How does gravity exactly counteract the expanding universe at most scales?
We get told that the expanding universe is like the surface of a balloon and we live on one of the little dots on the surface. As the balloon expands, every other dot looks further away. However the dot gets bigger too, and so do I at the same rate.
Why does it seem that gravity completely explains the attraction of objects at laboratory scale up to galactic scale, but then doesn't when you look beyond that? Why isn't the space between my toes getting bigger at exactly the same rate?
- gintableLv 710 years agoFavorite Answer
The balloon and dots is only an analogy. It isn't 100% accurate.
Just because the dots spread out on the balloon by no means implies that at most size-scales...that the objects/orbit systems should expand.
The expansion factor of the universe is so small that it requires consideration of distances of millions of light years for it to even become significant at all. At the scale of a planetary system, which is seldom any larger than 1 light year in radius...the universe might as well not be expanding.
If you did draw a collection of dots on the balloon that were millionths and billionths the size of just one square centimeter...you would see the centimeters of distances expand...but the nanometers of dot sizes and nanometers of localized dot sizes wouldn't expand much at all. Any actual forces among them are much more significant.
Nothing sudden happens when the size scale gets to 1 million light years. The relative amounts of gravity vs universe expansion are continuous functions of distance. Yes your toes would be getting more spread out if hypothetically electromagnetism were to cease to exist and it would do so at the same "rate" as recession of distant galaxies. The problem is that we have a conflicting definition of what rate means. Rate in this sense does not refer to velocity or acceleration. Rate in this sense refers to the Hubble constant.
The Hubble constant has the units of reciprocal time units. Instantaneous recession velocity equals Hubble constant multiplied with distance at that instant.
If all the four forces of nature were to cease to exist, and suppose also that all objects were to suddenly become stationary among each other...everything in the universe would follow the same expansion law as described above.
- Anonymous10 years ago
The universe is expanding ONLY at inter-galactic scales, not laboratory scales. Gravity is NOT canceling out the expansion of the universe completely because it is the weakest of the four forces. If it WAS exactly canceling out the expansion of space, the universe would not be expanding. The light from stars and galaxies would not be red-shifted. Matter is not expanding. The strong and weak atomic nuclear forces are stronger than gravity.Source(s): Capstone course in geography. "...The last paridigm shift in the discipline of geography [as of 1993] was the realization of the importance of the concept of scale." Quote or araphrase of what the geography department chairman said. B.S. in physical geography, 1994, B.S. in geology, 1980
- DLMLv 710 years ago
The strong and weak nuclear forces are probably keeping you together better than gravity does, those are a lot stronger on small scales.
Larger objects, like planets, stars, natural satellites, etc, rely on gravity to keep themselves together.
But if the value for Dark Energy is large enough (and we don't know its exact value) then, eventually, someday, the universe expansion will rip apart things held together by gravity. And eventually, even molecular bonds will separate, and atoms themselves will be ripped apart.
It's just one possible fate of the Universe... they call it the Big Rip, and between that or Heat Death, you have a very dismal very distant future.
If Dark Energy is not strong enough, or if there is more mass in the universe than initially estimated, then perhaps a A Big Crunch will occur, in which gravity wins, and all the matter in the universe will fall back into itself.
- Anonymous10 years ago
This is because space is expanding, not matter. If you place a ball on a rubber sheet and stretch the sheet, the sheet is going to expand and not the ball. Gravity is much much stronger then the expansion in small scales, but once you get to scales somewhat larger then galaxy clusters the expansion has much more of an influence.