Our observations of the vast Universe are, in many ways, the closest thing we’ll ever have to a time machine. We can view the Universe as it was when it was a large length of time ago, which is the next best thing because we are unable to travel back in time. Every time light is released from a far-off source, such as a star, galaxy, or quasar, it must first travel the huge cosmic distances between that source and the observer, which takes time.
These signals can take billions or perhaps more than 10 billion years to travel at the speed of light, thus the more away an object is from us, the closer in time we are to the Big Bang. The earliest light, however, was created when the atomic nuclei and electrons of the universe joined to form neutral atoms before there were any stars or galaxies. We can only perceive the Universe as it was so long ago thanks to a very precise quantum physics anomaly. The earliest signals wouldn’t exist without it, and we couldn’t look as far back in time and space as we can now.
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| The quantum fluctuations that occur during inflation get stretched across the Universe, and when inflation ends, they become density fluctuations. This leads, over time, to the large-scale structure in the Universe today, as well as the fluctuations in temperature observed in the CMB. New predictions like these are essential for demonstrating the validity of a proposed fine-tuning mechanism. (E. SIEGEL, WITH IMAGES DERIVED FROM ESA/PLANCK AND THE DOE/NASA/ NSF INTERAGENCY TASK FORCE ON CMB RESEARCH) |
We must travel back in time to the first moments of the Big Bang in order to comprehend the origin of the Universe’s oldest visible signal. The Universe was expanding very quickly when it was hot, dense, almost uniform, and full of a mixture of matter, antimatter, and radiation. There were areas of the Universe that were just a little bit denser than average and areas that were just a little bit less dense than average, but only by a factor of around one part in 30,000.
If it were up to gravity alone, the underdense regions would lose their matter to the denser surrounding parts while the overdense regions would expand, attracting more of the surrounding matter than the average or underdense regions. But other natural forces also play a significant role in the universe’s regulation, not just gravity. For instance, in the early Universe, radiation, especially in the form of photons, is incredibly energetic and has a significant impact on the evolution of matter in a variety of ways.
First of all, matter (and antimatter) will quickly scatter off of photons if it is electrically charged. This implies that any quantum of radiation will interact and exchange energy with any charged particle it comes into contact with. Encounters are more likely to occur with low-mass charged particles (such as electrons) than high-mass ones (such as protons or atomic nuclei).
Second, the energy density of the region increases above this average as matter tries to collapse gravitationally. However, in response to those greater energy densities, radiation flows from those high-density regions into the lower-density ones, resulting in a phenomenon known as a “bounce,” in which
- densities rise,
- photon pressure increases,
- photons flow out,
- the density drops,
- causing the photon pressure to drop,
- causing photons and matter to flow back in,
- increasing the density,
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| As our satellites have improved in their capabilities, they’ve probes smaller scales, more frequency bands, and smaller temperature differences in the cosmic microwave background. The temperature imperfections help teach us what the Universe is made of and how it evolved, painting a picture that requires dark matter to make sense. (NASA/ESA AND THE COBE, WMAP AND PLANCK TEAMS; PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS; PLANCK COLLABORATION (2018)) |
and the cycle keeps going. These “bounces,” or acoustic oscillations, that take place in the plasma of the early Universe correspond to a certain pattern of “wiggles” that we find in the fluctuations we observe in the cosmic microwave background.
However, the Universe is expanding, which is also happening at the same time as all of these other things. Due to the fact that the total number of particles within the universe remains constant as the volume grows, its density decreases as it expands. The wavelength of every photon, or quantum of electromagnetic radiation, also stretches as the Universe expands, which is a second phenomenon. The Universe also cools off as it expands because a photon’s wavelength determines its energy, with longer wavelengths corresponding to lower energies.
More than only gravitation will be produced by a universe that becomes less dense and cools from an originally hot and dense state. As long as there is sufficient energy present in each collision to produce massive particles (and antiparticles) via Einstein’s E = mc2, there is a chance that particle/antiparticle pairs will spontaneously form in every collision between two quanta at high energies.
This occurs frequently in the beginning, but as the Universe cools and expands, it ceases to occur, and instead, when particle/antiparticle couples collide, they annihilate away. Only a very small excess of matter will remain when the energy falls to low enough levels.
Numerous additional significant transitions take place as the Universe continues to expand and cool and as the density and temperature both decrease. As follows:
- Quarks and gluons form stable, bound states: protons and neutrons,
- neutrinos, which previously interacted copiously, no longer collide with other particles,
- the last of the antimatter pairs, electrons and positrons, annihilate away,
- The photons cool off sufficiently so that the first stable nuclear fusion reactions occur, creating the light elements in the immediate aftermath of the Big Bang,
- the oscillating dance between normal matter, dark matter, and radiation takes place, leading to the particular pattern of fluctuations that will later grow into the Universe’s large-scale structure,
- and, finally, neutral atoms can stably form, as the photons have cooled enough that they no longer immediately blast electrons off of the nuclei they’d bind to.
Only after this last stage, which takes more than 100,000 years, is finished will the Universe become transparent to the light that already exists within it. When neutral atoms form, the photons that were continually absorbed and reemitted by the previously present ionized plasma simply free-stream and redshift with the expanding Universe, producing the cosmic microwave background that we see today.
On average, that light arrives at our location 380,000 years after the Big Bang. This is extremely long compared to the preceding phases, which take place in the first nanosecond to the first few minutes following the Big Bang, yet it is extremely brief compared to the 13.8 billion years of our universe’s history. Even a modest quantity of super-energetic photons may maintain the ionization of the entire Universe since photons outweigh atoms by a factor of more than a billion to one. These neutral atoms can only finally form when they cool to a particular threshold, which corresponds to a temperature of roughly 3000 K.
If you think about it, there is an immediate issue with that last action.
A chain reaction occurs when electrons attach to atomic nuclei, cascading down the different energy levels. Eventually, those electrons will undergo the transition that requires the most energy: to the ground state. The most frequent transition, in which it produces an energetic Lyman-series photon, is from the second-lowest energy level (designated n=2) to the lowest state (n=1).
