Science Clock Series: Part III

Today we turn to the field of cosmology for our number, which happens to be three, and is given by:

\[\approx\ \text{background radiation of space (K)}\] The “background radiation of space” refers to the Cosmic Microwave Background Radiation, usually abbreviated “CMBR,” or just “CMB.” The CMBR is a diffuse ocean of electromagnetic radiation which pervades space evenly from every direction and which has its peak energy in the microwave portion of the spectrum. It has an almost uniformly frigid temperature of \(2.72548\pm0.00057\) K (about \(-270.42^\circ\)C, or \(-454.76^\circ\)F).

Because the CMBR pervades all of space, as far as we can tell, it defines the coldest temperature you can find naturally in the universe (at the current epoch). This is what people usually have in mind when they think about the temperature of space. If you were to travel to the intergalactic void in the incomprehensibly large bubbles of empty space between the faintly glimmering gossamer filaments of galaxy clusters, far away from any stars or sources of heat, this is the temperature you would measure. That's what I mean when I say that the SCUBA-2 instrument on the James Clerk Maxwell Telescope is currently the coldest place in the known universe, because its sensor arrays operate at a temperature of a mere \(0.07\) K (about \(-273.08^\circ\)C or \(-459.54^\circ\)F), 70 millikelvin above absolute zero.

Now, although the CMBR is incredibly uniform in all direction, it's not perfectly uniform. It is, however, pretty close, having no variations larger than 1 part in 10,000. For comparison, according to the regulations of the World Pool-Billiard Assocation, pool balls need only be smooth to 1 part in 500. To put that in perspective, the CMBR is twenty times smoother than a pool ball.

Analysis of the variations in the CMBR is a very active area in cosmology. In the last 24 years there have been no fewer than three satellites (COBE, WMAP, and Planck) dedicated to measuring and mapping the minute variations found across the sky in the CMBR. On a more personal note, it also holds a bit of a special place in my heart because it's partly responsible for me going into astrophysics.

You see, I've always been interested in astronomy as long as I can remember, but when I was young I was interested only in the planets and moons of our Solar System. Stars, galaxies, and the wider universe held no interest for me. It wasn't until I came across a book on cosmology sometime around the age of eleven that I became interested in the universe in general.

You see, any modern theory of cosmology needs to explain the existence of the CMBR. It is generally taken as evidence of the Big Bang theory of the universe's formation, and is explained as light from diffuse, homogeneous clouds of (primarily) hydrogen from early in the universe's history that has been highly redshifted over time by the expansion of the universe down to the energies found today. It is extremely difficult (if not impossible) to explain in the Steady State model of the universe (a competing theory during the early 20th century which was generally considered to be dis-proven upon the discovery of the CMBR), and is generally taken as proof of a finite age (and thus a beginning) of the universe.

There are, however, some problems with this particular interpretation. For one, the CMBR is too smooth; for it to be as smooth as it is today in the Big Bang theory, it would have required various parts of it to exchange heat energy between themselves and equalize their temperatures. However, if we pick two areas of the sky on opposites sides from us, there hasn't been time for them to have exchanged energy. This led to the postulation of inflation, a period of greatly-accelerated expansion of the universe, providing time beforehand for everything to equalize; however, there has yet to be any concrete evidence for such a period of rapid expansion. There's also the problem of galaxy clusters not casting as much of a “shadow” on the CMBR through the Sunyaev-Zel’dovich effect as they should, on average, and the anomalously weak quadrupole moment in the distribution of variations in the CMBR.

The point of the above is not necessarily to show that the Big Bang theory is wrong, per se, merely to point out that there remain unsolved problems with it (as there do in pretty much every area of science. It wouldn't be the “search for knowledge” if we already knew everything, would it?). As scientists we must always keep in mind the possibility that there may exist alternatives that fit the data as well or better, and it would be prudent to keep an open mind.

I encountered one such alternative as a young lad in the previously-mentioned book Starlight and Time, by Russell Humphreys, which contained an alternate cosmological theory consistent with the book of Genesis, General Relativity, and everything known about the CMBR up to 1994 when it was published. I remember that as a child the equations of General Relativity scattered liberally throughout the text were incomprehensible to me – I'm not sure I had even begun algebra at that point – but the accompanying text explained fascinating concepts like time dilation, gravitational red-shifts, and the expansion of the universe in language that I could grasp. It was, in all honesty, a pivotal point in my life. I was hooked on physics, and knew that, someday, I too wanted to spend my life studying the fundamental mysteries of the universe.

(If you're curious, in Humphrey's theory the CMBR is the primeval light created on the first day of creation, just stretched and red-shifted into the microwave region. If you're curious and not afraid of math [though as mentioned the writing stands on its own], you can find the book on Amazon.com, as I discovered while writing this post. I never owned a copy myself and it's been years since I read it, so I now have the Kindle edition to look forward to re-reading on my phone.)

Anyway, you now know what the cosmic microwave background is, and that where it comes from depends on your starting assumptions. Regardless, it's a fascinating topic and I could say a lot more about it, but this post is long enough already. Tune in next time for a very important number from biology! Click here to jump directly to it.