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A Reflection On Cosmology

9 Ekim 2021

We’ve been asking questions since as far as we invented language. How long is that? To be precise, a glimpse of an eye. Within that insignificant segment of time, the two sublime impulses of the intellect, doubt and curiosity, engendered an inconceivably large net of questions. Just like the scientific method, this is an overarching process which is not static, but expanding. Questions, unlocked by discovery, are entailments of others. See, Newton’s questions entailed those of Einstein, and Einstein’s questions entailed those of string theorists, and so forth. Then, what might be a question, or a set of questions, that any question can be linked to?

“If you wish to make an apple pie from scratch, you must first invent the universe.” — Carl Sagan

What is Universe made of? Until no long ago, the atomistic modeling that Universe consists of atoms and empty space looked fine, but that is almost totally incompatible with our current understanding: Universe barely consists of atoms and involves no “empty” space. Firstly, if we were to assert that (sub)atomic particles are the arche, we would disregard the gap between Quantum Field Theory (that is, Special Relativity + Quantum Mechanics) and General Relativity. The two involve different math, different realms, and hence different relations to gravity: One cannot be represented in terms of the other in a wholesome way. Secondly, even if we assumed atoms to be the underlying substances of Universe, we would ignore far more than we know about it. Regular matter (atomic bodies) make up roughly 4% of Universe: The Universe is 96% dark matter and dark energy. And as for the “absence of emptiness,” wherever and how far ever we go, there will be fields, and most importantly, the gravitational field. These being said, there is in some sense ongoing discussion on this question; but the only wholesome, consistent, and satisfactory answer seems to be the String Theory. For now, there is sound reason to believe that the whole fabric of Universe can be reduced to strings, which can manifest as both particles and gravity depending on their “shape” and resonance.

How do we conceive of the entirety of Universe? This question presupposes that Universe is the same entity no matter how we look at it, partially or holistically, and so do we. Put differently, as a principle (that is, the Cosmological Principle), scientists agree on that Universe is isotropic and homogenous, for the sake of scientific productivity: that is, if we didn’t assume that, we couldn’t draw conclusions about parts of Universe that are far, far from us upon what we know here and now, and there has been no contradicting evidence. Importantly, there is no “outside of Universe”; by its definition, Universe is the whole being. It’s difficult to envision this since our brain typically assesses things by recognizing and applying patterns. Still, there is an “outside” of the Observable Universe, the light of which has not yet reached us, if it will ever.

That being said, until no long ago, we would also assume that Universe is static, infinite, and primordial. In this model, galaxies are suspended where they are, like lanterns fixed on an unimaginably large dome that does “exist” but was never constructed. There used to be nothing irrational in this assumption, since again as a matter of principle, we choose the simplest hypothesis. Nonetheless, this assumption entailed one problematic question: if these three properties were valid, the infinitely many light beams of all bright celestial objects, which are found equitably (ish) at every direction, wouldn’t they have reached us ultimately, brightening the night sky like lanterns under a dome? This problem was known as Olbers’ Paradox and ceased to be a paradox when Hubble’s Law suggested that universe is expanding. He did so by analyzing starlight spectrographies, which involved redshifts. Together with Special Relativity, the observations showed that there is a proportion between the distance and speed of expansion (v = DH, “H” being Hubble’s constant); that is, there is an apparent recession between them. Furthermore, since this expansion is accelerating, light from the furthest parts of Universe will probably never reach us, keeping the Non-Observable Universe mysterious forever. The “edge” of the Observable Universe is called the “horizon”: the closer we look at it, the further we see in the past. Crucially, the expansion of Universe implies two other properties of Universe: that it was born at some time and that it is finite, respectively.

What’s happening in Universe? Two underlying principles guide Universe overall: determinism and randomnessEvolution is the manifestation of those two as a function of time. At a given instance of the course of the evolution, a number of laws and constants operate spacetime and matter. Cosmological Principle and General Relativity jointly suggest that spacetime might be positively curved, negatively curved, or flat. This is determined by the average density of Universe: If the density was greater than the critical density (about 6 H atoms per m^2), spacetime would have closed geometry, like a balloon, in which case we would end up at where we started if we moved at constant velocity; if the density was less than the critical density, spacetime would have open geometry, like a curved piece of paper. The average density of matter in Universe appears to be equal or almost equal to the critical density. As for matter, there are four classes distinguished according to their pressure and energy density in the Concordance Model: radiationbaryonic matterdark matter, and dark energy (that is, Cosmological Constant). Radiation is particles moving at the speed of light with low or no mass and high pressure. Baryonic matter is atomic bodies, which have negligible pressure. Radiation and Baryonic matter are considered to be regular matter. Dark matter interacts weakly with regular matter and has negligible pressure. Dark energy has high negative pressure and motives the expansion of Universe. Spacetime and matter constantly interact with each other; Black Holes, for instance, are products of these interactions.

Cosmos and Universe are equivalent, with the exception that “Cosmos” presupposes that Universe is ordered and can be represented in a wholesome and (scientifically) meaningful way, as the opposite of Chaos. Referring to Universe by “Cosmos” is a way of implicitly saying that the scientific phenomena we’ve discussed so far, together with some other beyond the scope of this essay, determine everything from Black Holes to social relationships to the flight of birds in an understandable way. Cosmology investigates the origin, evolution, and “fate” of Cosmos in a way to construct cosmological histories and modelings. Interestingly, those phenomena are subject to evolution as well, such that in different epochs of the evolution of Cosmos, things looked and behaved quite differently, if not for too long.

“We do not “come into” this world; we come out of it, as leaves from a tree.” — Alan W. Watts

The most widely accepted cosmological modeling is the Big Bang Theory. Importantly, Big Bang is not “why Cosmos exists,” it is rather a description of its evolution; Big Bang didn’t happened and isn’t happening in/to space, it is how spacetime came into being; and the essence of Big Bang is not necessarily an explosion but that Cosmos is expanding and cooling (Peebles, 2001). Such misconceptions rise from a craving to reconcile traditional cosmological ideas such as “Let there be light” with science, but science shouldn’t seek to prove what we would like to be true, it should rather seek the truth no matter what it is by falsifying false hypotheses. Crucially, a falsified hypothesis doesn’t lose its value or smartness (see, Einstein believed Cosmos was static), it is just no more practical. Moreover, the evidences for Big Bang are not limited to the expansion. According to the Big Bang Theory, Universe evolved from a terrifyingly dense and hot matter that didn’t resemble any piece of matter we might observe today. The CMB radiation is a remnant of that heat — it is 2.725 K today and is the best blackbody we know. Furthermore, the abundance of light elements such as H, He, and Li — in comparison with those which are more complicated like Au, who reasonably seem to have evolved during a further stage — is an indication for (sub)atomic particle evolution. Interestingly, humans occupy a (somewhat) special place in cosmological history: Many highly unstable elements such as Am, Cm, and Bk were synthesized in laboratory, thereby completing the Periodic Table.

During the first 400 million years, physical phenomena weren’t exactly the way they are today. In fact, especially during the first moments of Cosmos, Quantum Field Theory and General Relativity wouldn’t make sense that much, not to mention the Standard Model and Newton’s laws. The extreme conditions of the speculative Planck Epoch and Inflation were amazingly extreme, but it’s disappointing that we may not draw evidences from them, we even cannot conduct experiments on them, since they are no more present, at least in the way they used to be. During Planck Epoch, in those terrifyingly high temperatures, fundamental forces and particles had not yet diverged; and during Inflation, the expansion acceleration of Cosmos was far greater than it is today. As Inflation epoch reached its end, matter and radiation diverged. These two “epochs” took less than 10^-4 of a second, and towards the end of the first second, Cosmos was about 10 billion degrees. Most notably, some of highly unstable free neutrons that were present decayed into protons and electrons, and others combined with protons during the first minutes (that is, the Big Bang Nucleosynthesis). From this combination resulted deuterium, helium, and lithium. More than a fifth of the ordinary matter that is present today is thought to be helium that was generated during these moments. As for the others, those heavier than lithium, they were generated in stars — within their layers and in supernovae. The matter we living organisms are composed of, all carbon based structures, originate in stars. During the rest of the first 400 million years, the electrons of more complex atoms started orbiting their nuclei, as radiation diluted. Today, we are still in the star formation epoch.

Is the universe eternal? It took about 13.8 billion years for us to appear and to find all these out. Though, we still don’t know much about the origin and fate of Cosmos. A big reason is that, we simply cannot put Cosmos on a weighing machine. The expansion rate may be calculated with Hubble’s constant, but the role of gravity in this expansion would depend on the average density and pressure in Cosmos. In the matters we know about, pressure seems not to be a big deal. So, if the density is lower than or equal to the critical density, Cosmos will most likely continue expanding forever, going through a “degenerate era,” getting cooler and cooler, leading to what is named as the “Big Chill.” If it is greater than the critical density, gravitation will lead and after some point Cosmos will enter a process of collapsing, namely the “Big Crunch.” If it is the latter case, there may be sound reason to believe our Universe is an instance of a series of Big Crunches followed by Big Bangs (Steinhardt, 2002). To go further, a “heretical” discussion could be held on the soundness of our principles and cogency of our assumptions, but that must have been annoying. If we, say, were to suppose that Universe is more of a Chaos, than a Cosmos, we would have far less puzzles and aporias in cosmology or in any domain of science than we do, but we would know far less either. At the end, it seems like there is an inverse proportion between dull certainty and scientific advancement.

“The wise man will be as happy as circumstances permit, and if he finds the contemplation of the universe painful beyond a point, he will contemplate something else instead.” — Bertrand Russell


  • https://map.gsfc.nasa.gov/universe/uni_fate.html
  • https://map.gsfc.nasa.gov/universe/WMAP_Universe.pdf
  • http://www.talkorigins.org/faqs/astronomy/bigbang.html#origin — Douglas Scott & Martin White, 2000
  • https://www.astro.ubc.ca/people/scott/cmb_intro.html — Björn Feuerbacher and Ryan Scranton, 2006

December 2020, Istanbul

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