When the universe was born, it was a hot mess– literally. Filled with high energy atomic soup at a whopping 10ˆ32 Kelvin, tons of atomic particles were buzzing around a incredible speeds. To be completely honest, scientists don’t really understand what happened during this stage of cosmic development because things were… weird.
Introduction
Modern astronomers have worked hard to construct a timeline for the development of our universe. Through this endeavor, we have developed a scientific toolbox filled with ways to piece together our cosmic history. One of the techniques is called the 21 cm line, and has given us an insight into the evolution of our universe. However, it took billions of years for space to reach its current state, so here is how our cosmic story began.
The very beginning
At the dawn of time, researchers believe that our 4 fundamental forces of physics didn’t yet apply and exotic physics– like the illusive and theoretical concept of quantum gravity– were at play. After this stage of our universe’s development, everything undergoes a huge growth spurt that astronomers call cosmic inflation. At this point, the universe expands by a factor of 10^26 which allows for supercooling. In other words, the universe grows out of its awkward phase, and particles are cooled down enough that our normal rules of physics begin to apply. This is exciting as it allows for the next stage in our universe’s development called Big Bang Nucleosynthesis, or BBN for short.
Let there be light! (Sort of…)
At the beginning of BBN, all the hot soup of particles begins to condense into simple atomic nuclei. Next up is Recombination, which is when things have finally cooled off enough that there are neutral hydrogen atoms, as opposed to the hotter and more energetic particles from before. Because the universe is now neutral, everything becomes transparent, allowing the first waves of light to propagate freely throughout the universe. This ‘first light’ of the universe is called the Cosmic Microwave Background, CMB, and is important to scientists as it is the oldest light in the universe that we are able to still detect today! Ironically, not many light producing sources, such as stars, have formed yet, so astronomers call this era the Cosmic Dark age, aka the universe’s emo phase.
Once every 10 million years
Luckily for scientists, we are still able to learn about this time using some of the properties intrinsic to hydrogen atoms. Essentially, each hydrogen atom has a proton and electron, and each of these is assigned a spin state according to the laws of quantum mechanics. About once every 10 million years, the direction that the subatomic particles are spinning randomly changes. When a spin flip occurs, it spits out a photon that travels specifically at a 21 cm wavelength. Wow, once every 10 million years sounds like horrible odds! Luckily for
astronomers, the universe during its emo phase was just a massive blob of hydrogen, so this spin flip transition actually happened pretty frequently! Now, a cheeky 13 billion years later or so, this 21 cm line is still detectable to scientists, just at a much longer wavelength.
Cosmic redshift
To understand how a 13 billion year old photon is still visible, you can compare it to hair dye. The number one symptom of a middle school emo phase is green hair dye from the pharmacy. Just like the spin flip transition, scientists can tell how long ago various epochs of our universe took place based on how ‘faded’ the signal is. So just like green undertones that never seem to leave your hair, astronomers can date specific events in our cosmic history using a technique called redshift (see Figure 1). Because our universe is infinitely expanding, the 21 cm wavelength has become incredibly stretched overtime, but will continue to ripple through our universe forever. Just like the emo phase that never really left you, it never left space-time either.
Cosmic dawn and cosmic present
In conclusion, an exceedingly rare process of quantum physics is the key to looking back 13 billion years in our cosmic history. This phenomenon has led to exciting initiatives such as the EDGES project at MIT. By peering into the night sky in radio silent areas, such as the Australian outback, their team is able to detect the faintest signals from our universe's first stars and galaxies.
Despite all of these ultra ancient celestial bodies being long gone, we are able to find proof that they were once there. Through projects such as EDGES, astronomers are able to learn more about the earliest phases of our universe and piece together our cosmic origins.
References
“Chronology of the Universe.” Wikipedia, Wikimedia Foundation, 3 Aug. 2024, en.wikipedia.org/wiki/Chronology_of_the_universe.
“Cosmic Dawn.” Network for Exploration and Space Science, 26 Oct. 2017,
www.colorado.edu/ness/science/cosmic-dawn.
“Edges: Experiment to Detect the Global EOR Signature.” MIT Haystack Observatory, 5 Dec. 2023, www.haystack.mit.edu/astronomy/astronomy-projects/edges-experiment-to-detect-the-global-eor-signatu re/.
Hoffman, Erika. “Cosmic Dark to Cosmic Dawn.” Spin-Flip Background - Cosmic Dark to Cosmic Dawn, cosmicdawn.astro.ucla.edu/spin_flip.html. Accessed 13 Aug. 2024.