Let's take a quick tour of the universe and as we go along we'll take advantage
of scientific notation to get a better idea of the powers of ten or the orders
of magnitude that are involved. Beginning with Earth, our home, it has a diameter of
about twelve thousand 742 kilometers and if we want to write that out in
scientific notation we would just say that's one point two seven times ten to
the four kilometers. Likewise the moon has a diameter of just under 3,500
kilometers and again, for the sake of demonstration, we can write that out in
scientific notation as three point four seven times ten to the three. Let's zoom
out even farther and instead of ten to the four kilometers, we're going to make
a rather large jump. This is the actual distance between the Earth and the Moon
to scale, and now we've jumped a full order of magnitude. So we've gone from
ten to the four kilometers to ten to the fifth kilometers. And as you can see the
Earth and Moon are pretty far apart from one another there's enough room to
easily fit all of the major planets of the solar system. So let's zoom out even
farther the distance from the earths of the Sun is about 150 million kilometers.
So that takes us to one point five times ten to the eighth kilometers. We haven't
gone one order of magnitude, we've gone three orders of magnitude. This distance
is known as the Astronomical Unit and it serves as a kind of measurement standard
within our solar system. As a matter of fact, we'll step up one more order of
magnitude, that takes us out to the orbit of Neptune which is about 4.5 million
kilometers from the Sun. And notice we've gone from 1 AU to 30 AU so we've
actually increased an order of magnitude of Astronomical Units as well. Now the
farthest possible extent of any object in the solar system orbiting the Sun
lies at the outer edges of something called the Oort cloud; it's really just a
cloud of primordial or dirty snowballs, in other words comets, and nobody knows
for sure just how far these comets can be from
Sun and still remain in orbit, but some estimates put the radius of the Oort
cloud at anywhere from 50,000 to 200,000 Astronomical Units. Remember
we're talking about the radius we're talking about the distance from the Sun
to the outer edge of this sphere. It's also worth noting that the distances
between each of these snowballs is extremely vast in fact they're about as
far apart from one another as Earth is from Saturn, so there's no way you would
actually see these comets from this vantage point; we're turning up the
brightness knob to let you see these comets. Yeah, it's pretty far away
anywhere from about seven and a half times ten to the twelfth that's seven
point five trillion kilometers to as far as 30 trillion kilometers or three times
ten to the thirteenth; it's pretty far. As a matter of fact, we can start to even
think about the amount of time it would take a beam of light to travel from the
Sun out to one of these comets. It turns out that it takes light the better part
of a year maybe even a little over three years to reach the outer edge of the
Oort cloud. So we're talking light-years a Light-year is on the order of about
nine point five trillion kilometers. Now the Sun is just one of a local group of
stars that form our interstellar neighborhood. Many of the stars here are
stars you may already be familiar with such as Sirius, Wolf 359, Tau Ceti, and so
on. The closest star to the Sun is Alpha Centauri. Now it's at a distance of about
4.3 light years, or putting that into perspective, at least 40 trillion
kilometers distant. Now I'd like to introduce a new term of distance that we
often use in astronomy to indicate the distances between stars - this is called
the "parsec". Later on we'll learn how we define a parsec, but for now it's enough
to know that a parsec is equal to about three and a quarter light years.
The interstellar neighborhood, and indeed every star that we see, is really just
concentrated into a kind of a backwater region of the galaxy called the Orion
Spur. It's not even a full spiral arm, we're just kind of living off the
"Galactic Main Line" somewhere. This is of course the Milky Way galaxy and it's
home to about 400 billion suns and has a diameter of about well about a hundred
thousand light-years across or so this is a massive jump from the
interstellar neighborhood, three orders of magnitude, taking us all the way up to
nine and a half times 10 to the 17 kilometers so about 32.6 kiloparsecs
in diameter. Now the Milky Way is one of the two
largest galaxies of a local group of galaxies called the Local Group. It's not
the most exciting name, but there it is. Now the perhaps the largest galaxy in
the local group is the Andromeda galaxy. It's at a distance of about two times 10
to the 19 kilometers so now this is two more orders of magnitude. In terms of
light-years, it works out to about two and a half million light-years, or 2.5
times ten to the six, so we're at almost 800 kilo parsecs from Earth. The local
group is just one tiny little clump of galaxies belonging to a much larger
complex called the Virgo Supercluster, and the Virgo Supercluster spans a
diameter of about 10 to the 21st kilometers so a few more orders of
magnitude greater. In fact, when you think about this in terms of parsecs, it's
easier to just convert now to mega parsecs - 33 million parsecs across.
The Virgo Supercluster is one of several nearby super clusters, and it turns out
that all of these super clusters are a great complex called Laniakea. Laniakea
is Hawaiian for "immeasurable heaven", and this gravitational relationship was
discovered fairly recently. And it's about five times larger than the Virgo
supercluster so we thought the Virgo supercluster was once upon a time the
biggest structure that we belong to, but no, we in fact belong to a far larger
structure, Laniakea.
But everything that we see, all galaxies that we see in the
entire sky, they are as distant as ten billion light years from Earth or about
10 to the 22nd kilometers. Now astronomers estimate that there could be as many as
100 billion galaxies. And if you think about that for a second, each galaxy has
on the order of about a hundred billion or ten to the ten stars so 10 billion
stars times 100 billion galaxies gives us 10 to the 21st stars total; there are
more stars in the universe than there are grains of sand on all the beaches on Earth.
Yet every one of the galaxies lay before something called the "Cosmic
Microwave Background." It's the leftover radiation from the Big Bang itself and
it represents our cosmic horizon at a distance of 10 to the 23rd kilometers
and we don't know for sure what lies beyond this region but it's extremely
likely that the universe just keeps on going and going. In other words, the
observable universe stops at the Cosmic Microwave Background but there's almost
assuredly an infinite universe beyond. And speaking of infinities, it no longer
makes any sense to invoke powers of ten at this point, it's infinite. If the
universe is truly infinite beyond what we can see it's entirely possible that
there are pockets within the universe that Big Bang themselves into existence
all the time and perhaps some of them have their own laws of physics or
they're just slightly different in other words maybe what we think of as our
universe is really just a vast hyperdimensional multiverse. We don't
know this for sure but it actually makes the most sense when we start to think
about really difficult tricky cosmological problems. And so, if we want
to consider our cosmic address we would have to write a fairly lengthy address.
We can start with our basics, we know that we live somewhere on Earth which is
of course part of the solar system which is part of the Orion spur of the Milky
Way galaxy which resides in the local group of galaxies which is part of the
Virgo supercluster which is part of Laniakea and that is found somewhere in
the observable universe which is somewhere inside of the infinite
universe which perhaps may be part of some hyperdimensional multiverse.
Space is big.