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The Scale of Things
Cosmology is about the largest scales (and times) in the Universe. What the hell does that mean? Well, if you play with the interactive flash on the right, and you focus on the scales that are 1022 m and larger, you're looking at cosmology.
The weird thing is that understanding those scales requires we investigate the other end of the spectrum, too: not just the behavior of atoms and electrons (10-10 m) but all the way down to the Planck length (10-35 m). Cosmology is really the audacious study of everything which is why it's taken so long to get away from reassuring yet completely fabricated theological stories told around our ancestors' campfires and become a legitimate science. We have truly entered an amazing time in human history (which has only been about 1011 seconds so far) where our imaginations and intelligence have begun to comprehend it all. There's plenty of work left to do: we don't have a satisfying explanation for the Standard Model of Particle Physics yet (at the small end of the scale), nor a reasonable explanation of what dark matter is (also part of the small end of the scale, but it affects the biggest ends) or dark energy (we really don't know what this is yet!). So despite what Marx might have said, we are not reaching the "end of history." We are only beginning.
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Topics we will cover...
Cosmology is the study of everything. This seemingly ridiculous statement will make more sense as the semester progresses, despite the obvious hubris. It is amazing that our ancestors on the savannahs of Africa evolved brains clever enough to study and model the universe on vast and miniscule scales. After all, during our daily lives we can only experience scales on the order of fractions of a millimeter (10-4m or so) up to at most, hundreds of kilometers (105m). There's no a priori reason evolution should equip us to comprehend anything on the scale of galaxies (~1020m) or quantum mechanics (~10-10m), let alone the smallest conceivable scale (~10-35m, something we'll talk about later, the Planck scale) or the scale of all visible spacetime (currently ~1026m), a.k.a., "The Universe." The same applies to timescales. Our experience is limited to coarse fractions of a second (~10-1s) up to maybe 100 years or so (~109s), but the smallest concievable amount of time (~10-44s, the Planck time) and the age of the Universe (~1017s) should be alien to our brains.
And they are at one level. Yet advances in observational technology and theoretical physics in the last century have brought our understanding of the entire visible Universe within reach of our minds.
Here are some mind-blowing videos made with either real data from the Sloan Digital Sky Survey (SDSS) or made using N-Body simulations. The similarities between the two types of visualizations strongly suggest we're getting something right about our models of How the Universe Got That Way (apologies to Kipling).
Keep in mind that a galaxy is almost a "single dot" to cosmologists, though they vary in size from 105 to 1011 stars...
If the videos don't show up in the next row, here are the links:
Comparison of the size of XDF
Since you're the center of the universe, it's time you performed a little self examination. Here are some free downloadable programs that let you explore the visible universe in all its glory. Stellarium is free and also available for mobile apps and gives you a planetarium-like view of what the sky holds for any date and time from any place on Earth. Celestia actually lets you fly around (like the movies above) to investigate the crazy scales we deal with in class. Both are informed by real data, and have tons of metadata (i.e., click on a planet and it tells you details) loaded as part of the experience.
He was wrong to think he could now forget that the big, hard, oily, dirty, rainbow-hung Earth on which he lived was a microscopic dot on a microscopic dot lost in the unimaginable infinity of the Universe.
— Douglas Adams
In class, we covered a fair amount of history (which is actually not addressed in great detail in your book). The main points to make are that what we can see with the naked eye is remarkably limited: the Sun, the Moon, a lot of dots, some of them colorful and the occasional smudge. Five of the dots move. That's not a lot to go on, but from this, clever Greeks (and others) figured out an awful lot of astronomy.
We drew a chronological (and oversimplified) line from Aristarchus, Eudoxus, Aristotle to Ptolemy and drew up a self-consistent view of the Universe which turned out to be wrong in some parts. But figuring out the distance to the Moon in Earth radii, as well as the size of the Earth reflects some of the best of what was right: and how to get there.
Aristotelean physics and the Ptolemy's geocentric model stood firm for over 1600 years or so. Others had certainly suggested heliocentrism (Aristarchus for one!) as a possiblity, but religious bias and natural human ego consistenly thwarted those efforts. It's not until Copernicus and friends in the late 1400's start to apply Occam's razor (look it up!) that science begins to insist there's a better way than just force-fitting observations with a priori bias from doctrine and dogma.
Tycho Brahe (born three years after Copernicus dies; picture on right) elevates the art and technology of measuring those dots and smudges to the point where observations of the heavens actually conflict with those doctrines and dogma. He notices that the parallax distances of comets as they move clearly show they're far away and that they travel through those "perfect transparent spheres" without any difficulty.
Science is a process more than a thing (though the word translates as just "knowledge"). And that process requires very specific things to work, not the least of which is imagination. The other buzzwords are important, of course, but if you have no curiosity and limited desire to "figure things out," then science just becomes a boring list of facts which seem to have nothing to do with daily life. What a sad (and incorrect) point of view that is.
Enough proselytizing. Here is a link to the history and "figure it out" slides about the distance to the Moon, the distance to the Sun and the size of the Earth. You are encouraged to work through these approaches until they make sense. Beneath a little bit of math lies a picture, worth far more than 1000 equations.
As emphasized in class, we need to rethink what angle means. You learned at your mother's knee that an angle is formed when two lines cross.
That's true, but for our purposes, astronomical and cosmological, it's good if we also realize an angle is a size divided by a distance.
θ ≈ D/d
For that to be true, we can't use degrees anymore, and need to think about angles in terms of radians. We'll go back and forth unavoidably between degrees (and arcminutes and arcseconds) and radians (angle units as the Gods would have subtended...uh, I mean intended). It is up to you to make sure you can go back and forth as needed on your calculator between these two units.
Here's the conversion factor in a nutshell. "Radians" are the units of angle if you form the ratio of arclength to radius, in otherwords, a size (the length of arc) divided by a distance (from the center):
θ = arclength / r
The circumference of a circle is 2πr and the radius is r, so the ratio of circumference over radius is just 2π. In other words, all the degrees in a circle, 360o, must be equivalent to 2π radians (about 6.28 radians — hey! look the picture above, doesn't one radian look like about a sixth of the whole pie?). That's the conversion, then, between degrees and radians: 360o = 2 π radians = 6.28 radians (to three digit precision).
1o = 6.28/360 radians = 0.0174 radians
I am the Parallax, I speak for the stars...
Parallax distance is determined by noticing how much an object "wiggles" in our field of view as the Earth moves around the Sun compared to the background stars (which are so far away, we don't expect them to noticeably wiggle). The perfect analogy is that distant mountains don't seem to "move" even as you speed down the highway, but nearby telephone poles whiz by. Shown here is a star of frightening proximity (the parallax angle is large between the two viewpoints of the Earth six months apart).
Which brings up the question, what is the parallax angle of the closest exosolar star? Now you know what to do: the angle is the ratio of a size (the Earth's orbit's radius, "1 AU") to the distance to α-Centauri. A few caveats are in order here: as you can see in this diagram, the angle you use is half of what you might have expected. So the "size" part is exactly 1 AU (not 2 — i.e., use the radius of the orbit, not the diameter). The only problem is, we might have a mix of units: α-Centauri is 4 light-years away, and if we're to have a useful angle in radians, we need the same units for the "size" as for the distance.
The reason the Earth's orbit is "1 AU" in radius (ignore the ellipticity for a moment, any Keplerheads out there!) from the Sun is that people didn't know the exact distance to the Sun in meters. But even Kepler could figure out what the ratio of the various orbits was using parallax. So he did, and others have done so, using the Earth's orbit as a standard unit for comparison. In the modern era, we know the distance of 1 AU (look it up!) in meters, so we could convert 4 light-years into meters and get that parallax angle to α-Centauri. But let's dig deeper into conversion factors.
The Sun is also 8 light-minutes away (8.32 to be more exact — but keep it to one digit precision for now). That is, it takes 8 minutes for light to get here from the Sun. So the parallax angle to α-Centauri should be 8 lm/4 ly. All we need to do now to figure this out is convert light-minutes to light-years. How many minutes in a year? Why not ask the cast of Rent?
θ = 8 lm/4 ly × 1 year/525,600 min = 4 × 10-6 radians
WOW! That parallax angle for α-Centauri is a very small number, even for radians. So how many degrees is that? Here is where the degrees system gets very odd. We can finely divide degrees in decimals, or we also think of degrees as being made up of arcminutes ("minutes of arc") and each arcminute is divisible into arcseconds. But rather than base 10 as the French Revolution would have us do (i.e., the metric system), it's base 60: there are 60 arcminutes in a degree and 60 arcseconds in an arcminute. This makes drawing an arcsecond nearly impossible since it's a ridiculously small angle: 1/3600th of a degree!
θ = 4 × 10-6 radians × 57.3o/radian × 3600"/1o = 0.8"
So the parallax angle of even the closest star is less than one arcsecond (which explains why Tycho Brahe could never measure the parallax of any stars without a telescope!).
But let's say you're an Astronomer in the late 19th century and your equipment is good enough to measure arcseconds. However, you don't know the distance to the Sun (8.32 lm or 1.5 × 1011m, etc.). Then you might just decide to keep everything relative to 1 AU. If you measured your angle in radians, your distance would also be in AU since you would need your units to cancel in such a ratio:
p(radians) = 1 AU / D(AU)Wouldn't this be sensible? However, the formula in your book looks (almost) like this:
D (pc) = 1 AU / p(")or, rearranged:
p(") = 1 AU / D (pc)
That is, the relative distance to something which has a parallax angle measured in arcseconds (") is reported in "parallax arcseconds" or parsecs by taking 1 (AU) and dividing by the angle in arcseonds. Clearly this is a bizarre twisted set of units. But it does mean that even if we don't know any of these sizes or distances in meters, we can develop a conversion factor for AU and parsecs. Multiply both sides by the conversion factors for radians and arcseconds:
p(radians) × 57.3(o/radian) × 3600("/1o) = 1 AU / D(AU) × 57.3 × 3600
Is this star close?
Fun with Spectroscopy!
Make your own spectrometer!
Spectroscopy! Here's a "game" to look at emission (or absorption) lines.
The Size of Things
Recognizing we're not special has been a difficult sell throughout history. First we find out the Earth isn't the center of the Solar System (1500s), then we find out the Sun isn't the center of the galaxy (1800s), and finally we discover there are other galaxies so even the Milky Way isn't special (gasp!). This it the progression from the Copernican Principle to the Cosmological Principle. We're not actually there yet...but close.
For an overview of the Curtis-Shapely Great Debate, click on this link. The fourth link is Dr. Virginia Trimble's article.