Astrophysicists have systematically analyzed various methods for determining the rate of expansion of the Universe and have come to the conclusion that all methods except calibrated Cepheids of supernovae obtained with the Hubble telescope give a generally consistent result. In another work, astronomers re-measured the distances to Cepheids in the Milky Way using the data from the Gaia telescope instead of the Hubble: they turned out to be systematically larger than previously thought. Recalculation of the expansion rate of the Universe with the new data led to a consistent result and the absence of Hubble tension.
The expansion of the universe is measured using the Hubble constant. Contrary to the name, this value changes over time, but in the current era its value is approximately 70 kilometers per second per megaparsec. This means that if two bodies are located at a distance of one megaparsec, then the space expands between them at such a rate that it seems to the observer on one of the bodies that the other body is moving away from it at a speed of 70 kilometers per second.
The speeds calculated from the expansion of the Universe are not real speeds of movement and can exceed the speed of light without violating the postulates of the special theory of relativity. The explanation is that the effect of the expansion of space gradually occurs when the photons move outside gravitationally bound bodies and only for relatively close distances corresponds to the classical Doppler effect.
Astronomers have come up with many different methods for determining the Hubble constant, which are based on different objects that shone at different stages of the universe. There are two main methods: based on the brightness of standard candles, first of all, supernovae, and on the basis of relict radiation analysis. The first method shows a “local” rate of expansion in a nearby Universe, and the second, cosmological, talks about the properties of the world 380,000 years after the Big Bang. The first method gives a value of about 73 km / s / Mpc, and the second – about 68. This situation of inconsistency is called Hubble tension.
In a first paper, American astronomers from the University of North Carolina, led by Weikang Lin, compared the results of different methods of measuring the rate of expansion of the universe. The authors consider simultaneously estimates of both the Hubble constant and the fraction of matter in the modern Universe. This approach allows us to take into account more experimental data, as well as to get rid of some mutual correlations between the results, which allows us to make more reliable results.
Scientists consider the following sources of information: time delay between images of strong gravitational lensing, gamma-ray attenuation, a large overview of DES galaxies, age differences of passive galaxies at close redshifts (space chronometers), baryonic acoustic oscillations at low and high redshifts, relict analysis radiation according to WMAP and Planck satellites, the relative brightness of supernovae calibrated by Cepheids supernovae in the local Universe, as well as the age of the oldest stars.
It turned out that all methods, except for supernovae calibrated by Cepheids, intersect with reasonable accuracy in one region in the region of the Hubble constant at the level of 68-69 km / s / Mpc. The authors conclude that the problem lies precisely in this method, and not in something else. Scientists call additional arguments the results of replacing the Cepheid calibration with the one performed on the top of the branch of red giants – in this case, it is possible to achieve a much better agreement between the data.
In another work, a team of scientists from France, Italy, Germany and Chile, led by Louise Breuval from the French National Center for Scientific Research, re-evaluated the distance to Cepheids in the Milky Way. For stars of this type in our Galaxy, it is possible to measure the distance by an independent geometric method of trigonometric parallax. Since the Cepheids of the Milky Way are used as the first “step” in the corresponding calibration method, a change in the estimate of the distance to them affects all the values obtained on their basis.
The researchers used the second data catalog of the Gaia astrometric satellite for a new measurement of the Levitic law, that is, the relationship between the period and luminosity of Cepheids, which allows them to be used to estimate the distance. However, in order to avoid problems with errors in measuring distances to Cepheid variables, the authors used the data not about themselves, but about their companions in multiple systems. Scientists managed to find 23 such Cepheids.
The results of astronomers were systematically shifted relative to previous estimates made using the Hubble telescope. In particular, the measured parallaxes were larger by an average of 200 microseconds of arc. If we use the new data as the basis for calibrating distances to supernovae, then the estimate of the Hubble constant is obtained at 69 km / s / Mpc with errors of about 2, that is, significantly less than before and, more importantly, in agreement with the results of other methods . In other words, using Gaia measurements instead of Hubble has helped remove Hubble’s tension.
The authors note that this work represents the first alternative attempt to establish the Leviticus law, since all previous studies directly or indirectly relied on observations of Cepheids using the Hubble telescope. More accurate conclusions can be made after the publication of the third Gaia catalog, which will take into account the variability of the Cepheids themselves, which should make the estimate of the distance directly to them reliable.
Prior to this, astronomers had high hopes for neutron star fusions as an independent estimate of the expansion rate of the Universe, but so far only one such event has been recorded. There was also the idea of a giant void in the local Universe, which affects estimates of the Hubble constant, but scientists have already disproved the validity of this argument . Another new way could be to use the recently proposed standard rulers for universal dawn.