After a year of studying the laws of the universe
together, a year of studying and calculating,
learning about motion and fluids, thermodynamics,
electricity and magnetism, light and sound,
We find ourselves here:
Space, the final frontier.
We’re all voyagers on a mission to understand
the universe, using the power of physics.
And even though we’ve been doing it for
centuries, there’s so much we have to learn
about the cosmos!
Some of the most exciting research in physics today
is being done by astrophysicists and cosmologists.
Astrophysicists study the physics of celestial
bodies, such as planets, stars, and galaxies.
Their research takes us inside phenomena like
black holes and supernovae.
But we can use physics to try to answer even
bigger questions about the universe.
Cosmologists study the universe overall and
ask questions about the origin of everything,
as well as its future.
It’s their job, and yours, to continue looking
into the night sky, searching for answers using the
tools and knowledge that physics can provide.
[Theme Music]
Before we can talk about something as big
as the universe, we need to be able to describe
just how big it is.
When talking about things on Earth, we typically
use measurements in the range of nanometers
to kilometers.
But when we’re in space, we need something
a whole lot bigger.
For instance, I could say that the nearest
star to earth, besides the sun, is 4x10^13
kilometers from us.
But that’s a mouthful for something that’s
basically right next door, in cosmic terms.
So it’s easier to say that this star, Proxima
Centauri, is 4.2 light-years from Earth.
A light-year is a unit of length, with one
light-year equaling the distance that light would
travel in a vacuum in one year.
If you take the speed of light, about 300
million meters per second, and multiply it
by how many seconds are in a year,
you find that one light-year is approximately
10^16 meters, or 10 million, billion meters.
To give you a sense of scale, it takes light just
over 8 minutes to travel from the sun to the Earth,
and the Milky Way is roughly a hundred
thousand light-years in diameter.
Sometimes we also use a unit called a parsec,
which is equal to 3.26 light years.
Now, when we say that Proxima Centauri is
4.2 light-years away,
this also means that when we look at the
star through a telescope, we’re seeing what
Proxima Centauri looked like 4.2 years ago.
It takes light that long to get here from Proxima
Centauri, so we’ll never know what that star –
or any star or other distant
object – looks like at this exact moment.
This means that, as we observe celestial
objects far away, we’re looking into the past,
seeing what stars and galaxies looked like millions,
if not billions, of years ago.
While we’re observing these stars, we can use a
spectrometer, the device that separates wavelengths,
to reveal the star’s absorption
spectrum and its elemental composition.
But when we study very distant bodies, we find that their absorption spectrum is slightly different from what we’d expect, given our knowledge of typical star compositions.
Remember the Doppler effect?
How the pitch of an ambulance siren becomes higher
as it approaches you and lower as it moves away?
The same effect happens with light!
If an object is moving away from you, the
speed of light doesn’t change, but the peaks
of the electromagnetic wave that it emits
move farther apart.
This effect – which occurs with light emitted
by an object moving away from Earth – is
called redshift, because the longer the
wavelengths get, the closer they are to the
red part of the visible spectrum.
Once astronomers recognized and could
measure redshift, they found that the spectra from
nearly every distant galaxy was redshifted,
meaning that every galaxy
was moving away from us.
And if that wasn’t strange enough, astronomers such
as Edwin Hubble noted that the amount of redshift is
proportional to the distance from Earth.
So the farthest galaxies are moving away even
faster than the close ones!
Georges Lemaître, a Belgian physicist,
proposed this relationship as Hubble’s Law.
It expresses the velocity at which a galaxy
is speeding away from Earth, in terms of its
distance from us.
The equation uses a proportionality
constant called the Hubble parameter,
which says that for every million light-years of
distance from us, a galaxy is moving away at an
additional 21 kilometers per second.
Now, the fact is that nearby galaxies might
be going away from us or toward us, based on
how they’re moving within their local cluster.
But the overall tendency for distant galaxies
to recede from us is much more common, so
Hubble’s Law holds true in most cases.
And this trend of distant galaxies moving
away from us, and from one another, is called
cosmological redshift.
By the way, this expansion looks the same
whether you’re on Earth or not.
No matter where you are, all distant galaxies
appear to be moving away from you.
So this leads physicists to believe that at
some point in time, all the stars and galaxies
were closer to one another – a lot closer.
In the 1940s and 50s, Russian-American
physicist George Gamow, developed a theory of the
early universe that explained,
among other things, why so many light elements,
like hydrogen and helium, were observed throughout
the cosmos.
He suggested that the universe began in a
state of highly compressed hot plasma – a sort
of hot soup of elementary particles.
His theory became known, somewhat dismissively
by his colleagues, as the Big Bang Theory.
And this same theory ultimately predicted that
there should be radiation left over from the initial,
rapid expansion of that compressed plasma.
This is because hot plasma, like the plasma
in the flame of a candle, is not transparent.
But ordinary gas – like the air around a
candle’s flame – is transparent, and it lets
light travel freely through it.
So it would make sense that the early, hot
universe was originally opaque, until it cooled
down to the point where it became transparent.
Once that happened, the thinking goes, light
from the Big Bang was able to travel freely.
But its wavelength kept stretching out, redshifting
until it could only be detected as microwave radiation.
Gamow's theory didn’t gain much acceptance,
and it was largely forgotten.
Until, in 1964, American astronomers Arno Penzias
and Robert Wilson pointed a radio antenna into space,
and they discovered cosmic microwave background
radiation.
They basically discovered the radiation from
the Big Bang, by accident.
They found that a low-energy microwave radiation
persisted at all times, day and night,
and they concluded that the source of the
radiation was the universe itself.
This cosmic background radiation provides
support for the Big Bang Theory,
and it tells us a lot about the conditions of
the early universe.
Thanks to these insights, along with the observed
expansion of the universe and other evidence,
we have learned that the universe began in
a hot dense state, then cooled, and produced
galaxies and clusters that we see today.
However, many mysteries remain.
For instance, if the universe started with
such high density and temperature, wouldn’t
gravity make its expansion slow down?
The fact is, the rate of expansion would
slow down only if the universe was filled with
nothing but matter and radiation.
But that’s not the case!
Space is filled with a constant – or at least, slowly
varying – form of energy known as dark energy.
And because of the pervasive presence of this
energy, according to general relativity, gravity is
actually causing space to expand, and accelerate!
This isn’t just theoretical.
Recent evidence suggests that the universe
actually IS accelerating in its expansion,
showing no signs of slowing down.
But beyond the fact that it exists, there’s
not much that we know about dark energy.
Another one of the universe’s great mysteries
is the existence of mass that we can’t see,
but we know that it, too, exists.
When we study a galaxy’s rotation, we can
estimate how much mass is in it by measuring
its radius and rotational velocity.
But when we actually calculate that mass,
the result is far greater than what’s observable
as stars and gas.
The conclusion is that there’s an immense
amount of mass in the universe known as dark
matter, which doesn’t reflect or emit any light.
By current estimates, dark matter makes up
almost 85 percent of all the matter in the universe.
This means that all visible matter, including
stars and planets, make up just a small percentage
of all energy in the universe,
while the rest is mysterious dark energy
and dark matter.
Research in these fields is ongoing and
new evidence is found every year, refining our
understanding of the universe.
In the past year here on Crash Course, and
for thousands of years, we have used physics to
answer some of life’s most important questions.
Whether it’s a ball flying through the air
or the origin of the universe,
we can use our knowledge from Newton’s Laws
to special relativity in order to move closer to
the truth.
And there’s still so much to discover, both
in the stars and here on earth.
Some of the most groundbreaking research is
happening on the smallest scales,
as physicists seek to understand the building blocks
of our universe and the nature of matter itself.
It’s not an easy task, and it’s why we
need scientists and enthusiastic supporters
such as yourself to go out,
be curious, ask questions, and to find
answers through scientific methods.
Today we learned about light-years and how
looking in the distance is also looking into the past.
We discussed redshift and used Hubble’s
Law to calculate how much certain parts of
the universe are expanding away from us.
Finally, we introduced the Big Bang, cosmic
background radiation, and the mysteries of
dark energy and dark matter. Bye!
Crash Course Physics is produced in association
with PBS Digital Studios.
You can head over to their channel to check
out a playlist of their latest amazing shows like:
The Art Assignment, Gross Science, and Deep
Look.
This episode of Crash Course was filmed in
the Doctor Cheryl C. Kinney Crash Course Studio
with the help of these amazing people and
our equally amazing graphics team, is Thought Cafe.
