Hubble helps in precise measurement of universe's expansion

Astronomers, using the Hubble Space Telescope, have measured the precise rate at which the universe is expanding, by employing a technique completely independent of any previous method.

The researchers noted that knowing the precise value for how fast the universe expands is important for determining the age, size, and fate of the cosmos. Unravelling this mystery has been one of the greatest challenges in astrophysics in recent years, thy said.

The study adds evidence to the idea that new theories may be needed to explain what scientists are finding, NASA said in a statement. The finding strengthens a troubling discrepancy between the expansion rate, called the Hubble constant, calculated from measurements of the local universe, and the rate as predicted from background radiation in the early universe, a time before galaxies and stars even existed, it said.

This latest value represents the most precise measurement yet using the gravitational lensing method, where the gravity of a foreground galaxy acts like a giant magnifying lens, amplifying and distorting light from background objects. The study did not rely on the traditional "cosmic distance ladder" technique to measure accurate distances to galaxies by using various types of stars as "milepost markers." Instead, the researchers employed the exotic physics of gravitational lensing to calculate the universe's expansion rate.

The team derived the 'H0LiCOW' value for the Hubble constant through observing and analysis techniques that have been greatly refined over the past two decades. H0LiCOW and other recent measurements suggest a faster expansion rate in the local universe than was expected based on observations by the European Space Agency's Planck satellite of how the cosmos behaved more than 13 billion years ago, the researchers said.

The gulf between the two values has important implications for understanding the universe's underlying physical parameters, and may require new physics to account for the mismatch, they said. "If these results do not agree, it may be a hint that we do not yet fully understand how matter and energy evolved over time, particularly at early times," said H0LiCOW team leader Sherry Suyu of the Max Planck Institute for Astrophysics in Germany.

The H0LiCOW team used Hubble to observe the light from six faraway quasars, the brilliant searchlights from gas orbiting supermassive black holes at the centers of galaxies. Quasars are ideal background objects for many reasons; for example, they are bright, extremely distant, and scattered all over the sky, the researchers said. The telescope observed how the light from each quasar was multiplied into four images by the gravity of a massive foreground galaxy.

The galaxies studied are 3 billion to 6.5 billion light-years away. The quasars' average distance is 5.5 billion light-years from the Earth. The light rays from each lensed quasar image take a slightly different path through space to reach Earth. The pathway's length depends on the amount of matter that is distorting space along the line of sight to the quasar. To trace each pathway, the astronomers monitor the flickering of the quasar's light as its black hole gobbles up material. When the light flickers, each lensed image brightens at a different time.

This flickering sequence allows researchers to measure the time delays between each image as the lensed light travels along its path to Earth. To fully understand these delays, the team first used Hubble to make accurate maps of the distribution of matter in each lensing galaxy. Astronomers could then reliably deduce the distances from the galaxy to the quasar, and from Earth to the galaxy and to the background quasar. By comparing these distance values, the researchers measured the universe's expansion rate.

"The length of each time delay indicates how fast the universe is expanding," said Kenneth Wong of the University of Tokyo's Kavli Institute for the Physics and Mathematics of the Universe. "If the time delays are shorter, then the universe is expanding at a faster rate. If they are longer, then the expansion rate is slower," Wong said. The researchers calculated a Hubble constant value of 73 kilometres per second per megaparsec (with 2.4 per cent uncertainty).

This means that for every additional 3.3 million light-years away a galaxy is from Earth, it appears to be moving 73 kilometers per second faster, because of the universe's expansion, the researchers said. The team's measurement also is close to the Hubble constant value of 74 calculated by the Supernova H0 for the Equation of State (SH0ES) team, which used the cosmic distance ladder technique. The SH0ES measurement is based on gauging the distances to galaxies near and far from Earth by using Cepheid variable stars and supernovas as measuring sticks to the galaxies.