Seeking a signalTo look back at the cosmic dawn, HERA uses low-frequency radio waves to identify signals that are not easily observed. This is different from other telescopes, such as the Hubble Space Telescope, which observe structures like galaxies that comprise just 5 percent of the observable matter in space. The other 95 percent of matter is what is between galaxies, including low-density hydrogen. With HERA, scientists can look at what is going on between galaxies and use that information to infer what galaxies are doing that we cannot observe, and how galaxy formation influences the space around it.
We now know when cosmic dawn ended. For a period of roughly 100 million years in the early universe, starting about 380,000 years after the big bang, the cosmos was completely dark. Then, stars and galaxies began to form, emitting light and ionising the intergalactic hydrogen gas in a process called reionisation, or cosmic dawn. It ended, with all of the hydrogen ionised, 1.1 billion years after the big bang.
This non-detection allowed the researchers to make other determinations about the cosmic dawn, placing restraints on the first galaxies, enabling them to rule out scenarios including galaxies which were inefficient heaters of cosmic gas and efficient producers of radio emissions.
Working with collaborators in India, Australia and Israel, the Cambridge team used data from the SARAS3 experiment to look for signals from cosmic dawn, when the first galaxies formed. Using statistical modelling techniques, the researchers were not able to find a signal in the SARAS3 data.
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.
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.
The method involves looking at a spectral line equivalent to a wavelength of 121.6 nanometers (one nanometer is one-billionth of a meter). This wavelength belongs to the UV range and is the strongest hydrogen spectral line. However, the cosmic expansion shifts the quasar spectrum to longer wavelengths the farther the light travels. Therefore, the redshift of the observed UV absorption line can be translated into the distance from Earth. In this study, the effect had moved the UV line into the infrared range as it reached the telescope.
Normally I would complain about the ending not feeling like it was fully realized, but this is not an intellectual sci-fi film. This is a cosmic space trip and I adore the feeling. From a cool blue and green designed cult commune/clubhouse to a full blown voyage into the cosmos, it's like Panos Cosmatos with more storyline. Great soundtrack too. If the story ended better and it didn't jump back in forth in time so much, this would have easily been 4 stars.
Since the cosmic signal is extremely faint, buried in orders of magnitude brighter radiation from our own galaxy and man-made terrestrial interference, detecting the signal, even using the most powerful existing radio telescopes, has remained a challenge for astronomers. 781b155fdc