Reionization

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 comprised largely of 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, the predominant dark matter began to collapse into halo-like structures through its own gravitational attraction. Ordinary matter was also pulled into these halos, eventually forming the first stars and galaxies, which, in turn, released large amounts of ultraviolet light.  That light was energetic enough to strip the electrons out of the surrounding neutral matter, a process known as cosmic reionization.

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 structure of our universe. It also gives scientists a glimpse into the so-called dark ages themselves, a period sandwiched between 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, as well as 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 – the afterglow of the Big Bang and a powerful cosmic backlight to illuminate the universe’s deep past.

Click the images below to see a larger version

Timeline showing where reionization falls in the grand scheme of things, marking the transition from a dark and neutral universe to an ionized one, populated with stars and galaxies. Credit: NASA/WMAP Science Team.

Still images from a simulation of reionization, created by researchers at KIPAC. Small ionized “bubbles,” represented by blue regions, develop around denser regions of dark matter and eventually grow to ionize the entire universe, filling the whole box, which is 1 Gpc (3.26 billion light years) on a side. Yellow dots in the last image mark some of the present-day dark matter halos, where galaxies are believed to reside. Credit: Marcelo Alvarez, Tom Abel, Ralf Kaehler