5 Facts You Probably Don’t Know About the Cosmic Microwave Background

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“Science cannot tell theology how to construct a doctrine of creation, but you can’t construct a doctrine of creation without taking account of the age of the universe and the evolutionary character of cosmic history.” –John Polkinghorne

Out there in space, whether we look with our eyes or with a telescope — a far more powerful version of our eyes — we find that the Universe is full of stars, galaxies, clusters, and luminous objects everywhere we look.

But if we look in different wavelengths of light than what our eyes can see, we’re going to see the Universe in a whole new light, literally. X-rays show us where black holes, neutron stars, and ultra-hot gas is, ultraviolet light shows us the hottest, youngest stars in the Universe, near infrared shows us cooler stars and is transparent to all but the hottest neutral atoms that normally block visible light, while far infrared shows us warm and cool gas and dust, including the locations of future stars.

But if you look in the microwave part of the spectrum, you see something that is, perhaps, a little unexpected. You see, if you look in, say, the infrared, what you’ll see is completely dominated by the local group: our galaxy, the stars in it, and the nearest galaxies to us.

If you’re clever enough to subtract out those local sources from your sky map, what you’ll find is a slew of point sources that show off the structure of the Universe: galaxies, clusters and filaments lying beyond our own galactic neighborhood. When you look at a picture like the one below, we are looking at a map of the large-scale structure of the cosmos.

But what if we look in microwave wavelengths? Instead of seeing this rich structure that shows us point sources, galaxies, black holes, gas, dust, or something like this, what we instead see — once we subtract our galaxy out — is this.

Believe it or not, that’s the picture of our Universe in microwave wavelengths. The microwave sky shows us the same 2.725 Kelvin temperature radiation in all directions in the sky, a leftover relic from the hot Big Bang when our Universe was just 0.0027% of its present age! For perspective, if the Universe were scaled to be exactly one year long, so that right now is 11:59 PM on December 31st, this is a picture of what the Universe looked like at 12:14 AM on January 1st!

And this is a picture of the fluctuations in the cosmic microwave background, or the temperature differences in different regions of the sky. Just a few hundred microKelvin separate the hottest regions from the coldest here, with the coldest (bluest) regions actually showing us the regions of space from 13.82 billion years ago that have slightly more matter (and hence a deeper gravitational well for the photons to climb out of, making them appear colder) than average, while the reddest (hottest) regions are the least dense regions.

That’s what we see when we look at the Universe in microwave wavelengths: the Cosmic Microwave Background (CMB). But that was all background: here are 5 facts about the CMB that you might not know, even if you’re a professional astrophysicist!

1.) The Cosmic Microwave Background actually extends far into the infrared and radio spectrum!

That number that corresponds to the temperature of the CMB — 2.725 K — is the photon energy (converted into a temperature via Boltzmann’s constant) of the peak of this radiation. But the photons in the Universe come from a thermal bath, where matter, radiation and everything else from the young Universe was constantly colliding with every other particle it saw, exchanging energy and thermalizing. This produces a very special spectrum to the radiation, known as a blackbody spectrum. Every photon in the Universe cools as the Universe expands and stretches the wavelength of each one, but the shape of this spectrum is preserved!

The photons may still peak in the microwave, but they play a role in the infrared — particularly at wavelengths longer than about 300 microns — throughout the entirety of the microwave range and all the way into the radio, where wavelengths are the size of your hand!

2.) The Cosmic Microwave Background is a “surface” over 100,000 light-years thick!

The photons of the CMB smack into free electrons and protons all the time, whenever they see one. Once the Universe cools enough so that the atoms become neutral, the vast majority of these photons now stream freely for the next 13.8 billion years, until they run into something like our detectors. But the Universe didn’t become neutral all at once; nuclei and electrons have been finding one another for hundreds of thousands of years, only to be blasted apart by a high-enough energy photon! When enough time has passed and the photon background has cooled enough, collisions like this become more and more rare, and eventually, the Universe is cool enough so that photons will free-stream all the way to your eye. That’s why the CMB is sometimes called the “surface of last scattering.”

Only, it’s not quite a surface in any direction: it takes somewhere around 117,000 light years for the Universe to go from a completely ionized to a completely neutral state, and the photons we see come from all sorts of different points along the way in every direction.

But there is something remarkable about the CMB we see today…

3.) It only became neutral when it did because of a curiosity of chemistry!

The simplified picture I presented to you — a high-energy photon background cooling as the Universe expands — would explain why atoms became neutral and the CMB appears as a rough “surface,” despite having a finite thickness. But think about it: every time you get a neutral atom forming, it emits a photon, which can then be absorbed by another neutral atom, ionizing it again! Sure, eventually the Universe will have expanded enough so that we get our surface of last scattering, but that surface would have been a lot thicker than 117,000 light-years if this were the dominant effect.

In fact, there’s another effect that’s much more important!

4.) The hot-and-cold spots you see in the CMB today are completely unrelated to all the structure in the Universe!

Because the last-scattering surface has a thickness of about 117,000 light-years, that means with the passage of time, that structure changes! In fact, the last-scattering surface will look completely different 117,000 years from now, if we’re still around to see it. The structure we see in the Universe has evolved from a huge cosmic web of initial seed fluctuations, spread out all across the Universe. But the structure we see here is related to what the CMB looked like billions of years ago, not the CMB we see today!

Image credit: ESA, of a simulation of the CMB.

Sure, as far we can tell, the CMB would have looked different only in detail and distribution; the spectrum of fluctuations would be indistinguishable regardless of when we look.

But this spectrum tells us one final, very, very interesting thing…

5.) There’s a lower-limit to the size of gravitationally-bound structure in the Universe!

Due to the presence of photons in the early Universe, initially large fluctuations get washed out over time, and become smaller and smaller in magnitude on smaller and smaller scales. The lowest mass that can exist in its own bound structure at this time is on the order of a few hundred-thousand solar masses. If all of these were in the form of normal matter…

Image credit: John Nassr of Stardust Observatory, of Messier 4.

We’d get globular clusters, or collections of around 100,000 stars and up! We do get plenty of them, but remember, the Universe is also full of dark matter. And so we’d expect to also get structures dominated by this dark matter, where — after a little burst of star formation — only a tiny amount of stars remain.

The Universe, according to our understanding of the physics going all the way back to when the Universe is just a few hundred thousand years old, should be filled with not just globular clusters, but also tiny, dark-matter-dominated structures with just around 1,000 stars or even fewer! Good thing it’s 2013 and not 2005, because we’ve found them!

With just around 1,000 stars in a structure containing 600,000 solar masses (mostly dark matter, obviously), Segue 1 was the first one discovered, and now there are others! This is right around what’s predicted, and it tells us that there are likely hundreds to tens-of-thousands around every galaxy.

All of that comes from the physics underlying the cosmic microwave background, and now you know!