cosmology – Page 2 – Ted Bunn’s Blog

Accepted

Two papers I submitted a while ago were accepted for publication by the American Journal of Physics this week, within minutes of each other as a matter of fact. To be precise, they were “conditionally accepted,” meaning that they’ve successfully passed the review by external referees and the science content been deemed acceptable. There’s a further review by the editors for style, clarity, etc., before they’re finally accepted. Because AJP is a journal with a pedagogical slant, they place a heavy emphasis on clarity, which is probably why they have this “conditional acceptance” stage.

Both of these are less technical than the usual research paper: they’re intended for readers who know some physics but are not necessarily specialists in any particular field. The first one requires a bit of knowledge of relativity (students who took my Physics 479 course should be fine) , and the second one requires just undergraduate-level thermodynamics and statistical mechanics.

The first article is on the correct interpretation to place on the observed redshifts of galaxies in the expanding Universe. I blogged about it when we originally submitted it. These redshifts are usually described as being due to the “stretching of space,” but David Hogg and I argue that this conceptual model is misleading. We claim that, contrary to what you often see in introductory textbooks, it’s correct to think of the redshift as being due to a plain old Doppler shift.

Here’s the revised version of the paper. It doesn’t differ all that much from the one we originally submitted, although some aspects of the argument are expanded and clarified a bit in response to the referees’ comments.

The second article is on the relationship between entropy and the second law of thermodynamics. It’s a response to a very nice paper by Daniel Styer, which attempts to show quantitatively that the entropy production due to sunlight is more than enough to account for the entropy reduction required for biological evolution (contrary to claims often made by creationists). The original article had a serious gap in it: it depended on an assumption that was unjustified and, I argue, almost certainly wrong. My paper presents an argument that doesn’t depend on that assumption. The new argument shows quite rigorously that there is no conflict between evolution and the second law.

I blogged about the original Styer article and about my response a while back. Here’s the revised version of the paper. Thanks to the referees, I think the new version is much clearer than the original. It’s also much longer. I was thinking of the original as just a comment on Styer’s earlier paper, but the new version reads more like a stand-alone article.

Planck preparing for launch

The Planck Surveyor, the European Space Agency’s satellite-borne microwave background telescope, is preparing for launch on April 16. Andrew Jaffe has some pictures of the preparations.

Planck will map the microwave background with finer resolution than NASA’s WMAP satellite. It’ll also cover a considerably broader range of frequencies. This is helpful in separating the microwave background from other, more local sources of radiation.

I think I’m still officially a member of the Planck team in some sense, although I haven’t done any real work on it.

Accepted

The paper that I coauthored with Brent Follin (UR undergraduate) and Peter Hyland (Wisconsin grad student turned McGill postdoc) has officially been accepted for publication in Monthly Notices of the Royal Astronomical Society. I thought it would be, but it’s still nice to make it official. Congratulations to Brent especially, for becoming a published scientist.

Unlike the last one I posted about, this is a “real” refereed paper. We decided to submit it to Monthly Notices, not Astronomy and Astrophysics as I wrote in my earlier post, for reasons that aren’t at all interesting. Monthly Notices is a very good journal, and it has a way cooler name than A&A.

Direct to video

I wrote up a little piece for the proceedings of a conference I went to over the summer. To go into self-deprecating mode, this is the sort of thing that a colleague of mine used to call a direct-to-video paper (this was in the pre-DVD era), because it doesn’t go through the same level of scrutiny as a refereed journal article.

The article has to do with how to separate a map of the polarization of the microwave background into two pieces called the E and B components. Over the coming years, maps of microwave background polarization are likely to become more and more important in putting constraints on our theories of the early Universe. A polarization map can be thought of as two maps lying on top of each other. The B map is considerably weaker than the E map, and it contains information that’s much more useful than the E map, so cleanly splitting the map into the two pieces is going to be very important in extracting science from the data. This article is an overview of some of the issues involved in this separation. It contains an extension of some work I did a while ago on finding ways to do this separation more accurately and efficiently.

Phase shifts

Peter Hyland, Brent Follin, and I just submitted a paper for publication in the journal Astronomy and Astrophysics. You can see it here.

Peter is a postdoc at McGill now, but he was a graduate student at the University of Wisconsin when we did the work. Brent is a rising senior here at U.R.

In this paper, we’ve solved a problem that’s an important part of the construction of a kind of telescope known as an adding interferometer. In an adding interferometer, a bunch of different signals from different antennas are mixed together, resulting in an output signal that is the sum of all of the inputs. We want to be able to extract information about the individual signals (specifically, pairwise correlations between the inputs, if you must know), not just the overall sum. To get this information out, we need to modulate all of the inputs in different ways. Finding the optimal way to do this — that is, the way that results in the smallest errors in the result — turns out to be a tricky problem. We’ve found a general method for finding the solution.

The reason we wanted to solve this problem is that we’re part of a group that’s trying to build an adding interferometer for observing the polarization of the cosmic microwave background radiation. We tested a prototype out at Wisconsin recently. Eventually, a much larger version could map the polarization in great detail, giving us new windows onto the very early Universe.

Down with the rubber sheet

People like to visualize the expanding Universe as a sort of a stretching rubber sheet. Textbooks and popular cosmology books play up this analogy in a big way. Like most analogies, it’s useful in some ways, but taken too far it can lead to misconceptions. David Hogg and I have written an article in which we try to fight back against some of these mistakes.

The article is about how we should interpret the redshifts of distant objects. Most of the time, redshifts are Doppler shifts, indicating that something is moving away from you. In the cosmological context, though, a lot of people think that you’re not allowed to interpret the redshift in this way. The idea is that galaxies are “really” at rest with respect to the stretching rubber sheet. Since they’re not “really” moving, what we see is something different from a Doppler shift. The point of our article is to rehabilitate the Doppler shift interpretation.

The real reason I care about this is not that I think it matters much what we call the redshift, but because I think that this is a good example of the muddled thinking that the rubber sheet analogy causes. In particular, the analogy provides precisely the wrong intuitions about the nature of space and time in the theory of relativity. If you want to know more specifically what we mean by this, you’ll have to read the article!

To understand the guts of the article, you really need to have studied relativity a fair bit. (Students who took my Physics 479 course should be able to handle it.) Even if you don’t know enough relativity to understand all the technical details, the beginning and end might be interesting and accessible. (Certainly, this paper should be more accessible to non-specialists than the last one I wrote about, which is pretty technical.)

Puzzles in the microwave background

The maps of the microwave background radiation made by the WMAP satellite have been incredibly important in our understanding of the Universe. In most ways, the maps are amazingly consistent with the “standard model” of cosmology. In this model the Universe is made of mostly dark energy and dark matter, and the structure we see around us grew out of tiny density variations imprinted during a period of inflation.

But there are a few puzzles in the WMAP observations, mainly having to do with large-scale patterns in the maps. One of the puzzles is that large-angle correlations in the map are significantly weaker than expected. U.R. rising junior Austin Bourdon and I have written a paper analyzing some possible explanations for this puzzle. Our paper shows that a broad class of possible explanations can actually be ruled out, because they make the problem worse rather than better. The class of explanations we rule out includes some “exotic” models that have been proposed in the literature recently, but it also includes some much more mundane possibilities, such as various non-cosmological contaminants in the data.

In addition to posting it on the web, we’ve submitted the paper for publication in the journal Physical Review D. For any non-scientists who’ve read this far, the next step is that the paper will be sent out for review by experts, who will recommend for or against publication. In the mean time, most people who care about this subject will see it on the web archive.

Bonjour!

I’m spending this week in Paris, at a conference on Bolometric interferometry for the B mode search. This is a narrowly focused workshop on a specific technique that may be used for future measurements of the microwave background. (That’s why its name is so completely esoteric.) For a number of years now, I’ve been part of a collaboration trying to develop this technique.

This is the second conference I’ve been to in the past few weeks. St. Louis was perfectly nice, but I have to say it’s nicer to be in Paris, weak dollar notwithstanding.

American Astronomical Society Meeting

My research group and I are just back from the summer AAS meeting in St. Louis. Here we all are at the hotel just before leaving:

Our group presented four posters, with primary authors Austin Bourdon, Brent Follin, Ben Rybolt, and me. This means that pretty large fraction of the undergraduate presentations were by UR students. I suspect that we had more undergraduates presenting than any other college, although someone said that Wesleyan had a lot too. If you want to know about the research we were presenting, take a look at Austin’s, Brent’s, Ben’s, and my posters.

The fifth member of the group is Haoxuan Jeff Zheng, who wasn’t presenting this time because he only started research quite recently.

The meeting felt a bit small to me, compared to past AAS meetings I’ve been to: there didn’t seem to be that much going on. There were certainly some good talks, though. John Monnier talked about using interferometers (particularly CHARA) to produce images of rapidly rotating stars. In general, stars just look like points of light, even through the largest telescopes. But with these interferometers, you can actually resolve the stars well enough to see their overall shapes. Rapidly rotating stars bulge out at the equator, so they’re quite far from circular in appearance. Some are rotating so fast that they’re pretty close to breaking up. I had no idea how far the state of the art in this field had advanced in recent years.

The best talk was by Sean Carroll, on one of those questions that sounds stupid when you first hear it, but gets more interesting the more you think about it: Why does time flow in one direction and not the other? Why is the future different from the past? The reason it’s puzzling is that the microscopic laws of physics look the same whether you run time forwards or backwards, which makes it a bit strange that the large-scale universe doesn’t.

A conventional answer to this question is to invoke the second law of thermodynamics. Carroll argued that this only pushes the problem back one step, rather than really solving it. He argued further that none of the usual attempts at further explanation, including the theory of inflation, really solve the problem. He speculated a bit on what a true explanation might look like, but mostly had to admit that we have no idea.

Galileo

Apparently there’s a tradition here at the University of Richmond: when a faculty member gets tenured, he or she chooses a book for the library and writes a description of the importance of the book. The library inserts that description into the catalog, or into the book, or something like that.

Since I just got tenure, I had to choose a book to write about. After thinking about it for a while, I decided to go with Galileo’s Dialogue concerning the two chief world systems. Here’s the description I’m sending off to the library.

By the way, I really mean it when I say in here that the book is extremely readable. I can’t think of any other book that’s (a) anywhere near as important as this one and (b) actually fun to read. If you haven’t read it, check it out! (Although if you’re at UR, you’ll have to wait a week or so to get it out of the library, because I’ve got it at the moment.)

Anyway, here’s what I wrote:

The central idea of astrophysics is that the same laws of nature we discover in labs on Earth can be used throughout the Universe: there are not separate laws for heavenly bodies. It would be hard to overstate the importance of this insight to the emergence of modern science. Galileo did not invent this idea – like most really big ideas, this one cannot be attributed to any one person. But he deserves a large share of the credit for developing the idea and for persuasively and cogently championing it. Galileo is an all-too-rare figure among the giants of science: he wrote with clarity and even wit for an audience of non-specialists. He wrote in the common language, not in Latin, in a style that makes his work still readable and even enjoyable today.

Galileo has a lot in common with Einstein, so it is fitting that Einstein wrote the foreword to this English translation. In particular, Galileo's description of experiments performed below decks on a moving ship is the direct ancestor of Einstein's discovery of relativity, both in the scientific content of the ideas and in the ingenious use of thought experiments.