Just after the Big Bang birth of the Universe almost 14 billion years ago, there was a mysterious era without stars; without light. The primordial Universe was a featureless swath of incredible, unimaginable darkness. This long ago and faraway era is termed the Cosmic Dark Ages and it came to a brilliant end when the first stars were born to blast away the darkness with their fires. In November 2014, a team of astronomers using the Subaru Telescope’s Suprime-Cam to perform the Subaru Ultra-Deep Survey announced that they have looked back in time more than 13 billion years to find 7 very ancient galaxies that made their glittering debut quite suddenly within 700 million years after the Big Bang.
The team of astronomers was led by graduate student Akira Konno of the University of Tokyo’s Institute for Cosmic Ray Research (ICRR) and Dr. Masami Ouchi, an associate professor at the ICRR. The astronomers were on the hunt for a special kind of galaxy termed a Lyman-Alpha emitter (LAE), in order to understand the role such galaxies may have played in an ancient event called cosmic reionization. LAE galaxies are lit up by strong hydrogen excitation (called Lyman-alpha emission). The team’s discovery of these ancient LAE’s at the distance of 13.1 billion light-years indicates that LAE galaxies appeared very suddenly in the early Universe. Japnudes
Young galaxies in the ancient Universe can often be discovered by the prominent emission of Lyman-alpha photons. The Lyman-alpha line represents the 2-1 transition of neutral hydrogen. Galaxies that are experiencing ongoing, very active blasts of star-birth display strong Lyman alpha emission lines. This is because they host massive, hot, and youthful stars, and these searing-hot, fiery stars hurl out copious quantities of ultraviolet radiation, which ionizes the neutral hydrogen, breaking it up into a free proton and a free electron. These particles later recombine to create neutral hydrogen again. However, this hydrogen is in an excited state when formed, and as it relaxes back to the ground state, it emits a series of line photons. Most of the time, this series comes to an end with the emission of a Lyman-alpha photon.
Imagine how, in the most ancient epochs of the Universe’s existence, it overflowed with a searing-hot “soup” of charged protons and electrons. As the baby Universe expanded, its temperature decreased uniformly. When the Universe was about 400,000 years old, it was finally cool enough for protons and electrons to join together to form neutral hydrogen atoms. That great event in the Universe’s past is termed recombination–and it resulted in a Universe filled with a “fog” of neutral atoms. As time passed, the first generation of brilliant stars and galaxies began to emerge, and their ultraviolet light ionized (energized) the hydrogen atoms, ripping them apart into their component protons and electrons again.
The Dark Ages
At the instant of our Universe’s birth there was a fierce burst of wonderful, brilliant light. Photons (particles of light) of high-energy radiation were hurled out by the extremely hot matter of the ancient Universe. But at that long ago time, light could not travel freely because at the searing-hot temperatures of the ancient Universe, matter was ionized, and any atoms that had somehow managed to form were speedily torn apart soon after their birth. This is because the positively charged atomic nuclei could not keep a grip on their surrounding clouds of negatively charged electrons. Particles that possess an electrical charge are constantly absorbing and emitting photons. For the first 400,000 years of our Universe’s existence, light was perpetually being absorbed, then emitted, then absorbed, and then emitted again–in a cycle that went on for a much longer time than human civilization has existed on Earth. In fact, this cycle continued for literally hundreds of thousands of years, and only ended when the temperature of the Universe finally tumbled down to five thousand degrees Fahrenheit.
For the first several hundred thousand years of the Universe’s existence it glared with a fierce fire that was much more brilliant than that of our Sun. When atoms could finally form and survive during the era recombination, matter and light could finally separate and happily go their separate ways. The dancing light has been shining its way through the Universe ever since.
Our Universe today is transparent, expanding, and cooling off. But just before the era of recombination, the entire baby Cosmos looked very much like the surface of our Sun. It was opaque, ferociously hot, and suffused with a dazzling, brilliant, imprisoned light. The ancient Universe was smaller than what we observe today. The galaxies formed after the era of recombination.
Now, with your mind’s eye, envision that very ancient, mysterious era when there were no sparkling stars to light up the Cosmos with their wonderful fires. Likewise, no galaxies whirled around like immense, majestic cosmic, starlit pin-wheels in Space to brighten a very dark, murky expanse. The Cosmic Dark Ages began only a few hundred thousand years after the Big Bang. At this time, the relic radiation of the Big Bang had faded, and atomic nuclei had finally managed to merge to create neutral hydrogen. Neutral hydrogen atoms absorb radiation. The Cosmic Dark Ages lasted for about half a billion years, and it remains cloaked in ancient mystery. At the start of this early era, the very first atoms of hydrogen formed. By the time this era had ended, the very first light-emitting objects had started to blast their furious, burning light through space to shatter the bewitching blackness. However, all was not peaceful during this dark era. Matter was distributed smoothly throughout the baby Cosmos when it first formed–but by the end of the Dark Ages, it had somehow managed to clump together to create very massive large-scale structures.
Buried within the clumps of matter that possessed higher-than-average densities, some pockets formed clouds that started to bud off and then collapse. Those collapsing clouds served as the ancient nurseries for the very first generation of brilliant stars. The first generation of stars cast their raging fires through the dark Cosmos–and lit it up. Like the glittering rays of our Star at sunrise, that fabulous light, coming from the first generation of stars, soared through the darkness. The fabulous, sparkling fires from these neonatal stars caused the opaque gas of the primordial Universe to become transparent. The alteration from an opaque, foggy darkness to a transparent star-screaming Universe took hundreds of millions of years. But, ultimately, the most ancient stars burned away the darkness of the cosmic fog. During this transition, foggy and opaque regions of the Cosmos were interspersed with pockets of light and recently ionized, transparent gas.
A Sudden Appearance Of Galaxies In The Ancient Cosmos!
In order to study this ancient era of cosmic reionization, the Subaru scientists went on the hunt for early LAE galaxies at a distance of 13.1 billion light-years. Even though the Hubble Space Telescope (HST) has discovered even more remote LAE galaxies, the discovery of seven such galaxies at 13.1 billion light-years represents a distance milestone for the Subaru Telescope.