waiting for the dawn

A comparison might help drive that point home: If you compress those 14 billion years into a 24-hour day that begins at midnight today, that would take about 50 minutes in 500 million years. We’ll be at 12:50: still dead of night, long before dawn breaks. I mention “morning” here intentionally. Astronomers have long thought that what they commonly refer to as “cosmic dawn” occurred—the time when the universe first had light. Well, maybe there was a huge burst of light with the Big Bang. But after that, there was darkness for many millions of years, before the formation of luminous bodies – first stars, then galaxies. It is the cosmic dawn.

So when was it? Astronomers believe that this era began about 300,000 years after the Big Bang and continued for about 400 million years. By then, the first galaxies had formed and began to emit light. As surprising as it may sound, we can see some of that same light today. To understand this, think of looking at a galaxy a billion light years away. What this actually means is that the light reaching our curious eyes today left that galaxy billions of years ago. So it’s at least that old. Thus, the more distant a celestial body is, the older it is, and the farthest one we have observed is over 13 billion years old. Looking back at them, we’re actually looking back at that time. We are seeing objects as they existed during that cosmic dawn.

But it is worth remembering that these objects are also particularly difficult to observe. Because they are so far away, the light from them is extremely dim. Much of it is scattered and lost as it makes its way through everything – dust, other stars and galaxies, black holes – that lies between objects and us. Therefore direct observation of these proto-stars and galaxies is difficult. This is why astronomers must look for other ways to detect these dazzling things. One such detection tool is an understanding of what happened to the gas hydrogen in those early years of the universe. Hydrogen was everywhere at the time—distributed widely and fairly evenly throughout the universe. It was also electrically neutral. Irrespective of what this means, it is worth noting that gas has a natural state. a youtube clip It describes the universe at that time as “dark, cold, mostly hydrogen, a weak and unlit gas”.

But then stars and galaxies began to form. Astronomers suggest that the light they emitted “excites” the hydrogen, which means the gas absorbs the radiation. This changed the fundamental character of the hydrogen atoms, so they were no longer neutral. They were “ionised”. Importantly, this would allow hydrogen to be “seen” by current observers, at least as spectroscopic patterns at specific radio frequencies. These are characteristic of hydrogen in this state.

At least, that is the broad principle, in simple words. This tells us that if we do find those particular spectroscopic patterns, it is evidence of the formation and existence of early stars, and thus of the elusive cosmic dawn. Astronomers are therefore searching for the so-called “spin-flip signal”, spectroscopic lines at a wavelength of 21cm. This occurs because the hydrogen atom reacts when excited by emitting a photon at that wavelength. If they find it, this particular incarnation of hydrogen has a definite gift of existence. But finding it is not an easy task either. The same YouTube clip says it’s like detecting the flutter of a hummingbird’s wings inside the chaos and hoarseness of a storm. But a few years ago, some scientists at the Massachusetts Institute of Technology installed a relatively small but exceptionally sensitive radio antenna in the West Australian desert and set it to listen to the sky.

Result: They got something. They had to carefully filter through overlapping noises and other radio signals. They had to rule out other possible explanations for what Antenna found. But in the end, in a paper he published in the journal Nature, he remarked that what he found was “largely in line with expectations for a 21-centimeter signal induced by early stars”. In fact, they had stumbled upon the cosmic dawn. He dated it to about 180 million years after the Big Bang. “In terms of the direct detection of a signal from hydrogen gas,” said one of the astronomers, “it has to be the earliest”.

Naturally, the results caused a stir in celestial circles. Other researchers scrambled to do what they told science after the new findings were published: Try to replicate and expand on them. Various large radio telescopes were deployed around the world in an attempt to detect the spin-flip signal. If this was the first glimpse of the cosmic dawn, other teams of astronomers hope to fill in the details and eventually understand how the universe evolved after the Big Bang. However, there is a twist in the tail – this is also how science sometimes advances. No one has yet detected that 21 cm signal since Bowman et al.

Notably, a team of scientists from India’s Raman Research Institute tried to replicate Bowman’s experiment. Instead of the Australian desert, they floated their antennae on a “raft made of Styrofoam” in a “large body of water in southern India”. They chose carefully, taking into account the depth and salinity of the lake, so that the water could protect the antenna from radiation. They did not find what Bowman did (an absorption profile centered at 78 MHz in the sky-averaged spectrum, Judd D. Bowman et al., Nature, 1 March 2018). Thus they suggest that Bowman’s findings “were not of astrophysical origin. Perhaps Bowman’s instruments were faulty, or there were errors in his reported data. Further, “the inhomogeneous ground beneath the antenna … was attributed to systematic errors.” That is, the terrain in Australia did not provide adequate protection from noise and irrelevant data. Of course that is why Indian scientists chose to float their antennae on the water.

Sure, the Indians didn’t find the signal they were looking for. Nevertheless, they remain optimistic about both their methods and the end goal: “continuous observation with deployed sensors”. [on] Large water bodies in remote locations on Earth … will provide data free of systematics and lead to the discovery of a true redshifted 21-cm signal from cosmic dawn”.

But for the time being, “our nonlinearity bears out earlier concerns and suggests that the profile found by Bowman et al. is not evidence for new astrophysics or non-standard cosmology. This is why many others The teams are, despite the Indians’ findings, still conducting their own experiments. All are looking for dawn.

Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.