快猫短视频

LOST PROPERTY doesn鈥檛 come any bigger than this. If Harvard astrophysicist
Abraham Loeb is right, we are about to lose most of the visible Universe.
Billions of sparkling galaxies are flying away from us into regions we will
never see, leaving only a few thousand of them in our sky. Future Earthlings
will have no notion that anything ever existed beyond the Milky Way and its
immediate neighbours. 鈥淎fter that, there won鈥檛 be anything we can learn about
the distant Universe,鈥� says Loeb.

It鈥檚 not going to happen for 100 billion years or so, but Loeb has calculated
that distant objects, including nearly all the other galaxies in the Universe,
will have vanished for ever. They will have flown out beyond a truly final
frontier, one that is patrolled by nature鈥檚 speed cops.

The point is that we can only see a star or galaxy if its light can reach us.
That means the light must have taken no longer than the 14 billion years or so
since the beginning of the Universe to travel from the distant galaxy to Earth.
But what鈥檚 the problem? According to the big bang theory, everything started out
at a single point, and nothing can travel faster than light, so how could any
galaxy now be so far away that its light hasn鈥檛 got time to reach us?

Superfast expansion

Well, expansion doesn鈥檛 mean simply that galaxies are travelling outwards;
the very fabric of space is changing. It is possible for space to expand so fast
that some regions of the Universe fly apart faster than light. On top of that,
the distances light has to travel are changing too. Light might set out from a
galaxy with only a billion light years to travel to Earth, but find that space
is stretching out the distance before it so it takes 10 billion years to reach
its destination. The net result is that there are places so far removed from us
that we have never seen their light.

But surely we鈥檒l see it one day? Until a couple of years ago, the accepted
idea was that matter鈥檚 gravity has been slowing down expansion ever since the
big bang. The result it that our growing horizon ought to be overtaking those
distant worlds, so more and more galaxies should be heaving into view.

That changed in 1998. Saul Perlmutter of the Lawrence Berkeley Laboratory in
California, Alex Filippenko of the University of California at Berkeley and Adam
Reiss and Bob Kirshner at Harvard measured the brightness of a number of
supernovae. These explosions were of a kind that always emit the same amount of
energy. So the brightness we see should tell us how far away they are. Or to be
more precise, how much space their light travelled through to reach us.

The supernovae were fainter than expected, suggesting that the Universe鈥檚
expansion has speeded up while the light from the explosions was en route to Earth
(快猫短视频, 11 April 1998, p 26).
But why? One suggestion is
that empty space has a kind of vacuum energy that pushes galaxies apart.

Pondering this one morning, Loeb decided to work out exactly what that means
for the future. He took measured values of the rate of acceleration of the
Universe and the amount of matter it contains, and assumed that the acceleration
is indeed caused by a kind of constant vacuum energy. From this he calculated
that instead of gaining new territories, the visible Universe is losing them
鈥� and fast.

A rough measure of distance is 鈥渞ed shift鈥�. This reflects how much space has
expanded and stretched out the light from a galaxy or other source. Loeb
calculated that galaxies we see with a quite modest red shift of 1.8 are now
disappearing over our horizon. While we can still see them by the light they
emitted about 10 billion years ago, the light they are emitting now will never
reach us.

Light that is already on its way from these distant galaxies will continue to
reach us for several billion years yet. But after that, we鈥檒l start to see weird
things happening as the horizon-crossing becomes visible. Einstein鈥檚 general
theory of relativity implies that the galaxies won鈥檛 simply disappear. Instead,
each will leave behind a gradually fading snapshot of itself sitting on the
horizon.

These galaxies are astronomical Dorian Grays. Beyond our horizon they鈥檒l be
growing older naturally, but all we鈥檒l see is fading pictures of them in their
youth. The light from these ghosts will become fainter and stretch to
ever-longer wavelengths, until they鈥檙e impossible to detect at all.

Even more distant objects will vanish sooner. Take the farthest known object
in the Universe, a quasar reported in August to have a red shift of 6.28. The
light we see coming from this quasar started its journey when the Universe was
about a billion years old. Loeb calculates that the object is already well past
our horizon. Several billion years from now, we will see it turn into a frozen
image of a quasar aged only 6 billion years.

Eventually, in around 100 billion years, Earthlings will only be able to see
true images of what鈥檚 now in our galactic backyard鈥攁 swarm of a few
thousand galaxies called the Virgo cluster, which will remain fairly tight-knit
because gravity clasps it together. The most distant visible galaxy will lie
less than 100 million light years away, only 1 per cent of the distance we can
probe today. 鈥淏eyond that, everything will be dark,鈥� says Loeb. 鈥淭here won鈥檛 be
any sense in doing extragalactic astronomy then.鈥�

Disappearing ripples

The same problem will afflict the cosmic microwave background, the relic
radiation of the big bang fireball. The radiation carries ripples corresponding
to density fluctuations in the early Universe, and watching these ripples gives
astronomers crucial information that helps them measure the size, shape and
weight of the Universe. But in around 100 billion years, the apparent source of
the cosmic radiation will also cross our horizon, freeze and fade. After that,
there鈥檒l be no way of extracting its secrets.

If so, we are beneficiaries of a happy coincidence, treated to information
that will only be visible for a brief period in the possibly infinite life of
the Universe. Such apparent good luck makes some astronomers uneasy. 鈥淚t seems
too weird,鈥� says Filippenko. He suspects that a constant vacuum energy may not
be the force behind cosmic acceleration. Instead, he suggests that a force that
weakens over time is responsible, perhaps an exotic
type of vacuum energy called quintessence
(快猫短视频, 3 April 1999, p 29).
鈥淭his would give a much longer period of good
times for us,鈥� says Filippenko. It鈥檚 also possible
that some new universal force will gradually emerge that pulls everything back
together again.

快猫短视频s are working on how to find the correct model. An American team has
proposed building a new satellite called SNAP (Supernova/Acceleration Probe), a
telescope fitted with a billion-pixel camera, the largest digital astronomical
imager ever constructed. That could watch 2000 supernovae flare up in a single
year鈥�20 times as many as we鈥檇 expect to see in a whole decade if we were
observing them from the ground鈥攚ould tell astronomers just how the
expansion rate of the Universe changed in the past. That would in turn shed
light on what kind of force drives the acceleration of space, and whether it
will change in future. It may also answer the burning question: whether the
cosmos is about to turn out the lights for good.