Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson Read Free Book Online Page A

the temperature of the cosmic background radiation for themselves, long before we managed to so do, they should have found its temperature to be greater than 2.73 degrees Kelvin, because they would have inhabited the universe when it was younger, smaller, and hotter than it is today.
    Can such an audacious assertion be tested? Yup. Turns out that the compound of carbon and nitrogen called cyanogen—best known to convicted murderers as the active ingredient of the gas administered by their executioners—will become excited by exposure to microwaves. If the microwaves are warmer than the ones in our CBR, they will excite the molecule a little more effectively than our microwaves do. The cyanogen compounds thus act as a cosmic thermometer. When we observe them in distant, and thus younger, galaxies, they should find themselves bathed in a warmer cosmic background than the cyanogen in our Milky Way galaxy. In other words, those galaxies ought to live more excited lives than we do. And they do. The spectrum of cyanogen in distant galaxies shows the microwaves to have just the temperature we expect at these earlier cosmic times.
    You can’t make this stuff up.
    The CBR does far more for astrophysicists than to provide direct evidence for a hot early universe, and thus for the big bang model. It turns out that the details of the photons that comprise the CBR reach us laden with information about the cosmos both before and after the universe became transparent. We have noted that until that time, about 380,000 years after the big bang, the universe was opaque, so you couldn’t have witnessed matter making shapes even if you’d been sitting front-row center. You couldn’t have seen where galaxy clusters were starting to form. Before anybody, anywhere, could see anything worth seeing, photons had to acquire the ability to travel, unimpeded, across the universe. When the time was right, each photon began its cross-cosmos journey at the point where it smacked into the last electron that would ever stand in its way. As more and more photons escaped without being deflected by electrons (thanks to electrons joining nuclei to form atoms) they created an expanding shell of photons that astrophysicists call “the surface of last scatter.” That shell, which formed during a period of about a hundred thousand years, marks the epoch when almost all the atoms in the cosmos were born.
    By then, matter in large regions of the universe had already begun to coalesce. Where matter accumulates, gravity grows stronger, enabling more and more matter to gather. Those matter-rich regions seeded the formation of galaxy superclusters, while other regions remained relatively empty. The photons that last scattered off electrons within the coalescing regions developed a different, slightly cooler spectrum as they climbed out of the strengthening gravity field, which robbed them of a bit of energy.
    The CBR indeed shows spots that are slightly hotter or slightly cooler than average, typically by about one hundred-thousandth of a degree. These hot and cool spots mark the earliest structures in the cosmos, the first clumping together of matter. We know what matter looks like today because we see galaxies, galaxy clusters, and galaxy superclusters. To figure out how those systems arose, we probe the cosmic background radiation, a remarkable relic from the remote past, still filling the entire universe. Studying the patterns in the CBR amounts to a kind of cosmic phrenology: we can read the bumps on the “skull” of the youthful universe and from them deduce behavior not only for an infant but also for a grown-up.
    By adding other observations of the local and the distant universe, astronomers can determine all kinds of fundamental cosmic properties from the CBR. Compare the distribution of sizes and temperatures of the slightly warmer and cooler areas, for instance, and we can infer the strength of gravity in the early universe, and thus how quickly matter