快猫短视频

GHOSTS gather in the shadows. Just outside the cosy circle of our own
galaxy鈥攚hich is lit by the fires of a hundred billion stars鈥攊s a
host of wraith-like galaxies made of almost nothing but exotic invisible matter.
Instead of shining like celestial beacons across the Universe, they are
virtually indistinguishable from the blackness of space.

鈥淒ark galaxies might outnumber normal galaxies by a hundred to one,鈥� says
Neil Trentham of Cambridge University. Trentham, like most astronomers, believes
dark galaxies must be out there somewhere. He is attempting the almost
impossible task of trying to see these sinister clouds of darkness.

But other astronomers warn that the search is a waste of time. Like ghosts,
dark galaxies may be a figment of the imagination. If so, our theories of how
galaxies form are wrong, and we may have to change our ideas about what makes up
most of the matter in the Universe, or rewrite the story of the Universe鈥檚 first
moments.

Galaxies are thought to have formed from the sea of gas left behind by the
big bang. If some patches of gas were slightly denser than others, their gravity
would have pulled in surrounding material.

But with the gravity of ordinary gas alone, this process would have taken far
too long鈥攇alaxies would still be forming even today. So cosmologists have
been forced to assume that there is also a lot of invisible matter in the
Universe, outweighing the normal matter many times. This 鈥渄ark matter鈥� can also
explain how galaxies spin so fast without breaking apart. A large spherical halo
of dark matter surrounding each galaxy could provide enough gravity to balance
the spin, gluing the galaxy together.

Physicists鈥� attempts to merge the fundamental forces of nature have thrown up
any number of candidates for the stuff of dark matter
(快猫短视频,16 January 1999, p 24),
called weakly interacting massive particles, or WIMPS.
These hypothetical exotic particles are supposed to have been formed along with
normal matter in the furnace of the big bang. They are invisible because they
don鈥檛 feel all the forces that ordinary matter does, and light just ignores
them. Astrophysicists usually assume that these dark matter particles are
鈥渃old鈥濃�攖hat is, fairly heavy and slow moving, so they tend to clump
together.

According to the conventional theory, this clumpiness allowed cold dark
matter to give birth to galaxies. Way back in what astronomers call the dark
ages, when the Universe was a mere billion or so years old, cold dark matter
gathered itself under gravity into giant blobs called haloes. These then
attracted normal matter to form stars, turning into bona fide galaxies.

The problem is that in its simplest form, this theory says there should be an
awful lot of little galaxies鈥攖he failed relics of galaxy formation that
never managed to grow into giant elliptical galaxies or majestic spirals like
the Milky Way.

Astronomers already know of two species of galactic minnow. The small round
galaxies called dwarf spheroidals and their untidier cousins, the dwarf
irregulars, both weigh in at about ten million times the mass of the Sun, or
only one ten thousandth that of the Milky Way. But there are far too few of them
to agree with the theoretical models of cold dark matter, which predict ten or a
hundred times as many.

To save the theory, astrophysicists assume that these small galaxies do
exist鈥攐nly they鈥檙e invisible. Somehow, most of the smaller dark-matter
haloes must have been unable to form stars. 鈥淭he smallest dwarf galaxies could
be the very rare, one-in-a-hundred cases which, for some reason, do form stars,鈥�
says Trentham.

So what stops stars forming in the remainder? Here, dark-galaxy pundits split
into two camps. Either something stops the gas entering the dark matter halo, or
it falls in and then is somehow prevented from making stars.

Trentham believes that dark galaxies failed to attract any normal matter
because they missed out on a feeding frenzy during the dark ages, when gas was
plentiful. 鈥淭o produce normal galaxies, dark matter haloes grow in the early
Universe, pulling in more dark matter and gas,鈥� he says. And according to his
simulations, still unpublished, the smaller haloes would be celestial late
developers. 鈥淚magine a small dark halo growing five to ten billion years after
the big bang. The gas between the galaxies has been heated up by the light from
stars, and it is now moving so fast that it cannot be pulled in.鈥�

Others are not convinced. Frazer Pearce of Durham University believes that
even long after the dark ages, there was plenty of cold gas around to be
captured. Astronomers see traces of dark gas clouds throughout intergalactic
space, revealed because they absorb some of the light from the distant celestial
beacons known as quasars. And these clouds are relatively cool.

鈥淭he gas is at 20,000 K,鈥� says Pearce. And that鈥檚 too cool to escape even a
modest gravitational pull. 鈥淓ven when you have gas at tens of millions of
kelvin, you can鈥檛 keep it out of haloes forever.鈥�

Pearce thinks that these intergalactic clouds are already sitting inside
their own very small haloes of dark matter. So what stops them from forming
stars?

The tiniest galaxies might suffer a kind of boom-bust economy, he suggests.
When the small dark haloes form, they attract some gas and stars form. The most
massive of these burn quickly and explode as supernovae after just a few million
years. That heats the remaining gas and flings it in all directions. A few
supernovae could make the gas hot enough to eject it back into intergalactic
space. 鈥淲e are hoping that supernovae will blow these clouds to pieces and stop
stars forming,鈥� says Pearce.

The hot, scattered gas will eventually radiate its excess energy, slow down
and get pulled in again by the halo, beginning another boom-bust cycle. Each
short period of intense star formation, lasting a few tens of millions of years,
would be followed by a dormant period of up to a billion years.

But once again, there are dissenters. 鈥淭he idea that it is really easy to
blow the gas out of galaxies is problematic,鈥� says George Lake of the University
of Washington. 鈥淧eople have tried to do simulations, overestimating the effect
of supernovae, and they still can鈥檛 get the stuff flung out. I think the whole
idea is misguided.鈥�

Lake thinks dark galaxies are a mirage. 鈥淚t is not that these dark galaxies
lie below current detection limits, they are just not there. They do not exist
at all.鈥�

The only way to settle the argument is to look for these spectral galaxies.
Astronomers have a few ideas about how to catch them (see 鈥淕host hunting鈥�), but
what if, after the trawling is over, they return with empty nets?

Warm dark matter

Pearce believes that the conventional theory of galaxy formation could be
repaired. Instead of cold dark matter, the Universe may be filled with another,
slightly different strange substance.

The lighter a particle is, the faster it is likely to be moving. And
fast-moving particles are much less likely to clump together. Very lightweight
particles such as neutrinos would be a kind of 鈥渉ot dark matter鈥�. These zippy
particles would be so resistant to clumping that they would tend to smooth out
structures even on the scale of large galaxies鈥攕o they can鈥檛 make up most
of the dark matter in the Universe, or we wouldn鈥檛 be here.

Instead, Pearce thinks a dearth of little dark galaxies could be explained by
鈥渨arm dark matter鈥�, of an intermediate mass and speed. Warm dark matter would
happily form big clumps that make ordinary galaxies, but would move too fast to
be captured by the weak gravity of small haloes.

The mass of a warm dark matter particle would need to be around 10^-33
kilograms鈥攁 millionth the mass of a proton. This presents no problem for
particle physicists, who can tweak their speculative theories to produce
particles of virtually any mass required.

But Lake thinks this running repair is useless. He points out that the
conventional theory of structure formation also predicts too many biggish
objects, larger than galaxies but smaller than the most common kind of galaxy
cluster, which contains hundreds of galaxies. It will take more than tinkering
to repair this anomaly. Theorists may be able to use warm dark matter to wash
out little galaxies, but it won鈥檛 get rid of these heavier structures.

Lake believes that there is something fundamentally wrong with our theories
about the early Universe. For structures to form at all in the Universe, there
must be some initial variations in the density of gas. Cosmologists think that
these variations were created by quantum fluctuations in the first fraction of a
second after the big bang, and then magnified by a process called inflation
(快猫短视频, 16 December 2000, p 26). But how big were these
fluctuations? The usual assumption is that, like a fractal pattern, the Universe
was as lumpy on small scales as it was on large scales鈥攁 鈥渟cale-free
fluctuation spectrum鈥�.

This is a catch-all solution that Lake thinks is used to mask our almost
total ignorance of the early Universe. There is no strong evidence for it, and
yet it has become entrenched in cosmological orthodoxy. 鈥淭he strange thing is
that people now treat it as though it were a unique prediction of
颈苍蹿濒补迟颈辞苍.鈥�

Lake believes this is where the problem lies, because the assumption that
there are no special scales doesn鈥檛 fit the shortage of small galaxies and small
clusters. Galaxies and galaxy clusters mark distinct peaks in the fluctuation
spectrum, rather than being part of a smooth continuum of structures, he says.
鈥淚t seems like we have some notes or harmonics in the Universe.鈥� If he is right,
then we can use this knowledge to work out just how the fluctuations formed.
This could sound the death knell for dark matter. Powerful peaks in the
fluctuation spectrum suggest bigger variations in gas density on the right scale
to produce galaxies. And with a better head start, the gravity of ordinary gas
would have been enough to make galaxies, obviating the need for dark matter.

That still leaves the problem of why some galaxies manage to spin so fast
without falling apart, of course, but there may be an another explanation for
that. Some researchers maintain that Newton鈥檚 law of gravity might be slightly
different on large scales. According to the theory of Modified Newtonian
Dynamics, gravity pulls a little harder at large distances than conventional
wisdom dictates, enabling it to hold fast-spinning galaxies together.

If Lake is right, astronomers will have to let the whole notion of dark
matter slip away quietly into the night. The ghosts will have been banished for
good.

How do you look for a black cloud in space? It鈥檚 a riddle astronomers will
have to answer if they want to find dark galaxies.

There may already be some circumstantial evidence for these shadowy objects.
A few isolated galaxies look as though an invisible rival is pulling them to
bits. UGC 10214, for example, has a conspicuous bridge of material extending
into space towards鈥攁pparently鈥攏othing.

There are also the small, faint objects called Blue Compact Galaxies, which
are furiously forming stars. They cannot have been building stars so quickly for
long, otherwise they would be packed with stars and therefore shining far more
brightly. Trentham suggests that perhaps a dark matter halo has passed by each
of these galaxies, causing gas clouds to collapse prematurely and form
stars.

If nothing else, these shreds of evidence could help to narrow down the
search for dark galaxies to a few promising sites. And astronomers will need all
the help they can get.

If dark galaxies hold some gas or a few dead stars, conventional methods
might just work. Brown dwarfs鈥攕mall, failed stars鈥攎ight collect
within a dark galaxy, softly glowing with infrared light. The next generation of
infrared satellites, such as NASA鈥檚 Space Infrared Telescope Facility, will
survey the Universe in the right wavelength range, and could spot them. A few
white dwarfs, the cores of extinct stars, might also be around, but they would
be almost impossible to spot with any existing or planned instruments.

However, Neil Trentham of Cambridge University thinks that there will be
little or no ordinary matter in dark galaxies. If so, the search becomes
fiendishly difficult.

Gravitational lensing might be the only way. A dark-matter halo would bend
light slightly with its gravity. If it drifted between us and a more distant
source, it would slightly distort the image of the source. Unfortunately, the
technique used for spotting these distortions is still far too crude to see dark
matter halos. But all may not be lost.

As the light rays detour through the dark matter halo, they take paths of
different lengths. So rays of light emitted simultaneously no longer reach the
Earth at the same instant. If the original source is variable, those changes
will be staggered when viewed from Earth through the lens.

A rapidly varying source would be essential to detect these short time
delays. Nial Tanvir of the University of Hertfordshire thinks that distant
explosions called gamma ray bursts may do the job. When seen through a dark
galaxy, rays from a burst would rise to a peak of brightness, dim a little and
brighten again as the late-comers arrived.

The problem is that gamma ray bursts are few and far between, so we鈥檒l have
to wait a very long time before one happens to explode right behind a dark
galaxy. However, if in the future more sensitive gamma-ray satellites prove that
there is a plethora of weaker, currently undetectable gamma ray bursts, then
this method might be in business.