see it.


  55. complex field

    Just for the sake of completeness: turns out that I was, in fact, erm….misremembering. Please disregard my posts.

  56. me3po

    Wow! It doesn’t happen often that I read about the universe and actually get what I’ve just read. The comments were also quite informative. Thanks for them.

    I do have a question, though. How do we figure out how old a galaxy is? I know some of them are very old, but I’m not sure how we know. Is there some calculation done based on the brightness of light or the type of light? I’d like a relatively simple scientific explanation if you have a moment, please.

  57. Ron1

    56. icemith Said, “I also have trouble trying to incorporate the “Nowness” of Now. Yes, I know that the Sunlight that bathes us, that generates the weather, gives life to vegetation etc., is already 8.25 seconds old.”


    Don’t you just love, as Phil would say, the enheadbustingness of the real world?

    As for a concept of now, it get’s even a little more interesting when you bring human physiology and psychology into the mix. For example, while it takes about 8 seconds for light to travel from the sun to your eye, it also takes further time for your eye to transmit it to your brain, then for your brain to flip the light image and then interpret the image so that you see the sun and the surrounding image as you do (ie. right-side up), … and yet it may be not quite as others might experience the event.

    Groovy, huh!

  58. ecotto26

    What’s throwing me off is the fact that electrons don’t absorb photons. A photon itself & all visible light for that matter are created by electrons itself so saying the electron absorbs the photon makes no sense. A photon is created when an atom’s electron absorbs enough energy (radiation) that it jumps to higher orbital level; however the electron can only live briefly in this new orbital level so before returning to the original orbital level the new energy absorbed is then release into a photon of light (preventing violating the conservation of energy and momentum). The distance the electron falls is a function of the photon frequency/color that is produce. This is how all the visible lights we see originates which brings me to the main problem I have with this article. The article states: “A thick, dense soup of matter permeates space, formed in the first three minutes after the Big Bang…. During this time, electrons and protons were whizzing around on their own.” This implies hydrogen atoms weren’t even formed yet so how in the world were there photons in existence before the first ionization that cause the neutral hydrogen occurred since the loose electron did not have an orbital level in an atom to jump to & produce the photons in the first place? The only way this article makes sense to me is if the word “photon” is replaced with the word radiation. If I’m missing something here please someone en-light me.

  59. Ivar

    Regarding the long string of comments about electrons absorbing photons…

    I suppose an electron can’t absorb a photon in an isolated system consisting of only the electron and the photon. That doesn’t apply to this universe. Solitary charged particles emit radiation when accelerated in a magnetic field – cyclotron radiation – consequently the reverse can also happen! The key is the external field – external to the electron and the photon.

    The radiation gets more energetic with stronger fields – so with other charged particles nearby and thus strong fields – the plasma before recombination – the photons emitted and absorbed are of short wavelengths, with thinner matter – the current interstellar or intergalactic plasma – the emissions and absorptions happen mostly (or perhaps only) at longer wavelengths. But it happens! All it takes is something in the surroundings to interact with, even if it’s just the fields of solitary protons passing by now and then.

    I have absolutely no idea what the probabilities may be – but obviously they’re very very low, practically zero (or really zero, see previous parenthesis) for visible light in the interstellar medium.

    I guess the wavelengths that are absorbed efficiently, increase very rapidly with reduced density, so only a few hundred million years of expansion and clustering was enough to make the difference between ionized gas being opaque or transparent to visible light. Based on the ionospheric cutoff of radio waves from around 10-30m wavelengths – I’d say the present interstellar medium may be opaque from around, oh, I don’t know, a few kilometers wavelength perhaps.

  60. Thanks for taking the time; it’s long, but well worth reading…!

  61. Matt B.

    Chris @15, you screwed up one of your equations. (I’m replacing the names of Greek letters with the actual letters to make it more legible.)

    “E(photon) + E(electron_before) = E(electron_after)
    p(photon) + p(electron_before) = p(electron_after)

    hc/λ + 0 = γmc^2
    h/λ + 0 = γmv”

    hc/λ + 0 = γmc^2 should be hc/λ + mc^2 = γmc^2,
    so hc/λ + 0 = (γ – 1)mc^2


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