What Does Gravity Have To Do WithThe Expanding Universe?

[polymath257]

Gravity is a curvature of spacetime. 

Why do some physicists think that there may be a "graviton" particle through which gravity operates? If gravity is explained as a curvature of spacetime, what function would a particle have? Just curious.

That is trickier to answer, but the essence is that the *way* gravity is described mathematically in general relativity is best described quantum mechanically by a spin 2 particle when things are quantized. In essence, the interaction with that spin 2 particle is equivalent to the interaction with curved spacetime mathematically.

---

No. Electromagnetism is *way* stronger than gravity. Partly because of that, matter tends to be both magnetically and electrically neutral.

One way to see this: if you have a small magnet, it can produce sufficient force to counteract the gravity of the whole earth.

Gravity is a different thing than magnetism by orders of magnitude.

Can I use phonons to effect magnetism? This is difficult to study because of magnets in speakers. Any help would be appreciated. And should it be ultra(VHF, UHF) or infrasonic? I'm in way over my head!

First,

phononts=sound
photons=light

Yes, light (photons) is an electromagnetic wave, so light does, in fact, change the magnetic field. But the frequency is *very* high:on the order of 10^16 Hz for ordinary light. Also, the amount of change to the magnetic field tends to be small unless the light is of very high intensity. Even then, the electric forces will cause more effects than the magnetic because of the relative sizes of the two.

If you really are asking about phonons (sound), then the effects are different. if you can get the wavelength of the sound and the wavelength of the light (typically radio frequencies), then there can be a resonant effect. But you need a material that interacts strongly with the light (radio) as well as having the right wavelength of sound.

---

One way to look at dark energy is as the 'energy of empty space'. So, when space expands, there is more space, and hence more dark energy.

This extra energy has to be balanced, yes, but it is balanced by a decrease in the curvature of spacetime, which accelerates the expansion of space.

For ordinary matter and dark matter, when space expands, the total amounts of matter stay the same, but it is spread out more. This serves to gradually slow expansion.

For radiation (like light), expansion actually decreases the total amount of energy, which slows the expansion more.

So, in the very early universe, which was radiation dominated, the expansion was slowing, decelerating fairly fast. Later, when matter dominated, the expansion was still slowing, but not as fast. Now, with dark energy dominating, the expansion has started to accelerate. it's all a play-off between the different components of the universe and how the respond to expansion. And that is all about gravity.

Was spacetime ever curved? Light (photons, energy) curves when it comes within the vicinity of a body of matter. But in the beginning there was no matter, ergo, no gravity until  recombination.  Wouldn't that make spacetime/energy that isn't within the vicinity of a body of matter linear rather than curved and that would also account for acceleration.

The universe doesn't orbit around a center like matter does. It just moves outward

First, both mass and energy produce gravitational effects. So yes, there was gravity before recombination, but it was due to radiation, not matter.

Second, the universe is NOT moving outward from a center either. Or, equivalently, ALL points in space can be thought of as 'the center'.