Dark Energy: The greatest puzzle of modern cosmology

 

At the turn of the last century, observations from Supernova Type Ia confirmed the fact that our universe has entered a phase of accelerated expansion. Two separate teams respectively working on the Supernova cosmology project and the high-Z supernova search team reached this result independently in 1998. They were respectively headed by Saul Perlmutter and Brian P. Schmidt & Adam Riess.  This came as a shocking discovery to the scientific community since the outcome defied our orthodox understanding of the dynamics of the universe.

It is clearly known that gravitational interaction is attractive in nature. So an expanding universe that originated from the Big Bang would tend to slow down due to gravity interacting between the objects such as planets, stars, galaxies, etc. Thus a decelerated expansion of the universe will slowly bring the universe to a halt before the contraction phase sets in. But the discovery of accelerated expansion totally defies this idea, and we are left wondering about the mechanism that is responsible for this strange phenomenon.

It was clear to the scientific community that in order to explain this phenomenon theoretically, we will have to introduce a repulsive (anti-gravitational) interaction, which will counterbalance and even dominate the attractive gravitational interaction. But this repulsion will not jump out of nowhere, and there should be some source for it. So what is needed is basically a matter field that can sustain a repulsive interaction. This matter field is exactly what we term Dark Energy which is a hypothetical matter fluid with negative pressure.

Observations suggest that our universe is basically filled with three major components, namely normal or visible matter, dark energy, and dark matter. It is really very strange that normal matter (which includes all the visible entities around us) accounts for the smallest portion of the universe, which is around 6 %. Around 26 % of the universe is considered to be dark matter and the rest around 68% is dark energy. So it is evident that the present universe is dominated by dark energy and its queer exotic properties affect the universe at the cosmological scales, thus driving the accelerated expansion.

The density of dark energy is very low, estimated at around 7 x 10^ (-30) gm per cubic cm. This is considerably low compared to that of visible matter and dark matter, but still, it plays a major role in shaping the dynamics of the universe due to its uniformity across space. From the general relativistic point of view, Einstein’s field equations become inconsistent with the observations at the cosmological scales. This basically means that general relativity in its original form cannot account for the accelerated expansion of the universe in its framework. So we need to introduce modifications to the field equations in order to address this issue. The right-hand side of Einstein’s field equations comprises the matter fields, which can be modified by introducing dark energy. Such modifications have been attempted and found to be consistent with the observations.

There are basically two broadly proposed forms of dark energy, the cosmological constant and the scalar fields such as quintessence. The cosmological constant represents the constant vacuum energy density filling the space homogeneously, whereas scalar fields involve dynamic quantities, whose energy densities vary in space and time.

Cosmological inflation is an enormous and exponential expansion that took place just after the Big Bang. Dark energy can not only explain the dynamics of the late-time accelerated expansion of the universe but can also satisfactorily account for the early-time inflationary evolution of the universe. Alan Guth and Alexei Starobinsky in 1980 proposed that a matter field with negative pressure (similar to that of dark energy) can drive the cosmic inflationary mechanism in the early universe.

In physical cosmology, dark energy is studied extensively to explain cosmological observations and derive a proper cosmological model of the universe. This will ultimately help us get a description of the large-scale structure and the dynamics of the universe. Proper knowledge of dark energy will help us answer critical questions regarding the origin, structure, evolution, and ultimate fate of the universe. As of now we are still shrouded by the darkness of dark energy and trying to find our way out of the puzzle.

By Prabir Rudra

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