Professor Dave here, I wanna tell you about Edwin Hubble.
Earlier in the series, we learned about the incredible history of astronomy, from ancient
times all the way up until the 20th century.
A tremendous amount of knowledge was gathered about the solar system in that time, and as
a result, the center of our universe shifted from the Earth to the Sun, with the planets
moving around in elliptical orbits.
It was then realized that all the stars in the night sky are suns just like ours, potentially
with planets of their own, and this realization caused our perceived universe to grow tremendously
in the blink of an eye, to reveal the true scope of the Milky Way Galaxy.
Then, in the early 20th century, something even more incredible happened.
Telescopes became powerful enough for us to see that many objects we had thought were
nebulae or other such objects within our galaxy, are actually not those things at all.
These are entire galaxies unto themselves, just like our own Milky Way galaxy.
All the billions of stars we see in the night sky turn out to belong to just one little
island of stars out of hundreds of billions of similar and very distant little islands,
and what we had thought was the entire universe, was revealed to be just a speck of dust in
a far grander structure.
We didn’t figure this out until the 20th century, because these galaxies are just so
far away.
We already learned about the local group that the Milky Way is a part of, and even these
closest galaxies are millions of light years away, a distance so great that the stars in
our own galaxy suddenly seem a stone’s throw away by comparison.
It took huge, powerful modern telescopes to get images with good enough resolution to
see that these were not simple structures, but rather swirling masses of many stars,
and we began to categorize them as spiral galaxies, elliptical galaxies, and all the
other varieties we learned about earlier in the series.
One person who made considerable contributions to the task of categorizing galaxies was Edwin
Hubble, and he developed the categories and subcategories we use today.
But that’s not all that Hubble did.
He gathered data that would later allow for the interpretation that the galaxies are all
moving away from one another.
This was determined by looking at emission spectra for many galaxies.
To understand how this is possible, we must learn about a phenomenon called the Doppler
shift, or the Doppler effect.
Say we have some source of waves, which could be acoustic waves, or sound, just as well
as electromagnetic waves, or light.
Say that these waves are emitted with a constant frequency.
Now let’s say that the source of these waves is in motion with respect to our frame of
reference, like a car.
When the car is coming towards us, the sound waves will be of a shorter wavelength than
if the car was stationary.
This is because the car is moving closer to us in between the emission of each wave.
If the wavelength is shorter, the frequency is greater, and we will hear the sound as
being pitched up.
Once the car passes and is moving away, the wavelength will increase, because the car
will travel some distance away from us in between the emission of each wave.
A longer wavelength means a lower frequency, and we will hear the sound as being pitched down.
Higher pitch coming towards us, lower pitch moving away from us, and that’s why a vehicle
passing by will sound like this.
Precisely the same principle works with electromagnetic waves, or light.
A source of visible light that is moving towards us will have its frequency increase, and the
wavelength will therefore decrease, towards the blue end of the visible spectrum.
We call this a blue shift.
If it is moving away from us, the frequency will decrease, and the wavelength will therefore
increase, towards the red end of the visible spectrum.
We call this a red shift.
These shifts are easy to observe on an emission spectrum, because we know where the emission
lines for hydrogen and other elements ought to be.
If an object gives emission lines that are shifted from these values, it’s a very clear
indication of the motion of that object.
When Hubble observed the emission spectra for a variety of galaxies, it became apparent
that almost every single galaxy in the observable universe is redshifted, meaning they are almost
all moving away from us, with the only exceptions being a handful of galaxies nearby that are
gravitationally bound to the Milky Way.
Rather than assuming the Milky Way is an especially repulsive galaxy that the others are fleeing
from, astronomers realized that the universe itself must be expanding.
If we imagine that the dots on this balloon represent galaxies, and the surface of the
balloon represents the spacetime fabric of the universe, as the balloon expands, all
of the dots get spread apart.
All of them are moving away from all the others, and no matter which dot you’re standing
on, all the others will appear to recede from you.
If the universe is expanding, then it must have once all been very close together.
This fact was the first piece of evidence that suggested the birth of the universe from
a single point, at some specific time in the finite past.
But as if this realization wasn’t shocking enough, Hubble also realized that the recession
velocities of the galaxies increased with greater distance.
In other words, the farther away a galaxy is, the faster it is moving away from us.
This relationship is stated in Hubble’s law, a very simple equation relating recession
velocity, measured in kilometers per second, and distance, measured in megaparsecs, which
are equivalent to one million parsecs, or 3.26 million light years.
This equation also utilizes H, or the Hubble constant, equal to about 70 kilometers per
second per megaparsec, with some dispute over its precise value.
We can use this equation to find the approximate distance to any distant galaxy in megaparsecs,
given its recession velocity that we can determine from the red shift in its emission spectrum.
There is much more to discuss regarding Hubble’s law, which we will get to a bit later.
First, we want to make good on our promise from the beginning of this series.
Hubble’s realization that the galaxies are all receding from one another was the first
observation that suggested a cosmological model like the Big Bang.
But this isn’t nearly enough evidence to be conclusive.
So what else did we figure out to corroborate this model?
Let’s move forward and go over the evidence for the Big Bang.