Propulsion Disk, very interesting questions, and y
ou're on nearly the right track thinking about them.
In simple terms, mass "bends" spacetime, in the sense of making it curved (but in a four-dimensional way which is difficult to visualize, though made precise in the math of general relativity). Curved spacetime changes the way that objects move through it, which is the effect of gravitation. Mass curves spacetime, and curved spacetime tells mass how to move.
This means that any energy density associated with the vacuum (vacuum energy) will distort the geometry of spacetime as well. Since vacuum energy is the same everywhere, the curvature must also be the same everywhere, throughout the entire universe. We refer to this vacuum energy on a cosmological scale as the "cosmological constant", and its effect is to accelerate the universe's expansion (also called "dark energy").
Observations indicate the dark energy is real, but its strength is in very poor agreement with our understanding from quantum field theory. The actual energy density of the dark energy is only about 6x10[sup]-10[/sup] joules per cubic meter!
[sup]1[/sup] This sounds very small, but over very large distances the effect adds up to something measurable, and in fact it is the most significant form of mass/energy in the universe! (69% of the total density, compared to 26% which is dark matter and 5% regular matter.)
If the universe had no vacuum energy, then the result would be no additional acceleration. The expansion rate of the universe would instead slow down due to the combined gravitational effects of matter and radiation. It would never slow all the way down to zero though (and never collapse in a Big Crunch) because there isn't enough matter and radiation to halt the expansion completely.
Using the cosmology code (see
here for details), we can figure out exactly what the differences in evolution would be between our real universe (called the ΛCDM universe), and one containing no dark/vacuum energy. I think the results are perhaps most easily visualized on a spacetime diagram, which I've plotted here:
With vacuum energy:
Without vacuum energy:
Time is shown horizontally in billions of years since the Big Bang, and space is shown vertically (by
"proper distance" in billions of light years). The present time is marked by a vertical gray bar. The locations of galaxies are traced through time by the dotted white lines, with a thicker dashed white line marking the location of the most distant part of the universe that we can now see (the "particle horizon"). Finally, a solid yellow path extends backwards from our location at the present to the Big Bang, representing the path taken by light that is just now reaching us. This is our past light cone.
With vacuum energy, we see that the universe's expansion initially slows down, but then starts speeding up again after about 10 billion years. Galaxies are driven apart ever more rapidly in the future.
Without vacuum energy, it instead continues to slow down. The galaxies recede from one another, but at a diminishing rate.
Another tremendous difference is the present
age of the universe. Without vacuum energy, the current age of the universe must be younger: 11.6 billion years instead of 13.7! This is because it would take less time for the expansion rate to have dropped to the currently observed value without dark energy speeding it up.
The size of the observable universe would also be smaller, by about 40%! This is because of both the reduced expansion rate, and the decreased age of the universe, so that light would have less time to reach us.
One other subtle effect is that the curvature of the universe would be different -- precisely because of what we said earlier about how matter and energy curve spacetime. The curvature of the real universe is what we call "flat", in the sense that the geometry is Euclidean. Without the vacuum energy, the spacetime would instead be negatively curved. This means that parallel beams of light would spread away from each other, and the sum of angles in a triangle would be less than 180°. The "radius of curvature" for this vacuum-energy-less universe would be about 17 billion light years, which would be very noticeable in observations of the cosmic microwave background radiation!
1: To calculate the energy density of dark energy, we can multiply the total density of the universe which is very close to equal to the "critical density" of (3H[sup]2[/sup])/(8πG), multiply it by the fraction which is dark energy (about 69%), and then multiply by c[sup]2[/sup] to convert to energy density.