Introduction:

Our universe is subjected to greater impacts by unknown forms of energies known as Dark Energy. The study of supernovae gave us an idea about this phenomenon, which also suggested that the universe’s expansion is not constant but exponential or accelerating. Although previously it was assumed that all energies contributed to the universe’s expansion, the studies in the later years proved otherwise. The undiscovered energies were eventually termed dark energies. According to the Lambda CDM model, Dark energy corresponds to 68% of all energy present in the studied universe. Dark energy is further classified into two forms that are, 

  • Cosmological Constant
  • Scalar fields. 

Scalar fields account for a constant considerable input of energy, so they are classified under the Cosmological constants. 

History

Einstein’s field equation of general relativity is the inclusion of Cosmological constant in it. Einstein proposed the idea of a cosmological constant to be able to solve the Gravitational field equation. The cosmological constant is denoted by λ\ lambda, and it needs a condition where “negative masses are gravitated by empty spaces”. However, this mechanism which led to a static universe was later found to be not stable. Einstein acknowledged this as a blunder since he failed in explaining the expansion of the universe. 

Later in 1980, Alan and Alexei proposed that a negative pressure field could have caused cosmic inflation in the early universe. This model worked in explaining the Big Bang and the expansion that followed it. Ever since the inflationary model was widely accepted, the cosmological constant was considered irrelevant.

The models of inflation suggested that the density of the universe should be close to the critical density. The model’s success was questioned following a lower value for Hubble Constant in the late 1980s. The first proof of dark energy was following the supernovae observation in the year 1998. 

Characteristics of dark energy 

Unlike dark matter, dark energy is hypothetical to be understood, and many concepts remain in a state of uncertainty. Dark energy is assumed to be highly dense and homogeneous. It remains intact and is not found to interact with the fundamental forces except gravity. However less dense it is, the reason it fills much space in the universe is uniformly spread across the space of the universe. However, to explain the universal expansion, strong negative energy is supposed to be present in dark energy. The expansion effect is often termed as “Gravitational repulsion.” 

Theories of Dark energy 

The speculations of unexplained properties of Dark energy have made it the primary target of research interest. Many theories have been proposed to explain the characteristics of dark energy. The Cosmological constant concept addresses this as fundamental energy present in space. General relativity theory suggests that this energy will have effects of gravitational force. The problem with this idea is that quantum field theories suggest that such a huge cosmological constant is too large. 

The Quintessence model suggests that acceleration is caused by the potential energy of a dynamic field. The dynamic field is also known as the quintessence field. The Quintessence can vary in space and time, unlike the cosmological constant. Although there is no evidence for this model, this model nevertheless is not ruled out yet. Scientists worldwide believe that this model can be proved at the cost of violating a few principles of Einstein. The major question of why the expansion started when it did remains unsolved. Phantom energy is one special case of Quintessence where the density increases as time increases. 

The interacting dark energy theory encompasses both Dark matter and dark energy as a single entity phenomenon. They suggest that both are covariant theories from modified gravity. 

The evolution of the universe The expansion is estimated to have begun around 5 billion years ago. Dark energy dominates in an expanding universe since the density of dark matter decreases in an expanding universe. Technically when the universe doubles in volume, dark matter halves in density. In the same process, dark energy remains unchanged. For the models suggesting a continuous universal expansion, the result will be a continuous increase in the Galaxies’ velocity line outside the local group. This shall exceed the speed of light but will not be a violation of special relativity. As the Hubble parameter is decreasing, the chances that galaxies recede faster than light is higher. 

As the expansion is accelerating, many galaxies will cross the cosmological event horizon beyond which any light emitted by them will not reach us anytime in the infinite future. If dark energy remains as constant as now, the cosmological horizon is 16 billion light-years away. This means an event on the horizon will never reach us if it’s happening in the present and is 16 billion light-years away. 

Dark Energy and Dark Matter: 

By observations, we understand that space is composed of 27% of dark matter and 68% of dark energy. Analyzing Dark matter, we understand that it doesn’t exist in the form of planets, stars, etc. Also, we understand that it is not in the form of a cluster of normal matter. Eventually, this means they are not made up of Baryons. And they are not anti-matter either since they don’t produce the unique gamma rays. However, baryons can account for dark matter if they are clustered into small brown chunks. But so far, we can conclude that dark matter is not baryonic.

Conclusion 

The vastness of science never fails to amuse us. The universe, which we share with hundreds of generations, continues to amuse us with its unexplainable aspects. Continuous expansion of the universe is the major concern of science because humankind will have to adapt differently when the universe is no longer the same. The effects of natural forces, including gravity, will have different effects, unlike now. Exploring dark energy will contribute to a great difference in science as we’ll be able to explain what contributes to the change. Although the cosmological constant theory explains inflation, it fails to explain the enormous nature of energy. The supernovae observation is a great leap since it showed humankind that inflation is not constant but accelerating and will lead to different impacts. This led to a global focus on research towards dark energy. Although many theories in the recent past took us closer to understanding what dark energy is, we never really solved the puzzle of universal inflation. With the amount of technology that the world has today, maybe the near future would explain why our universe is expanding at this rate.