• Sean Carroll
  • The cosmological constant problem is a significant barrier and hints at merging quantum field theory and general relativity. It remains unsolved.
  • There is little reason to accept an absolute zero point energy scale in quantum field theory. Semiclassical treatment of gravity would include a vacuum energy contribution to the total stress-energy.
  • In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy density (the small matter of the cosmological constant) and the significant theoretical value of zero-point energy suggested by quantum field theory.
  • The simplest explanation for dark energy is the intrinsic, fundamental energy of space.
  • Galaxies seem to be heavier than all the visible matter inside them. We know this because we have observed the stars that orbit them and measured the speed of these stars. Using Newton’s law of gravity, we can calculate how much mass the galaxy must have to support these orbits, and we find about five times as much as the mass of all the stars (and other ordinary matter) together. Of course, it is hard to know precisely how much visible matter exists, but whenever we try to estimate this, we come up short of what’s needed to explain the speed of the orbits. Therefore, there must be more mass than what we can see, and the extra mass is called dark matter. Evidence for dark matter has been piling up for a long time, starting probably in 1933 with the Swiss astronomer Fritz Zwicky, working at Caltech.
  • NASA - Dark Energy Changes the Universe
  • The cosmological constant is the dark energy of the form Pdarkenergyρdarkenergy=−1, and its energy density remains constant as the Universe expands, though the dark energy of the form where the energy density increases and of the state where the energy density decreases are both allowed by the above.
  • The main attraction of the cosmological constant term is that it significantly improves the agreement between theory and observation. The most spectacular example of this is the recent effort to measure how much the expansion of the Universe has changed in the last few billion years.
  • Dark energy is one of the great mysteries of cosmology. It is now thought to make up 68% of everything in the Universe. 
  • Maybe you have seen those sprays of isolating foam used in buildings. For example, you spray foam into the space between two surfaces to isolate from heat and cold transfers. When you trigger the spray gun, that foam expands and starts occupying any space around it. If you want to stop the foam from developing, apply a force, such as containing it within a fixed volume, pushing a plate against the expanding foam, etc. Space is not static; it has a natural tendency to expand.
  • Interesting that you’re asking this question today. I just read an article that maybe dark energy doesn’t exist.

    The idea of dark energy comes from the fact that if the only force acting at large distances would be gravitation, the expansion of the Universe could not be accelerated; it would just tend toward a constant growth (if it wouldn’t be a contraction) as gravity, as we all know, is an attractive force.

    The fundamental research that produced this result is based on Type Ia supernovae observed in distant galaxies. These supernovae have constant luminosity (energy produced per second) wherever they occur, and the scientists involved were looking for a specific pattern of luminosity for the events observed. This pattern would be different, depending on the fact that the Universe would be in an accelerated, non-accelerated, or decelerated expansion. But what they found was a sped-up expansion. The pattern they found matched the sped-up expansion, and the sped-up expansion demanded the existence of dark energy.

    You noticed the requirement of constant luminosity for these supernovae invoked in this research. Recently, another team found that the luminosity of these supernovae could, in fact, not be consistent. Still, it would depend on some properties of stellar populations that would be different in distant galaxies. Now, when you plug in this variability, the result is that the expansion is no longer sped up but uniform, so there is no need for dark energy.