We often picture the Big Bang as an enormous explosion but it was actually a terrific expansion. For about 380,000 years after its initial appearance, the universe was a fiercely brilliant but fading fireball of light. It might be best imagined as a luminous fog similar to how heaven is depicted in Hollywood movies. Only this fog would burn your skin off!
The temperature was so high there were no atoms, only the particles that make atoms — electron and protons. Before the universe cooled enough for electrons to bond with protons and make atoms, free electrons scattered light this way and that, preventing it from streaming freely across the universe. If you were a particle of light called a photon, it would be like trying to make your way through a crowd but always getting turned around. In essence, trapped.
When the universe had cooled sufficiently for electrons to bind with protons, the first atoms formed (mostly hydrogen and helium), and light was set free to stream across the cosmos. This first light is called the cosmic background radiation (CMB), and it fills the universe to this day but only weakly. To “see” it you need a radio telescope. Back in its wild and free days, these early photons would have been visible briefly before quickly shifting out of visible range. Newly minted hydrogen atoms emitted light, too but it only of the radio variety invisible to human eyes. In a very short while, the universe went from impenetrable fog to pitch black, heralding the start of the cosmic “dark ages.”
Not until hydrogen atoms coalesced to form the very first stars millions of years later did that “let there be light” moment finally happen. Now, it looks like we have a better idea of just when that happened. In a study published recently in the journal Nature, astronomers from MIT and Arizona State University report that a table-sized radio antenna in a remote region of western Australia picked up faint signals of hydrogen gas from the primordial universe just 180 million years after the Big Bang, the earliest evidence of hydrogen yet observed.
They also determined that the gas was in a state that would have been possible only in the presence of the very first stars. These stars, blinking on for the first time in a universe that was previously devoid of light, emitted ultraviolet radiation that interacted with the surrounding hydrogen gas. As a result, hydrogen atoms across the universe began to absorb background radiation — a pivotal change that the scientists were able to detect as radio waves.
The findings provide evidence that the first stars may have started turning on around 180 million years after the Big Bang. Though a short time in long history of the universe, it gives us an idea how long cosmic night may have lasted before the first stars lit a candle in the dark.
“This is the first real signal that stars are starting to form, and starting to affect the medium around them,” says study co-author Alan Rogers, a scientist at MIT’s Haystack Observatory. “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.”
Perhaps most interesting is that the radio waves suggest that the hydrogen gas and the universe as a whole back then must have twice as cold as scientists previously estimated with a temperature of about –454°F (–270°C). Rogers and his colleagues are unsure exactly why the early universe was so much colder, but some researchers think that interactions with dark matter may have played some role.
Dark matter gets its name from the fact we can’t see it — it neither gives off nor absorbs radiation — but it does have pull. We know it exists from how it attracts everyday matter to it. Whatever it is, there’s five times more dark matter in the universe than the ordinary stuff.
The scientists detected the primordial hydrogen gas using EDGES (Experiment to Detect Global EoR Signature), a small ground-based radio antenna located in western Australia, and funded by the National Science Foundation. Scientists think that when that when the first stars turned on, the UV light they radiated affected electrons in hydrogen atoms that caused those atoms to either give off or absorb light of a specific energy — 1,420 megahertz — a frequency smack dab in the middle between the AM and FM radio broadcast bands. As the universe expanded, this light shifted to lower frequencies. By the 21st century at Earth, its frequency would have dropped to 100 megahertz.
Scientists initially used the little antenna to listen in between 100 to 200 megahertz but barely picked up a signal. They had assumed the gas was hot, but if it were much colder, then 50-100 megahertz was the ticket.
“As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” Rogers says. They saw a dip at about 78 megahertz, which corresponds to roughly 180 million years after the Big Bang. It also told us that the dawn of light in the early universe was preceded by bitter cold.