Astronomers using the NASA/ESA Hubble Space Telescope have discovered that the Universe is expanding between 5% and 9% faster than previously calculated.
Cosmic distance ladder. Image credit: NASA /ESA / A. Field, STScI / A. Riess, STScI & JHU.
The scientists, led by Prof. Adam Riess, a Nobel Laureate and an astrophysicist at the Space Telescope Science Institute and the Johns Hopkins University, used Hubble to measure the distances to stars in 19 galaxies more accurately than previously possible.
They found that the Universe is currently expanding faster than the rate derived from measurements of the Universe shortly after the Big Bang.
If confirmed, this apparent inconsistency may be an important clue to understanding three of the Universe’s most elusive components: dark matter, dark energy and neutrinos.
“This surprising finding may be an important clue to understanding those mysterious parts of the Universe that make up 95% of everything and don’t emit light, such as dark energy, dark matter, and dark radiation,” Prof. Riess said.
“One possible explanation for this unexpectedly fast expansion of the Universe is a new type of subatomic particle that may have changed the balance of energy in the early Universe, so called dark radiation.”
The team made the discovery by refining the measurement of how fast the Universe is expanding, a value called the Hubble constant, to unprecedented accuracy, reducing the uncertainty to 2.4%.
This measurement presents a puzzle because it does not agree with the expansion rate found by looking at the moments shortly after the Big Bang.
Measurements of the afterglow from the Big Bang from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the ESA’s Planck satellite mission yield smaller predictions for the Hubble constant.
“Comparing the Universe’s expansion rate as calculated by WMAP and Planck (for the time after the Big Bang) and Hubble (for our modern Universe) is like building a bridge,” Prof. Riess said.
“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.”
For the calibration of short distances Prof. Riess and co-authors observed Cepheid variables — pulsating stars which fade and brighten at rates that are proportional to their brightness (this property allows astronomers to determine their distances).
They calibrated the distances to the Cepheids using a basic geometrical technique called parallax. With Hubble’s Wide Field Camera 3, they extended the parallax measurements further than previously possible.
To get accurate distances to nearby galaxies, the team then looked for galaxies containing both Cepheids and Type Ia supernovae, which 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 astronomers 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.
“We compare those distance measurements with how the light from the supernovae is stretched to longer wavelengths by the expansion of space,” the astronomers said. “Finally, we use these two values to calculate the Hubble constant.”
The team’s findings have been accepted for publication in the Astrophysical Journal (arXiv.org preprint).
_____
Adam G. Riess et al. 2016. A 2.4% Determination of the Local Value of the Hubble Constant. ApJ, accepted for publication; arXiv: 1604.01424
