The Cosmology Large Angular Scale Surveyor (CLASS) project aims to make a unique measurement of the Cosmic Microwave Background (CMB) that will transform our understanding of the universe and fundamental physics with far-reaching impacts on the scientific community, the next generation of scientists, and the public. As electromagnetic radiation, the CMB has both an intensity and a polarization, and it is the polarization of the CMB that CLASS will map over 70% of the sky.

The CLASS telescopes are unique. We have developed new technologies that make CLASS stable and sensitive at a level necessary to achieve our science goals. The CLASS measurement strategy – mapping 70% of the sky with cross-checks against systematic errors from a premier site in the Atacama Desert of northern Chile – is also one-of-a-kind.

Key outcomes of CLASS are:

  • Characterizing the origin of the universe. Inflation theory posits that the universe grew exponentially from quantum fluctuations to astronomical scales producing gravitational waves. The primarily goal of CLASS is to detect and characterize the polarization pattern (“B-modes”) expected to be imprinted on the CMB by the inflationary gravitational waves. CLASS is unique in its goal to measure B-modes over the full range of angular scales affected by these primordial waves and also over a range of wavelengths (1.4–7 mm) that allows CLASS to distinguish the primordial B-modes from local polarized emission from our galaxy.
  • Pinpointing cosmic dawn. When the universe was less than a tenth of its current age, stars in the first galaxies formed. They emitted light that ionized the intergalactic gas during an epoch called reionization. This ionized gas adds further polarization to the CMB. By measuring this polarization, CLASS will determine when galaxies first lit up.
  • Clarifying the Big Bang Theory.
    In the popular imagination, the Big Bang Theory describes an event at the beginning of the universe (sometimes literally imagined as the “bang” of an explosion) from which the universe is born. In physics, the Big Bang Theory describes not the beginning of the universe (much less a “bang”) but rather the evolution of the universe. During the first half of the twentieth century Edwin Hubble and other scientists demonstrated that the universe is expanding. In its basic essence, the Big Bang Theory is the theory of the expanding universe.But what do we mean by “the expanding universe”? According to the Big Bang Theory, a given volume in the universe is literally growing in size. The distances between objects in the universe that are not bound to each other grow with time. Correspondingly, the matter (e.g., atoms), light and other “stuff” inside that volume are becoming less dense with time. Therefore, the density of universe was higher in the past.

    The heat in the universe also grows more diffuse with time causing an ever decreasing temperature. Long ago the universe was very hot — so hot that it glowed with high energy radiation. After much expansion this glow is measured today as microwaves called the Cosmic Microwave Background (CMB) and its existence was a prediction of the Big Bang Theory. Thus the announcement of the discovery of the CMB in 1965 was a pivotal triumph of the Big Bang Theory. The figure below illustrates the history of the expanding universe. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite measured the age of the universe to be 13.77 billion years. The CMB originates just 375,000 years after the beginning.

    But what happened at “the beginning”? We don’t know, but an idea called “Inflation” may answer this question. The Big Bang Theory successfully describes the evolution and cooling of the universe over billions of years. Within the Big Bang evolution framework tiny initial “primordial” density fluctuations grew under gravity to become the structure of the universe (e.g., galaxies and their stars) we see today. But the Big Bang Theory does not explain the origin of these primordial fluctuations — it does not explain the beginning. Inflation theory completes the picture by describing how the initial state of the universe was established by a rapid expansion at the very beginning stretching random submicroscopic quantum fluctuations into primordial density fluctuations — the seeds of cosmic structure. To quote physicist Brian Greene, “Galaxies are nothing but quantum mechanics writ large across the sky.”
    The Big Bang Theory describes the expansion history of the universe, here illustrated, with a timeline from left to right, by an expanding volume filled with matter and other forms of energy. At the far left, inflation theory describes a rapid expansion of space at the beginning of the universe during which microscopic quantum fluctuations in density and the fabric of space-time grew to macroscopic cosmological size. From these primordial cosmological fluctuations (at the left of the figure) grew galaxies and other cosmic structures (at the right). After Inflation slows, the Big Bang Theory expansion takes over. (Image from the WMAP website)

  • Constraining the mass of neutrinos. Neutrinos are among the lightest and most elusive particles in nature. Like the CMB, a cosmic neutrino background pervades space as a relic of the early universe and affects the growth of cosmic large scale structure (LSS). The CLASS result on reionization together with CMB intensity data constrain the primordial density fluctuations from which the LSS grew. Comparing these primordial seeds of LSS to measurements of the LSS itself provides powerful insights into the neutrino (specifically its mass).
  • Searching for new particles and interactions in ancient light. The CMB travels to us over 13.77 billion years from the edge of the observable universe. During this long journey, subtle processes, previously undetected, can alter the CMB polarization in measurable ways. Grand Unified Theories predict the generation of new forms of energy (quantum fields) in the early universe that could interact with the CMB (e.g., by a Chern-Simons mechanism) to rotate its linear polarization, an effect that CLASS is well suited to measure. In this way CLASS searches for new fundamental interactions and pushes the boundaries of physical theory.
  • Probing cosmic anomalies and new physics. Measurements of the CMB intensity have led to claims of cosmic anomalies at large angular scales. Whether these claims result from statistical flukes or point the way to new physics beyond current theories can be determined by checking whether the purported intensity anomalies have associated polarization anomalies at large angular scales. The CLASS polarization measurement is uniquely positioned to answer this exciting question.
  • Understanding our Milky Way. The CLASS survey will also improve our understanding of our Milky Way The proposed survey images 70% of the sky, an area including much of the Milky Way and the nearby Magellanic Clouds.
  • Providing legacy data. The CLASS data will be made publicly available. Because it covers nearly the entire sky, the CLASS data will combine with other astronomical surveys to extend the scientific impact beyond our project’s substantial intrinsic benefits.
  • Training young scientists. CLASS provides a rigorous research training ground for undergraduates and PhD candidates, and postdoctoral researchers develop leadership skills to start their own research groups. By fostering these young investigators, the proposed research is not only creating ground-breaking science, it is creating the next trail-blazing scientists.
  • Communicating with you. We engage the global public via on-line news releases, videos and regular website highlights. We also focus on local STEM development through presentations and interactions with students, teachers and the general public in Baltimore and other cities.


Further information about the science and technology of CLASS can be found under publications. In particular, general information suited to a public audience may be found on our Wikipedia page and in news articles by the Baltimore Sun and The Johns Hopkins News Network. An overview of the project more suited to professional scientists may be found in the latest CLASS summary paper.