Last week, we began talking about understanding the size of the Universe, and we continued this week with some information on distances and motion in the Universe. This brings us to my favorite application, which leads to the Hubble expansion:
Redshift. You see, whenever an atom or molecule emits light, it gives off that light at a very few particular wavelengths. For instance, if you have hydrogen, you’ll always get light at wavelengths of 656 nanometers (red), 486 nm (cyan), 434 nm (indigo), 410 nm (violet), and 397 nm (on the border of violet/ultraviolet):
Now there are three things — and only three things (unless you really want to get technical) — that can happen to this light to change the wavelengths that you see. Let’s go over what they are.
1. Gravitational Redshift. If you’re deep in a gravitational field (like close to a black hole), you have to use up energy to climb out of it. For light of all types, energy and wavelength are very closely related to each other. Smaller wavelength = higher energy and larger wavelength = lower energy, so if you need to climb out of a strong gravitational field, you lose energy, and therefore your light gets shifted towards the red. This is what we call redshift, where something happens to make the wavelength of your light longer and lower in energy. But gravitational redshift is rarely significant; two other effects are far more important.
2. Redshift due to motion. If an object that emits light moves away from you, the light from it gets redshifted. This is the same exact effect — the doppler shift — that causes police sirens to sound lower pitched when they move away from you. One thing that’s neat? If a light-emitting object moves towards you, the light gets blue-shifted, and becomes more energetic! (We see this happening for the Andromeda galaxy, one of the only ones in the Universe that moves towards us.) And although this is incredibly useful, this is not what’s happening to light in the Universe. Remember, I told you that these distant galaxies aren’t moving, the space between them is just expanding. Well, guess what?
3. Expanding space causes a redshift! (And thanks to av8n.com for the image!) You see, as space expands (above), the wavelengths of the light in it also expand, as you can see below.
And this last effect is so important for the expanding Universe. Why? Well, if we measure the light from many, many distant objects and determine their distances, we can — simply based on the objects’ redshifts — learn the entire history of how the Universe expanded. The redshift isn’t hard to measure, either:
It is from literally millions and millions of these individual measurements that we’ve been able to determine the entire history of how the Universe expanded. That, among other things, is how we discovered dark energy and the accelerating Universe! Pretty remarkable stuff, and yet, not intuitive at all.
So what should you take away from this? That as light travels through space and space expands, it causes the wavelength of that very light to expand, too. And that’s how we learn about the history of cosmic expansion in our Universe. Again, it’s expansion that’s causing this redshift, and not motion. Hope this helps shed some light on some of the most confusing stuff out there!