The first billion years after the big bang (out of about 13.7 billion years total since then) were among the most interesting in terms of giving birth to the kind of objects that still dominate the scene today. Mostly that means stars and galaxies, plus a few exotica such as quasars. Unfortunately, it’s very difficult for astronomers to actually see what was going on back then.
There are three reasons for this difficulty. First, astronomers can detect objects at very early times only at very large distances from us, due to the light travel time. So such objects are likely to be very dim, if detectable at all. Second, due to the expansion of the universe, light emitted by objects in the very early universe will be shifted in wavelength to much higher values. This places a large portion of the light into the infrared part of the spectrum, which is difficult or impossible to observe with ground-based telescopes.
However, the third reason that very early objects are difficult to observe is that conditions in the early universe, namely the presence of a great deal of neutral hydrogen gas in the space between galaxies, obscures light from the galaxies just like atmospheric fog. This is not only unfortunate but also ironic, since a good determination of just how much “fog” was present is one of the key pieces of information that astronomers need in order to understand what objects in the early universe were really like. Astronomers need to know how much “fog” there was in order to correct for it so the visual characteristics of early galaxies can be determined. Yet lack of understanding when and how the “fog” cleared makes this effort rather frustrating.
Research that’s just been published gives new information that helps clarify things a little. It’s based on observations of just 5 very early galaxies, which are among the earliest, most distant galaxies known. What the research is telling us is that the “fog” was clearing rapidly at the time the light we now see from these 5 galaxies was actually emitted.
Distant Galaxies Reveal The Clearing of the Cosmic Fog
An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang.
The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.
The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.
Astronomers are sure that they know what the “fog” consisted of: ordinary atomic hydrogen gas that is not ionized. A neutral (not ionized) hydrogen atom consists of an electron and a single proton. High-energy photons interact strongly with neutral hydrogen but not with ionized hydrogen. (As will be explained below.) In the early universe about 75% of the mass was in the form of hydrogen, and the rest was helium, with only a small trace of a few other elements. It’s known that this hydrogen “fog” dissipated as most hydrogen atoms became ionized. But it’s not known just when the process of ionization began or when it ended. Even less is it known exactly what caused the ionization. The new research, however, does indicate that the process was occurring rapidly at a specific point in the universe’s history.
Let’s take a closer look at the details of the process as they are currently understood.
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