DARK ENERGY

The Hubble constant, in conjunction with observations of other galaxies, might lead one to think that our galaxy is at or near the center of the universe. After all, everything else is moving away from us, right? Isn’t that what Hubble’s Law says? Sort of. Hubble’s law does say that, relative to our position, objects appear to be accelerating away from us at 71 km/s/Mpc. However, this applies to any position in the universe, not just ours. An apt comparison is to a loaf of bread expanding under heat. To a single particle in the loaf, it appears that all other particles are moving away from it, but from an external perspective, it’s evident that some of them are moving in the same direction--just at different speeds. In a somewhat similar vein, it can be helpful to think of expansion of the universe not as objects moving through space, but space itself stretching. This line of thought makes more sense with some understanding of Einstein’s theory of general relativity, but it might help you to make sense of the expansion of the universe in this context.

 

The symbol Ω (omega) is used for the total energy density of the universe. The critical density of the universe, where it will neither expand nor contract, is defined as Ω = 1. Any more and it will expand forever; any less and it will inevitably collapse at some point. Breaking Ω down into its basic components allows for the comparison of dark energy with matter of all types. The result of these calculations, which are derived from observations of radiation that originated from only 380,000 after the big bang, are as follows: first of all, dark energy makes up for about three times the energy density of matter in our universe, and secondly, Ω = 1.02±0.02. In other words, not only is dark energy the dominant force in our universe, but it will be for the foreseeable future. This is consistent with the properties of both dark energy and matter, and the fact that the universe is already expanding. The law of conservation of mass states that matter can’t be destroyed. In other words, if the universe is expanding without new matter coming into existence (which it won’t), the energy density due to matter will decrease. Since Ωλ, or energy density due to dark energy, is spread evenly throughout the universe, it will only increase as the universe expands. The universe isn’t just expanding rapidly, its rate of expansion will increase with time.

So what does this mean for the future of the universe? Basically, it will keep expanding forever. In the distant future, the universe will be a very dark, cold place. Unless some unforeseen change occurs or major fundamental discovery about the nature of universe is made, matter will ultimately become very sparse.