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Could Gaia shed light on the dark matter/energy questions? I mean, it's also a mapping of the gravity of the galaxy, besides a mapping of the visible stars of the galaxy, since it has much data on the movement of stars.
Yes, indeed. For dark matter an ESA press release said:
By watching for the large-scale motion of stars in our Galaxy, it will also probe the distribution of dark matter, the invisible substance thought to hold our Galaxy together.
Consider also that not only we are now capable of mapping the fine structure of dark matter in out galaxy but
also in broad terms thanks to the accurate motion of globular clusters in the halo
For the dark energy I don't have an answer. But considering the fact that DR2 includes a new catalog of 500.000 quasar detections (some are going to lensed sources
which means we are probing vast distances) maybe someone can come with some ideas to further constrain cosmological models.
Another interesting topic in fundamental physics that is going to emerge from DR2 is gravitational wave detection
(just by observing the changes in the path of light using the exquisite astrometry of the spacecraft).
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Is this database something SE could handle, on a computer with sufficient memory? My home server has 64 GB, which might be tight, I guess. But perhaps with a fast SSD and a clever database format with different detail layers it could work.
I don't know what sizes could SE actually reach but I can tell you something about the amount of data. Currently the DR2 main catalogue occupies 587 GB
of memory (All columns for the 1.692.919.135 stars). But it is true that not all the information would be necessary for SE. I've just done a reduction of the data such that we are now only considering stars that have parallax errors of around 5% (very very nice distances for SE) and those are 148.476.074 (the 8,8% of the entire DR2). This means that we are dealing with 51,5 GB
or so of data. But is also true that we don't need to know all the 96 columns for each of those stars. It is sufficient with 5 parameters for SE (Source ID, ra, dec, parallax and G magnitude) even if DR2 has Temperature and Radius for the stars also (using some models) which could be useful for SE. But yeah, taking only DR2 stars with good quality parallaxes and taking only those 5 parameters for each one we are going to deal with around 2.7 GB
of data. Any computer can handle this in memory but accessing the information and making it accessible from SE in terms of milliseconds is a feat that seems impossible to me. But who knows I've saw many things in SE that I though were completely impossible for one developer. The ideal would be to have some sort of SQL capability incorporated in SE for something like this.
In the next Gaia release, the DR3 there are going to be flags marking which are binary resolved systems and maybe even some orbital parameters (something interesting for SE). The parallax measurements are going to be better since the time-baseline increases and with it the number of observations for each star, which means that we are probably having many more stars with 5% accurate parallaxes. Also is very probable that DR3 might reach the 2 billion star milestone (expanding a "little" the 1.7 billion in DR2). So this will keep growing. DR3 is expected for 2020.