• bram

optical illusions, pulsars, optical illusions involving pulsars

Three things today.

  1. A riff on November 3rd's beautiful Astronomy Picture of the Day via the Hubble Space Telescope:

  2. A recent preprint on a puzzle about pulsars: the rates at which they spin down don't agree with our current theories.

  3. I don't know enough about pulsars floating in space on their own: I've studied mostly pulsars/neutron stars in binary systems. So I'll conclude with something I do know something about: a mash-up of the first two--optical illusions involving pulsars!

* * *

November 3rd's Astronomy Picture of the Day was on V838 Mon, a variable star. Since a 2002 outburst, the star's changing light has been lighting up surrounding gas.

Here you can see some "movies" of the nebula's appearance changing over time.

This is one of those neat "optical illusion" situations in astronomy, where apparent motion is actually the result of changes in the lighting.

Here is a more technical paper on this star system

Here is an Astronomy Picture of the Day on Hubble's Variable Nebula. Remember, most things in the sky take a long time to change, but Hubble's Variable Nebula can change its appearance over weeks. In this case, too, it's a trick of the light.

Below is a false color animated GIF showing changes in this nebula in 1998:

The source of this animation and explanations for the effect can be found at this U of Manitobe page.

Basically instead of actual gas moving around in the nebula, what's happening is that small blobs right next to the bright star move around blocking the light to the rest of the nebula.

* * *

The preprint on pulsars is by Yue, Xu, and Zhu! Haha Chinese names are funny. They get you so many points on Scrabble! It's also cute the way they talk about "magnetic momentum" when they mean "magnetic moment."

Pulsars, spinning neutron stars, should slow down over time. The radiation they give off comes at the expense of their rotational energy. If omega is the angular frequency (equivalently the angular velocity) of the pulsar, dot(omega) is the time variation (negative means the pulsar is slowing down).

If you assume

omega proportional to dot(omega)n

then you can define the "braking index" of the pulsar to be n.

For background I'll add to this preprint the old (1983) book by Shapiro and Teukolsky on Black Holes, White Dwarfs, and Neutron Stars.

Model or Star n
Magnetic dipole model 3
Gravitational wave model 5
PSR J1846-0258 2.65+/-0.01
PSR B0531+21 2.51+/-0.01
PSR B1509−58 2.839+/-0.003
PSR J1119−6127 2.91+/-0.05
PSR B0540−69 2.140+/-0.009
PSR B0833−45 1.4+/-0.2

If the pulsar's energy is carried off by gravity waves, then n=5. This means that for large omega, gravity waves are better at carrying off energy and spinning down the pulsar than the magnetic model, which has n=3. When the pulsar is young, omega is larger--so in the early days of the pulsar, gravity waves should spin down the pulsar faster than magnetism.

So the big surprise here is that all of the braking indices are <3, when the main mechanisms of energy loss should cause 5>n>3.

The preprint summarizes some recent attempts to solve this problem:

  • Include relativistic particle flow (the preprint I'm reviewing here)
  • The inclination angle of the pulsar changes (or is different from assumed??)
  • The magnetic field strength increases
  • Gas falls onto the pulsar, changes its spin
  • The magnetic field re-connects

I'm going to have to check that I understand the authors' English correctly!

The problem with n<3 is that the theory says that the pulsar would lose its spin to power its brightness slower than observed... Gravity waves give up energy fastest (n=5), then comes the magnetic energy (n=3)... So I think most of these explanations are adding energy sources that don't weaken so much as the pulsar slows down...

* * *

Optical illusions involving pulsars

This is the standard picture of what goes on around a pulsar:

From the 1969 paper by Goldreich and Julian on Pulsar Electrodynamics. Note that Goldreich was my thesis advisor's thesis advisor!

So the pulsar has a strong magnetic field because the field has collapsed along with the star that formed the pulsar into a small space (20 km across or so)... And this fast spinning magnetic field creates a strong electric field on the surface of the neutron star, which pulls off charged particles to form the "magnetosphere".

Within a specially defined "light cylinder" the magnetic field forces particles to rotate along with the pulsar... But not outside! Why not? Because to keep up with the pulsar's rotation from further away, you have to go faster... until you would have to move faster than the speed of light!

To qualify for my Ph.D. I wrote a thesis asking: what happens if a pulsar's beam reflects off of a surrounding accretion disk... and the accretion disk is outside the light cylinder?

This is one of those "paradoxes" in which a reflected beam can seem to move faster than light... But it doesn't violate relativity because it's only a beam... Nothing physically is moving from point to point...

And there are neat effects you could see... Even though only one point is lit up at a time, you can see 1+4 R nu / c points lit up at once, where nu is the frequency of the pulsar's spin, R is the radius where the beam is reflecting from, and c is the speed of light.

Sometimes the point will seem to move toward you at the speed of light--and its brightness gets amplified. If the beam weren't moving at light speeds, you'd see the spectral line move in a circle, as the beam lights up gas moving towards and away from you--with alternately positive and negative Doppler shifts. With speeds up to and greater than c, you instead get a cycloid, "looping back in time" so you see the several points at once.

From my qualifying thesis:

Neat, eh? Only this effect has not yet been observed. And in reality the beam would reflect off of not just one radius in the surrounding disk. And then the reflection would get smeared out in time from the back of the disk--if the beam really moved faster than light you'd only see one of those big brightness peaks as it broke light speed at the front of the disk only.
  • bram

(more cosmic rays--other stuff soon!)

I have a longer entry coming up about optical illusions in astronomy and about neutron stars (pulsars).

But first, I've been skimming through Physical Review Focus, which is great! A friend of mine from college used to be involved with this. The idea is that it's a semi-popular summary of physics papers in Physical Review, the fancy technical physics journals. And then it also links to the actual papers if you want to know more.

So here are a couple of links--unfortunately they're a few years old and I'll have to look up to see if the ideas are still current--apparently "doubly special relativity" is not the only game in town for explaining why we see cosmic rays with such high energy. Cosmic rays with energies > 1020 erg should be impossible, because particles with such high energy would collide with the photons of the cosmic microwave background, the radiation filling all space as a result of the Big Bang.

Massive neutrinos minimize the number of tooth fairies: crossing the thresshold of early 1999, massive neutrinos were still becoming accepted. I think the evidence is even more in their favor now. The idea is that these high energy cosmic rays could result from massive neutrinos that pass through space unimpeded and then collide with other neutrinos when they get close to us (hm, is that collision so likely??), releasing particles.

A later paper suggested that maybe intergalactic magnetic fields are stronger than previously thought. So super high energy cosmic rays could be deflected more as they pass intergalactic space--and even though so far we haven't seen them come more from one direction of space than another, that could be because of the deflection. And so maybe the cosmic rays come from relatively nearby active galaxies like M87.
  • bram

Cosmic Rays (Totally COSMIC, Man!)

Astronomy Picture of the Day (apod): "Little Dumbell Nebula" near Andromeda Galaxy in the sky:

Here's another thing I don't know enough about: cosmic rays!

I'm still reading Lee Smolin's The Trouble With Physics, a critique of string theory that suggests physicists instead of looking inward at the mathematics of superstring theory should look for experimental inspiration in astronomy observations that don't fit current theories.

So it's timely that yesterday a preprint was posted by Pasquale Blasi on "Open questions with ultra-high energy cosmic rays".

Here are the questions I'll try to answer:

  1. What is the general background knowledge?
  2. How is the current theory for the formation of cosmic rays holding up?
  3. What are the current state of the art cosmic ray detectors, and how generally do they work?
  4. What is this big problem with the "GZK cutoff"--the idea that we may be seeing such high energy cosmic rays that it would be impossible for them to cross the Universe for us to see?
  5. What is "doubly special relativity"? (or "deformed special relativity"?) (As an unconfirmed and somewhat vague hypothesis I don't expect many scientists to jump on its bandwagon)--I will have to go into more detail later!

It would be ironic if cosmic ray observations were to change our understanding from special relativity of how to change our view in different frames of reference! That's because one very basic and simple way to show that relativity works--that stuff moving near the speed of light has its "clock" made to run slow--is through observing muon particles in cosmic rays (here's a description of the standard experiment).

Basically, the muon is an unstable particle. When a cosmic ray (a particle coming from space onto our atmosphere--90% of the time a proton, 9% of the time a helium nucleus and about 1% of the time other) strikes a particle in our atmosphere, a "shower" of exotic particles like pions and muons are created. This schematic is from (although I don't know who runs that site!):

And here is a track in a bubble chamber of a pion moving to the right decaying into a muon--which then lives only a very short life--causing just a small blip!--and decays into an electron, which spirals around to the left:

I got that picture from this site.

I think the way it works is that a magnetic field causes a force on a particle with charge q (F=q vxB... for roughly circular motion this = q v B)... With a=v2/r for circular motion and F=ma (this is non-relativistic!), r=m v / q B = p / q B. So the cyclotron radius shouldn't change when one particle decays to another, as long as momentum is conserved...

(Heh, this checks out on Wikipedia though I forgot the charge q at first!)

So the radius changes slightly as you go from pion to electron because sometimes some momentum goes to another particle you don't see (because it doesn't have charge)... And when a charged particle accelerates it has to radiate too: so you don't get perfect circles, but decaying spirals.

(Another thing you'll notice from the r=p / q B equation is that it means that a cosmic ray with large momentum p will be curved in a larger circle by a magnetic field B. So even if most cosmic rays are isotropic, coming from all directions, maybe the higher energy ones will depend more on direction.)

Anyway, when I was a senior in college I tried an experiment similar to the one related above. Muons decay in only a very short time, so if it weren't for relativity, they wouldn't be able to make it down from the atmosphere to sea level. However because of relativity and because the muons move so close to the speed of light c, their lives are extended. Time for the fast muons slows down, and they don't die until they get down far enough for us to see them.

You can actually observe this by counting muons at the top of a mountain and at sea level.

For my senior experiment I tried counting muons in the top and bottom floors of the physics building... They came out equal... I wasn't that great at running experiments though I do remember carrying a bunch of lead bricks (to shield out non-muons from the detector!) up and down the building...

By the way, the great pioneer of cosmic ray physics was Bruno Rossi--after whom the Rossi X-ray Timing Explorer (RXTE), mentioned in a previous post, is named.

* * *

Cosmic ray astronomy is very different from telescope astronomy. We don't know where this stuff comes from--whereas astronomy based on light is all about knowing where the stuff comes from!

The problem is that cosmic rays--protons or helium nuelcii or heavier nucleii--are charged. So magnetic fields will deflect them as they pass through space. So we don't even find that cosmic rays come more strongly from the direction of our own galaxy. Instead they are "isotropic", coming from all directions of space pretty much equally.

What gives cosmic rays their energy?

The answer was theorized by Enrico Fermi, the great Italian physicist who worked on the Manhattan Project and directed the first controlled nuclear reaction.

Those of us who keep lab notes on yellow graph paper ruled notebooks can only pray that some day we will write a page of such enduring value!

Link to Symmetry magazine--which I think I will subscribe to!

The idea is that charged particles bounce back and force through magnetic fields in space--generated for example in supernova remnants--and along the way they pick up energy.

This has recently been more or less confirmed by observations! The idea is that the strong magnetic fields will not only accelerate charged particles but also those charged particles will radiate. So although we can't tell what direction cosmic rays come from, we can expect that supernova remnants will be bright in synchrotron radiation, which can be observed by X-ray telescopes.

For example, this paper shows from Chandra data that the supernova remnant Cas A--discussed here a couple of days ago because the Spitzer Infrared Space Telescope showed its explosion to be more symmetric than thought before--can indeed be responsible for many of the cosmic rays observed.

However, this preprint suggests that Cas A and other common supernova remnants couldn't cause the most energetic cosmic rays.

Things are pretty complicated because not only does the supernova remnant magnetic field accelerate the cosmic rays, but the cosmic rays themselves then change the energy and magnetic field of the supernova remnant.

* * *

Ok, What is the current state of the art of cosmic ray detectors, and how do they work?

I'm going to focus on the highest energy cosmic rays. That's where things are most exciting: we don't know where they come from, and these energies (1020 electron volts) are greater than those that can be produced in the fanciest and most expensive particle accelerators on Earth (the Large Hadron Collider should go up to scales of 1012 eV, in a more controlled and observable way than cosmic ray collisions!)

The latest and greatest cosmic ray detector is/will be the Pierre Auger Observatory, which is actually a pair of observatories, one in the Northern Hemisphere (Colorado) and the other in the Southern Hemisphere (Argentina). That way they can observe the entire sky. Otherwise, if there were a slight difference in cosmic rays from different directions, that would mess up the observation.

Each observatory will detect cosmic rays in two different ways.

By one method, 1,600 tanks of water are placed on the ground--each tank capable of holding 3,000 gallons. Here they are all together:

Haha. That so totally looks like a scene out of one of the Aliens movies! Like each tank is going to hold some kind of Alien reptile that's going to be shipped out to be a parasitic host and burst through someone's rib cage! Well, it's not like that actually.

Actually each tank is placed out in the rural middle of nowhere:

Then when a cosmic ray passes through the tank of water it produces Cherenkov radiation, which is what happens when a particle moves faster than the speed of light.

Ok, wait a minute--are we ready to throw out relativity yet? No, not yet!

You can go faster than light goes within some material. Relativity just says you can't go faster than light in a vacuum and light in a vacuum moves at c. In water, light is slowed down slightly, so a cosmic ray passing through water can go faster than the speed of light in water. And if it does, it'll give off this radiation, which is a little like a sonic boom given off by a supersonic airplane.

The other method the Auger Observatory uses is to look at the UV light emitted when a cosmic ray strikes the atmosphere (this is fluorescence). So the two methods are a check on each other. They expect a real cosmic ray to cause both UV light, seen by several telescopes, and also Cherenkov radiation seen inside these huge tubs of water.

The total cost of the Auger Observatory was $50 million, to be compared with other costs mentioned here (and in the NASA budget on the sidebar).

The Auger Observatories aren't ready to go public yet with their data--they are still calibrating.

Previous high energy cosmic ray observatories that have gone public with their data:

KASKADE looked at what the cosmic rays are made of for energies up to 1018 eV, where there maybe be a turnover (a "dip" or "ankle") in the graph of cosmic rays vs. energy. The number of cosmic rays from galactic supernova remnants may drop off at high energies, but that may be taken over by some unknown (perhaps extragalactic) source of high energy cosmic rays.

AGASA, a Japanese array

HiRes, the High Resolution Fly's Eye, run by U of Utah

Actually these last two observatories disagree with each other on the most important point!

Here's a debate on which experiment has it wrong!

That's why the Auger Observatory results are so eagerly anticipated.

* * *

So what? So two observatories disagree, one made a mistake, and the new telescope should set things right.

Well, if AGASA is right, maybe something funny is going on. (Funny-strange, not funny-haha.)

The thing is that high energy cosmic rays passing through empty space could get messed up by the radiation left over from the Big Bang (see earlier entries!) that fills all space. This should limit how far cosmic rays with such large energy can travel, and so we shouldn't see that many cosmic rays with energies like 5x1018 eV. This is called the GZK limit, after Greisen, Zatsepin, and Kuzmin who first theorized it. HiRes seemed to show that really high energy cosmic rays became rare as expected by this theory, but not AGASA.

Here is a paper by Floyd Stecker. I used to work down the hall from him at NASA. He was a fun guy--loved California (though he lived in Maryland), loved to drive fast (though he always got speeding tickets!)

Anyway, Floyd works on cosmic rays (though I never talked with him about them much!) and in this paper he mentions speculations that violations of special relativity could cause the GZK limit not to apply--could allow high energy cosmic rays to pass through the Big Bang radiation freely.

He suggests a new kind of cosmic ray detector--one that will look down on Earth from space for the ultraviolet fluorescence radiation when cosmic rays crash against the atmosphere.

Why would anyone expect special relativity to break down? Well, there's a suggestion that the Planck length (10-34 cm or so) is fundamental. It's the only length you can calculate based only on the fundamental constants of nature, like the speed of light c, Planck's constant h, the gravitational constant G. It's where quantum theory and gravity (General Relativity) collide to make space and time meaningless--requiring a new unknown theory of Quantum Gravity. So the new Doubly Special Relativity (or Deformed Special Relativity--DSR in either case) hypotheses (there are at least two versions) say that as in normal special relativity, everyone will observe the same speed of light c, but also that everyone will observe the same special Planck scale--there will be a special energy or momentum (related to the Planck length) that different observers will also agree on. The math is here but I haven't gone over it yet. Also, there's this paper.

DSR doesn't neccessarily mean the GZK cutoff will be changed, but it is possible according to some versions of DSR.

For more discussion on this, see this blog entry by Christine Dantas, with communication from Lee Smolin.
  • bram

Fast Variations in X-rays from Neutron Stars

[Astronomy Picture of the Day: Spirit on Mars]

I will have to make more sense of this preprint (preprint 1) from last Thursday. It's about a new neutron star system that's similar to a bunch of other mysterious systems--but it changes its brightness a lot more. So there's hope that by watching it go through its changes we'll learn more about the other systems.

And then there's preprint 2, this paper on a couple of accreting millisecond pulsars.

What's so great about accreting millisecond pulsars?

People had known that radio pulsars can spin fast--as fast as once a millisecond or so--even if they are very old. The brightness of radio pulsars is thought to be powered by their rotation so they should slow down over time. The theoretical solution? Pulsars are sped up if they go through a phase in their lives in which they are in a binary system and accrete spinning gas. This was just theory until 1998, when Deepto Chakrabarty found the missing link,
the first accreting millisecond pulsar.

Both papers make use of the venerable Rossi X-ray Timing Explorer, an X-ray telescope optimized to detect very fast time variability in X-ray sources. Optimized how? It has great sensitivity ("effective area") so it can see down to very low X-ray levels. Also when it reports its data back to Earth there's a lot of flexibility in how it does that--it can bin up the data according to a known period or bin the time of arrival into about a microsecond. It's not so great though when it comes to spectral resolution to spatial resolution: just time resolution!

(It also has an All Sky Monitor on board--it's awesome! Go to and have a look. The All Sky Monitor sweeps the sky a few times a day to determine how all the really bright X-ray stars and galaxies are changing their brightness over months... So RXTE really can see X-rays change on all time scales from microseconds to years (one year is about 3.15x107 seconds--so that's from microseconds to hundreds of megaseconds! A factor of something like 100 trillion!)

Jean Swank was one of the pioneers of RXTE, and her page says she's now working on detectors to see X-ray polarization.

Anyway, what about these two papers and fast variations in X-rays from neutron stars?

I think I will have to go into more detail later. The co-authors are all very experienced in this area. van der Klis was one of the pioneers in finding fast "quasi-periodic" oscillations from neutron stars. Walter Lewin was one of the pioneers in studying X-ray bursts, which are thermonuclear explosions on the surfaces of neutron stars. He's a real wild character--I was on a committee with him once. When I get broadband I'll download some of his MIT lectures from his page.

I will only point out that some of the explanations in Preprint 1 look a little "kludgey" but that I'm reluctant to discount them because of the prestige of the authors. Namely, they suggest that the changing amount of matter falling onto the neutron star is responsible for two kinds of systematic variability in the system--that struck me like saying, "Hm, rotation of the Earth explains the day/night cycle pretty well, maybe it can explain the seasons too?"

You can make something like an X-ray "HR diagram" for these mysterious neutron star systems, and you often find something like a "Z" in one star as it changes its X-rays over time. So they say that the motion along the Z is caused by changing amounts of matter falling onto the neutron star. But also the track of the Z itself changes in time and is different from star to star (5 stars are shown below in different colors, from this paper:

  • bram

Hubble Not Buried Yet!

Links: NY Times, NASA, NASA TV

The Shuttle will fix Hubble in May 2008, extending its life to 2013, if all goes well, the NASA administrator Michael Griffin has decided. After the Shuttle Columbia disintegrated in 2003, NASA planned to send Shuttle missions only to the International Space Station, which needed to be completed, and from which the Shuttle could be inspected for repair. A robotic mission to fix the Hubble without human assistance was proposed but deemed infeasible.

The new instruments to be placed aboard HST--otherwise they'd languish on the ground, as they are already prepared--are COS, the Cosmic Origins Spectrograph, and WFC 3, the 3rd version of the Wide Field [Planetary] Camera (used to be pronounced Wiff-Pick--when did they drop the P?).

Links to these new instruments:


Goddard Space Flight Center page on WFC3

COS mini-handbook, WFC3 handbook


COS provides spectra in the 1150 to 3200 Angstrom range (UV light that doesn't get down through Earth's atmosphere) with higher sensitivity for faint objects than ever before

How does it compare to STIS, the Space Telescope Imaging Spectrograph, that's currently on board? Well, it doesn't go to such high resolution but it has higher sensitivity. This is from the "mini-handbook" linked above:

This is a complicated table--how do I read it? Well, FUV is Far UV--I like to observe in the FUV! lambda/delta lambda is the resolution. I like to work with a resolution of 20,000 to see Doppler shifts of 15 km/s or so, which are important in the star systems I study. (A stellar wind is about 1,000-3000 km/s and X-ray binaries usually orbit with 100-500 km/s... an accretion disk spins at about 300 km/s at the edge).

If I used the COS G130M (the G130M is a configuration choice, probably having to do with gratings, etc.) near 1300 A, the sensitivity Slambda would be 1.0.
If I used the STIS E140M and added up each 2.3 wavelength bins, the sensitivity would be only 0.14! That's only 1/7th as sensitive! (I know that by heart b/c they used to say Pi was about 3 1/7.) But if I opted for 2 times lower resolution (R=10,000) with the STIS G140M, I'd still have only 0.66 of the sensitivity, compared with 2.0 for the COS binned by 2 to get the same resolution of 10,000. So either way it looks like COS should be better--if everything works!

WFC3 provides wide field images and spectra from the UV through the IR and compares favorably with older instruments:

I don't think that the upcoming deadline for proposals to use HST includes the chance to use these new instruments... I think that will be for next year... and future years!
  • Current Mood
    ecstatic ecstatic
  • bram

Neutron Stars and Black Holes in the Andromeda Galaxy

My research specialty has been X-ray binary stars: double star systems in which one of the two stars is a neutron star or a black hole. Many of the stars that are brightest in X-rays are neutron star or black hole systems like these.

I was excited to see this preprint available today, about XMM/Newton observations of X-ray binaries near the center of the nearest big galaxy, the Andromeda Galaxy (M 31). It used to be we could only see individual X-ray binaries within our own galaxy, but Chandra and XMM have changed that. Now it's like opening up the Americas to Europe--we have available all sorts of indigenous oddities like corn and potatoes and cocoa that we hadn't seen before. Studies of the nearby galaxies LMC (Large Magellanic Cloud) and SMC (Small Magellanic Cloud) have suggested that the X-ray binaries in these galaxies are brighter than the Milky Way's--maybe because these galaxies don't have as many heavy elements.

Anyway, I was excited also because I thought they would cite a paper I worked on and they did--Prestwich et al. 2003. (I just measured a bunch of X-ray spectra--nice and easy, as opposed to interpreting nature or other human beings, which is trickier!)

So Trudolyubov, Priedhorsky, and Cordova, with XMM, have found 123 X-ray binaries, and here's how they look:

And a close-up (the circle is 7 arc minutes radius):

The authors identify 7% of their 123 objects as good candidates for black holes and 24% of them as good candidates for neutron stars.
  • bram

Daily round-up 2: Viewing the Big Bang's Inflation

Two preprints caught my eye today:

Genus Topology of the Cosmic Microwave Background from the WMAP 3-Year Data


Topology of structure in the Sloan Digital Sky Survey: model testing

both by J. Richard Gott III as first author.

Gott, a professor at Princeton, has written a popular book called Time Travel in Einstein's Universe.

I met him once when I was 17--he was an interviewer when I competed for the Westinghouse Science Talent Search contest.

When I saw these research papers I thought they were about the topology of the Universe, which is an interesting field covered, for example, by Jana Levin in her book How the Universe Got Its Spots, which is a combination of scientific speculation and personal diary. (Levin has recently published a novel too.)

When you ask about the topology of the Universe you ask whether, for example, going in one direction will ever bring you back where you started. Or maybe it's like stacked repeating cubes: go left, up, or forward and you'll end up coming back from the right, down, or back, respectively. This is something people like Levin started to test by using the data from WMAP, the Wilkinson Microwave Anisotropy Probe, which looked at the radation from the edge of the visible Universe, at 300,000 years after the Big Bang (looking through that radiation would be like looking through the surface of the Sun--when you look back further in time you look to when the Universe was too dense to see through.)

That's not what Gott's two papers seem to be about, but they look very interesting anyway!

People usually try to understand the un-evenness of the Big Bang's radiation by computing the "power spectrum".

When I analyze data from a neutron star, I also look at its "power spectrum"--but a 1-D power spectrum describing how the light goes up and down from adding up sine waves. For the Big Bang radiation you have to use a 2-D "spherical harmonic" power spectrum, and then you add up one of those dimensions to give you an ordinary 1-D graph.

But even a regular power spectrum hides information! A power spectrum could show me a 1.24 second pulsation of the neutron star and a 1.7 day orbit effect too--but it wouldn't show me if they were in or out of phase.

So what Gott seems to be doing is testing the predictions of Inflation--the idea that goes beyond the Big Bang to suggest a period of extremely rapid early expansion to even out the
early Universe--by looking not only at the power spectrum but at Inflation's prediction that the phases should be random and the amplitudes (with the m subscript?) have Gaussian (or bell-shaped) distribution.

One neat thing he's done on the side in this work is to develop a new method of map projection that keeps areas equal (although distorts shapes away from the Prime Meridian).

Here's an image of The Moon's entire spherical surface projected according to his method:

Why has he developed this method? To show better the entire sky with the Big Bang radiation:

His conclusion from both data sets, WMAP and the Sloan Digital Sky Survey, is that everything's consistent with random phases and Gaussian initial conditions.

He also addresses something I'd just been reading about in Lee Smolin's new book The Trouble With Physics.

There seems to be a dip in the Big Bang radiation power spectrum at low frequencies (large angles). This means that the sky at the time of the Big Bang seems to not have varied much across large angles--at least compared with the expectation of Inflation.

Smolin conjectures in his book that the scale of R=(almost the size of the visible Universe) over which there's this deficit of variation in the Big Bang radiation could be related to the similar time t=R/c over which the "dark energy" has affected the rate of expansion (this wasn't so clear to me!) and the acceleration scale R/t2=c2/R at which dark matter affects the rotation of galaxies and the "Pioneer anomaly" (that the two Pioneer spacecraft launched out of the solar system are moving inconsistently with theory)...

It was a bit of coincidentology and numerology--but he was at least contrasting it with superstring theory which seems to be completely non-empirical...

Anyway, Gott et al. show that:

  1. There's evidence but very weak evidence that subtraction of our own galaxy's radiation and other objects in the "foreground" could have messed up the large angle data on the Big Bang
  2. The phases of the data at large angle are random as Inflation would predict

Note that I've only skimmed this paper which has only been available today!
  • bram

Daily Roundup (part 1)

Press releases from the Spitzer infrared space telescope:

Snake on a Galactic Plane: dark splotches of dust in the general shape of a "snake" along the galactic plane mark areas where young stars may not yet have blown away the gas that formed them

Spitzer reveals that supernova remnent Cas A exploded in a more orderly way than thought

Actual preprint on Spitzer Cas A results

Astronomy Picture of the Day: crescent Venus and Moon
High Energy Astrophysics Picture of the Week:

NGC 3242 (NGC means "New General Catalog"--a rather Old astronomical catalog by now!), a nebula also known as the Jupiter's Ghost Nebula--why it's been posted this Halloween season! This is a composite of X-rays (from the X-ray multiple mirror telescope XMM-Newton) and optical. The object is a "wind-blown bubble", the interaction of stellar winds and surrounding gas. My thesis advisor worked on the theory of these objects and You-Hua Chu (to whom the X-ray image is credited) was my first astronomy mentor...
  • bram

Will Hubble, Like a Zombie, Rise from the Grave?

Future of the Hubble Space Telescope to be Announced Tomorrow (Tuesday Oct. 31)

Although it's still not clear whether a further shuttle servicing mission will be able to fix growing problems with the Hubble Space Telescope and attach more advanced instruments, there's talk--only in the preliminary stages--of re-using the design of the Hubble with newer technology to make a much-cheaper Hubble "clone".

Astronomy Magazine article on the Hubble Origins Probe

Hubble Origins Probe home page
  • bram

International Space Station

This is something I don't know much about! Good chance to learn and share.

Last week I came across this "EarthKam" site that provides images of Earth from the International Space Station.

That's a view of a "drainage system" in Queensland, Australia.

Here's a view from above the surface of Titan, from the Cassini-Huygens probe that landed a year and a half ago:

Photo: ESA

More on the Cassini-Huygens mission from the European Space Agency

Shows that planetary exploration can make the familiar look strange and the alien worlds look familiar! Only the drainage system on Titan is not from water flow--I think it's from methane mush!

There are many more beautiful images (though with some computer "artifacts" like bad pixels) on the EarthKam site, showing both natural and artificial features (like highways, dams, etc.)

What has ISS done and has it been worth it?

Notes from Wikipedia article:

NASA site

Started 1998, projected completion 2010
Three astrononauts at a time, has included 4 space tourists
All long-term residents from US or Russia, visitors from 12 countries
Orbital period: 92 minutes

Mass and longevity of ISS compared with previous space stations:

ISS (1998-2016) Mir (1986-2001) Salyut 1 (1971) Skylab (73-79)
Mass200,783 kg 124,340 kg 18,425 77,088 kg

Dimensions: 73 m x 44.5 m x 27.5 m

425 m3 living area

I've skipped over the other 6 Salyut stations. More Wikipedia information in the Space Station entry.

I've read it argued that one problem with ISS is that although it is touted as a step on the way to human bases on the Moon and Mars, the relatively low altitude shields it from cosmic rays that may endanger astronauts

It's often argued that unmanned missions can discover more at lower cost and risk (but less glory!)

From 1980s to 2016 cost: 100 billion Euros (about 100 billion dollars)
Not including shuttle costs ($5 billion/shuttle each year, shuttle used almost exclusively for ISS since 1998)

Comparison of 100 billion to other costs:

Hubble Space Telescope: $1.175 billion in 1986 dollars (can you find a better link than Wikipedia?)--note that HST costs much much more than a large ground-based observatory...

Iraq war: Originally upper limit of 100-200 billion, now projected 1 trillion (or 1,000 billion), with 400 billion cost to US taxpayers

To learn about observing ISS from the Earth: