SPTpol and DES groups for the first time measured the mass of galaxy clusters using weak gravitational lensing of the oscillations of the polarization of relic radiation. The error of the results is several times greater than that of other methods, but a potentially new way of measuring may be more effective because it does not depend on the contamination of the signal by other sources.
Currently, observational cosmology has three more or less reliable methods by which it is possible to measure the parameters of our universe. The first method, the most accurate, relies on the heterogeneity of relic radiation. Roughly speaking, initially homogeneous radiation is distorted differently at different parameters of the universe, so the size and shape of distortions can restore the basic cosmological parameters. The second method measures baryon acoustic oscillations, global fluctuations in visible matter density caused by acoustic waves running through the young universe. Finally, the third method assesses how the number of galaxy clusters changes depending on their mass and redshift. Together, these methods allow us to get the most stringent constraints on the parameters of our universe.
Unfortunately, in practice, the accuracy of the third method is severely limited by the methods of measuring the mass of distant clusters. Astronomers usually estimate the mass of the cluster using weak gravitational lensing. In fact, massive objects warp space-time and with it distort the trajectory of the rays of light coming from more distant objects – hence the magnitude of this distortion can calculate the mass of the object. Obviously, lensing requires not only a lens but also a source that is close enough to the observation line (the angular distance between the lens and the source must be no more than one second of the arc). If the redshift of the lensing object is small, such a source is likely to be found, so this method works well for close clusters. However, for distant clusters sources are much more difficult to find (in the young universe they simply do not exist), and gravitational lensing in the conventional form becomes useless.
However, in addition to galaxies, relic radiation can also be used as a source of light, as it was formed about 400,000 years after the Big Bang, it serves as a good backdrop even for the most distant clusters. Moreover, two features of relic radiation – temperature and polarizing anisotropy – can be used to estimate the distortion of the signal and the mass of the cluster. On the one hand, the amplitude of temperature fluctuations is greater, and therefore they are easier to measure. To date, scientists have already estimated the mass of several clusters, receiving a margin of error of about ten per cent. On the other hand, on average, the radiation of astrophysical sources is poorly polarized, so the signal from polarization fluctuations is less polluted. This possibility has not yet been tested in practice.
The SPTpol (South Pole Telescope) and DES (Dark Energy Survey) groups for the first time measured the mass of galaxy clusters, relying on the polarization of relic radiation. To do this, the SPTpol team provided data on the polarization of relic radiation covering about 500 square degrees of the celestial sphere, and the DES team put this data on the map known clusters of galaxies. In total, physicists selected 17,661 clusters containing more than ten galaxies (3868 clusters containing more than twenty galaxies) and had a redshift in the interval between z = 0.1 and z = 0.95. Scientists evaluated the redshift of clusters photometrically, the error of the assessment did not exceed two per cent. The authors emphasize that they took into account small clusters (10 to 20 galaxies) to simplify the primary analysis by increasing the signal/noise ratio. Because the algorithm looking for clusters in a set of galaxies does not work well in such small clusters, and scientists do not advise to use the data in the cosmological analysis.
As the primary fluctuations of relic radiation were exponentially suppressed on the scale of clusters, scientists brought it closer to a gradient from some Scalar function. When lensing such a field, the observer sees a picture that resembles a dipole field oriented along the gradient. With this effect, physicists evaluated the intensity of the lensing and the mass of the cluster.
To be more precise, the researchers adhered to the following algorithm. First, the scientists cut out areas of 10 × 10 arc seconds in the CMB map. Along with the areas near the clusters, the scientists also looked at randomly carved areas. For each field, physicists found the median value of the gradient and then turned it along that direction. Then, in each area, the scientists assigned certain weights, depending on the variance of noise and the amplitude of the gradient. Taking these weights into account, the researchers averaged the signal by random areas and areas near clusters. Physicists then subtracted the average random signal from the average signal near clusters. Finally, scientists built a function of plausibility, which connected the mass of the cluster and the signal in its vicinity.
Using the built plausibility function, physicists estimated the average mass of clusters that hit the area in question and contained more than ten or more than twenty galaxies. In the first case, the researchers received a mass of order (1.43 ± 0.40) × 10 14 solar masses, and in the second case, (3.23 ± 1.01) × 10 14 solar masses. In general, these results are consistent with estimates of the weak gravitational lensing of other galaxies ((0.96 ± 0.07) × 10 14 and (2.06 ± 0.14) × 10 14 solar masses) and the temperature fluctuations of relic radiation of the Sun ((0 , 85 ± 0.16) × 10 14 and (1.80 ± 0.33) × 10 14mass of the Sun), although the error of the new method is several times worse. Thus, physicists for the first time saw the lensing of oscillations polarization of relic radiation. Perhaps in the future scientists will also be able to reduce the errors of the proposed method.
Relic radiation is one of the most reliable sources of information about the early years of our universe. It was with his help that astronomers measured Hubble’s constant and assessed the density of dark energy, dark matter, baryons, photons and neutrinos. Theoretically, even more, subtle phenomena, for example, relic gravitational waves, can also be traced from the fluctuations of the CMB. A significant contribution to the study of relic radiation was made by the American astrophysicist James Peebles, who theoretically predicted a large number of effects, subsequently Confirmed by Planck and WMAP satellites.