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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 measurable only for objects bound together by weak forces.

Your right and left eyes, for example, (about 4 inches or 0.1 metre 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 within about two 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," e-mails 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.

Further Reading:

• Narrated video of the expanding Universe , SpaceGeek.org
• The beginning and end of our Universe , Royal Observatory Greenwich
• 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

April Holladay lives in Albuquerque, New Mexico. Her column, WonderQuest, appears every second Monday of the month on globetechnology.com. To read April's past columns, please visit her website . If you have a question for April, visit this information page .

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