Cosmic Microwave Background Radiation Quiz

Answer: Big Bang Theory

The Big Bang Theory refers to the idea that the universe, in its earliest moments, was small, hot, and dense, and that over the subsequent billions of years it has expanded and cooled. It does not address how the universe began, or what kicked it into existence. By definition, science deals only with the natural laws of the universe, so anything that happened "before" or "outside" is a separate question entirely. Before the universe as we know it existed, we can only guess whether time, matter and energy had any meaning at all, let alone the ones we're familiar with. Science picks up a tiny fraction of a second after the beginning, with the universe in an extremely hot, extremely dense state, and follows how the universe changed as it expanded and cooled.

The Big Bang Theory was not widely accepted when it was first proposed by Belgian physicist Georges Lemaître. To many scientists, the idea that the universe had any beginning at all seemed like an awfully religious idea -- and their perception wasn't helped by the fact that Lemaître was a Roman Catholic priest! The successful prediction of a cosmic microwave background -- leftover thermal radiation from a much smaller and hotter universe -- by George Gamow, Ralph Alpher, and Robert Herman, was one of several pieces of evidence that forced physicists to sit up and pay attention. The Big Bang Theory is now generally accepted as the best description and explanation of the history of the universe.

Answer: They were trying to track down the persistent noise in their new, extraordinarily sensitive radio antenna.

The tale of Penzias and Wilson, radio astronomers at the great Bell Laboratories, is near and dear to experimentalists' hearts. Their story is one of the best examples that ever illustrated Isaac Asimov's famous observation that "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' but 'That's funny....'"

Their instrument was a horn reflector radio antenna, designed for use in satellite communications, and they spent over a year trying to figure out why it was reporting radio noise they could not explain. It was weak but persistent and the same from every direction, and at first -- unable to think of a suitable extraterrestrial source -- they assumed that the noise must be originating on Earth. They tried cleaning the instrument, taping over irregularities (such as rivets) in the telescope's surface, and removing a pair of pigeons that had nested in the antenna. (Penzias famously referred to the pigeon droppings as a "white dielectric material.") Finally, after having calculated that their instrument noise had a characteristic temperature of around 3 Kelvin, our heroes encountered a theory paper showing that the Big Bang would have left such background radiation behind, and the rest is history. The theorists (Robert Dicke and P.J.E. Peebles, based at Princeton University) and the experimentalists submitted a pair of papers to the "Astrophysical Journal Letters," and suddenly the Big Bang was the only big-league theory in town. It had led to a prediction about the universe, and that prediction had turned out to be true; competing theories could no longer compete.

Answer: Their energy spectrum is characteristic of thermal radiation from a source at that temperature.

Pure thermal radiation follows what is called a "black body spectrum" - so named because it's the spectrum of light that would be emitted by a perfectly black object that absorbs all the light that strikes it. The cosmic microwave background is the most perfect black body spectrum that has ever been measured - in fact, for the published COBE graph of the spectrum, the line that traces the theoretical calculation is famously thicker than the error bars on the data!

The cosmic microwave background dates from the moment when the universe first became transparent to light; in effect, it gives the thermal spectrum of the universe just before that moment, about 380,000 years after the Big Bang. Everything was quite hot then, of course, but since then even the background radiation has cooled: as the universe expands, so too do the wavelengths of the photons (a phenomenon called redshift, since it shifts the wavelengths toward the red end of the spectrum). Astronomers searching the sky eons from now might not ever see this background - the photons will have redshifted too much to be seen.

Answer: Neutral hydrogen and helium atoms formed.

One interesting effect of the cooling of the universe is that it allowed the formation of stable particles, then atomic nuclei, then neutral atoms. For these things to exist for any meaningful period of time, the universe has to be cool enough (that is, low-enough energy) that they won't be knocked apart immediately -- and that condition just wasn't true for a shockingly long time. The first hadrons -- composite particles like protons and neutrons -- weren't formed until about the universe was about a millionth of a second old, which may not seem long by human standards but is an eternity by particle physics rules. It took another three or so minutes for light nuclei (like deuterium, helium and lithium) to start forming; nearly all the light elements in the universe were made before the universe was twenty minutes old. Electrically neutral atoms took much longer, however: it doesn't take much energy to knock an electron off an atom. The process of forming neutral atoms, via the capture of negatively charged electrons by positively charged atomic nuclei, is called recombination, and is thought to have taken place when the universe was about 380,000 years old.

Before recombination, the universe was opaque. Any photons emitted by matter -- thermal radiation -- were quickly scattered and re-absorbed by charged particles. When the universe became electrically neutral, however, the photons were no longer scattered so frequently; they passed right through. The universe was transparent, and any photon emitted from that point on was likely just to keep on going. The cosmic microwave background radiation that we see comes from what's called "the surface of last scattering," namely the spherical surface of the then-universe at the moment when recombination decoupled matter from radiation.

Answer: Different parts of the universe should NOT be in thermal equilibrium, due to the vast distances between them.

If two areas of the universe are in thermal equilibrium, this means that they are the same temperature -- which, as students of thermodynamics know, can only happen if there's some heat exchange between them. This means an exchange of photons, and that's where things get dicey, because the speed of light is fixed. The cosmic microwave background photons that we see coming from the galactic north are 13.3 billion years old; the photons coming from the south are 13.3 billion years old; so the "distance" between the two is 26.6 billion light-years, meaning that it would take nearly twice the total age of the universe for a photon from one region to arrive in the other, and there simply hasn't been enough time for the two regions to reach thermal equilibrium. If you wind back the clock, the same is true of the "surface of thermal equilibrium" whence the CMB photons came -- the largest region of equilibrium would have been only 380,000 light-years wide, but we observe equilibrium on a vastly wider scale.

To solve this problem, astrophysicists have formulated Inflationary Theory, which posits that, in its very earliest moments, the universe was small enough to achieve thermal equilibrium among all its parts. Then, in an infinitesimal fraction of a second, it expanded exponentially -- a brief period of incredibly rapid inflation. Although this requires a new (and not well understood) type of energy and field, it explains a variety of strange phenomena, and even predicts the small variations that we DO see in the cosmic microwave background: small, quantum fluctuations in the inflation field effectively "seeded" the universe with density pockets that grew into large-scale structures.

Answer: Sound waves

What we call sound waves are carried via vibrations, which stretch some regions while compressing others. Most of the sounds we encounter in everyday life are carried by vibrations in the air, and new swimmers soon learn that vibrations in water also carry sound.

At this point, science fiction fans may be starting to mutter. After all, it's a well-known fact that space is silent: in a vacuum, there's nothing to vibrate. So how can we talk about sound waves traveling through the early universe? It turns out that this is not a problem: the early universe was so much denser that sound waves, triggered by density disturbances left by cosmic inflation, could indeed travel through. Since all the sound waves began at the same moment, the result was much like the tones and overtones of a musical instrument, forever recorded in the cosmic microwave background.

Answer: The motion of the solar system through space distorts our view of the CMB photons.

There's a great image of this at the Lawrence Berkeley Laboratory site; the set of three pictures was once used as the cover of an issue of "Physics Today", and it's well worth a Google. The top picture shows the "dipole anisotropy," a variation of one part in a thousand. The middle picture, which has the dipole filtered out, shows a band across the middle corresponding to the heat from the center of the Milky Way galaxy. The picture at the bottom has both the dipole and galactic emissions removed, and shows the underlying structure of the CMB. Here the differences between hot and cold are only one part in 100,000!

The dipole anisotropy is a result of the Doppler effect, which is familiar to anyone who's ever stood by the side of a road as an ambulance moves by at high speed. When the ambulance is coming towards you, the sound waves from its sirens appear compressed, and you hear them at a higher pitch than when it's standing still. As the ambulance moves away, the sound waves seem stretched, and sound lower-pitched. The same thing happens to light waves. If we're moving toward a region of the universe, its CMB photons appear to have a higher frequency and thus represent a hotter temperature; the reverse happens for regions we're moving away from.

Answer: The universe is basically flat.

One of the most basic questions in cosmology is the shape of the universe. We know it is vast, possibly infinite; but is it curved? It can be hard to imagine what a "curved" universe would mean in four dimensions (space and time), so let's think about it in two. Imagine a two-dimensional universe. It could be flat, like a tabletop; it could be positively curved, like the outside surface of a balloon; or it could be negatively curved, like the sides of a saddle. You can tell what the curvature is by measuring how geometrical figures (like triangles and parallel lines) behave. In a flat geometry, the angles of a triangle add up to 180 degrees -- but in a positively curved geometry, like the surface of the Earth, the angles of a triangle add up to more than 180 degrees.

When we look at the scale of variations in the cosmic microwave background, we're essentially measuring how vast triangles drawn on the universe behave. The angular size that we measure for the largest fluctuations depends on the geometry of that part of the universe which the photons have traversed on their long journey. Our measurements of the CMB tell us that the universe is quite flat -- at least, to within our ability to measure it!

Answer: The second Lagrange point (L2)

In any three-body gravitational system where the third body (a satellite, for example) is much less massive than the other two, there are five Lagrange points where the gravitational forces of the larger two bodies balance in such a way that the point is motionless relative to them. The Lagrange points rotate and revolve with the larger bodies; their position relative to those bodies is always fixed. Although they aren't completely stable (meaning that a small displacement can cause trouble), a satellite can stay on station relatively simply; gravity takes care of revolution and rotation.

L1 is the Lagrange point on a straight line between the two main bodies. Currently used for solar observatories (since their view is never blocked) in the Earth-Sun system, in the Earth-Moon system it's the top candidate for a way station for trips between those bodies. L2, behind the smaller body on a straight line from the larger, is perfect for deep-sky observations in the Earth-Sun system. L3, behind the larger body on a straight line from the smaller, naturally can't be observed from Earth; a popular (now disproven) idea in mid-twentieth century science fiction placed a "counter-Earth" at that point.

L4 and L5 -- also known as Trojan points -- are much less stable than the first three Lagrange points. L4 is ahead of the smaller body in its orbit, and L5 is behind it. These are most famous in the Sun-Jupiter system: clusters of asteroids lead and trail Jupiter in its orbit, orbiting themselves around L4 and L5. By convention, L4 asteroids are "Greeks" and L5 asteroids are "Trojans"; the asteroids themselves are named after heroes from the two opposing sides of "The Iliad."

Answer: As part of the static when you tune an analog television between channels

When an analog TV set is tuned between channels, about 1-2% of that familiar black-and-white "snow" is the cosmic microwave background radiation. It isn't much to make measurements from -- and, as television sets transition to digital, this behavior will be lost -- but it's downright amazing to think that an old television set can pick up signal from the Big Bang.

I hope you've enjoyed this tour of one of the most fascinating phenomena in modern cosmology. Thank you for joining me!