Structure
of the Universe
Does the Universe have an edge, beyond which there is nothing?
Are the galaxies arranged on the surface of a sphere?
Why can't we see the whole universe?
Does the term "universe" refer
to space, or to the matter in it, or to both?
Evolution of the Universe
Did the Universe expand from
a point? If so, doesn't the universe have to have an edge?
Then where did the idea that the universe was once a point come from?
If the universe started out so dense, why didn't it collapse into
a black hole?
Why does looking out in space mean looking back in time?
I've heard the expansion of
the universe may be speeding up. Is there an "anti-gravity" force?
More about the Big Bang
When they
say "the universe is expanding," what
exactly is expanding?
But if you can't see space, or
feel it or touch it- how can it be expanding?
Why did anyone ever think that space should be expanding? Isn't it
a far-fetched idea?
Then how do we know that space really is expanding?
Are the galaxies in the universe moving through space?
But I heard that our Milky Way galaxy may one day collide with a
neighboring galaxy. If galaxies are all moving apart from each other,
how can they collide?
Where did the Big Bang scenario come from?
How do we know when the Big Bang took place?
Do we know where, in space, the Big Bang took place?
How do we know there really was a Big Bang?
But I've heard on the news there
are problems with the Big Bang theory. Is it still just a "theory"?
Was the Big Bang the origin of the universe?
Are there theories that go beyond the Big Bang? Structure
of the Universe
Does the Universe have an edge, beyond which there is nothing?
Galaxies extend as far as we can detect... with no sign of diminishing.There
is no evidence that the universe has an edge. The part of the
universe we can observe from Earth is filled more or less uniformly
with galaxies extending in every direction as far as we can see
- more than 10 billion light-years, or about 6 billion trillion
miles. We know that the galaxies must extend much further than
we can see, but we do not know whether the universe is infinite
or not. When astronomers sometimes refer (carelessly!) to galaxies "near
the edge of the universe," they are referring
only to the edge of the OBSERVABLE universe - i.e., the part we can see.
^ back to top Are the galaxies arranged on the surface of a sphere?
No. Galaxies are not actually arranged
on the surface of a sphere. Many students and teachers mistakenly
believe that the galaxies in the universe are arranged on the surface
of a sphere. One origin of this misconception is the common demonstration
of blowing up a balloon to model the expansion of the universe.
Another is the (mistaken) belief that during the Big Bang, matter
expanded into space from a point (see below). A third is the finding
that many clusters of galaxies appear to be arranged around the
outside of "bubble-like" voids
in the universe. But on the largest scales that astronomers have observed,
each chunk of space appears to have just as much matter as any other equivalent
chunk.
^ back to top Why can't we see the whole universe?
We can see just about as
far as nature allows us to see. Two things prevent us from
seeing further. First, the universe has been evolving with time.
Stars and galaxies did not always exist. Therefore light from
MOST of the galaxies in the universe has not yet had time to
reach us. Second, the universe has been expanding with time.
Again, light from MOST of the universe has not yet had time to
reach us. If you could suddenly freeze time everywhere in the universe,
and magically survey all of creation, you would find galaxies
extending out far beyond what we can see today. But how far,
no one knows.
^ back to top Does
the term "universe" refer
to space, or to the matter in it, or to both?
Just a hundred years ago, scientists
thought of the universe in terms of matter. Space was just the "emptiness" in
which matter lived. Today, the situation is reversed. During the twentieth century,
scientists learned that space is not "nothingness." First,
Einstein showed that space has structure: It is flexible and
can be stretched. (In fact, when astronomers talk about the "expansion
of the universe," they are referring to the stretching of
space between clusters of galaxies - NOT to the motion of galaxies
through space.) Later, scientists found other properties of space.
For example, matter and anti-matter are routinely created in
the laboratory from space itself (and an energy source); the
kinds of particles that can exist reflect the structure of space.
In fact, there is now evidence that space itself MAY possess
some slight amount of energy of its own, of a form previously
unknown. If so, space may actually have weight! Discovering the properties of space remains one of the deepest
and most important problems in modern science.
^ back to top Evolution
of the Universe
Did the Universe expand from a point? If so, doesn't the universe
have to have an edge?
No. The Big Bang was not an explosion IN space. It was a
process that involved ALL of space. This misconception causes
more confusion than any other in cosmology. Unfortunately, many
students, teachers, and scientists(!) mistakenly picture the "Big
Bang" as an explosion that took place at some location in space, hurtling
matter outward.
In reality, ALL of space was filled with energy right from
the beginning. There was no center to the expansion, and no magical
point from which matter hurtled outward. The confusion arises
in part because of the amazing conclusion that the OBSERVABLE
portion of the universe was once packed into an incredibly tiny
volume. But that primordial pellet of matter and energy was NOT
surrounded by empty space... it was surrounded by more matter
and energy (which today is beyond the region we can observe.)
In fact, if the whole universe is infinitely large now, then
it was always infinite, including during the Big Bang as well. To put it another way, the current evidence indicates only
that the early universe - the WHOLE universe - was extremely
DENSE - but not necessarily extremely small. Thus the Big Bang
took place everywhere in space, not at a particular point in
space.
^ back to top Then where did the idea that the universe was once a point come
from?
For much of the twentieth century, astronomers and physicists
believed that space might NOT be infinitely large - that is,
space might actually curve around on itself to form a "closed
universe." This
unusual three-dimensional shape was discovered in the mid-1800's
by the great mathematician Bernhard Riemann. The shape was later
favored by Einstein as a possible shape for the universe. Such
a closed universe would have a finite volume, yet no boundaries
or edges. Although closed universes cannot be visualized from the outside,
they CAN be visualized from the inside. For example, the image at right gives
an idea of what a tiny closed universe might look like. (In a real closed universe,
you cannot see the back of your head, the way you can here.) If you shrink
such a space down, then everything in it gets closer together, and the volume
of the closed universe gets closer and closer to zero. But there is still nowhere
OUTSIDE the space for an observer.
Current evidence shows that our part of the universe appears
not to be curved. This tells us that either the universe is infinitely
large, or else is so large that we cannot detect its curvature
from the tiny portion we can observe -- just as we could not
tell that the Earth was curved if our measurements were confined
to a sandbox!
^ back to top If the universe started out so dense, why didn't it collapse
into a black hole?
A large enough clump of matter will collapse
to form a black hole, but ONLY if it is surrounded by (relatively)
empty space. During the Big Bang, there WAS NO empty space: ALL
of space was filled more or less uniformly with matter and energy;
there was no "center of attraction" around which matter
could coalesce. Under these circumstances, a cosmic-scale black
hole will not form (and lucky for us!).
^ back to top Why does looking out in space mean looking back in time?
Because
it takes time for light from distant objects to reach us. We
see the sun as it looked about 8 minutes ago... other stars as
they looked years ago... and distant galaxies as they looked
millions or even billions of years ago.
^ back to top I've
heard the expansion of the universe may be speeding up. Is there
an "anti-gravity" force?
Current studies of
distant exploding stars have led astronomers to conclude that
the universe is not only expanding - the expansion may be accelerating
with time. This is not due to an "anti-gravity
force" but rather to gravity itself. In fact, the effect
was predicted as a possibility on the basis of Einstein's theory
of gravity. (It may seem strange that gravity can be "repulsive" as
well as attractive. The secret is that the expansion applies
to the fabric of space itself - not to the matter within it;
space behaves very differently from matter. For example, no chunk
of matter can travel through space at the speed of light. Yet
SPACE itself can expand faster than the speed of light. Similarly,
while matter is attracted to other matter by gravity, space behaves
differently: Space can either expand or contract as a consequence
of gravity.)
^ back to top
More
about the Big Bang
When
they say "the universe is expanding," what
exactly is expanding?
As bizarre as it may seem, space itself is expanding - specifically,
the vast regions of space between galaxies.
^ back to top But if you can't see space,
or feel it or touch it - how
can it be expanding?
According to Einstein, space is not simply
emptiness; it's a real, stretchable, flexible thing. In fact,
understanding the properties and behavior of space is a major
goal of modern physics.
^ back to top Why did anyone ever think that space should be expanding? Isn't
it a far-fetched idea?
The notion that space is expanding is a
prediction of Einstein's theory of gravity, which describes a
simple but universal relationship between space, time, and matter.
But it was a prediction that Einstein didn't believe; in fact,
he tried to modify his theory to get rid of it.
^ back to top Then how do we know that space really is expanding?
In the late
1920's, the astronomer Edwin Hubble first observed that distant
galaxies are moving away from us, just as would be expected if
the space between galaxies were growing in volume - and
just as predicted by Einstein's theory of gravity. Since then,
astronomers have measured this recession for millions of galaxies.
But there's other evidence as well.
^ back to top Are the galaxies in the universe moving through space?
No, the
galaxies sit more or less passively in the space around them.
As the space between galaxies expands, it carries the galaxies
further apart - like raisins in an expanding dough.
^ back to top But I heard that our Milky Way galaxy may one day collide with
a neighboring galaxy. If galaxies are all moving apart from each
other, how can they collide?
The universe is a chaotic place - and
the gravity from one galaxy, or from a group of galaxies, may
disturb the motion of its near neighbors, causing them to collide.
However, on average, when you compare two large enough chunks
of space, the galaxies in one are moving away from the galaxies
in the other.
^ back to top Where did the Big Bang scenario come from?
If space (and everything
with it) is expanding now, then the universe must have been much
denser in the past. That is, all the matter and energy (such
as light) that we observe in the universe would have been compressed
into a much smaller space in the past. Einstein's theory of gravity
enables us to run the "movie" of
the universe backwards - i.e., to calculate the density
that the universe must have had in the past. The result: any
chunk of the universe we can observe - no matter how large -
must have expanded from an infinitesimally small volume of space.
^ back to top How do we know when the Big Bang took place?
By determining
how fast the universe is expanding now, and then "running
the movie of the universe" backwards
in time, using Einstein's theory of gravity. The result is that
space started expanding about 15 billion years ago, give or take
a few billion years. This number is uncertain, in part because
of uncertainties in our current measurements of how fast the
universe is expanding, how much matter and energy there is, and
even what kind of energy there is in the universe.
^ back to top Do we know where, in space, the Big Bang took place?
It's a common
misconception that the Big Bang was an "explosion" that
took place somewhere in space. But the Big Bang was an expansion
of space itself. Every part of space participated in it. For
example, the part of space occupied by the Earth, the Sun, and
our Milky Way galaxy was once, during the Big Bang, incredibly
hot and dense. The same holds true of every other part of the
universe we can see. Artists may find it more dramatic to draw a "fireball" expanding
into space, but as far as we know, there would have been no such "ball."
^ back to top How do we know there really was a Big Bang?
As mentioned above,
we observe that galaxies are rushing apart in just the way predicted
by the Big Bang scenario. But there are other important clues. Astronomers have detected, throughout the universe, two chemical
elements that could only have been created during the Big Bang:
hydrogen and helium. Furthermore, these elements are observed
in just the proportions (roughly 75% hydrogen, 25% helium) predicted
to have been produced during the Big Bang. This prediction is
based on our well-established understanding of nuclear reactions
- independent of Einstein's theory of gravity. Second, we can actually detect the light left over from the
era of the Big Bang. The blinding light that was present in our
region of space has long since traveled off to the far reaches
of the universe. But light from distant parts of the universe
is just now arriving here at Earth, billions of years after the
Big Bang. This light is observed to have all the characteristics
expected from the Big Bang scenario and from our understanding
of heat and light.
^ back to top But
I've heard on the news there are problems with the Big Bang theory.
Is it still just a "theory"?
The Big Bang is
actually not a "theory" at all, but
rather a scenario about the early moments of our universe, for
which the evidence is overwhelming. But the Big Bang scenario
cannot be the whole story, and its details are a subject of intense
research.
^ back to top Was the Big Bang the origin of the universe?
It is a common misconception
that the Big Bang was the origin of the universe. In reality,
the Big Bang scenario is completely silent about how the universe
came into existence in the first place. In fact, the closer we
look to time "zero," the
less certain we are about what actually happened, because our
current description of physical laws do not yet apply to such
extremes of nature. The Big Bang scenario simply assumes that space, time, and
energy already existed. But it tells us nothing about where they
came from - or why the universe was born hot and dense
to begin with.
^ back to top Are there theories that go beyond the Big Bang?
Yes, there are
theories that build on the Big Bang scenario by adding insights
from physics about the structure of space itself. Watch this
space for more details.
^ back to top |