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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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The Universe Then and Now

The early universe was a very different place from what we see today. Matter was packed together at temperatures of tens of millions of degrees, hotter than a nuclear explosion, or the interior of a star. Everywhere in the universe was the same: there were no separate galaxies, stars, or planets, just a featureless expanse of white-hot plasma. As the universe expanded and cooled, matter began to clump together, pulled in by the force of gravity. Bigger clumps had a larger gravitational pull, and became even more efficient at pulling in even more "stuff". Eventually, these clumps of matter grew large enough to form the familiar stars and galaxies that we observe today.

Universe then and now

The oval image at the upper left shows the distribution of matter and energy in the Universe shortly after the Big Bang, as measured by the WMAP satellite. The blue regions represent places with a tiny (few parts per million) excess of matter. These "seeds" would later grow to form the star and galaxies visible today (lower right).

How did these first stars and galaxies form? Did stars form first and later cluster together to form galaxies, or did great sheets of dust and gas form first and later fragment into individual stars? Images of the cosmic microwave background show us how matter and energy were distributed shortly after the Big Bang. We can compare these initial conditions to views of the modern universe to try to piece together how this complex process took place.

ARCADE measures the slight additional heating of the universe by the first stars and galaxies to form after the Big Bang. This heating -- from stars so far away that they can not be seen as individual objects -- will help determine how the infant universe began the transition to the familiar universe we see today.