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Chapter 16Dark Matter, Dark Energy,

&The Fate of the Universe

Dark Matter: An undetected form of mass thatemits little or no light but whose existence weinfer from its gravitational influence

Dark Energy: An unknown form of energy thatseems to be the source of a repulsive force causingthe expansion of the universe to accelerate

16.1 Unseen Influences

• “Normal” Matter: ~ 4.4%– Normal Matter inside stars: ~ 0.6%– Normal Matter outside stars: ~ 3.8%

• Dark Matter: ~ 25%• Dark Energy ~ 71%

Contents of Universe

• Dark matter and dark energy have never been directlyobserved, but each has been proposed to exist becauseit seems the simplest way to explain a set of observedmotions in the universe.

• Dark matter is the name given to the unseen masswhose gravity governs the observed motions of starsand gas clouds.

• Dark energy is the name given to whatever may becausing the expansion of the universe to accelerate.

What do we mean by dark matterand dark energy?

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16.2 Evidence for Dark MatterOur Goals for Learning

• What is the evidence for dark matter in galaxies?• What is the evidence for dark matter in clusters ofgalaxies?• Does dark matter really exist?• What might dark matter be made of?

Rotation curve

A plot of orbitalvelocity versusorbital radius

Solar system’srotation curvedeclines becauseSun has almostall the mass

Rotation curve ofMilky Way stays flatwith distance

Mass must be morespread out than insolar system

Mass in Milky Wayis spread out over alarger region than thestars

Most of the MilkyWay’s mass seems tobe dark matter!

Mass within Sun’sorbit:

1.0 x 1011 MSun

Total mass:

~1012 MSun

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The visibleportion of agalaxy liesdeep in theheart of alarge halo ofdark matter

We canmeasurerotationcurves ofother spiralgalaxiesusing theDopplershift of the21-cm lineof atomic H

Spiral galaxies all tend to have flat rotation curvesindicating large amounts of dark matter

Observations first made by Vera RubinObservations first made by Vera Rubin

Broadening of spectral linesin elliptical galaxies tells ushow fast the stars areorbiting

Elliptical galaxies alsohave dark matter

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We canmeasure thevelocities ofgalaxies in acluster fromtheir Dopplershifts

dark matter in clusters ofgalaxies

Almost everyobject is a galaxyin this picture

The mass we find from galaxy motionsin a cluster is about

50 times larger than the mass in stars!

Fritz Fritz ZwickyZwickydiscovered

dark matter ingalaxy clusters.

Clusters containlarge amounts of X-ray emitting hot gas

Temperature of hotgas (particlemotions) tells uscluster mass:

85% dark matter 13% hot gas 2% stars

Gravitational lensing, the bending of light rays bygravity, can also tell us a cluster’s mass

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A gravitational lens distorts our view of things behind it A gravitational lens distorts our view of things behind it

All three methods of measuring cluster mass indicatesimilar amounts of dark matter

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Our Options

1. Dark matter really exists, and we are observing theeffects of its gravitational attraction

2. Something is wrong with our understanding of gravity,causing us to mistakenly infer the existence of darkmatter

Does dark matter really exist?

1. Dark matter really exists, and we are observingthe effects of its gravitational attraction

2. Something is wrong with our understanding ofgravity, causing us to mistakenly infer theexistence of dark matter

Because gravity is so well tested, most astronomersprefer option #1

Our Options

… not as bright as a star.

How dark is it?

• Ordinary Dark Matter (MACHOS)

– Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS)

– Weakly Interacting Massive Particles:

mysterious neutrino-like particles

Two Basic Options

What might dark matter be madeof?

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• Ordinary Dark Matter (MACHOS)– Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS)

– Weakly Interacting Massive Particles:

mysterious neutrino-like particles

Two Basic Options

TheBestBet

• There’s not enough ordinary matter

• WIMPs could be left over from Big Bang

• Models involving WIMPs explain how galaxyformation works

Why Believe in WIMPs?

16.4 The Universe’s Fate

• Will the universe continue expanding forever?• Is the expansion of the universe accelerating?

Our Goals for Learning

Canon ball analogy

Does the universe have enough kinetic energy toescape its own gravitational pull?

Fate of universe depends on amount of dark matter

Lots ofdark matter

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Fate of universe depends on amount of dark matter

Lots ofdarkmatter

Notenoughdarkmatter

Critical densityof matter

Notenoughdarkmatter

Fate of universe depends on amount of dark matter

Lots of darkmatter

Amount of dark matteris ~25% of the criticaldensity suggesting fateis eternal expansion Not

enoughdarkmatter

But expansionappears to bespeeding up!

Not enoughdark matter

DarkEnergy?

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Estimated age depends on dark matter and dark energy

oldolder

oldest

Brightness of distant white-dwarf supernovae tell us howmuch universe has expanded since they exploded

Is the expansion of the universe accelerating?

Accelerating universe is best fit to supernova data