فصل 03کتاب: ستاره شناسی برای مردمی که در عجله هستند / فصل 3
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Let There Be Light
After the big bang, the main agenda of the cosmos was expansion, ever diluting the concentration of energy that filled space. With each passing moment the universe got a little bit bigger, a little bit cooler, and a little bit dimmer. Meanwhile, matter and energy co-inhabited a kind of opaque soup, in which free-range electrons continually scattered photons every which way.
For 380,000 years, things carried on that way.
In this early epoch, photons didn’t travel far before encountering an electron. Back then, if your mission had been to see across the universe, you couldn’t. Any photon you detected had careened off an electron right in front of your nose, nano- and picoseconds earlier.† Since that’s the largest distance that information can travel before reaching your eyes, the entire universe was simply a glowing opaque fog in every direction you looked. The Sun and all other stars behave this way, too.
As the temperature drops, particles move more and more slowly. And so right about then, when the temperature of the universe first dipped below a red-hot 3,000 degrees Kelvin, electrons slowed down just enough to be captured by passing protons, thus bringing full-fledged atoms into the world. This allowed previously harassed photons to be set free and travel on uninterrupted paths across the universe.
This “cosmic background” is the incarnation of the leftover light from a dazzling, sizzling early universe, and can be assigned a temperature, based on what part of the spectrum the dominant photons represent. As the cosmos continued to cool, the photons that had been born in the visible part of the spectrum lost energy to the expanding universe and eventually slid down the spectrum, morphing into infrared photons. Although the visible light photons had become weaker and weaker, they never stopped being photons.
What’s next on the spectrum? Today, the universe has expanded by a factor of 1,000 from the time photons were set free, and so the cosmic background has, in turn, cooled by a factor of 1,000. All the visible light photons from that epoch have become 1/1,000th as energetic. They’re now microwaves, which is where we derive the modern moniker “cosmic microwave background,” or CMB for short. Keep this up and fifty billion years from now astrophysicists will be writing about the cosmic radiowave background.
When something glows from being heated, it emits light in all parts of the spectrum, but will always peak somewhere. For household lamps that still use glowing metal filaments, the bulbs all peak in the infrared, which is the single greatest contributor to their inefficiency as a source of visible light. Our senses detect infrared only in the form of warmth on our skin. The LED revolution in advanced lighting technology creates pure visible light without wasting wattage on invisible parts of the spectrum. That’s how you can get crazy-sounding sentences like: “7 Watts LED replaces 60 Watts Incandescent” on the packaging.
Being the remnant of something that was once brilliantly aglow, the CMB has the profile we expect of a radiant but cooling object: it peaks in one part of the spectrum but radiates in other parts of the spectrum as well. In this case, besides peaking in microwaves, the CMB also gives off some radio waves and a vanishingly small number of photons of higher energy.
In the mid-twentieth century, the subfield of cosmology—not to be confused with cosmetology—didn’t have much data. And where data are sparse, competing ideas abound that are clever and wishful. The existence of the CMB was predicted by the Russian-born American physicist George Gamow and colleagues during the 1940s. The foundation of these ideas came from the 1927 work of the Belgian physicist and priest Georges Lemaître, who is generally recognized as the “father” of big bang cosmology. But it was American physicists Ralph Alpher and Robert Herman who, in 1948, first estimated what the temperature of the cosmic background ought to be. They based their calculations on three pillars: 1) Einstein’s 1916 general theory of relativity; 2) Edwin Hubble’s 1929 discovery that the universe is expanding; and 3) atomic physics developed in laboratories before and during the Manhattan Project that built the atomic bombs of World War II.
Herman and Alpher calculated and proposed a temperature of 5 degrees Kelvin for the universe. Well, that’s just plain wrong. The precisely measured temperature of these microwaves is 2.725 degrees, sometimes written as simply 2.7 degrees, and if you’re numerically lazy, nobody will fault you for rounding the temperature of the universe to 3 degrees.
Let’s pause for a moment. Herman and Alpher used atomic physics freshly gleaned in a lab, and applied it to hypothesized conditions in the early universe. From this, they extrapolated billions of years forward, calculating what temperature the universe should be today. That their prediction even remotely approximated the right answer is a stunning triumph of human insight. They could have been off by a factor or ten, or a hundred, or they could have predicted something that isn’t even there. Commenting on this feat, the American astrophysicist J. Richard Gott noted, “Predicting that the background existed and then getting its temperature correct to within a factor of 2, was like predicting that a flying saucer 50 feet wide would land on the White House lawn, but instead, a flying saucer 27 feet wide actually showed up.”
The first direct observation of the cosmic microwave background was made inadvertently in 1964 by American physicists Arno Penzias and Robert Wilson of Bell Telephone Laboratories, the research branch of AT&T. In the 1960s everyone knew about microwaves, but almost no one had the technology to detect them. Bell Labs, a pioneer in the communications industry, developed a beefy, horn-shaped antenna for just that purpose.
But first, if you’re going to send or receive a signal, you don’t want too many sources contaminating it. Penzias and Wilson sought to measure background microwave interference to their receiver, to enable clean, noise-free communication within this band of the spectrum. They were not cosmologists. They were techno-wizards honing a microwave receiver, and unaware of the Gamow, Herman, and Alpher predictions.
What Penzias and Wilson were decidedly not looking for was the cosmic microwave background; they were just trying to open a new channel of communication for AT&T.
Penzias and Wilson ran their experiment, and subtracted from their data all the known terrestrial and cosmic sources of interference they could identify, but one part of the signal didn’t go away, and they just couldn’t figure out how to eliminate it. Finally they looked inside the dish and saw pigeons nesting there. And so they were worried that a white dielectric substance (pigeon poop) might be responsible for the signal, because they detected it no matter what direction the detector pointed. After cleaning out the dielectric substance, the interference dropped a little bit, but a leftover signal remained. The paper they published in 1965 was all about this unaccountable “excess antenna temperature.”††
Meanwhile, a team of physicists at Princeton, led by Robert Dicke, was building a detector specifically to find the CMB. But they didn’t have the resources of Bell Labs, so their work went a little slower. And the moment Dicke and his colleagues heard about Penzias and Wilson’s work, the Princeton team knew exactly what the observed excess antenna temperature was. Everything fit: especially the temperature itself, and that the signal came from every direction in the sky.
In 1978, Penzias and Wilson won the Nobel Prize for their discovery. And in 2006, American astrophysicists John C. Mather and George F. Smoot would share the Nobel Prize for observing the CMB over a broad range of the spectrum, bringing cosmology from a nursery of clever but untested ideas into the realm of a precision, experimental science.
Because light takes time to reach us from distant places in the universe, if we look out in deep space we actually see eons back in time. So if the intelligent inhabitants of a galaxy far, far away were to measure the temperature of the cosmic background radiation at the moment captured by our gaze, they should get a reading higher than 2.7 degrees, because they are living in a younger, smaller, hotter universe than we are.
Turns out you can actually test this hypothesis. The molecule cyanogen CN (once used on convicted murderers as the active component of the gas administered by their executioners) gets excited by exposure to microwaves. If the microwaves are warmer than the ones in our CMB, they excite the molecule a little more. In the big bang model, the cyanogen in distant, younger galaxies gets bathed in a warmer cosmic background than the cyanogen in our own Milky Way galaxy. And that’s exactly what we observe.
You can’t make this stuff up.
Why should any of this be interesting? The universe was opaque until 380,000 years after the big bang, so you could not have witnessed matter taking shape even if you’d been sitting front-row center. You couldn’t have seen where the galaxy clusters and voids were starting to form. Before anybody could have seen anything worth seeing, photons had to travel, unimpeded, across the universe, as carriers of this information.
The spot where each photon began its cross-cosmos journey is where it had smacked into the last electron that would ever stand in its way—the “point of last scatter.” As more and more photons escape unsmacked, they create an expanding “surface” of last scatter, some 120,000 years deep. That surface is where all the atoms in the universe were born: an electron joins an atomic nucleus, and a little pulse of energy in the form of a photon soars away into the wild red yonder.
By then, some regions of the universe had already begun to coalesce by the gravitational attraction of their parts. Photons that last scattered off electrons in these regions developed a different, slightly cooler profile than those scattering off the less sociable electrons sitting in the middle of nowhere. Where matter accumulated, the strength of gravity grew, enabling more and more matter to gather. These regions seeded the formation of galaxy superclusters while other regions were left relatively empty.
When you map the cosmic microwave background in detail, you find that it’s not completely smooth. It’s got spots that are slightly hotter and slightly cooler than average. By studying these temperature variations in the CMB—that is to say, by studying patterns in the surface of last scatter—we can infer what the structure and content of the matter was in the early universe. To figure out how galaxies and clusters and superclusters arose, we use our best probe, the CMB—a potent time capsule that empowers astrophysicists to reconstruct cosmic history in reverse. Studying its patterns is like performing some sort of cosmic phrenology, as we analyze the skull bumps of the infant universe.
When constrained by other observations of the contemporary and distant universe, the CMB enables you to decode all sorts of fundamental cosmic properties. Compare the distribution of sizes and temperatures of the warm and cool areas and you can infer how strong the force of gravity was at the time and how quickly matter accumulated, allowing you to then deduce how much ordinary matter, dark matter, and dark energy there is in the universe. From here, it’s then straightforward to tell whether or not the universe will expand forever.
Ordinary matter is what we are all made of. It has gravity and interacts with light. Dark matter is a mysterious substance that has gravity but does not interact with light in any known way. Dark energy is a mysterious pressure in the vacuum of space that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would.
What our phrenological exam says is that we understand how the universe behaved, but that most of the universe is made of stuff about which we are clueless. Our profound areas of ignorance notwithstanding, today, as never before, cosmology has an anchor, because the CMB reveals the portal through which we all walked. It’s a point where interesting physics happened, and where we learned about the universe before and after its light was set free.
The simple discovery of the cosmic microwave background turned cosmology into something more than mythology. But it was the accurate and detailed map of the cosmic microwave background that turned cosmology into a modern science. Cosmologists have plenty of ego. How could you not when your job is to deduce what brought the universe into existence? Without data, their explanations were just hypotheses. Now, each new observation, each morsel of data, wields a two-edged sword: it enables cosmology to thrive on the kind of foundation that so much of the rest of science enjoys, but it also constrains theories that people thought up when there wasn’t enough data to say whether they were right or wrong.
No science achieves maturity without it.
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