Astronomers have found when and how the cosmic fog was lifted

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

Astronomers have found when and how the cosmic fog was lifted AstronomyI am an astronomer, writer, and skeptic. I likes reality the way it is, and I aims to keep it that way. My real name is Phil Plait, and I run the Bad Astronomy blog.astronomy222

Successful stars talk dead stars

I somehow missed it when it came out, but the folks at IRrelevant Astronomy have a great video about how stars die, and it has Sean Astin (Samwise!) and Sandeep Parikh (Zaboo!).

IRrelevant Astronomy is a very funny web series about infrared astronomy put together by folks at Spitzer Space Telescope, and they’re all pretty good. This one is a followup for <a href="; target="_blank"a great video about galaxies featuring Felicia Day. They also have a couple with a guy named Wil Wheaton. Never heard of him myself, but he has promise as an actor, I think.

If you have the time, you should watch ‘em all. They’re funny, and well done, and you just might learn something.

Tip o’ the beryllium mirror to Jennifer Ouellette on Google+.

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Felicia Day collides galaxies
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Robot Wil Wheaton takes over the Universe

Successful stars talk dead stars AstronomyI am an astronomer, writer, and skeptic. I likes reality the way it is, and I aims to keep it that way. My real name is Phil Plait, and I run the Bad Astronomy blog.astronomy222

Followup: FTL neutrinos explained? Not so fast, folks.

If you haven’t heard about the experiment that apparently showed that subatomic particles called neutrinos might move faster than light (what we in the know call FTL, to make us look cooler), then I assume this is your first time on the internet. If that’s the case, then you can read my writeup on what happened.

Basically, neutrinos move very very fast, almost at the speed of light. Some scientists created neutrinos at CERN in Geneva, and then measured how long it took them to reach a detector called OPERA, located in Italy. When they did the math, it looked like the neutrinos actually got there by traveling a hair faster than the speed of light! 60 nanoseconds faster, to be accurate.

Was relativity doomed?

Nope. In fact, relativity may very well be what saves the day here.

First, most scientists were skeptical. Even the people running the experiment were skeptical, and were basically asking everyone else for help. They figured they might have made a mistake as well, and couldn’t figure out what had happened. Relativity is an extremely well-tested theory, and doesn’t (easily) allow for FTL. Despite some headlines screaming that Einstein might be wrong, most everyone figured the problem lay elsewhere.

Most everyone zeroed in on the timing of the experiment, which has to be extremely accurate. The entire flight time of a neutrino from Switzerland to Italy is only about 2.4 milliseconds, and the measurement accuracy needs to be to only a few nanoseconds — mind you, a nanosecond is a billionth of a second!

The scientists used a very sophisticated GPS setup to determine the timing, so that has been the focus of a lot of scrutiny as well. And a new paper just posted on the Physics Preprint Archive may have the answer… and it uses relativity.

Basically, what Einstein found is that the speed of light is the same for all observers. If I’m moving at 0.9 times the speed of light toward you and turn on my flashlight, I see those photons moving away from me at the speed of light. The thing is, you see those photons moving toward you at the speed of light! This goes against common sense, which tells us that velocities add together; if I throw a baseball out car window, the velocity of the ball add to that of the car.

But light doesn’t behave that way. And this changes a lot of things, including how two objects moving relative to each other measure distance, and even how they measure time. I might measure a meter stick in my hand as being (duh) one meter long, but an observer moving past me at a significant fraction of the speed of light would see it being shorter. It’s just a consequence of the Universe making sure we all see the same speed of light.

And that’s where neutrinos come in. In this new paper, author Ronald A.J. van Elburg lays out his case. The timing was measured using a GPS satellite orbiting the Earth, and moving relative to CERN and OPERA. That means the distance traveled by the neutrinos would be less as measured by the GPS sat as it would be from the ground, and therefore wouldn’t take as long to cover it. Doing the detailed math, van Elburg calculates how much faster the neutrinos would be expected to arrive accounting for the satellite’s motion, and he gets… 64 nanoseconds. That’s almost exactly the discrepancy measured by the original experimenters.

Case closed!

Well, maybe. As I recall from the foofooraw that unfolded after the initial announcement, the original experimenters said they accounted for all relativistic effects. The paper they published, however, didn’t include the details of how they did this, so it’s not clear what they included and what they might have left out. It’s possible van Elburg might be right, but I expect we haven’t seen the end of this. After all, not long after the announcement, a physicist asked if they had accounted for gravitational time dilation — like relative velocity, gravity can also affect the flow of time, throwing off the measurement — and the experimenters said they had.

I had thought of something like this as well. CERN and OPERA are at different latitudes, and since the Earth rotates, they are moving around the Earth’s axis at different speeds. Could that be it? I did the math, and the answer is no. Too bad; it would’ve been fun to be the person to have figured this out!

The bottom line here is that this experiment is still very interesting. I don’t think we know exactly what’s going on here yet — my bet is still on the statistics, since they didn’t measure the speeds of individual neutrinos, but clouds of them, making the exact timing much harder — but it’s hard to say. Like most other scientists, I think somewhere down the line here a mistake was made, and the neutrinos, like everything else we know of made of matter, travel slower than light. But if we’re wrong, then we get new physics, which is great! And if we’re right and figure out how, it means that future experiments will benefit from this. Win/win.

Either way, my bet is that we’re not done here. This new result is interesting and may very well be right, and be the dampening field that bursts the neutrino FTL warp bubble. But I’ll wait for the reaction from the original experimenters to see what they say. If we’ve learned one thing from all this, it’s that it’s best not to jump to conclusions.

Related posts:

Faster-than-light travel discovered? Slow down, folks
A (very) smart kid and a solid theory
Wall Street Journal: neutrinos show climate change isn’t real
Followup on the WSJ climate denial OpEd

Followup: FTL neutrinos explained? Not so fast, folks. AstronomyI am an astronomer, writer, and skeptic. I likes reality the way it is, and I aims to keep it that way. My real name is Phil Plait, and I run the Bad Astronomy blog.astronomy222