Tag Archives: Astronomy

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).

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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)}

where

\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.

Map_0msSep1msUnc
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.

Map_8msSep100usUnc
Skymap of a gravitational wave with a time separation of 8 ms and a precision of 100 microseconds.
Map_8msSep1msUnc
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.
Map_8msSep2msUnc
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.

Map_10msSep1msUnc
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

Evidence for a New Planet?

In one of the most surprising recent science stories, some astronomers have announced that they may have found some evidence for an unknown large planet (possibly much larger than Earth) far beyond the orbit of Pluto. They studied the orbits of a number of objects in the solar system and found some unusual features that might be explained by a new planet. Of course, there’s still a chance that this is just a coincidence and nothing too strange is going on, but it’s an interesting hypothesis. In fact, such studies have been used in the past to predict new celestial objects, so this isn’t unprecedented. I’ve been told that the scientists involved are generally very serious people and so this does not seem to be a hoax and the work involved shouldn’t have any serious mistakes. But, we can’t start saying that there is definitely a new planet until we actually find it. Given the huge distances involved, it might take quite a while to find the planet if it really exists.

We Still Haven’t Found Aliens

The press is talking about that star that shows some odd features again and once again people are speculating that maybe it’s behavior is caused by aliens. Once again, there is basically zero chance that we’ve found evidence of something like a Dyson sphere. There is almost certainly some mundane explanation even if it’s something we haven’t really thought of or seen before.