Dark Matter and the Dark Matter and the Structure of the UniverseStructure of the Universe

Jaan EinastoJaan Einasto

Tartu ObservatoryTartu Observatory

04 October 20004 October 20077

OverviewOverview

1. Dark matter story: local dark matter2. Global dark matter: early studies3. Global dark matter: mass distribution in galaxies4. Dark matter in galaxies, further evidence5. Structure of the Universe6. Cold Dark Matter7. Matter/Energy content of the Universe8. Conclusions

Local dark matterLocal dark matter

Öpik (1915)Öpik (1915)Oort Oort (1932(1932, 1960, 1960))Kuzmin (1952, 1955)Kuzmin (1952, 1955)Eelsalu (1959), JEelsalu (1959), Jõeveer (197õeveer (1972, 2, 19741974))Bahcall Bahcall (1985)(1985)Gilmore et al (1989)Gilmore et al (1989)

Global dark matterGlobal dark matter

Zwicky (1933)Zwicky (1933)Kahn & Woltjer (1959)Kahn & Woltjer (1959)Stability Symp. 1961Stability Symp. 1961Einasto et al (1974), Ostriker et al (1974)Einasto et al (1974), Ostriker et al (1974)Bosma (1978), Rubin et al (1978)Bosma (1978), Rubin et al (1978)Blumenthal et al (1984)Blumenthal et al (1984)

1. The dark matter story1. The dark matter storyLocal Dark Matter: invisible matter in the Galaxy in Solar vicinityGlobal Dark Matter: invisible matter surrounding galaxies

Ernst Öpik with his wife and Grigory Kuzmin in 1970s author in 1970 IAU Brighton The amount of local dark matter is small, if anythingDiscussion continued until 1990; Tartu results confirmed

Local dark matterLocal dark matter

2. Global Dark Matter. Early Studies 2. Global Dark Matter. Early Studies

Zwicky (1933) – velocity dispersion of galaxies in Coma cluster

⇩M/L ≈ 300 – 900 M/Lsun

Groups and pairs of galaxies M/L ≈ 30 – 60 M/Lsun.

Galaxy populations M/L ≈ 1 – 10 M/Lsun.

In outskirts of galaxies the rotational speed should decrease according to Newton law.

Galaxy and M31 Models Galaxy and M31 Models

Galactic models predict the decrease of rotation velocity with distance. Actually the speed is almost constant. Two solutions:

1) Galaxies are surrounded by massive coronas;2) There exist large non-circular motions.

Corona cannot be stellar (properties differ from extreme halo stellar populations), consists probably of hot gas. (Einasto 1972)

Left: Roberts & Rots, AA 26, 483, 1973Right: Roberts, Whitehurst, ApJ 201, 327, 1975Using data by Rubin, Ford, ApJ 159, 379, 1970:

Rotation curves decline slowly, if at all, at large radii, implying significant mass density at large distances. Mass within 24 Kpc is 1.8 1011 Msun .

The internal mass and mass-luminosity ratio increase with radius. How much?

3. Mass distribution in 3. Mass distribution in galaxiesgalaxies

Companions of giant galaxies measure the internal mass inside their orbit

V2(r) = G M(r)/r

Winter School in Caucasus, January 1974

Einasto, Kaasik & Saar, Nature (1974)massive corona

Ostriker, Peebles & Yahil ApJ (1974)massive halo

All galaxies are surrounded by massive coronas/halos, total masses and radii of galaxies exceed previously accepted values about tenfolds. Dark matter is the dominant population in Universe.These papers triggered a ‘dark mass boom’

Burbidge (1975)• If companions are not physical, then they do not measure the internal mass

of the main galaxy, but characterize mean random velocities of galaxies.

Materne & Tammann (1975)• Groups of galaxies are bound with conventional masses M/LS = 4, M/LE = 30

• Mass determinations of spiral galaxies from rotation curves: in many cases rotation curves decrease, also deviations from circular motion possible.

• High masses of clusters may be explained by high masses of cluster dominant cD galaxies (there is no extra mass in clusters).

• Big Bang nucleosynthesis suggests a low-density universe = 0.05, the smoothness of the Hubble flow favors low- density universe.

Objections to Dark Matter Concept: Objections to Dark Matter Concept: Arguments for the Classical ParadigmArguments for the Classical Paradigm

Massive halo (corona) cannot be stellar (Saar) nor gaseous (Novikov).

Of what stuff halos are made of?

In nature everything has its purpose. If 90% of matter is dark, then this must have some purpose.

What is the purpose of dark matter?

Problems of the Dark Matter ConceptProblems of the Dark Matter Concept

4. Dark matter in galaxies: further evidence4. Dark matter in galaxies: further evidenceAre Companions Physical Members?Are Companions Physical Members?

Morphological type of companions depends on distance from main galaxy: mean companions elliptical, distant spiral or irregular. This is possible only if companions are physical. Einasto et al (1974)

New New evidenceevidence: M/L of : M/L of galactic populationsgalactic populations

M/L of bulges (dominant population of E galaxies) is ~ 3M/L of bulges (dominant population of E galaxies) is ~ 3 Masses of stellar populations of elliptical galaxies much Masses of stellar populations of elliptical galaxies much

less than thought previously less than thought previously (Faber et al. 1977) (Faber et al. 1977)

Bosma (1978) and Rubin et al (1978, 1979, 1980) observed extended rotation curves of spiral galaxies. All have flat rotation curves indicating the growth of internal mass until outer boundaries of galaxies.

New Evidence: rotation of galaxiesNew Evidence: rotation of galaxies

New evidence:X-ray Observations of ClustersNew evidence:X-ray Observations of Clusters

Coma cluster in optical and X-rays.X-ray gas is in hydrodynamical equilibrium: particle velocities correspond to the depth of the potential well. This allows to derive cluster mass. Results in agreement with virial masses.

New evidence: Gravitational LensingNew evidence: Gravitational Lensing

Cluster mass from lensing support high M/L as found by X-ray and velocity dispersion data

COSMOS weak gravitational lensing map of the dark matterMassey et al. Nature, 18 Jan 2007

Strong gravitational lensing divides the light of background quasar into 5 imagesAstronomy Picture of Day 24 May 2006 (Sharon et al.)

BarBaryonic dark matteryonic dark matterSome dark matter can be baryonic. How much?

Local DM must be baryonic: a flat disk can form only via dissipation of interstellar gas to form a flat disk. Its total mass is according to Kuzmin, Gilmore … rather small.

What about DM in galactic haloes?MACHO project: detect via microlensing invisible stellar objects in halo. Result: total mass contribution negligible.

Red halos (M31 …). Probably old population with bottom-heavy IMF. Total mass in the same range as masses of conventional stellar populations. Negligible contribution to dark halo.

Nucleosynthesis argument still valid: b = 0.04

If companions are not physical, then they do not measure the internal mass of the main galaxy, but characterize mean random velocities of galaxies. Burbidge (1975) Physical nature of companions confirmed

Mass determinations of spiral galaxies from rotation curves: in many cases rotation curves decrease, also deviations from circular motion are possible.

Rotation curves of almost all spiral galaxies are flat

Groups of galaxies are bound with conventional masses M/LS = 4, M/LE = 30 M/L of luminous populations is much lower

The Status of Objections (Late 1970)The Status of Objections (Late 1970)

High masses of clusters may be explained by high masses of cluster dominant cD galaxies (there is no extra mass in clusters)

Existence of diffuse matter in clusters confirmed BUT

Big Bang nucleosynthesis suggests a low-density universe = 0.05, the

smoothness of the Hubble flow favors low-density universe ?

Acceptance of Dark MatterAcceptance of Dark MatterAstro-Particle PhysicsAstro-Particle Physics

Tallinn, April 1981 – Conference on astrophysics & particle physics (birth of Astro-particle physics)

Eddington's test: No experimental result should be believed until confirmed by theory (cited after Turner 1999).

Chernin: If DM is non-baryonic (neutrinos), then this helps to explain the paradox of small temperature fluctuations of cosmic microwave background radiation.

The purpose of dark matter: Dark matter is needed to start early enough gravitational clustering to form structure. Initially it was thought that DM is due to massive neutrinos. This solves the Big-Bang Nucleosynthesis controversy

Now the presence of dark matter was accepted by leading theorists.

Jim Peebles and Scott Tremaine 1977 Yakov Zeldovich with his wife, late 1970s

When you have eliminated the impossible, whatever remains, however improbable, must be the truth (Sherlok Holmes, cited by Binney & Tremaine 1987)

Zeldovich (speech on Tallinn conference banquet 1981): Observers work hard in sleepless nights to collect data; theorist interpret observations, are often in error, correct their errors and try again; and there are only very rare moments of clarification. Today it is one of such rare moments when we have a holy feeling of understanding secrets of the Nature. Non-baryonic dark matter is needed to start structure formation early enough.

5. Structure of the Universe: 5. Structure of the Universe: Zeldovich question 1972: find evidence what Zeldovich question 1972: find evidence what

structure formation theory is correctstructure formation theory is correct

Our previous experience from galactic Our previous experience from galactic studies shows that galactic populations studies shows that galactic populations evolve slowly and the present structure evolve slowly and the present structure contains info on the formation and evolutioncontains info on the formation and evolution

Numerical simulations by Zeldovich group Numerical simulations by Zeldovich group suggest the formation of a cellular network suggest the formation of a cellular network with high-density knots, chains connecting with high-density knots, chains connecting knots, and low-density regions in betweenknots, and low-density regions in between

During the whole Hubble time a galaxy with During the whole Hubble time a galaxy with peculiar motion ~300 km/s has moved from peculiar motion ~300 km/s has moved from his place of origin a few Mpc. Thus the his place of origin a few Mpc. Thus the present distribution of galaxies is close the present distribution of galaxies is close the the distribution at formation.the distribution at formation.

If Zeldovich is right, we should observe If Zeldovich is right, we should observe high- and low-density regions forming a high- and low-density regions forming a cellular network cellular network

Looking for galaxy systemsLooking for galaxy systems

Distribution of galaxies (dots) and Abell clusters (filled circles) in the Perseus Supercluster (Joeveer, Einasto 1977).Galaxies and clusters populate identical regions (superclusters), regions between superclusters almost void of galaxies.

Conclusions: galaxies cannot cluster from random initial positions to chains – they must already form in supercluster chains

Distribution of galaxies in the Local Distribution of galaxies in the Local SuperclusterSupercluster

Supercluster-void networkSupercluster-void network

2dF Northern and Southern regions contain superclusters of various richness and voids

Structure of Structure of rich rich

superclusterssuperclusters

High- and low-resolution view of the richest supercluster in 2dF Northern part.Large number of compact density knots (DF-clusters) are visible

6. Cold Dark Matter6. Cold Dark MatterMany people thought the early universe was complex (mixmaster universe Misner, explosions Ostriker etc.)

Zeldovich assumed that it was simple – just scale-free spectrum of adiabatic fluctuations of

a) baryons,and when that failed [(ΔT/T)CMB < 10-4] and Moscow physicists thought they had discovered neutrino mass

(b) hot dark matter.Blumenthal and Primack thought simplicity is a good approach, and tried other simple candidates for the dark matter, first

(c) warm dark matter, and with Faber and Rees,(d) cold dark matter.

Hot dark matter has problems with fine structure and ages. Superclusters form too late and are structureless. Reason:Small-scale fluctuations are damped because of “free-streaming”.

Cold dark matter (axions or other weakly interactive particles) yields a model with very good agreement with observations. Model superclusters consist of galaxy and cluster chains as real superclusters (Melott, Saar, Einasto et al. 1983, Blumenthal et al. 1984).

Bullet-cluster – direct evidence for dark matterBullet-cluster – direct evidence for dark matter

Left: image of the merging-cluster 1E0657-558. Right: X-ray emitting gas. Bar is 200 kpc long, contours: gravitational potential reconstructed from weak lensing.

Due to collision of two clusters dissipationless stellar population & fluid (X-ray gas)

are separated. Gravitating potential follows stellar population → dark matter must be dissipationless, i.e. non-baryonic. Clowe et al. ApJ 648, L109, 2006

This excludes also all sorts of non-Newtonian gravity (MOND - Milgrom et al.)

7. Matter-energy density of the universe7. Matter-energy density of the universeTotal matter density estimates (dark and baryonic matter) yield values m ~ 0.3 of critical cosmological density.Baryonic matter density from baryon nucleo-synthesis constraints is b = 0.05 (h = 0.7)

From general theoretical considerations a closed model with total density equal to critical density is preferred.The difference 0.7 was attributed to cosmological constant Until recently there was little observational evidence for it.

Searching for Missing Energy/MassSearching for Missing Energy/MassDistant Supernova ProjectDistant Supernova Project

Comparing redshifts and magnitudes of nearby and distant type Ia supernovae it is possible to discriminate between different cosmological models

Left: the luminosity distance expressed as distance modulus for four cosmological models; in lower panel the empty Universe is subtractedRight: Hubble diagram for nearby and distant SNIa; in lower panel we plot residuals of distance moduli relative to the model M = 0.3, = 0.0 Model with M = 0.3, = 0.7 fits data best.Perlmutter, Schmidt 2003 (arXiv-0303428)

CMB dataCMB data

Comparison of CMB spectrum with model. The position of the 1st peak of the spectrum depends on the total matter/energy density. Data suggest that total = 1,i.e. there exist some cosmological constant or Dark Energy.

Model curve is calculated for Double Dark model: Dark Matter + Dark Energy + Baryonic Matter = Total Matter/Energy = 1 (critical density).

Combination of Supernove and CMB dataCombination of Supernove and CMB data

w = p/ is the equation-of-state parameter (p – pressure – density). w = 0 for matterw = -1 for vacuum energy

Why the Hubble flow is so quiet?Why the Hubble flow is so quiet?

Formation of the central Formation of the central cluster of a supercluster. cluster of a supercluster.

Due to multiple mergers Due to multiple mergers the central cluster is the central cluster is located at the very center located at the very center of the potential well of of the potential well of the supercluster and has the supercluster and has almost no peculiar almost no peculiar motion in respect to motion in respect to general Hubble flow.general Hubble flow.

The Role of Vacuum EnergyThe Role of Vacuum Energy

The local value of the Hubble constant (in the vicinity of the Local Supercluster) is almost equal to the global value in spite of the inhomogeneous mass distribution. Sandage et al. ApJ 653, 843, 2006

The role of vacuum energy to make the Hubble flow so quiet has been suggested by Chernin et al. (2001 – 2005)

Cluster evolution modeling and the discovery of vacuum energy removed the last arguments against Dark Matter.

Cosmic density pyramid

Primack 2007

8. Conclusion: Dark Matter8. Conclusion: Dark Matter

Dark Matter Story is a typical scientific revolution (Tremaine 1985).Dark Matter Story is a typical scientific revolution (Tremaine 1985). Evidence for dark matter has been collected independently in many

centres. There are 2 dark matter problems – dark matter in Galaxy disk, and

dark matter around galaxies and clusters. Dark matter in the disk is baryonic (faint stars or jupiters). The

amount is small. Dark matter around galaxies is non-baryonic Cold Dark Matter. It

constitutes about 0.25 of critical cosmological density.

Conclusion: Formation of the structure Conclusion: Formation of the structure in the Universein the Universe

■ Universe has structure: it contains of an infinite network of galaxy and cluster filaments which form enhancements – superclusters of galaxies

■ Structure formation started from small quantum fluctuations in the very early Universe. Initially the primordial matter was hot, we see this hot Universe as Microwave Background Radiation.

■ Expansion leads to cooling, atomic nuclei recombine with electrons forming neutral gas. By gravitational instability the density increases in high-density regions: rich become richer and poor poorer. Matter flows from various directions and collapses forming chains of galaxies, clusters and superclusters.

■ In low-density regions the matter density decreases – here cosmic voids form

Challenges for FutureChallenges for Future

WHAT IS THE DARK MATTER?WHAT IS THE DARK ENERGY?

Prospects for DIRECT and INDIRECT detectionof WIMPs and AXIONs are improving. But what kind of WIMP? SUSY LSP …

We can use existing instruments to measure w= P/ρ and see whether it changed in the past.Better telescopes (e.g. LSST, SNAP) willprobably be required both on the ground and inspace, according to the Dark Energy Task For.

Thank you!