We are all familiar with the famous opening of the TV show “The Big Bang Theory”. This is a song that begins with the verse: “The whole universe was in a hot dense state …” performed by the band BareNakedLadies. It turns out that this is not just a sweet line. The ladies are right – it describes exactly what happened to the universe a long time ago. After the Big Bang, space was very hot, thick, rapidly expanding plasma soup. It was also in a cosmic “dark age” because there were no sources of light. It was just dark. And hot.
A schematic representation of the view of cosmic history revealed by the light of distant quasars. Telescope observations provide information about the era of reionization or the cosmic dawn (bubbles, top right), which came after the Big Bang about 13.8 billion years ago. Credit: Carnegie Institution of Science / MPIA.
What happened next? Of course, the Earth did not yet exist, nor did the autotrophs, the Neanderthals, or anything else mentioned in the song. All this was far in time, space and cosmic evolution. There was not even light.
To come to light, the universe had to go through a period of hot soup. This extends to about 380,000 years. During all this time, the universe expands and cools, leading to its next state of existence. As it cooled, protons and electrons combined to form huge amounts of neutral hydrogen. The universe has spent about 100 million years in this period, dominated by neutral hydrogen. Then (reminiscent of a line from the 2010 movie) something wonderful happened. The first stars and galaxies begin to form.
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With the stars you get heat or, to be more scientifically correct, ionization. This happened when ultraviolet light from these hot, young massive stars tore hydrogen atoms and separated electrons from protons. The good news is that when you have ionization, you have light. And that, people, is a summary of how the universe was illuminated only a few hundred million years after the Big Bang. He entered what cosmologists call the “age of reionization,” the dawn of light in the universe.
Quasar Light reveals more about the dawn
Astronomers know when this “cosmic dawn” began, but they are still debating when it ended. This is not just an academic exercise. Getting an exact “date” for the end of the Cosmic Dawn is important. This can help them understand the first generations of stars who illuminated things. And since astronomers have more information about these early objects, many other cosmological questions can also be answered.
The search for this end point led Dr. Sarah Bosman and a group of astronomers from the Max Planck Institute for Astronomy in Heidelberg to look for clues in hydrogen. Not just some hydrogen, but that neutral version that existed long before Space Dawn. They thought they could find a way to observe a time when everything was ionized by early stars. If so, they would know the end of the Cosmic Dawn.
To find out, they sifted through the light streaming from 67 distant quasars. Some of the light they watched was emitted by 25 quasars observed during a project called the XQR-30 Survey. It uses the X-shooter spectrograph of the European Southern Observatory, installed on a very large telescope (VLT) in Chile. The X-shooter provides extremely high resolution spectra for its targets. The spectra show the wavelength of the components of the light that the target emits or reflects. (Learn more about spectra and spectroscopy here.)
Journey from Space Dawn through the Universe
The light from the quasar, captured by the X-shooter, passed through hydrogen clouds at different distances from us. The light passed through ionized clouds as well as neutral clouds. Neutral hydrogen left a characteristic “fingerprint” on the spectrum of quasar light at a wavelength of 121.6 nanometers. This is in the ultraviolet range of the electromagnetic spectrum. However, light appears to be “shifted” toward the red end of the spectrum due to the expansion of the universe. Astronomers say it is “redshifted” and the amount of redshift gives clues to the distance of objects emitting light.
From Earth we observe the past of the universe. The light from the quasars passes through the already partially ionized gas from the reionization era near the early galaxies. Neutral hydrogen gas between galaxies has imprinted its existence on light, which is revealed by the spectra. Courtesy of MPIA Graphic Department.
The distance data allows Bosman and colleagues to make an accurate measurement of the end of the Cosmic Dawn. This happened 1.1 billion years after the Big Bang. “I am fascinated by the idea of the various phases that the universe has gone through, leading to the formation of the Sun and the Earth. It is a great privilege to contribute a new part to our knowledge of space history, “she said. Bosman is the lead author of a research paper published in the Monthly Notices of the Royal Astronomical Society.
Modeling data to find the end of the cosmic dawn
The spectra studied by Bosman’s team pose some challenges as the quasar’s light travels through different regions of space on its way to Earth. This left a tangled set of fingerprints in the quasar spectra. For example, neutral hydrogen leaves a specific fingerprint. The same light passes through the so-called “cosmic web” of matter that connects galaxies and galaxy clusters. There is both ionized and neutral hydrogen. This also printed the light.
So astronomers had to unravel all the different fingerprints. To do this, they used a mathematical model that reproduces variations in light measured later in the universe, when intergalactic gas was already fully ionized. When they compared this model with their observations, they found a point at which the 121.6-nanometer light line they observed was shifted by a factor of 5.3 times. This corresponds to a cosmic age of 1.1 billion years (after the Big Bang). This is the last period in space history when neutral hydrogen gas must have been present in intergalactic space. The ultraviolet light from the stars then ionizes the intergalactic hydrogen. This moment of time marks the end of the Cosmic Dawn.
next steps
Now that a new date has been set for the “End of Space Dawn” comparison, what’s next? More observations, of course. “This new data set provides a crucial benchmark against which the numerical simulations of the first billion years of the universe will be tested for years to come,” said team member Frederick Davis. They will help to characterize the ionizing sources, the first generations of stars.
Knowing the end date of the Cosmic Dawn provides tools for more numerical simulations of the period when the stars first illuminated the universe. And he must open the door to explore the entire Cosmic Dawn. “The most exciting future direction for our work is to extend it to even earlier times, towards the middle of the reionization process,” said Sarah Bosman. “Unfortunately, greater distances mean that these earlier quasars are significantly weaker. Therefore, an extended collection area for next-generation telescopes such as ELTs will be crucial.
For more information
The end of the Cosmic Dawn
Hydrogen reionization ends with z = 5.3: Lyman-alpha optical depth measured from the XQR-30 sample
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