Diagram showing how Hubble's increased power has enabled astronomers to measure the distances of more far-away galaxies.Credit: NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
The modern Universe is expanding faster than previously thought, according to a new study using the Hubble Space Telescope.
Astronomers used Hubble to measure the distances to stars in 19 galaxies to a greater degree of accuracy than ever achieved.
These observations enabled the team to calculate that the Universe is currently expanding between 5 and 9 per cent faster than suggested by measurements of the Universe shortly after the Big Bang.
One possible explanation for this discrepancy could be the effect on the Universe of dark matter, dark energy or a type of subatomic particle called dark radiation.
These allusive, invisible elements are not fully understood as they cannot be observed, but discrepancies in our understanding of the Universe, such as the one posited in this study, can help astronomers learn more about their effect on the cosmos.
The discovery was made by refining a measurement of the expansion of the Universe known as the Hubble constant, which records the change in an object’s appearance as it moves away from the observer.
The team managed to reduce the uncertainty of this value to 2.4 per cent.
They looked for galaxies containing both Cepheid stars and Type 1a supernovae.
Cepheids pulsate at rates relative to their true brightness, and this measurement can be compared with their apparent brightness as seen from Earth to determine their distance.
Type 1a supernovae are explosions that can also be used to measure distances in the galaxy, by calculating the actual brightness of their explosion and comparing it to their apparent brightness.
Measuring about 2,400 Cepheid stars in 19 galaxies, the team compared the observed brightness of both types of stars and were able to calculate the distances to about 300 Type 1a supernovae in distant galaxies.
The new Hubble constant is 73.2km per second per megaparsec which, if true, would mean the distance between objects in the Universe will double in the space of 9.8 billion years.
The calculation is at odds with measurements of the afterglow of the Big Bang carried out by NASA and ESA, which has created a problem for astronomers.
Study leader Adam Riess of the Space Telescope Science Institute compared the problem to building a bridge:
“You start at two ends and you expect to meet in the middle if all of your drawings are right and your measurements are right.
But now the ends are not quite meeting in the middle and we want to know why.”
One explanation is that dark energy is increasing the rate of expansion of the Universe.
Another is that a subatomic particle from the early Universe called dark radiation is affecting how accurately astronomers can calculate the current expansion rate.
Or, perhaps dark matter operates in a way that we don’t currently understand and is causing the expansion of the Universe to increase in speed.
In order to get to the bottom of the mystery, the team are continuing to study the expansion of the Universe, hoping to reduce the Hubble constant to an uncertainty of one per cent.