Universe is expanding faster than expected

The Hubble Space Telescope has revealed that the modern Universe is expanding at a faster rate than it did shortly after the Big Bang.

For the calibration of relatively short distances the team observed Cepheid variables. These are pulsating stars which fade and brighten at rates that are proportional to their true brightness and this property allows astronomers to determine their distances. The researchers calibrated the distances to the Cepheids using a basic geometrical technique called parallax. With Hubble’s sharp-eyed Wide Field Camera 3 (WFC3), they extended the parallax measurements further than previously possible, across the Milky Way galaxy. To get accurate distances to nearby galaxies, the team then looked for galaxies containing both Cepheids and Type Ia supernovae. Type Ia supernovae always have the same intrinsic brightness and are also bright enough to be seen at relatively large distances. By comparing the observed brightness of both types of stars in those nearby galaxies, the team could then accurately measure the true brightness of the supernova. Using this calibrated rung on the distance ladder the accurate distance to additional 300 type Ia supernovae in far-flung galaxies was calculated. They compare those distance measurements with how the light from the supernovae is stretched to longer wavelengths by the expansion of space. Finally, they use these two values to calculate how fast the universe expands with time, called the Hubble constant.

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)

Advertisement

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.

Advertisement

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.