[thedoc (OP)]

Fred Richardson asked the Naked Scientists:
       
From Fred Richardson, Facebook.
   
1. Hi Chris and the team, I've 2 questions hoping to get in for question of the week: 1] Why did Einstein's theory of general relativity predict that the universe should be expanding? and 2] how is the energy of the vacuum calculated? Thanks as always for the show:)

1a. And has anyone observed the density of any objects reducing as a consequence of Dark Energy?




   
What do you think?

[PhysBang]

As for question 1, GR predicts that a universe that is effectively homogenously distributed in space should expand or collapse, such a universe would have to be amazingly well balanced in order to be static. When the matter is homogeneously distributed, then the effect of gravity can be reduced to one equation governing the average distance between ideal objects: their initial "velocity" apart from each other is counterbalanced by the gravity pulling them together.

This brings us to question 2, since we can't really see objects being pulled apart by dark energy because dark energy works mostly at the scale of cosmology. The "ideal objects" I mentioned above are the size of galaxy clusters or even superclusters for our universe at the scale of cosmology.

[jerrygg38]

As for question 1, GR predicts that a universe that is effectively homogenously distributed in space should expand or collapse, such a universe would have to be amazingly well balanced in order to be static. When the matter is homogeneously distributed, then the effect of gravity can be reduced to one equation governing the average distance between ideal objects: their initial "velocity" apart from each other is counterbalanced by the gravity pulling them together.

This brings us to question 2, since we can't really see objects being pulled apart by dark energy because dark energy works mostly at the scale of cosmology. The "ideal objects" I mentioned above are the size of galaxy clusters or even superclusters for our universe at the scale of cosmology.

   The only objection I have on your analysis is your statement that "gravity pulls things together". Many scientists cannot believe this to be true. The two lead balls in the lab have a vector force that pushes them together. there is no magical rope or spring between them. they are pushed together. The Earth and Sun are pushed together this is counterbalanced by the rotation of the Earth around the sun. This is similar to the way magnets are pushed together.
   As I see it, as the universe expands the counter-force of gravity occurs. Thus gravity is positive as objects are pushed together and negative as the universe expands. Equal and opposite forces are at play. What do you think?

[PhysBang]

Technically, if "dark energy" is merely the cosmological constant, then gravity alone does push at greater distances.

In the context of GR predicting expansion, the cosmological constant was not in play. Einstein added it in order to get a push so that GR could predict an unchanging universe.

[CPT ArkAngel]

Originally, General Relativity did not predict if the universe was expanding or not. Einstein could do whatever seemed to be appropriate according to observations. It is only after that Hubble observed a redshift of distant galaxies that Einstein fixed his equations for an expanding universe. At first, he added a mathematical term to his equation to make a static universe. He said that it was his biggest blunder.

Today, with the discovery of Dark Energy, this added mathematical term can be used to explain how Dark Energy behaves, at least approximately. So many physicists think that Einstein's biggest blunder was not a blunder after all. This was just an unknown and it is still an unknown to some extent.

The vacuum energy explanation is possibly just a big blunder. It is in my humble opinion. It is not observed in any way concerning Dark energy.

Vacuum energy is ill-defined in physics. No one knows what it is and how it works exactly. It is a concept used to explain some observations in Quantum theories. In fact it has no true meaning in General Relativity.

[Scott Mayers]

Only for the first part of the first question for how GR predicts expanding, it doesn't directly but you usually hear this spoken of as an indirect reflection of gravity prior to either the Steady State or Big Bang ideas. At that time the default was to a static universe to which Einstein and others would have asked how, given conservation of energy and matter throughout the universe, gravity could not have already 'pulled' (or 'pushed', depending on which interpretation) all of that fixed quantity to one place? This is where I believe Einstein actually added his 'fudge factor' number in his equations to simply account for this 'loss' in an accounting way.

Either way, GR wasn't focused on this except as a 'disproof' potential for his theory or to represent something incomplete about it.