Editing 2516: Hubble Tension

 

[[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.

 

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}}.

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.

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.

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. Doing that still needs very powerful radar systems like the former {{w|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 {{w|Ford Galaxy|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).

:[Cueball and Ponytail are walking to the right. Ponytail has her palm raised.]

:[They continue walking to the right. Ponytail points towards Dave who replies from off-panel to the right.]

:Dave (off-panel): ''Those galaxies are really booking it!''