Opening a Window of Discovery on the Dynamic Universe

Understanding Dark Energy

In the last decade, cosmologists have converged on a description of the birth and development of our Universe that is very successful at explaining what we have discovered using telescopes and scientific probes.

But the description is not yet complete. To explain decades of astrophysical observations, it posits two mysterious new components: dark matter and dark energy, which are thought to comprise 96% of the combined total of mass and energy in the Universe. The existence of dark matter is confounding enough, implying as it does the existence of a totally new type of subatomic particle that does not obey the same rules our familiar protons, neutrons, and electrons do. 

Even more striking is evidence for accelerating cosmic expansion. By combining the data from Type Ia supernovae observations with the examination of the large-scale structure of the Universe and the cosmic microwave background (CMB), which is the relic light from the universe's birth, scientists have confirmed that the rate at which the Universe is expanding is increasing, as if a cosmic accelerator pedal is stuck. Something is ripping the Universe apart.

This is clear and robust evidence for new physics at the interface between quantum mechanics and gravitation. The physics that produces the observed accelerating cosmic expansion is a complete mystery. Is it "dark energy" arising from quantum fluctuations in the vacuum, or is it new gravitational physics? We have no trusted dark energy theory guide, so the LSST will employ a diversity of cosmic probes to find out:

  • Weak lensing of galaxies vs redshift, which probes both the evolution of structure over cosmic time and ratios of distances vs cosmic time, thereby setting multiple independent strong constraints on the nature of dark energy.
  • Strong lensing of galaxies, quasars and supernovae, which probes the nature of dark matter in galaxy and cluster halos, as well as the evolution of the underlying geometry.
  • Correlations of galaxies in three-dimensional space vs cosmic epoch to reveal changes in the expansion of the Universe over time, which will help isolate the influence of dark energy.
  • Counts of clusters of dark matter via weak gravitational lensing combined with the optical data are a sensitive probe of dark energy.
  • Supernovae are a useful complementary technique for probing the cosmic era when dark energy becomes dominant.
  • By simultaneously measuring mass growth and space-time curvature, LSST data can tell us whether the recent acceleration is due to dark energy or modified gravity.

In following the signs toward new fundamental physics, the LSST will focus on observation, not experimentation, which, by its very nature, carries some risk. Telescopes can have flaws, data analysis methods can have bugs, and astrophysical observations can be misinterpreted. But the LSST is being specifically designed and engineered to minimize and control for these types of systematic errors. The diverse techniques listed above will provide checks on each other, helping to discriminate fundamental physics from artifacts.

LSST’s 18,000-square-degree wide and deep coverage of billions of galaxies has the power to test differences in fundamental properties of space and time itself in different directions across in the Universe. Much of that power comes from the fact that the measurements will be obtained from the same basic set of observations, using a powerful facility that is optimized for the purpose.

Image Credit: 
NASA

Financial support for LSST comes from the National Science Foundation (NSF) through Cooperative Agreement No. 1258333, the Department of Energy (DOE) Office of Science under Contract No. DE-AC02-76SF00515, and private funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an operating center under management of the Association of Universities for Research in Astronomy (AURA).  The DOE-funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory (SLAC). 


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