- 2924 - COSMIC INFLATION - understanding the Universe? Shortly after the Big Bang, the universe was a relatively small, nearly infinitely dense place. 13.8 billion years ago the expanding universe means the entirety of what we know is now incredibly large and is getting more immense every day. The Universe is expanding at an ever accelerating rate.
----------------- 2924 - COSMIC INFLATION - understanding the Universe
- Today the Universe is an immensely large place. Even distances between the nearest objects are staggering, and the distances across the Milky Way Galaxy and certainly between galaxies in the universe are astonishingly huge to us living beings stuck on a planet.
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- A model of the Milky Way wherein the Sun is a grain of sand stars being the sand grains, are 4 miles apart in the Milky Way’s disk and the disk is about 40,000 miles across.
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- The concept of the size of the universe has taken a huge stride forward in just the last few years. There was a time not too long ago when astronomers did not know even the approximate size of the cosmos with any degree of accuracy. We still don’t know with high precision.
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- The Big Bang theory tells us that once the universe was very small. We know the fastest that radiation or any information can travel is the speed of light, 186,000 miles per second.
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- We are confident that the universe is 13.8 billion years old. We also know that a light-year is equal to approximately 6 trillion miles . In nearly 14 billion years, we might expect radiation to expand radially outward to something like 30 billion light-years across.
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- But the Big Bang was not like an explosion that went off in a room. Following the Big Bang, space-time itself expanded radially outward at all points, meaning all of space expanded too, not just the stuff within it.
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- As the expansion of the universe began, just 1 centimeter of “empty space” interstitially became 2 centimeters over time, and so on. So the best ideas about the size of the universe allowing for its expansion over time point to a radius of slightly more than 46 billion light-years and therefore a diameter for the universe of approximately 93 billion light-years.
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- A thorough understanding of our neighborhood, our solar system, our area of the Milky Way, our galaxy, and so on is critical to comprehending how the universe works. And exploring the cosmic distance scale also unveils a slew of interesting objects astronomers use to determine distances to objects near and far.
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- The seeds of measuring the universe stretch back in time all the way to the Greek astronomer Aristarchus of Samos (ca. 310–230 B.C.), who had correct notions of parallax in mind with regard to distances of the Sun and Moon.
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- Parallax is the technique of measuring the offset of nearer bodies to the distant background of stars and geometrically calculating a distance.
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- Little progress took place after Aristarchus until Polish astronomer Nicolas Copernicus (1473–1543) proposed the heliocentric model of the cosmos, and it was one of the last great visual astronomers, Danish nobleman Tycho Brahe (1546–1601), who made the first parallax measurements of comets and helped define a more modern distance scale to nearby objects.
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- Imagine a scale solar system with the Sun on one end and 1 centimeter representing the distance between our star and Earth, called an astronomical unit (AU). That is, 1 AU = 1 centimeter. You actually can draw this out on paper to help crystallize it in your mind.
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- Tape several sheets of paper together. With the Sun at one end, Earth is 1 centimeter away, and Mercury and Venus are in there too at 0.4 centimeter and 0.7 centimeter, respectively. Outward from Earth, we have Mars at 1.5 centimeters, the main-belt asteroids centered around 2.5 centimeters, Jupiter at 5 centimeters, Saturn at 9.5 centimeters, Uranus at 19 centimeters, and Neptune at 30 centimeters. Pluto can be placed at 40 centimeters.
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- The outer solar system is sparse, consisting of the Kuiper Belt region from 30 to 50 centimeters from the Sun, and planets beyond Pluto, Haumea at 40 centimeters, Makemake at 45 centimeters, and Eris at 60 centimeters.
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- Now you can finish by indicating the region of the scattered disk, a sparse body of energetically “spun up” icy asteroids, between 50 and 100 centimeters from the Sun. This gives you a complete scale model of the solar system in a region spanning 1 meter, or 3 feet, across.
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- Now appreciate that on this scale, the inner edge of the Oort Cloud, the vast halo of 2 trillion comets on the solar system’s perimeter, is 100 meters farther away than the edge of your diagram. The outer edge of the Oort Cloud, on this scale, is 1,000 meters , more than 10 football fields) away.
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- Yet as human astronaut-explorers, we only have traveled as far away as the Moon, about 1/389 AU, or on our scale 1/389 centimeter, from Earth, which on this scale is about the size of a human red blood cell. That distance is imperceptibly close to our planet’s “dot” on the scale drawing.
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- And yet the distances to the nearest stars are larger than our imagined scale of the Oort Cloud. And then come perhaps 400 billion stars scattered across the bright disk of our Milky Way Galaxy, 150,000 light-years across, and a hundred billion more galaxies spread across a vast cosmos.
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- The next time you’re out under the stars, look up and think carefully about the enormity of the universe. It is one of the great humbling feelings of humanity. I can not explain how I exist?
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- The biggest questions humanity has ever pondered, the most profound is, “where did all of this come from?” For generations, we told one another tales of our own invention, and chose the narrative that sounded best to us.
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- The idea that we could find the answers by examining the Universe itself was foreign until recently, when scientific measurements began to solve the puzzles that had stymied philosophers, theologians, and thinkers alike.
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- The 20th century brought us General Relativity, quantum physics, and the Big Bang, all accompanied by spectacular observational and experimental successes. These frameworks enabled us to make theoretical predictions that we then went out and tested, and they passed with flying colors while the alternatives fell away.
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- But, for the Big Bang, it left some unexplained problems that required us to go farther. When we did, we found an uncomfortable conclusion that we’re still reckoning with today: any information about the beginning of the Universe is no longer contained within our observable cosmos.
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- Looking back to greater distances means looking back in time. The stars and galaxies we see today didn't always exist.
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- In the 1920s, our conception of the Universe changed forever as two sets of observations came together in perfect harmony. For the past few years, scientists led by Vesto Slipher had begun to measure spectral lines, emission and absorption features, of a variety of stars and nebulae.
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- Because atoms are the same everywhere in the Universe, the electrons within them make the same transitions: they have the same absorption and emission spectra. But a few of these nebulae, the spirals and ellipticals in particular, had extremely large redshifts that corresponded to high recession speeds: faster than anything else in our galaxy.
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- Starting in 1923, Edwin Hubble and Milton Humason began measuring individual stars in these nebulae, determining the distances to them. They were far beyond our own Milky Way: millions of light-years away in most instances.
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- When you combined the distance and redshift measurements together, it all pointed to one inescapable conclusion that was also theoretically supported by Einstein’s General theory of Relativity: the Universe was expanding. The farther away a galaxy is, the faster it appears to recede from us.
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- The redshift-distance relation, from Hubble to the present, and the expanding Universe.
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- If the Universe is expanding today, that means that all of the following must be true:
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--------------------- The Universe is getting less dense, as the fixed amount of matter in it occupies larger and larger volumes.
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--------------------- The Universe is cooling, as the light within it gets stretched to longer wavelengths.
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--------------------- Galaxies that aren’t gravitationally bound together are getting farther apart over time.
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- Those are some remarkable and mind-bending facts, as they enable us to extrapolate what’s going to happen to the Universe as time marches inexorably forwards. But the same laws of physics that tell us what’s going to happen in the future can also tell us what happened in the past, and the Universe itself is no exception.
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- If the Universe is expanding, cooling, and getting less dense today, that means it was smaller, hotter, and denser in the distant past.
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- The big idea of the Big Bang was to extrapolate this back as far as possible: to ever hotter, denser, and more uniform states as we go earlier and earlier. This led to a series of remarkable predictions, including that:
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----------------------- More distant galaxies should be smaller, more numerous, lower in mass, and richer in hot, blue stars than their modern-day counterparts,
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---------------------- There should be fewer and fewer heavy elements as we look backwards in time,
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----------------------- There should come a time when the Universe was too hot to form neutral atoms (and a leftover bath of now-cold radiation that exists from that time),
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------------------------ There should even come a time where atomic nuclei were blasted apart by the ultra-energetic radiation (leaving a relic mix of hydrogen and helium isotopes).
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- All four of these predictions have been observationally confirmed, with that leftover bath of radiation called the “cosmic microwave background” , discovered in the mid-1960s.
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- This means that we can extrapolate the Big Bang all the way back, arbitrarily far into the past, until all the matter and energy in the Universe is concentrated into a single point. The Universe would reach infinitely high temperatures and densities, creating a physical condition known as a singularity: where the laws of physics as we know them give predictions that no longer make sense and cannot be valid anymore.
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- The Universe began with a Big Bang some finite time ago, corresponding to the birth of space and time, and that everything we’ve ever observed has been a product of that aftermath.
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- We have a scientific answer that truly indicates not only that the Universe had a beginning, but when that beginning occurred. In the words of Georges Lemaitre, the first person to put together the physics of the expanding Universe, it was “a day without yesterday.”
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- Only, there remains a number of unresolved puzzles that the Big Bang posed, but presented no answers for:
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----------------------- Why did regions that were causally disconnected, i.e., had no time to exchange information, even at the speed of light, have the same temperatures as one another?
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---------------------- Why were the initial expansion rate of the Universe (which works to expand things) and the total amount of energy in the Universe (which gravitates and fights the expansion) perfectly balanced early on: “to more than 50 decimal places?”
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--------------------- Why, if we reached these ultra-high temperatures and densities early on, are there no leftover relic remnants from those times in our Universe today?
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- In 1979, a young theorist named Alan Guth had a spectacular realization that changed history. The 3 big puzzles, the horizon, flatness, and monopole problems, that inflation solves.
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- His new theory was known as cosmic inflation, and postulated that perhaps the idea of the Big Bang was only a good extrapolation back to a certain point in time, where it was preceded by this inflationary state. Instead of reaching arbitrary high temperatures, densities, and energies, inflation states that:
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------------------ The Universe was no longer filled with matter and radiation,
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------------------ Instead the universe possessed a large amount of energy intrinsic to the fabric of space itself,
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------------------ Which caused the Universe to expand exponentially where the expansion rate doesn’t change over time,
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----------------- Which drives the Universe to a flat, empty, uniform state,
until inflation ends.
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- When it ends, the energy that was inherent to space itself, the energy that’s the same everywhere, except for the quantum fluctuations imprinted atop it, gets converted into matter and energy, resulting in a hot Big Bang.
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- How did inflation and quantum fluctuations give rise to the Universe we observe today? Theoretically, this was a brilliant leap, because it offered a plausible physical explanation for the observed properties the Big Bang alone could not account for:
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- Causally disconnected regions have the same temperature because they all arose from the same inflationary “patch” of space.
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- The expansion rate and the energy density were perfectly balanced because inflation gave that same expansion rate and energy density to the Universe prior to the Big Bang.
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- And there were no left over, high-energy remnants because the Universe only reached a finite temperature after inflation ended.
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- Inflation also made a series of novel predictions that differed from that of the non-inflationary Big Bang, meaning we could go out and test this idea. As of today, in 2020, we’ve collected data that puts four of those predictions to the test:
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--------------------- The Universe should have a maximum, non-infinite upper limit to the temperatures reached during the hot Big Bang.
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-------------------- Inflation should possess quantum fluctuations that become density imperfections in the Universe that are 100% adiabatic (with constant entropy).
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-------------------- Some fluctuations should be on super-horizon scales: fluctuations on scales larger than light could have traveled since the hot Big Bang.
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-------------------- Those fluctuations should be almost, but not perfectly, scale-invariant, with slightly greater magnitudes on large scales than small ones.
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- With data from satellites like COBE, WMAP, and Planck, we’ve tested all four, and only inflation (and not the non-inflationary hot Big Bang) yields predictions that are in line with what we’ve observed.
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- But this means that the Big Bang wasn’t the very beginning of everything; it was only the beginning of the Universe as we’re familiar with it. Prior to the hot Big Bang, there was a state known as cosmic inflation, that eventually ended and gave rise to the hot Big Bang, and we can observe the imprints of cosmic inflation on the Universe today.
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- But only for the last tiny, minuscule fraction of a second of inflation. Only, perhaps, for the final 10^-33 seconds of it can we observe the imprints that inflation left on our Universe. 10^33 seconds is an exceedingly short period o time?
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- It is possible that inflation lasted for only that duration, or for far longer. It’s possible that the inflationary state was eternal, or that it was transient, arising from something else.
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- It’s possible that the Universe did begin with a singularity, or arose as part of a cycle, or has always existed. But that information doesn’t exist in our Universe. Inflation, by its very nature, erases whatever existed in the pre-inflationary Universe.
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- In many ways, inflation is like pressing the cosmic “reset” button. Whatever existed prior to the inflationary state, if anything, gets expanded away so rapidly and thoroughly that all we’re left with is empty, uniform space with the quantum fluctuations that inflation creates superimposed atop it.
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- When inflation ends, only a tiny volume of that space, somewhere between the size of a soccer ball and a city block, will become our observable Universe. Everything else, including any of the information that would enable us to reconstruct what happened earlier in our Universe’s past, now lies forever beyond our reach.
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- It’s one of the most remarkable achievements of science of all: that we can go back billions of years in time and understand when and how our Universe, as we know it, came to be this way.
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- But like many adventures, revealing those answers has only raised more questions. The puzzles that have arisen this time, however, may truly never be solved. If that information is no longer present in our Universe, it will take a revolution to solve the greatest puzzle of all: where did all this come from?
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- December 2, 2020 COSMIC INFLATION 2924
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--------------------- --- Wednesday, December 2, 2020 ---------------------------
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