(NASA) 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.
Does Cosmic Expansion Move Everything Apart, Even Parts of Our Bodies?
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JANUARY 08, 2009
April Holladay, HappyNews Columnist

Q: 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
A: 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.
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)