How does the universe work? Understanding the universe's birth and its ultimate fate are essential first steps to unveil the mechanisms of how it works. This, in turn, requires knowledge of its history, which started with the Big Bang.
Previous NASA investigations with the Cosmic Microwave Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) have measured the radiation from the universe when it was only 300, 000 years old, confirming theoretical models of its early evolution. With its improved sensitivity and resolution, ESA's Planck observatory probed the long wavelength sky to new depths during its 2-year survey, providing stringent new constraints on the physics of the first few moments of the universe. Moreover, the possible detection and investigation of the so-called B-mode polarization pattern on the Cosmic Microwave Background (CMB) impressed by gravitational waves during those initial instants will provide clues for how the large-scale structures we observe today came to be.
Observations with the Hubble Space Telescope and other observatories showed that the universe is expanding at an ever-increasing rate, implying that some day - in the very distant future - anyone looking at the night sky would see only our Galaxy and its stars. The billions of other galaxies will have receded beyond detection by these future observers. The origin of the force that is pushing the universe apart is a mystery, and astronomers refer to it simply as "dark energy". This new, unknown component, which comprises ~68% of the matter-energy content of the universe, will determine the ultimate fate of all. Determining the nature of dark energy, its possible history over cosmic time, is perhaps the most important quest of astronomy for the next decade and lies at the intersection of cosmology, astrophysics, and fundamental physics.
Knowing how the laws of physics behave at the extremes of space and time, near a black hole or a neutron star, is also an important piece of the puzzle we must obtain if we are to understand how the universe works. Current observatories operating at X-ray and gamma-ray energies, such as the Chandra X-ray Observatory, NuSTAR, Fermi Gamma-ray Space Telescope, and ESA's XMM-Newton, are producing a wealth of information on the conditions of matter near compact sources, in extreme gravity fields unattainable on Earth.