Category Archives: Astrophysics

Kepler Telescope Lost & Found

Over the past week or so, it looked like the Kepler telescope finally broke down for good. Luckily, it somehow recovered and may be able to run again. It’s already well past its original mission length but there’s still plenty of data to take if it’s able to. Even if it can’t take good data again, Kepler has already allowed scientists to make huge advances in the field of exoplanets (i.e. planets outside our solar system).


Edelweiss Releases New WIMP Limits

The Edelweiss experiment has released a new preprint including updated limits in the low mass region from their WIMP dark matter search using germanium bolometers. There are still some stronger limits, but one interesting thing is that this further bolsters the case that the various purported WIMP signals from other experiments are probably not actually dark matter. The results from experiments like CoGeNT, CRESST, and DAMA all lie above the limit shown here.

How Does LIGO Point Back to a Source?

LIGO published the position of their gravitational wave on the sky when they announced that they had found something, so I was wondering how this could be done. While LIGO has more information that just timing, I decided to consider the case of reconstructing a position based just on the relative timing from two detectors.

I started with a coordinate system with the origin at the center of Earth (assumed to be a perfect sphere here). The z-axis points toward the north pole, the x-axis toward the equator at the Greenwich meridian and the y-axis 90 degrees east of the x-axis. I can then place the two detectors in this coordinate system (easiest using spherical coordinates). The time separation for a gravitational wave along some axis given by a unit vector u(θ,φ) is just

\Delta t(\theta,\phi) = \frac{R_\oplus}{c}({\bf v_1}-{\bf v_2})\cdot {\bf u(\theta,\phi)}

where v1 and v2 are the unit vectors pointing toward the two detectors, R is the radius of Earth, and c is the speed of light (the speed of the waves). If each detector can measure the time of the wave to some uncertainty σ and the relative timing uncertainty (typically from GPS) is negligible, I can define some likelihood that u is the correct vector as

L(\theta,\phi) = e^{-\frac{1}{2}\chi^2(\theta,\phi)}


\chi^2(\theta,\phi) = \frac{(\Delta t_{\rm meas} - \Delta t(\theta,\phi))^2}{2\sigma^2}

Using this function for some test values, we can try to understand how pointing works. If the time separation is 0, then we expect to see most likely values along the great circle between the two detectors.

Skymap in standard spherical coordinates of a gravitational wave measured with 1 ms precision with 0 time separation. Red indicates a good fit, blue is poor.

I will also note that this is plotting position on the sky in Earth coordinates. Earth is not in an inertial reference frame: it rotates and moves in space, so actually mapping this against the stars in the sky gets very complicated and I’ll ignore that since it doesn’t affect what I want to show. The coordinates are also spherical coordinates, which are non-Euclidean, so shapes look strange if you’re not used to them. The curve above is actually the spherical version of a “straight” line. For a time separation of 8 ms, we see a thin ring that widens out as the resolution is degraded.

Skymap of a gravitational wave with a time separation of 8 ms and a precision of 100 microseconds.
Skymap of a gravitational wave with a time separation of 8 ms and a precision of 1 ms. The thin ring is getting wider, with even the center not having a terrible likelihood.
Skymap of a gravitational wave with a time separation of 8 ms and a precision of 2 ms. The ring is starting to turn into just a blob. The strange vertical band at low theta is just an effect of the coordinates. That corresponds to basically just a single point in the sky.

At 10 ms, we’re getting a large separation, and the ring has shrunk almost to just a blob.

At a time separation of 10 ms and a precision of 1 ms, we no longer see the ring and just see a roughly elliptical blob.

Basically, what seems to be going on is:

  1. The general shape of the likelihood distribution is a ring of positions corresponding to the measured time separation
  2. As the time separation increases, the size of the ring on the sky decreases
  3. When the separation is too large (requires a speed less than c), no position works well, so there is a sharp peak at the best (but still non-optimal) position.
  4. The ring is characteristic of having two detectors. The center of the ring never changes (there are actually two centers depending on the sign of the separation) and its position is determined by the positions of the detectors
  5. A third detector should break the degeneracy and allow for reconstruction of the position as a single point on the sky
  6. Since the measured position looks like a blob, it looks like the time separation is large enough for the timing uncertainty to prevent us from seeing a ring-like shape.

Finally Read the LIGO Paper

I finally found some time to read the LIGO paper. A couple things that I thought were interesting:

  1. The peak power from gravitational waves was 200 solar masses per second. The power didn’t stay there for very long since a total of 3 solar masses was radiated away.
  2. The rate of false positives the size of the signal seen is one in tens of thousands of years, so this is a signal that is enormously above any known backgrounds.
  3. LIGO also uses some complicated template fitting routine where they compare the measured signal to a library of pre-calculated theoretical curves. This only gives approximate results for physics parameters, so they then have to supplement this with an actual fit.
  4. The next biggest event had a false positive rate of only one every few years

Colbert & Brian Greene Discuss Gravitational Waves

Yesterday, Columbia professor Brian Greene was on Colbert’s show to talk about gravitational waves. Greene gives some nice explanations for laymen (with graphics!) about gravitational waves in general and about LIGO. He even brings out a Michelson interferometer to demonstrate how the LIGO setup works (though not with gravitational waves). Since Greene is a theorist, I would assume that someone else had to set up the interferometer for it to actually show some sensible results. You can find the video on Youtube here.

Antares Looks for Dark Matter Signals

The ANTARES neutrino telescope has a new result looking for “secluded” dark matter, where dark matter annihilation is mediated through some new mediator that then decays into Standard Model particles. They claim that this can explain the high energy bump in the positron/electron ratio and can also still be a thermal relic from the Big Bang.

They look at several different channels, including one where the mediator actually lives long enough to reach Earth and decay in the atmosphere, and others where neutrinos in the final state are measured.

For this model, the result is actually stronger than direct detection experiments for spin-dependent interactions and is stronger at very high masses in the spin-independent channel. While this result isn’t particularly groundbreaking, the paper mentions that it is the first search of this kind for this type of dark matter, and I think the model, which I hadn’t heard much of previously, sounds quite interesting.

DM-Ice Releases Results

The DM-Ice experiment has released their first results for a search for an annually modulating signal of dark matter. They’re currently about an order of magnitude off from the purported DAMA/LIBRA signal but hopefully will improve significantly in the future.

DM-Ice is a NaI(Tl)-based experiment looking for signals of dark matter in their scintillating crystals. The big purpose of DM-Ice is to test various theories for why the DAMA/LIBRA experiment, a NaI(Tl) experiment at Gran Sasso, has seen an annual modulation in its event rate for many years. Various people have proposed that maybe DAMA/LIBRA is just seeing a seasonal effect. DM-Ice is in Antarctica, so any seasonal effects will be very different. If DM-Ice sees the same modulation as DAMA/LIBRA, then that would rule out many of the proposed explanation, since dark matter will modulate in the same way no matter where the detector is but most other things will be location dependent.