(Astro)Physics

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When do extra gamma-rays mean dark matter?

Total number of gamma-rays (left) versus the excess gamma-rays (right) after all known sources are removed.

Image Credit: T. Daylan et al., Fermi Space Telescope, NASA. Image shows gamma-rays from the center of our Galaxy.

Remember a month or two ago when astronomers claimed to have detected an excess of gamma-rays from around the center of our Galaxy?

The image on the right shows that, after accounting for all known sources of gamma-rays in the region, there is still a lot of gamma-rays coming from this region. So what is creating these gamma-rays? Dark matter, of course!

 Credit: UVA Astr 5630/5640. Image shows the density of dark matter we expect to see when we look towards the center of our Galaxy — very high density right at the center followed by a quick decrease in density. The number of gamma-rays follows the dark matter density.

This slippery substance neither emits nor absorbs light. So naturally, they emit gamma-rays. Actually, according to some models, a particle of dark matter can destroy fellow dark matter particle, leaving gamma-rays and possibly other goodies. This is more of a hope than an established hypothesis.

The center of our Galaxy should be a good source of dark matter-created gamma-rays. It is the location in which the Galaxy’s gravity tries to pull everything. Dark matter definitely feels gravity, so it should clump there.

Lo-and-behold, we see gamma-rays from the center of the Galaxy which are not explained. Better still, the shape of this gamma-ray signal looks like the expected distribution of dark matter throughout our Galaxy. So … we’ve found dark matter!

Hold on one moment. Taking stock of all possible gamma-ray sources in that region is really hard. I mean really hard. A preprint posted just today on the astronomy archive (arXiv) claims that the gamma-ray excess can be explained without resorting to dark matter or other fancy things. All they need are protons flying around in space. These may have been supplied by past supernovae or previous activity of the supermassive black hole at the heart of our Galaxy — two events in which we are confident have happened in the past.

Credit: Scientific American - Cosmic Rays at the Energetic Frontier by Cronin, Gaisser, and Swordy. Image shows how a supernova can cause protons to fly around through space.

Every since findings of the extra gamma-rays have been reported, I have thought that we simply haven’t taken everything into account. The analyses which conclude in favor of dark matter are all done very well, but we are always missing some pieces of the puzzle. What can be taken to be a signature of dark matter is really only a proof that we’re missing something. Whether it is dark matter or not, I’ll let others argue this one out. I’ll go back to my own work for a while (which, curiously, is very related to this topic).

May 9

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13335.html

The full paper can be found here, though I do not have access. Alien life in movies are sometimes identified by the number of base pairs in its DNA. Life as we know it uses DNA with 2 base pairs to pass on genetic information. Now there is an organism that has 3 base pairs. Only it is not alien, it is engineered right here on Earth.

Since the mid 1990’s, we have known that additional molecules could potentially act as new base pairs, unnatural base pairs. Now, we have successfully encouraged some E.coli bacteria to take the new base pairs into it’s DNA. After many generations of reproduction, the new base pairs were still found in the young generations.

Life with DNA noticeably different from ours (3 base pairs instead of 2) is possible (aliens?). What does that tell us about life on Earth? How will these new base pairs affect the functionality of the bacteria?

Want to see what it is like 500 years in the future?

A group of fellow physics graduate students and I were chatting one evening. We tried to imagine what life might be like in 500 years. Considering the pace of technological and theoretical discoveries and innovation, what will the future bring? And so the inevitable question came up: given the option, would you travel far into the future to see what things are like? Because it will all be fantastical (we’re optimists — beer helps, I guess).

Some of us said ‘of course!’ A little more than 100 years ago, we didn’t have airplanes or automobiles. Recent memory has brought personal computers and the internet. We have cracked the code of DNA. What will the future bring? Don’t you want to see? Of course!

I cocked my head a bit. Of course I’d like to see. But I gave a dissenting answer to the question. “No,” I said, “I’d rather stay at this time.” I had to explain. “It would be great to see what happens, don’t get me wrong. But I want to be here to see how it happens.”

The results aren’t as interesting as the process, in my opinion. For example, I read non-fiction. I prefer the books that discuss how an idea came to be rather than the books that simply explain an idea. There are books like, ‘this is how the universe works!’ That’s all well and good, but for me, the real story is the story of how we came to know how the universe works. It is a story about people. People interacting with other people and people interacting with nature. The story of people helps bring the science home.

But the real reason why I’d rather stay put? Not only do I want to witness the genesis of these new technologies and scientific ideas. I want to help make it happen. The future is wide open. We face a lot of problems, but all of us also carries a lot of promise. I have a role to play in shaping the future, we all do. And I want to play my part. With everyone’s help, we will make a bright future.

"It would be great to see what happens, don’t get me wrong. But I’d rather be the one to make it happen.”

Understand … under - stand

To understand something to to really get it. I don’t see how we put together the words “under” and “stand” to get that. It clearly is not native to English.

The Online Etymology Dictionary sheds some light on the origin of this word. I previously took German: verstehen is the translation of “to understand.” Literally: to stand before or in front of. That kind of has the same problem as standing under…

Old English has things that resemble the word “understand” and means “among, in the presence of.” In that sense, we take a different meaning of the word “before.” This feels closer to the definition of the word. Once you understand a concept, things become clear. Sometimes there is this feeling of catharsis after a struggle; you’ve made it to the top and mingle with the concept.

Perhaps this is why Plato proposed his idea of “ideals.” Every idea has some perfect “ideal.” Everything we see in nature are imperfect examples of that ideal. Upon understanding, we find ourselves in the presence of the ideal. We have finally found the ideal. Of course, I take issue with that philosophy, too.

It is not to stand under. Understanding is more than simple knowledge. You have internalized a whole concept, it is now part of you. Understanding gives you a new point of view, a new place to stand.

If I have seen further it is by standing on the shoulders of giants.

Fermi Summer School 2014

I asked Dr. Perkins, “Why does Fermi have sunglasses?”

Response: “Because Fermi is cool.”

I don’t know what to say to that. On a related note, I am slowly (emphasis on the word slowly) working on my first first-author paper on a gamma-ray survey of small, nearby molecular clouds. It won’t revolutionize anything, but it is a good study and highlights some issues the community is talking about. Maybe I’ll get my butt in gear and write about it some day. And other things. I have a post on the Asimov Memorial Debate and another on a talk about inflation and the BICEP2 results from last week. But for now … enjoy Fermi with sunglasses:

And if anyone is interested, the Fermi Gamma-ray Space Telescope team is hosting another summer school! It lasts almost two weeks and costs $1000. You get to enjoy good company at a nice University of Delaware getaway while learning the ins-and-outs of the Fermi instruments and science done with the data. I went last year, and it was a blast.

The focus last year was on dark matter. This year will apparently focus on the center of the Galaxy: the bubbles, this whisper of too many gamma-rays (this excess fits a dark matter profile remarkably well), modeling a very busy region … it should all be good stuff.

Mar 4

Cosmos time

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Cosmos: A Spacetime Odyssey premiered at a special event today, and on television on Sunday.

The show aims to popularize science, and I think it will do a better job than the original did. Regardless your opinion on the host, Neil deGrasse Tyson, he is a great choice. He gives a face to the traditionally underrepresented minorities in the sciences. He isn’t just another white guy telling you what’s up. Here is a black man that everyone listens to and everyone respects.

Perhaps a year ago, I attended a seminar at the American Museum of Natural History where the speaker talked about underrepresented minority participation in astronomy. Early in the talk, she presented the following graph, which shows the number of Ph.D.s granted to underrepresented minorities year by year:

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From this paper concerned with the underrepresented minorities in astronomy.

Tyson gets up and points to the screen, “This one was me!” He pointed at the blue blip at 1991 (check his CV). That data point was him. No other black person in the entire country received their Ph.D. in astronomy that year. Talk about lonely.

I hope this series will do more than promote that science is as natural as breathing and that our world contains plenty of wonder. I hope that people become inspired to think clearly. Perhaps having Tyson as a host will show that being black or hispanic is not a bad thing, that you can do astronomy too. I hope that the established majority in astronomy (the white people) will start to actively encourage diversity rather than turning a blind eye to it. I hope it is time for a change.

The oldest star in the universe? Maybe, maybe not

I’ll talk about this article twice, if I get to the second one. I first want to comment on the “oldest star found.” The star found has the lowest fraction of iron ever found. Iron and other elements heavier than helium were all made in stars. Specifically, those elements were made after the beginning of the universe. Hydrogen and a lot of helium were created during the beginning: in the first few minutes of existence. So as a rule, gas with a lower fraction of elements heavier than hydrogen and helium is older gas. There has been less time for other stars to “pollute” the gas with heavier elements.

Does that mean this is a very old star? Not necessarily. If it is a low mass star, then it could potentially have formed near the beginning of the universe and still be burning bright. The fill paper (Nature) includes discussion on possible pollution by prior stars. This occurs when a massive star explodes and mixes its enriched guts into the neighborhood.

But do they consider different mixing? Inhomogeneous mixing, for example? Or the fact that galaxies, even today, have gas falling into the galaxy from outside. This gas often takes the form of pristine atomic hydrogen. Can these clouds get shocked during its passage through the Galaxy and start forming stars? It could be a reserve of unenriched gas from which the star could form.

What are the kinematics of this star — how is it moving? Have we performed stellar evolution simulations to see how old it is? The answer is … sort of. The fraction of lithium is significantly different from what was created in the Big Bang. They take this as evidence that the star must have gone through a particular stage of evolution which uses up lithium. Ok … but lithium is easy to kill.

Well, I’ll poke through the paper a bit more and see what I can find. I’m not entirely convinced this is the oldest star so far found. But I’ve been wrong before.

Black hole firewall solution?

There was a paper published an New Year’s day which claims to solve the black hole firewall paradox. All right, by now there are a handful of hopeful solutions. What is different about this one?

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I wonder what’ll happen if I go over there.

The paradox arose as a direct result of Stephen Hawking’s calculation that black holes give off radiation. Black holes aren’t black because of the strangeness of quantum mechanics. Four physicists from the Kavli Institute for Theoretical Physics at UC Santa Barbara found a contradiction: the theory of gravity predicts that anything passing into a black hole should not notice anything wrong, besides the usual stretching. In order for the black hole’s radiation to abide by physics, the object can NOT get into the black hole. Or else any information about the object will disappear.

So for physics to be physics, an person has to simultaneously enter the black hole without trouble AND smash into a wall and be prevented from entering the black hole. Hmm…

Recent headlines state that Stephen Hawking has proclaimed that black holes do not exist in the traditional sense. The media blew that out of proportion. He said that black holes do not have eternal event horizons. Instead, they have temporary “apparent” horizons, which allows for the black hole to evaporate and eventually release everything that ever went into it. That is not saying that black holes do not exist.

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Nobody understands me…

Among the many proposed solutions is the suggestion by Sabine Hossenfelder that the paradox stands on shaky grounds. There are certain assumptions which went into the formulation of the paradox. They include that the black hole Hawking radiation is “pure,” the radiation comes from near the surface of the black hole, and observers see nothing strange when passing through that surface. The second assumption comes from Hawking’s calculations using quantum mechanics near a black hole, and the last assumption is required by our theory of gravity.

It turns out the radiation is not pure. It is mixed and messier than previously thought. That was result of Hossenfelder’s paper mentioned in the previous paragraph, along with two other previous papers. In other words, there is no paradox, it is just wrong. Well, it certainly got people interested in black holes again and it pushed research forward significantly in the past year.

See Ethan Siegel’s blog for more explanation. I haven’t gone over Hossenfelder’s paper; I do not know quantum field theory in curved spacetime yet (I hope to learn soon), so I wouldn’t be able to appreciate the paper.

astronomnomy:

I won’t provide screenshots as that would be impolite to those participating in the conversation but - over the last few hours there has been frenzied discussion on the Professional Astronomers’ Facebook page about that Type 1a supernova in M82 and they may have already found the progenitor (star that exploded). There’s also been a lot of work on Twitter, too.

Scientists are using social media to do work on amazingly short timescales. Whoop!

It will probably be a while before the gamma-ray signature changes noticeably.

Project Idea

I know this has been done at other universities, but not at mine as far as I know. Some of my students last semester showed some interest in astrophotography (and why not? it is awesome). They can pursue it in a couple of ways.

York College holds a public observatory night typically the second Wednesday of every month during the fall and winter semesters. Professor Paglione brings an 8” computerized telescope (Schmidt-Cassegrain). I can encourage my students to go and then possibly take images by holding their smartphones up to the eyepiece. This is called afocal photography.

Please don’t point the telescope in anyone’s window.

Or I can have them use a publicly available robotic telescope. If they wanted, perhaps they could contact a privately owned robotic telescope and request time. Hopefully the telescope can make the raw images available, so the student can really play around with the image. Example telescopes available:

Admittedly, the last one would be a little involved for an introductory astronomy course, but the option is available. It requires writing a 2 page scientific proposal and knowing the details of observing conditions.

They can at least virtually hang out with observers. The google hangout “virtual star party" is good for those of us trapped in New York City when the local amateur astronomy clubs are not active.

Another option is if the student already has a telescope, then they can start doing things on their own. How about harvesting a store-bought webcam? They can be used to take phenomenal images of planets and the Sun. Or simply point their cameras up and take pictures of the night sky.

I think these options will go over well, even though most people will not do them. The fact that these options are available helps make the class more free and enjoyable. I’m sure I have missed other options. What else could I have the students do?

It’s likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe.

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New measurements from the BOSS study tell us the shape and size of our universe to within one percent accuracy. Turns out, it’s essentially flat, and probably infinite.  (via brookhavenlab)

No edge, like a BOSS

(via jtotheizzoe)

I searched most of the likes/reblogs and I am a little surprised the comment wasn’t already made. The fact that the universe is flat does not tell us that the universe is infinite in extent! After all, a torus (donut) is considered flat, too! However, if an ant were to crawl in a straight line in any direction, the ant would always come back to where it started. Flat, never ending, but not infinite.

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(wiki) Here we start with a flat sheet. Fold it into a tube, then bend the tube until the two ends meet. We have taken a flat sheet and turned it into a donut.So a donut is actually flat. Silly geometry.

This does say that if the universe repeats itself like the donut, then the size is significantly bigger than the observable universe, or else we would have seen correlations in the sky either by seeing similar patterns of galaxies or in the cosmic microwave background.

The infinite is still a hard pill to swallow. Nothing we deal with normally is infinite. Does the universe extend infinitely far in all directions? I cannot answer that. Nor does this new data.

Just saying.

[1401.2204] A Suggestion for MOND

I have not read this yet, so I should probably refrain from commenting, but I will anyway. I’ll quote some of the abstract:

We present an alternative theory with two forces, one the traditional Newtonian inverse-square, and the other that falls off inversely with distance at large distances.

Now regular Newtonian gravity is a single force that falls off as the inverse-square of the distance:

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This new force goes like:

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Where I don’t know A, they may have it or may simply leave it unknown. They claim that the effect of the new force is suppressed near stars. Fine; at small distances, the (1/r2) term dominates anyway. My point:

In the book The Physical Foundations of General Relativity by Dennis Sciama, the author considers such a new force. Sciama is well known in relativity theory as worked in astronomy, as well. His doctoral adviser was Dirac, and he was adviser to a number very talented and well known astronomers and physicists (see Sciama’s wiki). Note: I should also finish the book, I got about halfway through.

I’ll skip details. Sciama considers a 1/r force and concludes that this extra force simple acts like inertia! It is the reason for the equivalence principle (inertial mass = gravitational mass). In other words, consider Newton’s 2nd Law:

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According to Sciama, there is no “m" on the right side of this equation. Rather, this mass comes from the gravitational interaction with everything else in the universe! Or something like that.

Reaction to this paper: at the very least, a special case has been done. The special case recovers general relativity from Newtonian gravity. Therefore, the general case will necessarily modify general relativity. Perhaps this paper warrants a read?

Jan 6

infinity-imagined:

City lights photographed from the International Space Station and Neurons imaged with fluorescence microscopy.

Source images; Cities (1) (2) (3) (4) (5), Neurons (1) (2) (3) (4) (5)

Check out the concept of “centrality" in network theory. This is simply an efficient way of storing and transporting things, clearly. Most of the time, both cities and neurons grow organically, with connections to neighbors. These connections are meaningful and must, therefore, be as efficient as possible while maintaining the meaningfulness.

So while these images are amazing and captivating, it is not necessarily surprising to me. That gravity shows the same structure is more interesting. The following is perhaps not the greatest example, but it was easy to find:

The mass of a galaxy cluster may, then, grow in a similar fashion to neurons. An initial seed drawing in material from around it: the seed grows. The bigger object is able to draw more things to it. The filaments seen in the right image come naturally from simulations of how gravity works. It is all about the connections between two things: in this case, how one thing draws material from it’s immediate environment.

Jan 2

The nights sky was stunning! Unfortunately, I didn’t get as much Milky Way time as I wanted because of the moon (full moon to 3rd quarter during the trip) and weather. Beautiful landscape, though.

I met Jack Newton (his winter house is shown with two domes). He is super friendly; I definitely want to visit him and his wife’s bed and breakfast in Canada this summer (Bed and Breakfast in Osoyoos).

Expect more writing this semester as opposed to last! I felt sort of dumpy last semester. I think this vacation was a good thing. More writing and more photos to come.

Note: the middle photo is sunrise. The last photo, with the cloud flopping over the mountain: I hikes on that mountain just the day before. Portal, AZ is a wonderful place. The Milky Way photos were all taken at Casitas de Gila in Gila, NM (Gila is pronounced hee-lah).

Jan 2

vic-torl:

rabraha3:

I’ll add some more when I can. I went on a trip to Arizona and New Mexico where I was able to see the Milky Way for the first time in a while.

The first image features Jupiter and the Beehive (I think) poking over the mountains. The second image includes M 31, the Andromeda galaxy. There aren’t touched up or modified. Just a 20 second and 30 second exposure, respectively. Location, location, location. The night sky is amazing.

You should visit my little farm here in Australia. I live away from populated areas, and my night sky is incredible, all year.

Wow, that sounds amazing! After visiting these places, I want to go visiting places around the world. Maybe I’ll apply to the NSF fellowship which can pay me to do work in Australia (EAPSI)…