Philosophy Explorer wrote:It would seem that dark energy is the opposite of gravity where gravity pulls and dark energy pushes. Yet gravity is described as a force while dark energy is described as...well, energy.
Also with dark energy, within our solar system, it doesn't seem to operate while gravity does and gravity helps to keep our planets in orbit about our Sun.
Can anybody add to this?
PhilX
It's an excellent question and pretty hard to understand. Basically, the modern idea of a "force" is that particles emit (throw) "gauge bosons" (particles - photons are an example) at each other. Actually, they usually emit virtual gauge bosons at each other. Virtual particles are ones you never detect, but seem to exist all the same. So with electrically charged particles, they throw (virtual) photons at each other. Depending on the overall system and what charge they have, this will cause charges to be repelled or attracted to each other. A proton throws a photon at an electron, and the electron is attracted to the proton (due to the Heisenberg uncertainty principle and their overall energy, which is affected by charge). Electrons throw a photon at each other, and they electrons repel.
This is what three of the four fundamental forces do (electromagnatism, and the strong and weak nuclear forces). They throw (mostly virtual) particles at each other. This results in moving away or towards, and we call it a force. the gauge bosons are the force mediators.
Gravity, however, is different (and arguably not really a force) because objects move towards each other because they each warp space time, so bodies sort of "fall" towards each other. It's analogous to how two cannonballs on a trampoline would roll towards each other. There is no particle exchange here (probably, but gravitons might exist), so if gravity is properly called a force, it's probably different than the other three.
So, what about dark energy? The idea with this is there is some "substance" throughout the universe that does not interact with matter in any way except gravity. Electromagnetism and the nuclear forces don't interact with it. The reason we feel and can touch something is because electron clouds on our hands (or other parts of our bodies) repel the electron cloud in another substance, like a rock (through photon exchange). But this hypothesized "substance" doesn't interact with photons (virtual or real), they go right through, so we can't touch it or feel it. We can't see it either, because photons go right through (hence the name "dark" energy).
The overall effect of this substance is that it permeates the universe, as the universe gets bigger, more and more of it fills up the universe. Sean Carrol
describes it as a feature of space itself. AS space gets bigger, you get more of this energy (a weird "substance" if you can call it that). Why, because it's a part of space, sorta. It's like your balloon expands and keeps filling up with air. The effect of this is to keep the curvature of space constant. If the curvature of space is constant, then the universe expands at a constant rate. However, in GR cosmological parlance, this means that the doubling time (or tripling time, or whatever) for the universe stays constant. SO to get twice as big, we need say, 10 billion years. To get four times as big, we need, 20 billion years. TO get 8 times bigger, we need 30 billion. That's what constant rate means to universe expansion.
Since this "substance" has energy, and E=mc^2, you might expect that it contributes gravity to the universe, and that would cause more attraction and the expansion of the universe to slow down. The energy does do this, but the other effect overwhelmes this effect.
The other thing to note is that masses still attract each other through gravity, and even though there is lots of dark energy between them. If dark energy were a sort of opposite gravitational force, dark energy would inhibit the gravitational force, making the attraction between objects less, but it doesn't, another reason "dark energy" is not a force.