Tuesday, October 11, 2011

How We Know: The Expansion of the Universe

The universe is a big place.  The scale of it is nearly impossible to conceive except through series of pictures, or a video showing a zoom-out from a park in Chicago to the furthest reaches of what we can see, while increasing the zoom speed as we go.  Even with the rapid acceleration*, the journey takes some time, and by the time you get to the end, it's easy to forget the vastness of space in between landmarks.

The best visuals are probably these two:

Powers of Ten video

The Scale of the Universe flash tool

Now, the universe is so big that it can take several billion years (the record is about 13 billion) for light from one end of it to reach us, because light has a fixed speed**.  So far this is all just background information.  The real question boils down to how we measure the distance and speed of these objects that are billions of light years away from us.  There are a few steps to get there.

1.  Measure the Earth's orbit around the Sun.  This can be done by observing the planet Venus, measuring its angle at a specific point in orbit, comparing that to the relative sizes of the Earth's orbit, Venus's orbit, and the apparent size of the Sun, and then doing some fancy trigonometry.  Wikipedia's entry on the process (Warning: High Math Content).

2.  Measure the distances to the nearest (often brightest) stars.  You now know how far away the Earth is when it's on opposite side of the Sun from any starting point.  On a January night, make an incredibly detailed map of everything you can see in the sky.  Every star, even the ones you can barely make out.  Use a telescope to get the best detail.  Now do the same thing exactly six months later, in July.  You'll notice some of your stars have moved around a bit.  Your old map isn't inaccurate, since you'll see most of the stars are in the same place.  These are your "background" stars, and they're too far away to measure right now.  But as for the ones that moved, you can measure how much they moved, and calculate how far away they are, because they didn't really move, but you did, all the way around the Sun.  So draw a triangle with Earth at point A, Earth at point B, and the star, fill in the distances and angles you know, and use trigonometry again.  You should be able to accurately calculate every star within a thousand light years.  Wikipedia again.  (Less math this time.)

3.  Measure the distance of every Cepheid you can find.  Some of the stars you mapped out already were Cepheids, or variable stars.  They are named for one of the first discovered, a star called Delta Cepheid, and they are called variable because they dim and brighten with regularity.  They are special because they all do this at the exact same level of luminosity, or brightness.  So now that you know distance to a few local Cepheids, the ones within a thousand light years, you can now look at distant galaxies, find their Cepheids, measure the brightness, and because brightness diminishes over distance, you can accurately calculate the distance to those galaxies!  Kudos to Scientific American for this article.

4.  Now measure how fast those galaxies going, and in what direction.  So far, everything we've done is by direct observation by naked eye or telescope and math.  This next step requires a technique called spectrometry.  It's easier than it sounds.  First, take a prism and hold it out to sunlight coming in through a window.  Somewhere on the floor, wall, or ceiling you'll see a rainbow.  This is the spectrum of sunlight.  A fancy telescope with a prism attached can see it much more accurately and even detect faint gaps in the spectrum.  These gaps point to the chemical components of the light.  Light emitted by a neon sign and burning carbon-rich campfire will give off very different spectrums through a prism, and you tell the difference from the different patterns of gaps.  Most stars are full of hydrogen, which has its own special pattern.  This pattern can be recognized from sunlight and appears on very specific points on the spectrum.  So any star we look at should have the same pattern in the same places, because they all have hydrogen.  But they don't.  The pattern is almost always there, but sometimes we see it moved toward the red end of the spectrum, and sometimes toward the violet end.  This happens because of the Doppler Effect, and it shows that some stars are moving toward us (blueshift), and some are moving away from us (redshift).  But when we look at galaxies, the vast majority are redshifted, which means they are moving away.

5.  Time for logic.  Imagine you have a deflated balloon, and you draw a bunch of little dots on it with a marker.  You now have a polka-dotted balloon.  One of those dots is our galaxy, and the rest are all the other galaxies we can see.  If you blow up the balloon, every dot gets farther from every other dot as the balloon stretches.  This is the best model to explain our observations.  So if most of the galaxies in the universe are moving away from us, it logically follows they're all moving away from each other as well.

And so the universe must be expanding.

Some time in the future I'll cover the acceleration of this expansion, but I think we've had enough for today.

*Imagine you want to drive across the U.S., New York City to Los Angeles, accelerating by a factor of ten every minute.  You'd go 1 mph the first minute, your house is still in view, and other drivers think you're parked.  You'd go 10 mph the second minute, you'd still be in your neighborhood, and other drivers are honking at you and pulling around, maybe flipping you off.  The third minute you're going 100 mph, breaking most speed limits in the country.  You travel about a mile and a half in that time.  In the fourth minute, you've broken the sound barrier, going 1,000 mph.  In that minute, you travel 16.6 miles, and you're out of town.  In the fifth minute, 10,000 mph:  you've shattered the land speed record several times over and are even traveling faster than any man-made vehicle on Earth.  You get 166 miles closer to your destination.  You may literally be on fire at this point.  In the sixth minute, at 100,000 mph, you're moving faster than orbiting shuttles and satellites, and make good time, traveling 1,666 miles, you're halfway there!  Within the seventh minute, as you approach 1,000,000 mph, that's a million miles per hour, you reach your destination, unless you disintegrated from the wind resistance.  You were going several times faster than anything humans have ever built, relative to the Earth or Sun.  It's a good thing you made it when you did, as in another 3 minutes you'd be moving faster than light, and Einstein wouldn't be too happy.

**A "How We Know" for some other day.

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