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--------------- 2558 - ANTHROPIC PRINCIPLE - if the universe was different?
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- When you look at the Universe on the largest scales, only one force matters: the gravitation force. While the nuclear and electromagnetic forces that exist between particles are many, many orders of magnitude stronger than the gravitational force, they cannot compete on the largest cosmic scales.
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- The Universe is electrically neutral, with one electron to cancel out the charge of every proton in the Universe, and the nuclear forces are extremely short-range, failing to extend beyond the scale of an atomic nucleus.
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- When it comes to the Universe as a whole, only gravitation matters. The Universe expands at the rate it does throughout its history only because our laws of gravity and all the forms of energy that exist in the Universe. If things were slightly different from how they actually are, we wouldn't exist. This idea is called he anthropic principle.
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- The Universe appears to be enormously fine-tuned.
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- On the one hand, we have the expansion rate that the Universe had initially, close to the Big Bang. On the other hand, we have the sum total of all the forms of matter and energy that existed at that early time as well, including:
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--------------------------------- radiation,
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--------------------------------- neutrinos,
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--------------------------------- normal matter,
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--------------------------------- dark matter,
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--------------------------------- antimatter,
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--------------------------------- dark energy.
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- Einstein's General theory of Relativity gives us an intricate relationship between the expansion rate and the sum total of all the different forms of energy in it. If you know what your Universe is made of and how quickly it starts expanding initially, you can predict how it will evolve with time.
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- The expected fates of the Universe all correspond to a Universe where the... [+] matter and energy combined fight against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained.
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- All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it. There's an apparent fine-tuning issue here, but there may be an underlying physical cause.
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- A Universe with too much matter-and-energy for its expansion rate will recollapse in short order; a Universe with too little will expand into oblivion before it's possible to even form atoms.
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- Not only has our Universe neither recollapsed nor failed to yield atoms, but even today, some 13.8 billion years after the Big Bang, those two sides of the equation appear to be perfectly in balance.
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- If we extrapolate this back to a very early time, one nanosecond after the hot Big Bang, we find that not only do these two sides have to balance, but they have to balance to an extraordinary precision. The Universe's initial expansion rate and the sum total of all the different forms of matter and energy in the Universe not only need to balance, but they need to balance to more than 20 significant digits.
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- To balance to more than 20 significant digits is like guessing the same 1-to-1,000,000 number as me three times in a row, and then predicting the outcome of 16 consecutive coin-flips immediately afterwards.
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- If the Universe had just a slightly higher matter density , it would be closed and have recollapsed already; if it had just a slightly lower density, it would have expanded much faster and become much larger.
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- The Big Bang, on its own, offers no explanation as to why the initial expansion rate at the moment of the Universe's birth balances the total energy density so perfectly, leaving no room for spatial curvature at all and a perfectly flat Universe. Our Universe appears perfectly spatially flat, with the initial total energy density and the initial expansion rate balancing one another to at least some 20+ significant digits.
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- The odds of this occurring naturally are astronomically small.
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- It's possible, of course, that the Universe really was born this way: with a perfect balance between all the stuff in it and the initial expansion rate. It's possible that we see the Universe the way we see it today because this balance has always existed.
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- In science, when faced with a coincidence that we cannot easily explain, the idea that we can blame it on the initial conditions of our physical system is akin to giving up on science. It's far better, from a scientific point of view, to attempt to come up with a reason for why this coincidence might occur.
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- One option is to claim that there are a near-infinite number of possible outcomes, and a near-infinite number of possible Universes that contain those outcomes. Only in those Universes where our existence is possible can we exist, and therefore it's not surprising that we exist in a Universe that has the properties that we observe.
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- If you read that and your reaction was, "what kind of circular reasoning is that," congratulations. You're someone who won't be suckered in by arguments based on the anthrotropic principle.
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- We can imagine a great variety of possible Universes that could have existed, yet even if we enforce the laws of physics as they are known, there are still fundamental constants required to determine exactly how our Universe behaves and evolves. Quite a large number of fundamental constants are required to describe “reality” as we know it, and science cannot yet explain why they have the values that they do.
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- Science should provide a mechanism for creating these conditions that appear to be finely tuned to us. That mechanism would also make additional predictions that differ from, and are testable against, the predictions that arise from not having that mechanism present.
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- What is the condition as to what separates a non-scientific argument from a scientific one? If all you can do is appeal to the initial conditions of a problem, you'll have no way of testing whether your scenario any further. Other Universes might exist, but if we cannot observe them and determine whether they have the same initial conditions that our Universe has or not, there's no scientific merit there.
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- On the other hand, if some pre-existing phase of the Universe created these initial conditions while also making additional predictions, we'd have something of enormous scientific importance.
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- Inflation causes space to expand exponentially, which can very quickly result in any pre-existing curved space appearing flat. This flatness, when applied to the observable Universe, will create a balance between the observed expansion rate and the total amount of energy present in a given volume of space.
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- In the case of the energy balance of the Universe, where the expansion rate appears to match up with the total energy density perfectly, an idea like cosmic inflation is the perfect theoretical candidate. Inflation would stretch the Universe flat, yielding an energy density that matched the expansion rate, and then when inflation ended, the Big Bang's initial conditions would be set up.
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- Inflation also makes additional predictions that could be experimentally or observationally measured, putting the scenario to the rigorous scientific test we require.
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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 Cosmic Microwave Background radiation, the CMB. New predictions like these are essential for demonstrating the validity of a proposed fine-tuning mechanism.
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- To validate this theory the key is to tease out novel and unique predictions that can be put to the experimental or observational test; without it, our attempts at theorizing will remain divorced from reality.
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- The fact that our Universe has such a perfect balance between the expansion rate and the energy density (today, yesterday, and billions of years ago) is a clue that our Universe really is finely tuned.
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- With robust predictions about the spectrum, entropy, temperature, and other properties concerning the density fluctuations that arise in inflationary scenarios, and the verification found in the Cosmic Microwave Background and the Universe's large-scale structure, we even have a viable solution.
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- Further tests will determine whether our best conclusion at present truly provides the ultimate answer, but we cannot just wave the problem away. The Universe really is finely tuned, and our existence is all the proof e need. If it were any different I would not be able to tell you about it.
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- December 24, 2019 2558
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--------------------- Tuesday, December 24, 2019 --------------------
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