If the universe is expanding, are we getting further from the Sun?

15 August 2010

Question

Hi guys, love the show, listen to it at work to help make the days a bit more interesting. My question is: If the universe is expanding, but planets and stars are staying the same size, why doesn't the distance between Earth and the sun increase, and our orbit around the sun become longer, thus lengthening our year over time, and eventually cooling the Earth due to its distance?

Answer

We put this question to Dr Carolin Crawford:

Carolin - Well this is about, not so much from the effects of dark matter on our solar system, but looking at more the effects of dark energy - so staying on the dark side. Space is expanding and it's carrying the galaxies along with it for the ride. They're all receding from us, and we think they're being pushed apart by a force that we call dark energy, and this is currently accelerating the expansion of the universe.

But the curious thing is, that this dark energy, whatever it is, is a property of space. So the larger the distance between bodies, the stronger they push to drive them apart. Conversely, gravity - which we're a bit more used to - is a property of matter, and it's a pulling force, so that opposes the expansion, and the gravitational pull is stronger the more mass that's there, and depends on how close you are to it.

So, whether the pull of gravity, or the push of dark energy dominates over a given region of the universe, depends on how much mass is there, and how widely separated it is. If they're far apart, the push of the dark energy wins, but if they're close together, gravity is going to dominate. You have to remember, in astronomical terms, our solar system is absolutely tiny. The planets and the sun, and all the constituents of our solar system, are very close together, and there's no question that gravity wins in that circumstance. Even on the scales of the galaxy, gravity is the dominating force. Even between groups or clusters of galaxies, gravity is gluing them together. You're only going to get this expansion of space on the very largest scales, where you have sufficient space that the dark energy can dominate.

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