Why does the cosmic microwave background radiation have a length of about 1.9 mm and does the radiation from the first galaxies, visible through the Hubble Deep-Space images, still have a wavelength of between 780 and 380 nanometers?
1. What explains the difference between the wavelength of the cosmic microwave background radiation (about 1.9 millimeters) and the wavelength of light from the very distant galaxies? (As revealed by the Hubble Ultra Deep Field images, these are between 780 and 380 nanometers.)
2. How can there be such a huge difference between the cosmic microwave background radiation with a wavelength of 1.9 mm (started as gamma radiation and now microwave radiation, after all, this is electromagnetic radiation from about 300,000 years after the Big Bang, so now 13.7 billion years underway) and the radiation of the first galaxies (approx. 13 billion years underway), whose visible light is between 780 and 380 nm.
Both types of electromagnetic radiation have been traveling for almost the same number of years, but have very different wavelengths.
3. Is the light from the very distant galaxies visible in the Hubble Ultra Deep Field images from the Hubble telescope ‘ordinary’ light waves, ie light visible to the eye? (It is argued that these are images of galaxies that would have formed less than 400 and 800 million years after the Big Bang.)
4. Or has the light from those very distant galaxies left as high-energy (gamma) radiation (with a short wavelength) and then stretched due to the expansion of space and thus acquired a longer wavelength that now manifests itself as visible light with a wavelength between 780 and 380 nm?
Answer
The mathematical answer to your question is that the redshift is not simply proportional to the distance the light has traveled. That is close to us, but at greater distances the redshift increases faster with distance. At the Big Bang itself (13.7 billion years ago), the redshift is infinite, but the Big Bang is not infinitely distant.
The most distant galaxies we see have a redshift of the order of 10, which corresponds to an age of less than 1 billion years. For a redshift z, the light is redshifted by a factor (1+z), so also about 10. The light from galaxies that a telescope like Hubble can see comes from stars, and they typically radiate in the optical-ultraviolet part of the planet. spectrum: the optical UV light reaches the near infrared at high redshift.
The background radiation as we see it now comes with a redshift of about 1000! It is the residual radiation from the big bang itself, which started with a temperature of ‘infinity’ and cooled with the expansion. Whence the redshift 1000 and the 380,000 years? Before that, the universe was so hot that all matter was charged, and charged matter reacts strongly with radiation. Many interactions means that a uniform temperature occurs, and therefore a well-defined spectrum for the radiation, that of the so-called black radiator. Once that temperature has dropped to such an extent that atoms become neutral (capture electrons and no longer give them up), the interaction between matter and radiation is over, and they each go their own way. The radiation then does nothing more than expand and cool down. This phase transition has taken place after 380,000 years (‘decoupling’), at the temperature at which hydrogen becomes neutral. It has been a gradual transition, which ended when the temperature was about 3000 K, about 1000 times the background temperature today, and that 1000 is the very redshift corresponding to decoupling.
It is somewhat coincidental that the temperature of decoupling (determined by the ionization temperature of hydrogen) is of the same order as the temperature of stars, and thus also that the redshifts of the background radiation and of the most distant galaxies are approximately the same as the wavelengths at which they be observed.
Answered by
Prof. dr. Christopher Waelkens
Astronomy
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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