Dark Matter: Some Observational Evidence

Mike Gladders

“Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.

Magicians and scientists are, on the face of it, poles apart. Certainly, a group of people who often dress strangely, live in a world of their own, speak a specialized language and frequently make statements that appear to be in flagrant breach of common sense have nothing in common with a group of people who often dress strangely, speak a specialized language, live in ... er ...”

----Terry Pratchett

“Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.

Magicians and scientists are, on the face of it, poles apart. Certainly, a group of people who often dress strangely, live in a world of their own, speak a specialized language and frequently make statements that appear to be in flagrant breach of common sense have nothing in common with a group of people who often dress strangely, speak a specialized language, live in ... er ...”

----Terry Pratchett

A Starting ThoughtA Starting Thought

OutlineOutline• Introduction and Context

1. Cosmological principles

2. The Universe in photons

3. The seeds of structure

4. Light, the deceptive messenger

1. Evidence for Dark Matter (a partly historical review)1. Fritz Zwicky and the Coma Cluster

2. Vera Rubin and galaxy rotation curves

3. Strong lensing

4. The cosmic microwave background

5. Alternate explanations?

2. What Dark Matter is not – astrophysical and cosmological constraints

• Introduction and Context1. Cosmological principles

2. The Universe in photons

3. The seeds of structure

4. Light, the deceptive messenger

1. Evidence for Dark Matter (a partly historical review)1. Fritz Zwicky and the Coma Cluster

2. Vera Rubin and galaxy rotation curves

3. Strong lensing

4. The cosmic microwave background

5. Alternate explanations?

2. What Dark Matter is not – astrophysical and cosmological constraints

Some Principles of CosmologySome Principles of Cosmology• THE Cosmological Principle:

Averaged over sufficiently large distances, there are no preferred directions or preferred places in the Universe

• We cannot conduct experiments. Only observations. • Looking across a large distances implies looking back in time.

• THE Cosmological Principle: Averaged over sufficiently large distances, there are no preferred directions or preferred places in the Universe

• We cannot conduct experiments. Only observations. • Looking across a large distances implies looking back in time.

seen as 1 year old boy

69 light years

70 year old man70 year old man 70 year old man

The Universe in Photons

(…is a beautiful place!)

The Universe in Photons

(…is a beautiful place!)

Primordial Seeds of StructurePrimordial Seeds of StructureUniverse

at t = 380,000 yrsUniverse

at t = 380,000 yrst = 13.7 billion yrs

(today)t = 13.7 billion yrs

(today)

Deep Hubble ImageDeep Hubble Image Galaxy like Milky WayGalaxy like Milky Way

t = 8 billion yrst = 8 billion yrs

Cosmic Microwave BackgroundCosmic Microwave Background

Cosmic Ripples grow into

enormous waves via gravity

Cosmic Ripples grow into

enormous waves via gravity

But… Light Can Be DeceivingBut… Light Can Be Deceiving

NASA

Some Evidence for Dark Matter

(a partly historical review)

Some Evidence for Dark Matter

(a partly historical review)

The Virgo ClusterThe Virgo ClusterNoted by Charles Messier (1781)Noted by Charles Messier (1781)

The Coma ClusterThe Coma Cluster‘discovered’ by William Herschel (1785)‘discovered’ by William Herschel (1785)

•large scale excesses of galaxies have been known noted for several centuries, but only entered into scientific discussion in the early 20th Century, once it was demonstrated (by Edwin Hubble and others) that galaxies were truly distant objects

•large scale excesses of galaxies have been known noted for several centuries, but only entered into scientific discussion in the early 20th Century, once it was demonstrated (by Edwin Hubble and others) that galaxies were truly distant objects

… on the 40 inch telescope at Mt. Wilson … on the 40 inch telescope at Mt. Wilson

Enter Fritz Zwicky…Enter Fritz Zwicky…

Fritz ZwickyFritz Zwicky (from the Caltech archives)(from the Caltech archives)

• Zwicky and Sinclair Smith measured the mass of the Coma Cluster from this effect in the 1930s

• Zwicky and Sinclair Smith measured the mass of the Coma Cluster from this effect in the 1930s

Dark Matter in the Coma ClusterDark Matter in the Coma Cluster•Doppler shifts of galaxies measure velocity

•scatter in velocities of galaxies encodes the mass

•Doppler shifts of galaxies measure velocity

•scatter in velocities of galaxies encodes the mass

Dark Matter in the Coma ClusterDark Matter in the Coma ClusterZwicky’s initial result: mass based on 8(!) galaxies:

•if galaxy clusters are bound, then they are far too massive and hence some form of ‘dark matter’ must exist

•Zwicky also noted that galaxy clusters are probably the best place to look for strong lensing (more on this later)

“It will, nevertheless, be advisable to obtain more data on the velocities of both cluster nebulae and field nebulae in order to arrive at accurate values of

the dispersion…”

•this result gained little credence for decades: reasons are complex, but the best reason I have heard discussed is the change in the Hubble constant over intervening decades due to recognition of two types of Cepheid variables (which reduced the discrepancy) as well as various astrophysical concerns – dust and gas in galaxies, and whether clusters are gravitationally bound

Zwicky’s initial result: mass based on 8(!) galaxies:

•if galaxy clusters are bound, then they are far too massive and hence some form of ‘dark matter’ must exist

•Zwicky also noted that galaxy clusters are probably the best place to look for strong lensing (more on this later)

“It will, nevertheless, be advisable to obtain more data on the velocities of both cluster nebulae and field nebulae in order to arrive at accurate values of

the dispersion…”

•this result gained little credence for decades: reasons are complex, but the best reason I have heard discussed is the change in the Hubble constant over intervening decades due to recognition of two types of Cepheid variables (which reduced the discrepancy) as well as various astrophysical concerns – dust and gas in galaxies, and whether clusters are gravitationally bound

… measuring spectra… measuring spectra

Enter Vera Rubin…Enter Vera Rubin…

Vera RubinVera Rubin(from Vera Rubin)(from Vera Rubin)

• mass keeps spinning objects together! • mass keeps spinning objects together!

Rotation Curves of Spiral GalaxiesRotation Curves of Spiral Galaxies

•If light traces mass…

•…we expect the rotation speed of galaxies to fall with radius

•If light traces mass…

•…we expect the rotation speed of galaxies to fall with radius

(image from John Lanoue)

A `Keplerian’ Rotation Curve

• Consider the Solar System: from a mass perspective, only the Sun counts as it is more than 99.9% of all the mass. What behavior does this produce in the orbits of the planets?

• Rubin and Ford measured the mass of the Andromeda galaxy this way in 1970: the rotation curve does not fall!

• Rubin and Ford measured the mass of the Andromeda galaxy this way in 1970: the rotation curve does not fall!

Rotation Curves of Spiral GalaxiesRotation Curves of Spiral Galaxies

(from Rubin and Ford 1970)

DARK MATTER

Galaxies and Dark MatterGalaxies and Dark Matter

The presence of a living fish implies the presence of a bowl of water – even if we can only see the fish! Similarly, the presence of rapid rotation in the outer regions of spiral galaxies implies the presence of lots of mass at large distances – even if we cannot see it directly.

The presence of a living fish implies the presence of a bowl of water – even if we can only see the fish! Similarly, the presence of rapid rotation in the outer regions of spiral galaxies implies the presence of lots of mass at large distances – even if we cannot see it directly.

Gravitational LensingGravitational Lensing

Can we ‘see’ mass?Can we ‘see’ mass?

Gravity Warps Space

By carefully considering the perturbations to background sources imposed by a foreground mass distribution (such as a cluster of galaxies) we can measure the mass distribution in detail.

By carefully considering the perturbations to background sources imposed by a foreground mass distribution (such as a cluster of galaxies) we can measure the mass distribution in detail.

Strong Gravitational Lensing: Galaxy Clusters

Strong Gravitational Lensing: Galaxy Clusters

This effect, predicted to occur in galaxy clusters by

Zwicky in 1933, was discovered in two clusters

in 1986

This effect, predicted to occur in galaxy clusters by

Zwicky in 1933, was discovered in two clusters

in 1986

Giant Lensed ArcsGiant Lensed Arcs

Weighing the Dark Matter

Images from Broadhurst et al. (2005)

With sufficiently well-observed lenses we can ‘weigh’ the dark matter, in detail. ‘Sufficiently well’ means observations from big telescopes and HST!

light versus mass

Mass Cluster of galaxies: 1015 Suns ( about 1048 g !)

Physical Size 1 Megaparsec (31024 cm) ~ 20 size of Milky Way

Type of Matter Dark matter dominated

Density 10-25 g cm-3 (air = 10-3 g cm-3; best lab vacuum = 10-20 g cm-3)

Pulling together dynamical measurements and gravitational lensing results, what can we infer about dark matter in clusters of galaxies?Pulling together dynamical measurements and gravitational lensing results, what can we infer about dark matter in clusters of galaxies?

Dark Matter: PropertiesDark Matter: Properties

Dark Matter is dominant but NOT dense: locally – e.g., on scales of the Solar System, it isn’t relevant! But, on scales of galaxies and larger, it is!Dark Matter is dominant but NOT dense: locally – e.g., on scales of the Solar System, it isn’t relevant! But, on scales of galaxies and larger, it is!

Primordial Seeds of StructurePrimordial Seeds of StructureUniverse

at t = 380,000 yrsUniverse

at t = 380,000 yrst = 13.7 billion yrs

(today)t = 13.7 billion yrs

(today)

Deep Hubble ImageDeep Hubble Image Galaxy like Milky WayGalaxy like Milky Way

t = 8 billion yrst = 8 billion yrs

Cosmic Microwave BackgroundCosmic Microwave Background

Cosmic Ripples grow into

enormous waves via gravity

Cosmic Ripples grow into

enormous waves via gravity

Cosmological Evidence

• the cosmic microwave background gives strong evidence for dark matter

• the evidence comes from an analysis of the power spectrum of the CMB – not from an analysis of individual astronomical objects

• what exactly do we mean by the power spectrum in this case...?

What is this??

photos by John Carlson

Keisler et al. 2011

Cosmological Evidence• these bumps in the power spectrum of the CMB are small – enhancements of only a

few parts in 100,000 – but are a critical line of evidence for dark matter• in the early universe, normal matter (baryons – protons are what matter) are

closely coupled to photons (a so-called photon-baryon fluid). Radiation pressure keeps this fluid rather smoothly distributed – small acoustic perturbations are what we see as power in the CMB

• however, if the underlying matter density – of all mass – were actually this smooth we’d not have galaxies around us today! Something must be clumpier at that epoch; that something is dark matter (it has to be dark –i.e. not interacting with photons – otherwise it too would be subject to the smoothing effects of radiation in the early Universe...)

• a careful modeling of the CMB power spectrum provides limits on both the baryonic and dark matter components of the Universe

MACHOs ? (nope!)• astronomers have also explored the possibility that dark matter is

composed of massive, compact, but fundamentally baryonic objects : perhaps brown dwarfs, neutron stars, unassociated planets. This class of models – so called Massive Compact Halo Objects – have basically been ruled out by micro-lensing experiments, which exclude the existence of a significant population of such object in the halo of our own galaxy (and by cosmological principle elsewhere in the Universe – remember we are not special!)

Alternate Explanations?: MoNDAlternate Explanations?: MoNDCan we explain current data using ‘Modified Newtonian Dynamics’?Can we explain current data using ‘Modified Newtonian Dynamics’?

Clowe et al. ‘direct’ detection

of Dark Matter

NO NO

Note the positional disconnect between the mass inferred from lensing and the hot x-ray emitting gas; can’t just crank up the mass associated with the light to get this!

Note the positional disconnect between the mass inferred from lensing and the hot x-ray emitting gas; can’t just crank up the mass associated with the light to get this!

RecapRecap

• the existence of dark matter is now well established

• dynamics of galaxies, masses of clusters, and gravitational lensing all tell the same story; most of the clustered mass in our universe IS dark!

• alternate explanations – modifications of the gravitational force law – do not appear viable

• the existence of dark matter is now well established

• dynamics of galaxies, masses of clusters, and gravitational lensing all tell the same story; most of the clustered mass in our universe IS dark!

• alternate explanations – modifications of the gravitational force law – do not appear viable

RecapRecap

• purely baryonic dark matter has been ruled out

• baryonic dark matter would not explain the CMB, and the compact objects it would form do not exist!

• we are left with a particle physics explanation: weakly interacting massive particles (WIMPs)

• purely baryonic dark matter has been ruled out

• baryonic dark matter would not explain the CMB, and the compact objects it would form do not exist!

• we are left with a particle physics explanation: weakly interacting massive particles (WIMPs)

“They both savored the strange warm glow of being much more ignorant than ordinary people, who were ignorant of only ordinary things.”

--- Terry Pratchett, Equal Rites

“The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it.”

--- Terry Pratchett, Diggers

“They both savored the strange warm glow of being much more ignorant than ordinary people, who were ignorant of only ordinary things.”

--- Terry Pratchett, Equal Rites

“The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it.”

--- Terry Pratchett, Diggers

Parting ThoughtsParting Thoughts