Looking at the dark side

Henk HoekstraDepartment of Physics and Astronomy

University of Victoria

Looking at the dark sideLooking at the dark side

It is clear that part of the contents of the universe consist of “ordinary” baryonic matter and photons.

Observations show, however, that the matter we are made of is not the most important ingredient in the universe. These “dark” components do not interact through electro-magnetic interactions and therefore do not produce detectable radiation.

In this talk I will outline why we have to take such a crazy idea serious!

What dark side?What dark side?

The study of the contents of the universe is part of cosmology.

Cosmology is the study of the global properties of the Universe. Hence we need to measure quantities on large scales.

This is easier said than done…

CosmologyCosmology

Mean density of the Universe Geometry of the Universe Age of the Universe Future of the Universe What is the Universe made of?

Some fundamental numbers/questions:

Major cosmological Major cosmological questionsquestions

Our view of the universe has changed a lot since Galilei...

Cosmological Principle:We are not in a special place in the universe

Universe is isotropic (looks the same in all directions)

Universe is homogeneous (same stuff everywhere)

Changing CosmologyChanging Cosmology

Our view of the universe has changed much in the last 100 years...

Small, island universe (Milky Way) Static Universe (Einstein’s early model) Expanding Universe (Hubble, Big Bang) Inflationary Universe Accelerating Universe something completely different…?

Changing CosmologyChanging Cosmology

To understand the universe and answer some of the fundamental questions, it is important to measure masses and distances.

Unfortunately this is very difficult:

We can only measure direct distances to the nearest objects; other methods are indirect measures.

Most of the matter in the universe is invisible: the “dark matter”. In addition there is “dark energy”

What do we need to What do we need to measure?measure?

The universe expands and therefore more distant objects appear redder: redshift

We can measure redshifts very well!

What can we measure?What can we measure?

Measuring large distances using “standard candles”

The more distant an object, the dimmer it appears. If we would happen to know the total energy output of the object, we can infer the distance!

This technique has been used for nearby variable stars and very distant supernovae.

Using 10Using 103737 W “lightbulbs” W “lightbulbs”

1. discover supernova2. get spectrum3. is it type Ia?4. follow multicolor lightcurve5. model lightcurve6. go to 1

Eventually……

Using 10Using 103737 W “lightbulbs” W “lightbulbs”

The 1998 supernova results were surprising: the rate of expansion is accelerating! This implies that the dynamics of the Universe is dominated by the “dark energy”.

This measurement has profound implications for our understanding of particle physics: the observed (small) amount of “dark energy” is not easily explained.

A runaway universe?A runaway universe?

(and how much is out there….)

To answer this question we would like to measure masses of astronomical objects. This can tell us about:

Formation of structure Evolution of galaxies Properties of dark matter Cosmological parameters

What is the Universe made What is the Universe made of?of?

Comparison of masses for objects in the universe to the amount of light they emit suggests there is much more matter than meets the eye.

Dark matterCould the emission be missed? (unobserved wavelength)

NO!

How to measure masses?How to measure masses?

Comparison of masses for objects in the universe to the amount of light they emit suggests there is much more matter than meets the eye.

Dark matterCould the masses be overestimated?

NO!


Masses can be determined through various techniques

“direct”:Measure the strength of the gravitational force

theory of gravity way of measuring the force

“indirect”:Infer the mass from the dynamics of the system

theory of gravity tracer of the potential assumptions about dynamical state (equilibrium)


Optical image of the Coma cluster

1000s of galaxies many elliptical galaxies ~1 Mpc radius

Clusters of galaxiesClusters of galaxies

“On the masses of nebulae and of clusters of nebulae”

Fritz Zwicky (1937)

“The Coma cluster contains about one thousand nebulae. The average mass of one of these nebulae is therefore M > 4.5x1010 solar masses.

… This result is somewhat unexpected, in view of the fact that the luminosity of an average nebula is equal to that of about 8.5x107 suns. The conversion factor from luminosity and mass for nebulae in the Coma cluster would be of the order 500 as compared with about 3 for the local Kapteyn stellar system.”

First evidence for dark First evidence for dark mattermatter

Hot cluster gasHot cluster gas

More evidence for dark More evidence for dark mattermatter

galaxies (in particular outer parts) groups of galaxies clusters of galaxies super clusters of galaxies

log(M/Msun)

8-1212-13.513.5-15>15

But where/what is the dark matter?

Where is dark matter Where is dark matter relevant?relevant?

or … Nature’s own weighing scales

Zwicky (1937): “… The gravitational fields of a number of “foreground” nebulae may therefore be expected to deflect light coming to us from certain background nebulae. The observations of such gravitational lens effects promises to furnish us with the simplest and most accurate determination of nebular masses. No thorough search for these effects has as yet been undertaken.”

The first gravitational lens was discovered in 1979

Gravitational lensingGravitational lensing


Nowadays it is seen everywhere!


Gravitational lensing provides a powerful tool to study the dark matter distribution in the universe.

It does not require assumptions about the dynamical state of the system under investigation.

It can probe the dark matter on scales where other methods fail, as it does not require visible tracers of the gravitational potential.

or … Nature’s own weighing scales


Image splitting by massive galaxies

Strong lensingStrong lensing

We observe that the images of distant galaxies are aligned.

Weak lensingWeak lensing

Example of “everyday lensing”.

Weak gravitational Weak gravitational lensinglensing

A measurement of the shape of a galaxy provides an unbiased but noisy measurement of the lensing signal.

Weak lensingWeak lensing

Mass distribution in clustersMass distribution in clusters

mass

dis

trib

uti

on

Visualizing the “invisible”Visualizing the “invisible”

mass

dis

trib

uti

on

Visualizing the “invisible”Visualizing the “invisible”

To measure the weak lensing signal we need to measure the shapes of large numbers of galaxies

We need to observe a large area on the sky Measure shapes accurately Compare the results to numerical simulations

Cosmological parameters

Cosmological weak lensingCosmological weak lensing

Megacam: FOV 1 square degree

CFHT Legacy SurveyCFHT Legacy Survey

We will observe 140 square degrees on the skymultiple exposures in 5 different filters

MegaCam has ~ 350 Megapixels!

~5500 images, ~ 1.5GB each…

More than 8TB of data!

CFHT Legacy SurveyCFHT Legacy Survey

Comparison with simulationsComparison with simulations

Comparison with simulationsComparison with simulations

The Universe started with a Big Bang

For about 300,000 years the universe was ionized and opaque. Then protons and electrons combined and the “fog” lifted.

The surface of last scattering gives rise to the Cosmic Microwave Background (CMB), which is now observed to have a temperature of 2.7K.

Small seeds of structure give rise to small temperature fluctuations, which allow us to do cosmology.

The ultimate probe: CMBThe ultimate probe: CMB


The temperature fluctuations are tiny: one part in 100,000 and hence very accurate measurements are needed.

After subtracting the mean temperature, the motion of Earth through the Universe and removing the emission of the galaxy, one obtains a map of the “ripples” in the CMB


WMAP image of the CMB


Cosmic Microwave Background Type Ia supernovae Large Scale Structure

all provide strong evidence for the existence of

Dark Energy

in addition to the dark matter…

The Dark UniverseThe Dark Universe

The Dark UniverseThe Dark Universe

The progress made in recent years is amazing!

The result, however, is embarrassing:

The more we measure, the less we understand!The more we measure, the less we understand!

• ~70% is dark energy, which we do not understand• ~25% is dark matter, which we do not understand

The result?The result?

Understanding the nature of dark matter and dark energy are among the most important questions of this decade (and coming ones…)

Ongoing Canadian-French effort:Canada-France-Hawaii-Telescope Legacy Survey

largest weak lensing survey (cosmic shear) largest type Ia survey

Next step: space based missions Dark UNiverse Explorer (DUNE) SuperNova Acceleration Probe (SNAP) Joint Dark Energy Mission (JDEM)

The Future?The Future?

The evidence for dark matter from observations of various objects (galaxies, clusters) in the universe is convincing.

These results are supported by studies of the global properties of the universe (CMB)

Alternative theories of gravity cannot explain the results

Dark matter exists!

Now we (only) need to detect it directly

ConclusionsConclusions

The accuracy with which we can measure cosmologicalparameters is increasing rapidly, thanks to new, well understood techniques.

The improved measurements lead to new puzzles that need to be solved before we understand the Universe we live in!

It is a good time to be a It is a good time to be a cosmologist!cosmologist!

ConclusionsConclusions

Documents

Looking at the dark side