explain xkcd - User contributions [en]

2516: Hubble Tension

2021-09-16T01:41:41Z

172.69.42.21: /* Explanation */

{{comic
| number = 2516
| date = September 15, 2021
| title = Hubble Tension
| image = hubble_tension.png
| titletext = Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!
}}

==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 redshift and thus the Hubble Constant calculated. Measuring the distance turns out to be feindishly difficult because a distant bright star looks the same as a dim star that is closer.

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, or about 4 light years.

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 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 calculated.

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, using 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, gravity, and possibly shed light on the origin of the universe and a 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.

This also disagrees with Dave.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
Physicist: There are three main estimates of the universe's expansion rate and they all disagree.

Physicist: Measurements of star distances suggest [that] the universe is expanding at 73 km/s per megaparsec.

Physicist: Measurements of the cosmic microwave background suggest [that the universe is] expanding at 68 km/s per megaparsec.

Physicist: And Dave, who has a radar gun, says it's expanding at 85 [miles per hour] in all directions. <br>
Dave: Those galaxies are really booking it. <br>
Physicist: Thanks, Dave.

Title text: Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!
{{comic discussion}}

Talk:2464: Muller's Ratchet

2021-05-18T14:58:12Z

172.69.42.21:

I do this frequently: Seeking out the best quality of two media I've seen put together, in an effort to improve the quality of the resultant combined media.

I had no idea, all this time I'd been creating recombinant PNG.
[[User:ProphetZarquon|ProphetZarquon]] ([[User talk:ProphetZarquon|talk]]) 20:58, 17 May 2021 (UTC)

This is the first time I've encountered recombination, but I've seen the many versions of images scattered across the internet for sure --[[User:Char Latte49|Char Latte49]] ([[User talk:Char Latte49|talk]]) 21:33, 17 May 2021 (UTC)

On a number of occasions I've had to edit images given to me to submit to a resource. The original photographer ''insists'' on having a 'burnt-on' timestamp directly on the image (he's always done that, and won't accept metadata does the job); meanwhile, the resource's policies are heavily against such 'decoration' and moderators often reject such. When I can't easily clone a 'coverage' area from within the same image - and feather its edges to blend in - I sometimes find another original (but slightly different viewpoint) submitted image that has a sufficiently unsullied 'patch' to let it pass muster. That's a slightly meta-example, I suppose. [[Special:Contributions/141.101.99.161|141.101.99.161]] 23:17, 17 May 2021 (UTC)

Sexual reproduction pictures on teh Interwebz mkay, but I lament a certain underrepresentation of amoeba pr0n. [[Special:Contributions/141.101.105.134|141.101.105.134]] 08:33, 18 May 2021 (UTC)

im guessing this time traveller ^^ just came back from 2008. dont go to 2020. ever. just dont. [[Special:Contributions/162.158.79.59|162.158.79.59]] 14:36, 18 May 2021 (UTC)Bumpf

That's a squirrel?? Look at its tail - squirrels have thick fluffy tails, not that skinny one. And if it is a squirrel, it's by far the largest I've ever seen - unless Hairbun and Cueball are children. I was 100% certain when I saw it that it was a cat. --mezimm [[Special:Contributions/172.69.42.124|172.69.42.124]] 14:53, 18 May 2021 (UTC)
:: And we do know how Randall draws squirrels: [[1503]], [[776]], and [[1156]] all contain examples. I vote that this be changed. --mezimm [[Special:Contributions/172.69.42.21|172.69.42.21]] 14:58, 18 May 2021 (UTC)

2456: Types of Scientific Paper

2021-04-28T20:34:20Z

172.69.42.21: added title text

{{comic
| number = 2456
| date = April 28, 2021
| title = Types of Scientific Paper
| image = types_of_scientific_paper.png
| titletext = Others include "We've incrementally improved the estimate of this coefficient," "Maybe all these categories are wrong," and "We found a way to make student volunters worse at tasks."
}}

==Explanation==
{{incomplete|Created by a BOT. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
Titletext: "Others include "We've incrementally improved the estimate of this coefficient," "Maybe all these categories are wrong," and "We found a way to make student vlounters[sic] worse at tasks""
{{comic discussion}}