DCMSTMay 22nd, 2007

Dark matter and dark energy

Gavin LawesWayne State University

DCMSTMay 22nd, 2007

Outline

•Physics on cosmic length scales: special and general relativity

•Astrophysical measurementsMeasuring distanceMeasuring velocity

•History of the universe (on a single viewgraph)

•Open problems in cosmologyDark matterDark energyLarge scale structure of the universe

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Special relativityc=300,000 km/sec is a hard speed limit for the universe.

The time interval or space interval between events depends on the speed of the observer.

Space

Tim

e

C

A

B Slope=c

Causality

•The interval between A and B is timelike; everyone agrees that A occurs before B (so A could cause B).

•The interval between A and C is spacelike; some people think A happened first and some people think C happened first (so A cannot have caused C).

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General Relativity

abababab TgRgR 821

•This equation describes how space is distorted by mass and energy.

• is called the cosmological constant. Einstein put this in to force a static universe.

DCMSTMay 22nd, 2007

Cosmological Distance Units

Astronomical unit (AU): Earth-Sun distance (way too small to be useful).

Lightyear (ly): Distance light travels in a year (still too small).

Parsec (ps): 3.26 lightyears (ditto).

Megaparsec (Mpc): One million parsecs (this is about right for talking about distant galaxies).

Our galaxy (Milky Way) is about 100,000 ly in diameter, the Andromeda galaxy is about 1 Mpc away, and the “edge” of the universe is ~10,000 Mpc away.

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How do we measure distance?

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•Parallax can be used to find the distance to close objects (~300 ly).

•Astronomers often measure cosmic distance using apparent luminosity measurements on Cepheid variables and Type 1a supernovae.

Cepheid variable“Pulsing” star

works to ~20 MPC

Type 1a supernova“Exploding” star

works to >1000 MPC

DCMSTMay 22nd, 2007

Cepheid Variables•Period of variable luminosity (brightness) is related to actual brightness of star (Henrietta Leavitt, 1908).

•Problems: This estimate is affected by space dust, and these stars are not bright enough to see huge distances.

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Type 1a supernovaThese supernovae are believed to follow a standard intensity vs time curve (after taking into account well-known “fudge factors”).

Problems: There are not too many Type 1a supernovae known.

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Supernovae seen by SDSS

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How do we measure velocity?

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•Elements in distant stars emit light at very specific wavelengths (spectrum).

•The wavelength of light we measure on Earth (measured) will be different than the wavelength emitted by the star (star) is the star is moving relative to the Earth (just like a Doppler shift).

•This shift in wavelength is parameterized by “z” defined by:

star

measuredz 1 If z<<1 then z=v/c

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Hubble’s Law

dHv 0H0=70 km s-1/Mpc

Age of the universe is 1/H0, or about 13 billion years.

DCMSTMay 22nd, 2007

NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe.

Hubble’s law tells us that the universe is expanding.

DCMSTMay 22nd, 2007

NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe.

Hubble’s law tells us that the universe is expanding.

...

...

DCMSTMay 22nd, 2007

NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe.

Hubble’s law tells us that the universe is expanding.

...

...

...but what is it expanding from? What is it expanding into?

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How did the universe start?

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Big Bang•15 billion years ago, the universe was very small, very hot ball of matter and energy.

•In the intervening time, the universe has expanded, but this original energy can be seen in the cosmic microwave background.

In 1963 isotropic background radio waves were measured. These could be fit to 2.7 degree background radiation, and proved strong evidence for the Big Bang theory.

DCMSTMay 22nd, 2007

History of the Universe

?

Matter/antimatter annihilate (0.001 sec)

Protons/neutrons form (3 minutes)

Neutral atoms form(380,000 years)

3000 K

First galaxies form (1 billion years)

Today (15 billion years)

2.7 K

(Inflation)

DCMSTMay 22nd, 2007

Just one more thing....

DCMSTMay 22nd, 2007

Why was there more matter than antimatter?

•The early universe should have had equal amounts of matter and antimatter, which would then annihilate completely.

•This didn’t happen.

•About 1 out of every 1 billion matter particles survived to the universe today.

DCMSTMay 22nd, 2007

Why do galaxies show flat rotation curves?

Galactic rotation curve(measures the rotation velocity as a function of distance from the center of the galaxy)

22 )(

rrGMv

•Most of the light from galaxies comes from the center.

•The flat rotation curves suggest that there is a uniform mass distribution.

•This led to the suggestion of missing mass, later called “dark matter”, because it provides gravitational attraction, but doesn’t emit light.

DCMSTMay 22nd, 2007

This dark matter is believed to surround most galaxies, and the mass-to-light ratio for certain galaxies can exceed 300 times that of the sun.

(What is dark matter?)•Doesn’t really exist―the theory of gravity doesn’t work on very, very large length scales.

•Black holes, neutron stars, brown dwarfs (collectively MACHOS).

•Neutrinos.

•Weakly interacting massive particles (WIMPS).

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Will the universe keep expanding forever?

Einstein put an extra term (called the cosmological constant) in his equations for General Relativity to produce a static universe.

Universe collapses

Universe expands forever

“Flat” universe

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The intensity of the most distant objects (largest z) is smaller than we expect using Hubble’s Law, so they must be moving away faster.

Accelerating Universe

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Why is the universe accelerating?

•Einstein’s theory of general relativity is wrong (or incomplete) and gravity works differently at long length scales.

•The speed of light has changed since the Big Bang.

•A cosmological constant (first proposed by Einstein) that provides a negative pressure causing the universe to accelerate.

•Quintessence: like the cosmological constant, but not uniform in space and time.

The last two are forms of dark energy, which would need to have an energy density of 10-29 g/cm3 (NB: E=mc2)

DCMSTMay 22nd, 2007

Why did galaxies and clusters form?

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Small density variations right after the Big Bang could coalesce into galaxies, clusters, and superclusters.

Pillars of creation~4 ly long dustclouds

Great Wall~500 Mly long cluster of galaxies

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Cosmic microwave background

The cosmic microwave background shows temperature differences of about 1 part in 100,000 over the universe.

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This plot allows astronomers to analyze the microwave background, and gives information about the geometry (flat, open, closed) and matter and energy density of the universe.

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What is the universe made of?

Atoms: Normal matter. We can see this.

Dark Matter: Invisible mass inferred by measuring speed of galaxy rotations.

Dark Energy: Provides negative pressure for accelerating universe.

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Further research directions in cosmology

DCMSTMay 22nd, 2007

May be possible to observe dark matter using gravitational lensing.

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Intersection of particle physics and cosmology

•There was a lot of energy present during the Big Bang.

•Looking at the very distant parts of the universe allows astronomers to take a snapshot very early universe to understand how matter and energy interact at these high energies.

•Some specific theories also predict particles that can provide the dark matter and dark energy seen in the universe.

•The Large Hadron Collider (LHC) set to start this year will allow physicists to investigate matter and antimatter at high energies, similar to .

DCMSTMay 22nd, 2007

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