Dave Green asked:
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?
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 Ė so staying on the dark side. Space is expanding and itís 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 weíre a bit more used to - is a property of matter, and itís a pulling force, so that opposes the expansion, and the gravitational pull is stronger the more mass thatís 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 thereís 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.
Dave Green asked the Naked Scientists: 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 cooling the Earth? What do you think? Dave Green, Tue, 26th Jan 2010
Sweet question :)
No. the expansion of space effectively passes by all strongly gravitating bodies right up to the scale of clusters of galaxies.
"By characterizing the detailed structure of the cosmic microwave background fluctuations, WMAP has accurately determined the basic cosmological parameters, including the Hubble constant, to better than 5%. This measurement is completely independent of traditional measurements using Cepheid variables and other techniques. The current results show the Hubble Constant to be 73.5 (km/sec)/Mpc (give or take 3.2 (km/sec)/Mpc). If the WMAP data is combined with other cosmological data, the best estimate is 70.8 (km/sec)/Mpc (give or take 1.6 (km/sec)/Mpc). These results assume that the universe is spatially flat, which is consistent with all available data. However, if we do not make this assumption, the uncertainty in the Hubble constant increases to 4 (km/sec)/Mpc, or slightly over 5%.2"
How about this then?