Cosmology: Hubble's constant becomes more and more mysterious


For the discovery of range, astronomers use a whole series of measures known as "ladder of cosmic distance". Each individual method, each "step" of the ladder, is based on the preceding one. The lowest rung is based on the triangulation of nearby stars. Larger shoots, on the other hand, are always assisted by simultaneous explosions of supernovae or pulsating giant stars, called Cepheids, in which the distance from the light curve can be reconstructed.

The uncertainties of each shoot add up, so that distances from distant objects are always subject to systematic uncertainties. Meanwhile, astronomers have made significant progress and believe they have understood every step of the distance ladder well.

The work of Riess and his colleagues is the culmination of this work so far: with the help of the Cepheids in the Great Magellanic Cloud, the error of the distance ladder method has been reduced by another 0.5%, write the researchers in the Astrophysical Journal. Now, one of them has 1.9 percent error size. So the 74 km / s / Mpc value found by Riess and his colleagues is well established – and the discrepancy with Planck's measurement is even more impressive.

The result of the ESA satellite is based on a completely different principle. Radiation from the microwave background provides no way to directly measure the Hubble constant. Instead, it is possible to reconstruct the properties of today's cosmos from the ultrafine temperature fluctuations of the radiation and assume a particular cosmological model – including the Hubble constant. For example, in the widely accepted cosmological standard model, it is determined how great the proportion of dark matter and dark energy, which, cosmologists believe, is crucial to the evolution of the universe.

If the standard model describes the universe correctly, then the Hubble constant calculated from it using the Planck measure must be the same measure directly. But she does not. Background-based measurement is now considered so accurate that it only has a percentage error. Systematic or statistical measurement errors can not really explain the difference to the measurement of Riess and his colleagues.

Above all, there are not just two research teams: the value of the Planck team and Adam Riess and colleagues is supported by other groups and other measures. For example, the comparison of large-scale structures in the current universe with density patterns in cosmic background radiation reaches a similar result to that of Planck's team. On the other hand, the quasars considered by gravitational lenses, for example, also speak by the distance ladder method, even though the uncertainty of measurement is somewhat greater here.

Measurement error or new physics?

In general, more and more scientists believe that it is possible that no one has made a mistake and that both fields are correct. The standard cosmological model would be wrong or at least incomplete at a crucial point. In this case, the evolution of the Big Bang universe until today would have been a bit different. For example, dark matter may have slightly different properties than expected. An additional type of neutrinos could also be responsible. Both could have influenced the evolution of the initial universe and lead to a value different from the Hubble constant from the background microwave radiation. Unfortunately, so far none of these ideas can explain all observations without contradiction.

The team around Riess is now strong for another explanation: the enigmatic Dark Energy, which constantly distinguishes the universe, can change over time. In the standard cosmological model, the puzzling antigravity appears as a constant. According to this image, it is a form of energy welded to the vacuum, whose density is always the same for each volume of space.

In recent years, scientists are increasingly discussing the scenario that dark energy may change over time. Riess and his team now speculate on three different phases that could have happened since the Big Bang. According to Riess, the dark energy in the space nursery could have caused an extraordinarily strong impulse, which would explain the discrepancy in Hubble's values.

If this suspicion were confirmed, it would not be the first time that more precise observations would lead to a radical revolution in cosmology. Already the discovery of dark energy in 1998 marked a turning point, the same was true for the discovery of the expansion of the universe in the 1920s.

Are the measurements really comparable?

From the point of view of many astrophysicists, it is too early to proclaim yet another revolution. Finally, it should not be forgotten that the two methods of measuring the Hubble constant consider two very different states of the cosmos: The distance staircase method examines the current universe and measures its instantaneous expansion rate. It is limited to a comparatively small and "local" part of the cosmos.

The microwave background, on the other hand, dates back to a time when the universe was only 380,000 years old and instead of galaxies, it consisted only of gas and radiation. In analyzing this radiation in today's night sky, Planck has in mind the radiation that has come a long way and has experienced all the ages of space. In both temporal and spatial dimensions, the two measurements are therefore not directly comparable.

Spectrum Compact: Dark Energy - Enigmatic Propulsion in the Expansive Universe

In fact, cosmologists assume that Hubble's constant has changed in the course of cosmic evolution: on the one hand, by the braking effect of mutually attractive matter that initially dominated, and, on the other, by the effect of dark energy, the universe with distance growing. between galaxies – even though it should be constant as in the standard cosmological model.

The above-mentioned line in the Hubble diagram becomes a curve when you look just enough for the galaxies and therefore for the past of the cosmos. In addition, astrophysicists believe that parts of the cosmos can expand in different rhythms: if our part of the universe is a representative example of the rest of the universe, this is far from certain.

Perhaps it would be better for cosmology if the new Hubble constant could not be resolved simply by fitting the standard cosmological model. Because as elegant as the model is – what is behind dark matter and dark energy can not explain it.


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