Precise surveying of the universe intensifies cosmological puzzle

What is the universe made of - and how fast is it expanding? An international research team has found the most accurate answers to these questions to date. To do this, the astronomers evaluated data from over 1,500 stellar explosions up to 10.7 billion light-years away. The problem: The speed of cosmic expansion determined in this way is in clear contradiction to a value determined using another method. This points to the existence of a previously unknown physical phenomenon in the young cosmos, the scientists write in the Astrophysical Journal.

As part of the Pantheon project, the team collected and analyzed the brightness of stellar explosions of a certain type over time. When a star like our sun has completely used up its nuclear energy supply, it ends up as a white dwarf. Only about the size of the earth, such a remnant of stars slowly cools down over billions of years. However, if such a white dwarf forms a binary system with a second large star, it can snatch matter – mainly hydrogen gas – from it.

At some point, a white dwarf will have accumulated so much fresh hydrogen that a thermonuclear explosion occurs - a type Ia supernova. Such explosions are extremely valuable to astronomers because they all light up equally brightly. How brightly such a supernova shines in the terrestrial sky therefore depends only on its distance. From the observed brightness of the starburst, astronomers can calculate the distance of the supernova.

Armed with this cosmic yardstick, sky explorers can determine what the universe is made of and how fast it is expanding by observing many supernovae at different distances. First of all, the data from Pantheon confirm the previous cosmological model with unprecedented accuracy: The matter of which stars, planets and also we humans are made makes up only a tiny proportion of about five percent of the cosmos. The universe is dominated by dark matter and dark energy – two previously mysterious substances.

Dark matter contributes about 29 percent to the cosmos and ensures that galaxies and galaxy clusters are held together by gravity - the visible normal matter alone would not be enough for this. Without dark matter, stars, planets and also no life would never have arisen in the cosmos. The researchers suspect that dark matter consists of previously unknown particles. But all attempts to track down such particles have so far been unsuccessful.

Dark energy is even more mysterious. The universe was created 13.8 billion years ago with the Big Bang and has been expanding ever since. This cosmic expansion should be slowed down slowly by the attraction of matter. In fact, the opposite is the case: it's accelerating. The researchers see a type of internal energy in the room as the cause. Pantheon data now shows that this dark energy has probably not changed throughout cosmic history, it is constant.

Pantheon's answer to the question of how fast the universe is expanding today is explosive. Astronomers describe the rate of expansion using the Hubble constant, named after Edwin Hubble, the discoverer of cosmic expansion. Pantheon delivers a value of 73.4 for the Hubble constant with an uncertainty of only 1.3 percent.

However, there is a second, independent method to determine the Hubble constant. It is based on a detailed study of the cosmic background radiation - a kind of radiation echo from the Big Bang - and gives a value of 67.4 with an uncertainty of 0.7 percent. The numerical values ​​indicate by how many kilometers per second the distance between two objects that are 3.26 million light years - one megaparsec - apart increases.

The difference between the two values ​​is called the Hubble voltage. So far, researchers have still hoped that the difference would simply turn out to be a statistical error. But with the new data from Pantheon, the probability of that happening has dropped to well under a ten-thousandth of a percent.

"We hoped to find a possible solution to the problem with our data," says co-author Dillon Brout from the Harvard Smithsonian Center for Astrophysics in the USA. "Instead, we have to discard many remaining explanations and the differences are more serious than ever." The Hubble voltage indicates a new physics in the young cosmos - and there are now many theoretical considerations. "But these theories still have to withstand the scientific process," says Brout - so they can be checked by further observational data.