The Cosmic Microwave Background

Still taking data at work, so I figured I might as well start hammering out some more blog entries.  So much to say!  Sure, this thing is supposed to be about my experience at the Pole, but I'm going for good reasons - why not explain what those reasons are?  Sorry if I get a bit technical or I start to ramble.  I'll try my best to keep my audience in mind, but don't hesitate to tell me in comments if I fail or you want more explanation, or less.  :) I love being verbose about what I do...

As I mentioned in the first post, we're interested in learning about the very early Universe.  It fits best in a branch of astronomy called cosmology - the study of the origin, history, evolution, and eventual fate of the Universe.  How we've perceived the history and structure of the Universe has changed drastically and many times over the course of human history, and just about every step forward has brought with it a new perspective on our place in the Universe.  At first we thought Earth was the center of everything.  Then we learned the Earth revolves around the Sun.  Then we learned the Sun was just one of hundreds of billions of stars orbiting the center of a massive collection of stars and gas - the Milky Way.  Each time an advancement in cosmology brought a new perspective, taking us further and further from the center of attention.

In the 1920s, the Universe changed again, and in a big way.  The astronomer Edwin Hubble (a University of Chicago graduate, for those of you interested in such things), was observing a class of stars called cepheid variables.  These stars have a special property: how bright they are changes in a regular way.  You can observe these stars "pulse," and how fast they pulse is related to how intrinsically bright they are.  You can't actually observe how bright a star is intrinsically, at least not directly.  Just as the brightness of a candle or a flashlight depends on how far away it is, so too does starlight.  The farther the star is from us, the dimmer it appears. This uncovers a particularly thorny part of astronomy: determining distances to objects.  If we don't have prior knowledge of how bright something is, how can we disentangle its intrinsic brightness from the distance it is from us?  Well, that's the beauty of cepheid variable stars!  Since we know that their intrinsic brightness is related in a known way to how fast they pulse, we can compare how bright the stars appear (their apparent brightness), with how bright they should be and back out how far away they are.  Using this technique astronomers can get accurate measurements of distance to any cloud of gas and stars or whatever that has a cepheid variable in it.

Edwin Hubble

This is exactly what Hubble did, and his findings rocked the world. (Indeed, his work played such an important role that NASA named a telescope after him...).  Looking at cepheid variables, Hubble discovered that many were vast distances away, so far, in fact, that they couldn't possibly be in the Milky Way.  Giant gas clouds, such as the Great Andromeda Nebula, turned out to be their very own galaxies, as big or bigger than the Milky Way.  This fact astounds me - the human race was unaware of other galaxies until my great grandmother, (whom I talked to just the other day: fantastic lady.  She still bowls every Tuesday, makes a mean raspberry jam, and taught me how to make some seriously awesome crocheted slippers), was in grade school!  That's absolutely mind-boggling to me.

Hubble didn't stop there, and here's the real meat if you ask me: Hubble realized that distant objects were moving away from us, and the farther they were away from us the faster they sped away.  The Universe was expanding!  (Einstein actually figured this out years before Hubble discovered it.  It pops out naturally from the equations of General Relativity, something I found quite magical when I saw it derived for the first time, but he didn't like the idea of a Universe that changed sizes, so he put a fudge factor into his equations to make the expansion go away.  He later called this mistake the biggest blunder of his life.  Then again, the current leading theory is that his fudge factor, now called the cosmological constant, represents roughly 75% of all the stuff in the Universe, so maybe it wasn't such a bad thing to dream up).  

Anyway... Hubble plotted up galaxy velocities (v) versus their distance away from us (d) and fit a line to the data and BAM!, he discovered the constant of proportionality, H, known today as Hubble's constant (even though it's not really a constant...) : v = H*d.  This equation is now known as Hubble's law and H is the rate of expansion of the Universe.  Hubble's original value was a bit off due to calibration errors, but today we know the value to be about 70 km/s/Mpc.  One Mpc (megaparsec) is equal to 3.26 million light years, and a light year is roughly 6 trillion miles.  So, Hubble's law says that for every ~20 million trillion miles something is away from you, it is receding at a rate of 70 kilometers per second just due to the space between you expanding.  That distant galaxy over there is 100 Mpc away?  We're receding from it, (and it from us) at 7000 km/s.  That's pretty fast.  Like, circle the Earth in 6 seconds fast.  Well, just as gas expands and cools off when we open a bottle of pop (making that nice mist of condensation), so too does gas in the Universe as it expands.  As the Universe gets bigger, the same amount of gas in it has more room to fill so it gets less dense and cools off.

Hubble's original plot.

(A quick aside: if you notice, the units of the Hubble constant are 1/time.  Take the inverse of the constant to get a rough age of the Universe: 14 billion years.  Pretty close!  The current best estimate is 13.76 billion years.)

The next logical thing was to think backwards.  If the Universe is expanding and will be larger and less dense and cooler tomorrow than it is today, than it must have been smaller and more dense and hotter in the past.  Go back far enough in time and the visible Universe is a tiny point, with nearly infinite density and extreme temperatures.  And with that train of thought, the Big Bang Theory was born.

Today, the Big Bang Theory, apart from being a CBS sitcom that I find almost embarrassingly lifelike in its "made up" scenarios, is one of the cornerstones of modern cosmology.  It says that at some time, let's call it t = 0, the Big Bang happened: some unknown event that started the expansion of the Universe.  Okay, so, you have this super hot dense blob of stuff.  In that blob you have matter and anti-matter, but they're a bit out of balance.  For every billion anti-matter particles, you have a billion and one matter particles.  Well, those two don't play nice together.  They annihilate and produce radiation (photons), but that one extra matter particle is left over.  Today, we're made up of those extra matter particles.  What about all those photons?  Well, they helped thermalize the Universe: they made all parts of the Universe the same temperature.  But everything with a temperature emits light, called blackbody radiation.  Hot things emit more light than cool things.  If you know the temperature of an object, and that temperature is stable, than you know exactly how much light that object is emitting at a given wavelength/frequency/color.

So what does this mean for the Big Bang?  Since the Universe started out very hot and very dense, and it was well thermalized by matter/anti-matter annihilation (it had a very stable temperature), then it too should have emitted blackbody radiation.  The radiation started out very hot and very intense, with peak intensity in the highly energetic gamma rays, but as the Universe aged, expanded, and cooled, so too did the radiation.  This background glow of gamma rays redshifted into a glow of x-rays, then ultraviolet rays, then visible light, then infrared.  Today, this thermal radiation glow left over from the Big Bang is so cold it peaks in the microwave portion of the electromagnetic spectrum.  Hence, Cosmic Microwave Background.  Cosmic, because it comes from the Universe itself, Background because it's everywhere.  And I do mean everywhere.  There are a lot of photons in the CMB.  If you do the math, you'll find that every cubic centimeter of space (the amount of space 1 mL of water takes up) is filled with 411 CMB photons.  Turns out this is about a billion times the average density of matter in the Universe, just like the original ratio of anti-matter to matter (actually, this is how that ratio is calculated).

There's a ridiculous amount of information we can learn from the CMB, which I'll more than likely cover in way too much detail in other posts, but I should drive the point of all of this home before I finish.  In the early 1990s scientists measured how bright the CMB is at a range of colors and they found that it matched the spectrum you would expect for a theoretical blackbody almost perfectly.  It's the best blackbody in the Universe... by a lot.  The error bars on the measurements are smaller than the plotted curve.  This fact alone ensures that at some point in the past the Universe was hot and dense, proving the Big Bang happened.  A Nobel prize was awarded for these CMB measurements a few years back.

The blackbody spectrum of the CMB, as measured by the COBE FIRAS experiment, along with the expected blackbody spectrum of a 2.725 K blackbody. Error bars have been multiplied by 400 to make them big enough to see. When these results were first presented at the January 1990 American Astronomical Society conference, there was a standing ovation. It's one of the most beautiful and powerful results in the history of science (and the topic of an xkcd cartoon).

So, there's a CMB and it proves the Big Bang happened.  But that's not the whole story...