Until about 400,000 years after the Big Bang, the Universe was mostly full of electrons and protons, zipping in random directions. It was only when the Universe cooled down enough, because of expansion, that electrons and protons had a chance to combine to form neutral hydrogen (the lightest element in the Universe) for the first time. This epoch is known as the epoch of recombination. The Universe then enters and remains in what we call the Dark Ages until the formation of the first luminous sources -- first stars, first galaxies, quasars, and so on. During this period, the Universe was full of neutral hydrogen, and thus completely opaque to any ultra-violet (UV) radiation because neutral hydrogen is very efficient at absorbing UV radiation. Intense UV ionizing photons from the first stars and first galaxies then start to ionize their surrounding, forming ionized bubbles. These bubbles grow with time, and eventually the entire Universe was filled with ionized bubbles. The epoch during which this change of phase or transition occurred i.e., the ionization of most of the neutral hydrogen to ionized hydrogen -- is called the epoch of reionization (see Figure below). This was the last major transition in the history of the Universe, and had a significant impact on the large scale structure of the Universe. Therefore, this is one of the frontier research areas in modern observational cosmology.
|Time line history of the Universe from Big Bang (left) to the present day Universe (right). Before the process of reionization, the Universe was completely filled with neutral hydrogen. It is only after the formation of first sources including first stars, first galaxies, that the neutral hydrogen in the Universe started ionizing, and by about one billion years after the Big Bang, most of the neutral hydrogen in the Universe was vaporized marking the end of the epoch of reionization (Image credit: NASA, ESA, A. Fields (STScI).|
Probing the Epoch of Reionization
One of the most powerful and practical tools to probe the epoch of reionization is the Lyman-alpha emission test. Lyman-alpha photons are a n=2 to n=1 transition in neutral hydrogen which emits a photon with a wavelength of lambda=1215.67 Angstroms. In the presence of neutral hydrogen, Lyman-alpha photons are scattered again and again and eventually many of the Lyman-alpha photons are scattered away form our line of sight . As a result, we expect to see fewer and fewer galaxies with Lyman-alpha emission as we probe higher and higher redshifts (closer to the Big Bang).
To study the epoch of reionization, we did exactly this using a large sample of very distant (high-redshift) galaxy candidates selected from the Hubble Space Telescope (HST) CANDELS survey -- the largest galaxy survey ever undertaken using HST. To know the exact distance of a galaxy, it is critical to obtain spectroscopic observations of these galaxies. We did this using a near-infrared spectrograph, MOSFIRE, on the Keck Telescope located at 13,000 ft on top of Mauna Kea, a dormant-volcano mountain in Hawaii.
To our surprise, we discovered that most of the galaxies we observed did not show Lyman-alpha emission. The figure below shows our results combined with previous studies. This figure shows the Lyman-alpha equivalent width, the ratio of strength of Lyman-alpha emission from a galaxy to its underlying blue stellar light continuum (non Lyman-alpha light), as a function of redshift (or age of the Universe on the top axis), as we probe closer and closer to the Big Bang. As can be seen, there are fewer galaxies, and at the same time the strength of Lyman-alpha emission also decreases as we go to higher redshifts. While this can be a result of a few different things, upon careful inspection, we think that this is likely because of the Universe becoming more neutral as we go beyond redshift ~7, and we are witnessing the epoch of reionization in-progress.
|This Figure shows the evolution of strength of Lyman-alpha emission in galaxies, as we get closer and closer to the Big Bang. As can be seen, the strength of Lyman-alpha emission appears to be decreasing or in other words we are missing vetry strong Lyman-alpha emitting galaxies as we go towards higher redshifts. This is likely a consequence of increasing neutral hydrogen, as expected from theoretical studies (Image credit: Tilvi et al 2014).|
Currently, Lyman-alpha emission provides the best tool to discover and confirm very distant galaxies. While there are a few other emission lines that could be used to confirm distance to a galaxy, their strengths compared to the Lyman-alpha emission is much weaker. Despite this, we have made quite a significant progress in understanding the first billion years of the Universe.
The figure below shows the summary of progress astronomers have made over the past few years, understanding the transition of Universe from a completely neutral to an ionized phase. Below redshift of about 6, that is about 1 billion years after the Big Bang, the Universe is almost completely full of ionized hydrogen—only one part in 10,000 is neutral. At redshifts greater than 6, the Universe becomes more and more neutral. The James Webb Space Telescope (JWST) will be very instrumental in discovering galaxies within the first 600 Myrs, and will help us gain even more insight into the details of the crucial epoch.
|This figure shows the evolution of neutral hydrogen fraction as a function of redshift (or age of the Universe shown on top axis). Only one part in 10,000 is neutral below redshift of about 6 which implies that the Universe is mostly ionized and the process of reionization has occurred at redshifts greater than six, where the Universe is becoming increasingly neutral (Image credit: V. Tilvi).|