- 2917 - ASTRONOMY - measurements in astronomy? When we try to comprehend the Universe, there’s a whole lot that doesn’t add up. All the matter we observe and try to measure, from planets, stars, dust, gas, plasma, and exotic states and objects, can’t account for the gravitational effects we see in the orbits of stars in galaxies and galaxies in clusters.
--------------------------- 2917 - ASTRONOMY - measurements in astronomy?
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- When we observe galaxies and measure both their distances and “redshifts“, it reveals the expanding Universe, and yet there are two recent surprises:
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-------------------- Observations that indicate the expansion is accelerating (attributed to dark energy),
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-------------------- The fact that different measurement methods lead to two different sets of expansion rates.
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- The “redshifts” refer to the measurement that the wavelength the longer it travels through empty space. Longer wavelengths are seen closer and into the red end of the light spectrum. A blue light from far away appear as a redlight to astronomers.
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------------------- Three is no evidence of a ‘Universe before the Big Bang’,
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------------------- The laws of physics are the same, everywhere, for all observers at all times,
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------------------ General Relativity, put forth by Einstein, is our “theory of gravitation“,
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----------------- The Universe is isotropic, homogeneous, and expanding,
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----------------- Light obeys Maxwell’s laws of electromagnetism
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------------------- The quantum rules that govern light (quantum electrodynamics) apply when it exhibits quantum behavior.
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- The equation, known as the first “Friedmann equation“, can be derived directly from General Relativity under the above assumptions. It tells you that if you can measure the expansion rate of the Universe today and at earlier times, you can determine exactly what’s in the Universe in terms of matter and energy. The result: 30% matter and 70% energy.
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- Conversely, if you can measure the expansion rate today and the contents of the Universe, you can determine the expansion rate at all times in the past and future.
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------------------------- Measure some quantity that’s related to either the observed size or the observed brightness of an object (like a star or galaxy),
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------------------------- Infer from some other measured quantity or from some known property of the object how intrinsically large or bright the object actually is,
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-------------------- Measure the redshift of the object, or how much the light has been shifted from its rest-frame wavelength.
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- There are two general methods are known as “standard candles” (if they’re based on intrinsic brightness) and standard rulers (if based on size), as they are both based on simple concepts:
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- If you take an object like a candle or a light bulb, and placed it a certain distance away, you will be able to see it with a particular brightness. For every candle or light bulb in the Universe, if we put it at that same distance, it would have a specific brightness that you’d see associated with it. That’s because, intrinsically, it has a property inherent to it that causes it to be luminous: an ‘intrinsic brightness‘.
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- If you move it farther away, it will appear fainter: twice as far away means one-quarter the brightness; three times as far away means one-ninth the brightness; four times as far away means one-sixteenth the brightness, etc.
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- Light emitted from a source spreads out in a spherical shape, and so the farther away you go, the less light you can see with the same amount of collecting area. That is the way that sunlight spreads out as a function of distance.
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- A similar story happens for the sizes of objects: the farther away they are, the more their apparent size changes. The details of the story are slightly more complicated in the expanding Universe because the geometric properties of space change as time unfolds, but the same principle applies.
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- If you can make a measurement that reveals the intrinsic brightness or size of an object, and you can measure the apparent brightness or size of an object, you can infer its distance from you.
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- These cosmic distances are important because knowing how far away the objects you’re viewing allows you to determine how much the Universe has expanded over the time that the light was emitted from when it arrives at our eyes.
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- If the laws of physics are the same everywhere, then the quantum transitions between atoms and molecules will be the same for all atoms and molecules everywhere in the Universe. If we can identify patterns of absorption and emission lines and match them up to atomic transitions, then we can measure how much that light has been redshifted.
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- A small part of that redshift (or blueshift, if the object is moving towards us) will be due to the gravitational influence of all the other objects around it: what astronomers call “peculiar velocity.”
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- The Universe is only isotropic (the same in all directions) and homogeneous (the same in all locations) on average: if you were to smooth it out by averaging over a very large volume.
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- In reality, our Universe is clumped and clustered together, and the gravitational over densities, like stars, galaxies, and clusters of galaxies, as well as the under dense regions, exert pushes and pulls on the objects within it, causing them to move around in a variety of directions.
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- Objects within a galaxy move around at tens-to-hundreds of kilometers / second relative to one another because of these effects, while galaxies can move at hundreds or even thousands of km/s because of peculiar velocities.
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- That effect is always superimposed over the expansion of the Universe, which is primarily responsible, especially at large distances, for the redshifts we observe.
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- There are three things we should worry about in making these measurements:
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--------------------------- If our distance estimates to any of these astronomical objects are biased nearby, we could be mis-calibrating the expansion rate today: the Hubble parameter (also called the Hubble constant).
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-------------------------- If our distance estimates are biased at large distances, we could be fooling ourselves into thinking that dark energy that is expanding he universe is real, where it might be an artifact of our incorrect distance estimates.
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-------------------------- If our distance estimates are incorrect in a way that translates equally, or proportionally, to all galaxies, we could get a different value for the Universe’s expansion by measuring individual objects as compared to measuring, say, the properties of the leftover glow from the Big Bang: the cosmic microwave background.
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- We see that different methods of measuring the Universe’s expansion rate actually do yield different values, with the cosmic microwave background and a few other “early relic” methods yielding a 9% smaller value than all the other measurements.
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- There are a myriad of independent ways to measure distances to galaxies, as there are a total of 77 different “distance indicators” we can use. By measuring a specific property and applying a variety of techniques, we can infer something meaningful about the intrinsic properties of what we’re looking at.
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- By comparing something intrinsic to something observed, we can immediately know, assuming we’ve got the rules of cosmology and astrophysics correct, how far away an object is.
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- Earlier this month, November, 2020, exactly that test was performed to tabulate multiple distances for 12,000 separate galaxies, using a total of six different methods. In particular, a couple of key galaxies frequently used as “anchor points” in constructing the cosmic distance ladder, like the Large Magellanic Cloud and Messier 106.
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- The results were all six methods (spanning 77 various indicators) yielded consistent distances for each of the examined cases.
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- Our understanding of the expanding Universe, our methods for measuring cosmic distances, the existence of dark energy, and the discrepancy between measurements of the Hubble constant using different methods are all robust results.
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- Any one measurement will have large uncertainties associated with it, but a large and comprehensive data set should enable us to render those uncertainties irrelevant by providing sufficient statistics, so long as they’re unbiased.
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November 27, 2020 ASTRONOMY - measurements of expansion 2917
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