- 2874 - ASTRONOMY - is universe expansion real? - When we study the Universe, there is a whole lot that doesn’t add up. All the matter we observe and infer, from planets, stars, dust, gas, plasma, and exotic states and objects, can’t account for the gravitational effects we see.
--------------------------- 2874 - ASTRONOMY - is universe expansion real?
- When we observe galaxies and measure both their distances and redshifts, it reveals the expanding Universe, and yet there are two recent surprises: observations that indicate the expansion is accelerating (attributed to dark energy), and the fact that different measurement methods lead to two different sets of expansion rates.
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- Are these problems actually real or could they be due to problems with the measurements ?
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- For the expanding Universe, we assume the following:
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------------------ There 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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------------------ That General Relativity, as put forth by Einstein, is our theory of gravitation,
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------------------ That the Universe is isotropic, homogeneous, and expanding,
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------------------ That light obeys Maxwell’s laws of electromagnetism when it behave classically, and the quantum rules that govern it (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.
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- Conversely, if you can instead 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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- You can measure some quantity that’s related to either the observed size or the observed brightness of an object (like a star or galaxy),then 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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- You also measure the redshift of the object, or how much the light has been shifted from its rest-frame wavelength.
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Astronomers use standard candles and standard rulers to measure the expanding Universe. These two general methods are known as standard candles (if they’re based on brightness) and standard rulers (if based on size), as they’re based on simple concepts.
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- If I take an object like a candle or a light bulb, placed at a certain distance away, you will be able to see it with a particular brightness. That’s because it intrinsically has a property inherent to it that causes it to be luminous, ie: an intrinsic brightness.
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- If I 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. 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.
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- A similar story happens for the sizes of objects: the farther away they are, the more their apparent size changes.
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- The details of the measurements are slightly more complicated in the expanding Universe because the geometric properties of space change as time unfolds, but the same principle applies. 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. 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 large volume.
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- Our Universe is clumped and clustered together, and the gravitational over densities, like stars, galaxies, and clusters of galaxies, as well as the underdense regions, exert pushes and pulls on the objects within it, causing them to move around in a variety of directions.
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- Typically, 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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- But that effect is always superimposed over the expansion of the Universe, which is primarily responsible, especially at large distances, the redshifts we observe.
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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, sometimes 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 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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- Because 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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- We can check using independent ways to measure distances to galaxies, as there are a total of a 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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- We look at numerous different, independent methods for measuring the distances to the same sets of objects, and to see whether these distances are consistent with one another. Only if the different methods all yield similar results for the same objects should we consider them trustworthy.
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- The NASA/IPAC Extragalactic Database of Distances was used 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, were included.
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- The results were spectacular: all six methods (spanning 77 various indicators) yielded consistent distances for each of the examined cases. It’s the largest independent test like this we’ve ever performed, and it shows that we don’t appear to be fooling ourselves about cosmic distances.
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- We can confidently state that 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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- October 23, 2020 2874
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--------------------- --- Sunday, October 25, 2020 ---------------------------
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