COSMOLOGICAL PARAMETERS :

GALAXY SURVEY

SUBHASIS SHIT15PH40040

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR

INTRODUCTION

The term ‘cosmological parameters’ is forever increasing in its scope, and nowadays oftenincludes the parameterization of some functions, as well as simple number describing propertiesof the Universe.

The original usage referred to the parameters describing the global dynamics of the Universe,such as its expansion rate and curvature. Also now of great interest is how the matter budget ofthe Universe is built up from its constituents: baryons, photons, neutrinos, dark matter, and darkenergy.

There may also be parameters describing the physical state of the Universe, such as theionization fraction as a function of time during the era since recombination.

1. Direct measures of the Hubble Constant: The slope of the relation between the distance and recession velocity is

defined to be the Hubble constant H0.

The Hubble Space Telescope project found by using the empirical period-luminosity relations for Cepheid variable stars to obtain distances to 31galaxies, and calibrated a number of secondary distance indicators—TypeIa Supernovae (SNe Ia), the Tully–Fisher relation, surface-brightnessfluctuations, and Type II Supernovae—measured over distances of 400 to600 Mpc. They estimated H0 = 72 ± 3 (statistical) ± 7 (systematic)kms−1Mpc−1.

A recent study of over 600 Cepheids in the host galaxies of eight recentSNe Ia, observed with an improved camera on board the Hubble SpaceTelescope, was used to calibrate the magnitude–redshift relation for 240SNe Ia. This yielded an even more precise figure, H0 = 73.8 ± 2.4kms−1Mpc−1 (including both statistical and systematic errors).

The indirect determination of H0 by the Planck Collaboration found a lowervalue, H0 = 67.3 ± 1.2 kms−1Mpc−1.

2. Supernovae as cosmological probe:

Two major studies, the Supernova Cosmology Project and the High-z Supernova Search Team, found evidence for an accelerating Universe, interpreted as due to a cosmological constant or a dark energy component.

Combined with the CMB data (which indicates flatness, i.e., Wm + WL = 1), the best-fit values were Wm ≈ 0.3 and WL ≈ 0.7.

Taking w = −1, the SNLS3 team found, by combining their SNIa data with baryon acousticoscillation (BAO) and WMAP7 data, Wm = 0.279+0.019

−0.015 and WL = 0.724+0.017−0.016,

including both statistical and systematic errors.

3. Cosmic Microwave Background:

The CMB is the electromagnetic radiation left over from the time of recombination in Big-Bang cosmology.

It is the oldest light of the Universe. This CMB was discovered by Penzias and Wilson in 1964 accidentally.

Black body curve of the CMB. The DMR was able to detect the anisotropy of cosmic

background radiation, but the fluctuations are extremely faint. The DIRBE instrument was able to conduct studies on

interplanetary dust (IPD) and determine if its origin was from asteroid or cometary particles.

The second contribution DIRBE made was a model of the Galactic disk as seen edge-on from our position.

COBE: FIRAS-DMR-DIRBE

The age of the Universe: 13.772±0.059 billion-year. Hubble constant:69.32 ± 0.80 km s-1 Mpc-1

Baryon density (Wb): 0.04628±0.00093 Cold dark matter density (Wc): 0.2402+0.0088

−0.0087

Dark energy density (WL):0.7135+0.0095

-0.0096

The curvature of space is less than 0.4 percent of "flat" andthe universe emerged from the cosmic Dark Ages "about400 million years" after the Big Bang.

WMAP:

The Universe is 13.798±0.037 billionyears old.

It is currently theorised that theseripples gave rise to the present vastcosmic web of galactic clusters anddark matter.

It contains 4.82 ± 0.05% ordinarymatter, 25.8 ± 0.4% dark matter and69 ± 1% dark energy.

The Hubble constant was alsomeasured to be 67.80±0.77(km/s)/Mpc.

PLANCK (Spacecraft):

Cosmic Microwave Background comparison

4. Galaxy Clustering:

BAO: In cosmology, baryon acoustic oscillations (BAO) are regular, periodicfluctuations in the density of the visible baryonic matter (normal matter) of theuniverse. The power spectra of the 2-degree Field (2dF) Galaxy Redshift Survey andthe Sloan Digital Sky Survey (SDSS) both surveys showed evidence for BAOs. TheBaryon Oscillation Spectroscopic Survey (BOSS) of Luminous Red Galaxies (LRGs) inthe SDSS found consistency with the dark energy equation of state w = −1 ± 0.06.

Redshift Survey: In astronomy, a redshift survey is a survey of a section of the skyto measure the redshift of astronomical objects: usually galaxies, but sometimesother objects such as galaxy clusters or quasars. These observations are used tomeasure properties of the large-scale structure of the universe. The Great Wall, avast supercluster of galaxies over 500 million light-years wide, provides a dramaticexample of a large-scale structure that redshift surveys can detect.

5. Cluster of Galaxies:

A cluster of galaxies is a large collection of galaxies held together by their mutual gravitationalattraction.

Clusters are an ideal application in the present Universe. They are usually used to constrain thefluctuation amplitude σ8.

A theoretical prediction for the mass function of clusters can come either from semi-analyticarguments or from numerical simulations. The same approach can be adopted at high redshift(which for clusters means redshifts of order one) to attempt to measure σ8 at an earlier epoch.The evolution of σ8 is primarily driven by the value of the matter density Wm, with a sub-dominant dependence on the dark energy properties.

The Planck observations were used to produce a sample of 189 clusters selected by the SZeffect. For an assumed flat LCDM model, the Planck Collaboration found σ8 = 0.77±0.02 and -Wm = 0.29±0.02.

Bringing all the observations together:

The CMB data gives the age of the universe is 13.798 ± 0.037 billion years old which is in goodagreement with the ages of the globular cluster and radioactive dating.

Inclusion of BAO data, plus the assumption that the dark energy is a cosmological constant, withthe CMB data yields a constraint on Wtot = Wi + WL =1.0005±0.0033.

From the CMB and BAO data the cosmological constant w = −1.13+0.13−0.11 .

The data provide strong support for the main predictions of the simplest inflation models: spatialflatness and adiabatic, Gaussian, nearly scale-invariant density perturbations. But it isdisappointing that there is no sign of primordial gravitational waves.

Outlook For the Future:

In the future developments we can expect the one of two directions. Either the existing parameter set will continue to prove sufficient to explain the data, with the parameters subject to ever-tightening constraints, or it will become necessary to deploy new parameters.

There are many possibilities on offer for striking discoveries, Detection of primordial non-Gaussianities would indicate that non-linear processes influence

the perturbation generation mechanism, Detection of variation in the dark-energy density (i.e., w ≠ −1) would provide much-needed

experimental input into the nature of the properties of the dark energy.

There are many proposals for the nature of the dark matter, but no consensus as to which iscorrect. The nature of the dark energy remains a mystery. Even the baryon density, nowmeasured to an accuracy of a percent, lacks an underlying theory able to predict it within ordersof magnitude.

THANK YOU