FAQ on Dark Energy

Many people assume that due to their similar names these two mysteries are connected. Currently, we do not know of any connection between the physics that may be responsible for each of the phenomena. Indeed, some researchers would prefer that dark energy be called vacuum energy or a negative equation of state (NEOS) component to avoid this naming confusion.

Let's look at them more closely:

• Dark matter concentrates around galaxies and galaxy clusters.
  Dark energy is smoothly distributed throughout the universe.

• Dark matter is pressureless, with rest mass dominating the energy density.
  Dark energy has a strongly negative pressure, of the same order as its energy density.

• Dark matter has been important to the evolution of the universe since around the epoch of matter-radiation equality, when the universe was less than 100,000 years old.
  Dark energy has been important to the evolution of the universe since the universe was about 7 billion years old.

• Dark matter interacts through gravity and presumably the weak nuclear force.
  Dark energy interacts only through gravity.

• Dark matter makes up about 20% of the total energy density in the universe today, and about 70% at the time of the last scattering of the cosmic microwave background radiation.
  Dark energy makes up about 75% now, and a negligible amount at CMB last scattering.

So other than their names, there really isn't much similarity.

Of course it is attractive to solve two mysteries with one explanation, so we would like to find a way to tie them together. Speculations involving more complicated connections exist, but in the simplest pictures dark energy and dark matter are independent.

Both the current epoch and the inflation epoch in the early universe are times of accelerating expansion of the universe. Both may be due to physical components described by scalar fields (possessing a value, but not direction at each point in space. Think of a field of tall grass: if you trample it down, then at each point you can measure the length of the stalk of grass and the direction it is lying on the ground -- that is a vector field. If instead you cut the grass raggedly without trampling it then at each point all you measure is the length of the grass -- that is a scalar field.)

But:

• The energy scale of inflation is possibly 10²⁴eV, while the energy scale of dark energy is around 10^-3eV. This is as different as the weight of a flea and that of the whole Earth! Since physicists actually talk about cosmological components in terms of the energy density, which is proportional to the fourth power of the energy scale, it's even worse -- then the ratio is 10¹⁰⁸!

• Inflation occurred when the age of the universe was perhaps 10^-35 seconds, while dark energy acceleration began when the age was about 7 billion years.

So it's a long way to go to connect one to the other.

As before though, it is an attractive idea to try to tie these two phenomena together. Speculations involving some connection exist, but in the simplest pictures dark energy and inflation are independent.

Casimir energy is a quantum energy known to exist that arises when the degrees of freedom of a field are restricted by geometry. The most well known example is that of a force arising between two parallel plates as they get very close together. Since there is more "freedom" in the electromagnetic field outside the plates than in the restricted region between the plates, the fluctuations in the field between the plates exhibit a quantum zeropoint energy with negative pressure. (Think of being bombarded from all sides by tennis balls. If another person stands near you, no balls will hit you from that side, and you likewise shield the other person. So the remaining force pushes the two of you together.) Of course negative pressure is one key property of dark energy.

But:

• The Casimir energy sometimes has positive pressure, opposite to dark energy, depending sensitively on the geometry of the boundaries.

• Casimir energy depends on physical boundaries, e.g. conducting plates, while the universe has no physical edges.

• Casimir energy, as detected, is a property of the electromagnetic field (a vector field), while dark energy involves not a vector, but possibly a scalar, field.

• Casimir force dies off rapidly with distance, while dark energy repulsion (tension) increases with distance.

So there is no clear connection of one to the other.

As before though, it is an attractive idea to try to tie these two phenomena together. Speculations involving some connection exist, but in the simplest pictures dark energy and Casimir energy are independent.

Overall, dark energy appears to be a fundamentally new phenomenon in physics, and we are working with great excitement to figure out its mysterious nature and unknown origin.