Expansion of the universe: Discrepancy in Hubble’s constant deepens


With a new method, a group of astrophysicists has once again measured the so-called Hubble constant and again determined a value that contradicts that of the ESA Planck space telescope. The discrepancy in the basic constant thus remains and continues to puzzle science.

Based on the fluctuations in the surface brightness of 63 large elliptical galaxies, they come to a value of 73.3 kilometers per second per megaparsec, with an uncertainty of 2.5 km / sec / Mpc. That fits pretty well with other values ​​that were recently measured variously for the local universe, but not at all with the value that was not only determined with Planck.

The Hubble constant (H.0) is a fundamental quantity for understanding the universe and indicates the speed at which the universe is currently expanding. It means that an object is moving away from us at this speed at a distance of one megaparsec (3.26 million light years) simply because of the expansion of the universe. For comparison: The Andromeda Galaxy is about 0.89 megaparsecs away from us. The Hubble constant was calculated for the first time by the US astronomer Edwin Hubble. However, the increasingly precise measurements of recent years do not provide a uniform value. Instead, different measurement methods provide two different methods that are now quite clearly outside the respective error rate.

Measurements of comparatively close objects in the universe consistently provide a value of around 74 km / sec / Mpc, while the Planck space telescope measured a whole 9 percent less when looking back to the beginnings of the universe (a little over 67 km / sec / Mpc). The Planck analyzes are considered to be the most accurate of all and the “gold standard”. The space telescope made it possible to analyze the cosmic background radiation, i.e. the afterglow of the Big Bang. Then it can be calculated how long this light was on its way to us, how old the universe is and finally the speed of propagation. The discrepancy between the value determined in this way and the measurement of the speed with which other galaxies are currently moving away from us has deepened recently.

Overview of the determined values ​​and their accuracy (the current result light green in the lower third).

(Image: Di Valentino et.a., Alessandro Melchiorri)

The team led by John Blakeslee from the National Optical-Infrared Astronomy Research Laboratory in Arizona has now contributed to this deepening of the discrepancy. As they explain now, they measured the escape velocity of dozen galaxies using a technique that is independent of other approaches. In doing so, they took advantage of the fact that huge elliptical galaxies (so-called giant ellipses) are comparatively old and combine a constant number of old stars. With high-resolution infrared images of the galaxies, they would have determined how much each pixel deviates from the respective average. The smaller this deviation, the further away the galaxy. Your study was accepted for publication in the Astrophysical Journal.

Although their measurement technology works independently of others, the researchers want to separate it even more from other analyzes in the future, they now explain. With the upcoming James Webb Space Telescope (JWST) it would have the potential to provide the best value for the local Hubble constant. It should probably no longer lead any closer to the value that was determined using the background radiation. Even when presenting a scientific analysis to confirm this, experts had surmised that the growing discrepancy heralds a fundamental discovery that is changing our understanding of the universe. There have been attempts to explain the conflict, but no favorite has emerged.


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