NASA's Hubble Space Telescope has helped astronomers achieve yet another new milestone: the most precise measurements of the universe's expansion ever calculated. The results revealed that the cosmos is expanding much faster than a previously expected rate, which was measured based on its trajectory witnessed soon after the violent event called Big Bang that created the early universe about 13.8 billion years ago.
But, the discrepancy between the latest Hubble data and the old measurements is not easy to grasp. In fact, astronomers consider the new findings to be unexpected, something that may not be properly explained without new physics. The researchers have described the results of their observation in a study, which has been accepted for publication in The Astrophysical Journal.
The new Hubble measurements highlight the inconsistency with the previous data derived from observations of the early universe's expansion that took place 378,000 years after the big bang. The difference between the old measurements, made by the European Space Agency's Planck satellite, and the latest values is about 9 percent.
According to Planck's results, which was based on the analysis of the cosmic microwave background, the universe is expanding at 67 kilometers per second per megaparsec (3.3 million light-years). The latest Hubble measurements, on the other hand, revealed a value of 73 kilometers per second per megaparsec, suggesting that galaxies are moving at a faster rate than previously thought.
"Both results have been tested multiple ways, so barring a series of unrelated mistakes," Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University said in a statement. "It is increasingly likely that this is not a bug but a feature of the universe."
A few possible explanations
According to Riess, one possible explanation could be related to the mysterious dark energy, which is believed to be accelerating the universe by constantly pushing galaxies away from each other. Another theory talks about new subatomic particles, collectively called "dark radiation", which can travel at a speed similar to that of light. There is also a third possibility dealing with dark matter, which is believed to interact with normal matter in a stronger way than previously assumed.
If any of these scenarios turns out to be accurate, it will lead to inconsistencies in existing theoretical models that will result in an erroneous value for the Hubble constant, inferred from observations of a young universe.
Studying Cepheid variable stars
The astronomers used Hubble to examine objects called Cepheid variable stars, which can be considered to be a reliable galactic yardstick. These are pulsating stars whose distances can be deduced by comparing their natural brightness with their apparent brightness as observed from Earth.
Unlike the existing data that was based on only a few Cepheid stars between 300 and 1,600 light-years from Earth, the latest Hubble results are based on eight more newly analysed Cepheid stars that are 6,000 to 12,000 light-years away from Earth.
In order to measure their exact distance, the astronomer concentrated on changes in their positions as the Earth moved around the Sun. Since these wobbles are extremely small -- 1/100 of a single pixel on Hubble's camera – the astronomers developed a new scanning method to ensure accuracy of measurements.
"This method allows for repeated opportunities to measure the extremely tiny displacements due to parallax," Riess said. "You're measuring the separation between two stars, not just in one place on the camera, but over and over thousands of times, reducing the errors in measurement."