Galactic Lenses Confirm Universe’s Age, Size
The B1608+656 galactic system blocks a distant galaxy and causes the distant galaxy's light to detour around the blockage. But instead of taking a single path, light can bend around the object in one of two, or four different routes, thus doubling or quadrupling the amount of information scientists receive. As the brightness of the background galaxy nucleus fluctuates, physicists can measure the ebb and flow of light from the four distinct paths.
Credit: Sherry Suyu of the Argelander Institut für Astronomie in Bonn, Germany

The size and age of the universe, as well as how fast it is expanding, has been confirmed with a new, precise method that uses galaxies as lenses to look at other galaxies.

The new measurement confirmed the age of the universe as 13.75 billion years old, to within 170 million years, and also confirmed the strength of dark energy, which is responsible for the accelerating expansion of the universe.

When looking out at the cosmos, it can be difficult for scientists to distinguish between a very bright light far away and a dimmer source much closer to Earth.

To circumvent this problem, a team of researchers used a technique called gravitational lensing to measure the distances light traveled from a bright, active galaxy to the Earth along different paths, along with data from the Hubble Space Telescope. Researchers can use the observations to infer not just how far away the galaxy lies but also the overall scale of the universe and some details of its expansion.

Size and age

The size of the universe is often expressed by astrophysicists in terms of a quantity called Hubble's constant, which describes the rate at which galaxies in the universe are flying away from each other.

"We've known for a long time that lensing is capable of making a physical measurement of Hubble's constant," said team member Phil Marshall of the researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). But gravitational lensing had never before been used in such a precise way.

The new measurement provides an equally precise measurement of Hubble's constant as long-established tools such as observation of supernovas (often used as "standard candles" of cosmic distance) and the cosmic microwave background, the remnant radiation of the Big Bang.

The most widely accepted value for the Hubble constant right now is 72 kilometers per second per megaparsec, obtained by the Hubble Space Telescope.

"Gravitational lensing has come of age as a competitive tool in the astrophysicist's toolkit," Marshall said.

How it works

Gravitational lensing works like this: When a large nearby object like a galaxy blocks a distant object, such as another galaxy, the light can detour around the blockage.

But instead of taking a single path, light can bend around the object in one of two, or four different routes, thus doubling or quadrupling the amount of information scientists receive. As the brightness of the background galaxy nucleus fluctuates, physicists can measure the ebb and flow of light from the four distinct paths.

Though researchers do not know when the light left its source, they can still compare arrival times.

?Marshall likens it to four cars taking four different routes between places on opposite sides of a large city. And like automobiles facing traffic snarls, light can encounter delays, too.

"The traffic density in a big city is like the mass density in a lens galaxy," Marshall said. "If you take a longer route, it need not lead to a longer delay time. Sometimes the shorter distance is actually slower."

The gravitational lens equations account for all the variables such as distance and density, and provide a better idea of when light left the background galaxy and how far it traveled.

The technique is now more precise, astronomers say, and so more accurate values for Hubble's constant and the uncertainty of the value of that constant can be reliably determined.

The research will be detailed in a March issue of The Astrophysical Journal.