This Just In: No Evidence of Cosmic Dawn

The big bang model posits that the universe abruptly appeared 13.8 billion years ago. According to the big bang, the universe was birthed in a very hot, dense, expanding state. Many things supposedly happened in the early universe, such as cosmic inflation. However, most popular accounts of the big bang history of the universe pick up about 370,000 years after the big bang with an event called the time of recombination. Prior to this time, the matter of the universe was ionized. The free electrons in the universe would have blocked the passage of photons of light. Hence, photons were repeatedly absorbed and remitted, making the universe opaque. Consequently, light could not travel very far. Cosmologists say that matter and light were coupled up to the time of recombination. However, once the universe cooled enough that electrons could assume stable orbits around protons and form hydrogen atoms, the universe became transparent, and matter and energy were decoupled. It is the unincumbered radiation from this time that is supposed to be the cosmic microwave background.

The sudden and rapid appearance of the first stars and galaxies is often called the cosmic dawn.

But First, the Dark Age

The time of recombination ushered in the cosmic dark age. It’s not that there was no light, for the radiation field of the decoupling of matter and radiation still existed. This radiation initially was a 3,000 K blackbody spectrum that filled the universe. However, as the universe continued to expand, the radiation was redshifted, gradually cooling that radiation field (its temperature now is 2.73 K). Rather, being the cosmic dark age meant that there was no other significant source of radiation. Today, the universe is filled with radiation from galaxies that contain stars, but during the cosmic dark age, stars and galaxies did not yet exist. The sudden and rapid appearance of the first stars and galaxies is often called the cosmic dawn.

Astronomers and cosmologists who are committed to the big bang model are very interested in how and when the cosmic dawn happened that eventually brought the cosmic dark age to a close. This is why the recently launched James Webb Space Telescope was optimized in a way to make study of the most distant galaxies one of its primary objectives. How long did the cosmic dark age last? That is not clear. Some estimates are that the first stars and galaxies began to form 200–300 million years after the big bang. It is widely believed that by one billion years after the big bang, the most distant normal galaxies we can see today had formed. Given the clumpy nature of matter in the universe, it may be that the cosmic dark age ended at different times in different regions of the universe. However, cosmologists are confident that stars and galaxies were in all regions of the universe a few hundred million years after the big bang.

A large part of the problem in probing the end of the cosmic dark age is that light from the first galaxies had to travel so far to reach us that it is too faint to be detected by telescopes today.

A large part of the problem in probing the end of the cosmic dark age is that light from the first galaxies had to travel so far to reach us that it is too faint to be detected by telescopes today. However, in 2018 a study claimed to have found a way around this problem. The universe is made mostly of hydrogen. Before stars formed, hydrogen would have been in atomic form. Atomic hydrogen at low density can emit radiation at 21 centimeters wavelength (1,420 MHz frequency). This emission occurs when electrons in the ground state of hydrogen atoms flip from the parallel to antiparallel spin state with respect to the protons in atomic nuclei. Radio detection of 21-cm radiation has been used to probe galactic structure since the 1950s. Today there are mechanisms to excite electrons to the antiparallel spin state from which they can flip and emit 21-cm radiation, but during the cosmic dark age, these mechanisms were not available. Consequently, cosmologists do not expect to receive 21-cm emission from hydrogen atoms during the cosmic dark ages.

On the other hand, as the cosmic dark age came to an end, a powerful mechanism for exciting hydrogen atoms in the ground state came about so that their electrons could emit 21-cm radiation for the first time. Astronomers and cosmologists think that the first stars were massive and formed very quickly. According to the theory of stellar evolution, as these first stars fired up, they ionized much of the hydrogen near them, and as those hydrogen ions attempted to recombine with electrons to form hydrogen atoms again, they emitted Lyman-alpha photons. The ultraviolet Lyman-alpha photons were absorbed by neutral hydrogen some distance away from the first stars, whereupon they returned to the ground state, with many of them in the slightly higher energy parallel spin state. This process, called the Wouthuysen-Field coupling, would have initiated the first significant 21-cm emission in the history of the universe. The 2018 study claimed to have measured this 21-cm emission from the early universe, albeit at a much redshifted wavelength, or frequency. Since the universe has expanded tremendously since the emission, the observed 21-cm would be observed at a much longer wavelength, or a much lower frequency. Instead of a frequency of 1,420 MHz, the 2018 study found emission at 78 MHz. This corresponds to a redshift, z, of about 17, near the middle of the expected range of z = 15–20. At the time, this was hailed as confirmation of astronomers’ expectation of when the first stars formed.

Repeatability

A new study has failed to reproduce the earlier study’s result. What went wrong? Astronomers don’t know.

However, a new study has failed to reproduce the earlier study’s result.1 What went wrong? Astronomers don’t know. The part of the spectrum where the emission is expected is noisy, suffering from contamination of signals from natural and manmade sources. Care must be taken to avoid and eliminate those spurious signals as much as possible. It may be that the earlier study inadvertently misidentified the emission as being 21-cm emission redshifted by z =17 when it had come from another factor that the recent study was able to eliminate.

Of course, we at Answers in Genesis do not agree with the big bang model as the origin of the universe. Our view is that God made everything in six normal days just a few thousand years ago. In the creation view, there is much left unsaid, and certainly we creation astronomers have much to work out. Consequently, I’m not very surprised that this newer study failed to confirm the earlier result.