Spectroscopy lies at the heart of astrophysical inference. Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe. Detailed information on all of these properties can be gleaned from high-quality spectroscopic data. For distant objects, some of these properties (e.g., motions and composition) can only be measured through spectroscopy.

Ultra-violet (UV) spectroscopy provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter. The UV offers access to spectral features that provide key diagnostic information that cannot be obtained at other wavelengths.

COS will help answer some fundamental questions:

• What is the large-scale structure of matter in the Universe?
• How did galaxies form out of the intergalactic medium?
• What types of galactic halos and outflowing winds do star-forming galaxies produce?
• How were the chemical elements for life created in massive stars and supernovae?
• How do stars and planetary systems form from dust grains in molecular clouds?
• What is the composition of planetary atmospheres and comets in our Solar System (and beyond)?

A primary science objective for COS is to measure the structure and composition of the ordinary matter that is concentrated in what is known as the ‘cosmic web’: long, narrow filaments of galaxies and intergalactic gas separated by huge voids. The cosmic web is shaped by the gravity of the mysterious, underlying cold dark matter, while ordinary matter serves as a luminous tracery of the filaments. COS will use scores of faint distant quasars as ‘cosmic flashlights,’ whose beams of light have passed through the cosmic web. Absorption of this light by material in the web will reveal the characteristic spectral fingerprints of that material. This will allow Hubble observers to deduce its composition and its specific location in space.

Observations like this, covering vast distances across space and back in time, will provide information on both the large-scale structure of the universe and the progressive changes in chemical composition of matter, as the universe has grown older