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Orbits, Universe Expansion, Center
Name: Marc
Status: other
Grade: other
Location: CT
Country: N/A
Date: 10/16/2005
Question:
It has always been said that the Galaxies are moving away
from us, and that space is "stretching". But is that really the case?
From what I know, mass floating in space always tends to orbit something
due to gravitational pull. Starting on a small scale, Moons and other
Satellites orbit Planets. Planetary Systems orbit the sun. The Solar
Systems that make up the Galaxies appear to move in a circular motion as
if orbiting a central mass. Could it be that the Galaxies (including
ours) are orbiting a central mass? Possibly the center of the universe?
Is it possible that Galaxies are not moving away from us, but merely
traveling along their orbits?
Replies:
On smaller local scales, various celestial bodies can/do orbit as you
describe. However, on the inter-galactic scale this is not the case. No
matter in what direction one observes galaxies, they are moving away from
us. We know this because of the Doppler shift (also called the "red
shift"). When one measures the wavelength of light (no matter what
wavelength) the wavelengths are shifted to longer wavelengths compared to
the wavelength one measures for the same emissions in a laboratory on
Earth. If all galaxies were rotating about a single central point of
reference one would expect to see some galaxies moving away and some
galaxies moving toward us in a relative sense. So roughly speaking half
the galaxies should be "red" shifted and half should be "blue" shifted. In
addition, the "center of the Universe" model could not explain the (now
well established) cosmic background radiation. This is not to say that the
motions of galaxies is a "solved" problem. There are definitely unanswered
questions. For example, if you calculate the orbit of stars in a galaxy,
you would "expect" that stars further away from the center should be
moving slower than those near the center, because the force of gravity is
less (It falls of like 1/R^2). However, that is not the case. The distant
stars move at the same speed as nearer stars, so that the galaxy is
behaving like "a rigid body". But no one has been able to find the "stuff"
that would make this motion follow from the laws of physics. Hence the
introduction of "dark matter" and "dark energy". These topics are too long
to take up in a format such as NEWTON BBS but there are any number of
books at any level of sophistication you desire that discuss these
unanswered unobserved quantities.
Vince Calder
Marc,
First of all, we need to understand how we determine that galaxies are in
motion relative to us. The main indicator -as I am sure you already know-
is the Doppler Effect on light. Thus, objects that are moving away from us
are red-shifted, and objects moving toward us are blue-shifted.
Second, not all galaxies are red-shifted. Some members of the Milky Way's
local group (the Magellanic Clouds, etc.) are blue-shifted because, as you
noted, the local gravity is affecting them and causing them to move around
each other.
Third, while it is true that gravity is a very long-ranged force, it is
also the weakest. If some earlier effect (such as the Big Bang) caused an
expansion, then galaxies could be moving away from us at a decreasing
rate. If there is some other force (it is being theorized that vacuum
itself has an expansive nature) then that force could easily overpower
gravity. Thus, only members of our local group are affected strongly by
our local gravity.
Finally, if all the galaxies are moving around a universal center, then
around half the galaxies that we can see should be moving toward us while
the other half should be moving away from us. The fact that practically
all of the galaxies that we can see are red-shifted suggest that there is
an expansion.
Addendum: It is probably not a good idea to visualize the expansion of the
universe as a 3D expansion into 3D space - but I will leave that for later.
Greg (Roberto Gregorius)
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