2516: Hubble Tension

Explain xkcd: It's 'cause you're dumb.
(Redirected from 2516)
Jump to: navigation, search
Hubble Tension
Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!
Title text: Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!

Explanation[edit]

Ambox notice.png This explanation may be incomplete or incorrect: Created by Dave - Please change this comment when editing this page. Do NOT delete this tag too soon.
If you can address this issue, please edit the page! Thanks.
Ponytail is telling Cueball about the 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.

The fact that most galaxies are receding from us, and that the distance to the galaxy is directly proportional to the speed (as measured by red-shift) 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 localized movements can influence the speed of recession — though less significantly, for multiple reasons, the further away are the objects 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 Cosmic 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 (3x1013) kilometers.

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 Inverse-square law where not otherwise obscured) the distance can be measured.

The final rung is Type Ia supernova, 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.

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, analyzing makeup of the 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.

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 '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.

The third method introduced in this comic is a guy named Dave who is trying to use a 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 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. Doing that still needs very powerful radar systems like the former Arecibo Telescope to be able to get any useful information from that far away; a hand-held radar gun would not be able to 'lock on' across those distances, let alone distant galaxies.

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.

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 Galaxy that he can more easily find out the velocity of.

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.

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).

Transcript[edit]

[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.
[They keeping walking to the right.]
Ponytail: Measurements of star distances suggest the universe is expanding at 73 km/s/megaparsec.
[They are still walking to the right.]
Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec.
[They continue walking to the right. Ponytail points towards Dave who replies from off-panel to the right.]
Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions.
Dave (off-panel): Those galaxies are really booking it!
Ponytail: Thanks, Dave.


comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!

Discussion

Having noticed that 85 KPH is slower than 85 MPH, it took me a while to work out that 85 MPH is much slower than 68 km/s (and I was blindly assuming that the universe is at least one megaparsec in radius), after which the title-text joke started making sense. Congratulations on being almost too subtle for me.00:46, 16 September 2021 (UTC)

Is the 85 mph number significant in any way? Why does "Dave" who points radar guns in random directions get this number? 108.162.245.167 03:41, 16 September 2021 (UTC)

Well, it's probably over the speed limit in most places. Maybe Dave is a traffic cop? --172.68.129.137 04:55, 16 September 2021 (UTC)
Possibly coincidental, but part of the National Maximum Speed Law required speedometers to "peg" at 85 mph, which gives it a connotation of "maximum speed" (perhaps the "maximum" of the radar gun?) for a certain generation who grew up with those cars, but this could be a coincidence. This is why the DeLorean has to go 88 to time travel ("beyond maximum"). 172.70.126.211 17:55, 4 October 2021 (UTC)

Unfortunately, the explanation only explains the things everyone can read on the internet anyway. 85 mph or 85 km/h have the wrong unit, because for the expansion speed we need to look at two points of space, measure how fast they move away from each other. Obviously this should be a number that increases linearly with the distance of the two points (if space is created equally everywhere in the universe). Thus the 85 km/h misses the length. Is the joke here that a random dudes results are reported equally (false equivalence)? --162.158.93.142 04:41, 16 September 2021 (UTC)

Dave again: "But when they hit 88 mph, we're gonna see some REALLY weird shit!" RAGBRAIvet (talk) 06:50, 16 September 2021 (UTC)

Just a thought: maybe Dave is talking about Fords Galaxies? -- 162.158.183.222 08:39, 16 September 2021 (UTC)

I had already added that before I saw your comment. --Kynde (talk) 10:37, 16 September 2021 (UTC)

Relating (coincidentally) with several of the above comments, I added in a "what Dave could be measuring" paragraph (ultimately: just traffic!), via a diversion where I totally messed up a factor and it sent me down a rabbithole of completely the wrong distance! ((Sanity-check my new figures, please: e.g. 85mph => ~0.038km/s => ~0.0005588(of the 68km/s/Mpc figure) => therefore 558.8pc, etc and onwards)) Anyway, perhaps Dave just is/wants to be a traffic-cop? (If you can find humour in the 'all directions' - presumably away from - then obviously supercede the 'both directions' bit.) 172.70.134.47 09:16, 16 September 2021 (UTC)

I believe a radar speed gun pointed at the sky would actually display something like "no reading" when it doesn't receive any radar echo, rather than 85 miles/h. It wouldn't do to fine drivers for speeding when the measurement fails. 162.158.183.208 09:39, 16 September 2021 (UTC)

Dave is measuring Ford Galaxies that are all speeding away from him, either way too fast, or if kph just moderately speed for a normal road between cities. --Kynde (talk) 10:37, 16 September 2021 (UTC)
Unless it is actual some kind of a Ford MPV Owners' Club 'burn out' session of some kind, I read it as more general "large, relatively unwieldy family-style vehicle passing in conspicuous quantities and not actually holding up the rest of the traffic" rather than truly committing traffic violations (in excess of any other vehicle on the road). To "book it" is to hurry, but (more colloquial understandings allowing) more as in the "not dawdling" sense. (They'd be noteworthy only for narrative punning reasons, really, as one of the comic's hooks. But that's only meta.)
Though (outwith the most obviously speed-trapped areas) my personal experience is indeed that driving at the posted speed limit often means being treated as an inconvenient mobile-roadblock, by more than half the rest of the vehicles that come up behind and tailgate or pass, I would still consider hugging-the-limit (whatever it is) as going fast and not actually slow. 162.158.159.11 12:49, 16 September 2021 (UTC)

I think we've missed an important joke in the last panel. Dave says the galaxies are moving at 85mph/kph and "booking it". However, KM/S/Mpc is going to be on the scale of several thousand times faster than 85mph/kph. Dave's reference to "booking it" is actually moving quite slow. Pconwell (talk) 12:57, 16 September 2021 (UTC)

Dave is used to measuring car speeds, and 85 MPH is very fast (significantly over the speed limit) in most places. The joke is that he's totally out of context and measuring the wrong thing, but it happens to be a number in the same ballpark as the Hubble Constant. Of course, this is just a coincidence, since the units are different. Barmar (talk) 14:20, 16 September 2021 (UTC)

How is the expansion rate different from an acceleration? The (correct) units surprise me. 172.70.130.87 14:15, 16 September 2021 (UTC)

In acceleration, the velocity increases over time. In expansion, the velocity increases over distance. Barmar (talk) 14:20, 16 September 2021 (UTC)

With all this talk about units, I would like to mention that Google ever-so-helpfully says that 1 km/s/Mpc is "3.24077929e-20 hertz". 172.70.126.195 16:22, 16 September 2021 (UTC)

Google also says 68 km/s/Mpc is 2.20372992 × 10-18 hertz, which is perhaps unsurprisingly about one over the age of the universe, also known as the "Hubble time". 162.158.91.106 11:35, 20 September 2021 (UTC)

I see two things to comment on about Dave's response. First, if everything measured always shows the same velocity, then the "per megaparsec" part of the result becomes largely irrelevant, as Dave's results are the same regardless of this value. This means that Dave's results are "it doesn't matter how far away, it is all 85 MPH!" If taken seriously, this would be a challenge to the standard model of an expanding universe much bigger than the actually existing controversy.

Second, I believe the joke here is that Dave's radar gun is a police model that consistently reports that anything being measured is speeding. A recurring accusation, and occasional actual problem that requires police radar units to be tested regularly to avoid, is that radar guns that report speeds higher than they actually are are used to issue tickets unfairly. When used as an accusation, or in fictional media, it sometimes comes with an accusation that this happens with the full knowledge of a cop who cares more about issuing tickets than doing his job properly. In this case it appears that either every Ford Galaxy, all traffic, or even possibly the sky itself, is consistently "speeding" in this fashion.Geek Prophet (talk) 20:34, 16 September 2021 (UTC)

Dave's radar could have been pointed at a Ford Galaxie: https://mystarcollectorcar.com/the-out-of-this-world-ford-galaxie1959-to-1974/ 172.70.178.191 09:09, 17 September 2021 (UTC)