sirbiotech
01-08-2002, 11:19 PM
january 8, 2002
in dark matter, new hints of a universal glue
by dennis overbye
ometimes, defying its wont, science makes the cosmos look a little
simpler. recently it seems as if astronomers have been sprung from a
long cosmological nightmare. last month a consortium of astronomers
announced that an analysis of some 130,000 galaxies showed that the
the universe, at least on large scales, is structured pretty much the
way it looks.
that might sound unremarkable, but it didn't have to come out that
way.
"it was not a mad idea that galaxies don't trace the matter," said
dr. licia verde, an astronomer at rutgers and princeton universities,
who was the lead author of a paper submitted last month to the
journal monthly notices of royal astronomical society.
the reason is something called dark matter.
for centuries people have found meaning ? or thought they did
? in
the sky, in the forms of the constellations, the sudden careering of
comets, the stately dance of the planets, the filigree of galaxies,
spanning space as far as the telescope can see, like an old jeweled
fishing net cast across the void.
but what if all this is just an illusion? suppose the real universe
is something we can't see and all the glittering chains of galaxies
are no more substantial, no more reliable guides to physical reality,
than greasepaint on the face of a clown?
that was the humiliating prospect that astronomers faced in the
1980's, as they grudgingly came to accept that decades of
astronomical observations were telling them that most of the universe
was invisible. they could deduce that dark matter was there by its
gravitational effect on the things they could see. if newton's laws
of gravity held over cosmic distances, huge amounts of it were needed
to provide the gravitational glue to keep clusters of galaxies from
flying apart, and to keep the stars swirling around in galaxies at
high speed.
cosmologists concluded that it was in fact dark matter, slowly
congealing under its own weight into vast clouds that provided the
scaffolding for stars and galaxies. and it was dark matter that would
determine the fate of the universe: if there was enough of it,
gravity would eventually reverse the expansion of the universe and
cause a "big crunch." if not, the universe would expand forever.
most gallingly, astronomers didn't even know whether the dark matter
was distributed the way stars and galaxies are. they had no clue to
the whereabouts of most of the universe. luminous matter, the story
went, is like snow on mountaintops or foam on waves, but there could,
in theory, be whole mountain ranges not quite high enough to be
whitecapped, hiding in the darkness.
noting that dark matter heavily outweighed the visible galaxies, four
astronomers analyzed the results of an earlier galaxy mapping
project, in 1980. there was no reason the ratio of dark to light
matter should be the same everywhere "and there may well exist
massive systems that emit essentially no light," read the report in
the astrophysical journal, written by marc davis, john huchra, david
latham and john tonry, all then at the harvard-smithsonian center for
astrophysics.
or as dr. vera c. rubin, an astronomer at the carnegie institute of
washington and a pioneer of dark matter research, said a year
later: "we know very little about the universe. i personally don't
believe it's uniform and the same everywhere. that's like saying the
earth is flat."
the new results suggest that the universe, as mysterious as it
essentially is, may not be entirely perverse. as einstein once
said, "the lord god is subtle, but malicious he is not." but it was a
close call.
"in principle galaxies could bear no resemblance to the underlying
dark matter distribution," explained dr. verde, who performed the
analysis with dr. alan f. heavens of the university of edinburgh.
"we were right to be worried," dr. heavens said.
the notion that the luminous universe might only be greasepaint was
born of the quest for beauty. in the 1980's astronomical surveys
showed that the galaxies were not distributed more or less uniformly
around the sky, as had been thought, but were concentrated in sheets
and clusters and long looping chains separated by huge, black,
presumably empty spaces millions of light-years across.
but the gravitational pull from such striking disparities in the
distribution of mass would tug the galaxies to and fro violently,
distorting the orderly expansion of the universe, if the most
fashionable cosmological theories were right.
those theories held that the density of matter and energy in the
universe was just high enough so that the gravitational attraction
between the contents of the cosmos would eventually just balance the
energy of their outward rush. as a result, space on the largest
scales would show no geometrical warp: it would be "flat," in
cosmological jargon.
whatever wayward velocities the galaxies had, however, was relatively
modest. rather than give up the mathematically beautiful notion of a
high density universe, some theorists suggested that astronomers
might have to give up an equally beautiful and seemingly bedrock
notion, namely that the universe is what we see when we look up at
the sky.
if the voids were just an illusion, and were not empty but just dark,
cosmologists reasoned, there would be no gravitational fields tugging
at the galaxies, which would explain why their so-called peculiar
velocities were so low. they could keep their beautiful universe.
as the primordial clouds of dark matter grow and congeal, so this
theory went, ordinary matter sinks to the center and lights up. but
vast stretches of dark matter outside the center would go unflagged
by visible galaxies, like mountains not quite high enough to attract
snow, or reefs unmarked by buoys.
figuring out exactly why galaxies would have formed in this pattern
was another matter that engaged theorists' imaginations. dr. martin
rees, a cosmologist at cambridge university and the astronomer royal
of england, said he could imagine that galaxy formation could be
catalyzed or impeded by some environmental event. fierce radiation
from the first quasars, for example, could ionize the protogalactic
gas over large swaths of space, affecting its ability to collapse and
light up.
in the 1990's, though, evidence began to mount, from the cobe
satellite, which studied faint radio emanations from the big bang
itself, and from other studies, that the density of matter was less
than a third of the magic critical value needed for a
perfectly "flat" universe.
in that case the clusters do not have the gravitational oomph to
cause trouble and the lack of high velocities is not a problem.
luckily, theorists could still have a flat beautiful universe because
the gap in the matter density was made up by the so-called dark
energy that astronomers have recently discovered seems to be
accelerating the expansion of the universe. but it is no longer a
universe in balance; if the dark energy continues to prevail,
astronomers say, the cosmos will blow apart, chilling all life.
in recent years, therefore, dark matter has surrendered some of its
cachet to dark energy, but the identity of the dark matter is as
mysterious as it ever was. some of it may be ordinary matter, like
rocks and dead stars. but most of it must be more exotic stuff ?
perhaps elementary particles left over from the big bang ?
according
to a study published last week in the journal nature by dr. robert
rood of the university of virginia, and his colleagues. they measured
the abundance of a rare form of helium in the milky way to determine
the amount of "normal" matter produced in the big bang.
nevertheless, the relationship between the light and the dark, once
raised, continued to haunt astronomers. "it was perfectly reasonable
if galaxies didn't cluster the same way as mass," said dr. heavens,
adding, "the genie was out of the bottle."
he and dr. verde set out to measure the degree, technically known as
bias, to which the distributions of luminous and dark matter were
mismatched, using statistical techniques she had developed for her
ph.d. dissertation under his supervision.
for a database, they turned to a catalog of the relative distances
and positions on the sky of 130,000 galaxies that had been compiled
by an international consortium of astronomers known as the 2-degree
field galaxy redshift survey, or 2df for short, using the 12-foot-
diameter anglo-australian telescope near coonabarabran, australia.
by the time it is finished, the survey, which takes its name from the
field of view of the telescope, should have mapped 250,000 galaxies
out to a distance of around 500 million light-years.
as dr. verde explained, she and dr. heavens used statistics to
analyze the shapes of the galaxy clusters in the sky. according to
gravity theory and computer simulations, she said, dark matter, which
only interacts gravitationally, should start out in rounded lumps and
then gradually shape itself into filaments and sheets as these lumps
collapse along their shortest axes first.
"the signature of gravity is filaments," dr. verde said. "if there is
biasing you get a distribution that is not sheets and filaments ?
you
get a different pattern."
the results, she and dr. heavens said, were clearly consistent with a
filamentary structure, "like a web, not round hills and mountains."
"you would have to come up with a theory quite mad to get this
pattern with biasing," dr. verde said. "taken together, these
measurements argue powerfully that the 2dfgrs galaxies do indeed
trace the mass on large scales," she and 29 co-authors concluded in
the recent paper.
the mountains are where the snow is. the universe is where the light
is. dr. rees added: "2df shows that things hang together. it could
have not been that way. there is no evidence for enormous dark
somethings with no galaxies associated with them."
at least in the present universe.
"five billion years ago we would have gotten a different answer,"
said dr. heavens, explaining that galaxies probably did form first in
concentrations at the centers of dark matter clouds but gradually
spread into the hinterlands over cosmic history to reflect more
accurately the overall distribution of matter, the as yet unknown
cosmic stuff.
on average, galaxies today trace mass, and the astronomy of the
invisible is thus also the astronomy of the visible.
of course it was by following the light that astronomers were led
into the darkness. like the proverbial drunk looking under the
streetlight for his keys, they never had any choice about where to
look for the universe. "thirty years ago we thought the universe was
all stars. now stars are just the tip of the iceberg," said dr.
michael turner, a cosmologist at the university of chicago. "there
was a worry that the light in the sky was not faithfully tracing the
distribution of matter. large surveys went out looking for clumps of
matter that didn't correspond to light.
"that story is now starting to come to an end."
now if someone would just do something about that dark energy.
http://www.nytimes.com/2002/01/08/science/08dark.html
in dark matter, new hints of a universal glue
by dennis overbye
ometimes, defying its wont, science makes the cosmos look a little
simpler. recently it seems as if astronomers have been sprung from a
long cosmological nightmare. last month a consortium of astronomers
announced that an analysis of some 130,000 galaxies showed that the
the universe, at least on large scales, is structured pretty much the
way it looks.
that might sound unremarkable, but it didn't have to come out that
way.
"it was not a mad idea that galaxies don't trace the matter," said
dr. licia verde, an astronomer at rutgers and princeton universities,
who was the lead author of a paper submitted last month to the
journal monthly notices of royal astronomical society.
the reason is something called dark matter.
for centuries people have found meaning ? or thought they did
? in
the sky, in the forms of the constellations, the sudden careering of
comets, the stately dance of the planets, the filigree of galaxies,
spanning space as far as the telescope can see, like an old jeweled
fishing net cast across the void.
but what if all this is just an illusion? suppose the real universe
is something we can't see and all the glittering chains of galaxies
are no more substantial, no more reliable guides to physical reality,
than greasepaint on the face of a clown?
that was the humiliating prospect that astronomers faced in the
1980's, as they grudgingly came to accept that decades of
astronomical observations were telling them that most of the universe
was invisible. they could deduce that dark matter was there by its
gravitational effect on the things they could see. if newton's laws
of gravity held over cosmic distances, huge amounts of it were needed
to provide the gravitational glue to keep clusters of galaxies from
flying apart, and to keep the stars swirling around in galaxies at
high speed.
cosmologists concluded that it was in fact dark matter, slowly
congealing under its own weight into vast clouds that provided the
scaffolding for stars and galaxies. and it was dark matter that would
determine the fate of the universe: if there was enough of it,
gravity would eventually reverse the expansion of the universe and
cause a "big crunch." if not, the universe would expand forever.
most gallingly, astronomers didn't even know whether the dark matter
was distributed the way stars and galaxies are. they had no clue to
the whereabouts of most of the universe. luminous matter, the story
went, is like snow on mountaintops or foam on waves, but there could,
in theory, be whole mountain ranges not quite high enough to be
whitecapped, hiding in the darkness.
noting that dark matter heavily outweighed the visible galaxies, four
astronomers analyzed the results of an earlier galaxy mapping
project, in 1980. there was no reason the ratio of dark to light
matter should be the same everywhere "and there may well exist
massive systems that emit essentially no light," read the report in
the astrophysical journal, written by marc davis, john huchra, david
latham and john tonry, all then at the harvard-smithsonian center for
astrophysics.
or as dr. vera c. rubin, an astronomer at the carnegie institute of
washington and a pioneer of dark matter research, said a year
later: "we know very little about the universe. i personally don't
believe it's uniform and the same everywhere. that's like saying the
earth is flat."
the new results suggest that the universe, as mysterious as it
essentially is, may not be entirely perverse. as einstein once
said, "the lord god is subtle, but malicious he is not." but it was a
close call.
"in principle galaxies could bear no resemblance to the underlying
dark matter distribution," explained dr. verde, who performed the
analysis with dr. alan f. heavens of the university of edinburgh.
"we were right to be worried," dr. heavens said.
the notion that the luminous universe might only be greasepaint was
born of the quest for beauty. in the 1980's astronomical surveys
showed that the galaxies were not distributed more or less uniformly
around the sky, as had been thought, but were concentrated in sheets
and clusters and long looping chains separated by huge, black,
presumably empty spaces millions of light-years across.
but the gravitational pull from such striking disparities in the
distribution of mass would tug the galaxies to and fro violently,
distorting the orderly expansion of the universe, if the most
fashionable cosmological theories were right.
those theories held that the density of matter and energy in the
universe was just high enough so that the gravitational attraction
between the contents of the cosmos would eventually just balance the
energy of their outward rush. as a result, space on the largest
scales would show no geometrical warp: it would be "flat," in
cosmological jargon.
whatever wayward velocities the galaxies had, however, was relatively
modest. rather than give up the mathematically beautiful notion of a
high density universe, some theorists suggested that astronomers
might have to give up an equally beautiful and seemingly bedrock
notion, namely that the universe is what we see when we look up at
the sky.
if the voids were just an illusion, and were not empty but just dark,
cosmologists reasoned, there would be no gravitational fields tugging
at the galaxies, which would explain why their so-called peculiar
velocities were so low. they could keep their beautiful universe.
as the primordial clouds of dark matter grow and congeal, so this
theory went, ordinary matter sinks to the center and lights up. but
vast stretches of dark matter outside the center would go unflagged
by visible galaxies, like mountains not quite high enough to attract
snow, or reefs unmarked by buoys.
figuring out exactly why galaxies would have formed in this pattern
was another matter that engaged theorists' imaginations. dr. martin
rees, a cosmologist at cambridge university and the astronomer royal
of england, said he could imagine that galaxy formation could be
catalyzed or impeded by some environmental event. fierce radiation
from the first quasars, for example, could ionize the protogalactic
gas over large swaths of space, affecting its ability to collapse and
light up.
in the 1990's, though, evidence began to mount, from the cobe
satellite, which studied faint radio emanations from the big bang
itself, and from other studies, that the density of matter was less
than a third of the magic critical value needed for a
perfectly "flat" universe.
in that case the clusters do not have the gravitational oomph to
cause trouble and the lack of high velocities is not a problem.
luckily, theorists could still have a flat beautiful universe because
the gap in the matter density was made up by the so-called dark
energy that astronomers have recently discovered seems to be
accelerating the expansion of the universe. but it is no longer a
universe in balance; if the dark energy continues to prevail,
astronomers say, the cosmos will blow apart, chilling all life.
in recent years, therefore, dark matter has surrendered some of its
cachet to dark energy, but the identity of the dark matter is as
mysterious as it ever was. some of it may be ordinary matter, like
rocks and dead stars. but most of it must be more exotic stuff ?
perhaps elementary particles left over from the big bang ?
according
to a study published last week in the journal nature by dr. robert
rood of the university of virginia, and his colleagues. they measured
the abundance of a rare form of helium in the milky way to determine
the amount of "normal" matter produced in the big bang.
nevertheless, the relationship between the light and the dark, once
raised, continued to haunt astronomers. "it was perfectly reasonable
if galaxies didn't cluster the same way as mass," said dr. heavens,
adding, "the genie was out of the bottle."
he and dr. verde set out to measure the degree, technically known as
bias, to which the distributions of luminous and dark matter were
mismatched, using statistical techniques she had developed for her
ph.d. dissertation under his supervision.
for a database, they turned to a catalog of the relative distances
and positions on the sky of 130,000 galaxies that had been compiled
by an international consortium of astronomers known as the 2-degree
field galaxy redshift survey, or 2df for short, using the 12-foot-
diameter anglo-australian telescope near coonabarabran, australia.
by the time it is finished, the survey, which takes its name from the
field of view of the telescope, should have mapped 250,000 galaxies
out to a distance of around 500 million light-years.
as dr. verde explained, she and dr. heavens used statistics to
analyze the shapes of the galaxy clusters in the sky. according to
gravity theory and computer simulations, she said, dark matter, which
only interacts gravitationally, should start out in rounded lumps and
then gradually shape itself into filaments and sheets as these lumps
collapse along their shortest axes first.
"the signature of gravity is filaments," dr. verde said. "if there is
biasing you get a distribution that is not sheets and filaments ?
you
get a different pattern."
the results, she and dr. heavens said, were clearly consistent with a
filamentary structure, "like a web, not round hills and mountains."
"you would have to come up with a theory quite mad to get this
pattern with biasing," dr. verde said. "taken together, these
measurements argue powerfully that the 2dfgrs galaxies do indeed
trace the mass on large scales," she and 29 co-authors concluded in
the recent paper.
the mountains are where the snow is. the universe is where the light
is. dr. rees added: "2df shows that things hang together. it could
have not been that way. there is no evidence for enormous dark
somethings with no galaxies associated with them."
at least in the present universe.
"five billion years ago we would have gotten a different answer,"
said dr. heavens, explaining that galaxies probably did form first in
concentrations at the centers of dark matter clouds but gradually
spread into the hinterlands over cosmic history to reflect more
accurately the overall distribution of matter, the as yet unknown
cosmic stuff.
on average, galaxies today trace mass, and the astronomy of the
invisible is thus also the astronomy of the visible.
of course it was by following the light that astronomers were led
into the darkness. like the proverbial drunk looking under the
streetlight for his keys, they never had any choice about where to
look for the universe. "thirty years ago we thought the universe was
all stars. now stars are just the tip of the iceberg," said dr.
michael turner, a cosmologist at the university of chicago. "there
was a worry that the light in the sky was not faithfully tracing the
distribution of matter. large surveys went out looking for clumps of
matter that didn't correspond to light.
"that story is now starting to come to an end."
now if someone would just do something about that dark energy.
http://www.nytimes.com/2002/01/08/science/08dark.html