dark matter and dark energy.ppt - ursinus...

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Dark matter: An undetected form of mass that emits

Dark Matter and Dark Energy

little or no light but whose existence we infer from its gravitational influence

Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing

Copyright © 2010 Pearson Education, Inc.

p gthe expansion of the universe to accelerate

Contents of Universe

• Normal matter: ~ 4.6%— Normal matter inside stars: ~ 0.7% — Normal matter outside stars: ~ 3.9%

• Dark matter: ~ 23%D k 72%


• Dark energy: ~ 72%

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What is the evidence for dark matter in galaxies?


The broadening of spectral lines in elliptical galaxies tells us how fasttells us how fast the stars are orbiting.

These galaxies also have dark matter.


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We can measure the velocities of galaxies in a gcluster from their Doppler shifts.


The mass we find from galaxy motions in a cluster iscluster is about 50 timeslarger than the mass in stars!


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Clusters contain large amounts of X ray–emitting hot gas.

The temperature of hot gas (particle motions) tells us cluster mass:


85% dark matter13% hot gas2% stars

The Bending of the Path of a Light Ray as it Passes a Massive Object (Star, Neutron Star, Black Hole)

E

S

The diagram shows Earth (E), a massive object (C), and a distant star (S). C could be a star, neutron star, or black hole.

C


From Earth, the star appears to be located in the direction indicated by the dotted line.

The solid green line shows the path of a light ray from S. Instead of being a straight line as it would if C were not present, it is curved because C bends the space-time in its neighborhood.

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


All three methods of measuring cluster mass indicate similar amounts of dark matter.

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Our Options

1 Dark matter really exists and we are observing1. Dark matter really exists, and we are observing the effects of its gravitational attraction.

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


Our Options

1 Dark matter really exists and we are observing1. Dark matter really exists, and we are observing the effects of its gravitational attraction.

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


Because gravity is so well tested, most astronomers prefer option #1.

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• Ordinary Matter (MACHOs)

Two Basic Options

• Ordinary Matter (MACHOs)— Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Exotic Particles (WIMPs)


— Weakly Interacting Massive Particles:mysterious neutrino-like particles

• Ordinary Matter (MACHOs)

Two Basic Options

• Ordinary Matter (MACHOs)— Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Exotic Particles (WIMPs)The Best


— Weakly Interacting Massive Particles: mysterious neutrino-like particles

Bet

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Compact starlike objects occasionally make other stars appear brighterappear brighter through lensing…

… but there are not enough


not enough lensing events to explain all the dark matter.

• There’s not enough ordinary matter

Why WIMPs?

• There s not enough ordinary matter.

• WIMPs could be left over from the Big Bang.

• Models involving WIMPs explain how galaxy


formation works.

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⇒


Gravity of dark matter is what caused protogalactic clouds to contract early in time.

WIMPs can’t contract to the center bbecause they don’t radiate away their orbital energy.


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Maps of galaxy positions reveal extremely large structures: superclusters and voids.


Models show that gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time.

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Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs.

Fate of universe depends on the amountamount of dark matter.


Critical density of matter

Not enough dark matter

Lots of dark matter

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But expansion appears to be speeding up!


Dark energy?Not enough dark matter


Estimated age depends on both dark matter and dark energy.old older oldest

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Is the expansion of the universe accelerating?



The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded.

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An accelerating universe is the best fit to supernova data.

dark matter and dark energy.ppt - ursinus...

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