Universe's end frozen in time
Posted: December 13, 2001

Astronomers often investigate the beginning of the Universe, starting with the Big Bang. New data is shedding light on the opposite end of the arrow of time - how the Universe might end.

In the past, astronomers have theorized about what we might see if we watched the Universe billions of years from now. Some thought the expansion of the universe would slow and reverse, compressing all matter back in a "Big Crunch." Others said the expansion would continue forever and we would see the stars in all the galaxies age and die, leaving us in darkness.

But now, a calculation by Professor Abraham Loeb, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics, paints a different picture regarding the fate of the universe, and it looks quite lonely. As the universe ages and expands, fewer and fewer galaxies will be visible to us. Even weirder, as we watch the galaxies fade, their appearance will freeze in time. No matter how long we watch, like celluloid heroes in the cinema, they will never grow older or change. They will only grow dimmer as they recede from us.

Seen from the outskirts of our own Milky Way Galaxy, at lower left in this artist rendering, seven billion years from now the Universe will appear frozen in time as we look out onto space. Only the light from the local group of galaxies will remain visible to any creatures inhabiting our galaxy. The galaxies included in this view are M77 located upper left, M33 upper center, M74 lower left of M33, M31 lower right center, and NGC 147, lower right. Credit: Harvard-Smithsonian Center for Astrophysics
These strange results are the consequence of Einstein's general theory of relativity, combined with current knowledge of the parameters of the universe. Studies of distant exploding stars have shown that the expansion of the universe, rather than slowing down from the inexorable pull of gravity, instead is speeding up under the influence of a vacuum energy dubbed "the cosmological constant". Eventually, distant galaxies will simply be moving too fast for us to see.

Over the next 100 billion years, this accelerating force will shrink our cosmic horizon, reducing the number of galaxies we can see to only about a thousand members of the local Virgo Cluster and surrounding areas. As distant galaxies cross our horizon, their image will get frozen. The light they emit after the moment of horizon crossing will never be able to reach us.

"This process is analogous to what you see if you watch a light source fall into a black hole," states Loeb. "As an object crosses the black hole's event horizon, its image seems to freeze and fade away because you can't see the light it emits after that point."

Similarly, we will see distant galaxies freeze into an unchanging vista. We will never see new stars being born or old stars dying. The galactic snapshots will simply fade away to invisibility.

This has grim consequences for our study of the universe. Not only will the number of galaxies we can see shrink away, but we will not be able to watch the evolution of these galaxies later in their history. The amount of information available to us about the distant universe is finite.

For example, light from the most distant quasar yet seen left that quasar when the universe was only a billion years old. (The universe is now estimated to be 14 billion years old.) Loeb's calculations show that if we watch this quasar for the next several billion years, we will see it freeze at an age of six billion years and stop changing. Its frozen image will only grow fainter as the universe expands.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into seven research divisions study the origin, evolution, and ultimate fate of the universe.