First Stars and Galaxies

Roughly 400,000 years after the Big Bang, the Universe—bathing in the afterglow of radiation that we see today as the cosmic microwave background—began to enter the cosmic “dark ages,” so named because the luminous stars and galaxies we see today had yet to form. Most of the matter in the cosmos at this stage was dark matter, with the scant remaining ordinary matter comprising mostly neutral hydrogen and helium.

Over the next few hundred million years, the Universe entered a crucial turning point in its evolution, known as the Epoch of Reionization. During this period, dark matter began to collapse into roughly spherical structures called haloes through its own gravitational attraction. Ordinary matter was also pulled into these halos, eventually forming the first stars, which lit up as the first luminous objects in the Universe.

These stars were very different than stars like our Sun. They were significantly more massive, for one thing; they also burned hotter, bluer, and brighter, releasing large amounts of ultraviolet light that was energetic enough to strip the electrons out of the neutral matter in a process known as cosmic reionization.

The first stars contained only hydrogen, helium, and trace amounts of lithium—the three lightest elements—which were produced shortly after the Big Bang. The stars' much higher luminosity also made them much shorter-lived than sun-like stars, since they consumed burned through all their fuel in a much shorter time-span (millions of years vs. tens of billions of years for stars like our Sun) and exploded in violent supernovae at the end of their lives. By that time, their fusion furnaces had produced many heavier elements that the exploding stars released into their environment. This facilitated the formation of subsequent generations of stars that more closely resembled our Sun.

Though the Epoch of Reionization took place deep in the Universe’s past, it lies at the very frontier of our current cosmological observations.  The more researchers learn about this period, in fact, the more it reveals about the end of the cosmic dark ages, the first stars and galaxies, and the evolution of cosmic structure. It also gives scientists a glimpse into the so-called dark ages themselves, a period sandwiched between the release of the cosmic microwave background and reionization. Thus, by studying reionization, researchers get closer to probing the dark ages themselves.

There are numerous approaches researchers at KIPAC have embarked upon to better understand the process of reionization. Currently, KIPAC researchers are expanding their understanding through state-of-the-art computer simulations using supercomputers. These simulations illuminate the formation of the first galaxies and their role in reionizing the intergalactic medium. Researchers are also creating models that connect the Universal fossil record—stars in nearby galaxies—to these events, which took place as far back as 13 billion years ago.

The eventual challenge lies in seeing the very faint and distant light from these first stars and galaxies. The upcoming James Webb Space Telescope is expected to bring researchers closer than ever before to that goal. Another goal is to detect the even fainter radio glow from neutral hydrogen in the dark ages. To this end, KIPAC scientists are building next-generation instrumentation to probe the imprint of reionization upon the cosmic microwave background—afterglow of the Big Bang and a powerful cosmic backlight to illuminate the Universe’s deep past.