Where Did That Galaxy Go?

In the "not very far future" the universe will be a darker and lonelier place, and astronomers will look to the skies to see only dimming, frozen images of distant galaxies, like the fading photographs of friends who don't call anymore.
Because the rate of expansion of the universe exceeds the speed of light, remote objects will eventually slip out of Earth's potential viewing circle, said Abraham Loeb, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

But strangely, rather than fading like a movie reel shown on a projector with a dying light bulb, the galaxies’ images will become static at a certain point. While not a violent process, the light effect is analogous to watching a luminous object fall into a black hole, Loeb said. Once past the rim, or event horizon, of a black hole, its image seems to freeze and fade away because fresh light can no longer escape.

“What we can know of the universe is finite,” Loeb said.

Loeb’s bleak take on the cosmic future has been accepted for publication in the peer-reviewed journal, Physical Review.

Fortunately, a cosmologist has a different sense of time from the rest of us. Grad students eager to capture deep field images won’t have to claw past each other for Hubble telescope time.

“The image of the most distant galaxy will freeze and fade away about 50 billion years from now,” Loeb said Tuesday. “Its image will be frozen at an age of 5 billion years (roughly the age of the sun today). This means that we will never be able to see how stars age in this galaxy to having an age larger than the current age of the sun.” We’ll see no more births or deaths of stars in that realm, though those processes will continue.

The image doesn’t simply blink out, or fade in motion, because “from the point of view of a distant observer, the source never crosses the horizon. Rather, the source approaches the horizon slowly but never crosses it. As it approaches the horizon, its image fades and freezes. You can understand this intuitively, because the observer cannot receive any signal that was emitted after the source crossed the horizon. What it sees is an extended view of the last bit of this process, stretched over an infinite period of time,” Loeb said.

By the time the first galaxy slips over the event horizon, our sun will be long dead, and small, weak stars called brown dwarfs and red dwarfs will dominate the Milky Way galaxy.

Loeb’s conception falls into the mainstream, serving as an extrapolation of the latest data.

“It is really more of a conclusion of the currently favored theory,” said Charles L. Bennett, head of the Infrared Astrophysics Branch at NASA’s Goddard Space Flight Center. “Theories other than the currently favored theory will have different consequences. These theories will be sorted out via future observations.”

But any conclusion is likely to be weird. “When you investigate the deep theoretical issues underlying this effect, it looks even more outrageous than that,” said University of California at Davis cosmologist Andreas Albrecht.

Stanford’s Vahe Petrosian sees little new in Loeb’s work.
“This fading of the sources has been known for some time,” Petrosian said. “Rindler, in his book Essential Relativity, page 240 to 242, discusses the possibility of existence of ‘event horizon’ for an accelerating universe. It seems that Dr. Loeb is aware of Rindler’s work although he does not refer to the book.”

The steady-state model of the universe has over the past century given way to an expanding cosmos hurling out from a Big Bang 14 billion years ago. In 1998, the expansion was first observed to be accelerating under the force of a mysterious energy, sometimes called a Cosmological Constant, and shows no sign that it will stop.

Because the light speed limit Einstein made a cornerstone of his work applies only to matter and energy, not the fabric of space itself, we’re left with a cosmos in which distant objects without gravitational ties will be hurled away from each other across dark voids.

Results of two studies announced this week confirm that the amount of unidentified, but gravitationally apparent, dark matter in the universe isn’t enough to halt the expansion either. Dark matter, which bends light and seems to act as a cohesive to help draw normal matter together into orderly galaxies, appears to be distributed in familiar, clumpy ways.

“Knowing how clustered the dark matter is also reveals how much of it there is,” said Dr. Licia Verde, a co-author of one of the studies at Rutgers University, in a statement. There’s about seven times as much dark matter as ordinary matter, but only a quarter of what is needed to halt the expansion of the universe, as indicated by data produced by the Anglo-Australian Telescope in eastern Australia.

But all is not lost—the universal acceleration isn’t a strong force, and gravity overcomes it easily. That’s why the Earth isn’t stretching apart beneath your feet.

“Future astronomers might want to focus on things closer to home,” Loeb said.

Even as distant galaxies fade from view, the local scene will remain recognizable. The Virgo Cluster of galaxies, of which the Milky Way is a part, will hold together, as will our orbital pirouette with the Andromeda Galaxy.

Well, that is, until the age of stars has passed and the universe is filled with black holes.