Tiny Galaxies After the Big Bang Shed Light on the Early Universe

Learning what happened right after the Big Bang is important for understanding our early universe. Now, scientists have made a new, surprising discovery. They've found that the properties of the early universe were determined by the smallest galaxies.

Shortly after the Big Bang occurred, the universe was ionized; ordinary matter consisted of hydrogen with its positively charged protons stripped of their negatively charge electrons. Eventually, though, the universe cooled enough for electrons and protons to combine and form neutral hydrogen. This cool gas eventually formed the first stars in the universe.

Yet for millions of years, this gas didn't form stars, which means that astronomers can't see how the cosmos evolved during these "dark ages" using conventional telescopes. About 200 million years after the Big Bang, though, UV light from stars began to split neutral hydrogen into electrons and protons. It took another 800 million years to complete the process. Although it's certain that this particular phenomenon occurred, though, astronomers have long disagreed on what type of galaxies played the most important role in this process.

That's why the researchers created computer simulations in order to find out. In this case, the scientists discovered that the faintest and smallest galaxies in the early universe were essential. The tiny galaxies, while being 1,000 times smaller in mass and 30 times smaller in size than our own Milky Way, still contributed nearly 30 percent of the UV light during the process.

"It turns out that these dwarf galaxies did form stars, usually in one burst, around 500 million years after the Big Bang," said John Wise, the leader of the new study, in a news release. "The galaxies were small, but so plentiful that they contributed a significant fraction of UV light in the re-ionization process."

The new simulations actually provide a timeline that tracks the progress of re-ionization over hundreds of millions of years. This, in turn, tells researchers a bit more about the early universe and shows the major role that these small galaxies played.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.