|
 Cosmology
probes mysteries humans have wondered about since we first looked at stars. Some
answers are emerging.
Is space infinite? Is it defined by how far light has traveled since
the big bang? And if I am not at the center of the universe, what is?
Is space infinite?
Current models (supported by experimental evidence) assume the Universe is
infinitely big and has been for the 13.7 billion years since it sprang into
existence.
But language throws much confusion into the picture. We can only see part of
the Universe ― only within a sphere, centered at Earth with a radius the
distance light travels in the 13.7 billion-year age of the Universe. The rest of
the Universe is invisible to us.
What readers asked:
-
Do you have insights about where space ends? It's mind boggling to me.
Brian, Wisconsin, USA
-
They say space is infinite, but is that because they don't know where it
ends? What's at the end of space? Edith, Houston,
Texas, USA
-
Is the size of the Universe defined by how far light has traveled since
the big bang? Is the Universe constantly expanding like a balloon?
Rebekah, Middleton, Idaho, USA
-
Is there an actual center of the Universe? Ernest,
Abilene, Texas, USA
"So, I would say that stating the Universe is definitely infinite may be
slightly overstating our claim, since we don't really know that it is infinite
beyond our observable universe, but we have no strong reason to believe it is
not infinite," astrophysicist Tamara
Davis emails.
Also, the expanding nature of our Universe complicates understanding. A
photon traveling for 13.7 billion years traverses more than 13.7 billion light years in
distance, because the Universe is expanding. The photon moves through a
space about three times larger (46 billion light years).
So, we should speak of two Universes ― the Entire Universe and the Visible
Universe but often sloppily call them both by the same term: the Universe.
That's the rub. So, when cosmologists speak of a "grapefruit" size
early-time Universe, they mislead us, big time.
What cosmologists mean
is the part of the Universe we now see (present Visible Universe) used
to be grapefruit sized, astrophysicists
Charley Lineweaver
and Tamara
Davis write in Scientific American. But the Entire Universe then (as now)
could be
infinite. "Shrink an infinite space by an arbitrary amount, and it is
still infinite."
We must, however, exclude what happened at the time of the 'Big Bang', since
we know nothing about that instant. 'Big Bang' time is when time began
and our theories have validity only up to Planck time (10^-43 seconds) after
'Big Bang' time.
Is there an actual center of the Universe? No there isn't a
center of the Entire Universe. Just a center (at Earth) of our Visible Universe.
What's at the end of space? The Universe doesn't end, as counter-intuitive
as this seems. Once
again, language confuses concepts. We speak of a 'Big Bang' but don't
mean a 'bang' like an explosion, which has a center and a shock wave that
moves spherically out into air from the explosion.
Instead, the 'Big Bang' happened everywhere in the Universe at once, with no
center. Shortly after the 'Big Bang', density and pressure of the
Entire Universe were the same everywhere. So, pressure difference
could not possibly create an explosion. The 'Big Bang' wasn't a bang at all.
About 13.7 billion years ago, the Entire Universe increased by 10^30 (a million trillion trillion) times,
in less than a second. We call this remarkable phenomenon, the 'Big
Bang.' The Universe simply expanded ― not into anything because, even at
the beginning, the Universe was probably infinite. (We can't say for sure
the Universe was infinite at the exact beginning since our theories are valid
only up to Planck time after the Big Bang). Thus, the Universe probably has no
center.
Sometimes analogies help understanding strange concepts like 'no end' and 'no
edge.' Imagine we're a couple of ants living on a big red balloon.
We think as two-dimensional creatures. We can't go up or down
and move only on the balloon's surface. I, unfortunately, am stuck in
a spot of honey and cannot move. But we wonder how big our world
is. Where does it end? I'm fearful you'll fall off the edge but you
insist on exploring the balloon. No matter which direction you set off,
though, you always come back to me but can find no end to our world.
To complicate our life, but not enough to defeat us, a kid is blowing up the
balloon as you wander. So, each circuit you make around our world
takes longer than the one before (even though you crawl at the same ant speed)
but you always come back to me and never encounter an edge. We conclude
our world is expanding, distances are increasing, but our world has no
end.
It's tempting to think we can increase the number of dimensions of our
real-life space by
one to explain why there is no end to the Universe. It works for the ants
― going from the 2-dimensional surface to the 3-dimensional balloon.
The balloon is embedded in a 3-dimensional space and can expand into the 3-D
space. We might conclude that the expansion of our space requires a fourth
dimension for the Universe to expand into.
But the
analogy breaks down for our 3-dimensional space and us. We do, indeed,
live in a 4-D continuum. Time, however, is our fourth dimension ―
different from the dimensions of space ― and the Universe changes with
time. The fourth dimension helps express how the Universe changes, not
what it expands into. We use the four dimensions (Space-time) to locate
any physical object or event in the Universe ― where and when something is.
Einstein says space is dynamic. The
Universe can expand, shrink and curve without being embedded in a higher
dimensional space. Our space is self-contained, needing nothing else ― no
edge, no end and no nothing to expand into.
Is space constantly expanding? You ask if the size of the Universe is defined by how far light has
traveled since the Big Bang, and if the Universe is constantly expanding. The
Visible Universe is defined by how far light has traveled since the Big
Bang
(see above).
The Entire Universe is continuously expanding but not at a constant rate.
Expansion is
accelerating.
As NASA reports, "...the results of the Wilkinson Microwave Anisotropy
Probe (WMAP) mission and observations of distant
supernova have suggested that the expansion of the universe is actually
accelerating..." We knew the expansion is accelerating even before
the early WMAP results were published," Lineweaver emails. In 1998,
Lineweaver used cosmic microwave background measurements to verify
experimentally the acceleration.
"The real clincher in the evidence for acceleration was observations of
supernovae by the High-z Supernova Team and the Supernova Cosmology Project, who
each published evidence for the acceleration of the universe, the first in 1998
by the High-z team, and then in 1999 by the SCP," Davis says. "Those
observations are the ones that were considered decisive. They used supernovae
as standard candles to measure distance, and combined that with the red shift to
measure their recession velocity. From that they could tell that recession
velocities have been increasing over time ― thus the acceleration."
An accelerating expansion implies the existence of a gravitationally
repulsive form of energy. Cosmologists term this unknown form, 'dark energy.'
"If dark energy in fact plays a significant role in the evolution of the
universe, then in all likelihood the Universe will continue to expand forever,"
NASA says.
Unfortunately, we have little idea what dark energy is even though we estimate it
comprises 74% of the Universe's mass-energy.
Whatever causes our Universe to expand at an accelerating rate,
the mere concept gives us deep insight. In 1929, astronomer
Edwin
Hubble formulated the expanding-universe idea into "Hubble's Law". The law states that that
the rate at which the distance between us and a galaxy increases is directly
proportional to its distance from us. So, a galaxy twice as distant from
us recedes twice as fast as one only half so far away. About forty-five years ago, we
verified Hubble's Law with experimental evidence.
Hubble was first to show that the Universe is expanding, and is considered by
many to be the father of observational cosmology.
"Hubble law has been gathering increasingly precise observational validation
ever since it was proposed," Lineweaver says. Furthermore, "the expansion
and cooling [of the Universe] has been the unifying theme of cosmology, much as
Darwinian evolution is the unifying theme of biology. Like Darwinian
evolution, cosmic expansion provides the context within which simple structures
form and develop over time into complex structures" ― the mind-boggling
Universe we know today.
More exploring:
Does
cosmic expansion move everything apart, even the various parts of our bodies?
WonderQuest, December 2008
Misconceptions about the big bang, by astrophysicists Charles L. Lineweaver
and Tamara M. Davis, Scientific American, March 2005
The Visible Universe, Universe adventure, Lawrence Berkeley National
Laboratory, August 2007
The Cosmic Microwave Background and Observational Convergence in the Ωm-ΩΛ Plane,
by Charles H. Lineweaver, The
Astrophysical Journal letters, August 1998
Experimental evidence
supporting current cosmology models of the Universe, Wilkinson Microwave
Anisotropy Probe (WMAP), NASA, April 2009
Dark energy, dark matter, NASA, September 2009
Ned
Wright's Cosmology Tutorial, University of Los Angeles
(Answered 14 December 2009)
April Holladay lives in Albuquerque, New Mexico. Her column, WonderQuest,
appears every second Monday of the month on WonderQuest.com. To read April's
past columns, please visit her website. If you have a question for April, visit this
information page.
Comment
- You recently wrote: "Also, the expanding nature of our Universe
complicates understanding. A photon travelling for 13.7 billion years
traverses more than 13.7 billion light years in distance, because the Universe
is expanding. The photon moves through a space about three times larger (46
billion light years)."
I was under the impression that the speed of light is constant, and don't
understand how a photon could travel three light years in one year, when a
light year is how fast light always travels. Joe Toronto Ontario, Canada
- REPLY: The speed of light is a constant, but the speed at which
space expands is not limited by the
speed of light. Space expands faster than the speed of light. Only matter
and information are restricted to going no greater than the speed of light.
I hope this helps. These concepts are bizarre. And here's an article that
might also help clarify notions:
http://www.wonderquest.com/ExpandingUniverse.htm
- I don't understand: regardless of whether 2D, 3D or 4D, doesn't the
expansion needs space beyond to expand into? If so, then the Universe
contains space, thus a boundary as you can't have an open-ended space.
Sin, Vancouver, BC, Canada
- REPLY: Sin, I don't know how to state it better than this:
Einstein says space is dynamic. The Universe can expand, shrink and curve
without being embedded in a higher dimensional space. Our space is
self-contained, needing nothing else --- no edge, no end, and no nothing to
expand into.
And I know that's not completely satisfying. But to get into the nitty
gritty of why takes us into higher
mathematics, which merely proves a theory, given the correct assumptions.
Math doesn't satisfy our intuitions. Einstein comes the closest to actually
understanding what space and time mean.
- In mathematics the symbol for infinity does not represent a number. It
merely indicates that division by zero is not allowed. The word infinite is
confusing. Do we mean that the universe cannot be measured and is outside the
realm of science? Do we mean that the universe is very large and some day we
may measure it? Do we mean that mathematics is not allowed with respect to
measurements of the universe? Cosmology must relate to measurements and to
calculations that agree with the measurements. Calculations can only be
extended slightly beyond the measurements. The calculations may be extended
only to the extent of guiding the next experiment. Only one precise and
confirmed new experimental result is needed to change the theory. Let's
remember that before we speculate about an infinite universe appearing
instantaneously. We only have the measurements, nothing else. Bill Seattle
WA USA
|
|
