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.

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

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.

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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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.