The last post on thermal equilibrium was rather long and involved, so I will try and recap in briefer terms. The early universe was very hot, so that everything was in thermal equilibrium. In particular, because reactions were constantly taking place, the universe was strongly “ionized” or charged. Therefore, photons were constantly scattering off the charged particles.
Eventually the universe began expanding and cooling.* As it did so, the ions and free electrons “recombined” (during the time romantically referred to as the era or epoch of recombination) to form neutral atoms, …

In the 1940s and 50s, a few scientists (George Gamow, Ralph Alpher and Robert Herman among others) predicted the continued existence of the photons that last scattered in the very early universe. Theoretically, those photons had continued to travel through the universe, cooling as the universe expanded. The early theorists tried to predict what the temperature of these photons would now be (with varying degrees of success). These photons should be all over the place and hence providing a constant “background” to any antenna on earth. In addition, they should …

Carnivals are like online Zines, you know, those independent creative publications you created in high school or college. Carnivals are a collection of blog articles about a topic. Like a magazine, there is a publication date – some are published quarterly, monthly, or weekly ; an editor – which usually rotates among interested parties; and a theme.
I participate in a few Carnivals (see my bottom side bar). It helps me share my work with larger audiences. It’s also a great way for non-bloggers to get into blogs and see how …

In the last post, I finally finished the first “thread” about galaxy rotation curves. My dad (who apparently also reads this blog, although not as consistently as my mom) wasn’t quite sure how everything tied together (I believe he missed some entries in the middle). So to briefly recap: I started by explaining the Doppler effect, which was then followed by a series of posts on the wave/particle nature of light. I then discussed the Bohr model of the atom, because it provides a nice framework for understanding the emission …

Ok, so finally I think we can look at rotation curves. We’ll make the simplifying assumption that the objects we are interested in are in a perfectly circular orbit about the center of the galaxy, an assumption which doesn’t really change anything so it’s ok (another larger point about physics: quite often [in fact, almost always], we take a complicated problem and approximate it into something smaller that we can solve [often called the “spherical cow” approach – we would approximate a cow to be a sphere and go from …

I’m writing this entry from 6800 feet below ground. I am wearing a baby blue jumpsuit, safety glasses, steel toed boots, a hair net and a hard hat. At some point, my mom commented that hearing about working in the mine might be more interesting than posts on physics, and so I am going to give the human interest piece a try.

I have been working up in Sudbury, Ontario for the past two and a half weeks at the underground lab I mentioned in the overview posts (linked from the right …

This post is a follow-up to: Newton’s Theory of Gravity (Part 1)
I have not posted (I can’t quite bring myself to use “blogged” as a verb in the past tense, but I should probably get over such squeamishness) in almost a month now, for which I apologize. My excuses are standard – my work intruded. In the last month I have started looking at data from a new run and been to a conference (I’ve also been to my 10th high school reunion), and I am now visiting the mine …

This post is followed by: Newton’s Theory of Gravity (Part 2)
I fear that I got a bit ahead of myself at the end of the last post on the spectral lines of hydrogen. To fully close the circle between dark matter and everything I’ve been talking about in the last few entries, we do need to cover Newton’s theory of gravity. Therefore, I will try to do so now, so that we can put this particularly sequence to rest.
First, on talking with a friend earlier today, I was asked, “I …

As mentioned in the last post, the Bohr atom is not correct, quantum mechanically speaking. It does, however, do an excellent job in modeling the simplest atom, that of hydrogen. Hydrogen is the lightest element, consisting of one proton with one electron in orbit. With the Bohr atom model, we know that the orbiting electron can exist in various discrete orbits, corresponding to different energies. In addition, we know that when the electron jumps between these levels, it emits or absorbs a photon. Finally, we know that the energy of …