Lecture 37: Dark Matter & Dark Energy

Readings: Sections 26-8 and 28-7

Key Ideas

Dark Matter

Matter we cannot see directly with light

Most of the matter in the Universe?

Detected only by its gravity

Rotation Curves of Spirals

Velocity “Dispersion” in Ellipticals

Velocity Dispersion in Clusters of Galaxies

Gravitational Lensing by Clusters of Galaxies

Possible Dark Matter Candidates

Baryonic (detected with microlensing)

Non-baryonic (has to be some!)

Dark Energy

Vacuum energy of the Universe

Responsible for acceleration of the Universe’s expansion

Detected in Hubble diagram of distant Type Ia Supernovae

Dark Matter/Energy or New Form of Gravity

Solar System Examples

Galaxy Rotation Curves Revisited

Spiral Galaxies rotate such that:

Speed rises from the center to the inner disk

Speed becomes constant (flat) in the outer disk

Requires Dark Matter to explain such high speeds so far away from most of the visible matter in the galaxy.

Mass Distribution in Galaxies

Most of the stars are in the inner 10 kpc

If stars provided all of its mass we expect

Rotation speed should rise to a maximum in the inner parts

Then fall steadily with radius outside of R~10 kpc

But the rotation curve stays flat!

Outer parts are rotating faster than expected

Need more mass at large radii than is observed in the stars and gas alone.

Dark Matter in Elliptical Galaxies

Velocity dispersion=spread in speeds of stars. Greater if there is more matter. Way to detect matter that is “dark” in elliptical galaxies (or clusters of galaxies) that don’t have systematic rotation.

Dark Matter in Galaxy Clusters

1933: Fritz Zwicky measured the motions of galaxies in the Coma cluster

Found velocities of +/- 1000 km/sec relative to the cluster center

This is greater than the escape velocity computed by adding up the light of the cluster!

Zwicky suggested that “dark matter” adds extra gravity to hold the cluster together.

Subsequent observations show that galaxy clusters are 90-99% dark matter.

Coma Cluster

X-ray gas in cluster is extremely hot=particles moving very fast

Need lots of matter to keep X-ray gas in clusters from escaping.

Gravitational Lensing in a Rich Cluster

Einstein’s theory predicts that light will be bent by matter.

Again, lots of matter (more than seen in visible light) is needed to explain the amount of bending we observed.

Dark Matter

Called “Dark Matter” because it cannot be detected directly using light.

It is only detected by its Gravitational Effects”

Outer parts of galaxies rotate faster than expected from the starlight

Galaxies in clusters orbit faster than expected from the starlight

Hot X-ray gas that would otherwise evaporate from a galaxy cluster stays confined.

What is Dark Matter made of?

Baryonic Dark Matter

Ordinary matter (“baryons”) made of protons & neutrons

Candidates

Brown Dwarfs & Jupiter-sized planets

Cold stellar remnants (black holes, neutron stars & white dwarfs)

Primordial black holes (Big Bang leftovers)

Frozen hydrogen snowballs

Collectively called

Massive Compact Halo Objects (MACHOs)

Gravitational Microlensing

If a MACHO passes between Earth & a more distant background star

GR predicts that the MACHO’s mass bends the starlight of the more distant star.

Get a “Gravitational Microlens” that briefly magnifies the background star.

Chance alignments are very rare

Most should last only for a few weeks

Must monitor millions of stars for many years

Handy animation of microlensing model at www.jpl.nasa.gov/releases/2004/103.cfm

Handy animation of an actual microlensing event at bulge.princeton.edu/~ogle/ogle3/big235-53.html

Microlensing Searches

Multi-year monitoring of LMC & SMC to search for microlensing from MACHOs

Watched ~12 Million stars for 6 years

Found only 13-17 halo microlensing events

Most likely mass range ~0.15-0.9M_Sun making them Jupiters up to white dwarfs

MACHOs can make up only 20% of the halo of the Milky Way. So they are not the whole answer to the dark matter puzzle.

Not all Dark Matter is Baryonic

Big Bang nucleosynthesis sets a firm bound on the amount of baryonic matter in the Universe.

W_b~0.04

If there was more “normal matter”, then the ²H would all fuse with other p, n, ²H, etc. and there would be no deuterium left in the Universe.

Therefore W_nb ~0.26

Nucleosynthesis Depends on the Density of Baryons

Deuterium is very sensitive to the density of baryons in the first three minutes.

Higher Density=More Encounters

Non-Baryonic Dark Matter

Fundamental particles that only interact via gravitation and the weak force

Massive neutrinos

Produced in large numbers in the Big Bang??

Exotic new particles

Predicted by some particle theories

Possible candidates makes dark matter a more attractive theory

Weakly Interacting Massive Particles: WIMPs

Example of New Physics

Supersymmetry

For every standard particle, there is a corresponding supersymmetric particle

Electron—Selectron

Quark –Squark

Photon—Photino

Some of these particles are predicted to have masses, but to be weakly interacting.

Exactly what we want!

Conservation Laws

Basic Idea:

Before=After

Energy conservation

Energy before=Energy after

Other quantities conserved

Lepton number

Nucleon number

Momentum

Led to the discovery of the neutrino

Axions

A type of particle proposed to conserve CP-symmetry

Frank Wilczek proposed to name the new particle after a brand of detergent, because it “cleaned up” this problem with QCD theory.

Properties of axions

Low-mass (but not zero!)

Very low probability of interaction

Just what we want!

Dark Matter Annihilation

Depending on the kind of particle it is, the dark matter may be its own anti-particle.

Particle-particle annihilation changes mass into energy.

Detect this radiation as gamma-rays

Searches currently underway but

Lots of sources of gamma-rays other than dark matter

How often dark matter annihilates is unknown

Particle Dark Matter Searches

Attempts to directly dark matter

Estimate of the masses of neutrinos (must be very tiny)

Particle accelerator experiments searching for new massive particles in collisions

Searches for “cold dark matter” particles hitting the Earth from space and for gamma-rays from annihilation

So far, no convincing detections have been reported, but the searches go on…

Dark Matter Summary

~90% of the matter in the Universe is unknown at the present

Active searches underway for the dark matter particles, both for DM-related reasons and to test new theories of physics.

It is possible that the dark matter could be not even weakly interacting with ordinary matter, but non-interacting. Detecting it in that case would be really tough!

Dark Energy

Type Ia Supernova distances to galaxies have suggested that our Universe is:

Infinite

Spatially Flat (W₀=1)

Accelerating (W_L>W_m)

The extra expansion is due to Dark Energy

Extra energy filling the Universe

Acts to inflate the Universe against gravity

What is Dark Energy?

We know even less about Dark Energy than we do about Dark Matter.

Cosmological Constant (L)

Vacuum energy component whose density is constant over cosmic history.

Generic Dark Energy

Could vary in density over cosmic time

Candidates include scalar fields, quintessense….

Very Low Density (10^-29gm/cm³) but exists everywhere

Matter & Energy Content of the Universe

W_m divided into

W_b=0.04

stars=0.004

gas=0.036

W_dm=0.26 (mostly non-baryonic)

W_L=0.70

A Radical Suggestion

Maybe Dark Matter & Dark Energy don’t exist at all!

Is our theory of gravity wrong on large scales?

Problems:

None of the alternative theories of gravity have survived key tests of detailed predictions

Some alternatives aren’t very testable!

Hard to reconcile these theories with observed gravitational lensing

Examples from the Solar System

Motion of Mercury did not agree with Newtonian mechanics

Answer: General Relativity

Motions of Uranus did not agree with Newtonian mechanics

Answer: New matter (aka Neptune)

The Precessing Orbit of Mercury

New Theory of Gravity for the Solar System

Nineteenth century explanation

Small planet in the inner solar system affecting the orbit of Mercury

Planet called Vulcan, but never detected

Einstein’s explanation

Newton’s Law of Gravity gives incorrect answers in strong gravitational fields

General Relativity explains the orbit

Discovery of Extra Matter in the Solar System

Uranus was discovered by Herschel in 1781.

Its motions deviated slightly from the predictions of Newton’s Laws.

In 1845, Urbain Leverrier & John Couch Adams independently argued that a new planet’s gravity could explain Uranus’ motion.

In 1846, Neptune was discovered by astronomers at the Berlin Observatory.

Studying Dark Matter & Energy

An extremely active area of research

Experimental

Observational

Theoretical

We’d really like to know what makes up the Universe!