Dark Energy

the higher the density of matter, themore gravitational pull counteractingthe overall expansion, which couldeventually slow or stop the expansion ...

Both the “shape” of the universe andits fate depend on how much matter there is...

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...but will the overall gravitational pull succeed in stopping the expansion, or not?

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Possible futures of the universeAssumingthat gravity is the only determiningfactor, thereare threepossiblefates

Closed: end in a Big Crunch; gravity winsFlat: exactly the right amount of matter to balanceOpen (or coasting): keep on expanding forever

(Note: an oversimplification... rate of expansion can change with time)

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Gravity will 'win' if the overall density of the universe exceeds about 10-29 g/cc = Ωc(One hydrogen atom per 100 cubic centimeter!)

Is there enough dark matter to make up the critical density?

The CRITICAL DENSITY (assuming gravity & kinetic energy of all matter)

Right now, the total amount observedmatter, including dark matter, is about30% of critical density

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So giventhat observeddensityis less thancritical, whichfate is it?

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So giventhat observeddensityis less thancritical, whichfate is it?

Our current observations predict

none of these...

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Hubble in the 1920's:-Measure velocity of galaxy with Doppler shift of spectral lines in a galaxy-Measure distance to galaxy

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The Universe (space itself) is expanding

Any raisin sees all the others moving away

note: raisin itselfdoes not expand

Think of the raisins as galaxies

Distance to further Away raisins increases more rapidly than those nearby

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Hubble found that everything on the galactic scale is moving away from everything else. The modern value for the expansion rate is

H0 = 70 (km/s)/Mpc

Note: Hubble’s constant (Ho) implies the age of the universe

1/H0 = 13.7 billion years

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Observe white dwarf (1a) supernovae (standard candles) to extend Hubble's

plot out to very far distances

Automated survey:

Take pictures ofmany distantgalaxies & compare them by computer

(requires huge data processing capability)

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Very far way, the light is red shifted and was created very long ago

-A test...- • Measure luminosity → distance via inverse square law• Measure Doppler shift → distance via Hubble’s Law

Brightness vs time

brightness vs wavelength

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The results from distant supernovae

The most distant supernovae are fainter than you would expect for expansion

(hubble+gravity)

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The red data point is new from Fall 2012

Rate of expansion vs timeSN + quasar absorption lines (new)

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It will eventually become very cold and dark...

This means ... the Universe's expansion is accelerating!

“Well, this is weird.” Hitchhiker’s Guide 15

How can this accelerating expansion happen? Something other than gravity must be acting

We call it 'DARK ENERGY'... causes a repulsive force of some kind 16

'Einstein's Greatest Blunder'?

Einstein originally (1915, before Hubble) thought that the universe was static, so he added a 'fudge factor', the 'cosmological constant', to his equations of general relativity: this describes an effective repulsive force in that counteracts collapse of the universe due to gravity

He supposedly considered it a 'blunder‘because Hubble later found that the universe is expanding

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Because we observe that the universe's expansion is accelerating ...

A repulsive force is needed...

The cosmological constant may describe it.

'Einstein's blunder' is back in vogue!

(but it's still basically a fudge factor... It doesn’t tell us the nature of the dark energy and, it is not the only way to describe the repulsion...)

A series of telescopes and missions are planned to study the effects of Dark Energy

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Nobody has any really good ideas about what dark energy really is!

It's probably the biggest unsolved problem in physics & cosmology today

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