We are all familiar with the famous opening of the TV show “The Big Bang Theory”. It 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 it’s not just a pretty line. Ladies are right: she describes exactly what was going on with the Universe a long time ago. After the Big Bang, the cosmos was an intensely hot, dense, and rapidly expanding plasma soup. It was also in a cosmic “dark age” because there were no light sources. It was just … well … dark. And hot.
A schematic idea of the vision of cosmic history revealed by the light of distant quasars. Telescope observations provide information about the time of reionization or cosmic dawn (bubbles, top right) that came after the Big Bang about 13.8 billion years ago. Credit: Carnegie Institution for Science / MPIA.
What came next? Certainly the Earth did not yet exist, neither the autotrophs nor the Neanderthals or anything else mentioned in the song. All this was far away in time, space, and cosmic evolution. There was not even light.
To come to light, the universe had to go through the Hot Soup period. This lasted for about 380,000 years. During this time, the universe expanded and cooled, leading to its next state of existence. As it cooled, protons and electrons were able to combine and create large amounts of neutral hydrogen. The Universe spent about 100 million years in this hydrogen-dominated neutral period. Then (reminiscent of a line from the 2010 movie), something wonderful happened. The first stars and galaxies began to form.
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With the stars, heat is obtained — or, to be more scientifically correct — ionization. This happened when the ultraviolet light from those hot, young massive stars tore apart the hydrogen atoms and separated the electrons from the protons. The good news is that when you have ionization, you have light. And this, folks, is the executive summary of how the universe lit up just a few hundred million years after the Big Bang. He entered what cosmologists call the “era of reionization”: the dawn of light in the universe.
Quasar Light reveals more about dawn
Astronomers know when this “cosmic dawn” began, but they still debate when it ended. It’s not just an academic exercise. It’s important to get an accurate “date” for the end of Cosmic Dawn. It may help them to understand the first generations of stars that illuminated things. And once astronomers have more information about these early objects, many other cosmological questions could also be answered.
The search for this endpoint led Dr. Sarah Bosman and a group of astronomers at the Max Planck Institute for Astronomy in Heidelberg to look for clues about hydrogen. Not just any hydrogen, but that neutral version that existed long before the cosmic Dawn. They thought they could find a way to observe a point in time where everything was ionized by the first stars. If so, they would know the end of Cosmic Dawn.
To find out, they sifted through the flowing light from 67 distant quasars. Some of the light they looked at came from 25 quasars observed during a project called the XQR-30 Survey. He used the X-shooter spectrograph from the European Southern Observatory installed at the Very Large Telescope (VLT) in Chile. The X-shooter provides high-resolution spectra of its targets. Spectra shows the component wavelengths of light emitted or reflected by a target. (Learn more about spectra and spectroscopy here.)
Traveling from cosmic dawn through the universe
The quasar light captured by the X-Shooter passed through hydrogen clouds at different distances from us. The light passed through ionized clouds and also through 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 “displaced” at the red end of the spectrum due to the expansion of the universe. Astronomers say it is “shifted to red” and the amount of redshift shows clues about the distance of light-emitting objects.
From Earth, we observe the past of the Universe. The light from the quasars passes through the already partially ionized gas of the time of reionization near the first galaxies. Neutral hydrogen gas between galaxies imprinted its existence on light, which is revealed by spectra. Courtesy of the MPIA Graphics Department.
Distance data allows Bosman and his colleagues to make an accurate measurement of the end of Cosmic Dawn. It happened 1.1 billion years after the Big Bang. “I am fascinated by the idea of the different phases that the Universe went through that led to the formation of the Sun and the Earth. It is a great privilege to bring a little new piece to our knowledge of cosmic history. “Bosman is the lead author of a research paper published in the Monthly Notices of the Royal Astronomical Society.
Modeling the data to find the end of Cosmic Dawn
The spectrum that Bosman’s team studied presented some challenges as the light from the quasar passed through various regions of space on its way to Earth. This left a tangled set of fingerprints on the spectrum of the quasar. For example, neutral hydrogen leaves a specific fingerprint. The same light also passed through the so-called “cosmic network” of matter that connects galaxies and galaxy clusters. It has neutral, ionized hydrogen. This also imprinted the light.
Therefore, astronomers had to untangle all the different fingerprints. To do this, they used a mathematical model that reproduces the variations in light measured at a time after the universe, when the intergalactic gas was already fully ionized. When they compared this model with their observations, they found a point where the 121.6 nanometer light line they observed shifted by a factor of 5.3 times. This corresponds to the cosmic age of 1.1 billion years (after the Big Bang). It is the last period in cosmic history in which hydrogen neutral gas must have been present in intergalactic space. After that, the ultraviolet light from the stars ionized the intergalactic hydrogen. This point in time marks the end of Cosmic Dawn.
Next steps
Now that a new “End of the Cosmic Dawn” reference date has been set, what happens? More observations, of course. “This new dataset provides a crucial benchmark for testing numerical simulations of the first billion years of the universe over the next few years,” said team member Frederick Davies. They will help characterize ionizing sources, the first generations of stars.
Knowing the end date of Cosmic Dawn provides tools for more numerical simulations of the period in which the stars first illuminated the universe. And he should open the door to explore the whole cosmic Dawn. “The most exciting future direction for our work is to expand it even to earlier times, to the middle of the reionization process,” Sarah Bosman said. “Unfortunately, the greater distances make previous quasars significantly weaker. Therefore, the extended collection area of next-generation telescopes such as the ELT will be crucial.”
For more information
The end of the cosmic dawn
Hydrogen reionization ends at z = 5.3: Lyman-alpha optical depth as measured by the XQR-30 sample
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