- #36
Chalnoth
Science Advisor
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Well, sort of. The way that this measurement is actually done, though, is by drawing triangles. Two measurements for this are the cosmic microwave background and baryon acoustic oscillation measurements.Skolon said:Thank you again, but I already knew that.
My question was about that: to measure the flatness of Universe the mean density of Universe if measured and it is compared with critical density. If I understood correct, this is how we can tell today that the Universe is "very close to flat".
The "triangle" drawn from the CMB is composed of the typical distance between the "acoustic peaks" which is set by the age of the universe at the time the CMB was emitted. Comparing this distance to the angle that we see and the distance to the CMB gives us our triangle.
The "triangle" drawn from BAO measurements comes from the fact that these measurements are measuring a typical distance between galaxies in the more nearby universe, a distance that is correlated to the same distance seen in the CMB. We can thus draw a different triangle as the typical separation between galaxies compared to the distance to those galaxies and their angular separation.
By contrast, other measurements of the contents of the universe that don't end up effectively drawing triangles, such as supernovae to measure the expansion rate as a function of time, don't measure the curvature at all.