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-------------------- 2524 - UNIVERSE - Structure of the universe?
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- When you picture the universe, you probably start thinking of individual objects like stars and galaxies, where they’re located in space relative to one another and what those objects are doing today. You can think about the origin and evolution and growth of everything in the universe, from the smallest cosmic scales up to the scale of the entire observable universe.
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- The quantum fluctuations that occur during the universe beginning with 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 CMB.
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- The growth of structure from these seed fluctuations, and their imprints on the power spectrum of the Universe and the CMB’s temperature differentials, can be used to determine various properties about our Universe. The quantum fluctuations that occur during inflation get stretched across the universe.
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- On a logarithmic scale, the universe nearby has the solar system and our Milky Way galaxy. But far beyond are all the other galaxies in the Universe, the large-scale cosmic web, and eventually the moments immediately following the Big Bang itself.
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- Although we cannot observe farther than this 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. It is 46.1billion lightyears because the universe has been expanding during the time the light is traveling to each us over 13.8 billion years.
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- The observable Universe contains 2 trillion galaxies today, but as time goes on, more Universe will become observable to us, perhaps revealing some cosmic truths that are obscured to us today.
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- Some 13.8 billion years ago, the fabric of space-time was empty but full of energy inherent to space itself: a period of cosmic inflation. Then at one particular moment in time, inflation came to an end, converting that energy into matter, antimatter and radiation, and giving rise to the hot Big Bang that started it all.
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- Our Universe, as we know it, arose from this state, and was also born filled with dark matter, dark energy and with tiny density-and-temperature imperfections that departed from a perfectly uniform universe by about 1-part-in-30,000.
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- The universe ruled by the laws of quantum physics that govern matter and the gravitational force govern the curvature and evolution of space-time expanded and cooled and gravitated, give rise to a bath of leftover radiation, a universe filled with light and heavy elements, stars, galaxies, clusters, the cosmic web and more.
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- Our entire cosmic history is theoretically well-understood in terms of the frameworks and rules that govern it. It’s only by observationally confirming and revealing various stages in our universe’s past that must have occurred, like when the first elements formed, when atoms became neutral, when the first stars and galaxies formed and how the Universe expanded over time, that we can truly come to understand what makes up our universe and how it expands and gravitates in a quantitative fashion.
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- This is the story that we know to be true today, but only the barest bones of this framework were in place back in the early 1960s when I graduated from college. Not only wasn’t inflation, dark matter or dark energy part of the story yet, but the Big Bang was only one of a few competing ideas about the universe’s origin.
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- We knew how successful General Relativity was, but we were still working out the details of the nuclear forces. We did not even know the particle content of our universe.
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- By applying the laws of physics to the system of the entire universe we began to work out details of what the universe would have been like in its early stages and how those details would evolve, over time, to produce visible signatures we could look for today.
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- The universe, particularly on smaller scales, is not perfectly homogeneous, but on large scales the homogeneity and isotropy is a good assumption to better than 99.99% accuracy.
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- The tiny, initial imperfections that the universe was born with would try to gravitationally grow from the moment they were created, but the intense radiation pressure in the early, hot, dense universe smooths out the structure on scales that are too small. Instead, particles and antiparticles collide, blasting any complex structure apart, and eventually annihilating as the universe expands and cools.
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- As the universe expands and cools, more and more things become possible. Protons and neutrons can fuse into atomic nuclei, and we can use the laws of physics to calculate what the ratios of the different elements and isotopes produced should be, and then observe the universe to test it.
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- As the universe cools farther, neutral atoms can stably form, and all that radiation produced from annihilation should freely stream through the neutral universe, presenting an observable signature of a leftover blackbody signal just a few degrees above absolute zero. Now called the Cosmic Microwave Background.
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- The relative heights and positions of these acoustic peaks, derived from the data in the Cosmic Microwave Background, are definitively consistent with a universe made of 68% dark energy, 27% dark matter and 5% normal matter.
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- Gravitational growth should at last occur, as matter attracts other matter and begins to collapse on all scales. As the cosmic web grows, it is counter acted by the physical effect of the expansion, and only regions that become over dense enough soon enough will eventually grow into structure.
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- The structures that form will be very sensitive to the contents of the universe, and how that structure clusters together on large scales can allow you to learn about what the cosmos is made of. Those signals should then also be present in the detailed fluctuations in the Cosmic Microwave Background; signals that were at last verified with satellites like COBE, WMAP and Planck.
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- A standard cosmic timeline of our universe’s history. Our Earth didn't come to exist until 9.2 billion years after the Big Bang, requiring many generations of stars to live and die before planets with rocky and metallic cores could exist.
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- Today the universe should be rich in stars with exoplanets, and they have come in forms and distributions that have forced us to reevaluate how planetary systems form and evolve. Coming down from cosmic scales to Solar System scales, we need to go through billions of years of cosmic evolution.
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- Stars live and die and explode, recycling their now-fused elements into future generations of stars. When enough generations have passed, and the material found in star-forming regions is rich enough in heavy elements, stars can form with massive planets around them.
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- Those planets should come complete with metallic and rocky cores, just like all the planets in our Solar System. They should orbit their parent star in an ellipse, governed by the laws of gravity and having observable effects on the spectrum of the star they’re orbiting.
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- The gravitational planetary tug should redshift-and-blueshift the star periodically, while planets that are aligned with the star’s line of sight to Earth will transit in front of it, blocking a portion of its light.
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- As a planet orbits its parent star, both the star and planet will orbit in ellipses around their mutual center of mass.
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- Along our line of sight, the star will appear to move in an oscillatory fashion: moving towards us (and having its light blueshift) followed by it moving away from us (and seeing a corresponding redshift). This method, in 1995, yielded us the first exoplanet orbiting a Sun-like star.
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- Thirty years ago, only the Sun was known to have planets around it. Soon after, though, technology advanced to the point where the shift in a star’s spectral lines from “wobbling” back-and-forth would show up in long-period observations of that particular star.
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- While a controversial detection was first made in 1988 and the first noncontroversial detection came for planets around pulsars (a type of dead star) in 1992, neither heralded the exoplanet revolution quite like the next giant leap.
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- The first “normal” planet around a “normal” (Sun-like) star came in 1995. Exoplanets became all the rage. This stellar wobble method has since been augmented by other techniques like direct imaging, microlensing and planetary transits, revealing a total of over 4,000 confirmed exoplanets so far. With TESS currently flying and additional space telescopes on the horizon, the field is richer than ever.
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- Today we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Keplerdata. These planets range in size from larger than Jupiter to smaller than Earth.
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- Yet because of the limitations on the size of Kepler and the duration of the mission, the majority of planets are very hot and close to their star, at small angular separations. TESS has the same issue with the first planets it’s discovering: They’re preferentially hot and in close orbits.
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- Only through dedicated, long-period observations (or direct imaging), will we be able to detect planets with longer period (i.e., multiyear) orbits. New and near-future observatories are on the horizon, and should reveal new worlds where right now there are only gaps.
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- It will take longer-duration missions with excellent light-gathering power and sensitivity to reveal the first Earth-like world around a Sun-like star. There are plans in both NASA’s and ESA’s timelines for such missions. Some of these missions, like James Webb and WFIRST, will also be extraordinary for their cosmological capabilities.
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- With only a small percentage of the universe and the nearest exoplanets currently revealed to us, the coming decades should see scientists in these fields push the frontiers forwards into unknown territory.
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- Over 90% of the 2 trillion galaxies present in our observable Universe remain undiscovered; Only 4,000 exoplanets are known in a galaxy that should contain trillions of them, including billions that may be Earth-like.
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- What living in these distant worlds could be human like. Or, even life like? Our home is so unique with so many probabilities stacking up to get to where we are. I don’t know about you , but , being impossible makes me feel pretty important, or, more accurately just lucky.
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- December 4, 2019 2524
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--------------------- Wednesday, December 4, 2019 --------------------
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