Hey Crazies.
I don’t think we’ve done enough cosmology videos yet.
There’s still one big question we need to answer.
How do we know the universe is expanding?
Well, we’ve got a lot of data to back up that claim.
The Planck Telescope really came through for us
but the most famous is probably the red-shift of light.
What’s a red-shift?
It’s a shift in wavelength toward the... What’s a wavelength?
Ugh.
Alright, the things we measure about waves are kind of unusual,
so let’s start with a quick review.
Waves have two basic properties: Wavelength, which is the length of a complete
cycle measured with a ruler
and Frequency, which is how often peaks pass one spot.
These two properties combine to give you a speed, but how quickly a wave moves
really only depends on the stuff that’s doing the waving.
For light, that’s the electromagnetic field.
I’m doing this because, you know, it’s everywhere.
Anyway, since the fundamental properties of the electromagnetic field never change,
neither does the speed of light.
That means we can draw something like this: The visible spectrum.
All the colors that can be matched with a single wavelength of light.
But since the speed of light is constant, we can also match them with frequencies of
light.
Under some conditions, the color of light can actually change.
If it shifts toward the blue end of the spectrum, we say it blue-shifted.
If it shifts toward the red end, we say it red-shifted.
It’s that simple.
We keep with this same concept of shifting when we write it mathematically.
Since light is emitted with a certain wavelength, we expect it to be received or observed with
the same wavelength.
In other words, we expect this ratio to be equal to one.
But sometimes it's not.
If not, we say a “red-shift” has been added.
The “Doppler Effect” is one way that light can change color,
which is something you’re familiar with, but with sound.
Listen to the sound of the engine.
As the car approaches, the frequency is higher than it should be.
But once it passes and is moving away, the frequency is lower than it should be.
The same kind of thing is happening with light from the car, but it’s harder to notice.
If you relate the wavelengths to the car’s speed, you can see the problem.
Since light is so fast, this ratio is roughly one for any normal car speed.
Our eyes aren’t that sensitive.
These two squares are different colors, but how many of you can tell?
I certainly can’t.
After playing around with a few wavelength to RGB converters,
I found that I can tell the difference between 1 nano-meter.
Just barely, but I can do it.
To see that kind of change with the car, it would need to be going about a million miles
per hour.
Luckily we have better tools than our eyes for detecting this when it comes to stars.
Cosmological red-shift is not caused by the Doppler Effect.
I know, there’s a method to my madness, OK?
As annoying as he is, nerd clone is usually right, and this is no exception.
I sure am!
Besides the Doppler effect, there are 2 other sources of red-shift.
gravity and the expansion of space.
Light red-shifts as it escapes from a source of gravity.
This is more apparent when the gravity is strong
like the light emitted by the accretion disk of a black hole.
The expansion of space can also cause a red-shift.
But that expansion is slow, so the light needs to be traveling a long time for us to notice.
With the light coming from stuff close by, you get it mixed up with the other two sources of red-shift.
Like we said in a previous video:
Yes, super-clusters.
We don't see the expansion on scales any smaller than that because other forces like gravity
are strong enough to things together in spite of the expansion.
My point is that cosmological red-shift is not the result of a galaxy’s speed away
from us.
It is NOT the Doppler Effect!
You’ve been lied to.
It’s the result of the space stretching along the trip.
When light travels between galaxy super-clusters, that light gets stretched
as the space it’s traveling through stretches.
The more time that light travels, the more expansion it experiences.
Remember, that scale factor from the flower metric?
That thing that tells you how big those space infinitesimals have gotten?
We can actually write the red-shift in terms of that.
It compares the scale factor back then to what it is now.
The data shows that, the more time we look, the bigger the red-shift
meaning there’s more expansion between us.
The cosmic microwave background is the oldest light in the universe.
It was emitted so long ago, that it isn’t even visible light anymore.
It’s microwave light.
That’s a red-shift of over 1000!!!!
We can even see how fast the scale factor is changing, which shows us the universe is
expanding faster now than it was in the past.
It’s accelerating!
This adds another thing to the list of info we get from light.
Unfortunately, to know there’s even been a red-shift,
we have to know what color the light is supposed to be.
But that’s a topic for another video.
Don’t worry A Thrasher, I started a list for every time I've said that.
Anyway, crazies, if you’ve got any questions about red-shift, please ask in the comments.
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And until next time, remember, it’s OK to be a little crazy.