Phys 1160 - Concept of Dark Energy & Idea of Dark Matter

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1. Introduction

Everyone is impressed by the beauty of the vast universe but nobody can answer this simply question - 'What is the universe made of?'. However, through the observation and our understanding of the nature of the universe, scientists suggest that the amount of ordinary matter in the universe only makes up part of the universe. It highlights that there should be a completely invisible and undetectable type of mass and energy in order to make up the rest of the universe. Scientists give them a scientific fancy name called 'dark matter' and 'dark energy'. That is, to answer the question, it is necessary to explore the nature of both dark matter and dark energy through the observation and hypothesis.

2. Concept of Dark Energy & Idea of Dark Matter

Dark energy is described as a form of energy which could exist anywhere of space and that leads the universe to expand at an accelerating rate (Pickover 2011). The standard model of cosmology illustrates that 70% of the universe is filled by dark energy. Dark energy is thought to be related to the concept of cosmological constant and the quintessence model. The cosmological constant 'Λ' was introduced by Albert Einstein as a repulsive force to counteract the gravitational attraction force in order to achieve a static universe (Baldi 2012) whereas the quintessence model suggests that dark energy's attractiveness or repulsiveness is depended on its ratio of kinetic and potential energy (Wanjek n.d.). From this, dark energy has two different explanation in terms of dynamicity. In cosmological constant, dark energy is constant and do not change over time. Contrastingly, dark energy is dynamic and changes over time in the quintessence model. Although this ambiguous nature of dark energy allow scientists to make sense of the cosmos, it is very challenging  to understand where it comes from, as "if cosmologists could only explain where the dark energy comes from... they could claim to have uncovered a fundamental secret of the universe" (Pickover 2011, cited by Tyson,N., Goldsmith, D., 2005).

Despite most scientists accept the idea that both dark energy and dark matter are closely related to each other which shares a common origin, they symbolise two totally different ideas as Sato (2009) highlighted that their relationship is like two faces of the same coin. Dark matter remains a mysterious object in our universe. It cannot be observed directly even by the latest telescopic technology due to the fact that it does not emit and reflect any form of electromagnetic radiation, including visible light. Another nature of dark matter is that it does not interact with baryonic matter which refers to all normal atomic matters. By this definition of dark matter, it is not antimatter which is composed of antiparticles with same mass as ordinary particle but have opposite charge properties (Pickover 2011). Furthermore, large galaxy-sized black holes can be ruled out as dark matter (NASA n.d.). According to the standard model of cosmology, the universe contains 25% dark matter (National Geography n.d.). Although dark matter cannot be easily detected, it is proposed to explain observed motions in the universe in the most simplest way. For example, scientists advocate the strong relationship between dark matter and its gravitational effects on visible objects which is inferred from the big error of the measurement of orbiting speeds of stars when it comes to theoretic prediction and experimental observation (Pang & Wong 2010).