|MadSci Network: Astronomy|
Indeed there are plans for a spacecraft that will use a Solar Sail to achieve thrust. The idea is simple. The Sun blows off a wind of charged particles from its surface. The density of this wind near the Earth is enough such that if a large sail is deployed it can act the same as a sail for a boat on Earth. The spacecraft thus rides the wind. While the density of the Sun's wind near Earth is sufficient to do this it essentially stops at a place known as the Heliopause. At this location the solar wind mixes with the interstellar medium and no longer has any net flow. The density of matter out at this distance is extremely low (about 1 hydrogen atom per cubic centimeter). Space here is more empty than the best vacuum ever created on Earth. The distance to the Heliopause is estimated to be roughly about 150 AU (where an AU is the average distance between the Sun and Earth). To give you some sense of the immensity of space the nearest star to the Sun is about 272,000 AU.
Clearly then, if the solar wind ceases to have cosmic influence at such a small distance from itself the same must be true for all other stars. The density of matter outside of galaxies is even less dense than the interstellar medium and again has no net sense of flow. Therefore stellar winds could not be the cause of comic expansion.
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