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When it opens its eyes to the cosmos, NASA’s next extraterrestrial observatory, the Nancy Grace Roman Space Telescope, will peer back to a distant period in the universe’s history known as the “cosmic dawn .”
Although Rome’s predecessor telescopes, the Hubble Space Telescope and the James Webb Space Telescope (JWST), take full advantage of the fact that the cosmos is now transparent to light, the universe was not always so.
Up until about 400,000 years after the Big Bang, the cosmos was opaque, full of a vague “fog” of particles absorbing photons, particles of light. Cosmic dawn, between 50 million and a billion years after the Big Bang, represents the period when this fog began to clear and light began to travel freely.
It is also one of the most important periods in the history of the 13.8 billion year old universe, as it was also around the time when the first stars, galaxies, and black holes were born. The Nancy Grace Roman Space Telescope (Roman), to be launched in May 2027, will investigate the impact of these celestial objects during this crucial cosmic climax.
“Something very fundamental about the nature of the universe changed during this time,” Michelle Thaller, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland, said in a statement. “Thanks to Rome’s large, sharp infrared view, we can finally figure out what happened during a critical cosmic turning point.”
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Let there be light!
During the formative years of the universe, it was filled with a hot, dense sea of particles, including free electrons. These negatively charged particles endlessly scatter photons, making the universe opaque.
As the cosmos continued to expand, it also cooled, reaching a point at which electrons were able to combine with protons to form the first neutral atoms and the first elements, hydrogen and helium. This led to the formation of the first stars and galaxies. With the removal of the free electron, the first light was allowed to travel through the universe. We see this light today as a “celestial fossil” known as the cosmic microwave background (CMB).
Although the light was not endlessly scattered by free electrons at this point, it was still not free to travel far. This was because the photons quickly hit neutral atoms which absorbed them.
This period, which lasted between 380,000 and 200 million years after the Big Bang, was called the Cosmic dark ages. It ended over a period of several hundred million years as the neutral atoms broke down or were ionized, resulting in the cosmic dawn.
The question is: What caused the ionization of neutral atoms?
“We’re very curious about how the process happened,” said Aaron Yung of the Space Telescope Science Institute in Baltimore, who is part of the early Roman globe observation team. “The great crisp Roman view of deep space will help us weigh different explanations.”
One possible source of the radiation that provided the energy to ionize early neutral atoms is the early galaxies themselves. Another possible source of this high-energy light is the environment around the first black holes.
Roman will look at these two suspects.
“Romania will excel at finding the building blocks of cosmic structures like galaxy clusters that form later,” said Takahiro Morishita, an assistant scientist at the California Institute of Technology in Pasadena. “It will quickly identify the densest regions, where more ‘fog’ is clearing, making the Romans a prime mission to explore the galaxy’s early evolution and cosmic dawn.”
The stars of the cosmic dawn were different from those in the universe today, because the density of the early cosmos allowed them to grow to a mass hundreds or even thousands of times that of the sun. The enormous mass of these early stars meant that they had much shorter lifetimes than the sun’s estimated 10 billion year lifespan, but also meant that they blasted radiation more intensely than modern stars.
Huddled together in dense early galaxies, energy from these stars stripped electrons from protons in bubbles of space around them.
“You could call it the party at the beginning of the universe,” Thaller said. “We never saw the birth of the first stars and galaxies, but it must have been amazing!”
Related: The 1st stars in the universe were formed earlier than expected
Black holes join the cosmic dawn party
As these short-lived massive stars collapsed when their nuclear fuel was exhausted, they generated the first black holes. In the dense environments that were common in the early universe, these black holes collided and merged over and over again.
This led to the creation of supermassive black holes with masses thousands or billions of times that of the sun, although how black holes got so big so quickly is a pressing cosmic mystery.
Although black holes do not emit any light themselves because they are surrounded by a boundary called the “event space,” which represents the point at which even light cannot escape, these supermassive black holes could still added to ionization.
When a supermassive black hole is surrounded by gas and the dust it feeds on, this material settles into a fast cloud called an accretion disk. The massive gravitational influence of the black hole causes intense tidal forces in the accretion disk, generating friction and heating gas and dust, causing it to lie brightly across the electromagnetic spectrum.
In addition, the black hole’s magnetic fields can send matter to its poles, from which it is blasted out as double jets at near-light speeds. These jets are also accompanied by blasts of electromagnetic radiation. Stretching out for hundreds of thousands of light-years, a supermassive black hole’s jets are more than capable of ripping electrons from neutral atoms.
These active regions of this supermassive black hole are called quasars, or active galactic nuclei, and WST is discovering them at distances corresponding to a period of less than a billion years after the Big Bang. The powerful space telescope is finding far more quasars than expected as it searches for the cosmic dawn.
When the Romans operate, their wider field of view could provide a clearer picture of how common quasars were during the cosmic dawn, perhaps thousands of these supermassive black hole-powered regions. find.
“With a statistically stronger sample, astronomers will be able to test a wide range of theories inspired by the JWST observations,” explained Yung.
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One question that researchers will try to answer with the Romans is what kind of galaxies were responsible for the ionizing radiation at the cosmic dawn. One major indicator of this is the size of the ionization bubble carved out by radiation.
“Young galaxies may have started the process, and then quasars finished the job,” Yung concluded. “Galaxies would form huge clusters of bubbles around them, and quasars would form large spherical heads. We need a large scene like the Roman scene to measure their size because, in either case, they are likely to be up to millions of light-years wide. – often larger than the field of view of the JWST.”