https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&user=162.158.89.228&feedformat=atomexplain xkcd - User contributions [en]2024-03-28T15:27:51ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=2516:_Hubble_Tension&diff=2181542516: Hubble Tension2021-09-17T19:19:36Z<p>162.158.89.228: /* Explanation */ Given the previous grammar 'fix' (not sure it was actually wrong, just not the phrasing that editor expected to read), making it a proper aside.</p>
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<div>{{comic<br />
| number = 2516<br />
| date = September 15, 2021<br />
| title = Hubble Tension<br />
| image = hubble_tension.png<br />
| titletext = Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!<br />
}}<br />
<br />
==Explanation==<br />
{{incomplete|Created by Dave - Please change this comment when editing this page. Do NOT delete this tag too soon.}}<br />
[[Ponytail]] is telling [[Cueball]] about the {{w|expansion of the universe}} telling him that there are three main estimates of the rate of expansion, and that they all disagree. She then tells him of the two well known (and very complicated) methods, and finally the joke is that the third method is performed by a guy named Dave (who replies from off-panel), and he claims to measure the speeds with a radar gun, as if the galaxies were speeding here on Earth.<br />
<br />
The fact that most {{w|galaxies}} are receding from us, and that the distance to the galaxy is directly proportional to the speed (as measured by {{w|red-shift}}) was discovered in the 1920s by {{w|Edwin Hubble}} and others. This constant of proportionality is known as the {{w|Hubble Constant}}.<br />
<br />
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 localized movements can influence the speed of recession &mdash; though less significantly, for multiple reasons, the further away are the objects that you study.<br />
<br />
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 {{w|Cosmic distance ladder}}. <br />
<br />
The first rung is {{w|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 {{w|Parsec}} &mdash; about 3¼ light years or 30 trillion (3x10<sup>13</sup>) kilometers. <br />
<br />
The next rung is {{w|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}} where not otherwise obscured) the distance can be measured. <br />
<br />
The final rung is {{w|Type Ia supernova}}, which occur when an accreting {{w|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.<br />
<br />
Putting these together, the best measurement of the Hubble Constant is 73 km/s/Mparsec.<br />
<br />
This is in conflict with the other main way of measuring the Hubble Constant, analyzing makeup of the {{w|Cosmic Microwave Background}} (CMB) radiation, which yields a value of 68 km/s/Mparsec. The difference is statistically significant, and well outside the error bounds of each measurement.<br />
<br />
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, once that is fixed, the two results will be consistent.<br />
<br />
The third method introduced in this comic is a guy named Dave who is trying to use a {{w|radar speed gun}} (as used by the police for detecting speeding cars) 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 (a type of {{w|Earth–Moon–Earth communication|Moon bounce}}) and other objects orbiting the Earth (or ''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.<br />
<br />
Going by back-calculating grossly 'idealized' universe models, as suggested by the other two estimates, a receding velocity of 85 miles per hour ('mph'; about 137 kilometers per hour, 'kph' or 'km/h') should be seen at a distance of roughly 1700-1850 light-years, on the order of the thickness of our galactic disc. Much too far to use a radar gun on, also much too close to exclude any significant galactic stellar motions. Much the same is true if the figure is actually 85 kph (1050-1130 ly), as suggested it might be in the title text.<br />
<br />
Aside from being practically incorrect, that value of 85 kph relates to around 53 mph, which might be the normally observed traffic speed on certain roads (especially if 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. Dave may have been referring to the kind of {{w|Ford Galaxy|Galaxy}} that he ''can'' more easily find out the velocity of.<br />
<br />
The comic is likely making fun of the common internet phenomenon of amateur (wannabe?) scientists seeking to discredit established scientific facts by reporting the results of experiments made using everyday tools. Dave has probably heard of the fact that there is no agreement in the scientific measurements of the Hubble constant and decided to try to settle the controversy using the tools at his disposal, without remotely realizing that the margin of error required in the measurements is well outside the range of what can be used with conventional objects.<br />
<br />
Dave might also lack an understanding of units of measure and dimensions. Ponytail describes the measurements of the rate of universal expansion, a speed that varies with distance, in km/s/Mparsec, having dimension 1/T or 1/time. Dave made his measurements in miles/hour or km/h, which have dimension L/T or length/time. These are not comparable with the official units. Dave does not appear to be aware of this (and Ponytail does not draw Cueball or Dave's attention to it).<br />
<br />
==Transcript==<br />
:[Cueball and Ponytail are walking to the right. Ponytail has her palm raised.]<br />
:Ponytail: There are three main estimates of the universe's expansion rate and they all disagree.<br />
<br />
:[They keeping walking to the right.]<br />
:Ponytail: Measurements of star distances suggest the universe is expanding at 73 km/s/megaparsec.<br />
<br />
:[They are still walking to the right.]<br />
:Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec.<br />
<br />
:[They continue walking to the right. Ponytail points towards Dave who replies from off-panel to the right.]<br />
:Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions.<br />
:Dave (off-panel): ''Those galaxies are really booking it!''<br />
:Ponytail: Thanks, Dave.<br />
<br />
{{comic discussion}}<br />
<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Comics featuring Ponytail]]<br />
[[Category:Astronomy]]<br />
[[Category:Physics]]</div>162.158.89.228https://www.explainxkcd.com/wiki/index.php?title=Talk:2514:_Lab_Equipment&diff=218032Talk:2514: Lab Equipment2021-09-14T19:09:02Z<p>162.158.89.228: /* Annealing and Tempering */ Close-paren</p>
<hr />
<div><!--Please sign your posts with ~~~~ and don't delete this text. New comments should be added at the bottom.--><br />
What kind of lasers are used in mass spectroscopy? [[Special:Contributions/172.70.110.173|172.70.110.173]] 10:58, 11 September 2021 (UTC)<br />
:SFAIK, none. Electron beams can be used, but not sure laser-ablation is a big thing in this subfeld. So I just edited that detail out and added what a plain (i.e. ''light'') spectrometer is. (More likely, this being a laser-lab perhaps making use of novel materials, not a primarily materials-analysis one)<br />
:Any actual Laser Lab-Persons reading this might know otherwise, of course, if they can stop melting things for fun and bother to explain things to us... [[Special:Contributions/162.158.158.88|162.158.158.88]] 18:09, 11 September 2021 (UTC)<br />
: https://en.wikipedia.org/wiki/Matrix-assisted_laser_desorption/ionization#Laser I don't know any other uses for lasers in mass spectroscopy, but MALDI (Matrix-Assisted Laser Desorption Ionization) is commonly used in biochemistry and polymer chemistry (and chemistry of any other fragile macromolecule) to ionize molecules in the sample without breaking (fragmenting) them. It is called a "soft" ionization method due to it's propensity to leave the ions in one piece - something shared with ElectroSpray Ionization (ESI), which is commonly used for same purposes, but doesn't use lasers. [[Special:Contributions/162.158.88.21|162.158.88.21]] 20:15, 11 September 2021 (UTC)<br />
::So, yeah, your link says that lasers are used for ionization (I think of large molecules?) prior to mass spectrometry. The information removal would have been in error. [[Special:Contributions/108.162.219.123|108.162.219.123]] 20:59, 11 September 2021 (UTC) (addendum:unless mass spectrometers don't look like the drawing?)<br />
:::Why are we even assuming "mass spectroscopy" when it's just a "spectrometer" mention, in the context of laser-light research? If it's not the obvious (to me) application, it could be {{w|Spectroscopy#Other_types|one of many other disparate developments}}. [[Special:Contributions/141.101.107.247|141.101.107.247]] 21:52, 11 September 2021 (UTC)<br />
::::Sorry, that was my fault, I was the one who put "mass spectrometer." This is why we have multiple editors.[[Special:Contributions/172.69.22.13|172.69.22.13]] 00:39, 12 September 2021 (UTC)<br />
This could have been my lab in the late 1980's. We used a Nd:YAG laser as a light source for Raman Spectroscopy with a stepper motor controlled diffraction grating spectrometer (I think that was what it was called - was over thirty years ago) controlled via a GPIB (IEEE-488) bus by an Apricot PC running custom APL code (don't ask).<br />
One issue was that as it was an IR beam it had its own black painted room that was closed before a spectrum was run (took I think maybe half an hour). [[User:Dhericean|Dhericean]] ([[User talk:Dhericean|talk]]) 09:14, 14 September 2021 (UTC)<br />
<br />
=== Annealing and Tempering ===<br />
Annealing and tempering are used in wider contexts than chocolate and glass, specifically both copper and iron can be annealed and tempered (and most of their alloys). for more information a good place to start is a knife making video where you anneal the knife and then harden just the edge before final sharpening.[[Special:Contributions/108.162.221.191|108.162.221.191]] 18:09, 12 September 2021 (UTC)<br />
:I don't even know where the glass thing came from. I thought a perfectly good link to annealing (definitely including metal, it even says this in the bit I wrote) might be over-explaining what ''might'' be related to chocolate tempering, and then suddenly we're talking about glass which is ''far'' more complicated (just ask Prince Rupert). But that's amorphous solids for you, right..? [[Special:Contributions/141.101.107.115|141.101.107.115]] 23:14, 12 September 2021 (UTC)<br />
::If you have an example of annealing in scientific research then give one, otherwise the phrase "but more likely Randall means a use of annealing in scientific research" sounds like you are speculating about the existence of annealing in scientific research. [[Special:Contributions/172.70.82.111|172.70.82.111]] 01:18, 14 September 2021 (UTC)<br />
:::That was added by(/after?) the jewelry comment-adder, not the initial annealing reference that I (being the above IP, etc) made. I thought that was distracting too (I didn't think that its use in jewelry added anything to the fact that it was a (general) metal-treatment) but if I was to complain about everything added/changed by people other than me I should at least get myself a named-login to build up a reputation for being a grumpy old man about everyone else 'spoiling' things. (And, honestly, I like the actual improvements/corrections that others do, but there's [https://www.enago.com/academy/top-10-journals-publish-negative-results/ even less reason to wax lyrical about those...]) Right, no more from me, at least here on this commentary. Have fun, y'all... [[Special:Contributions/162.158.89.228|162.158.89.228]] 19:07, 14 September 2021 (UTC)<br />
<br />
=== Cloudflare ===<br />
This is new:<br />
This page (https://www.explainxkcd.com/wiki/index.php?title=2514:_Lab_Equipment&action=submit) is currently offline. However, because the site uses Cloudflare's Always Online™ technology you can continue to surf a snapshot of the site. We will keep checking in the background and, as soon as the site comes back, you will automatically be served the live version. Always Online™ is powered by Cloudflare | Hide this Alert<br />
...popover on the page. Didn't seem to stop me editing/previewing. Is it in response from all the (other) Cloudflare errors we've been getting recently, someone activiting a safety-net? [[Special:Contributions/162.158.158.88|162.158.158.88]] 18:09, 11 September 2021 (UTC)</div>162.158.89.228https://www.explainxkcd.com/wiki/index.php?title=Talk:2514:_Lab_Equipment&diff=218031Talk:2514: Lab Equipment2021-09-14T19:07:42Z<p>162.158.89.228: /* Annealing and Tempering */</p>
<hr />
<div><!--Please sign your posts with ~~~~ and don't delete this text. New comments should be added at the bottom.--><br />
What kind of lasers are used in mass spectroscopy? [[Special:Contributions/172.70.110.173|172.70.110.173]] 10:58, 11 September 2021 (UTC)<br />
:SFAIK, none. Electron beams can be used, but not sure laser-ablation is a big thing in this subfeld. So I just edited that detail out and added what a plain (i.e. ''light'') spectrometer is. (More likely, this being a laser-lab perhaps making use of novel materials, not a primarily materials-analysis one)<br />
:Any actual Laser Lab-Persons reading this might know otherwise, of course, if they can stop melting things for fun and bother to explain things to us... [[Special:Contributions/162.158.158.88|162.158.158.88]] 18:09, 11 September 2021 (UTC)<br />
: https://en.wikipedia.org/wiki/Matrix-assisted_laser_desorption/ionization#Laser I don't know any other uses for lasers in mass spectroscopy, but MALDI (Matrix-Assisted Laser Desorption Ionization) is commonly used in biochemistry and polymer chemistry (and chemistry of any other fragile macromolecule) to ionize molecules in the sample without breaking (fragmenting) them. It is called a "soft" ionization method due to it's propensity to leave the ions in one piece - something shared with ElectroSpray Ionization (ESI), which is commonly used for same purposes, but doesn't use lasers. [[Special:Contributions/162.158.88.21|162.158.88.21]] 20:15, 11 September 2021 (UTC)<br />
::So, yeah, your link says that lasers are used for ionization (I think of large molecules?) prior to mass spectrometry. The information removal would have been in error. [[Special:Contributions/108.162.219.123|108.162.219.123]] 20:59, 11 September 2021 (UTC) (addendum:unless mass spectrometers don't look like the drawing?)<br />
:::Why are we even assuming "mass spectroscopy" when it's just a "spectrometer" mention, in the context of laser-light research? If it's not the obvious (to me) application, it could be {{w|Spectroscopy#Other_types|one of many other disparate developments}}. [[Special:Contributions/141.101.107.247|141.101.107.247]] 21:52, 11 September 2021 (UTC)<br />
::::Sorry, that was my fault, I was the one who put "mass spectrometer." This is why we have multiple editors.[[Special:Contributions/172.69.22.13|172.69.22.13]] 00:39, 12 September 2021 (UTC)<br />
This could have been my lab in the late 1980's. We used a Nd:YAG laser as a light source for Raman Spectroscopy with a stepper motor controlled diffraction grating spectrometer (I think that was what it was called - was over thirty years ago) controlled via a GPIB (IEEE-488) bus by an Apricot PC running custom APL code (don't ask).<br />
One issue was that as it was an IR beam it had its own black painted room that was closed before a spectrum was run (took I think maybe half an hour). [[User:Dhericean|Dhericean]] ([[User talk:Dhericean|talk]]) 09:14, 14 September 2021 (UTC)<br />
<br />
=== Annealing and Tempering ===<br />
Annealing and tempering are used in wider contexts than chocolate and glass, specifically both copper and iron can be annealed and tempered (and most of their alloys). for more information a good place to start is a knife making video where you anneal the knife and then harden just the edge before final sharpening.[[Special:Contributions/108.162.221.191|108.162.221.191]] 18:09, 12 September 2021 (UTC)<br />
:I don't even know where the glass thing came from. I thought a perfectly good link to annealing (definitely including metal, it even says this in the bit I wrote) might be over-explaining what ''might'' be related to chocolate tempering, and then suddenly we're talking about glass which is ''far'' more complicated (just ask Prince Rupert). But that's amorphous solids for you, right..? [[Special:Contributions/141.101.107.115|141.101.107.115]] 23:14, 12 September 2021 (UTC)<br />
::If you have an example of annealing in scientific research then give one, otherwise the phrase "but more likely Randall means a use of annealing in scientific research" sounds like you are speculating about the existence of annealing in scientific research. [[Special:Contributions/172.70.82.111|172.70.82.111]] 01:18, 14 September 2021 (UTC)<br />
:::That was added by(/after?) the jewelry comment-adder, not the initial annealing reference that I (being the above IP, etc) made. I thought that was distracting too (I didn't think that its use in jewelry added anything to the fact that it was a (general) metal-treatment) but if I was to complain about everything added/changed by people other than me I should at least get myself a named-login to build up a reputation for being a grumpy old man about everyone else 'spoiling' things. (And, honestly, I like the actual improvements/corrections that others do, but there's [https://www.enago.com/academy/top-10-journals-publish-negative-results/ even less reason to wax lyrical about those...] Right, no more from me, at least here on this commentary. Have fun, y'all... [[Special:Contributions/162.158.89.228|162.158.89.228]] 19:07, 14 September 2021 (UTC)<br />
<br />
=== Cloudflare ===<br />
This is new:<br />
This page (https://www.explainxkcd.com/wiki/index.php?title=2514:_Lab_Equipment&action=submit) is currently offline. However, because the site uses Cloudflare's Always Online™ technology you can continue to surf a snapshot of the site. We will keep checking in the background and, as soon as the site comes back, you will automatically be served the live version. Always Online™ is powered by Cloudflare | Hide this Alert<br />
...popover on the page. Didn't seem to stop me editing/previewing. Is it in response from all the (other) Cloudflare errors we've been getting recently, someone activiting a safety-net? [[Special:Contributions/162.158.158.88|162.158.158.88]] 18:09, 11 September 2021 (UTC)</div>162.158.89.228