The-Universe

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Passage 5

The Universe's Invisible Hand

By Christopher J. Conselice

Dark energy (暗能量) does more than hurry along the expansion of the universe.

It also has a stranglehold on the shape and spacing of galaxies

What took us so long? Only in 1998 did astronomers discover we had been

missing nearly three quarters of the contents of the universe, the so-called dark

energy--an unknown form of energy that surrounds each of us, tugging at us ever so

slightly, holding the fate of the cosmos in its grip, but to which we are almost totally

blind. Some researchers, to be sure, had anticipated that such energy existed, but

even they will tell you that its detection ranks among the most revolutionary

discoveries in 20th-century cosmology. Not only does dark energy appear to make

up the bulk of the universe, but its existence, if it stands the test of time, will

probably require the development of new theories of physics.

Scientists are just starting the long process of figuring out what dark energy is

and what its implications are. One realization has already sunk in: although dark

energy betrayed its existence through its effect on the universe as a whole, it may

also shape the evolution of the universe's inhabitants--stars, galaxies, galaxy clusters.

Astronomers may have been staring at its handiwork for decades without realizing

it.

暗能量不仅仅会加速宇宙膨胀。它也对星系的形状和间隔有所束缚。

是什么让我们花费了那么长时间？直到1998年宇航员才发现我们几乎忽略了宇宙中四分之三的能源，所谓的暗能量—我们周围一种不为人知的能量，轻轻将我们拉住，掌控了宇宙的命运。但是，对此我们一无所知。事实上，一些研究者早已预料到，这种能量是存在的，但是，他们将告知你，这是20世纪宇宙学上革命性的发现。暗能量不仅是宇宙的一大组成部分，而且如果能经受时间的考验，它的存在很可能需要物理学最新理论发展来作为支撑。

科学家们正着手研究暗能量的本质和影响，其中一项发现是：暗能量通过对整个宇宙的影响暴露了它的存在，它也可以使宇宙中星系，银河系产生变化。宇航员们数几十年来也许一直致力于它的影响，而忽略了它的存在。

Ironically, the very pervasiveness of dark energy is what made it so hard to

recognize. Dark energy, unlike matter, does not clump in some places more than

others; by its very nature, it is spread smoothly everywhere. Whatever the

location--be it in your kitchen or in intergalactic space--it has the same density,

about 10-26 kilogram per cubic meter, equivalent to a handful of hydrogen atoms.

All the dark energy in our solar system amounts to the mass of a small asteroid,

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making it an utterly inconsequential player in the dance of the planets. Its effects

stand out only when viewed over vast distances and spans of time.

讽刺的是，暗能量无处不在，使人很难发现。暗能量不同于物质，它不是在空间中某处群聚成团，而是依据其特有的性质均匀地分散在各个地方。无论是你家的厨房里还是星系间，它的密度都一样，约每立方米10-26

公斤，相当于几个氢原子的重量。太阳系里所有的暗能量相当于一个小行星块，所以在行星的运行中，它是微不足道的角色。只有在遥远的地方长时间观察，暗能量的影响才突出。

Since the days of American astronomer Edwin Hubble, observers have known

that all but the nearest galaxies are moving away from us at a rapid rate. This rate is

proportional to distance: the more distant a galaxy is, the faster its recession. Such a

pattern implied that galaxies are not moving through space in the conventional sense

but are being carried along as the fabric of space itself stretches [see

"Misconceptions about the Big Bang," byCharles H. Lineweaver and Tamara M.

Davis; Scientific American, March 2005]. For decades, astronomers struggled to

answer the obvious follow-up question: How does the expansion rate change over

time? They reasoned that it should be slowing down, as the inward gravitational

attraction exerted by galaxies on one another should have counteracted the outward

expansion.

自从美国天文学家爱丁文哈伯那个时代开始，研究人员就已经知道那些除了最近的星系之外，其他星系都在以很快的速率远离我们。速率与距离是成比例的，离我们越远的星系，移动越快。这暗示了在传统的意义上，星系没有从空间移动，而是顺应着空间本身的结构延伸着〔参见Charles H. Lineweaver和Tamara M. Davis在《科学美国人》2005年3月号上撰写的〈你也误会了大霹雳〉〕数十年来，科学家努力地回答明显的随之而来的问题。膨胀的速率会随着时间如何改变呢？他们的推断是当彼此星系对内部的外有引力施以影响，那么外部扩张就会受到阻碍，宇宙的膨胀速率就会减慢。

The first clear observational evidence for changes in the expansion rate

involved distant supernovae, massive exploding stars that can be used as markers of

cosmic expansion, just as watching driftwood lets you measure the speed of a river.

These observations made clear that the expansion was slower in the past than today

and is therefore accelerating. More specifically, it had been slowing down but at

some point underwent a transition and began speeding up [see "Surveying

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Space-time with Supernovae," by Craig J. Hogan, Robert P. Kirshner and Nicholas

B. Suntzeff; Scientific American, January 1999, and "From Slowdown to Speedup,"

by Adam G. Riess and Michael S. Turner; Scientific American, February 2004]. This

striking result has since been cross-checked by independent studies of the cosmic

microwave background radiation by, for example, the Wilkinson Microwave

Anisotropy Probe (WMAP).

第一个膨胀速率变化的明显观测证据是涉及遥远的超新星的。大量爆炸的恒星可以被当做宇宙膨胀的标志，就像我们盯着浮木可以测量水速。观测结果清楚显示过去的膨胀速度比现在慢，因此它现在正在加速。更确切的说，宇宙膨胀确实曾经一度变慢，但在某个时刻经历一段过渡期后，便开始加速了。〔参见Craig J. Hogan, Robert P. Kirshner and Nicholas B. Suntzeff撰写的〈测量超新星时空〉和撰写的〈从减速到加速〉〕。这个显著的成果已经被其他关于宇宙微波背景辐射的个别研究反复检验过了，例如，威金森微波异向性探测器（WMAP）

Dark energy may be the key link among several aspects of galaxy formation that

used to appear unrelated.

One possible conclusion is that different laws of gravity apply on supergalactic

scales than on lesser ones, so that galaxies' gravity does not, in fact, resist expansion.

But the more generally accepted hypothesis is that the laws of gravity are universal

and that some form of energy, previously unknown to science, opposes and

overwhelms galaxies' mutual attraction, pushing them apart ever faster. Although

dark energy is inconsequential within our galaxy (let alone your kitchen), it adds up

to the most powerful force in the cosmos.

暗能量是连接曾看似不相关的星系的主要纽带

一项可能的结果是，不同的重力定律应用于超星系范畴，而不是较小星系范畴，所以星系的重力实际上不用抑制膨胀。但是人们更普遍接受的假设是，重力定律是普遍适用的，不过一些在科学界前所未有的能量形态，足以反抗并压制星系间的相互吸引力，促使它们分离的更快。尽管暗能量在我们的星系里无足轻重（更别提你家厨房了），但是它的总量却是宇宙中最强大的力量。

Cosmic Sculptor

As astronomers have explored this new phenomenon, they have found that, in

addition to determining the overall expansion rate of the universe, dark energy has

long-term consequences for smaller scales. As you zoom in from the entire

observable universe, the first thing you notice is that matter on cosmic scales is

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distributed in a cobweblike pattern--a filigree of filaments, several tens of millions

of light-years long, interspersed with voids of similar size. Simulations show that

both matter and dark energy are needed to explain the pattern.

That finding is not terribly surprising, though. The filaments and voids are not

coherent bodies like, say, a planet. They have not detached from the overall cosmic

expansion and established their own internal equilibrium of forces. Rather they are

features shaped by the competition between cosmic expansion (and any

phenomenon affecting it) and their own gravity. In our universe, neither player in

this tug-of-war is overwhelmingly dominant. If dark energy were stronger,

expansion would have won and matter would be spread out rather than concentrated

in filaments. If dark energy were weaker, matter would be even more concentrated

than it is.

The situation gets more complicated as you continue to zoom in and reach the

scale of galaxies and galaxy clusters. Galaxies, including our own Milky Way, do

not expand with time. Their size is controlled by an equilibrium between gravity

and the angular momentum of the stars, gas and other material that make them up;

they grow only by accreting new material from intergalactic space or by merging

with other galaxies. Cosmic expansion has an insignificant effect on them. Thus, it

is not at all obvious that dark energy should have had any say whatsoever in how

galaxies formed. The same is true of galaxy clusters, the largest coherent bodies in

the universe--assemblages of thousands of galaxies embedded in a vast cloud of hot

gas and bound together by gravity.

Yet it now appears that dark energy may be the key link among several

aspects of galaxy and cluster formation that not long ago appeared unrelated. The

reason is that the formation and evolution of these systems is partially driven by

interactions and mergers between galaxies, which in turn may have been driven

strongly by dark energy.

To understand the influence of dark energy on the formation of galaxies, first

consider how astronomers think galaxies form. Current theories are based on the

idea that matter comes in two basic kinds. First, there is ordinary matter, whose

particles readily interact with one another and, if electrically charged, with

electromagnetic radiation. Astronomers call this type of matter "baryonic" in

reference to its main constituent, baryons, such as protons and neutrons. Second,

there is dark matter (which is distinct from dark energy), which makes up 85 percent

of all matter and whose salient property is that it comprises particles that do not

react with radiation. Gravitationally, dark matter behaves just like ordinary matter.

According to models, dark matter began to clump immediately after the big

bang, forming spherical blobs that astronomers refer to as "halos." The baryons, in

contrast, were initially kept from clumping by their interactions with one another

and with radiation. They remained in a hot, gaseous phase. As the universe

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expanded, this gas cooled and the baryons were able to pack themselves together.

The first stars and galaxies coalesced out of this cooled gas a few hundred million

years after the big bang. They did not materialize in random locations but in the

centers of the dark matter halos that had already taken shape.

Since the 1980s a number of theorists have done detailed computer simulations

of this process, including groups led by Simon D. M. White of the Max Planck

Institute for Astrophysics in Garching, Germany, and Carlos S. Frenk of Durham

University in England. They have shown that most of the first structures were small,

low-mass dark matter halos. Because the early universe was so dense, these

low-mass halos (and the galaxies they contained) merged with one another to form

larger-mass systems. In this way, galaxy construction was a bottom-up process, like

building a dollhouse out of Lego bricks. (The alternative would have been a

top-down process, in which you start with the dollhouse and smash it to make

bricks.) My colleagues and I have sought to test these models by looking at distant

galaxies and how they have merged over cosmic time.

Galaxy Formation Peters Out

Detailed studies indicate that a galaxy gets bent out of shape when it merges

with another galaxy. The earliest galaxies we can see existed when the universe was

about a billion years old, and many of these indeed appear to be merging. As time

went on, though, the fusion of massive galaxies became less common. Between two

billion and six billion years after the big bang--that is, over the first half of cosmic

history--the fraction of massive galaxies undergoing a merger dropped from half to

nearly nothing at all. Since then, the distribution of galaxy shapes has been frozen,

an indication that smashups and mergers have become relatively uncommon.

In fact, fully 98 percent of massive galaxies in today's universe are either

elliptical or spiral, with shapes that would be disrupted by a merger. These galaxies

are stable and comprise mostly old stars, which tells us that they must have formed

early and have remained in a regular morphological form for quite some time. A few

galaxies are merging in the present day, but they are typically of low mass.

The virtual cessation of mergers is not the only way the universe has run out of

steam since it was half its current age. Star formation, too, has been waning. Most of

the stars that exist today were born in the first half of cosmic history, as first

convincingly shown by several teams in the 1990s, including ones led by Simon J.

Lilly, then at the University of Toronto, Piero Madau, then at the Space Telescope

Science Institute, and Charles C. Steidel of the California Institute of Technology.

More recently, researchers have learned how this trend occurred. It turns out that

star formation in massive galaxies shut down early. Since the universe was half its

current age, only lightweight systems have continued to create stars at a significant

rate. This shift in the venue of star formation is called galaxy downsizing [see "The

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Midlife Crisis of the Cosmos," by Amy J. Barger; Scientific American, January

2005]. It seems paradoxical. Galaxy formation theory predicts that small galaxies

take shape first and, as they amalgamate, massive ones arise. Yet the history of star

formation shows the reverse: massive galaxies are initially the main stellar birthing

grounds, then smaller ones take over.

The universe has run out of steam since it was half its current age. Mergers have

ceased, and black holes are quiescent.

Another oddity is that the buildup of supermassive black holes, found at the

centers of galaxies, seems to have slowed down considerably. Such holes power

quasars and other types of active galaxies, which are rare in the modern universe;

the black holes in our galaxy and others are quiescent. Are any of these trends in

galaxy evolution related? Is it really possible that dark energy is the root cause?

The Steady Grip of Dark Energy

Some astronomers have proposed that internal processes in galaxies, such as

energy released by black holes and supernovae, turned off galaxy and star formation.

But dark energy has emerged as possibly a more fundamental culprit, the one that

can link everything together. The central piece of evidence is the rough coincidence

in timing between the end of most galaxy and cluster formation and the onset of the

domination of dark energy. Both happened when the universe was about half its

present age.

The idea is that up to that point in cosmic history, the density of matter was so

high that gravitational forces among galaxies dominated over the effects of dark

energy. Galaxies rubbed shoulders, interacted with one another, and frequently

merged. New stars formed as gas clouds within galaxies collided, and black holes

grew when gas was driven toward the centers of these systems. As time progressed

and space expanded, matter thinned out and its gravity weakened, whereas the

strength of dark energy remained constant (or nearly so). The inexorable shift in the

balance between the two eventually caused the expansion rate to switch from

deceleration to acceleration. The structures in which galaxies reside were then

pulled apart, with a gradual decrease in the galaxy merger rate as a result. Likewise,

intergalactic gas was less able to fall into galaxies. Deprived of fuel, black holes

became more quiescent.

This sequence could perhaps account for the downsizing of the galaxy

population. The most massive dark matter halos, as well as their embedded galaxies,

are also the most clustered; they reside in close proximity to other massive halos.

Thus, they are likely to knock into their neighbors earlier than are lower-mass

systems. When they do, they experience a burst of star formation. The newly formed

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stars light up and then blow up, heating the gas and preventing it from collapsing

into new stars. In this way, star formation chokes itself off: stars heat the gas from

which they emerged, preventing new ones from forming. The black hole at the

center of such a galaxy acts as another damper on star formation. A galaxy merger

feeds gas into the black hole, causing it to fire out jets that heat up gas in the system

and prevent it from cooling to form new stars.

Apparently, once star formation in massive galaxies shuts down, it does not

start up again--most likely because the gas in these systems becomes depleted or

becomes so hot that it cannot cool down quickly enough. These massive galaxies

can still merge with one another, but few new stars emerge for want of cold gas. As

the massive galaxies stagnate, smaller galaxies continue to merge and form stars.

The result is that massive galaxies take shape before smaller ones, as is observed.

Dark energy perhaps modulated this process by determining the degree of galaxy

clustering and the rate of merging.

Dark energy would also explain the evolution of galaxy clusters. Ancient

clusters, found when the universe was less than half its present age, were already as

massive as today's clusters. That is, galaxy clusters have not grown by a significant

amount in the past six billion to eight billion years. This lack of growth is an

indication that the infall of galaxies into clusters has been curtailed since the

universe was about half its current age--a direct sign that dark energy is influencing

the way galaxies are interacting on large scales. Astronomers knew as early as the

mid-1990s that galaxy clusters had not grown much in the past eight billion years,

and they attributed this to a lower matter density than theoretical arguments had

predicted. The discovery of dark energy resolved the tension between observation

and theory.

An example of how dark energy alters the history of galaxy clusters is the fate

of the galaxies in our immediate vicinity, known as the Local Group. Just a few

years ago astronomers thought that the Milky Way and Andromeda, its closest large

neighbor, along with their retinue of satellites, would fall into the nearby Virgo

cluster. But it now appears that we shall escape that fate and never become part of a

large cluster of galaxies. Dark energy will cause the distance between us and Virgo

to expand faster than the Local Group can cross it.

By throttling cluster development, dark energy also controls the makeup of

galaxies within clusters. The cluster environment facilitates the formation of a zoo

of galaxies such as the so-called lenticulars, giant ellipticals and dwarf ellipticals.

By regulating the ability of galaxies to join clusters, dark energy dictates the relative

abundance of these galaxy types.

Space is emptying out, leaving our Milky Way galaxy and its neighbors an

increasingly isolated island.

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This is a good story, but is it true? Galaxy mergers, black hole activity and star

formation all decline with time, and very likely they are related in some way. But

astronomers have yet to follow the full sequence of events. Ongoing surveys with

the Hubble Space Telescope, the Chandra X-ray Observatory and sensitive

ground-based imaging and spectroscopy will scrutinize these links in coming years.

One way to do this is to obtain a good census of distant active galaxies and to

determine the time when those galaxies last underwent a merger. The analysis will

require the development of new theoretical tools but should be within our grasp in

the next few years.

Striking a Balance

An accelerating universe dominated by dark energy is a natural way to

produce all the observed changes in the galaxy population--namely, the cessation of

mergers and its many corollaries, such as loss of vigorous star formation and the end

of galactic metamorphosis. If dark energy did not exist, galaxy mergers would

probably have continued for longer than they did, and today the universe would

contain many more massive galaxies with old stellar populations. Likewise, it would

have fewer lower-mass systems, and spiral galaxies such as our Milky Way would

be rare (given that spirals cannot survive the merger process). Large-scale structures

of galaxies would have been more tightly bound, and more mergers of structures

and accretion would have occurred.

Conversely, if dark energy were even stronger than it is, the universe would

have had fewer mergers and thus fewer massive galaxies and galaxy clusters. Spiral

and low-mass dwarf irregular galaxies would be more common, because fewer

galaxy mergers would have occurred throughout time, and galaxy clusters would be

much less massive or perhaps not exist at all. It is also likely that fewer stars would

have formed, and a higher fraction of our universe's baryonic mass would still be in

a gaseous state.

Although these processes may seem distant, the way galaxies form has an

influence on our own existence. Stars are needed to produce elements heavier than

lithium, which are used to build terrestrial planets and life. If lower star formation

rates meant that these elements did not form in great abundance, the universe would

not have many planets, and life itself might never have arisen. In this way, dark

energy could have had a profound effect on many different and seemingly unrelated

aspects of the universe, and perhaps even on the detailed history of our own planet.

Dark energy is by no means finished with its work. It may appear to benefit life: the

acceleration will prevent the eventual collapse that was a worry of astronomers not

so long ago. But dark energy brings other risks. At the very least, it pulls apart

distant galaxies, making them recede so fast that we lose sight of them for good.

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Space is emptying out, leaving our galaxy and its immediate neighbors an

increasingly isolated island. Galaxy clusters, galaxies and even stars drifting

through intergalactic space will eventually have a limited sphere of gravitational

influence not much larger than their own individual sizes.

Worse, dark energy might be evolving. Some models predict that if dark energy

becomes ever more dominant over time, it will rip apart gravitationally bound

objects, such as galaxy clusters and galaxies. Ultimately, planet Earth will be

stripped from the sun and shredded, along with all objects on it. Even atoms will be

destroyed. Dark energy, once cast in the shadows of matter, will have exacted its

final revenge.

From Scientific American, Jan. 2007