The infant universe just got a little dimmer. A phenomenon called gravitational lensing may be causing astronomers to overestimate the brightness of the earliest galaxies, according to a study published in the 13 January edition of Nature.
Gravitational lensing occurs because massive objects, like galaxies, distort spacetime. Light passing through the distortion will end up on a curving trajectory instead of a straight line. This, in turn, can both alter the shapes and enhance the brightness of distant sources, just as a lens might do. The effect turns the universe into “a cosmic house of mirrors,” says Rogier Windhorst, an astronomer from Arizona State University in Phoenix, who presented the new findings at the American Astronomical Society (AAS) meeting on 12 January.
Looking at images from the Hubble Space Telescope’s Ultra Deep Field—which includes objects so distant the universe was less than 10% its current age when their light was emitted—Windhorst and his colleagues expected to see both close and distant galaxies randomly distributed across the image. Instead, they found many background galaxies lying near foreground galaxies. The likely explanation for this, they say, is that the foreground galaxies are lensing more distant objects, allowing astronomers to see more early galaxies than they should.
But this would imply that galaxies in the early universe are both fainter and more numerous than previously suspected, says Haojing Yan, an astronomer from Ohio State University in Columbus who works with Windhorst. He estimates that 20% to 40% of galaxies in the sample may be affected by lensing, significantly altering astronomers’ picture of cosmic history, he adds.
“It means that everything started out in smaller pieces,” says Windhorst. This is important because a larger number of smaller galaxies would play a somewhat different role during the era of reionization, which occurred around one billion years after the Big Bang. During this period, neutral hydrogen atoms that formed in the aftermath of the Big Bang were blasted apart into their constituent electrons and protons by radiation from newly emerging sources, creating the ionized plasmas that astronomers see throughout the universe today.
THe conventional picture is that the earliest galaxies produced this radiation, but if these objects were fainter than previously suspected, they wouldn’t have had quite the same effect, Windhorst says. Instead, he posits that black holes—which would have formed within the smaler, faint galaxies—could have accreted enough gas and dust around them to produce the ultraviolet radiation that ionized the hydrogen.
Because the findings are at the limits of what Hubble can see, the conclusions of the study remain controversial. Even Windhorst and Yan disagree on the contributions of black holes to the reionization era. More detailed data from Hubble’s yet-to-be-launched successor, the James Webb Space Telescope, will be needed to confirm the effect of gravitational lensing on the observed universe, they say.