PBS Space Time | Space Used to Be Orange!! | Season 1 | Episode 7

Outer space looks black, but the entire universe used to be this color.

How's that possible?

Let's find out.

Stars and galaxies notwithstanding, space is pitch black.

So pick a dark spot in the sky and point an analog satellite dish at it.

You might expect to get nothing, but you don't.

You get static.

Pick another point and more static.

Move your dish yet again, static.

Even accounting for all possible types of interference, no matter how you orient your dish, there's this constant underlying microwave band static that's just always there in the darkness of space, emitting the same pattern over and over.

Now since we pick up this mysterious static from every direction we look, it would seem to be coming from a source that exists literally everywhere on the sky.

Problem is, we don't know of any source anywhere that would emit this observed pattern of microwave emission, so where's it coming from?

Aliens.

No, it's not aliens.

It's never aliens.

But what if I told you that the source of static, which we call the cosmic microwave background, or CNB, was the process that formed the first atoms in the universe almost 13 and 1/2 billion years ago?

And what if I also told you that the source of the CNB also caused all of space to look orange for millions of years?

That's right, the universe used to be orange.

To understand how this could be true, we first need to take a brief detour into your toaster.

Turn on your toaster.

The heating elements glow a pale, reddish color.

That glow isn't ambient light reflecting off the toaster, it's light being emitted by the toaster itself.

If you were to analyze that glow with instruments less limited than human eyes, you would realize that the toaster isn't just emitting pale, red light.

It's emitting electromagnetic waves of all wavelengths.

Moreover, the intensity at different wavelength is in very specific proportions that trace out a graph very close to this.

That emission pattern represented by the graph is called the toaster's thermal spectrum or really an idealization of a thermal spectrum called a black body spectrum.

Now, everything has a temperature, so everything has a thermal spectrum, and it emits all electromagnetic wavelengths.

You, a taco, the sun, everything.

In fact, it's called a thermal spectrum because the light is generated by the random motions of particles in the material.

And those random motions are themselves a reflection of temperature.

Now, if you go really low in temperature, down to 2.7 degrees above absolute zero, the peak shifts way into microwave wavelengths and, lo and behold, exactly matches the CNB, and I mean exactly.

The CNB is one of the closest things to a mathematically perfect thermal spectrum that has ever been observed.

Problem is, space is pretty much empty.

There's nothing really in there to have a temperature, much less the very specific temperature of 2.7 Kelvin.

So why does the CNB look like a thermal spectrum at all?

To answer that and to see why space used to be orange, we need to turn the clock back to about 400,000 years after the Big Bang, give or take.

During that era, a supercharged particle with a temperature of several thousand degrees permeated all of space.

At this temperature, it's too hot for electrons and protons to even coalesce into atoms, let alone stars, planets or galaxies.

This ionized soup is called a plasma.

And just like toasters, people and tacos, it was emitting a thermal distribution of electromagnetic waves.

But because there were no neutral atoms yet, the light the plasma emitted just couldn't travel very far before it would run into an electron and ricochet like in a pinball game.

So if you took the Tardis back to this era and could somehow keep it from melting, you wouldn't be able to see very far on the viewscreen, maybe a few thousand lightyears, which sounds like a lot, but it's basically zero visibility in astronomical terms.

So at this moment, it was as if flash bulbs were constantly going off everywhere in space, but the light was being snuffed out by a fog.

Now as this plasma cooled, its temperature eventually dropped below the 3,000 or so degree mark, where neutral atoms could finally form.

With no more free electrons to redirect the light, the universe became, for the very first time, transparent.

The light that the plasma had emitted then just before neutralized was one last hurrah, one final flash of an infinite number of orange bulbs going off at every point in the universe more or less simultaneously.

And now, that light could free stream through the universe forever.

Before, during and after this event, space was expanding.

That's what thinned out the plasma and made it cool down in the first place.

But as we talked about in a prior episode that you can revisit here, expanding space stretches the wavelength of free streaming light through a process called cosmological redshift.

So over the course of a few million years, that orangey thermal spectrum of light was redshifted to longer and longer wavelengths, becoming toaster read and eventually infra-red, so that to human eyes, the sky eventually turned dark.

If you throw in another 13 plus billion years of space expansion, all that light has redshifted into the microwave band to become what we today perceive as the CNB.

And all those atoms from that plasma?

Well, they managed to clump, become stars, galaxies, and through a complicated process of cosmic recycling, us.

So the CNB, or more specifically, the shape of its thermal spectrum, is pretty compelling evidence that when it comes to the color of space, black is the new orange.

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