The cosmic staircase 1 - Intro
Items of knowledge only very rarely exist in isolation from all the other things we know. In fact almost all knowledge resides in an intricate network of connections, much like each neuron in our brains connecting to thousands of others, and one of the consequences of these interconnections is validation. There are many ways that this happens, and for the bigger concepts like evolution or the age of the universe, mutually validating interactions can track across dozens disciplines and hundreds of independent techniques. Theoretically this should make their rejection as a true description of the world difficult, although a lot of people still manage to do it.
For this piece I want to focus on what is generally known as the Cosmic Distance Ladder. It would be better described as the Cosmic Open Staircase, because its most useful characteristic is the overlapping of different techniques to calculate cosmological distance. This overlapping allows the distance to specific celestial objects to be measured by multiple techniques. The chart below shows 5 overlapping techniques for measuring distances to cosmic objects. For each one I have shown the range of distances that technique covers with a few interesting objects in between. So for instance Triangulation shows the maximum reach of various space telescopes.
Each technique will be covered in further articles. Cepheid Variables for example will include the story of its discoverer Henrietta Swan Levitt, which will lead in turn to a series of articles on women in science.
Here I will concentrate on two objects. First, the Large Magellanic Cloud (LMC), which is a small galactic cluster just outside our own Milky Way. It’s a sort of Milky Way groupie. The second is our nearest galactic neighbour, Andromeda. The distance of both of these objects has been measured using multiple techniques, and I have shown five of them above. You can see the two vertical lines for these objects running through the 5 techniques.
The only difference between the two is the use of triangulation for the LMC, and this was only possible because a supernova occurred, followed almost a year later by the appearance of a gas ring formed by the impact of supernova debris travelling near the speed of light. The easily calculated size of this ring made it uniquely possible to measure its distance using triangulation. This is unlikely to be repeated. More importantly the distance of 168,000 light years corresponds to the results from the other four techniques.
On their own, any one of these five techniques could be challenged as a true measure. Practically everything we know contains margins of error and gaps in the evidence base, as well as on-going debates among scientists about interpretation. This is true even for the things we know the most about, like electricity. To understand how more than one technique can dramatically increase confidence in our conclusions, I will use a thought experiment.
Suppose you want to measure the distance between opposing walls in a room. You have a number of choices.
Scale off a drawn layout
Pace it out
Shoe lengths
Steel measuring tape
Laser device
The last four of these have progressively smaller margins of error. Even a steel tape is both manufactured to a tolerance and has variable actual length depending on the ambient temperature. The laser device is the most accurate, but will still have a margin of error in its manufacture. Human error in using all the techniques also makes a contribution.
Suppose all of the last four give you a very similar answer, but the scaling from a drawing is wildly different. You see that the scale is given as 100:1, and gives you a measurement about half that of the other four. Obviously something is wrong. Is it the other four that are wrong or was there a mistake on the drawing? You could go back to the other four and run them again, or you could do the sensible thing and check the drawing. You suspect that the scale is wrong, but just to be sure you measure other things on the layout, like the thickness of the wall or the length of the garage, which should be long enough for a car. This confirms that the scale is wrong and almost certainly 50:1. The surveyor confirms his error and naturally apologises. You can now sleep peacefully.
This is pretty much how it happens in science. If any of the five techniques in the distance ladder throw up discrepancies they are rigorously examined until the problem is resolved.
The Andromeda Galaxy. Source - Wikimedia, Kees Scherer
Triangulation is very basic geometry and not really subject to much error. The error arises from the instrumentation, in its manufacture and use. As the angles being measured get smaller with greater distance the margins of error become too great to provide a credible result. There is therefore a limit to how far out into the universe you can go with triangulation.
The other four techniques in the chart above can also measure the distance to the LMC. Each new technique adds to our confidence in the result, but it also has another very important consequence. All of the other four techniques can measure distances much further away than triangulation, allowing us to extend our confidence in distance measurement in steps almost to the edge of the visible universe.
The distance to our closest galactic neighbour, Andromeda is determined by the other four techniques, each with their own margins of error. This is summarised in the chart below, and you can see a range of distance that is common to all four.
For the furthest distance we have yet measured with confidence we turn to the Hubble Space Telescope (HST). In 2016 the HST used red shift to detect the furthest galaxy yet, 13.4 billion light years away, and we are seeing it as it was only 400 million years after the big bang. So far red shift is the last of the steps in our cosmic staircase, although the recent discovery of gravity waves could well replace it. The video here is by Ryan Ridden, who is a physics and maths student in New Zealand. He shares his learning through his youtube channel. Well worth a look.