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

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

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

Loeb’s paper on this subject was accepted for publication in Physical
Review D. For more information see

An image of the final view from the Milky Way is online at

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.


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Harvard-Smithsonian Center for Astrophysics

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Harvard-Smithsonian Center for Astrophysics

(617) 495-7463