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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!


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.

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. And that still needs powerful radar systems like the former 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 '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).


[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.

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