Does cosmic expansion move everything apart, even parts of our body?
Gravitation tends to reduce distances between objects with mass, whereas
cosmic expansion tends to increase distances. But the diameter of galaxies, for
example, remains constant. So, at what scale does cosmic expansion start
dominating gravitation? Can we assert, for example, that our local galaxy
cluster will remain unaffected and that cosmic expansion will only make very
distant galaxies disappear from our sky? Daniel, Menerville, France
The cosmic expansion works on all scales. But its force is
tiny and, therefore, its effect is only measurable for objects bound together by
weak forces.
Your right and left eyes, for example, (about 4 inches or 0.1 meter apart) won't drift
farther apart (although acted on by space
expansion) because your skull is easily strong enough to overcome the cosmic
expansion force. The skull holds the eyes in place.
The force of
gravity within our solar system (dominated by the Sun's gravity out to about 2
light years) is likewise stronger than the cosmic expansion
force. Thus, our planets, dwarf planets, moons, asteroids, Kuiper belt
objects, comets, meteoroids and interplanetary dust — stay in place, roughly in
the orbits Newton determined over 300 years ago.
Likewise, gravitation force overcomes space expansion force within the
bounds of our Milky Way galaxy (about 100,000 light years in diameter).
But, the gravitational force on two objects decreases with the square of the distance between
them.
When galaxies are separated by the huge distances found within superclusters (several
hundred million light years across), the gravitational force generally is small compared to
cosmic expansion force. Therefore, the galaxies within the supercluster eventually move apart
like blueberries in rising muffins.
 This
infrared image from NASA's Spitzer Space Telescope shows hundreds of
thousands of stars crowded into the swirling core of our spiral Milky Way
galaxy. Eventually these will be almost the only stars we see, as the space expansion
force moves the more distant galaxies in our Local Supercluster far, far away.
After a long time, distant galaxies within our Local Supercluster will disappear from
view of
our most powerful telescopes.
One relatively nearby galaxy cluster, however, is so
massive that it will not recede from us. The attractive force of Virgo's
enormous mass is great enough to overcome cosmic expansion and pull our galaxy,
the Milky Way, and, perhaps, our entire Local Group of galaxies into its region.
After that, even more distant large clusters within the
Local Supercluster will recede and dim until they are "no longer visible",
emails astronomer
Jim O'Donnell of the Royal Observatory Greenwich.
In answer to your question: cosmic expansion dominates gravity for
objects several hundred million light years apart.
Over distances of hundreds of millions of light years, mass is spread more
thinly because there's nothing between the clusters, so the average density is
lower, gravity is weaker and expansion dominates at large scales," says
O'Donnell.
By the way, the expansion force is a real force. At Earth’s surface, the
outward acceleration away from the planet’s center is but a fraction (0.1 with
29 zeros after the decimal point) of inward gravitational acceleration — but
it’s present. A yet unexplained force due to what we term "dark energy", which
may make up 65% of the Universe.
Further Reading:
Narrated video
of the expanding Universe, SpaceGeek.org
The beginning and end of our Universe, Royal Observatory Greenwich
Misconceptions about the big bang, by astrophysicists Charles L. Lineweaver
and Tamara M. Davis, Scientific American
The Virgo
Cluster of galaxies, SEDS, the Students for the Exploration and Development
of Space
The expanding Universe, by physics professor Gary Felder, Smith's College
Hubble Tutorial, the University of Wisconsin Physics Department
(Answered Dec. 3, 2008)
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