Q & A – Measuring the Universe

Mini-Science logo At the conclusion of each Mini-Science lecture, audience members submit their questions to the evening’s presenter, who answers as many as possible on the spot. Three of the unanswered questions are sent to the presenter for posting here. In addition, a quiz is held each week based on material from the lecture. Here are questions and the quiz from Prof. Robert Rutledge’s lecture ‘‘…in a Galaxy Far, Far Away: Measuring the Size of the Universe’ (April 29, 2009).

Q: What is the ‘thumb theory’?

A: If I understand the context of your question correctly, “thumb theory” would be a humorous way of referring to the parallax effect – if you hold your thumb out in front of your face, and use first your right eye, and then your left eye, your thumb will appear to jump back and forth, in comparison with distant, background objects. If your thumb is closer to your face, it will appear to jump more; if it is further away from your face, it will appear to jump less.

Q: What is a cepheid? How does it work?

A: Some stars – depending upon their mass – during a short period of their life, will go through a stage in which they act as a “cepheid variable”. First, the name comes from the fact that the best known examples were found in the constellation Cepheus. There is nothing unusual about these stars, they are pretty much just like all other stars, except they happen to lie in a very particular mass range which means that during a short period of their evolutionary lifetime, there are competing physical effects which will conspire against each other, so that the stars brighten and fade in a periodic way, about every 1 day to several weeks, depending on how bright the star is. The competing effects are gravity, ionization and opacity. When the star is compact, it is very hot, and as a hot ball of gas it will expand and cool; at its largest, the photosphere (that is, what we can see directly with our eyes) will be cool enough that hydrogen can take on an extra electron, removing free electrons from the photosphere. This decreases the opacity, permitting photons to stream free from the photosphere, and removing a source of pressure – like letting the air of of a balloon. Lower pressure means the star’s photosphere will contract (it will appear to get smaller), and it will continue to do so, heating up, until it is hot enough to re-ionize the Hydrogen, which then adds a source of pressure – the balloon re-inflates! This causes the star to appear to pulse in brightness, and it does so very regularly, with a period which is slower if the star is more luminous. And this permits us to use them as “standard candles” – objects whose brightness we know, so that we can use the Distance-Luminosity-Flux relationship to figure out their distances.

Q: Could the high density of matter and resulting strong forces of gravity that existed in the early universe have caused light to travel at a reduced speed in that early environment?

A: This gets to what we mean by “the speed of light.” As an example, think of a man who walks ten blocks home from work every day, and he likes to take a drink now and then. When he takes one path which is ten blocks, there are no bars, so he just walks directly home. However, on another path, which is also ten blocks, there’s a bar on every block, and he stops at every one of them for a quick one, before continuing on his way. Now: assuming the man does not feel the cumulative effects of alcohol (something many of us have wished for on an early morning), one can ask – is his speed any different on the two paths? It would seem like the total amount of time to get home is shorter on the path with no bars, but longer on the path with a bar on every block. So, in the sense when you take his distance travelled and divide it by time, his speed may seem slower on the path with bars, and faster on the path without bars. In this analogy, the man is a photon, and the bars are matter. Photons like to interact with matter (charged matter, actually – like electrons and protons) and, in doing so, they will appear to move slower. However, what physicists usually mean by “the speed of light” is how fast light moves in free space, in the absence of matter. So, in the sense that physicists talk about light, its speed does not change at all. But, if you send light through matter (like glass, or water) and time it, it will have moved more slowly through the matter, as it interacts with it – just like the man walking home and past (or, not past) the bars.

The “strong forces of gravity” is a bit trickier to address. The short answer is, no, gravity doesn’t change the speed of light. But, again, the speed of light is what a person who is right next to it would measure. Gravity, however, does change the shape of space, and can change the speed with which time appears to flow – for example, if I am in orbit around the earth; and I compare a clock on earth with one I have with me, it will appear that the clock on earth is moving slower.

So, in the early universe, we expect that light had the same speed as it does today, but it would have interacted with matter, and the shape of the universe was also more compact, and both of these would have affected how light traveled.


Q: What is the name of the method for measuring distances, in which you use a thumb, and both your eyes – or, things which take the place of your thumb and both your eyes?

A: Parallax.

Q: In the parallax method, what is the name of the unit of distance to the stars (for example, to a star which produces one arcsecond of parallax observed from the earth-Sun system)?

A: parsec

Q: What is the approximate distance to the center of our own Galaxy?

A: 8000 parsec or 24 thousand light years.

If you have any related comments or questions, please feel free to post them. We cannot promise a reply to every question but will answer what we can.

2 responses to “Q & A – Measuring the Universe”

  1. Maggie W says:

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