#reionization

Starts With A Bang Podcast #60 - The End Of The Dark Ages

When we look out at the Universe today, we see that it's full of stars and galaxies. And yet, we can only see those stars and galaxies because the space between those galaxies and ourselves doesn't block that starlight before it gets to our instruments, observatories, telescopes, and eyes. But early on, that's an enormous problem: there is light-blocking gas and dust, and the record-holder for most distant galaxy ever discovered is still not a pristine, first-generation galaxy at all.

But there are new observatories and cutting-edge techniques that will reveal them, teaching us how the Universe grew up: from a collection of neutral atoms with no stars and galaxies at all to the structure-rich Universe we see today. Joining me on this special, bonus edition of the Starts With A Bang podcast (because don't we all need a bonus?) is extragalactic astronomer and PhD candidate Rebecca Larson from the University of Texas - Austin, in a rich conversation that takes us all the way back to the edge of the Universe as we can observe it.

The visualization shows galaxies, composed of gas, stars and dark matter, colliding and forming filaments in the large-scale universe providing a view of the Cosmic Web.

When Did The Universe Become Transparent To Light?

“The Universe became transparent to the light left over from the Big Bang when it was roughly 380,000 years old, and remained transparent to long-wavelength light thereafter. But it was only when the Universe reached about half a billion years of age that it became fully transparent to starlight, with some locations experiencing transparency earlier and others experiencing it later.

To probe beyond these limits requires a telescope that goes to longer and longer wavelengths. With any luck, the James Webb Space Telescope will finally open our eyes to the Universe as it was during this in-between era, where it's transparent to the Big Bang's glow but not to starlight. When it opens its eyes on the Universe, we may finally learn just how the Universe grew up during these poorly-understood dark ages.”

There are two ways that astrophysicists talk about the Universe becoming transparent. The first is when the particles from the Big Bang finally form neutral atoms, becoming “transparent” to the leftover photons from that era. The second is hundreds of millions of years later, when those same neutral atoms are reionized, and starlight can travel freely through intergalactic space. Which one is right? When did the Universe become transparent to light? 

Ask Ethan: Why Was The Universe Dark For So Long?

“One thing I wonder though is why did the dark ages last hundreds of millions of years? I would have expected an order of magnitude smaller, or more.”

There’s a troubling puzzle when it comes to the Universe: the so-called ‘dark ages’ don’t come to an end until 550 million years after the Big Bang. But this is a big problem when you consider that we’ve already imaged a galaxy from when the Universe was only 400 million years old, and that we fully anticipate the first stars to form when the Universe is only 50-100 million years old. So what’s with all the darkness, then? And how do we expect the James Webb Space Telescope to see back to the very first galaxies? The answer lies in two parts. First, even though you have stars, the Universe is still filled with neutral atoms, which block visible and ultraviolet light. We need to ionize those atoms in order to have a transparent Universe, and that takes lots of time. But the second key is that the Universe, even with neutral atoms, is quite transparent to other wavelengths of light, like infrared light. And that’s where an observatory like James Webb is going to be looking!

The First Galaxies: What We Know And What We Still Need To Learn

“As we look farther back in time, we find that younger galaxies formed stars at faster rates than galaxies do today. We can measure the star-formation rate, and find that at earlier and earlier times, it was more intense. But then we find it hits a peak when the Universe is about two billion years old. Go younger than that, and the rate goes down again.”

We’ve come incredibly far in our quest to learn how the Universe came to be the way it is today. We can see out in space for tens of billions of light years, to galaxies as they were when the Universe was only a few percent of its present age. We can see how galaxies evolve, merge and the stars inside change. And we can see to even before that, when no stars or galaxies existed at all. But how did we get from there to here? There are still plenty of gaps in the story. We’ve never seen the first stars or galaxies; we’ve never witnessed the start of cosmic reionization; we’ve never seen the star formation rate jump from zero to a real, finite number. Yet with James Webb and WFIRST on the horizon, these gaps in our knowledge may -- if we’re lucky -- all disappear.