The
shape of the universe has been a very theoretical topic in the field of
physics. Well, after all, no human species has ever had the opportunity to
observe the entity of space. Because of this, many scientists have developed
multiple theories on what they think the universe currently looks like.
However, through images and reliable facts, we are able to estimate the shape
of the universe.
The universe is infinite and it
is rapidly expanding.
One
thing we know about the universe is that it is continuously expanding. Thus, the
fate of the universe is determined by a struggle between the momentum of expansion and the pull of gravity. The rate of expansion is
expressed by the Hubble Constant, Ho, while the strength of gravity depends on the density and pressure of
the matter in the universe. If the pressure of the matter is low, as is the
case with most forms of matter we know of, then the fate of the universe is
governed by the density. If the density of the universe is less than the
"critical density" which is proportional to the square of the Hubble
constant, then the universe will expand forever.
This equation represents the critical density of the
universe where, H represents the Hubble Constant, and, G represents the
universal constant for gravity.
So, is the universe infinite...?
The density of the universe
determines its geometry. If the density of the universe exceeds the critical
density, then the geometry of space is closed and positively curved like the
surface of a sphere. This implies that initially parallel photon paths converge
slowly, eventually cross, and return back to their starting point (if the
universe lasts long enough). In this case, the universe is not infinite,
but it has no end (just as the area on the surface of a sphere is not infinite
but there is no point on the sphere that could be called the “end”). The
expansion will eventually stop and turn into a contraction. Thus, at some point
in the future, the galaxies will stop receding from each other and begin
approaching each other ass the universe collapses on itself. This is called a
closed universe.
If the density of the universe is
less than the critical density, then the geometry of space is open (infinite) and
negatively curved like the surface of a saddle. If this is the case, then
the space has negative curvature; there is insufficient mass to cause the
expansion of the universe to stop. In such a case, the universe has no bounds,
and will expand forever. This is called an open universe.
If the density of the universe
exactly equals the critical density, then the geometry of the universe is flat
like a sheet of paper, and infinite in extent. In this case space has no
curvature (it is flat), there is exactly enough mass to cause the expansion to
stop, but only after an infinite amount of time. Thus, the universe has no
bounds and will also expand forever, but with the rate of expansion gradually
approaching zero after an infinite amount of time. This is termed a flat
universe or a Euclidian universe (because the usual geometry of non-curved surfaces
that we learn in high school is called Euclidian geometry).
The simplest version of the inflationary theory, an extension of the Big
Bang theory, predicts that the density of the universe is very close to
the critical density, and that the geometry of the universe is flat, like a
sheet of paper. It can be depicted by the image below:
would be greater than one degree
across. This is an example of microwave background fluctuations:
Recent
measurements (c. 2001) by a number of ground-based and balloon-based
experiments, including MAT/TOCO, Boomerang, Maxima, and DASI,
have shown that the brightest spots are about 1 degree across. Thus the
universe was known to be flat to within about 15% accuracy prior to the WMAP
results. WMAP has confirmed this result with very high accuracy and precision.
We now know (as of 2013) that the universe is flat with only a 0.4% margin of
error. This suggests that the Universe is infinite in extent; however, since
the Universe has a finite age, we can only observe a finite volume of the
Universe. All we can truly conclude is that the Universe is much larger than
the volume we can directly observe.
By: Denny Na
By: Denny Na