Reconstructing the large scale structure

Laszlo DobosDept. of Physics of Complex System

dobos@complex.elte.huE 5.60

April 20, 2018

Structures of the large scale distribution

Beyond the scale of galaxy clusters: the cosmic web

Voids

I diameter in the 10-150 Mpc range

I appear almost empty (there’s primordial gas + DM)

I almost entirely devoid of galaxies

Superclusters

I structures way larger than galaxy clusters

I not bound gravitationally

Filaments and voids

I walls around voids

I filaments connect superclusters

Reconstructing the matter distribution

Galaxy bias: galaxies are only the tip of the iceberg

I there’s an underlying, invisible structure

I this structure organizes the cosmic web

I galaxies only in the deepest potential wells

Galaxies: discrete points in the deepest potential wells

I the goal is to reconstruct the continuous potential field

I geometric triangulation methods

Delaunay triangulation1

I connect discrete points with simplexes2 in a predefined way

I where points are dense, simplexes are small in volume

I local density is estimated by the inverse volume of simplexes

1Dalunay Tessellation Field Estimator (DFTE)2triangles, tetrahedrons, etc.

Cosmic structures3

3Forras: Willem Schaap & Rien van de Weijgaert

Reconstructing the velocity field

Motion of galaxies in the local universe

I collective motion with approx. ∼ 600 km s−1

I towards a “Great Attractor”

Dark flow: collective motion of galaxies towards certain directions

I independent of Hubble flow

I due to the large scale structure

I peculiar velocities cannot independently be measured

I important future goal: reconstruct the velocity field

Cosmic velocity field from simulations

Different flavors of matter

I radiation (not significant, mostly from the cosmic background)

I light-emitting baryonic matter (stars in galaxies)

I dark baryonic matter (mostly H-He clouds)

I dark matter

I dark energy

Detecting the neutral Hydrogen: Lyman-α cloudsAbsorption lines of neutral hydrogen

I Lyman and Balmer series

I shortest wavelength: Lyman-α (1216 A)

Lyman-α forest:

I light ray from distant quasar intersected by H clouds

I quasar spectrum shows Ly-α line at (1 + z)λLy-α wavelength

I H clouds between quasar and as at various redshifts

I absorb light from quasar at different wavelengths dependingon z of cloud

Can be used to map the distribution of hydrogen

I only in directions of bright quasars

I resulting map is in accordance with cosmic structure ofgalaxies

I clouds in shallower potential wells than galaxies

Lyman-α forest

Lyman-α forest

Hydrogen in the early universe

around z ' 5–6: universe was significantly smaller

I the scale factor was (1 + z)−1

I galaxies in a swiss cheese of neutral H-He gas

I still total Lyman-α absorption in all directions

Reionization

I first stars and quasars: UV radiation

I ionized the surrounding gas

I at early times only ionized “bubbles”

I much later all gas is ionized but also less density

I epoch and length of reionization is an important parameter ofmodels

Quasars in the early universe

We see them at extremely large redshifts also

I z > 6, but there are quasars at z = 7

I at these early time the gas was only ionized in bubbles

I total cut-off in quasar spectrum below (1 + z)λLy-αI Gunn–Peterson trough: z > 6 quasars

Direct detection of neutral hydrogen

Neutral atomic hydrogen emits microwave radiation

I hyperfine structure of energy levels of the H-atom

I two different electron spin states wrt. the proton’s spin

I transition between the two states with 5,87 µeV

I f = 1420 MHz, λ = 21 cm, but highly redshifted

The transition is strongly prohibited

I lifetime of excited state ∼ 107 yr

I cannot be observed in the lab

I collision of H atoms in tenuous gas is very unlikely

I due to Heisenberg’s uncertainty principle, the line is very thin∆E∆t & ~

I (excellent to measure Doppler shift of gas within the MilkyWay)

Distribution of neutral H before reionization

21 cm radio observations at z � 6 redshift

I investigate the dark ages of the universe

I observations at a few hundred MHz

I very problematic due to background noise

I UHF TV stations, ionosphere

Observational projects

I LOFAR: Low Frequency Array, the Netherlands

I SKA: Square Kilometre Array, Western Australia, South Africa

In search of dark matter

Based on the rotation curve of galaxies, there’s more mass in thehalo than what emits light.

Possible candidates for the missing mass

I non-emitting compact object

I exotic matter

Observations: via its gravitational effects

I rotation curve

I virial mass via velocity dispersion

I gravitational lensing

Massive compact halo objects

MACHOs: massive compact halo objects

I non-emitting, compact objects in galaxies

I giant planets (failed stars)

I stellar mass black holes

I cooled down, faint white dwarfs

Observing faint white dwarfs

I Hubble space telescope

I only a weak upper limit on their total mass

I at most 5-10 % of the mass of the total halo

Gravitational microlensingsearching for MACHOs:

I gravitational microlensing bymassive objects

I follow changes of brightnessof background stars

I idiosyncratic brightening atall wavelength

Microlensing events are rare

I background star must alignwith MACHO

I long, systematicobservations are necessary

Weakly interacting massive particlesWeakly interacting massive particles (WIMPs)

I exotic particles not described by the standard model

I mostly interact gravitationalyy

I take part in weak interaction with extremely small effectivecross section

Possible candidates:

I neutrinos, but they don’t have the sufficient mass

I nautralinos from super symmetric theories

I strangelets made up of strange quarks

WIMPs might decay

I e.g.: AMS4 experiment on the international space station

I look for positrons originating from annihilation of WIMPs

I annihilation could also produce gamma photons, Fermisatellite

4Alpha Magnetic Spectrometer

Experiments to detect the decay of WIMPsWIMP interact only via gravity and weak (or weaker) interaction

Indirect detection

I detect decay products from annihilation

I gamma photons from annihilation (Fermi LAT)

I high energy neutrinos as byproduct of annihilation (IceCube)

Direct detection

I WIMPs might interact very weakly with baryonic material

I phonons (oscillations) produced in crystal detectors

I scintillation detectors, bubble chambers etc.

I need extremely large detector volume

Look at directions where a lot of DM detected gravitationally

I but electromagnetic background is low

I centers of nearby satellite galaxies

Gravitational lensing

Galaxy clusters

I we discussed them before

I significant mass: 1015 M� in larger clusters

I 90% dark matter

I but most galaxies are not in clusters

Weak gravitational lensing

I voids, filaments are formed by dark matter

I try to measure their weak gravitational lensing effect

I images of background galaxies are slightly distorted

I cosmic shear

Visualization of weak lensing

Effect of lensing foreground on images of background galaxies

I only background galaxies are shown

I apparent effect is magnified several times

I in reality, can only be detected statistically

Distortions caused by the weakly lensing foreground

Measuring weak gravitational lensingDirectly measurable quantities

I redshift (what’s in the foreground, what’s in the background)I complex excentricity of images of galaxies:

χ =a2 − b2

a2 + b2e2iφ,

where a and b are the major and minor axes, φ is positionangle

I complex number contains position angleI with no weak lensing, average over an area of the sky must be〈χ〉 = 0

Needs precise measurements of morphological parametersI requires good seeingI smallest possible point spread functionI at the same time, large field of viewI PanSTARRS didn’t have the seeingI Dark Energy Survey (DES), LSST

Complex excentricity

χ =a2 − b2

a2 + b2e2iφ,

I invariant to 180◦ rotations

I distortions of a circle, as infigure

Describing cosmic shear

Start from complex excentricity of background galaxies

χ =a2 − b2

a2 + b2e2iφ = γe2iφ,

Connect to a so called shear tensor:

A = (1− κ)

[1 00 1

]− γ

[cos 2ξ sin 2ξsin 2ξ − cos 2ξ

]

I first term: κ, magnification (brightening)

I second term: γ, shear (distortion)

I ξ is the angular distance from the lens

I shear tensor is defined on the surface of the sphere

I if known, distribution of lensing mass can be computed