colloquium, physics

Avi Loeb’s Colloquium

Avi gave a very good and broad colloquium on 2+ topics: 21 cm cosmology, super massive black holes, many other things.  He was allotted an hour, went over.  That was fine, he had many interesting things to talk about.  Although Avi is a theorist, much of his talk concerned experiments, or predictions for experiments, which I found very interesting.  To sum up, I liked the talk.

21 cm cosmology

The main thrust of this seemed to be that you can observe the reionization of the universes hydrogen at different times in the past by looking at different red-shifed 21 cm photons.  He liked it to taking a slice of swiss cheese, depending on your slice (what \lambda you observe), you will see a different amount of the universe ionized.  Here is a figure:

slices image

slices image

We are far off the right side of the image.  I’m not sure I liked the swiss cheese analogy too much, you couldn’t take it very far.  I think all that he meant was that if you slice the cheese, the hole are different for each slice.  Similarly, if you took at a slice of the universe (say \lambda_1) the amount of the universe that has been reionized will be different than the amount in a different slice (at say, \lambda_2).

However, if you take the analogy a little bit further, the cheese holes open and close.  This doesn’t happen in the universe.  Once a region has been re-ionized, it stays re-ionized.  It is more like taking slices of a group of stalagmites, starting at the top and moving toward the ground.  Or, drawing from a recent experience of mine, like slicing the Badlands from the top toward the bottom in an area that ends in a trough rather than the plains.  But, the swiss cheese is a little more common, and easy to recognize.  You just can’t take it too far.

He covered so much material, I don’t remember it all in this topic that I don’t remember it all!

Super Massive Black Holes

The idea is to image the largest black hole on the night sky.  That is the one at the core of the Milky Way.  It may not be the closest, but because of it’s great mass, it subtends that largest angle on the sky.  And when I say largest angle, I mean 50 micro radians (a tiny amount). That’s like looking at a pea at 200 meters!

We won’t be able to actually see the black hole, but we can see it’s shadow.  It will be blocking the hot accretion disk behind the hole.  Avi’s part of that project was to predict what it might look like, depending on various models of the accretion disk, the mass of the black hole and general relativity.  That is to say his job was to make some pretty pictures.  He showed us those, and they were pretty.


I came up with a couple of questions.

  1. how did the last scattering surface evolve?
    In the end I decided not to ask this question.  I asked a couple of my fellow students and they were able to clear it up some for me.  In the formulating of that question I realized I was confused about the cosmic dark age.  That is the topic that Avi was talking about in the first section.
  2. what angle are we looking at the black hole in the pretty pictures you showed?
    I did ask this one, and Avi took us back and explained more about that.  Also about how the pictures vary with more than just the angle we see the object at: the model for the accretion disk too.  They have produced a library of such images and movies.

At the end of the talk there was a general Q&A time, then a students only Q&A time.  I liked the student only Q&A time because I didn’t feel quite as embarrassed to ask my simple question.  Also the questions from the other students were good.  Eric asked about if the 21 cm observations could help distinguish between models for the first stars that had dark matter as their power source (called Dark Stars).  Avi explained that he didn’t work on that, but suspected that they could.  Dark Stars are a very new idea.


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