Observations made with the Hubble Space Telescope suggest that the universe is expanding faster than expected

The galaxy UGC 9391 seen by the Hubble Space Telescope. Cepheid variable are in red circles, a type Ia supernova is marked by a blue cross (Image NASA, ESA, and A. Riess (STScI/JHU))
The galaxy UGC 9391 seen by the Hubble Space Telescope. Cepheid variable are in red circles, a type Ia supernova is marked by a blue cross (Image NASA, ESA, and A. Riess (STScI/JHU))

An article accepted for publication in “The Astrophysical Journal” describes a research based on observations made with the Hubble Space Telescope that suggests that the universe is expanding at a faster rate than expected. A team of astronomers led by Nobel laureate Adam Riess measured the distance of stars in nineteen galaxies with the best accuracy ever achieved to obtain this surprising result.

The measurement of the expansion rate of the universe has been refined over the decades since it was confirmed by astronomer Edwin Hubble’s observations. That discovery, based on a work by Georges Lemaître, made a revolution in astronomy comparable only with the Copernican one. For this reason, Hubble earned a huge reputation and, in addition to the space telescope, the estimated speed of expansion of the universe was called Hubble constant after him.

Another surprise came in 1998, when it was proved that not only the expansion of the universe is not slowing down but is even accelerating. Research in this field earned a number of scientists the Nobel Prize including Adam Riess. To explain this phenomenon scientists proposed the hypothesis of the existence of dark energy but we know only this effect and little else about it.

This new calibration of the Hubble constant measurement was carried out with great precision by measuring the distances of stars in several galaxies both near and far away. For these observations, they used the Hubble Space Telescope’s Wide Field Camera 3 (WFC3) instrument.

In particular, the researchers used what are called standard candles, variable stars called Cepheid variables that have a very strong correlation between their period of variability and their absolute brightness. Some type Ia supernovae were also used for this purpose.

The possibility that the universe is expanding at a rate even higher than expected makes the problem even more complex. One possible explanation is based on a further hypothesis, that of dark radiation. It’s a hypothetical subatomic particle that acts as a mediator in the interaction between dark matter particles in a manner similar to the mediation of the photons between ordinary matter particles.

The interactions mediated by dark radiation could have changed the balance of energy in the early universe. The effect would be in the expansion of the universe, which according to this hypothesis has become even faster. Now we can only see this effect but according to Adam Riess this could provide important clues for the understanding of dark matter, dark energy and dark radiation. They could form 95% of the universe but we can’t detect them directly.

There are various hypotheses related to the expansion of the universe and a lot of discussions about their validity and the problem of their verification. In these types of research the studies of the universe cross path with those of subatomic particles but they may take years. Those are studies that are expanding the frontiers of our knowledge and will continue with the next-generation instruments.

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