- 3221 - UNIVERSE - did it have a beginning? The Big Bang wasn’t the beginning of time and space, and cosmic inflation, which preceded it, cannot be the beginning either, unless it went on for an eternity. After a century of cosmic revolutions, we’re right back where we started: unable to answer the most fundamental question we can ask, “how did it all begin?”
------------------ 3221 - UNIVERSE - did it have a beginning?
- We humans have a logical thought process we call “cause and effect“. Whenever we see anything occur, that’s the “effect‘. The processes that occurred earlier and led to the effect occurring are what we typically refer to as the “cause“ The reason for the effect that is occurring.
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- We can extrapolate this phenomena back in time in an unbroken chain of cause-and-effect events. This can’t go back in an infinite chain, rather there has to be a “first cause” that led to the very existence of the Universe itself. The classical Big Bang theory seemed to imply that the Universe began from a “singularity”. The first cause.
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- A “singularity” is an infinitely hot and dense state from which space and time themselves emerged. The Big Bang was just another effect, and we think we know what caused it.
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- The Big Bang was an idea that attempted to explain the Universe we observed based on two pieces of evidence:
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----------------------- The demonstrated validity of our current theory of gravity, that is “General Relativity”
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----------------------- The observed fact that the more distant a galaxy was observed to be from us, on average, the greater the amount its light appeared to be “redshifted” before arriving at our eyes. The further away it was the faster it was receding.
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- General Relativity was shown to imply certain inevitable consequences. One was that the Universe could not be evenly, uniformly filled with matter and remain stable; a static, matter-filled Universe would inevitably collapse into a blackhole.
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- A second consequence was that a Universe that was evenly filled, not merely with matter but any type of energy, would either expand or contract according to a particular set of physics rules.
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- A third consequence was that when the Universe expanded or contracted, the wavelength of any waves (including de Broglie waves, for matter particles) would also expand or contract by the exact same proportional amount.
- The more distant an object is from us, the longer it takes the light it emits to reach our eyes. If the Universe is expanding as the light travels through it, then the longer it takes that emitted light to complete the journey to our eyes, the greater the amount that light’s wavelength will lengthen due to this Universe’s expansion.
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- The farther away we look, the farther back in time we’re seeing. At the greatest distances of all, we’re seeing the Universe as it was:
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---------------------- earlier in time
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---------------------- back when it was smaller, denser, and expanding faster
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---------------------- when it was in a more uniform, less clumpy state.
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- The first person to realize this expansion was Georges Lemaître, in 1927. He put together some early distance-determining data from Edwin Hubble with Vesto Slipher’s spectroscopic observations showing the redshifted light from distant galaxies, and concluded that the Universe must be expanding today.
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- Moreover, if it’s getting cooler, larger, and less dense today, then it must have been hotter, smaller, and denser in the past. Lemaître immediately extrapolated this as far as he could: to infinite temperatures and densities and an infinitesimal size.
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- He called this initial state the “primeval atom,” and noted that space and time could have emerged from a state of non-existence from a “singularity” at the very beginning.
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- It wasn’t until the 1940s that George Gamow uncovered the key predictions of this “Big Bang” scenario:
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-------------------- There would be a growing “cosmic web” over time, preceded by an early era without any galaxies or stars: a ‘cosmic dark ages‘,
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--------------------- Before the dark ages, the Universe would have been so hot that neutral atoms couldn’t form, and so when the Universe cools enough, we should see that leftover background of radiation, a few degrees above absolute zero, with a particular, blackbody spectrum,
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--------------------- Even before that, the temperatures and densities should have allowed nuclear fusion, meaning that we should have a mix of hydrogen, helium, and other light elements and isotopes that could be precisely calculated using nuclear physics.
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- In the mid-1960s, Bell Labs scientists Arno Penzias and Bob Wilson discovered this all-sky glow temperature at just 3,000 Kelvin. This was initially called the “primeval fireball” and known today as the “Cosmic Microwave Background“ radiation.
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- Even as the evidence supporting the Big Bang was mounting throughout the 1960s and 1970s, there were some puzzles that emerged as well.
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----------------------- The “monopole problem“: if the Universe got arbitrarily hot in the past, there should be high-energy relics from that very early state still remaining in our Universe, but none have ever been observed.
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----------------------- The “horizon problem“: if the Universe began from an extremely hot, dense state, then there should be an upper limit to the size of structures and to the scale of uniformity in the Universe, but the observed scales of both are larger than the predicted limits.
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------------------------ The “flatness problem“: assuming the Universe came into existence with a certain density and a certain expansion rate, those rates must balance perfectly to avoid the Universe either immediately recollapsing or expanding into total, empty oblivion, yet there’s no explanation for this perfect balance.
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- The Universe was simply “born” with the properties we observe it to have. Like all of the 10^24 star systems in the observable Universe, ours was born from a protostar with a nebula and a disk around it, which then spawned planets, asteroids, and frozen, icy, outer bodies, leading to the system we inhabit today.
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- Lots of chances will lead, inevitably, to some low-probability outcomes, like the emergence of intelligent life, on some of them. You are reading this. That is evidence enough.
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- This approach relies on there being a large number of possible outcomes, all with their own likelihoods, and a large number of chances for those outcomes to occur. The other approach is frequently more fruitful: to search for a mechanism that could set up and give rise to the initial conditions we’ve observed.
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- The idea of cosmic inflation, pioneered by Alan Guth, proposed that there was an epoch to the Universe prior to the hot Big Bang where space expanded differently from how it expands today. In a Universe filled with stuff, the expansion rate is directly proportional to the energy density of that “stuff,” whatever it is.
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- So that means if your Universe is filled with:
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------------------------- Matter, the expansion rate drops as the volume of the Universe increases, since matter’s energy density is the number of particles divided by the volume they occupy,
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---------------------------- Radiation, the expansion rate drops extra compared to matter, since radiation’s energy density is the number of particles divided by their occupying volume divided by their wavelength, which stretches as the Universe expands,
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--------------------------- Quantum Field inherent to space, then both the expansion rate and the energy density remain constant, since space cannot “dilute” as the Universe expands.
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- That was the big idea behind “inflation“: that the Universe was dominated by some form of energy inherent to space, that it underwent a period of exponential expansion, and that when the quantum field behind inflation decayed into matter-and-radiation, inflation came to an end and the Universe “reheated,” and the conditions that we identify with the hot Big Bang then arose.
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- How does “cosmic inflation” solve the horizon, flatness, and monopole problems?
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---------------------- The Universe reached a maximum temperature at the end of inflation, preventing the “monopole problem” pathologies,
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---------------------- The Universe has larger-scale uniformity and structure than anticipated because inflation “stretched” various regions of space to larger scales that the traditional (non-inflationary) cosmic horizon,
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--------------------- The Universe is flat, today, because inflation’s dynamics determined both the initial energy density and the initial expansion rate.
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- In addition, there were four new predictions that were made concerning cosmic inflation where the predictions differed from the hot Big Bang:
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-------------------- The Universe achieves a maximum temperature that’s orders of magnitude below the Planck scale of temperature.
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-------------------- The Universe possesses an initial spectrum of fluctuations where the fluctuations are slightly stronger on large scales than small ones.
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--------------------- The Universe is born with imperfections that are 100% “adiabatic” and 0% “is curvature” in nature.
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--------------------- The Universe possess super-horizon fluctuations, exhibiting structure on cosmic scales that exceed the distance that light could have traveled since the Big Bang.
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- Where did inflation come from? Was it eternal, or did it only last for a finite amount of time? In 2003, the Borde-Guth-Vilenkin (BGV) theorem showed that inflating spacetimes are what we call “past-timelike incomplete,” which means that inflation cannot describe a “beginning” to the Universe.
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- That doesn’t necessarily mean the Universe had a non-inflationary beginning; it only implies that if inflation was not an eternal state, it must have arisen from a previous state that, perhaps, did have a beginning. It is also uncertain whether the BGV theorem will apply to a fully quantum theory of gravity.
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- If inflation did arise from a pre-existing state, then what was that state like? Using the rules of quantum field theory it could have arisen from a non-inflationary spacetime with a condition very much like a Bunch-Davies vacuum, and then gave rise to the inflationary state that set up the hot Big Bang.
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- Both experimentally and observationally there’s no information accessible in our visible Universe, that would allow us to determine how inflation arose, or even whether inflation arose at all. In fact, because of the relentless expansion of the Universe during inflation, it can take a region as small as the Planck length on all sides, the smallest possible size at which the laws of physics make sense, and that region will be stretched to larger than the presently observable Universe in under 10^-32 seconds.
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- This fraction-of-a-second of inflation is the only interval that has any way of imprinting itself onto our Universe. Anything that occurred prior, including earlier phases of inflation, the beginning of inflation , or whatever occurred previously, has been wiped clean from our Universe by the dynamics of inflation itself.
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- The Big Bang wasn’t the beginning of time and space, and cosmic inflation, which preceded it, cannot be the beginning either, unless it went on for an eternity. After a century of cosmic revolutions, we’re right back where we started: unable to answer the most fundamental question we can ask, “how did it all begin?”
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- Thank you for completing the circle with me. How did I get here?
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- July 15, 2021 UNIVERSE - did it have a beginning? 3221
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--------------------- --- Saturday, July 17, 2021 ---------------------------
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