The Solar System is not expanding with the universe. The expansion of the cosmos only occurs on the grandest scales, where the force of gravity is too weak to resist the stretching of space. Our local system, including the Sun, planets, asteroids, and comets, is held together by an immensely powerful gravitational force that dominates the local environment.
Gravity’s Dominance
The Solar System remains stable because of the overwhelming strength of gravity at close range. The Sun contains over 99.8% of the entire mass of the Solar System, and this immense concentration of matter creates a powerful gravitational field. This field acts as an anchor, holding all orbiting bodies in fixed, stable paths.
The strength of gravity follows the inverse square law, meaning the gravitational pull between two objects is inversely proportional to the square of the distance separating them. This relationship ensures that the gravitational force exerted by the Sun on the planets is extremely powerful across the relatively small distances of the Solar System.
This local gravitational binding energy is vastly greater than the minute stretching effect of cosmic expansion across a few billion miles. The expansion of space acts like an incredibly subtle tug, but the Sun’s gravitational hold is a constant, strong force preventing the planets from drifting away. The same principle applies to Earth itself, where electromagnetic forces holding atoms together resist any expansion within the planet’s structure.
Defining Cosmic Expansion
The universe expands due to the stretching of the fabric of space itself, not a force actively pushing objects apart. This expansion is observed by analyzing the light from distant galaxies, which appears shifted toward the red end of the spectrum, a phenomenon called redshift. This redshift occurs because the space between galaxies stretches as the light travels across immense distances.
The observational evidence for this stretching is formalized in Hubble’s Law, which states that the velocity at which a galaxy moves away from us is directly proportional to its distance. This relationship confirms that the entire universe is expanding uniformly. Furthermore, observations revealed that the expansion of the universe is accelerating, not slowing down. Scientists attribute this acceleration to dark energy, a mysterious influence theorized to be a property of space itself that drives the expansion.
The Scale of Universal Expansion
The contradiction between a stable Solar System and an expanding universe lies entirely in the scale of the structures involved. Gravitationally bound systems, such as solar systems, galaxies, and small clusters, are dense enough that their internal gravity overcomes the expansion of space. Within these structures, the expansion is negligible.
The stretching of space only becomes a noticeable effect over vast, intergalactic voids where matter is sparse and gravity is weak. The distance between unbound galaxies, which can be millions of light-years, is where the expansion is clearly measurable. The boundary where gravity loses its dominance is generally considered to be at the scale of galaxy clusters, roughly a few tens of millions of light-years. Beyond this threshold, the weak tug of dark energy wins the competition, causing the space between gravitationally isolated structures to expand.