THE UNIVERSE IS DOING WHAT?

Hiya - Sean Lally, physics guy here. Today on "How do we know that?" we're talking about the universe at large. And I do mean 'large.'

We've just seen how speeds can be determined based on so-called "Doppler Shifts." Let's revisit what that is exactly. The Doppler Shift (z) is defined by the difference between the expected and observed wavelengths, divided by the expected wavelength. Better yet:

z = (λobs - λexp) / λexp

We can also represent z by this expression:

z = v / c

(This is a non-relativistic expression - that may not mean much to you now. Suffice it to say that galactic motion, while fast, does not rise to the level of being relativistic - which means super fast, near the speed of light.)

Also FYI - a negative z indicates a 'blue shift', while a positive z indicates a 'red shift.' Does that make sense to you? Think about it for a moment - a negative z means that observed wavelength is lower than the expected one. You could also say that the observed frequency is higher than the expected one - this is classic blue shift. Got it?

So, let's imagine that you have some data - a bunch of z-values for galaxies. And let's also imagine that they were nearly all greater than zero*. Quick, what could this mean?

* It's worth noting that a few galaxies have z's that are less than zero (blue shifted). These are in the local neighborhood of galaxies, the local group. We needn't worry about these now - they are, by far, in the minority of galaxies. Also - strictly speaking, we shouldn't exactly refer to the galaxy being red or blue-shifted. Typically, one looks at spectral lines from the galaxy. And importantly, we should mention that this pioneering work was first done by Vesto Slipher in the early 1900s.

A-ha - if nearly all galaxies are moving away from us, it strongly suggests that the universe is expanding.

Here's a tricky question - we know what things look like from OUR perspective. What would it look like if you were observing from somewhere else in the universe? For example, would you expect to see the Milky Way galaxy red-shifted if you were on a planet orbiting a distant star outside this galaxy?

Hmmmm.... so, everywhere you observe in the universe, it seems to be expanding away from everywhere else. What in the world can this mean? Is there a "priveledged position" in the universe? Or better yet - is there a "center" to the universe? Is it expanding from "somewhere?"

This idea is so profound and important that we call it the Cosmological Principle (or, occasionally, the Copernican Principle):

On large scales (say, 300 light-years or so), there is nothing special about any location in the universe.

Or if you prefer, the universe at large is homogenous and isotropic.

What does that mean, exactly? In short, the universe has no center or preferred direction at all (isotropic). Furthermore, it looks pretty much the same no matter where you look (homogeneous).

Here's one other very peculiar thing to consider. In 1929, Edwin Hubble had some data at his fingertips - both redshifts of galaxies (from which could be calculated speeds) and distances to these galaxies. What would you do with this data, if you had it? What would a scientifically curious person do with this kind of data? One could graph it, yes? But how, and what would any relationship mean?

Think about this - you have numerical data for redshifts of galaxies and their distances from us. How would you create a graph to show any relationship between them? Think about and discuss it with your friend.

Well, let’s cut to the chase - Hubble graphed redshift (basically, the velocity of the galaxy) versus its distance from us. What do you think he found?

Here's the basic graph he arrived at:

(See image above.)

What in the world can this mean?

Note also that this is a linear graph. Linear graphs have slopes (or steepness) that usually have some significance - what is the significance of this slope?

A widely used value for this slope, also called the Hubble Constant, is:

Ho = 70 +/- 7 km/s/Mpc

That's 70 kilometers per second per megaparsec. In other words - the further away the galaxy, the faster it is receding. Oh my.....

That's all for now, my friends - see ya soon, see ya on the Moon!

All text and images copyright 2009 Sean Lally, except where noted.