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What is the universe made of? How did it begin? How has it evolved over the 13.8 billion years since its origin?  And how will it end? These are the questions addressed by cosmology, the study of the universe as a whole. Research in cosmology involves astronomy, but also gravitational physics, particle physics, and challenging questions about the interpretation of phenomena we can’t see directly — such as the possible existence of something before the Big Bang.

Our Work

Center for Astrophysics | Harvard & Smithsonian cosmologists study the universe in many ways:

  • Connecting theoretical models for the very early universe — cosmic inflation or an alternative scenario — with observable effects. Whatever happened in the first split-second after the Big Bang, it occurred while the universe was opaque to light, so we have to infer its properties indirectly. Some effects might show up in the CMB, while others might be visible in the large-scale structure of the universe. In both these cases, it’s because inflation leaves an imprint on the fluctuations of mass and energy in the universe, which grow as spacetime expands.
    Scientists Are Using the Universe as a ‘Cosmological Collider

  • Looking for signs of the first stars in the universe. These were likely much more massive than the average star around today, which made them unstable. The supernova explosions of these stars might be observable even that far away, either directly or indirectly through its effects on early galaxies.
    Explosion Illuminates Invisible Galaxy in the Dark Ages

“Dark” percentage of the universe’s mass-energy content according to the standard cosmological model
  • Mapping the positions of galaxies to reconstruct the effects of dark energy. The Baryon Oscillation Spectroscopic Survey (BOSS) is an ongoing project to map baryon acoustic oscillations using tens of thousands of galaxies halfway across the universe.
    A One-Percent Measure of Galaxies Half the Universe Away

  • Tracing the structure of galaxies as produced by dark matter. According to dark matter theory, galaxies should come in a wide range of sizes, but many of those are too small to see easily. The next-generation Giant Magellan Telescope (GMT) will detect dwarf galaxies that are currently too faint to observe, providing a new realm for observing the effects of dark matter.
    Mapping Dark Matter

Starting at the Beginning

The universe began 13.8 billion years ago in the event we call the Big Bang. We know this is true by a number of different lines of evidence. The current expansion of the universe demonstrates that it was much smaller in the past, as measured by how galaxies are moving away from each other at increasingly faster rates. The cosmic microwave background is evidence that the cosmos was much hotter and denser. The relative amounts of hydrogen and helium compared to other elements tells us the cosmos wasn’t hot and dense long enough to fuse heavier elements. And so on.

Cosmologists at the Center for Astrophysics | Harvard & Smithsonian work to fill in the details of that big picture.

  • The earliest moments after the Big Bang are still mysterious. According to the cosmic inflation hypothesis, quantum interactions drove the universe to expand by a factor of 1026 — more than a trillion trillion times — in a tiny split-second after the Big Bang. Among other things, inflation explains why the universe has the same contents and temperature in every direction, and how matter on the largest scales became organized. However, a lot about inflation is still mysterious, so theorists and observers work to describe it, search for any observable traces, as well as test for alternative possibilities.

  • For the first 380,000 years or so after the Big Bang, the cosmos was filled with a hot opaque plasma. The expansion of the universe spread that plasma out until it cooled enough to form the first stable atoms, and the cosmos became transparent. Light leftover from that transition formed the cosmic microwave background (CMB), a low-energy bath of light filling the universe. Tiny variations in the CMB temperature reveal the relative amounts of the various contents of the universe, as well as revealing the density fluctuations that produced galaxies later on.

  • The first stars in the universe were probably born around 700 million years after the Big Bang. However, we don’t know for sure, since the earliest stars are too faint to see from such a great distance. Astronomers look for the earliest galaxies to find signs of those primordial stars, and study indirect clues about their nature.

  • The observable universe contains about 100 billion galaxies. These aren’t scattered randomly across the sky: gravity gathered them into a huge cosmic web, known as the large-scale structure of the universe. On an even grander scale, galaxies show traces of the sound waves known as baryon acoustic oscillations (BAO) that swept across the universe before the CMB formed. Cosmologists study the large-scale structure and BAO to measure the rate of cosmic expansion and understand how galaxies are organized on the largest scales.

  • All the atoms in the universe only make up about 5% of its total contents. The rest is dark matter and dark energy. Dark matter, which is about 27% of the contents of the universe, provides the gravitational foundation for building galaxies and galaxy clusters. The large-scale structure of the universe is produced by dark matter, but we still don’t know what it’s made of. Dark energy, making up the remaining 68% of the universe’s contents, causes the expansion of the universe to accelerate. The push-pull of dark matter and energy are what makes the universe look the way it does, so understanding exactly what these mysterious substances are and how they work is a major challenge of modern cosmology.


Artist’s impression of the history of the Universe

Artist’s impression of the history of the Universe, from the Big Bang on the left to the present day at the right. The scale is exaggerated to emphasize earlier eras of cosmic history.

Credit: NASA