dark energy

The concept of dark energy gives explanation to an unexpected aspect of expansion of the universe: the expansion has been observed to be accelerating, even though a deceleration in the expansion would make sense due to the gravity of the entire universe (just as a rock thrown straight up decelerates on its way up according to the law of gravitation). This acceleration suggests something must be providing an outward push and dark energy is a term for whatever is doing that, presumably whatever form of energy is fueling that push. A (mathematical) term representing such dark energy can be inserted into general relativity's field equation in the manner of the cosmological constant (Λ) that Einstein originally included to model a universe basically staying as is. Such a term can be calculated that would cause the observed effects, but whether the presumed term is constant over time is of research interest. Einstein used the symbol Λ (lambda) for his cosmological constant and it is now used to indicate dark energy, implying it does indeed act according to such a constant term (in the current favored model, the Lambda-CDM model), but lambda is often used to mean any kind of dark energy (i.e., even if such a term is not constant).

The observational evidence for dark energy is the population count of supernovae seen at various redshifts, and in characteristics of the cosmic microwave background (CMB). The supernova evidence was discovered in the late 1990s and since then, the CMB's indication of a flat universe has been interpreted as evidence: the mass of the theorized dark energy (given that e=mc²) makes up the difference between the mass density necessary to make the universe flat and the mass density determined by other means (i.e., that of observed galaxies plus deduced dark matter). The concept of dark energy is very well accepted.

One theory of dark energy that does not presume a cosmological constant presumes w (the cosmological EOS parameter) is constant, w being the ratio of pressure to mass density of the universe as a whole. Equations of state (EOSs) are equations that relate such values and the cosmological equation of state is w = p/ρ (w equals pressure divided by mass density). w is derived from measurement, and a value based upon 2015 Planck data is -1. Additional alternative dark energy theories often propose some equation regarding how w (or Λ) changes with the universe's evolution.

Alternatives to the dark energy concepts include adjustments to general relativity (e.g., DGP gravity), or that the expansion is an illusion, such as might result from selection bias in the gathering of the evidence.

Dark energy is conceived as some kind of vacuum energy, i.e., energy present even in a vacuum. An obvious candidate is the zero-point energy predicted by quantum theory but the measured acceleration of the universe's expansion requires a vacuum energy sixty orders-of-magnitude (or more) smaller than this theoretical zero-point energy, a situation which has been described as a strikingly large gap between theory and measurement.

The accelerating expansion is specifically the (positive) acceleration of the rate at which the scale factor is growing. The Hubble parameter is presumed to (slowly) decrease even though this acceleration is positive: the Hubble parameter is not the rate of the scale factor's growth, but that rate divided by the scale factor at the time; such a rate (i.e., speed) divided by distance-traveled can be falling even if acceleration is positive. For example, given something (constantly) accelerating by 32 ft/s/s:

The above demonstrates the rising/falling phenomenon when acceleration is positive and constant: the last column is analogous to the Hubble parameter and falls with time even though the actual speed is increasing with time. Below is a similar example, but in which this "speed divided by distance" remains constant with time (distance D ∝ dD/dt, which implies both grow exponentially):

Any acceleration less than this type (i.e., less than exponential) will show the phenomenon of "speed divided by distance" falling while speed is rising.

The term early dark energy (EDE) has been coined for whatever is causing the Hubble tension (an inconsistency between the results of two current methods of measuring the Hubble constant). The term presumes this difference in measurement-results is due to an additional outward force in the early universe rather than our measurement error. Such early dark energy powering this presumed force might or might not be closely related to that associated with the later acceleration of the universe's expansion.