The Universe’s First-Born Stars Had A Cold Nursery
By Judith E Braffman-Miller
Just after our Universe was born 13.8 billion years ago in the Big Bang, there was a mysterious era when it was black, and there were no stars around to cast their streaming, sparkling light into this swath of darkness. Today, when we stare up at our sky at night, we see a vast dark background lit by the star-fire emitted by its billions upon billions of brilliant stellar inhabitants. However, the way that our Universe’s first-born stars came into existence remains one of the most tantalizing mysteries haunting the dreams of astronomers. Where did the first stars come from, and when did they appear on this ancient swath of blackness–lighting up what was originally featureless and dark? In February 2018, astronomers from the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts and Arizona State University in Tempe, reported that a table-sized radio antenna in a remote region of western Australia has detected faint signals of hydrogen gas from the primordial Universe. This signal indicates the existence of the Universe’s first-born stars only about 180 million years after the Big Bang.
The primordial Universe cooled and then went dark for millions of years after its birth. Ultimately, gravity pulled matter together until stars were born and burst brilliantly into life, bringing the Cosmic Dawn that chased away the blackness of the Cosmic Dark Ages. The new-found signal marks the closest astronomers have been able to see of that ancient, brilliant moment when the first stars blasted away the Universe’s primeval, featureless night.
“Finding this minuscule signal has opened a new window on the Universe,” study lead author, Dr. Judd Bowman, commented in a March 1, 2018 Commonwealth Scientific and Industrial Research Organization (CSIRO) Press Release. Dr. Bowman is of Arizona State University. The CSIRO Observatory is in Western Australia.
Dr. Bowman has been conducting his Experiment to Detect the Global EoR (Epoch of Reionization) Signature (EDGES) for 12 years. Nine years ago, he began performing the observations from CSIRO’s Murchison Radio-astronomy Observatory (MRO), after searching for the best spot on Earth for his work.
The radio signal that Dr. Bowman and his colleagues discovered was extremely faint, arriving as it did from 13.6 billion years back in the Universe’s history.
It also fell in the region of the electromagnetic spectrum used by FM radio stations. This made the detection of this weak signal from most Earth-based sites literally impossible.
Fortunately, the MRO observatory is in a naturally “radio-quiet” area. This valuable and unique location is protected by a legislated “radio quiet” zone up to 260 kilometers across, which keeps human-made activities that manufacture interfering radio signals to an absolute minimum.
The MRO’s development was managed by Antony Schinckel, CSIRO’s Head of Square Kilometer Array (SKA) Construction and Planning.
“Finding this signal is an absolute triumph, a triumph made possible by the extreme attention to detail by Judd’s team, combined with the exceptional radio quietness of the CSIRO site,” Schinckel commented in the March 1, 2018 CSIRO Press Release.
“We worked hard to select this site for the long-term future of radio astronomy after exhaustive investigations across the country. We believe we have the gold standard in radio quietness, the best site in the world,” he added.
Schinckel further noted that “This is one of the most technically challenging radio astronomy experiments ever attempted. The lead authors include two of the best radio astronomy experimentalists in the world and they have gone to great lengths to design and calibrate their equipment in order to have convincing evidence for a real signal.”
The MRO was developed by CSIRO for its Australian Square Kilometre Array Pathfinder (ASKAP).
Dr. Robert Braun noted in the March 1, 2018 CSIRO Press Release that “(T)his is a powerful demonstration of what can be achieved with the combination of an excellent site and world-class engineering, boding well for the great discoveries that will be enabled by the SKA.” Dr. Braun is Science Director at the SKA Organisation.
The Darkness Before The Dawn
In your “mind’s eye” imagine the newborn Universe. The baby Cosmos was filled with an extremely hot “stew” of charged protons and electrons. But, as the ancient Universe expanded, its temperature plummeted uniformly. When the Universe was approximately 400,000 years old, it was finally cool enough for these charged protons and electrons to merge together to create neutral hydrogen atoms. That great era in the Universe’s history is termed recombination, and during this epoch the Universe was suffused by a strange “fog” composed of neutral atoms. As time went by, the Universe’s first-born stars and the galaxies that hosted them began to form, and their ultraviolet light ionized (energized) the hydrogen atoms. This means that the hydrogen atoms were torn apart into their component protons and electrons again.
At the instant of our Universe’s birth there was a powerful burst of brilliant light. Photons (packets of light) of high-energy radiation were blasted out by the searing-hot matter of the ancient Universe. But, during that very ancient era, light was not permitted to travel throughout the Cosmos freely. This is because at the extremely hot temperatures of the ancient Universe, the ionized atoms that had managed to form were rapidly ripped apart soon after their birth, since the positively charged atomic nuclei could not hold on to their encircling clouds of negatively charged electrons. Particles that possess an electrical charge are in a perpetual state of absorbing and then emitting electrons. For this reason, during the Universe’s first 400,000 years, light was continually being absorbed, and then emitted, over and over and over again. This cycle continued for a much longer time than human civilization has existed on our planet. Indeed, this cycle went on for literally hundreds of thousands of years, and only came to an end when the temperature of the Universe finally plummeted to five thousand degrees Fahrenheit.
For the first several hundred thousand years of the Universe’s existence, it dazzled with raging fires that glared much more brilliantly than our Sun does today. When atoms could finally congeal and survive during the era of recombination, matter and light could at last go their free and separate ways. The streaming, dancing light has been shining its lovely way through Spacetime ever since.
Today, the Universe is transparent, cooling off, and expanding towards its own death. But, just before the era of recombination, the entire newborn Cosmos looked like the surface of a star, like our Sun. It was extremely hot, opaque, and suffused with brilliant, glaring, imprisoned light. The primordial Universe was considerably smaller than it is today. The galaxies formed after the era of recombination.
Imagine that black, mysterious, primordial era before the stars were born, and there were no galaxies around to brighten up this murky expanse with the flames of their constituent stars. The Cosmic Dark Ages began only a few hundred thousand years after the Big Bang. At this time, the lingering, left-over radiation of the Big Bang itself had faded, and atomic nuclei had at last managed to congeal to create neutral hydrogen. Neutral hydrogen atoms absorb radiation. The Cosmic Dark Ages lasted for about half a billion years, and this murky ancient era remains veiled in mystery. At the beginning of this primordial epoch, the first atoms of hydrogen formed. By the time this era had ended, the very first light-emitting objects had begun to send their light streaming through space to chase the bewitching blackness away. However, this era was not peaceful. Matter was smoothly and evenly distributed throughout the baby Universe when it first formed. However, by the conclusion of the Cosmic Dark Ages, this matter had somehow clumped together to construct extremely massive large-scale structures.
Hidden deep within the clumps of matter that contained higher-than-average densities, some regions formed clouds that began to bud off and then collapse. Those collapsing primeval clouds were the ancient cradles of the Universe’s first-born stars. The first generation of stars sent their stellar fires raging through the darkness of the Universe–and lit up the entire swath of Spacetime. Like the rays of our sparkling Sun, when it rises at dawn, that lovely newborn starlight overwhelmed the universal darkness. The stellar fires of these baby stars forced the opaque gas of the primeval Universe to become transparent. The sea-change from foggy, opaque darkness to a transparent star-blasted Universe, took hundreds of millions of years. However, at last, the Universe’s first-born stars burned away this foggy universal blackness. During this lengthy transition, foggy and opaque regions of the Universe were interspersed with regions of light and recently ionized, transparent gas.
Astronomers currently think that the Universe’s first-born stars were not like the stars we see today. This is because they were born directly from primeval gases that billowed out of the Big Bang itself. These very ancient gases were mostly hydrogen and helium, and these two lightest of atomic elements are thought to have pulled themselves together to create ever tighter and tighter objects. The cores of the first stars (protostars) to inhabit the Cosmos started to light up within the mysterious cold, dark hearts of these extremely dense knots of pristine primordial hydrogen and helium–and they then collapsed under their own relentless gravitational pull. Many astronomers think that the Universe’s first-born stars were giants–compared with later generations of stars–because they did not form the same way, or from the same mix of atomic elements, as stars do now. The Universe’s first-born stars, called Population III stars, were likely “megastars”. Our Sun is a member of the most recent generation of stars, which are called Population I stars. Between the first and most recent generation of stars are, of course, Population II stars.
The extremely massive Population III stars were also glaring and brilliant, and their existence is responsible for triggering the sea-change of our Universe from what it was to what it is. These huge, brilliant stars changed the dynamics of our Universe by heating it and thus ionizing the ambient hydrogen and helium gases.
Youthful galaxies in the ancient Universe are often discovered by their prominent emission of Lyman-alpha photons. The Lyman-alpha line represents the transition of neutral hydrogen. Galaxies undergoing very powerful bouts of star-birth show strong Lyman-alpha emission lines. This is because they host searing-hot, massive baby stars, and these young stars hurl out enormous amounts of ultraviolet radiation–which ionizes the neutral hydrogen, ripping up its atoms into a free proton and a free electron. These particles later recombined 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 concludes with the emission of a Lyman-alpha photon.
Cold Nursery For First-Born Stars
The astronomers, who picked up the first signals of hydrogen gas in the primordial Universe, also determined that the gas was in a state that would have been possible only in the presence of the Universe’s first-born stars. This is because these stars–igniting for the first time in a Universe that had previously been devoid of light–emitted ultraviolet radiation that interacted with the ambient hydrogen gas. Because of this, hydrogen atoms throughout the entire Universe started to absorb background radiation–an important event that the astronomers were able to spot in the form of radio waves. These new findings provide important evidence that the first generation of stars may have switched on only about 180 million years after the Big Bang.
“This is the first real signal that stars are starting to form, and starting to affect the medium around them. What’s happening in this period is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies,” explained study co-author, Dr. Alan Rogers, in a February 28, 2018 MIT Press Release. Dr. Rogers is a scientist at MIT’s Haystack Observatory.
There are also certain indications, seen in the radio waves, that hydrogen gas is floating around within the entire ancient Universe. This means that the Universe as a whole must have been twice as cold as astronomers had estimated earlier–with a temperature of about 3 Kelvins (-454 degrees Fahrenheit). Dr. Rogers and his team are not certain why the primeval Cosmos was so much colder than expected, but some scientists have proposed that interactions with a mysterious form of matter, called dark matter, may have played some kind of role. Most of the matter in our Universe is thought to be dark matter, that is not composed of atoms like the familiar “ordinary” matter that we are used to in our world–the stuff of stars, planets, moons, trees, cats, and people, for example. Dark matter is bizarre stuff–it does not interact with light or any other form of electromagnetic radiation. This makes it invisible. Yet it is generally thought that the largest structures in the Universe are made of this strange, dark, non-atomic stuff. Even though “ordinary” atomic matter accounts for literally all of the elements listed in the familiar Periodic Table, there is much less of it than there is of the dark matter.
“These results require some changes in our current understanding of the early evolution of the Universe. It would affect cosmological models and require theoriests to put their thinking caps back on to figure out how that would happen,” commented Dr. Colin Lonsdale in the February 28, 2018 MIT Press Release. Dr. Lonsdale is director of the Haystack Observatory.
The astronomers spotted the primordial hydrogen gas using EDGES–the small ground-based radio antenna, located in western Australia. EDGES gets its funding from the National Science Foundation.
The antennas and parts of the receiver were created and constructed by Dr. Rogers and the Haystack Observatory team, along with the Arizona State University team. The scientists added an automated antenna reflection measurement system to the receiver, outfitted a control hut armed with electronics, constructed the ground plane, and conducted the field work as well for the ambitious and successful project. Australia’s CSIRO provided on site infrastructure for the EDGES project.
The EDGES instrument was originally designed to detect radio waves sent forth from the ancient Epoch of Reionization. During this primordial era, the first luminous objects, such as galaxies, quasars, and stars, were born in the Universe. Quasars are a particularly brilliant form of active galactic nuclei, inhabiting the hearts of ancient galaxies. They are thought to be the dazzling accretion disks surrounding enormous supermassive black holes that weigh-in at millions to billions of solar-masses. It is thought that every large galaxy in the Universe hosts a supermassive black hole in its hidden, hungry heart–including our own Milky Way.
During the Cosmic Dark Ages, hydrogen, the most abundant atomic element in the Universe, was virtually invisible–embodying an energy state that cannot be distinguished from the ambient cosmic microwave background (CMB) radiation–the relic radiation left from the Big Bang itself.
Astronomers think that when the Universe’s first-born stars ignited, they produced the ultraviolet radiation that caused sea-changes in the hydrogen atoms’ distribution of energy states. These dramatic alterations induced hydrogen’s solitary electron to spin in alignment or opposite to the spin of its lone proton, thus causing hydrogen (as a whole) to decouple from the CMB. This means that hydrogen gas began to either emit or absorb that radiation, at a characteristic wavelength of 21 centimeters, which is equivalent to a frequency of 1,420 megahertz. But as the Universe expanded over time, this radiation became red-shifted to lower and lower frequencies. By the time the 21-centimeter radiation managed to reach our planet at present, it landed somewhere in the range of 100 megahertz.
Dr. Rogers and his team have been using EDGES to spot hydrogen that floated around the ancient Cosmos in order to precisely determine when the first stars ignited.
“There is a great technical challenge to making this detection. Sources of noise can be a thousand times brighter than the signal they are looking for. It is like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing,” noted Dr. Peter Kurczynski in the February 28, 2018 MIT Press Release. Dr. Kurczynski is program director for Advanced Technologies and Instrumentation, in the Division of Astronomical Sciences at the NSF.
The antenna of this instrument detects radio waves from the entire sky, and the scientists originally tuned it to listen in at a frequency range of 100 to 120 megahertz. However, when they looked within this range, they initially did not detect much of any signal. The researchers then realized that theoretical models had predicted that primordial hydrogen should emit within this range if the gas was hotter than the ambient medium. But what if the gas was in fact colder? Models predict that the hydrogen should then absorb radiation more strongly in the 50 to 100 megahertz frequency range.
“As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” Dr. Rogers commented in the February 28, 2018 MIT Press Release.
Specifically, the astronomers observed a flattened absorption profile–a dip in the radio waves, at about 78 megahertz.
“”We see this dip most strongly at about 78 megahertz, and that frequency corresponds to roughly 180 million years after the Big Bang. In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest,” Dr. Rogers added.
This dip in radio waves was both much deeper and stronger than theoretical models had predicted. This suggests that the hydrogen gas at the time was much colder than previously believed. The radio waves’ profile also matches theoretical predictions of what would be produced if hydrogen was influenced by the Universe’s first-born stars.
“The signature of this absorption feature is uniquely associated with the first stars. Those stars are the most plausible source of radiation that would produce this signal,” Dr. Lonsdale explained in the February 28, 2018 MIT Press Release.
“It is unlikely that we’ll be able to see any earlier into the history of stars in our lifetimes. This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries,” study lead author Dr. Bowman noted in the same Press Release.
The scientists believe that this new detection lifts the veil from a previously mysterious era in the evolution of the Universe.
Dr. Lonsdale commented that “This is exciting because it is the first look into a particularly important period in the Universe, when the first stars and galaxies were beginning to form. This is the first time anybody’s had any direct observational data from that epoch.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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