The Universe Is an Inside-Out Star

the universe is an inside-out star!
The universe as an inside-out star. The stellar surface is the cosmic microwave background. Behind it is plasma. And behind that, the center of the star is the Big Bang. (Thanks to Douglas Scott for generously providing these images.)

No, not really. But as we’ll see, it’s a useful analogy. Today we’ll learn about sound waves in the sun and how, if we imagine that the universe is the sun but inside-out, these are the same as the sound waves that filled the early universe.

DISCLAIMER: This is a pedagogical exercise only! I am not claiming the universe is ACTUALLY an inside-out star or that scientists think of it as one.

Sound Waves in the Sun

I’m sure you won’t be surprised when I say that the sun is a complicated beast. A nuclear furnace burning at tens of millions of degrees powers a burning ball of turbulent hydrogen gas and plasma. All sorts of crazy things happen in the sun. Magnetic fields reconnect and plasma flows on the surface, neutrinos fly out of the nuclear reaction in the core, et cetera. But let’s ignore all that for now. Let’s say that the sun is “just” a gigantic ball of superheated hydrogen gas.

But hydrogen gas is… well, a gas. And if something makes a noise, sound can travel through it. Moreover, how the sound travels,  and the frequencies that make up the sound, can tell us a lot about the interior of the sun. Fortunately for us, lots of things in the sun make sound. For example, if a bit of gas is hotter than its surroundings, it will create a pressure wave through the sun. And this pressure wave is nothing more than a sound wave.

But if we want to use these sound waves to understand the interior of the sun, we have to measure them. How on Earth do we measure sound in the sun?

Wiggles Beget Wiggles

Fortunately, we don’t need to measure the sound waves directly. All we need to do is measure the color of the light coming off the surface of the sun. A sound wave is just a fluctuation in the velocity of the particles that make up a gas. So, as a sound wave reaches the surface of the sun (called the photosphere), it will accelerate the atoms in that area. This in turn slightly changes the color of the light these atoms emit, thanks to something called the Doppler effect. (I’ve spoken about the Doppler effect before in the context of the expanding universe.) Atoms moving toward us emit light that is more blue than it otherwise would be, while atoms moving away emit light that is more red. Since not all light coming from the sun is emitted at the surface, the change in the color of the sunlight that reaches us is small but measurable.

Therefore, all we have to do is look at the surface of the sun and measure the changes in the color of the light emitted from different points on the solar surface. These changes in color correspond to the peaks and troughs of a sound wave traveling through the sun. The scientific field that studies the sun’s interior using the color fluctuations on its surface goes by the awesome name of helioseismology.

The Universe

So what does all of this have to do with universe at large? Well, as I’ve remarked before, we know that the early universe was filled with an extremely hot plasma—so hot that atoms and molecules couldn’t form. And this plasma glowed incredibly brightly. As the universe expanded and cooled, atoms and molecules formed, but the glow remained. It still remains today in the form of a bath of microwave radiation filling the universe, which we call the cosmic microwave background, or CMB for short.

That’s one way to look at things. But there’s another way, too.

Looking Back in Time

The speed of light is finite. Indeed, it’s the speed limit of the universe. This means that the light from a star four lightyears away from us is four years old. In other words, when we look out into space, we look into the past. And greater distances take us further back in time.

As we peer away from Earth, things are mostly empty for a while. Stars and galaxies are incredibly far apart, after all. But eventually we peer far enough away, into the extreme past, that we see the hot plasma of the early universe. The plasma is opaque, though, so we can’t see inside it. What we can see is the point when the plasma cools enough for atoms to form. The distance at which we see this happen is called the surface of last scattering. The corresponding time in the history of the universe is called recombination.

Since we can’t see inside the plasma, it might seem impossible for us to learn what happened before recombination. But it’s plausible that the plasma fluctuated and moved… and maybe sound waves even traveled through it. Fortunately, we can measure that! The fluctuations in the pre-recombination plasma change the color of the light in the cosmic microwave background!

And now we’re at the punchline. One way to understand this is to imagine that the universe is an inside-out version of the sun, as shown in the figure. As we look away from the Earth, backwards in time, there’s empty space. Then we reach the surface of the universe-sun, which is nothing more than the surface of last scattering. Behind it is the plasma which makes up the interior of the universe-sun. The sound waves in the interior change how the atoms and molecules on the surface (the surface of last scattering) move and thus change the color of light that’s emitted and eventually reaches us!

And thus, by measuring the fluctuations in the CMB, we can measure the dynamics of the very early universe!

The Big Bang Wasn’t a Point

One thing I like about this analogy is that it takes the center of the sun, which is a single point, and smears it out so that it becomes the surface of a very large sphere, one with the same radius as the observable universe. I like this because it reverses a common misconception.

People usually imagine that the Big Bang, the beginning of the universe, was  a single point from which everything emerged. This is completely wrong. The beginning of the universe happened about fourteen billion years ago at every point in space. So, in our inside-out sun analogy, the smeared stellar center is the Big Bang.

(Of course, there may not have been a Big Bang if, for example, cosmic inflation is correct. But that’s a story for another time.)

Related Reading

What I described in this post is a weird and crazy way of looking at the cosmic microwave background. But I’ve discussed the more “standard” understanding of the CMB several times. Most recently, I described the nitty-gritty of how cosmologists measure the CMB and how this is related to the failed BICEP2 “primordial gravitational waves” measurement.

I also wrote a three-part series on the early universe:

  • In the first post, I describe how the cosmic microwave background helped convince scientists of the existence of the Big Bang.
  • In the second post, I describe some problems with the Big Bang theory.
  • Finally, in the third post, I describe how the model of cosmic inflation fixes the problems with the Big Bang.

Further Reading

  • This post is inspired by—and borrows heavily from—a pedagogical paper by Crowe, Moss, and Scott, which you can find for free here. It’s very readable, even for the layperson, so I recommend checking it out if you’re interested.
  • Astrophysicist Brian Koberlein has a beautiful (pun intended!) blog post on how we probe the interior of the sun, in which he describes helioseismology and some other techniques. You should definitely check it out.
  • There’s a nice piece in Scientific American on the CMB here.

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