By the way, I prefer T.S. Eliot over Portishead to the trumpet over you: http://www.hyperlexic.com/ts_eliot_portishead.php

Check it out. It’s pretty cool. And greetings to JO…

I would say that dark energy is not gravity: wouldn’t you? Take the cosmological constant out of Einstein’s equations and you will see that gravity certainly is working against (acting to decelerate) the expansion.

Martin

Well, I suppose to be more precise I should say that gravity is instrumental in changing the expansion, while the initial conditions of our region of the universe are also important. But whichever way you slice it, the expansion goes away once you’re in an orbit around anything. And furthermore, the existence of acceleration in the expansion indicates that gravity is decidedly not working against the expansion, except on small scales.

This has been my question for several years. If space is expanding as evidenced by Hubble’s consistent red shift in every direction, space is expanding from every point in the universe. Let’s build a model that assumes that matter is also expanding from the center of each individual particle in the universe. The expansion of space and matter is perpetual. As each object expands, the space between each object also expands, and everything stays relative. The only thing though is, you’ll get this strange illusion that material objects are being attracted to each other by some invisible force, or it will seem like the presence of matter is causing a curve in space-time that causes things that think they are moving in straight lines to move in curved lines.

Inertia is the tendency of mass to resist acceleration. If each atom or sub-atomic or primordial particle of matter is expanding in volume, that means the entire Earth is expanding. No matter what the rate of expansion is, the surface of any object would be accelerating away from the center of the object. That means that the surface of the Earth is accelerating upwards at a constant rate. That’s what we perceive as gravity.

Levi, all I know is, I pay a shit load of taxes so the NSF can dish out grants, and while they will seriously debate simulation theories and string theories, they constantly miss the point about perpetually expanding matter. No, you can’t measure the expansion of matter because your ruler is made of matter. That doesn’t mean that matter is not expanding. If you build your model based simply on matter that expands at the same constant rate everywhere, you will find that you don’t need dark energy anymore.

The idea that now is the time in history that will be looked back upon as the time when we figured out what the universe is made of, is just like the time they thought the patent office should be closed because everything had already been invented.

Sorry, perhaps I didn’t explain properly. The expansion rate itself is constant, independent of distance. The relevant point is rather that it’s an average. That is to say, on average, the universe as a whole is expanding everywhere at the same rate. But when you start looking closer, you find that local structure mucks things up. Since the expansion is driven by gravity, and since local structure generates gravitational fields, once you get to small enough scales, dependent upon the mass of the local structure, the local structure dominates over the average expansion of the universe.

On small scales, you’re not averaging over much area, so the local structure is more important. On large scales you’re averaging over much more area, so it isn’t so much, and we see the expansion. Thus, no matter how detailed your experiment, you simply cannot measure the expansion of the universe, say, in the vicinity of our solar system.

The comparison of velocities I gave earlier was an attempt to show how much area you have to average over before the average expansion of the universe becomes the dominant gravitational factor.

#22

I have always been perplexed by gravity. We know it exists, and we can measure it, but I still have not seen once decent theory as to why matter exerts a force upon other matter. I believe current theory just explains gravity away as “a property of matter” but does not explain why matter has this property. Please correct me if I am wrong. I would love to hear the bleeding edge explanation of why matter has the property of gravity. In my opinion, humanity will make a huge leap forward if we can understand why the apple falls from the tree.

We have such a theory – General Relativity – and it is one of the best tested theories we’ve ever had; far better than decent. While the concept of matter exerting forces on other matter can help in certain circumstances, in GR gravity is very well understood (including explaining the scales on which expansion makes sense and those on which it doesn’t) by mass and energy warping the geometry of spacetime. The motion of other matter or energy on that background i what we mean by gravity.