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--------------------- 2484 - BIG BANG - how do we know what happened?
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- Most people intuitively hear that word "bang" and picture an explosion, and then conceive of expansion like they would visualize shrapnel hurled outwards in all directions. It's true that the matter and energy in the Universe began in a hot and dense state all at once, and then expanded and cooled as all the various components sped away from one another. However there are differences from an “explosion“:
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- An explosion always begins at a specific location in space.
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- An explosion initially occupies a small but finite volume.
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- An explosion expands rapidly outward in all directions, limited only by the external forces and barriers it encounters.
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- In the case with our Universe when you look farther and farther away, you also look farther and farther into the past. The farthest we can see back in time is 13.8 billion years: our estimate for the age of the Universe. It's the extrapolation back to the earliest times that led to this idea of the Big Bang.
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- The Universe looks the same here as it does a few million or even a few billion light-years away. It has the same densities, the same energies, the same number of galaxies in a given volume of space, etc.
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- The objects that are very far away do indeed appear to move away from us at greater speeds than the nearby objects, but they also don't appear to be the same age as the slower, closer objects. Instead, as we go to extreme distances, the farther ones appear younger, less evolved, greater in number, and smaller in size and mass.
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- Despite the fact that we can see galaxies out to distances in excess of 30 billion light-years, if we track how everything is moving and reconstruct their trajectories back to a common origin, we see the most unlikely of outcomes: the perceived "center" lands right on us. We are at the center of everything we can see.
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- Our galaxy is one of all the trillions of galaxies in the Universe, what are the odds that we would just happen to be right at the center of the explosion that began the Universe?
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- What are the odds that the initial explosion was configured in just such a way, complete with irregular, inhomogeneous densities, varying start times for star formation and galaxy growth, energies that vary tremendously from place-to-place in just the right, fine-tuned fashion, and a mysterious 2.7 K background glow in all directions, to conspire so that we are exactly at the center?
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- Instead, the law of gravity that governs our Universe, Einstein's General theory of Relativity, predicts that a Universe full of matter and energy doesn't explode, but instead expands.
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- A Universe that's full of equal amounts of stuff everywhere, with the same average densities and temperatures, must either expand or contract; since we observe an apparent recession, the expansion solution is the only one that is physical.
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- There's a misconception that an expanding Universe can be extrapolated back to a single point; this is not true! Instead, it can be extrapolated back to a region of finite size with certain properties, filled with matter, radiation, the laws of physics, etc, but then must evolve according to the rules that our theory of gravity lays out.
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- What this leads to, inevitably, is a Universe that has similar properties everywhere. This means that in any finite, equally-sized region of space, we should see the same density to the Universe, the same temperature to the Universe, the same number of galaxies, etc.
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- We qould also see a Universe that appeared to evolve with time, as more distant regions should appear to us as they were in the past, having expanded less and having experienced less gravitational attraction and smaller amounts of clustering.
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- Because the Big Bang happened everywhere at once a finite amount of time ago, our local corner of the Universe will appear to be the oldest corner of the Universe that there is. From our vantage point, what appears to us nearby is almost as old as we are, but what appears at great distances is much more similar to what our nearby Universe was like many billions of years ago.
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- When you look at a region of the sky with an instrument like the Hubble Space Telescope, you are not simply viewing the light from distant objects as it was when that light was emitted, but also as the light is affected by all the intervening material, and the expansion of space, that it experiences along its journey.
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- Hubble has taken us farther back than any other observatory to date, and has shown us a Universe that evolves in galaxy type, size, and number density with time.
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- The distant galaxies that exist are constantly emitting light, and we are seeing the light that has arrived only after it has completed its journey to get to us through the expanding Universe. Galaxies whose light took a billion or ten billion years to get here appear as they were a billion or ten billion years ago.
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- If we go all the way back, towards almost the moment of the Big Bang itself, we'd find that the Universe when it was that young was dominated by radiation, and not matter. It has to expand and cool for matter to become more important, energy-wise. Remember matter and energy are two forms of the same thing.
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- Over time, as that Universe expands and cools, neutral atoms can finally become stable without being immediately blasted apart by radiation. The radiation that once dominated the Universe, however, still persists, and continues to cool and redshift, become wider wavelengths, due to the expansion of space.
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- What we perceive today as the Cosmic Microwave Background is consistent with being the leftover glow, much wider wavelengths, from this Big Bang radiation that started out with much narrower wavelengths.. But, the same thing can also observable from anywhere in the Universe.
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- The large-scale structure of the Universe changes over time, as tiny imperfections grow to form the first stars and galaxies, then merge together to form the large, modern galaxies we see today.
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- Looking to great distances reveals a younger Universe, similar to how our local region was in the past. Going back past the earliest galaxies we can observe, we find the leftover glow from the Big Bang itself, which appears in all directions and should be visible from anywhere in the Universe.
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- The Universe began expanding from everywhere, all at once because the Big Bang isn't referring to a special location in space, but rather a special moment in time.
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- The Big Bang is a condition that affects the entire observable Universe all at once at one specific moment. It's the reason why looking at objects that are farther away in space means that we're seeing that object as it was at a moment in the distant past. It's why all directions appear to have rough properties that are uniform regardless of where we look. And it's why we can trace back our cosmic history, through the evolution of the objects we see, as far back as our observatories enable us to go.
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- Despite all that we have access to, despite all that our theories and observations tell us, there's still a tremendous amount that remains unknown to us. We don't know what the actual size of the entire Universe is; we only have a lower limit that it must now be at least 46.1 billion light-years in radius in all directions from our perspective.
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- We don't know what the shape of the fabric of space is, and whether it's positively curved like a sphere, negatively curved like a saddle, or perfectly flat, like a sheet or a cylinder. We don't know whether it curves back on itself or whether it goes on forever.
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- Although we cannot observe farther than the “cosmic horizon” which is presently a distance of 46.1 billion light-years away, there will be more Universe to reveal itself to us in the future. The observable Universe contains 2 trillion galaxies today, but as time goes on, more Universe will become observable to us.
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- The reason we cannot know the true nature of the Universe, the entire, unobservable Universe, is because the portion that we have access to is finite. There's a finite amount of information we're capable of gleaning about our cosmos, even if we develop arbitrarily powerful instruments and detectors.
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- The Universe we can access is likely only a tiny component of whatever it is that actually exists. What is observable to us only sets a lower limit on the entirety of what is out there. The Universe could be finite or infinite, but the things we are certain of is that it's expanding, getting less dense, and that more distant objects appear as they were a long time ago.
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- The Universe is expanding the way your mind is expanding. It's not expanding into anything; you're just getting less dense. The older I get the smarter I was.
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- There are several more Reviews available about the Big Bang and the birth of the Universe. Available upon request.
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- November 14, 2019
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--------------------- Friday, November 15, 2019 -------------------------
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