- 3273 - UNIVERSE - how did it all begin? 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?”
--------------------- 3273 - UNIVERSE - how did it all begin?
- Most of us are perfectly willing to extrapolate the phenomena we see back in time in an unbroken chain of cause-and-effect events. However, this didn’t go back in an infinite chain, but rather there was a “first cause” that led to the very existence of the Universe.
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- For a long time, this picture was supported by the notion of the classical Big Bang, which seemed to imply that the Universe began from a “singularity“: an infinitely hot and dense state from which space and time emerged out of “nothing“.
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- The Big Bang was the start of our Universe as we know it, but not of space and time themselves. The Big Bang was just another “effect“ We think we know what caused the Big Bang. It reopens the question of whether the Universe had a beginning at all, and the answer so far is that we aren’t sure.
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- The first observation of the cosmic redshift which were receding spectral lines of light
First noted by Vesto Slipher in 1917. The Big Bang, originally, was an idea that attempted to explain this expanding 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.
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- General Relativity implied certain inevitable consequences. One of them 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. Gravity says so.
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- A second one 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 physical rules.
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- And third, 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. As the fabric of the Universe expands, the wavelength of radiation stretches out lower less energy frequencies.
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- Putting these pieces of information together led to a phenomenal possibility. 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 the Universe’s expansion. And the farther away we look, the farther back in time we’re seeing.
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- 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 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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- If the universe is 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. 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 came along and uncovered the key predictions of this “Big Bang” scenario:
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---------------- there would be a growing cosmic web over time,
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---------------- 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,
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--------------- when the Universe cools enough, we should see that leftover background of radiation 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 equations.
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- In the mid-1960s, Bell Labs scientists Arno Penzias and Bob Wilson discovered that all-sky glow at just 3,000 Kelvin: what was initially called the “primeval fireball” and what’s known today as the “Cosmic Microwave Background“.
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- The three big puzzles that emerged in the aftermath of the Big Bang’s widespread acceptance were:
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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“.
- The Universe was simply “born” with the properties we observe it to have: This line of thought sometimes applies, as it does in the case of our Solar System. Just 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. Lots of chances will lead, inevitably, to some low-probability outcomes, like the emergence of intelligent life, on some of them.
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- But 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.
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- 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. Such a mechanism needs to rise to the threefold challenges of reproducing all of the successes of the theory it’s attempting to supersede, of explaining the problems or puzzles the prevailing theory cannot, and of making testable predictions that are different from the pre-existing idea.
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- 40 years ago the idea of “cosmic inflation” was introduced by Alan Guth and others including Alexei Starobinskii, Andrei Linde, Paul Steinhardt, and Andy Albrecht.
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- That inflation 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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- 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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------------------- or a quantum field inherent to space, then both the expansion rate and the energy density remain constant, since space and the fields present within it 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 cosmic inflation solves the horizon, flatness, and monopole problems?
This possible solution was brilliant, but would it work? It took some substantial theoretical work to modify Guth’s original, promising idea until it could reproduce the Big Bang’s successes.
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- It was immediately clear how it resolved the monopole, horizon, and flatness problems:
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-------------------- the Universe reached a maximum temperature at the end of inflation, preventing the “monopole problem”,
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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.
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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% isocurvature in nature.
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----------------------- The Universe should 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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- All four of these predictions have now been tested, and inflation, as compared to the non-inflationary hot Big Bang, is 4-for-4 in its successes.
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- If inflation did arise from a pre-existing state, then what was that state like? It could have arisen from a non-inflationary spacetime with a condition and then gave rise to the inflationary state that set up the hot Big Bang.
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- There is no information accessible to us 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 final 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 (if it had one), 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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- How did I get here? What does it all mean? How will it all end?
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- September 12, 2021 UNIVERSE - how did it all begin? 3273
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--------------------- --- Tuesday, September 14, 2021 ---------------------------
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