Context: Scientists’ predictions about how much dark energy is in the universe, based on their theories, are way off.

What is Dark Energy?

• It’s a theoretical form of energy that produces a negative and repulsive force, acting in the direction opposite to that of gravity. 
• Impact: It causes the fabric of space itself to stretch and expand. It is thought to be responsible for the accelerating expansion of the universe.
• There are four fundamental forces in our universe, and speculative theories have proposed a fifth force – something that can’t be explained by the four forces. The four forces are: 
  • Gravitational force.
  • Electromagnetic force.
  • Strong nuclear force.
  • Weak nuclear force.

What is Dark Matter?

• It is made up of particles that do not have a charge — which means they do not interact through electromagnetic interactions. 
• Characteristics: These are particles that are “dark”, namely because they do not emit light, which is an electromagnetic phenomenon, and “matter” because they possess mass like normal matter and hence interact through gravity.

Dark Energy Vs Dark Matter

• Over 95% of the universe comprises dark matter and dark energy, with visible matter making up less than 5%.

Cosmological Constant

• The cosmological constant is a term introduced by Albert Einstein into his field equations of general relativity. 
• It represents a constant energy density that fills space homogeneously, even in the absence of matter or radiation. 
  • However, the value of the cosmological constant that would be needed to explain dark energy is much smaller than what is predicted by quantum mechanics.
• Einstein initially introduced the cosmological constant to allow for a static universe, but later abandoned it when the expansion of the universe was discovered by Edwin Hubble calling it his “biggest blunder”.

What is the Cosmological Constant Problem?

• The cosmological constant is a macroscopic parameter which controls the large-scale structure of the Universe. 
• All observations to date have shown that it is very small. 
• However, our modern microscopic theory of particle physics and gravity suggests that the cosmological constant should be very large. 
• This discrepancy between theoretical expectation and empirical observation constitutes the cosmological constant problem. 

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Source: The Hindu