Stars Dance to the Music of Parallax
I thought I’d share this cute video from the European Space Agency about the Gaia mission I blogged about last week. It shows the effect of parallax, as measured by Gaia, on the positions of stars on the sky. As the Earth orbits the Sun stars do a dance in the sky; the shift in their position greater for closer stars rather than distant ones. To make the video, parallaxes measured by Gaia have been exaggerated by a factor 100,000 and proper motions have been speeded up by one trillion (1012). The effect is rather hypnotic, and gives a sense of the three-dimensional nature of the distribution of stars. At the end of the video you can see the effect of proper motions too, i.e. the change in position of a star due to its actual motion rather than that of the observer.
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May 4, 2018 at 12:34 pm
How difficult is to use the shift in positions of large numbers of stars to detect gravity wave?
By using the motion of the stars what are recent constraints on dark matter in our galaxy?
May 4, 2018 at 2:28 pm
First question: Extremely difficult. An exceptional case is to detect gravitational waves with a pulsar-timing array; that will probably be possible within our lifetimes.
Second question: The constraints on (or, rather, detection of) dark matter in our galaxy based on the motions of stars probably haven’t changed much since Kapteyn’s work on this in 1922 (who might have been the first person to use the term “dark matter” in this context).
May 4, 2018 at 2:40 pm
First Q: Why is it extremely difficult? If relative distances of millions of stars are monitored over a time period any incoming gravity wave will distort the relative distances in a coherent way. They can be used as test masses? Of course, they will not be very precise like measuring the distances using a laser but there are huge number of stars acting as a test mass instead of just handful like LISA/eLISA etc.
Second Q: I am not quite sure if GAIA can’t improve the constraints on dark matter or its clumpiness in galaxy? Clearly, proper motion of starts can be used to construct their orbits more precisely which is affected by the presence/absence of dark matter.
May 4, 2018 at 3:03 pm
First Q: First, a gravitational-wave event lasts just a couple of seconds. So you would have to monitor this huge number of stars all at once. Second, the effect is tiny, well below the noise, even if it is coherent.
Second Q: Sure, it can improve things, but if you think of it as “how much of the galaxy is made of dark matter” then I don’t think that we can expect a huge improvement in our knowledge from Gaia. Of course, this is not what Gaia was designed for. On the other hand, Planck, say, has significantly improved our knowledge of certain cosmological parameters, including the amount of dark matter.
May 4, 2018 at 3:10 pm
There can be gravitational waves from early universe phase transitions with longer wavelength that can be detected by earth-based detectors and having longer base line can help. Also, not all sources are astrophysical som gravity waves may last longer. About noise; if it is too high or not only a detailed analysis can tell.
Planck gives global values of DM as a percentage of total mass
not for an individual galaxy which if possible will be one step closer to direct detection.
May 4, 2018 at 3:44 pm
I don’t doubt that it is possible in principle, but it won’t happen any time soon, except perhaps in the context of a pulsar-timing array.
Rotation curves of galaxies already give good evidence for departure from what one expects based only on visible mass. At best, astrometry could refine these results, but not significantly change them.
May 4, 2018 at 4:03 pm
How can rotation curve can anything beyond average radial profile?
May 4, 2018 at 4:04 pm
It can’t, but the question is how much detail do you need and why.
May 4, 2018 at 4:16 pm
How much detail we need and why is a fundamental driving force of all of the sciences. We know Higgs boson exists so why we need to know if its composite or not? 120 years back we could have stopped with the Swiss-cheese model of atoms why would Rutherford do the Gold-leaf experiment? I just simply don’t understand your question.
May 4, 2018 at 4:27 pm
Money, time, and human effort are limited. With finite resources, one has to guess which studies will turn out to be most valuable.
May 4, 2018 at 4:37 pm
“Money, time, and human effort are limited. ”
yes, that’s why not exploitation of the existing data will be waste of all the money time and human effort that has already gone into designing, launching and executing the mission. it’s nice that it is in public domain and anyone can download it. I don’t know if the basic software developed by the team needed to understand the raw data is also in the public domain – but I would think so.
May 4, 2018 at 3:11 pm
Correction: that can be detected -> can not be detected.