Astronomy

22 ASTRONOMY • DECEMBER 2015

IT’S ABSOLUTELY AMAZING

to know that shortly after

the Big Bang, the universe

was a relatively small,

nearly infinitely dense

place. It boggles the mind.

But that was 13.8 billion years ago. The
expanding universe means the entirety of
what we know is now incredibly large —
and is getting more immense every day.
This is one area that two generations
of science-fiction movies have seriously
distorted in the minds of the public. The
general feeling that technology is pretty
good and will know almost no bounds,
and that we can almost certainly one day
travel between star systems, is pretty much
taken on faith.
But what the sci-fi movies have failed to
communicate, among other things, is that
the universe is an immensely large place.
Even distances between the nearest objects
are staggering, and the distances across the
Milky Way Galaxy and certainly between
galaxies in the universe are astonishingly
huge to living beings stuck on a planet. A
model of the Milky Way wherein the Sun
is a grain of sand brings this home. On this
scale, stars — sand grains — are 4 miles
(6 kilometers) apart in the Milky Way’s
disk and the disk is about 40,000 miles
(60,000km) across. Now who wants to go
traveling from grain to grain?
The concept of the size of the universe
has taken a huge stride forward in just the
last few years. There was a time not too
long ago when astronomers did not know
even the approximate size of the cosmos
with any degree of accuracy. We still don’t
know with high precision.

Incredible expansion
The Big Bang theory tells us that once the
universe was very small. We know the fast-
est that radiation or any information can
travel is the speed of light, 186,000 miles
per second (300,000 km/s). We’re confident
that the universe is 13.8 billion years old.
We also know that a light-year is equal to
approximately 6 trillion miles (9.4 trillion
km). In nearly 14 billion years, on first
blush, we might expect radiation to expand
radially outward to something like 30 bil-
lion light-years across.

But remember that the Big Bang was not

like an explosion that went off in a room.

Following the Big Bang, space-time itself

expanded radially outward at all points,

meaning all of space expanded too, not just

the stuff within it. (The term space-time

refers to the mathematical model that com-

bines space and time into a single, interwo-

ven medium.)

As the expansion of the universe began,

just 1 centimeter of “empty space” inter-

stitially became 2 centimeters over time,

and so on. So the best ideas about the size

of the universe allowing for its expansion

over time point to a radius of slightly more

than 46 billion light-years and therefore a

diameter for the universe of approximately

93 billion light-years.

But there’s a major proviso to this result.

That diameter refers to the visible universe

we can see from Earth. Inf lation theory, if

correct — and it has widespread support

among cosmologists — suggests the portion

of the universe we can see is by no means

the entire cosmos. Some cosmologists pro-

pose that the universe is infinite. But let’s

work with what we really have and say the

cosmos, at least the part that we can observe,

is about 93 billion light-years across.

A thorough understanding of our

neighborhood, our solar system, our area

of the Milky Way, our galaxy, and so on is

David J. Eicher is editor of Astronomy maga-
zine. He has marveled at the cosmic distance
scale since the mid-1970s.

M74: NASA/ESA/THE HUBBLE HERITAGE TEAM (STS

CI/AURA)-ESA/HUBBLE COLLABORATION