Among the most interesting and mysterious concepts in modern cosmology are dark matter and dark energy. They account for a good 95% of the universe, yet scientists know little about them. These phenomena are, therefore essential in piecing together the cosmic puzzle, which will enlighten one on the structure, evolution, and the ultimate fate of the universe.
Dark matter is invisible and takes up 27% of the universe. It cannot be viewed, not directly anyway. Instead, scientists infer its presence based on how its gravity impacts on normal matter, such as stars and galaxies. For instance, consider the rotation curves for spiral galaxies. Their rotations are far more rapid than would be suggested just by observable mass. The implication of this absence of mass is that additional, hidden mass surrounds galaxies, which is giving the gravitational pull to hold them together.
There are lines of evidence for the existence of dark matter in the universe, including gravitational lensing, where massive objects bend the light from distant galaxies, and cosmic microwave background radiation, an insight into the structure of the early universe. Candidates for dark matter abound, and WIMPs (weakly interacting massive particles) represent one of the leading theories. However, despite extensive searches, dark matter remains undetected directly.
Dark energy, however, is some elusive and mystical force, comprises 68% of the universe-and that's what propels it to expand with an accelerated rate. There were observations of distant supernovae, and these said that expansion of the universe is not slowing down like it was thought to do so but rather speed up. This eventually led to a conceptual formulation of dark energy as a theoretical construct in an attempt to rationalize the phenomenon.
In fact, several models exist to describe dark energy, with perhaps the most simple being the cosmological constant, a term invented by Einstein. Other theories suggest that dark energy could instead be dynamic, changing over time. Research into what is known as dark energy and what this might mean for the universe's future is currently an active area of investigation.
While dark matter and dark energy are hostile players, the interaction of these two drives the evolution of the cosmos. Dark matter creates the skeleton within which galaxies and galaxy clusters form and attach to, while dark energy affects the larger structure and long-term fate of the universe. Their interrelation could unlock many of the cosmos's largest questions: where did it come from, how is it structured, and to what end does it exist?.
Some of these missions are intended to probe dark energy and dark matter more deeply, such as the Euclid mission designed by the European Space Agency and NASA's Wide Field Infrared Survey Telescope (WFIRST). Such missions will map the distribution of galaxies and measure the cosmic expansion history in order to give crucial data to several testing schemes for cosmological models.
In conclusion, whereas dark matter and dark energy probably are the two most central phenomena that prevail in our universe, their nature and interactions are still shrouded in much mystery. It is perhaps more than anything else a great challenge for modern cosmologists to understand nature and interactions. Further discoveries into the cosmos may only make fundamental changes in our perception of the universe with breakthroughs into dark matter and dark energy.
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