If you’ve ever stood outside on a clear night, in a location far from city lights, consider yourself lucky. There are few things as majestic and awe-inspiring as a star-studded night sky, emblazoned with the Milky Way, possibly peppered now and again by the fleeting blink of a shooting star.
For millennia, people have marveled at the heavens, inspiring some to think of the stars as timeless and unchanging. Indeed, it is difficult to imagine a time when the sky looked differently. Except that it’s not true. There was a time before the first star winked into existence. Astronomers are working hard to try to see what that early universe once looked like.
The history of the universe consisted of at least three phases. There is the period following the Big Bang, when the universe was smaller and full of a hot and glowing plasma. Early on, the temperature was hot enough that even protons and neutrons couldn’t exist. But, by about three minutes after the universe began, it had cooled enough to begin to look familiar – a bath of disassociated protons, neutrons, electrons and helium nuclei.
The universe was still hot and glowing – so hot that electrons could not stick to protons. Every time they tried, the proton and electron would run into something and be ripped apart. This environment persisted until about 380,000 years after the Big Bang. By that time, the universe had cooled to a temperature of approximately 2,700ºC (4,900ºF). That’s the temperature at which protons and electrons can pair up and make electrically-neutral hydrogen atoms. Since the electrons were no longer mobile and couldn’t emit light, a momentous thing happened.
The universe went dark. Completely and totally dark. This was the true Dark Ages.
Time passed. Initially, the universe was still hot enough that it was hard for gravity to overcome the motion of the hydrogen and helium that existed at the time. But, over perhaps 100 – 150 million years or so, the universe cooled and gravity’s inexorable tug pulled together enough gas to make the first stars. Slowly and inexorably, star after star blinked into existence. And each star poured out energy in the form of light, both visible and in wavelengths we can’t see. Some of that invisible light is in the form of ultraviolet and the ultraviolet light undid what the cooling of the universe had accomplished. The growing bath of ultraviolet light was strong enough and bright enough to start breaking apart hydrogen atoms into their constituent protons and electrons. This process is called ionization.
By about 500 or 550 million years after the beginning of the cosmos, the universe’s hydrogen gas went from the neutral stage to this ionized form. This period in the history of the universe is called the reionization era.
While we know a lot about the conditions immediately following the Big Bang and, of course, the current universe, the Dark Ages are still pretty mysterious. The only handle astronomers have on it is to measure the emission of light from neutral hydrogen. The easiest way to do that is to see the radio waves emitted when the spin of the electron flips from being parallel to the spin of the proton, to being antiparallel. In that transition, radio waves are emitted with a frequency of 1.420 gigahertz or wavelength of 21 centimeters. Thus, searches for this particular radio frequency is the way to explore the behavior of neutral hydrogen.
However, it’s more complicated than that. The universe has expanded since the Dark Ages, stretching the 21 cm line to more like two meters long. So that’s what astronomers are searching for.
While there are a number of groups searching for radio waves of this wavelength (including which claims a very tentative discovery), a consortium of astronomers using the Murchison Widefield Array (MWA) have made a recent announcement of their measurements. MWA is located in western Australia and uses 256-element array of radio receivers to search for radio waves in the frequency range of 80 – 300 megahertz, corresponding to wavelengths in the range of 1.0 – 3.8 meters. This is the range of radio waves that corresponds to neutral hydrogen during the Dark Ages.
The problem is that there are many cosmic sources of radio waves in this range that are much more powerful and “louder” than the desired signal (i.e. 10,000 – 100,000 times brighter). Further, there are several terrestrial sources radiation of this nature, including digital television. Separating out these mundane sources of radiation from the desired signal from the Dark Ages is key to a successful measurement.
So, what did astronomers announce? They were successful in suppressing much of the foreground contamination, but not all. The contamination was reduced to about ten times the expected signal in the most successful part of the radio spectrum, which means they have not yet successfully seen the neutral hydrogen of the Dark Ages. However, the researchers could rule out an unexpectedly-bright source of primordial neutral hydrogen. Furthermore, the MWA is not yet operating at full capacity and this initial result used only forty hours of observation. They have already recorded far more data, and have formulated a robust plan to improve their capabilities. Future announcements will come closer to observing Dark Ages neutral hydrogen and could be successful. Meanwhile, bigger facilities, like the Square Kilometer Array and the Hydrogen Epoch of Reionization Array (HERA), are being constructed. Eventually, one of them will unambiguously see this signal from the very early universe.
And the Dark Ages won’t be dark anymore.