Space is expanding. While I make no pretense at understanding this completely (if at all), there is something which puzzles me. Most dumbed-down-for-non-physicists discussions usually state that, while space is expanding way the fuck out there, here in local land space is not expanding.
That’s my puzzlement; why isn’t our local space expanding as well? Why am I not drifting away from my computer screen? Maybe not measurably, only gazillionth of a micron per minute, but some amount.
Because we are all stuck relatively motionless by the big lump of mass we are sitting on. The Earth’s gravity is strong enough to keep our individual atoms from spreading out due to universal expansion, the sun’s gravity is strong enough to keep the planets from spreading out from universal expansion, and the Milky Way’s gravity is strong enough to keep all its stars in relatively close contact with each other.
Space may be expanding, but the distribution of matter is still lumpy.
(Edited to add - I know that electromagnetic forces are stronger than gravity on the human scale [chair/self/computer] but the explanation stands. Forces sticking us together are stronger than universal expansion on the small scale of galaxies on down.)
Gravity prevents you from floating away from your keyboard. Gravity is also slowing the metric expansion of space.
I am expanding, although I concede that may be due to too many hot dogs rather than any cosmic principle.
This years Nobel prize in physics was awarded these gentlemen because they had discovered that the expansion of the universe is actually accelerating instead of retarding.
It’s not. The expansion of space is best described as new space being created in between the other bits of space. This isn’t affected by gravity.
What is affected by gravity is what objects* in* space do. On a large scale, they are carried away from each other by the expansion of the space they inhabit; on a smaller scale, as you say, the gravity between them is strong enough that it more than compensates for that motion. The nuclear forces between particles, thousands of times stronger than gravity, are what hold them together.
Maybe the question can be phrased like this:
[ol]
[li]Is space itself expanding?[/li][li]Or are the objects in space moving farther apart?[/li][/ol]
If it is #1, then I don’t see any difference between the distance betwen the Milky Way galaxy and the Andromeda galaxy, vs. the space between two adjacent molecules in my shirt. And in fact, maybe even the molecules themselves and the quarks they’re made up of are expanding.
But if it is #2, then I can see why gravity would inhibit the expansion of small objects very much, and it would hardly inhibit the expansion of great distances at all.
#1. You don’t notice it on even a sub-galactic level because it’s only expanding at a very small fraction of an atom’s width per second at a planetary, or probably even solar-system-wide, level. Easily swamped by electromagnetic and gravitational effects, which are tens of orders of magnitude larger.
It is definitely case #1. Let’s think of it using the old rubber sheet example.
On the rubber sheet, we have two couples. One is angry with each other and they are facing away from each other, arms crossed. The other couple is in love and holding hands. Now we start pulling the rubber sheet. As the sheet stretches, we’re simulating the expansion of the universe. The first couple is pulled apart by this movement; what used to be only 2 inches of distance is now 4, then 8, etc. The second couple are not pulled apart. The forces exerted by holding onto each other is sufficient that the rubber sheet just slides under their feet.
The holding on component could be atomic, molecular, gravitational, etc. but without sufficient force, you’ll definitely be moved apart at any scale.
Been reading Orphans of the Sky again?
A post-cataclysmic human living in a generational spaceship claims that the “law of gravity” spoken of in ancient books refers to lovers.
How do we know the objects in space are not also expanding? Like, what if all space, even that between subatomic particles, is expanding? That growth would be very small on the level of the individual particle, but its cumulative expansion over the scale of a galaxy or more would begin to be measurable.
The expansion of space only really works at large scales, in many ways the particular part of space we are in is contracting rather than expanding as our small section of space is particularly dense. Unfortunately it can get pretty subjective though.
IMO it’s best to talk about the metric expansion of space as the distance between objects increasing. That’s not the same as saying they are moving relative to each other though (in fact expansion of space refers to ‘co-moving’ objects at rest to the CMBR).
Because you’re held in place by gravitational and electromagnetic forces. Gravity is obvious, electromagnetic forces are what hold objects together (for non-spherical objects smaller than planets, at least), keep you from falling through your chair, and so on. Those forces are stronger than the force causing the expansion of the universe, so they win over it and things stay together.
Sit in your chair and blow air out of your mouth toward your computer, like you were blowing out a candle. Do you start moving backward? Probably not (I didn’t). Electromagnetic forces, here in the form of friction, hold you in place. If you were sitting in space and blew air out, you’d move as a result of that force. But the force of the air blowing out of your mouth was too weak to overcome the friction that’s holding you in place. The same thing is happening with the force causing the expansion of the universe.
The expansion of the universe is accelerating (if you know why, PM me- I’m sure Mr. Neville will be willing to share the Nobel Prize with you), so the force driving the expansion of space is getting stronger. There’s a theory that eventually it will be stronger than gravity and will tear apart gravitationally bound bodies (things like planets), in something called the Big Rip. It’s debatable whether this will actually happen, though, and in any case, it wouldn’t happen for a long time.
So if I would say that the couple doesn’t notice anything because they are growing at the same rate as the sheet, then I’d be totally wrong?
Well, yes, but you’re far too smart (and good looking) so say such a thing, right?
I understand that the speed of expansion doesn’t become significant until you are talking about huge intergalactic expanses.
But is the tiny, tiny amount of space expansion going on in a planetary scale detectable in any way whatsoever?
In principle? Sure. But it’d be one Hell of an engineering problem-- I can’t think of any way to actually do it with any remotely plausibly foreseeable technology.
That’s actually a good argument to say that it isn’t detectable at all on a planetary scale.
Of course Einstein and Straus were unaware of dark energy and possibly that may still have an effect on even a planetary scale. For example if the distribution of dark energy was homogenous even on a planetary scale and whilst that could quite plausibly be true, there’s no reason to assume it is.
No one knows what dark energy is, but it seems to be a property of space itself. It gets stronger in areas with more space, so that it can then create even more space. In other words, voids get bigger while the small dense regions (galaxies, planets, etc) remain tied together as they are carried along by the big empty regions expanding.
As a result of this, in the far future, all our local group galaxies will have combined into one mega galaxy, but every other galaxy beyond that will be so far away that they will no longer be visible.
Which is an overly-fancy way of saying “really big areas”. It doesn’t need to be empty space.