Editing 2516: Hubble Tension
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==Explanation== | ==Explanation== | ||
+ | {{incomplete|Created by Dave - Please change this comment when editing this page. Do NOT delete this tag too soon.}} | ||
− | + | The fact that most galaxies are receeding from us, and that the distance to the galaxy is directly proportional to the speed (as measured by redshift) was discovered in the 1920s by Edwin Hubble and others. This constant of proportionality is known as the Hubble Constant. | |
− | + | One way of measuring the Hubble Constant is to measure the distance to (relatively) nearby galaxies. Once distance is obtained, speed can be easily obtained by measuring the red-shift and thus the Hubble Constant calculated. Measuring the distance turns out to be fiendishly difficult because a distant bright star looks the same as a dim star that is closer, and localised movements can influence the speed of recession — though less significantly, for multiple reasons, the further away are the objects are that you study. | |
− | + | In practice, astronomers have a number of ways of measuring distance that work at different scales, and they can be built upon to measure distance to far away galaxies. This is known as the cosmological distance ladder. | |
− | + | The first rung is parallax. As the Earth orbits around the Sun, nearby stars appear to move slightly relative to distant stars; a star that moves by one second of arc is said to have a distance of 1 Parsec — about 3¼ light years or 30 trillion (3x10<sup>13</sup>) kilometres. | |
− | The | + | The next rung is Cepheid variables, which periodically brighten and dim. The frequency of variation is related to the absolute brightness of the star, and thus by comparing the absolute to the relative brightness (subject to the {{w|Inverse-square law|inverse square}} where not otherwise obscured) the distance can be measured. |
− | + | The final rung is Type 1a Supernovae, which occur when an accreting white dwarf exceeds 1.4 solar masses. Because the initial mass is always identical, the absolute brightness of the explosion is as well, so the distance can be similarly calculated. | |
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− | The final rung is | ||
Putting these together, the best measurement of the Hubble Constant is 73 km/s/Mparsec. | Putting these together, the best measurement of the Hubble Constant is 73 km/s/Mparsec. | ||
− | This is in conflict with the other main way of measuring the Hubble Constant, | + | This is in conflict with the other main way of measuring the Hubble Constant, analysing makeup of the Cosmic Microwave Background radiation, which yields a value of 68 km/s/Mparsec. The difference is statistically significant, and well outside the error bounds of each measurement. |
− | Since the CMB technique relies on our understanding and assumptions about the early universe, as well as on the cosmological effects of General Relativity on large scales, if this discrepancy proved real it could be the gateway to new discoveries in cosmology and gravity, as well as possibly shed light on the origin of the universe and a '{{w|Theory Of Everything}}'. Cosmologists got quite excited about this. It might also be that there was a previously unaccounted-for error in any of the rungs of the cosmological distance ladder and | + | Since the CMB technique relies on our understanding and assumptions about the early universe, as well as on the cosmological effects of General Relativity on large scales, if this discrepancy proved real it could be the gateway to new discoveries in cosmology and gravity, as well as possibly shed light on the origin of the universe and a '{{w|Theory Of Everything}}'. Cosmologists got quite excited about this. It might also be that there was a previously unaccounted-for error in any of the rungs of the cosmological distance ladder, and that once that is fixed, the two results will be consistent. |
− | The third method introduced in this comic is a guy named Dave who is trying to use a | + | The third method introduced in this comic is a guy named Dave who is trying to use a police radar gun to try to measure the movement of astronomical bodies. A radar system works by sending electromagnetic radiation from the gun and then measuring the returned radiation to determine how far away or how fast a moderately distant object is moving. Because of the transmission and return times required (and the inverse-square law), a radar device will only be able to get information about the very closest objects, such as the Moon and other objects orbiting the Earth (or the ''perhaps'' the Sun), where the influence of being in orbit utterly dominates over any possible Hubble-shift. And that still needs powerful radar systems like the former {{w|Arecibo Telescope}} to be able to get any useful information that far away, a hand-held radar gun would not be able to 'lock on' across even those distances. |
− | Going by back-calculating grossly ' | + | <!--Going by back-calculating grossly 'idealised' universe models, as suggested by the other two estimates, a receding velocity of 85MPH should be seen at a distance of roughly 16-17 thousand kilometres, about half the distance to geosynchronous orbit (where satellites do ''not'' move much at all, on average, on daily or even yearly timescales), and there seems no reason to believe the radar gun could discover anything at that distance at that speed. The same is true if the figure is actually 85KPH (for around 10,000km), as suggested in the titletext, although this velocity relates to around 50-55MPH, which might be the normal traffic speed on certain roads (perhaps when someone is conspicuously using a radar gun!) if by 'all directions' you effectively mean 'both directions' of traffic flow that Dave could possibly be measuring.--> |
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==Transcript== | ==Transcript== | ||
− | :[Cueball and Ponytail are walking to the right. Ponytail has her palm raised | + | {{incomplete transcript|Do NOT delete this tag too soon.}} |
+ | :[Cueball and Ponytail are walking to the right. Ponytail has her palm raised] | ||
:Ponytail: There are three main estimates of the universe's expansion rate and they all disagree. | :Ponytail: There are three main estimates of the universe's expansion rate and they all disagree. | ||
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:Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec. | :Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec. | ||
− | :[They continue walking to the right. Ponytail points | + | :[They continue walking to the right. Ponytail points to Dave, off screen] |
:Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions. | :Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions. | ||
− | :Dave (off- | + | :Dave (off-screen): ''Those galaxies are really booking it!'' |
:Ponytail: Thanks, Dave. | :Ponytail: Thanks, Dave. | ||
{{comic discussion}} | {{comic discussion}} | ||
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[[Category:Comics featuring Cueball]] | [[Category:Comics featuring Cueball]] | ||
[[Category:Comics featuring Ponytail]] | [[Category:Comics featuring Ponytail]] | ||
− | [[Category: | + | [[Category:Astronomy]] |
[[Category:Physics]] | [[Category:Physics]] |