Seeking the Fundamentals of the Universe: Cosmological Physics with LSST

The goal of LSST observations is to deepen our understanding of the Universe: its components, its origins, its fundamental nature and evolution, and the framework on which all these rest. Observations with a survey as comprehensive as LSST, one that samples an enormous volume of the Universe with billions of galaxies, can produce unsurpassed measurements and evidence to test our underlying theories and hone our cosmological framework, a framework that has undergone significant changes in the last two decades.

By the 1960s, when the cosmic microwave background (CMB) was discovered, the Big Bang universe model had gradually emerged as the winner among the many cosmological models proposed by scientists. Later on, inflation was proposed to address several problems of the Big Bang theory. However they realized that even if they counted the invisible dark matter, the total mass density of the Universe was insufficient to explain its observed flatness and isotropy. In the early 1990s, statistics of galaxy spatial distribution also showed preference for a low-density Universe, posing a challenge to inflation theory. The accelerated cosmic expansion revealed by Type Ia supernovae data in the late 1990s presented yet another challenge to the then accepted cosmological model, which could only produce deceleration of the expansion. Today, cosmologists continue to find the acceleration of cosmic expansion perhaps the most perplexing puzzle ever encountered.

The puzzle of accelerating expansion has prompted scientists to pursue innovative associations of data to understand what is going on. Combining data from Type Ia supernovae observations with the examination of large-scale structure and CMB confirms that the Universe is expanding at a greater rate over time. These observations have led scientists to propose dark energy as the phenomenon that reconciles the measured geometry of space with the total amount of matter in the Universe. Today, scientists recognize that fully 96% of mass and energy is dark, and dark energy is propelling the expansion at an ever-increasing rate.

What is dark energy, how does it create acceleration of the expansion of the Universe, and how does it affect the final fate of the Universe? In 2005, NSF-NASA-DOE created the Dark Energy Task Force (DETF) to advise on future research of this “new” phenomenon. DETF stated “nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration.” LSST’s survey data will produce significantly larger and more uniform data on the powerful probes of the Universe to investigate the cosmic acceleration and the ultimate fate of the Universe.

Probes of geometry and growth of structure, such as weak lensing (LSST E-News, Volume 4, Number 4, April 2012), baryon acoustic oscillations (LSST E-News, Volume 4, Number 3, October 2011), Type Ia supernovae (LSST E-News, Volume 4 Number 1, April 2011), and cluster counts, will provide multiple precision observations to provide cross-checks of results and methods. Because different cosmic probes are subject to different systematic effects, researchers can compare the measurements and reduce systematic errors. Scientists will be able to achieve more robust and tighter constraints on cosmological parameters.

Several investigations combining LSST data sets and non-LSST data sets will bring new insight into cosmological questions: constraining dark energy properties, determining neutrino mass, testing gravity, large-scale measurements, and the like.

A joint analysis of weak lensing and baryon acoustic oscillations (BAO) dramatically improves the constraints on the dark energy equation of state, which relates dark energy’s pressure to its density. Weak lensing extracts cosmological knowledge from shear, the distribution of minute distortions of the background galaxies caused by foreground mass. Weak lensing measures all matter, both luminous and dark. The shear statistics reflect the clustering of dark matter, although teasing the results from data requires tight control of various systematic effects. BAO studies use a characteristic scale in galaxies’ special distribution, a “cosmic ruler,” to translate angles into distances and so determine the expansion history versus cosmic epoch.

Constraining neutrino mass helps better define the model of the Universe. Joining the LSST survey with CMB observations can offer constraints on neutrino properties complementary to parameters determined from particle physics experiments. Constraints on the masses could improve by a factor of two using complementary data sets such as the measurement of expansion from Type Ia supernovae and BAO to determine other cosmological parameters.

Scientists ask, “Is dark energy a new dilute type of mass-energy with negative pressure or a new, beyond-Einstein form of gravity?” To answer such questions, one also needs to utilize multiple probes to examine the consistency between the expansion history, the growth rate of fluctuations in mass density, and the mass density-light bending relationship within the framework of each gravity theory. All four major LSST dark energy probes – weak lensing, BAO, Type Ia supernovae, and cluster counts – probe the expansion history. Weak lensing and cluster counts probe all three areas.

LSST’s 18,000 square degree (deg²) wide and deep coverage of billions of galaxies has the power to test differences in characteristics across various directions in the Universe. Weak lensing, BAO, and other observables can be measured in patches of sky across the whole survey area. Scientists will be able to address specific questions about fluctuations of cosmological quantities (e.g., distances) over different directions from these observations. Although scientists don’t yet have any plausible models of what dark energy is or how it works, LSST’s wide survey area is particularly suited to measure dark energy properties in many patches and to detect variations in the dark energy equation-of-state parameters to several percent level (depending on the patch size, equation-of-state parameterization, and other factors) across the sky to improve our understanding of the phenomenon.

The LSST survey will allow rigorous testing of the cosmological framework, precise measurements of cosmological parameters including the dark energy equation of state, and comprehensive investigation of the cosmic acceleration. LSST’s utilization of a diversity of cosmic probes will provide unique opportunities to answer fundamental questions of the Universe.

Article By Anna H. Spitz, Hu Zhan and J. Anthony Tyson.