As if in a dream where we swam but could not reach the shore, the universe likewise recedes as we study it, destined to disappear at the whim of time, space and the laws of physics. All that will be left are fading ghosts of distant galaxies, each an afterimage preserving a final moment as a swarm of stars slips into a netherworld of cosmic invisibility.
We've got less than 100 billion years left to study it all. And already, some galaxies are beyond reach, their potential inhabitants impossible to contact.
This is the universe as Abraham Loeb sees it.
Accelerating problem
Loeb, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics, peers through Einstein-colored glasses. His view of the end of the visible universe is rooted in the General Theory of Relativity and based on the notion that everything is expanding at an ever-increasing pace.
All distant galaxies are moving away from us and moving faster all the time. Few researchers debate this point. Few have predicted its ultimate consequence quantitatively as Loeb did.
Eventually, Loeb says, galaxies will recede at the speed of light, making it impossible for their light -- or any other radiation or information -- to traverse the cosmos to our home in the Milky Way Galaxy.
"Any given source accelerates away from us and eventually reaches a speed larger than the speed of light so that photons emitted from it cannot catch up with the cosmic expansion, relative to us," he said.
Already, galaxies more than 6 or 7 billion light-years away are beyond contact, Loeb figures. Such galaxies, measured by astronomers to have a redshift of 2 or more, will not be able to transmit any signal to us in the future due to the accelerated expansion of the universe.
"Suppose there are extraterrestrial civilizations in these galaxies," Loeb said in a telephone interview. "If we send a signal to them now, they will never see it."
Like black holes
The point of no return for these galaxies in called an event horizon, a concept more commonly used to describe the hypothesized sphere around a black hole beyond which nothing, not even light, can escape.
Matter falling into a black hole, according to theory, should also leave a final image for the outside world to see.
Loeb calls this final image near a black hole "a frozen image," analogous to the suspected eventual image that we will have of distant galaxies in the universe. We won't be able to see subsequent emissions from the galaxy.
Loeb's calculations have been accepted for publication in the journal Physical Review D.
Time running out?
The upshot should not overly worry present-day astronomers, though their proteges billions of generations hence may get frustrated.
"As a result, there is a finite amount of information that we can collect about the distant universe," Loeb said. "Within 50 billion to 100 billion years, we will not be able to gather any new information about all extragalactic sources."
Even the frozen images will eventually fade away. And then the universe will be a truly lonely place. Only a handful of galaxies, perhaps a thousand or so that are presently nearby and gravitationally bound to our own, won't succumb to Loeb's mathematics.
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