- 2893 - UNIVERSE - how did it begin? Creating the theory of the Big Bang is one of the most remarkable achievements of science of all time. We can go back billions of years in time and understand when and how our Universe, as we know it, came to be this way. Like many adventures, revealing those answers has only raised more questions. Where did all this come from? How did we get here?
------------------------------ 2893 - UNIVERSE - how did it begin?
- Here is how the search started: The Andromeda, M31, is the closest galaxy to our own Milky Way. But before it was known as a galaxy, it was called the Andromeda “Nebula“. We didn’t actually know if other galaxies even existed. Think about that! As recently as a hundred years ago, we thought the Milky Way might be the ENTIRE Universe.
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- Even then that is pretty big. The Milky Way is on the order of 150,000 light years across. A light year is about 10 TRILLION kilometers so even at the speed of light it would take nearly the same length of time to cross the Milky Way as humans have existed on planet Earth.
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- M31 star in Andromeda has the designation “V1” because it is known as a “cepheid variable“. Cepheid variables can be used as a “standard candle” to measure distances across the Universe. We know generally how bright variable stars get. So, if we compare two of them, and one is significantly dimmer than another, we can infer it is farther away in space.
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- In 1924 using this technique, Hubble measured the light of V1 and 35 subsequent variable stars to measure the distance to Andromeda at an incredible 900,000 light years, much too far to be a part of our own galaxy.
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- With improved imaging techniques and more accurate measurements, we now know Andromeda is more like 2,400,000 light years away. But Hubble’s value of 900,000 light years was enough to reveal our galaxy was but one “island universe” in a much vaster universe.
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- And just how many galaxies are out there? With Andromeda we knew at least two. But since then we’ve discovered that there are not two, or ten, or hundreds, or thousands, or millions, but likely TRILLIONS of galaxies each filled with hundreds of billions of stars.
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- Our own Milky Way is a collection of between 100 to 400 billion stars. One that we orbit. There are likely more stars in the Universe than grains of sand on all the beaches of all the Earth combined. But how can we know?
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- Since those days of Hubble measuring a handful of variable stars in one galaxy, the ‘Sloan Digital Sky Survey’ released a new map on July 19, 2020 that is the most comprehensive pictures of the Universe ever made. It took twenty years and contains 4 MILLION charted galaxies!!
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- Using a specialized telescope in New Mexico, the “Sloan Digital Sky Survey “(SDSS) has created a series of catalogues of distant galaxies to create this map of the Universe. The catalogues contain large red (older) galaxies closer to the Milky Way, more distant blue (younger) galaxies, and the most distant are galaxies whose central supermassive blackhole, which we think resides at the core of most galaxies, is actively feeding on dust, gas, and stars.
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- These feeding blackholes can become the most luminous objects in the Universe. They are known as “quasars“.
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- Hubble made another incredible discovery, the “Hubble Constant“. Hubble realized that distant galaxies are all moving AWAY from us. This was the first evidence that our Universe is actually expanding. That expansion itself can be used to measure our distance from these galaxies.
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- The SDSS uses different techniques than those used to measure the distance to Andromeda. A standard candle like a “cepheid variable” star works on the order of millions of lightyears but we can’t resolve individual stars in very distant galaxies.
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- Instead, the SDSS measures a galaxy’s “redshift.” As light from a distant galaxy travels across space, it is traveling through an expanding Universe which literally stretches the light out causing it to become more red. The amount of how red shifted the light is by the time it reaches us gives us an idea of how far the light has traveled.
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- Tracking these galaxies also helps track the expansion of the Universe over time, like running a film backwards. Called the “look back time” the farther into space we’re looking, the farther back in time we’re seeing as it takes time for light from the distant Universe to reach us.
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- For example, imagine if I mailed you a photo of me but the mail took twenty years to reach you because I was so far away. You’re seeing me as I appear twenty years ago. Similarly, the SDSS map looks back in time to about 400,000 years after the birth of the Universe and how it has expanded over time.
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- Until recently, a large gap in this timeline existed in the middle 11 billion years between the ancient-ancient past and the present. This is a big gap considering the Universe is 13.8 billion years old.
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- That gap was filled in by the most recent SDSS catalogue called the “eBOSS” (extended Baryon Oscillation Spectroscope Survey). Beyond having a new map of the Universe, SDSS is filling in pieces to another ultimate question, why and how is the Universe expanding?
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- Currently, the “force” that causes the expansion of the Universe is referred to as a mysterious and unknown “Dark Energy”. The new map helps determine if the influence of Dark Energy has changed over time. Based on SDSS measurements it seems that the rates of the Universe’s expansion are different across the Universe’s history which may be a clue as to how Dark Energy works.
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- Of all the questions humanity has ever pondered, perhaps the most profound is, “where did all of this come from?”
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- The 20th century brought us General Relativity, quantum physics, and the Big Bang, all accompanied by spectacular observational and experimental successes. These frameworks enabled us to make theoretical predictions that we then went out and tested, and they passed with flying colors while the alternatives fell away.
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- But, at least for the Big Bang, it left some unexplained problems that required us to go farther. When we did, we found an uncomfortable conclusion that we’re still reckoning with today: any information about the beginning of the Universe is no longer contained within our “observable” cosmos.
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- Looking back to greater distances means looking back in time. In the 1920s, just under a century ago, our conception of the Universe changed forever as two sets of observations came together in perfect harmony.
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- For the past few years, scientists led by Vesto Slipher had begun to measure spectral lines, emission and absorption features, of a variety of stars and nebulae. Because atoms are the same everywhere in the Universe, the electrons within them make the same transitions: they have the same absorption and emission spectra.
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- But a few of these nebulae, the spirals and ellipticals in particular, had extremely large redshifts that corresponded to high recession speeds: faster than anything else in our galaxy.
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- Starting in 1923, Edwin Hubble and Milton Humason began measuring individual stars in these nebulae, determining the distances to them. They were far beyond our own Milky Way: millions of light-years away in most instances.
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- When you combined the distance and redshift measurements together, it all pointed to one inescapable conclusion that was also theoretically supported by Einstein’s General theory of Relativity, the Universe was expanding. The farther away a galaxy is, the faster it appears to recede from us.
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- If the Universe is expanding today, that means that all of the following must be true:
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------------------------ The Universe is getting less dense, as the (fixed amount of) matter in it occupies larger and larger volumes.
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------------------------ The Universe is cooling, as the light within it gets stretched to longer wavelengths.
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------------------------ Galaxies that aren’t gravitationally bound together are getting farther apart over time.
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- Those are some remarkable and mind-bending facts, as they enable us to extrapolate what’s going to happen to the Universe as time marches inexorably forwards. But the same laws of physics that tell us what’s going to happen in the future can also tell us what happened in the past, and the Universe itself is no exception.
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- If the Universe is expanding, cooling, and getting less dense today, that means it was smaller, hotter, and denser in the distant past. The big idea of the Big Bang was to extrapolate this back as far as possible: to ever hotter, denser, and more uniform states as we go earlier and earlier. This led to a series of remarkable predictions:
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---------------------- More distant galaxies should be smaller, more numerous, lower in mass, and richer in hot, blue stars than their modern-day counterparts,
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---------------------- There should be fewer and fewer heavy elements as we look backwards in time,
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----------------------- There should come a time when the Universe was too hot to form neutral atoms (and a leftover bath of now-cold radiation that exists from that time),
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----------------------- There should even come a time where atomic nuclei were blasted apart by the ultra-energetic radiation (leaving a relic mix of hydrogen and helium isotopes).
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- All four of these predictions have been observationally confirmed, with that leftover bath of radiation, originally known as the “primeval fireball” and now called the “cosmic microwave background“, discovered in the mid-1960s often referred to as the smoking gun of the Big Bang.
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- You might think that this means that we can extrapolate the Big Bang all the way back, arbitrarily far into the past, until all the matter and energy in the Universe is concentrated into a single point. The Universe would reach infinitely high temperatures and densities, creating a physical condition known as a singularity: where the laws of physics as we know them give predictions that no longer make sense and cannot be valid anymore.
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- At last! After millennia of searching, we had it: an origin for the Universe! The Universe began with a Big Bang some finite time ago, corresponding to the birth of space and time, and that everything we’ve ever observed has been a product of that aftermath.
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- For the first time, we had a scientific answer that truly indicated not only that the Universe had a beginning, but when that beginning occurred. In the words of Georges Lemaitre, the first person to put together the physics of the expanding Universe, it was “a day without yesterday.”
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- Only, there were a number of unresolved puzzles that the Big Bang posed, but presented no answers for:
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---------------------- Why did regions that were causally disconnected and had no time to exchange information, even at the speed of light have the same temperatures as one another?
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--------------------- Why were the initial expansion rate of the Universe (which works to expand things) and the total amount of energy in the Universe (which gravitates and fights the expansion) perfectly balanced early on: to more than 50 decimal places?
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--------------------- Why, if we reached these ultra-high temperatures and densities early on, are there no leftover relic remnants from those times in our Universe today?
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- Throughout the 1970s, the top physicists and astrophysicists in the world worried about these problems, theorizing about possible answers to these puzzles. Then, in late 1979, a young theorist named Alan Guth had a spectacular realization that changed history.
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- The new theory was known as “cosmic inflation“, and postulated that perhaps the idea of the Big Bang was only a good extrapolation back to a certain point in time, where it was preceded by this inflationary state. Instead of reaching arbitrary high temperatures, densities, and energies, inflation states that:
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-------------------- The Universe was no longer filled with matter and radiation,
but instead possessed a large amount of energy intrinsic to the fabric of space itself,
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-------------------- Which caused the Universe to expand exponentially (where the expansion rate doesn’t change over time),
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--------------------- Which drives the Universe to a flat, empty, uniform state,
until inflation ends.
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--------------------- When it ends, the energy that was inherent to space itself, the energy that’s the same everywhere, except for the quantum fluctuations imprinted atop it, gets converted into matter and energy, resulting in a hot Big Bang.
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- Theoretically, this was a brilliant leap, because it offered a plausible physical explanation for the observed properties the Big Bang alone could not account for. Causally disconnected regions have the same temperature because they all arose from the same inflationary “patch” of space.
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- The expansion rate and the energy density were perfectly balanced because inflation gave that same expansion rate and energy density to the Universe prior to the Big Bang. And there were no left over, high-energy remnants because the Universe only reached a finite temperature after inflation ended.
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- In fact, inflation also made a series of novel predictions that differed from that of the non-inflationary Big Bang, meaning we could go out and test this idea. As of today, in 2020, we’ve collected data that puts four of those predictions to the test:
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----------------------------- The Universe should have a maximum, non-infinite upper limit to the temperatures reached during the hot Big Bang.
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---------------------------- Inflation should possess quantum fluctuations that become density imperfections in the Universe that are 100% adiabatic (with constant entropy).
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---------------------------- Some fluctuations should be on super-horizon scales: fluctuations on scales larger than light could have traveled since the hot Big Bang. Those fluctuations should be almost, but not perfectly, scale-invariant, with slightly greater magnitudes on large scales than small ones.
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---------------------------- The fluctuations from inflation get stretched across the Universe, creating overdensities.
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- With data from satellites like COBE, WMAP, and Planck, we’ve tested all four, and only inflation (and not the non-inflationary hot Big Bang) yields predictions that are in line with what we’ve observed.
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- But this means that the Big Bang wasn’t the very beginning of everything; it was only the beginning of the Universe as we’re familiar with it. Prior to the hot Big Bang, there was a state known as cosmic inflation, that eventually ended and gave rise to the hot Big Bang, and we can observe the imprints of cosmic inflation on the Universe today.
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- But only for the last tiny, minuscule fraction of a second of inflation. Only, perhaps, for the final 10^-33 seconds of it can we observe the imprints that inflation left on our Universe. It’s possible that inflation lasted for only that duration, or for far longer.
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- It’s possible that the inflationary state was eternal, or that it was transient, arising from something else. It’s possible that the Universe did begin with a singularity, or arose as part of a cycle, or has always existed. But that information doesn’t exist in our Universe. Inflation, by its very nature, erases whatever existed in the pre-inflationary Universe.
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- Inflation is like pressing the cosmic “reset” button. Whatever existed prior to the inflationary state, if anything, gets expanded away so rapidly and thoroughly that all we’re left with is empty, uniform space with the quantum fluctuations that inflation creates superimposed on top of it.
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- When inflation ends, only a tiny volume of that space, somewhere between the size of a soccer ball and a city block, will become our observable Universe. Everything else, including any of the information that would enable us to reconstruct what happened earlier in our Universe’s past, now lies forever beyond our reach.
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- It is one of the most remarkable achievements of science of all: that we can go back billions of years in time and understand when and how our Universe, as we know it, came to be this way.
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- But like many adventures, revealing those answers has only raised more questions. The puzzles that have arisen this time, however, may truly never be solved. If that information is no longer present in our Universe, it will take a revolution to solve the greatest puzzle of all: where did all this come from?
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- This is a new paradigm for understanding the earliest eras in the history of the universe has been developed by scientists, quantum physics farther back in time than ever before, all the way to the beginning.
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- The new paradigm of “loop quantum origins” shows that the large-scale structures we now see in the universe evolved from fundamental fluctuations in the essential quantum nature of "space-time," which existed even at the very beginning of the universe over 14 billion years ago.
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- The new paradigm provides a conceptual and mathematical framework for describing the exotic "quantum-mechanical geometry of space-time" in the very early universe. The paradigm shows that, during this early era, the universe was compressed to such unimaginable densities that its behavior was ruled not by the classical physics of Einstein's general theory of relativity, but by an even more fundamental theory that also incorporates the strange dynamics of quantum mechanics.
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- The density of matter was huge then, 10^94 grams per cubic centimeter, as compared with the density of an atomic nucleus today, which is only 10^14 grams.
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- In this quantum-mechanical environment, where one can speak only of probabilities of events rather than certainties, physical properties naturally would be vastly different from the way we experience them today. Among these differences are the concept of "time," as well as the changing dynamics of various systems over time as they experience the fabric of quantum geometry itself.
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- Cosmic background radiation has been detected in an era when the universe was only 380,000 years old. By that time, after a period of rapid expansion called "inflation," the universe had burst out into a much-diluted version of its earlier super-compressed self.
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At the beginning of inflation, the density of the universe was a trillion times less than during its infancy, so quantum factors now are much less important in ruling the large-scale dynamics of matter and geometry.
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- Observations of the cosmic background radiation show that the universe had a predominantly uniform consistency after inflation, except for a light sprinkling of some regions that were more dense and others that were less dense.
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- The standard inflationary theory for describing the early universe, which uses the classical-physics equations of Einstein, treats space-time as a smooth continuum.
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- Yet this model is incomplete. It retains the idea that the universe burst forth from nothing in a Big Bang, which naturally results from the inability of the paradigm's general-relativity physics to describe extreme quantum-mechanical situations.
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- A quantum theory of gravity, like “loop quantum cosmology“, is needed to go beyond Einstein in order to capture the true physics near the origin of the universe.
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- When scientists use the inflationary theory together with Einstein's equations to model the evolution of the seed-like areas sprinkled throughout the cosmic background radiation, they find that the irregularities serve as seeds that evolve over time into the galaxy clusters and other large-scale structures that we see in the universe today.
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- In human terms, it is like taking a snapshot of a baby right at birth and then being able to project from it an accurate profile of how that person will be at age 100.
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- The theory of “cosmic inflation” states that our universe expanded rapidly in the moments after its birth, resulting in the immense expanse we see today.
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- Cosmic inflation explains why the universe is billions of years old, as well as why the universe is nearly flat. The theory's conclusions about how the universe should look match observations by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).
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- But, is inflation the only model that can explain the beginnings of the universe?
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- According to the physicists' calculations, viable early universe theories must incorporate either an accelerated cosmic expansion (inflation); a speed of sound faster than the speed of light; or energies so high that scientists would need to invoke a theory of quantum gravity such as string theory, which predicts the existence of extra dimensions of space-time.
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- Inflation doesn't require any exotic physics. It's just standard particle physics. Cosmic inflation accounts for the distribution of the matter in the universe by incorporating quantum field theory, which states that under "normal" circumstances, particles of matter and something called antimatter can pop into existence suddenly before meeting and annihilating each other almost instantly.
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- According to cosmic inflation, materializing pairs of matter and antimatter particles flew apart so quickly in the rapidly expanding early universe that they did not have time to recombine. The same principle applied to gravitons and antigravitons, which form gravity waves.
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- These particles became the basis of all structure in the universe today, with tiny fluctuations in the matter in the universe collapsing to form stars, planets and galaxies. The concept relies on widely studied ideas to explain how the universe began and evolved.
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- Of all the questions humanity has ever pondered, perhaps the most profound is, “where did all of this come from?” For generations, we told one another tales of our own invention, and chose the narrative that sounded best to us.
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- The idea that we could find the answers by examining the Universe itself was foreign until recently, when scientific measurements began to solve the puzzles that had stymied philosophers, theologians, and thinkers alike.
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- November 7, 2020 2893
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--------------------- --- Sunday, November 15, 2020 ---------------------------
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