The Center for Astrophysics | Harvard & Smithsonian
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Cosmic Microwave Background

For the first 380,000 years or so after the Big Bang, the entire universe was a hot soup of particles and photons, too dense for light to travel very far. However, as the cosmos expanded, it cooled and became transparent. Light from that transition could now travel freely, and we see a lot of it today. This light is called the cosmic microwave background (CMB), and it carries information about the very early universe. Astronomers use the patterns in CMB light to determine the total contents of the universe, understand the origins of galaxies, and look for signs of the very first moments after the Big Bang.

Our Work

Center for Astrophysics | Harvard & Smithsonian researchers study different aspects of the CMB:

  • Looking for signs of the rapid expansion of the universe called inflation, by measuring the polarization of CMB light. These projects use instruments such as the South Pole Telescope (SPT) and the BICEP telescopes.

  • Using the South Pole Telescope (SPT) and other observatories to measure the Sunyaev-Zel’dovich effect, which is fluctuations in the CMB produced by galaxy clusters.
    New Cosmological Insights from the South Pole Telescope

First Light

No stable atoms existed in the early universe, because collisions between particles of light called photons and matter kept knocking electrons away. Those collisions also meant the cosmos was opaque, because photons couldn’t travel far.

But about 380,000 years after the Big Bang, the expansion of the universe allowed the first stable atoms to form. That event is called “recombination”, and it made the cosmos transparent. The joining of electrons to nuclei produced a lot of light, which could now travel through the less soupy universe and even reach us, 13.8 billion years later. Light from recombination was very energetic, but it cooled off with the rest of the universe, until it reached the microwave portion of the spectrum. This light corresponds to a temperature today of 2.7 Kelvin: 2.7º C above absolute zero, or -455º F.

Today, we see this light as the cosmic microwave background. Because recombination happened everywhere in the universe, we see CMB light coming from all directions. The CMB provides the best data we have on the early universe, and the structure of the cosmos on the largest scales.

the cosmic microwave background: a map of the entire sky in microwave light

The cosmic microwave background is a snapshot of the oldest light in our universe, from when the cosmos was just 380,000 years old. The colors of the map represent small temperature fluctuations that ultimately resulted in the galaxies we see today.

Credit: ESA and the Planck Collaboration

The Baby Picture of the Universe

The CMB looks almost exactly the same, no matter what part of the sky we look at. The term for that in cosmology is “isotropic”, and the small deviations from perfect sameness are called anisotropies. Measuring the larger-sized anisotropies reveals how much dark energy, dark matter, and ordinary matter are contained in the universe.

The smaller anisotropies reveal the tiny fluctuations in density that gave rise to the pattern of galaxies and galaxy clusters we see today, which astronomers call the large-scale structure of the universe. Without those small irregularities, there wouldn’t be any galaxies, and we wouldn’t be here to observe them. Likewise, larger anisotropies wouldn’t produce the universe we see.

The overwhelming sameness of the CMB also tells us something about the early universe. Two points on the CMB on opposite sides of the sky shouldn’t have almost exactly the same temperature, since they weren’t close together at recombination. The most popular explanation for this is “inflation”: a tiny fraction of a second after the Big Bang, quantum fluctuations caused the universe to expand at an extreme rate. Points that were far apart at recombination today were neighbors before inflation, so they have nearly the same temperature.

According to theory, inflation left its mark on the CMB in the form of the twisting of light known as polarization. Astronomers use modern telescopes to look for that polarization, in hopes of seeing the behavior of the universe when it was only a fraction of a second old.

The Center for Astrophysics | Harvard & Smithsonian is home to the Kovac Lab, which has developed the BICEP program in collaboration with NASA’s Jet Propulsion Laboratory and other institutions. The various BICEP telescopes measure CMB polarization to a high degree of precision, with the goal of identifying the physical processes at work in the first instants of the cosmos. The current iteration of the project includes the BICEP3 telescope and the Keck Array, both located at the South Pole.

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Physics of the Primordial Universe

Harvard University Department of Astronomy, Institute for Theory and Computation
The Big Bang theory of cosmology successfully describes the 13.7 billion years of evolutionary history of our Universe. However, it is known that the current Big Bang theory cannot self-consistently explain its initial conditions. We are interested in finding out what caused the Big Bang, and the physics involved in this primordial epoch. We study physics, build models and propose observables for the primordial universe using quantum field theory, general relativity and/or string theory, and test them with data from astrophysical observations.
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Telescopes and Instruments

BICEP

The first moments after the Big Bang are literally hidden from us: the entire cosmos was too hot and dense for any light to pierce. However, signs of what happened then could be imprinted on the cosmic microwave background (CMB), a cold sea of light filling the modern universe. The BICEP Program is an international collaboration looking for those signs using telescopes at the South Pole. Researchers from the Center for Astrophysics | Harvard & Smithsonian play a leading role in this project, which could provide the first discovery of gravitational waves produced when the universe was just a fraction of a second old.

Visit the BICEP Website
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Giant Magellan Telescope

Is Earth unique, or are there other planets with life in the Milky Way? To answer this question and many others, astronomers need larger and more sensitive observatories than anything we currently have. For that reason, the Center for Astrophysics | Harvard & Smithsonian is collaborating with a number of other institutions around the world to create the Giant Magellan Telescope (GMT), currently under construction in Chile. The GMT will consist of seven large mirrors acting in concert as one giant telescope 80 feet across. That large size provides an unprecedented view of the sky and the ability to detect the chemical composition of exoplanet atmospheres. Like NASA’s Hubble Space Telescope, the GMT will be a powerful tool across the field of astronomy, providing insights into the formation of planets, the structure of galaxies, and the evolution of the universe itself.

Visit the GMT Website
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Magellan Telescopes

The twin Magellan Telescopes in Chile are each 6.5 meter optical telescopes. These telescopes are both equipped with instruments to take images and spectra of light from a wide variety of astronomical sources, including exoplanet systems, star-forming regions, supernova remnants, and interacting galaxies. The Magellan Telescopes — named Baade and Clay —  are hosted at the Las Campanas Observatory and are operated by a consortium of institutions, including the Center for Astrophysics | Harvard & Smithsonian, Carnegie Institution for Science, University of Arizona, the University of Michigan, and the Massachusetts Institute of Technology. In addition, CfA scientists and engineers provided a wide field f/5 focal system (including secondary mirror and corrective optics), and a powerful astronomical camera for use at the Clay telescope.

Visit the Magellan Telescopes Website
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MMT Observatory

The MMT Observatory is the premier visible-light and infrared telescope jointly managed by the Smithsonian Astrophysical Observatory, part of the Center for Astrophysics | Harvard & Smithsonian, and the University of Arizona. Located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, this 6.5-meter (21 foot) telescope is used to study objects across the field of astronomy, from the Solar System to distant galaxies. The MMT has provided a testbed for new telescope technologies developed by scientists and engineers at the CfA and the University of Arizona.

Visit the MMT Website
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South Pole Telescope, Antarctica

The South Pole Telescope (SPT) is a submillimeter observatory in Antarctica that performs measurements of the cosmic microwave background (CMB) and the dark energy driving the acceleration of the expansion of the universe. The observatory is also part of the Event Horizon Telescope (EHT), a globe-spanning multi-telescope project that captured the first image of a black hole at the center of a nearby galaxy. The SPT project is a collaboration between the University of Chicago, the University of California at Berkeley, Case Western Reserve University, the University of Illinois, and the Center for Astrophysics | Harvard & Smithsonian.

Visit the South Pole Telescope, Antarctica Website
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Spitzer Space Telescope

Infrared light is the key to studying a wide range of astronomical systems, including newborn stars and planets, dust in distant galaxies, and material swirling around black holes. The Spitzer Space Telescope was NASA’s orbiting infrared observatory, and part of NASA’s Great Observatories program, along with the Chandra X-ray Observatory and the Hubble Space Telescope. Scientists and engineers at the Center for Astrophysics | Harvard & Smithsonian led the design, testing, and construction of the Infrared Array Camera (IRAC). The Spitzer Space Telescope was launched in 2003 and retired in 2020.

Visit the Spitzer Space Telescope IRAC Page
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