Cosmological evidence for dark matter. Cosmic microwave background: t ~ 300,000 yrs Galaxy distribution `now : t ~ 13 billion yrs

Cosmological evidence for dark matter

Cosmic microwave

background: t ~ 300,000 yrs

Dark matter history :

● 1930s – 1960s: large velocities of galaxies in Coma cluster (Zwicky).

Andromeda galaxy is approaching us – large total mass for Local Group of galaxies. Not treated as a major problem.

● 1970s: spiral galaxy rotation speeds (Vera Rubin, Bosma, others).

Dark matter started to be taken seriously – but generally assumed to be “ordinary” matter in hard-to-see form, e.g. very dim stars, Jupiter-like objects, etc.

● 1980s: “new particle” dark matter theories arise – large computer

models show these help galaxy formation. Gravitational lensing observations support dark matter.

● 1998 – 2003: new cosmological observations lead to a “standard

model of cosmology”, Lambda-CDM: the cosmic recipe is 5% ordinary matter, 25% “Cold Dark Matter, 70% “dark energy”.

● 2003 – now : more observations pin down Lambda-CDM to about 1

Dark matter history :

● 1930s – 1960s: large velocities of galaxies in Coma cluster (Zwicky).

Andromeda galaxy is approaching us – large total mass for Local Group of galaxies. Not treated as a major problem.

● 1970s: spiral galaxy rotation speeds (Vera Rubin, Bosma, others).

Dark matter started to be taken seriously – but generally assumed to be “ordinary” matter in hard-to-see form e.g. very dim stars, Jupiter-like objects, etc.

● 1980s: “new particle” dark matter theories arise – large computer

models show these help galaxy formation. Gravitational lensing observations support dark matter.

● 1998 – 2003: new cosmological observations lead to a “standard

model of cosmology”, Lambda-CDM: the cosmic recipe is 5% ordinary matter, 25% “Cold Dark Matter, 70% “dark energy”.

● 2003 – now : more observations pin down Lambda-CDM to about 1

Rotation of spiral galaxies :

●

Over half of all galaxies are “spiral” galaxies

●

For a spiral galaxy, most of their stars are in a fairly “thin”

rotating disk.

●

It’s not “solid-body” rotation – outer stars take longer to orbit

than inner stars.

●

Most of the stars (and gas) are on roughly circular orbits in a

common direction.

●

We can measure the (towards/away) component of velocity

quite easily, using the Doppler shift of spectral lines. (but

sideways motions across the sky are too small to measure).

●

Some typical numbers are 200 km/sec , or 0.07 percent of

Gravitational lensing :

●

According to Einstein’s general relativity, massive objects

bend light-rays passing nearby - “gravitational lensing”.

●

First measurement in 1919 : stars near the Sun shifted in

position during a total eclipse – made Einstein famous

overnight.

●

Today, verified to about 0.1 percent accuracy.

●

Clusters of galaxies contain lots of mass (10

solar) in a

Hubble Space telescope image:

Origin of the Cosmic Microwave Background

Universe cools as it

expands.

The critical density

 _{Gravity tends to decelerate expansion.}

 _{If Universe is “matter only” there is a critical density : Universe will} expand forever if matter density < critical density, or collapse to a Big Crunch if density > critical.

Convenient to express all densities of species X

by

Big bang – mysteries pre-1980.

 _{Horizon problem – why do separate patches of the CMB sky look the} same to 1 part in 100,000 ?

 _{Flatness problem: Ωtot = 1 is an unstable point; since it’s larger than} 0.1 today, back in the distant past we had

0.99999999… < Ωtot < 1.0000000…01 ; why ?

 _{Why is the universe lumpy, i.e. why do galaxies exist at all, rather than} the universe containing a uniform ultra-thin gas with no galaxies,

stars, planets ? Must have been some early density variations to kick-start formation of galaxies.

 _{Dark matter : does it exist, and if so what is it ?}

The mass-energy content vs time

Cosmological density fluctuations

Power spectrum: P

(k) = P

(k) T

(k; 

m

,h,

...)

Galaxies, clusters of galaxies and

superclusters exist:



_{There must have been seed fluctuations at early times.}

Gravity makes things grow (dense regions get denser), .

so “small” initial ripples can be enough.

(But “randomly placed atoms” gives far too small

ripples: can’t make galaxies by today).

Growth to present

depends on k & matter

content etc.

Initial seeds

1992: COBE

satellite measured

CMB over full sky:

Same picture with

contrast turned up:

dipole at 0.1%

Subtract dipole,

contrast up

another 100x :

CMB anisotropy !

The WMAP satellite:

2003: The WMAP map (Galaxy-subtracted).

WMAP is much sharper than COBE was.

Planck spacecraft –

Spherical Harmonics

Planck spacecraft: 2013

Curvature

Now six acoustic peaks visible …. Red dots = Planck ;

Reason for CMB acoustic peaks



_{Before recombination, CMB was hot, > 3000 K, hydrogen}

was fully ionized: protons + electrons.



_{Photons regularly scattered off free electrons, therefore did}

a “random walk” and could not move far overall.



_{In an overdense region, gravity tried to compress the}

matter and radiation, but a compressing region got hotter ,

and photon pressure resisted the compression.



_{This led to huge “sound waves” of all lengths; get an}

enhancement when one wavelength ~ size of universe at

recombination, and harmonics of this, i.e. acoustic peaks.



_{Dark matter only feels gravity, so ordinary + dark matter}

CMB pattern depends on densities

Animations by W.Hu, U Chicago

Dark matter density increasing: smaller atoms/DM ratio,

peaks go down

Curvature changes: peak is same actual size but seen at a different angle.

Photons, dark matter, baryons all significant. Gravity is squashing

overdense regions: photon pressure pushes against baryons but not DM. Velocities have an influence too… it’s complicated.

Fine detail: 2-deg NGP slices (1-deg steps)

2dFGRS: b_J < 19.45

2dF Galaxy Redshift Survey gives two

routes to 

:



Power spectrum shape: 

h  0.20  0.03



_{Line of sight squashing of structure:}

Density of dark matter + dark energy:

Matter density → Dark energy

The strange concordance universe

 _{Flat universe:}

5% normal matter + 25% dark matter + 70% dark energy = 100% of critical density.

 _{Just 6 numbers are enough to}

describe all the large-scale features of the Universe:

 _{Any two densities above} (3rd adds to 1)

 _{Hubble constant ~ 70 km/s/Mpc.}  _{Size and slope of inflation ripples}  _{“Optical depth” to CMB surface.}

The strange concordance universe

 _{Concordance universe works amazingly well and doesn’t rely heavily on} any one observation.

 _{The ratio of (dark matter)/(normal matter) is known to be near 5.3, with} uncertainty about 0.1.

 _{But, everything is far from “solved”. 95% of the universe is unidentified,} and there are many strange coincidences and open questions….

 _{Major unsolved questions -}

– What is the dark matter - lightest supersymmetric particle ? If so, maybe make it at LHC, or find it down a deep mine…

– Why are the densities of atoms, dark matter, dark energy roughly similar ?

– Is the dark energy Einstein’s “cosmological constant” or something more complicated ?