Universe consists of three types of substance: normal matter, dark matter and dark energy. Normal matter is made up of atoms. These atoms together form planets, stars, human beings, gasses and every other visible objects present in this universe. Mathematically speaking the universe contains 70% is Dark energy, 25% is Dark matter and the remaining 5% is Normal matter. Quiet astonishing right? We now know what normal matter is, but the curiosity is about dark energy. What exactly is dark energy?
In astrophysics Dark energy is, unknown form of energy. It is a hypothesised as a part of space which is tending to accelerate the expansion of universe. We know the amount of dark energy present in universe as we know how it affects the expansion of universe. Other than that we know nothing about dark energy. It is complete mystery. According to scientists dark energy is nothing but property of space. Albert Einstein was the first to find out that empty space is just not empty, it has something beyond. The proposed form of dark energy is cosmological constant and quintessence. The formulation of cosmological constant can be done by equalising it to vacuum constant. Cosmologists postulated that dark energy does not interact with any fundamental forces other than the gravity. Dark energy is believed to be homogeneous and not very dense.
What does the two models cosmological constant and quintessence?
- Cosmological constant is also called as lambda. According to Einstein’s general relativity theory the dark energy is sometimes called as vacuum energy. It has a negative pressure with is same as energy density. Thus it accelerates the expansion of universe. The classical thermodynamics explains why the cosmological constant has a negative energy. As stated in laws of thermodynamics, the change in volume dV involves work done equal to energy change. It is given as –PdV. Here P is the negative energy.
- Quintessence: The quintessence field is the acceleration caused by potential energy of dynamic field. Quintessence differs with time and space. The quintessence model has no proper evidence yet. This model predicts the slower expansion of universe compared to the cosmological constant. There are quintessence models which have tracker behaviour. This behaviour tracks the radiation density. This in turn triggers quintessence to behave as dark energy.
So dark energy is present everywhere. This dark energy keeps on increasing as universe expands. Gravity is the only force which the dark energy reacts to. The gravity is a force which is stronger when bodies are near and becomes weaker as they move away. Since the universe is expanding, the gravity is decreasing.