Cosmology Primer

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Cosmology Primer

Sean Carroll

Twentieth-century science completely overturned our view of cosmology. We now knowthat our solar system is one of many in our galaxy, and our galaxy is one of many in theuniverse. These galaxies are spread throughout space in a nearly uniform distribution,and distant galaxies are mutually moving apart from each other as the universeexpands. Over ten billion years ago the entire collection emerged from an incredibly hotand dense state: the ig ang.

!n the twenty-"rst century, new discoveries are o#ering new challenges to ourunderstanding. Ordinary matter comprises only about four percent of the stu# in theuniverse$ the rest is dark matter and dark energy. %tructures in the universe grew from

a very smooth primeval state$ the tiny deviations from perfect smoothness may havebeen caused by a period of in&ationary expansion in the very early universe. 'ewexperiments are being designed to extend our understanding further into the unknown.

This primer provides a brief introduction to these ideas, the basic picture of moderncosmology. The intended audience includes anyone with curiosity about science$ notechnical background is assumed.

Cosmology Primer: Overview

The universe is a big place, "lled with surprises, and including realms far beyondeveryday experience. To be honest, it(s a lot to contemplate in one sitting. On this page

of the )rimer, we give a *uick overview of the basic features of our universe, asunderstood by contemporary cosmologists. The pages to follow go into more detailabout the speci"c ideas mentioned here.

The +ilky Way galaxy, our home.

lick image to enlarge.

The most important features of the universe on very large scales can be summari/ed*uite concisely: it(s big, it(s smooth, and it(s getting bigger. When we look into the skyon a clear night, the "rst things we notice besides the +oon and possibly the planetsare the stars. 0ach star is an ob1ect comparable to our %un, although there is *uite arange of star types. We live in a collection of stars bound together by their mutualgravitational attraction: the +ilky Way galaxy.
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ut the +ilky Way is 1ust one of billions of galaxies in our observable universe$ the basiccharacteristics of this collection are described in the page on the luminous universe.2ortunately, on the largest scales the distribution of galaxies is very similar throughoutthe universe$ this is what is meant by the smoothness of the universe. +ost remarkably,distant galaxies are all moving away from each other, as discovered by 0dwin 3ubble in4565. This mutual moving-away is described in the page on the expanding universe.

The universe was a di#erent place early in its history. %ince the universe is expandingtoday, in the past it was much more hot and dense, as its constituents were packedmore tightly. Traced back su7ciently far, it should be too hot for atoms to exist, aselectrons are continually 1ostled away from atomic nuclei$ even earlier, nucleithemselves would be torn into individual protons and neutrons, and the universe shouldbe an incredibly hot plasma of subnuclear particles. !f we try to go all the way to thebeginning, we would reach a singular point called the ig ang. 8 timeline of importantevents in the history of the universe is provided in the page on the evolving universe.

Winding the picture forward from the ig ang, we evolve from a hot and dense plasmato the cooled-o# universe we observe today. 8long the way, there is a brief period when

the universe is about one minute old in which protons and neutrons are beingsynthesi/ed into light nuclei helium, deuterium, and lithium, along with individualprotons comprising hydrogen nuclei. 9ater, electrons combine with these nuclei intoatoms when the universe is about ;? of the universe is @ordinary matter@ the atoms of which stars, gas, dust, and planetsare made. 8bout 6>? of the universe is @dark matter,@ a new kind of particle never yetdetected in any laboratory here on 0arth. 8 full ;


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itself, or it might be something even more exotic. The pieces of evidence we haveaccumulated for dark matter and dark energy, as well as theories proposed to accountfor them and experiments ongoing to constrain them, are discussed in the page onthedark universe.

The conventional ig ang model provides an excellent "t to observations, as long as

we impose very speci"c initial conditions at early times: an expanding universe withalmost the same density at all points in space, but with small perturbations thateventually grow into galaxies today. Why did the universe start like thatA 8 possibleanswer is provided by @in&ation,@ which posits a brief period of accelerated expansion atvery early times. 8 related problem concerns the imbalance of matter and antimatter inthe universe: why are there more particles than antiparticlesA The page on the reallyearly universediscusses the idea of in&ation as well as mechanisms to explain thematterBantimatter asymmetry.

!t goes without saying that we still have a lot to learn about the universe, but scientistsare planning an exciting array of new experiments to push the frontiers of ourunderstanding forward. The page on the measured universedescribes the di#erent

kinds of experiments under development, as well as some of the most important onesalready in operation. We have every reason to believe that new discoveries about theuniverse will continue to surprise and delight us.

Cosmology Primer: The Expanding Universe

!n thinking about the expanding universe, it is tempting to appeal to some sort of simile:distant galaxies are like raisins in a baking loaf of bread, or dots drawn on the surface ofa balloon. ut the universe is a uni*ue place, and similes tend not to do it 1ustice orworse, to suggest something misleading. !t(s actually best 1ust to think about theuniverse itself, and what it looks like.

%o imagine standing outside on a clear night and looking into the sky. !magine furtherthat you have perfect vision, including not only ordinary light but all other kinds ofradiation radio waves, infrared and ultraviolet light, C-rays and gamma-rays. The "rstthing you notice are stars$ each star is much like our %un, but further away andcorrespondingly fainter. ut the stars aren(t distributed e*ually throughout the sky$ theyare arranged into a disk, and our solar system is near one edge of the disk. This disk ofstars, orbiting slowly under their mutual gravitational attraction, is our galaxy, the +ilkyWay. There are almost one trillion stars in the +ilky Way$ in the night sky it shows up asa faint band stretching from one hori/on to the other.

ut stars aren(t the only thing we see$ there are tiny patches of fu//y light, which standout in contrast to the pointlike stars. The patches are @nebulae@, and were a source of

controversy earlier in the twentieth century -- were they clouds of gas and dust withinour galaxy, or separate galaxies in their own rightA 0ventually 0dwin 3ubble showedthat many although not all of the nebulae were in fact distant systems of stars,comparable in si/e to our own +ilky Way galaxy. !n our observable universe, there areapproximately one hundred billion such galaxies. 2or more discussion, see the page ontheluminous universe.
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Dalaxy map from the %loan Eigital %ky %urvey.


osmology, as the study of the entire universe, would be a completely intractablesub1ect if it weren(t for one crucially simplifying feature: if we look over large enoughdistances, the distribution of galaxies is basically the same everywhere. !n technicalterms, we say that the universe is both @homogeneous@ the same at every point and@isotropic@ the same in every direction. Of course these statements are not strictly true$

the center of a galaxy has a higher average density than the space in between galaxies.ut as we look over larger and larger distances, the deviations from place to placebecome smaller and smaller$ once we are considering distances of hundreds of millionsof light years and more, the universe looks extremely uniform.

3owever, although galaxies are spread evenly throughout space, they are not static asa function of time$ 3ubble(s second great discovery is that the universe is expanding.

The concept of an expanding universe can be a tricky one, so it is worth being carefulabout what we mean. !t is best to think of space itself stretching, so that the amount ofspace between any two distant galaxies is increasing. The observable phenomenon thatleads us to this conclusion is the redshift: as light travels from one galaxy to another, itswavelength is stretched as the universe expands, reaching the second galaxy havingbeen shifted to the red longer wavelengths. We therefore see relatively nearbygalaxies slightly redshifted, and very distant galaxies extremely redshifted. Thecosmological redshift is similar in result although di#erent in underlying cause to thewell-known Eoppler shift that results when an ob1ect is moving away from you. !t istherefore convenient to assign a @velocity@ to this redshift$ 3ubble(s 9aw states that thisapparent velocity is directly proportional to the distance to the galaxy, with the constantof proportionality being the 3ubble constant. 8t a deep level, the expansion of space isdi#erent than the motion of ob1ects through a static space$ however, the di#erences arenegligible when the apparent velocities are much less than the speed of light, so theabuse of language is acceptable.

%ince we see distant galaxies moving away from us, it is tempting to think that we arein the center of something big. ut that(s an incorrect impression$ if we were living on

any one of the other galaxies, we would still see all the galaxies moving directly awayfrom us, as a conse*uence of the general expansion. There is no center to the universe,nor any preferred location$ it(s basically the same everywhere. 9ikewise, the universe isnot so far as we know expanding @into@ anything$ it(s 1ust that the amount of space inour single universe is growing with time.

0instein(s general theory of relativity, which states that spacetime is curved and thatcurvature is what we perceive as @gravity,@ provides a dynamical framework forunderstanding the expansion of the universe. !n cosmology, the curvature


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of spacetimecomes from two contributions: the curvature of spaceby itself, and theexpansion rate of the universe. The curvature of space is the same throughout space,and can be positive, negative, or /ero$ recent observations of the osmic +icrowaveackground indicate that it is close to /ero. Deneral relativity then relates the expansionrate and spatial curvature to the energy density of the universe -- the amount of energyin each volume of space. !f we know the spatial curvature, and we know the energy

density, and we know how the energy density changes as the universe expands, we canreconstruct the entire expansion history of the universe.

!n particular, we can extrapolate backwards from our current situation to describe thevery early universe. %ince it is expanding now, it was smaller in the past$ galaxies werecloser together, and the universe was both hotter and more dense. Doing backsu7ciently far, the universe was so hot that galaxies and stars could not exist$ furtherback, individual atoms could not exist$ further still, it was too hot for atomic nucleithemselves to exist. !f we extend ourselves fearlessly all the way back, the universe wasof essentially /ero si/e about 4.; billion years ago -- the ig ang. Of course there is alot we don(t know about this period, although there is some that we do$ see the pageson the early universeand the really early universefor details.

!f we can extrapolate into the past, we can also extrapolate into the future. The problemthere, of course, is that we have no observational data to check our speculations.%trictly speaking, therefore, we really don(t know what the far future history of theuniverse will bring. Our best current models, in which the universe is dominated by darkmatter and dark energy, predict that it is likely for the universe to continue expandingforever, becoming increasingly cold and dark as time goes by. %ee the page on the darkuniversefor more speci"cs. 3owever, our current ideas are still speculative, so it pays tokeep an open mind.

Cosmology Primer: The Evolving Universe

time = 10-43sec size = 10-30today temp = 1032KelvinThe Planck era.Fuantum gravity is important$ current theories are inade*uate. Wecan(t get any closer to the ig ang at t=0and say anything with con"dence or evenwith informed speculation.

time = 10-35sec size = 10-26today temp = 1028Kelvin

Infation.8 temporary period of domination by a form of dark energy at an ultra-highenergy scale. 8 speculative theory, but one that has so far been consistent withobservations.


Electroweak phase transition.8t high temperatures, electromagnetism is uni"edwith the weak interactions. This is the temperature at which they become distinct.

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Quark-gluon phase transition.Fuarks and gluons become bound into the protonsand neutrons we see today.

time = 10 sec size = 10-9today temp = 109Kelvin

Primordial nucleosynthesis.The universe cools to a point where protons andneutrons can combine to form light atomic nuclei, primarily 3elium, Eeuterium, and9ithium.

time = 3.7105years size = 10-3today temp = 3103Kelvin

Recombination.The universe cools to a point where electrons can combine with nucleito form atoms, and becomes transparent. Gadiation in the osmic +icrowaveackground is a snapshot of this era.

time = 108years size = 10-1today temp = 30 Kelvin

The dark ages.%mall ripples in the density of matter gradually assemble into starsand galaxies.

time = 9109years size = 510-1today temp = 6 Kelvin

Sun and Earth orm.2rom the existence of heavy elements in the %olar %ystem, weknow that the %un is a second-generation star, formed about "ve billion years ago.

time = 13.7109years size = 100today temp = 2.7 Kelvin

Today.

Cosmology Primer: The Luminous Universe

Our %un is a star, much like the other stars in the night sky except much closer to us.8stronomical distances are so immense that scientists measure them in terms of thetime it takes light to travel between ob1ects. 9ight from the %un takes about eightminutes to reach the 0arth, so we say the %un is eight light-minutes away$ this is e*ualto ninety-three million miles. The nearest star, in contrast, is over four light-years away-- almost twenty-four trillion 6HI4


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addition to the stars and planets, galaxies contain large clouds of gas and dust, often inthe process of collapsing to form new stars.

Dalaxies in the 3ubble Eeep 2ield.


Dalaxies come in many di#erent forms, depending on their age and history as well asthe distribution of stars, gas, and dust. The image on the right is part of the Eeep 2ieldimage from the 3ubble %pace Telescope, and shows a wide variety of galaxies. 8mongthe di#erent kinds of galaxies, three varieties are especially fundamental: spirals,ellipticals, and irregulars. %piral galaxies are large disks containing both stars and dustclouds, typically orbiting around a central bulge of stars. %een face-on, these galaxiesexhibit dramatic spiral arms, indicating regions where stars are being formed. 0llipticalgalaxies, in contrast, are dense with stars but have relatively little dust, and are similarto the central bulges of the spirals. !rregular galaxies do not feature the well-de"nedshapes of spirals and ellipticals, and are often smaller galaxies.

While stars orbit each other in galaxies, galaxies themselves often orbit withincollections of other galaxies. Our own +ilky Way has a number of satellite galaxies,including the +agellanic louds visible in the %outhern 3emisphere sky. There are alsolarger collections of galaxies: groups, clusters, and superclusters. The +ilky Way is amember of the 9ocal Droup, in which the other large member is the 8ndromeda Dalaxy,+4. %uperclusters are the largest gravitationally bound systems, but the distribution ofgalaxies on scales larger than superclusters is nevertheless not completely uniform$ thedeviations from perfect regularity form the large-scale structure of the universe.

The galaxy mapfrom the %loan Eigital %ky %urvey shows evidence of large-scalestructure.

%tars and galaxies are the most obvious features of the universe, at least if we areobserving it using ordinary visible light. !n an e#ort to learn as much as we can,

astronomers will often turn to other forms of light. 9ight can be thought of as either anelectromagnetic wave, or as individual particles called @photons.@ Gadiation with alonger wavelength than visible light infrared and radio waves correspond to lower-energy photons, while shorter wavelengths ultraviolet light, C-rays and gamma raysare high-energy photons.

Observations in di#erent wavelengths give evidence of a violent universe in a constantstate of &ux. !nfrared and radio observations show regions of gas and dust collapsing to
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form new stars. C-rays and gamma rays, at shorter wavelengths and thus higherfre*uencies, are formed by very high-energy processes in extreme environments. !t isfrom such observations that we infer the existence of black holes -- regions ofspacetime where the gravitational "eld is so strong that nothing entering can everescape. !n the observed universe, black holes seem to come in two ma1or types:supermassive black holes over a million times the mass of the %un at the centers of

large galaxies, and smaller black holes a few times the mass of the %un scatteredthroughout our galaxy. We suspect that these smaller black holes result fromsupernovae -- the explosions of stars at the end of their life cycles. %upernovae are rareperhaps one per galaxy per century, but by observing thousands of galaxies at oncewe are guaranteed to discover a respectable number on demand$ surveys of 1ust thissort have been crucial in making the case for dark energy, as explained in the page onthedark universe.

+ost of the photons in the universe are actually not emitted by stars, gas, or any otherob1ect in the contemporary universe. Gather, they are the low-energy photons left overfrom the ig ang, as described in the page on the early universe. This relic radiation isprimarily in the microwave region of the spectrum, and is known as the osmic

+icrowave ackground +. )hotons from the + pervade space, providing abackground bu// that serves as a constant reminder of the hot, dense state in which ouruniverse began.

%ince ancient times, almost everything we have learned about the universe has comethrough observing light of various forms. +odern astronomers, however, areincreasingly taking advantage of other ways to probe the universe. !n addition toordinary photons, numerous other kinds of particles are constantly bombarding the0arth: protons and atomic nuclei that make up cosmic rays, ultra-light neutral particlesknown as neutrinos, and the gravitational e*uivalent of electromagnetic waves, knownsimply as gravitational waves. osmic rays have proven to be an invaluable windowonto energetic processes in the universe, leading to important insights into particlephysics such as the discovery of the muon, a heavier cousin of the electron$ today, the

origin of the highest-energy cosmic rays remains a deep mystery. 'eutrinos fromastrophysical sources have likewise led to signi"cant discoveries$ it was the shortage ofneutrinos emitted from the %un which gave the "rst clue that these particles might havesmall masses, rather than being strictly massless. Dravitational waves have never beendetected directly, although new observatories are coming online with the goal of doingexactly that$ however, the e#ect of gravitational waves has been observed in the loss ofenergy from orbiting neutron stars, for which 3ulse and Taylor were awarded the 'obel)ri/e.

Cosmology Primer: The Dark Universe

Our knowledge of the universe comes from looking at it in various ways -- di#erentwavelengths of light, neutrinos, cosmic rays, and hopefully some day gravitationalradiation. ut how do we know that we are seeing everything there isA 3ow could wedetermine whether there were substances in the universe not directly visible in ourtelescopesA

The answer lies with gravity. 9ong ago, Dalileo determined that every ob1ect falls thesame way in a gravitational "eld. 9ater, 0instein extended this idea: every substancewith any kind of energy will create a gravitational "eld. 'ote that mass is a kind of
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energy, since E=mc2. %o we can, in principle, detect everything in the universe, bymapping out the gravitational "eld throughout spacetime. To our surprise, there is muchmore out there than we can see directly. The invisible stu# comes in two forms: darkmatter and dark energy.

Eark matter is some kind of particle that we have not yet detected in experiments here

on 0arth, but nevertheless comprises most of the matter in the universe. The "rstevidence for its existence came from studying the dynamics of galaxies and clusters ofgalaxies. The basic point is that something in orbit around a massive ob1ect moves morerapidly, the more mass the ob1ect has. 2rit/ Jwicky was the "rst to put this idea intoaction, studying the motion of galaxies in the oma cluster$ their motions were toorapid to be accounted for by the visible matter in the galaxies. 9ater, Kera Gubin lookedat matter orbiting at the edges of individual galaxies, and noticed an similar e#ect -- therotation speeds of the galaxies did not fall o# with distance as they should if thegravitational "elds were being caused by the visible matter alone.

These discrepant motions, and more modern methods con"rming this behavior withgreater precision, are strong evidence for unseen matter in galaxies and clusters. Of

course, it is natural to imagine that the extra matter is *uite ordinary, but simplyinvisible and transparent much like air on a clear day. 8nd indeed, observations indi#erent wavelengths have provided evidence for previously unseen gas in galaxies andclusters. 3owever, we have very good evidence that the substantial ma1ority of darkmatter is something exotic, rather than ordinary matter that we can(t see. This evidencecomes from two independent sources -- the abundance of light elements fromprimordial nucleosynthesis, and temperature anisotropies in the cosmic microwavebackground. oth phenomena are described in the page on the early universe. Theresult is that the abundance of exotic dark matter, some kind of particle that has neverbeen directly observed here on 0arth, is about "ve times the abundance of ordinarymatter in the universe.

What we know about dark matter, then, is that it is a new kind of unseen particle thatfalls readily into galaxies and clusters. The fact that it is dark means that it is neutral,rather than electrically charged$ in general, charged particles interact readily with light,and would not be dark. The fact that the dark matter is concentrated in galaxies andclusters indicates that it is slowly moving$ particles of this form are referred to as @cold.@What could the cold dark matter be made ofA We are not at all sure, although numeroustheories have been proposed. Two ideas are especially popular: neutralinos and axions.@'eutralinos@ are a kind of particle predicted by supersymmetry, a popular but as yetpurely con1ectural theory in particle physics. 8ccording to supersymmetry, each kind ofknown particle has a @superpartner@ with a di#erent intrinsic spin$ the neutralino issimply a massive, neutral, stable superpartner of one of the known particles, an is anatural dark matter candidate. %uch a particle would interact predominantly throughthe weak nuclear force, and is therefore an example of a Weakly !nteracting +assive

)article, or W!+). 8xions, on the other hand, are another kind of hypothetical particle,originally postulated to explain certain symmetries of the strong nuclear force. !n thecase of both neutralinos and axions, active experimental programs are underway todetect these dark matter candidates in the laboratory, either by producing them directlyor by observing the e#ects of ambient particles &oating through the %olar %ystem.2inally, it may be possible to detect dark matter particles indirectly, if they annihilateinto photons in high-density regions of the universe$ the resulting radiation would havea characteristic form that would signal the existence of a new kind of particle.
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Of course, there is a kind of known particle which is neutral, stable, and as we nowknow massive -- the neutrino. 'eutrinos are abundant in the universe, approximatelyas abundant as photons. 3owever, they are not good dark matter candidates, becausethey are not very cold. 0ven if neutrinos today are moving relatively slowly, in the earlyuniverse they were moving near the speed of light. 8s a result, they would stream freelyaway from galaxies and clusters, rather than settling into them as dark matter is

observed to do. !ndeed, a useful way to put an upper limit on the mass of the neutrino isto insist that it not comprise such a signi"cant fraction of the mass of the universe thatit would interfere with the evolution of large-scale structure.

8 supernova at the edge of a distant galaxy.


8s if dark matter weren(t exotic enough, dark energy is even more mysterious. We knowvery little about dark energy, other than two characteristic features: it is spreaduniformly throughout space, and maintains an approximately constant density as theuniverse expands. !t must be nearly uniform throughout space, since otherwise itwould clump into galaxies and clusters and a#ect local motions 1ust as dark matterdoes. !nstead, the dark energy only a#ects the overall curvature of spacetime. 3ow,then, do we know that dark energy existsA !ts a#ects on spacetime curvature show up intwo ways -- it makes the universe accelerate, and it contributes along with matter tothe curvature of space alone.

8ccording to 0instein, the rate of expansion depends on the average energy density ofthe universe. To measure the expansion rate, we use standard candles, ob1ects whoseintrinsic brightness is known. The further away a standard candle is, the dimmer it willappear, allowing us to accurately determine its distance. We want standard candles thatare very bright, so they can be observed at cosmological distances$ good candidates areprovided by supernova explosions, which can rival the entire light output of the galaxythey are in. !t turns out that supernovae come in various forms, of di#erent brightness$

but a certain kind, the Type !a supernovae, have a brightness which depends directly onhow fast they explode and fade away. Observations of Type !a supernovae at highredshifts by two groups the %upernova osmology )ro1ect and the 3igh-Gedshift%upernova Team in 455L provided the "rst direct evidence that the universe isaccelerating rather than slowing down.

Why does dark energy make the universe accelerateA ecause, unlike matter andradiation, it does not dilute away as the universe expands -- the density of dark energyremains close to constant. Therefore, according to 0instein, the 3ubble parameter


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remains close to constant. ut remember that the apparent velocity of a galaxy is givenby the 3ubble parameter times the distance, as explained in the page on the expandinguniverse. %ince the distance to any given galaxy is increasing, a nearly-constant 3ubbleparameter implies that the apparent velocity will also be increasing -- in other words,the galaxies are accelerating away from us.

The other piece of evidence for dark energy comes from the curvature of space, asmeasured through temperature &uctuations of the osmic +icrowave ackground. 8sdescribed in the page on the expanding universe, the curvature of spacetime can bethought of as a combination of the curvature of space, and the expansion of spacethrough time. Through observations of temperature &uctuations in the + describedin the page on the early universe, we can measure the overall curvature of space, and"nd that it is close to /ero. +eanwhile, the total amount of matter in the universe bothordinary and dark falls well short of what is needed to explain the &atness of space. !tturns out that the amount of dark energy re*uired to explain the acceleration of theuniverse is 1ust right to explain the fact that space is &at. We therefore seem to have acomplete inventory of the constituents of our contemporary universe:

"ve percent ordinary matter,

twenty-"ve percent dark matter,

seventy percent dark energy.

The completion of this inventory is one of the most impressive successes of moderncosmology.

We don(t know what the dark energy is. %ince observations constrain its density to beclose to uniform throughout space and approximately constant in time, the simplestcandidate would be something that is exactlyconstant in space and time. %uch a

substance is called vacuum energy -- an energy density that is inherent in empty spaceitself, unchanging from point to point in the universe. ut once we admit the possibilityof vacuum energy, we can go back and estimate how large such energy should be. Theresult, according to our best understanding of *uantum "eld theory, is 4< 46


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relativity on cosmological scales. 8ll of these possibilities are in play, and futureobservations will help us decide between them once and for all.

Cosmology Primer: The Early Universe

Diven our understanding of the current state of the universe, and our knowledge of theappropriate laws of physics, we can extrapolate backwards in time to say what the earlyuniverse must have been like. 2ortunately, we can then use current observations to testwhether such an extrapolation is valid$ the answer is that it is remarkably accurate.

Our improving view of the cosmic microwave background, from 4556 to 6


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radiation from the ig ang "lls all points of space, no matter where we are in theuniverse$ but since we can only observe it from our location, we perceive the thechanges from place to place as pro1ected onto the sphere of the sky.

The smooth, slightly perturbed early universe visible in the + anisotropies grew intothe lumpier universe of stars and galaxies we see today. This should come as no

surprise. The hot and cold spots of the + correspond to regions of slightly higher orlower density than average. !n the regions that were overdense, the pull of gravitybrought matter closer together, further emptying out the regions that were less dense.

The evolution of the universe under the in&uence of gravity thus acts to increase thecontrast of the matter distribution, from a nearly featureless plasma to an intricatecollection of galaxies. This process takes longer over larger distances, which is why theuniverse remains approximately smooth on very large scales.

There is a treasure trove of information contained in the + &uctuations. !n particular,statistical properties of the &uctuations depend on two things: the original primordialperturbations from which they arose, and the recipe of ingredients in our universe thatcontrols the subse*uent evolution of the perturbations between early times and now.

Gemarkably, an extremely simple speci"cation of primordial perturbations works verywell -- simply imagining that the perturbations are on average of e*ual strength at alldistance scales. 2rom this guess, and the observed &uctuations in the + sky, we canderive very tight constraints on interesting cosmological parameters. !n particular, the+ provides independent support for the ideas that there is more matter in theuniverse than can be accounted for by ordinary atoms thus implying the need for darkmatter, and that there is more total energy than be accounted for by matter alongthus implying the need for dark energy.

The + provides a valuable picture of what the universe was like when it was ;


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sensitively on two things -- the amount of ordinary matter in the form of protons andneutrons, and the expansion rate of the universe when it was 1ust a few seconds old. Weobtain perfect agreement with observations if two things are true -- the amount ofordinary matter is much less than the total amount of matter we deduce in the currentuniverse thus providing evidence for dark matter, and the expansion rate is exactly aspredicted by 0instein(s theory of general relativity thus assuring us that our best

theories can be safely extrapolated back to early times.

The agreement between the predictions of the ig-ang cosmological model and ourobservations of primordial light-element abundances was by no means guaranteed -- itis certainly conceivable that the observed amounts of deuterium, helium and lithiumcould not be explained by conventional general relativity for any value of the matterdensity. The fact that we do get good agreement, indicating that we understand thebehavior of the universe when it was only a few seconds old, is one of the mostprofound achievements of modern science.

Cosmology Primer: The Really Early Universe

We have no direct knowledge of what the universe was like before the ig-angnucleosynthesis era, when the universe was between a few seconds and a few minutesold as described in the page on the early universe. !t is worth emphasi/ing that anyideas we have about earlier times are only that -- ideas. 'evertheless, 1ust as we cansuccessfully extrapolate the laws of physics from the present day back to the time ofnucleosynthesis, we may also extrapolate these laws even further back, to construct apicture of what the very early universe may have been like.

!n a universe dominated by matter and radiation as opposed to dark energy, themutual gravitational pull of all the particles tends to slow down the expansion rate asthe universe expands. When the universe was smaller and more dense, it thereforefollows that the expansion rate was much larger than it is today. !ndeed, as we

extrapolate the universe further back in time, we reach a point where the density,temperature, and expansion rate were all in"nitely large. This point is a singularity,which we refer to as the ig ang although that term is also used for the entirecosmological model that includes the later universe as well. 8t the ig ang, ourknowledge of what happens gives out$ the fact that physical *uantities become in"niteis a sign that we don(t know what is going on. )resumably, in the real world there is nosingularity$ instead, something happens that cannot be described by physics as wecurrently understand it.

Nust because we don(t understand the ig ang itself doesn(t mean we can(t usefullytalk about the period immediately afterwards, when the universe was in a hot, dense,rapidly expanding state. !n the absence of a sensible theory of the origin of the

universe, cosmologists ask what initial conditions are necessary to explain the observedfeatures of our universe today. ut in fact we want more than that$ we would like tobelieve that these initial conditions are somehow natural, rather than arbitrarily "nely-tuned. This desire may or may not be accommodated by reality, but has led to a greatdeal of interesting speculation about the very early universe.

One pu//le we have about the universe is the apparent dominance of matter overantimatter. 0very type of elementary particle electrons, protons, neutrons, and so onhas a corresponding type of antiparticle positrons, antiprotons, antineutrons of e*ual
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mass and opposite electric charge. ut what we observe in the universe isoverwhelmingly matter and not antimatter, which we know because matter andantimatter tend to explosively annihilate when they come into contact with each other.!f other galaxies, for example, were made of antimatter, there would be regions inbetween where particles would intermix, giving rise to high-energy radiation that hasnot been detected. !t is possible that this asymmetry between matter and antimatter is

simply a feature of the initial conditions of the universe, but it would seem moresatisfying if we could explain how it arose dynamically as the universe evolved. %uch ahypothetical process is known as @baryogenesis,@ since the observed imbalancebetween matter and antimatter is actually an imbalance between baryons protons andneutrons and antibaryons. There are numerous models of baryogenesis, many of whichmay be testable at upcoming particle accelerators$ to date, however, no single modelhas proven so successful that it has been accepted as a standard picture.

8nother unusual feature of our universe is its smoothness -- the distribution of galaxiesis uniform on large scales, and the microwave background provides strong evidencethat matter was even more smoothly distributed at earlier times. This uniformity shouldstrike us as unusual, since small deviations tend to grow with time, as overdense

regions collapse to form stars and galaxies. ut the "nite speed of light makes thesituation even more surprising. The + shows us what the universe was like ;


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an expanding universe. Thus, while in&ation does its best to make the universeabsolutely uniform, *uantum mechanics prohibits it from doing so$ there is always asmall amount of &uctuation in the amount of energy from place to place that no amountof in&ation can erase. !ndeed, we can use the rules of *uantum mechanics to predictwhat kinds of &uctuations should arise from in&ation. The result is a set of perturbationsof approximately e*ual strength at all distance scales. 8s mentioned in the page on

theearly universe, these are precisely the kind of &uctuations needed to explain whatwe observe in anisotropies of the +. This doesn(t mean that in&ation is necessarilycorrect, but certainly provides some evidence in its favor.

ut there remains a great deal that we don(t understand about in&ation. !n particular,while the general framework remains attractive, no speci"c model of in&ation hasbecome popular. !n other words, we don(t know exactly what this mysterious darkenergy was that dominated the universe at very early times, nor how it converted intoordinary matter and radiation. 8 possible clue could come from another prediction ofin&ation: gravitational waves. Nust as *uantum mechanics predicts irreducible&uctuations in the density of matter during in&ation, it also predicts &uctuations in thegravitational "eld, which manifest themselves as gravitational waves. These waves can

lead to a speci"c unmistakable signature in the polari/ation of the microwavebackground. nfortunately, we don(t know for sure how strong these gravitationalwaves will be, and they might be so weak as to be undetectable. ut cosmologists areplanning experiments to look for them, and if they are detected it will be a greattriumph for in&ation.

!n a sense, in&ation hides from view anything that came before it. 'evertheless, we arestill curious about the very origin of the universe, and the conditions that gave rise toin&ation if indeed it happened. )resumably any sensible description of this epoch willinvolve *uantum gravity the long sought-after reconciliation of *uantum mechanicswith 0instein(s general relativity, and perhaps re*uire an understanding of moreesoteric physics such as superstring theory.

Cosmology Primer: The Measured Universe

8stronomy, arguably the oldest science, has traditionally depended on observations ofphenomena in the sky. The observational nature of the discipline distinguishes it fromexperimental sciences like physics or chemistry, in which we can control the conditionsunder which we measure the systems of interest, gradually altering con"gurations andrepeating the experiments as necessary. When it comes to phenomena outside our%olar %ystem, we have to take what the universe gives us. The di#erence betweenexperimental and observational science is much like the di#erence between askingsomeone *uestions versus eavesdropping on their conversations -- asking our own*uestions allows us to be precise and ask for clari"cation, but eavesdropping can allowus to learn secrets that people would never have revealed under direct interrogation.

!n looking out at the universe, our most straightforward tool is ordinary light, the samemessenger that allowed our ancestors to chart the stars and planets. We now know, ofcourse, that visible light is 1ust one form of electromagnetic radiation. Other formsstretch from very long wavelengths infrared light and radio waves, through visiblelight, down to very short wavelengths ultraviolet light, C-rays, and gamma-rays. 8ll ofthese wavelengths of light are emitted by ob1ects in the universe, and astrophysicists
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take advantage of all of them to get as complete a picture of the universe as possible,as discussed in the page on the luminous universe.

Dalileo, the "rst person to observe the sky through a telescope, demonstrated theexistence of ob1ects such as the moons of Nupiter that would never have been found bythe naked eye, and we continue to follow in his footsteps. The basic principle of a

telescope working in ordinary visible or infrared light is simply to collect a largenumber of photons light particles and focus them to a detector. 0ach individual photonis distinguished by three characteristics: its fre*uency or e*uivalently wavelength, orenergy, the direction from which it arrives, and the time at which it is observed.Ei#erent detectors may keep track of some or all of this information$ studying thedistribution of photon fre*uencies is spectroscopy, the precise directions from which thedi#erent photons arrive is of course imaging, while studying the amount of light arrivingas a function of time isphotometry. +odern telescopes strive for better views of theuniverse both by increasing the si/e of their light-collecting area -- typically with largemirrors -- and by being located in places where interference from the 0arth(satmosphere is minimi/ed -- either in dry, high-altitude climates, or somewhere in orbitoutside the atmosphere entirely.

Telescopes using visible light continue to produce startling discoveries. The %loan Eigital%ky %urvey %E%%, for example, makes use of a dedicated telescope at 8pache )oint in8ri/ona to survey a large area of the deep sky for galaxies and *uasars as well asordinary stars. The %E%% "rst images the sky, and then does spectroscopy on the mostinteresting ob1ects found so as to determine their cosmological redshift as explained inthe page on the expanding universe. %ince redshift is proportional to distanceaccording to 3ubble(s 9aw, the result is a three-dimensionalmapof the large-scalestructure of the universe. oth in the %E%% and in future surveys, an important goal willbe to study large-scale images of galaxies to search for the slight distortions of imagesdue to the gravitational lensing of background ob1ects by galaxies in the foreground.

!n a complementary vein, a satellite such as the 3ubble %pace Telescope 3%T can peerextremely deeply into one region of the sky to reveal galaxies in the early stages oftheir evolution. The great advantage of being in orbit is not that you observe morephotons, but rather that the absence of atmospheric distortion allows for the collectionof more precise and detailed images. %uch improved resolution can be crucial, forexample, in the study of distantsupernovae,where it is important to distinguish thesupernova event from the light of the surrounding galaxy. 8 successor to the 3%T, the

Names Webb %pace Telescope, is currently under development by '8%8$ it will be able tolook even deeper into the universe, to help us understand how early galaxies wereassembled from relatively smooth distributions of gas and dark matter.

8t longer wavelengths, radio telescopes have become an indispensable part ofobservational astronomy. %everal 'obel pri/es have gone to discoveries made with radio

telescopes, from the discovery of pulsars rapidly rotating neutron stars to the "rstobservations of the microwave background itself. 8long with visible light, the radio bandis one in which radiation can readily penetrate the 0arth(s atmosphere, so ground-basedtelescopes are extremely valuable. 'evertheless, 1ust as with visible light, there is muchto be gained by minimi/ing interference with the atmosphere. !t is therefore common toplace radio telescopes in locations where the atmosphere is both thin and stable$popular choices include the high plains of hile, and the extreme cold of the %outh )ole.+ore dramatic methods of minimi/ing atmospheric interference include placing radiotelescopes on long-duration balloon &ights, or simply putting them on satellites
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completely above the atmosphere. These techni*ues are especially useful whenobserving the microwave background$ the + is at such a low temperature that tinyamounts of contamination from the 0arth itself can be troublesome to experiments. The)lanck satellite, a planned collaboration between '8%8 and the 0uropean %pace8gency, will hopefully provide maps of the + of even higher precision than thoseproduced to date. 8nother goal is to obtain high-precision measurements of the

polari/ation of the microwave background$ as explained in the page on the really earlyuniverse, such observations may provide crucial clues to the nature of in&ation.

!n contrast to radio waves and visible light, higher-fre*uency waves of ultraviolet lightand C-rays are unable to penetrate through the atmosphere, and it is necessary to gointo orbit to observe them directly. '8%8(s handra satellite has provided anunprecedented view of the C-ray sky$ future missions are planned that will inventoryblack holes throughout the universe, map out hot gas in clusters of galaxies, and imagethe innermost regions close to black holes themselves. Once we consider extremelyhigh-energy gamma rays, however, the number of photons emitted by interestingsources becomes very small. The planned D98%T satellite will feature a large areatelescope to collect as many gamma-ray photons as possible. 8lternatively, we can take

advantage of the fact that high-energy gamma rays give o# a kind of secondaryerenkov radiation when they collide with the atmosphere. This radiation can bedetected, and from that we can reconstruct information about the original gamma rays$this techni*ue has been successfully used at the Whipple observatory in 8ri/ona, and isthe basis for its successor, the K0G!T8% observatory.

8s mentioned in the page on the luminous universe, photons are not the only means bywhich we can observe the universe. Other kinds of particles which we are able toobserve include cosmic rays and neutrinos, with very di#erent techni*ues applicable ineither case. osmic rays, which are thought to be individual protons or atomic nucleithat have been accelerated to tremendous energies, are similar to very high-energygamma rays, in that the number of particles is extremely low. ut because the energy ofeach particle is so large, it is again possible to observe secondary e#ects when the

cosmic rays interact with the atmosphere. The )ierre 8uger observatory, which aims atunderstanding cosmic rays at the very highest energies, uses two complementarytechni*ues: direct detection of erenkov radiation in an array of detectors on theground, and observations of &ashes in the atmosphere caused when secondary particlescause nitrogen to &uoresce.

'eutrinos are also observed using several di#erent techni*ues. One type of detectoruses the fact that a neutrino can interact with an atomic nucleus to turn it into adi#erent element entirely, such as chlorine being converted to argon. !t was a detectorof this type at the 3omestake mine in %outh Eakota that "rst provided evidence for ananomalously low &ux of neutrinos from the %un. 8lternatively, a neutrino can knock anelectron out of an atom, and the electron in turn can travel through a medium such as

water and give o# detectable erenkov radiation. This techni*ue is only sensitive tohigher-energy neutrinos, but has the advantage of providing information about thedirection and timing of the event$ the %uper-=amiokande observatory in Napan used thistechni*ue to detect neutrinos from %upernova 45L;a in the 9arge +agellanic loud. 8nadvanced version of this techni*ue, used by the %udbury 'eutrino Observatory, usesheavy water in which ordinary hydrogen is replaced by deuterium and is sensitive tothe dissociation of the deuterium nucleus by a high-energy neutrino. %imilar techni*ueson a very large scale look for neutrinos passing through large bodies of water or throughthe 8ntarctic ice sheets, such as the planned !ce ube facility.
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+eanwhile, physicists continue to work on ways to detect particles and waves that havenever been directly observed in the laboratory. 8n obvious example is provided bygravitational radiation, which is emitted by rapidly-accelerated masses such as veryclose compact binary stars or ob1ects falling into black holes. The 9!DO observatory,currently in the early stages of collecting data, consists of facilities in 9ouisiana andWashington, each with two evacuated arms four kilometers in length, arranged at right

angles. 9aser light is bounced o# of mirrors suspended at the ends of each arm, andsensitive detectors search for tiny displacements due to the stretching of spacetimecharacteristic of gravitational waves. 8 future pro1ect is the 9!%8 observatory, similar inspirit to the ground-based observatories but consisting of three satellites &ying information at distances of "ve million kilometers from each other. 9ike the )lanck missionto observe the +, 9!%8 is a 1oint e#ort between '8%8 and the 0uropean %pace8gency.

!t goes without saying that we would like to directly observe the mysterious darkconstituents of the universe: dark matter and dark energy. nfortunately there aremany di#erent models for what these might be, and correspondingly many ways wemight try to detect them and the very real possibility that they might never be directly

detectable. 8s discussed in the page on the dark universe, the leading candidates fordark matter are either supersymmetric neutralinos or axions$ in both cases there areactive programs underway to detect such particles directly. 2or neutralinos, aside from"nding evidence for supersymmetry at high-energy particle accelerators, the best bet isto build extremely sensitive devices that would register each time a passing neutralinoscattered o# a nucleus in the detector -- a very rare event indeed. To shield as best asone can from sources of external noise, such detectors are typically built deepunderground, so that the dirt above serves as a barrier to unwanted particles such ascosmic rays. 8xions, meanwhile, can be converted to photons in a precisely-tunedmagnetic "eld, and experiments are underway to search for such an e#ect.nfortunately, the magnetic "eld must be tuned in a way that depends on the mass ofthe axion, which is one of the thing we don(t know with great precision.

2or dark energy, meanwhile, there are three main possibilities: it is a strictly constantvacuum energy, a slowly-varying dynamical component, or a breakdown of generalrelativity on cosmological scales. !f the dark energy is simply vacuum energy, there isno way to detect it directly$ the best we can hope for is to attain a better understandingof why it has the value it does, perhaps through hints provided by particle physics. !f itis dynamical, however, it may be possible to detect long-range forces due to gradualvariations in the dark energy. 2inally, if gravity is to blame, we may be fortunate enoughto detect a breakdown of general relativity in the %olar %ystem due to the same e#ectsthat are making it break down in cosmology$ unfortunately, the speci"c e#ects wemight see will depend on the details, which are far from clear at this point.

Cosmology Primer: Frequently Asked Questions'ote: this primer has been translated into elorussian.

What is the universe expanding intoA

8re distant galaxies moving faster than the speed of lightA Wouldn(t that violate

relativityA
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Eoes the universe have a centerA

ould we detect the expansion of the universe by trying to measure the

expansion of the solar systemA

!s the universe "nite or in"niteA Will it recollapse or expand foreverA

!s space &at or curvedA !(ve heard both.

!s energy conserved in an expanding universeA

What is the di#erence between dark matter and dark energyA

Will we ever be able to detect dark matter or dark energy directlyA

!sn(t @dark energy@ 1ust like the older concept of the @ether@A

3ow do you know that dark matter isn(t 1ust ordinary matter that we can(t seeA

ould the inferred existence of dark matter and dark energy be due to a modi"ed

behavior of gravityA

!s in&ation testableA

What came before the ig angA

!s our universe the only one, or are there othersA

!hat is the uni"erse e#panding into$8s far as we know, the universe isn(t expanding @into@ anything. When we say theuniverse is expanding, we have a very precise operational concept in mind: the amountof space in between distant galaxies is growing. !ndividual galaxies are not growing, asthey are bound together by gravity. ut the universe is all there is again, as far as weknow, so there(s nothing outside into which it could be expanding. This is hard tovisuali/e, since we are used to thinking of ob1ects as being located somewhere in space$but the universe includes all of space.

%re distant gala#ies mo"ing aster than the speed o light$ !ouldn&t that

"iolate relati"ity$8 profound feature of relativity is that two ob1ects passing by each other cannot have arelative velocity greater than the speed of light. 8n even more profound feature, onewhich has received much less publicity, is that the concept of @relative velocity@ doesnot even make sense unless the ob1ects are very close to each other. !n 0instein(sgeneral theory of relativity which describes gravity as the curvature of spacetime,there is no way to de"ne the velocity between two widely-separated ob1ects in anystrictly correct sense. The @velocity@ that cosmologists speak of between distantgalaxies is really 1ust a shorthand for the expansion of the universe$ it(s not that the
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galaxies are moving, it(s that the space between them is expanding. !f the distance isn(ttoo great, this expansion looks and feels 1ust like a recession velocity, but when thedistance becomes very large that resemblance breaks down. !n particular, it(s perfectlyplausible to have distant galaxies whose @recession velocity@ is greater than the speedof light. We couldn(t see such galaxies directly, since light from them would neverreach us, but that doesn(t mean they aren(t there. The resolution to this paradox is

simply that we have taken a convenient analogy too far, and there isn(t a well-de"ned@speed@ between us and distant ob1ects.

'oes the uni"erse ha"e a center$'o. Our observable universe looks basically the same from the point of view of anyobserver. We see galaxies moving away from us in all directions, but an astronomerliving in any one of those galaxies would also see all the galaxies including our ownmoving away from them. !n particular, the ig ang is not an explosion that happenedat some particular point in space$ according to the ig ang model, the entire universecame into existence expanding at every point all at once.

(ould we detect the e#pansion o the uni"erse by trying to measure the

e#pansion o the solar system$'o. 8ny system that is bound together by internal forces -- whether it is a table, thesolar system, or the galaxy -- does not expand along with the universe. 'ot 1ust that itonly expands slightly$ it really doesn(t expand at all, or at least not because of theexpansion of the universe. To observe the expansion, we need to study ob1ects that arevery distant, not directly bound to us by gravity or anything else.

Is the uni"erse )nite or in)nite$ !ill it recollapse or e#pand ore"er$We don(t really know in either case. %ince the ig ang happened a "nite time agoabout 4H billion years, and since light travels at a "nite speed, there is an unbreakableupper limit to how far away we can see in the universe. p to the limits of theobservable universe, what we observe is consistent with a uniform distribution of matterand energy that could easily extend forever. On the other hand, it might eventually turninto something very di#erent, beyond what we can see$ indeed, this might arisenaturally as a result of in&ation see the really early universe. %imilarly, we canstraightforwardly extrapolate the current evolution of our universe, dominated by darkenergy, to predict a future in which the universe continues to expand for all time seethedark universe. 3owever, the dark energy might someday change its character intosomething di#erent, in which case the universe might very well collapse. %o, given howlittle we currently understand about the nature of dark energy, we can(t say anything forsure about the ultimate fate of our universe.

Is space fat or cur"ed$ I&"e heard both.There is an important distinction between @space@ and @spacetime,@ and also adistinction between exact statements and useful approximations. Our universe is a four-

dimensional spacetime -- to describe the location of an event, you need to specify threecoordinates of space and one of time. 8ccording to 0instein, spacetime can be curved,and gravitation is the manifestation of that spacetime curvature. %ince there is certainlygravity in the universe, there is no *uestion that the universe is curved. ut forcosmological purposes it is useful to model spacetime as a three-dimensional spaceexpanding as a function of time$ then the total curvature is a combination of thecurvature of space by itself, plus the expansion of the universe. Observations indicatethat space by itself is very nearly &at, rather than having an overall positive or negativecurvature see the expanding universe$ that is the origin of the statement that we live
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in a @&at universe.@ Of course this is only an approximation, since the real world featuresgalaxies and voids in large-scale structure, rather than perfect smoothness$ but it(s agood approximation. %o @space@ is approximately &at, while @spacetime@ is de"nitelycurved.

Is energy conser"ed in an e#panding uni"erse$

This is a tricky *uestion, depending on what you mean by @energy.@ sually we ascribeenergy to the di#erent components of the universe radiation, matter, dark energy, notincluding gravity itself. !n that case the total energy, given by adding up the energydensity in each component, is certainly not conserved. The most dramatic exampleoccurs with dark energy -- the energy density energy per unit volume remainsapproximately constant, while the volume increases as the universe expands, so thetotal energy increases. ut even ordinary radiation exhibits similar behavior$ thenumber of photons remains constant, while each individual photon loses energy as itredshifts, so the total energy in radiation decreases. 8 decrease in energy is 1ust asmuch a violation of energy conservation as an increase would be. !n a sense, theenergy in @stu#@ is being transferred to the energy of the gravitational "eld, asmanifested in the expansion of the universe. ut there is no exact de"nition of @the

energy of the gravitational "eld,@ so this explanation is imperfect. 'evertheless,although energy is not really conserved in an expanding universe, there is a very strictrule that is obeyed by the total energy, which reduces to perfect conservation when theexpansion rate goes to /ero$ the expansion changes the rules, but that doesn(t meanthat anything goes.

!hat is the di*erence between dark matter and dark energy$Eark matter behaves much like a collection of ordinary matter made of particles, exceptthat it(s dark. !n particular, dense regions of dark matter tend to become even moredense, as the mutual gravitational force of the matter pulls it together. 2or this reason,we suspect that the dark matter is some sort of new, massive particle, 1ust one wehaven(t yet discovered in the laboratory yet. Eark energy, on the other hand, doesn(tact anything like particles: it doesn(t cluster together, nor does it dilute as the universe

expands. !ts density remains constant so far as we can tell throughout space and time.%o whatever the dark energy is, it(s something di#erent than dark matter.

!ill we e"er be able to detect dark matter or dark energy directly$3opefully. Ei#erent candidates for what the dark matter particles are lead to di#erentstrategies for detecting them, either directly in laboratories here on 0arth or indirectlythrough high-energy particles from space. ut numerous e#orts are being undertaken,and we might "nd dark matter in the near future. Eark energy is an even longer shot$ ifit is a strictly constant vacuum energy, we could never detect it directly, while adynamical "eld could conceivably be detected. )robably we will have to contentourselves with understanding dark energy indirectly, through its gravitational e#ects onthe expansion of the universe. %ee the page on thedark universe.

Isn&t +dark energy+ ,ust like the older concept o the +ether+$'o$ in fact, it(s 1ust the opposite. The ether was supposed to be an invisible substancethat determined the rest frame of the universe. !t was expected by theorists, buteventually abandoned when experimenters could not "nd any evidence for it and0instein "gured out that it wasn(t necessary. Eark energy, meanwhile, was not at allexpected by most working cosmologists$ we need it to explain observed facts, like theacceleration of the universe and the mismatch between matter and total energy. 8nd
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the dark energy appears the same to all observers, so there(s no sense in which itdetermines a rest frame.

ow do you know that dark matter isn&t ,ust ordinary matter that we can&tsee$We can measure the total amount of matter through the gravitational "eld it creates,

both in galaxies and in clusters of galaxies. ut we can separately measure the amountof ordinary matter by less direct means. The traditional method is to study theabundances of light elements hydrogen, deuterium, helium, and lithium in the earlyuniverse. These elements are produced by primordial nucleosynthesis see the earlyuniverse, and the amount of ordinary matter in the universe directly a#ects the relativeamounts of di#erent elements that we predict. Observations of these elements areconsistent with ordinary matter comprising only >? of the total energy of the universe,whereas the total amount of matter is closer to


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also a @singular@ point, at which our theories break down. !t is possible that some futurereconciliation of general relativity with *uantum mechanics will help us understand theorigin of the ig ang, 1ust as it is possible that we may come to believe that theuniverse had an interesting history even before what we now call the ang. othpossibilities are being actively pursued by cosmologists.

Is our uni"erse the only one/ or are there others$3opefully you won(t be disappointed if we say that we don(t know. There are di#erentkinds of @other universes@ that one could reasonably imagine -- other regions of spacethat are very far away and look very di#erent, or regions that are separated from ourown by extra dimension of space, or di#erent branches of the *uantum-mechanicalwavefunction of the universe. These are all profound ideas which we won(t discuss indetail here. %u7ce it to say that these kinds of other universes are perfectly plausible,and are sometimes even predicted by ambitious theories of fundamental physics.3owever, it is hard to see how we could test their existence experimentally. %o we don(tknow one way or the other, but speculations along these lines play an important role inthe attempt to construct a uni"ed framework of physics and cosmology$ perhaps in thefuture we will be able to be more de"nite.

Geturn to )rimer 3ome
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Cosmology Primer