the fifth element, by scott tremaine -

8/14/2019 The Fifth Element, by Scott Tremaine - WWW.OLOSCIENCE.COM

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The fifth element: astronomicalevidence for black holes, dark matter,

and dark energy


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A brief history of astrophysics

Greek philosophy contained five classicalelements: earth

air fire

water

ether

terrestrial; subject to

change

heavenly; unchangeable

in Greek astronomy, the universewas geocentric and contained eightspheres, seven holding the knownplanets and the eighth the stars


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A brief history of astrophysics Nicolaus Copernicus (1473 1543) argued that the Sun, not the Earth, was

the center of the solar system the Copernican Principle:

We are not located at a special place in theUniverse, or at a special time in the historyof the Universe

Greeks Copernicus


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Isaac Newton (1642-1747) the law of gravity that makes apples fall to Earth

also governs the motions of the Moon and planets(the law of universal gravitation) thus the square of the speed of a planet in its orbit

varies inversely with its radius

the laws of physics that can be investigated in the labalso govern the behavior of stars and planets

(relative to Earth)


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Joseph von Fraunhofer (1787-1826) discovered narrow dark features in the

spectrum of the Sun realized these arise in the Sun, not the

Earths atmosphere

saw some of the same lines in thespectrum of a flame in his lab

each chemical element is associated witha set of spectral lines, and the dark linesin the solar spectrum were caused by

absorption by those elements in theupper layers of the sun

the Sun is made of the same elementsas the Earth
http://antwrp.gsfc.nasa.gov/apod/image/0604/solarspectrum_noao.jpg


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in 1868 Fraunhofer lines not associatedwith any known element were found: avery decided bright line...but hitherto notidentified with any terrestrial flame. Itseems to indicate a new substance, whichwe propose to call Helium

helium was only isolated on Earth in 1895

astronomy can teach us new physics
http://antwrp.gsfc.nasa.gov/apod/image/0604/solarspectrum_noao.jpg


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Earths atmosphere


each chemical element is associated witha set of spectral lines, and deduced thedark lines in the solar spectrum were

caused by absorption by those elementsin the upper layers of the sun


however, the mix of elements in the Sunis not the same as Earth:

75% hydrogen 23% helium all other elements only 2%

hydrogen and helium are the mainconstituents of the universe, in about a3:1 ratio


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Earths atmosphere


each chemical element is associated witha set of spectral lines, and deduced thedark lines in the solar spectrum were

caused by absorption by those elementsin the upper layers of the sun


however, the mix of elements in the Sunis not the same as Earth:

75% hydrogen 23% helium all other elements only 2%

hydrogen and helium are the mainconstituents of the universe, in about a3:1 ratio

Cecilia Payne-Gaposchkin (1900-1979) first Ph.D. in astronomy fromHarvard

first woman promoted to tenuredprofessor at Harvard (1956) first woman to head a department atHarvard


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about 150 years after Darwin, we knowmuch more about the origin of matter thanabout the origin of life


once astronomers knew what elements thestars were composed of, they began to thinkabout the origin of these elements

It is mere rubbish thinking, at present, of origin oflife; one might as well think of origin of matter.

(letter from Charles Darwin, March 29, 1863)
http://images.google.co.kr/imgres?imgurl=http://www.liblab.it/var/news/storage/images/scienza_e_tecnologia/scheda_biografia_di_sir_charles_darwin_e_l_antidarwinismo_in_italia/30120-1-ita-IT/scheda_biografia_di_sir_charles_darwin_e_l_antidarwinismo_in_italia_large.gif&imgrefurl=http://www.liblab.it/portale/scienza_e_tecnologia/scheda_biografia_di_sir_charles_darwin_e_l_antidarwinismo_in_italia&h=300&w=227&sz=52&hl=ko&start=13&tbnid=8yNwEhMGRoPVtM:&tbnh=116&tbnw=88&prev=/images%3Fq%3DCharles%2BDarwin%26gbv%3D2%26svnum%3D10%26hl%3Dko%26newwindow%3D1%26sa%3DG


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factorof onemillion


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The Milky Way is a galaxy,composed of 100,000 million

stars...

Small Magellanic Cloud

Large Magellanic Cloud


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The Milky Way is a galaxy, composed of 100,000 million stars...


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The Milky Way is a galaxy,composed of 100,000 million

stars, just like many othergalaxies


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Galaxies like the Milky Waycontain stars and gas; the gas

is slowly but steadily turnedinto new stars...


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Galaxies like the Milky Way contain

stars and gas; the gas is slowly butsteadily turned into new stars; inturn, some stars explode at the endof their lives, producing brilliantsupernovae and returning theirmaterial to the interstellar gas

before and afterimages of a

supernova in adistant galaxy

before and afterimages of the

1987 supernovain the LargeMagellanic Cloud

the expanding gas cloudremaining from asupernova observed byChinese and Arabastronomers in 1054 AD


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all stars, including the Sun, are powered by nuclearreactions:

neutrinos produced by nuclear reactions deep in the Sunsinterior are detected in underground labs

the decay of supernova brightness is determined by theradioactive decay of nickel and cobalt produced in the explosion

some stars show Fraunhofer lines of technetium, which is a

short-lived nucleus (half-life 2 million years) so must have beenmade recently


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these elements aredestroyed by nuclear

reactions in stars

most stable nucleus most common ash fromnuclear burning in stars

abundances get smallerbecause nuclei have to bebuilt up successively fromhydrogen and helium

sawtooth patternarises becausenuclei with oddatomic number are

less stable thanthose with evennumber

initially the star iscomposed of onlyhydrogen and helium

everything other than hydrogen and helium wasmade deep inside stars

we are stardust


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Where did the hydrogen and helium come from?

the universe is expanding extrapolating backward in time, wefind that it emerged from a singularity(the Big Bang) 13.7 billion years ago

Einsteins theory tells us that thegeometry of the universe is determinedby its average matter content:

if the average density at present

is less than the critical density, theuniverse is infinite

if the average density exceedsthe critical density, the universe is

finite critical density = 1.0 X 10-26

kg/cubic meter or about 1 atom percubic meter 10 million

light years100 millionlight years

10 millionmiles/hour


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Where did the hydrogen

and helium come from?

the universe is expanding extrapolating backward in time, we findthat it emerged from a singularity (the BigBang) 13.7 billion years ago

at earlier times the universe was hotterand denser

hydrogen and helium were created in thefirst three minutes after the Big Bang,along with trace amounts of other lightelements (deuterium, lithium, etc.) the correct relative amounts (e.g., 23%helium, as in the Sun) are obtained only ifthe density of matter is 4.0% of the

critical density


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the principles of twentieth-century astrophysics

we are not located at a special place in the Universe, or at aspecial time in the history of the Universe (the Copernicanprinciple)

the laws of physics that we investigate in the lab also governthe behavior of astronomical objects

the study of astronomical objects can reveal new laws ofphysics

the Sun and stars are made of the same elements as theEarth

these elements were made in the Big Bang (hydrogen andhelium) and in the centers of stars (everything else)

astronomy + physics = astrophysics


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Either

Newtons law of gravity doesntwork for galaxies, or

galaxies must have largeamounts of matter in someunseen form in their outer parts(at least2-3 times as much asin stars and gas)


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dwarf galaxies near the MilkyWay have up to 100 X moremass in dark matter than theydo in stars


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The masses of clusters ofgalaxies such as this one are five

times as large as the mass instars and gas


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Gravitational lensing

the gravitational field from theintervening mass bends light andtherefore:

- splits image into two- magnifies one image anddemagnifies the other- if source, lens and observer are

exactly in line the image appearsas an Einstein ring

lensing massEinstein ring

source track


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mass determinations from gravitational lensingconfirm mass determinations from Newtons law ofgravity

the masses of clusters of galaxies such as this

one are five times as large as the mass in stars andgas


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allmeasurements of galaxies and galaxy clusters on large scales (more than300,000 light years) show that the mass in stars and gas is only 1/5 of thetotal mass

the vast majority of the matter in the universe must therefore be in someunknown and invisible form

what is it?

rocks? very faint stars?

planets?

black holes?


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stars can also be gravitationally

lensed- image splitting is too small tosee but magnification can bedetected over a few months ofmonitoring

- stare at dense fields of starsand look for flares in thebrightness of individual starsdue to passing planets, blackholes, faint stars, etc.


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- stars near the center of theMilky Way are lensed byintervening stars

- stars in the Large Magellanic

Cloud shouldbe lensed byintervening black holes, planets,faint stars, etc. but are not

lensing mass


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what is it?

rocks? very faint stars?

planets?

black holes?

1. No gravitational lensing is detected

from the dark matter in the Milky Way

2. The right amount of helium is created

in the Big Bang only if the density ofordinary matter is 4% of the criticaldensity, but the average density of darkmatter is 5 X larger, or 20% of thecritical density


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the dark matter must be some exotic form of matter never found on Earth,in the stars, in laboratory experiments, etc.

most likely the dark matter is some unknown elementary particle that wasproduced in huge amounts in the Big Bang

Fermi Gamma-Ray


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Fermi Gamma RaySpace Telescope

Large Hadron Collider

Ice Cube

Cryogenic Dark Matter Search

The history and geometry of the universe


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the total density of ordinary matter(stars and gas) and exotic matter is 25%of the critical density

Einsteins theory tells us that the

geometry of the universe is determined byits average matter content:

if the average density exceeds thecritical density, the universe is finite, andspace is curved like the surface of the

globe if the average density at present is lessthan the critical density, the universe isinfinite and space is curved like a saddle

if the average density equals the critical

density, the universe is infinite and space isflat

critical density = 1.0 X 10-26 kg/cubicmeter or about 1 atom per cubic meter

since the total amount of matter isconserved, we can also deduce theexpansion history of the universe



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We can measure the history and geometryof the universe in several ways:

relation between distance and brightness ofsupernovae

distance

brightness

25% of critical density



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We can measure the history geometry ofthe universe in several ways:


large-scale distribution of galaxies


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the age of the universe (deduced from theoldest stars)



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the age of the universe

small irregularities in the background radiationleft over from the Big Bang



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We can measure the history and geometry ofthe universe in several ways:




small irregularities in the background radiationleft over from the Big Bang

All these estimates agree that theuniverse is flat, not curved.

Einsteins theory then says that the total

density must equal the critical density:ordinary matter: 4%

exotic matter: 20%

??? : 76%

total : 100% of critical density

The geometry of the universe


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The geometry of the universe

All these estimates agree that theuniverse is flat, not curved.

Einsteins theory then says that the totaldensity must equal the critical density:

ordinary matter: 4%exotic matter: 20%

??? : 76%


The remaining 76% is variously known as darkenergy, the cosmological constant, or vacuum

energy (energy of empty space)

Dark energy


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Dark energy

first introduced by Einstein in 1917 as an ad hocaddition to general relativity to permit staticcosmological models (my biggest blunder) dark energy exerts negative gravitational force sothe expansion of the universe is now accelerating

theoretical estimates imply that either the darkenergy should be exactly zero or 60-100 orders ofmagnitude larger than what we observe



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he h story and geometry of the un erse

ordinary matter: 4%

exotic matter: 20%

dark energy : 76%


Although we dont understand the properties ofexotic matter or dark energy, a model of theuniverse with these properties does aremarkably good job of fitting the data:

relation between distance and brightness of

supernovae large-scale distribution of galaxies


small irregularities in the background radiation

left over from the Big Bang

brightness

distance



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y g y f

ordinary matter: 4%

exotic matter: 20%

dark energy : 76%










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y g y

ordinary matter: 4%

exotic matter: 20%

dark energy : 76%








average density is equal to thecritical density to within 1%, so thegeometry of the universe is verynearly flat

age of the universe is 13.7 billionyears to within a few %

density of ordinary matter is4.2% of critical with an uncertaintyof 0.2%

on the largest scales, theuniverse is isotropic (the same inall directions) to within about 10parts per million

Black holes Quasars


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Black holes

a region of space in which thegravitational field is so strong thateven light cannot escape

first suggested by John Michell in

1783 consistent description was only

possible through general relativity

interior of a black hole (inside the

event horizon) is invisible, but theblack hole may reveal its presencethrough its actions on nearby matter:

gravitational lensing

orbits of nearby stars

heating gas orbiting the black hole tovery high temperature

black holes are very small, e.g., eventhorizon for a black hole with the massof the Sun is only 3 kilometers

Quasars

some strong astronomical radio sourcesare galaxies; others are star-like(quasi-stellar objects)

quasars were first thought to be

unusual stars in the Milky Way butwere discovered to be 100 million timesfurther away

quasars turn out to be intense sourcesof radiation located in the centers ofgalaxies

usually far brighter than the galaxyitself so they mask the light from thegalaxy

up to 10 billion times the energy outputof the Sun

quasars are the strongest steadypower sources in the universe


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s s m h


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quasars were muchmore common when

the universe wasyoung

factor

of

ten

13.6 5.9 3.3 2.2 1.6 1.2 0.9

(billion years since the Big Bang)


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quasars are black holes with masses of one million toone billion times the mass of the Sun, shining from

super-heated gas that is slowly spiraling into theblack hole and making it grow

directional stability of radio


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yjets maintained for a million

years or more

velocities in radio


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jets are close to the

speed of light


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quasars vary on timescales of months implies size less than the distance light can travel in a month, or

about 100 X the size of the solar system

tighter constraint is about equal to the size of the solar system(distance to outermost planets)


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quasars vary on timescales of months implies size less than the distance light can travel in a month, or

about 100 X the size of the solar system

tighter constraint is about equal to the size of the solar system(distance to outermost planets)

efficiency E=Mc2 if mass is converted entirely to energy; efficiency of any

engine is E/Mc2

gasoline 0.0000000003 nuclear reactors 0.001

black holes 0.1 0.3

all U.S. energy needs could be met with a fuel supply of 100

pounds per day


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if black holes are the power source for quasars

the present abundance of quasars is much less than the

abundance early in the history of the universe quasars are found in the centers of galaxies

then

many nearby galaxies must contain black holes (dead quasars) attheir centers

The Milky Way


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1000 light years


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The Milky Way


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The Milky Way

the stars must be orbitingan object of mass 4 million

times the mass of the Sun,of size less than thedistance from the Earth toPluto

no known astronomicalobject other than a blackhole has these properties

if this object is a blackhole, its event horizon isabout 10% of the Earth-Sundistance


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mass of centralobject is 40 million

solar masses with anuncertainty of only 2%

no known objectother than a black hole

could be so massiveand so small


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total mass of ash that must be left over from all the quasars in the pasthistory of the universe, if efficiency is 0.1 Mc2:

5,000 10,000 solar masses in a box of a million light-years on a side

total mass of black holes found in the centers of nearby galaxies:

8,000 14,000 solar masses in a box of a million light-years on a side

Summary


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we are not located at a special place in the Universe, or at a specialtime in the history of the Universe (the Copernican principle)

the laws of physics that we investigate in the lab also govern thebehavior of astronomical objects

the study of astronomical objects can reveal new laws of physics

the Sun and stars are made of the same elements as the Earth

Summary


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nor are we made of a special material everything we have ever seen orfelt comprises only 4% of the mass and energy in the universe




Summary


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there is now indisputable evidence that black holes exist, of masses aslarge as a billion solar masses

the question for the next decade is whether they behave according toEinsteins theory

one of the most remarkable predictions of theoretical physics black holes is confirmed by one of the most dramatic discoveries of observationalastronomy quasars



Summary


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and may well be the most powerful method we have to discover newphysics in the twenty-first century


Summary


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but most of the universe is not

Platos immutable, unchangeable fifth element comprises 75% of the massand energy of the universe, and its nature is one of the central problems fortwenty-first century physics

the fifth element, by scott tremaine -

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