George's Cosmic Treasure Hunt (6 page)

THE USER'S GUIDE TO THE UNIVERSE

A VOYAGE ACROSS THE UNIVERSE

We will now go on a voyage across the Universe.

Before setting out we must understand what we mean by the terms
voyage
and
universe
. The word
universe
literally means “everything that exists.” However, the history of astronomy might be regarded as a sequence of steps, at each of which the Universe has appeared to get bigger. So what we mean by “everything” has changed.

Nowadays most cosmologists accept the Big Bang theory, according to which the Universe started in a state of great compression around fourteen billion years ago. This means that the farthest we can see is the distance that light has traveled since the Big Bang. This defines the size of the
observable
Universe.

So what is meant by a
voyage
? First we must distinguish between
peering
across the Universe and
traveling
across it.
Peering
is what astronomers do and, as we will see, involves looking back in time.
Traveling
is what astronauts do and involves crossing space. This also involves another kind of voyage. For as we travel from the Earth to the edge of the observable Universe, we are essentially retracing the history of human thought about the scale of the Universe. We will now discuss these three journeys.

The Voyage Back Through Time

The information astronomers receive comes from electromagnetic waves that travel at the speed of light (186,000 miles per second). This is very fast, but it is finite and astronomers often measure distance by the
equivalent light travel time. Light takes several minutes to reach us from the Sun, for instance, but years from the nearest star, millions of years from the nearest big galaxy (Andromeda), and many billions of years from the most distant galaxies.

This means that as one peers across greater
distances
, one is also looking farther into the
past
. For example, if we observe a galaxy ten million light-years
away
, we are seeing it as it was ten million years
ago
. A voyage across the Universe in this sense is therefore not only a journey through
space
; it is also a journey back through
time
—right back to the Big Bang itself.

We cannot actually observe all the way back to the Big Bang. The early Universe was so hot that it formed a fog of particles that we cannot see through. As the Universe expanded, it cooled and the fog lifted about four hundred thousand years after the Big Bang. However, we can still use our theories to speculate what the Universe was like before then. Since the density and temperature increase as we go back in time, our speculation depends on our theories of high energy physics, but we now have a fairly complete picture of the history of the Universe.

One might expect that our voyage back through time would end at the Big Bang. However, scientists are now trying to understand the physics of creation itself and any mechanism that can produce our Universe could, in principle, generate others. For example, some people believe the Universe undergoes cycles of expansion and re-collapse, giving us universes strung out in time. Others think that our Universe is just one of many “bubbles” spread out in space. These are variants of what is called the
multiverse
proposal.

The Voyage Across Space

Physically
traveling
across the Universe is much more challenging because of the time it would take. Einstein's special theory of relativity (1905) suggests that no spaceship could travel faster than the speed of light. This means it would take at least one hundred thousand years to cross the Galaxy and ten billion years to cross the Universe—at least as judged by someone who stays on Earth. But special relativity also predicts that time flows more slowly for moving observers, so the trip could be much quicker for the astronauts themselves. Indeed, if one could travel at the speed of light, no time at all would pass!

No spaceship can travel as fast as light, but one could still approach this maximum speed. The time experienced would then be much shorter than that on Earth. For example, a journey across the Galaxy at nearly the speed of light would seem to take only thirty years, while much more time would have passed on Earth. One could therefore return to Earth in one's own lifetime, although one's friends would have died long ago. If one continued to accelerate beyond the Galaxy for a century, one could, in principle, travel to the edge of the currently observable Universe!

Einstein's general theory of relativity (1915) could allow even more exotic possibilities. For example, maybe astronauts could one day use wormholes or space warp effects—just like in
Star Trek
and other popular science-fiction series—to make these journeys even faster and get home again without losing any friends. But this is all very speculative.

The Voyage Through the History of Human Thought

To the ancient Greeks, the Earth (
geos
) was the center of the Universe (
cosmos
), with the planets, the Sun (
helios
), and the stars being relatively close. This
geocentric
view was demolished in the sixteenth century, when Copernicus showed that the Earth and other planets move around the Sun. However, this
heliocentric
picture did not last very long. Several decades later, Galileo used his newly invented telescope to show that the Milky Way—then known only as a band of light in the sky—consists of numerous stars like the Sun. This discovery not only diminished the status of the Sun, it also vastly increased the size of the known Universe.

By the eighteenth century it was accepted that the Milky Way is a disk of stars (the Galaxy) held together by gravity. However, most astronomers still assumed that the Milky Way comprised the whole Universe, and this
galactocentric
(
galactos
= milk) view persisted well into the twentieth century. Then, in 1924, Edwin Hubble measured the distance to our nearest neighboring galaxy (Andromeda) and showed that it had to be well outside the Milky Way. Another shift in the size of the Universe!

Within a few more years, Hubble had obtained data on several dozen nearby galaxies, which showed that they are all moving away from us at a speed that is proportional to their distance from us. The easiest way to picture this is to think of space itself as expanding, just like the surface of an inflating balloon onto which the galaxies are painted. This expansion is known as Hubble's Law, and it has now been shown to apply up to distances of tens of billions of light-years, a region containing hundreds of billions of galaxies. Yet another huge shift of scale!

The
cosmocentric
view regards this as the final shift in the size of the Universe. This is because the cosmicos expansion means that as one goes back in time the galaxies get closer together and eventually merge. Before that, the density just continues to increase—back to the Big Bang fourteen billion years ago—and we can never see
beyond
the distance traveled by light since then. However, recently there has been an interesting observational development. Although one expects the expansion of the Universe to slow down because of gravity, current observations suggest that it is actually
accelerating
. Theories to explain this suggest that our observable Universe could be a part of a much larger “bubble.” And this bubble could itself be just one of many bubbles, as in the multiverse proposal!!

What Next?

So the end point of all three of our journeys—the first back through time, the second across space, and the third retracing the history of human thought—is the same: those unobservable universes that can only be glimpsed through theories and visited in our minds!

I wonder what tomorrow's astronomers will discover…

 

Bernard