One of the most EXCITING observational discoveries of the past decade is that the Universe is Accelerating!!
The source responsible for Cosmic Acceleration is presently unknown and has been called "Dark Energy". Dark Energy has large negative pressure and could account for upto 70% of the total matter density in the Universe.
The roots of Dark Energy lies way down during 1905-1915 when Albert Einstein was developing the field equations for General Relativity. The General Relativity field equation is given by:
Einstein was not satisfied with the first outcome of his field equations since the equations were denoting a contracting universe due to gravitational effects. Therefore, he put a constant in his equation which we now call the "Cosmological Constant" for compensating the effect of gravity and create a static model of the Universe. Einstein regretted adding the constant in his equation and considered this as the "Biggest Blunder" of his life.
We now use Einstein's field equation with the cosmological constant but with a different value accounting for the expansion rate of the Universe!
The Big Picture:
Dawn of Dark Energy:
The first evidences of Dark Energy were probed through observations of distant Type Ia Supernovae.
Supernovae are among the brightest objects in our Universe. Type Ia supernovae are standardized candles since their luminosity is fairly constant and known to us. Thus, they can be used to probe the Universe on the larger scales.
Distant supernovae are systematically fainter than they would appear if the universe were decelerating. Therefore, the dimming of light from distant supernovae must have cosmological origin.
The above graph shows redshift(Z) on X-axis and the luminosity distance of supernovae DL on Y-axis. If we consider the curve for dark matter, we see that the luminous distance increases slowly as we go towards higher redshifts.
However, for the curve of dark energy a rapid rise in luminosity distance is achieved at smaller redshifts. Thus, the dimming of light by a distant supernovae is caused by the total energy density of the universe including:
Baryons, Leptons, Dark Matter & Dark Energy
Indirect evidence for an Accelerating Universe also comes from it's Age.
Since the cosmological constant is considered to be a property of space-time itself, as the universe expands so does the cosmological constant. Hence, the density in cosmological constant as the universe evolves, while density in other forms of matter decreases. The cosmological constant can dominate the Universe at late times.
The presence of cosmological constant leads to an older Universe!!
In absence of the cosmological constant, the age of our universe is estimated to be about 9 billion years. But this is too short since globular clusters have ages more than 11 billion years!
These observations give concrete evidences for the presence of dark energy as an account of the cosmological constant.