Cosmology (Astrophysics)

Bill Rutherford

Dec 12 2014

What is Cosmology?

Cosmology is the study of the origin, evolution and fate of the universe

The Hubble parameter H characterizes the observed expansion which all models are obliged to explain

H = v/d recession velocity over distance (often measured by red shift)

or H = ȧ/a the change in scale factor over the scale factor of the universe

Big Bang with Inflation

1 Initial Anomaly2 Quantum Fluctuations3 Inflation4 Cosmic Background Radiation

4 -> 5 Dark Ages5 First Stars & Reionization6 Formation of Galaxies7 Acceleration of Expansion

Image courtesy NASA

What is the Big Bang theory?

Theory that the universe began as a hot, dense, uniform soup of particles that filled space uniformly, and was expanding rapidly

How the early universe expanded and cooled

How the light chemical elements formed

How matter congealed to form stars, galaxies, and clusters of galaxies

Big Bang

ΩM + ΩR + ΩΛ = 1 a(τ) parametrises uniform

expansion of the universe τ = conformal distance/speed of light

Dark Energy

Radiation Dominated for ~47,000 yrs

Matter Dominated for ~ 8.8 B yrs

Dark Energy Dominated~5 B yr ago

now

What is Cosmic Inflation?

a(t) increases by ~e60

Inflation is a modification of the standard big bang theory, providing a very brief prequel

What caused the expansion? (big bang theory describes only the aftermath of the bang)

Where did the matter come from? (big bang theory assumes that all matter existed from the very beginning)

Inflation via Gravitational Repulsion

The positive energy of the repulsive gravity material (inflaton) was compensated by the negative energy of gravity.

The total energy of the universe may very well be zero.

Matter + Dark Matter + Radiation

Gravity

Total Energy = + ~ 0

Status 2014

The cosmic microwave background (CMB) has a faint pattern in it preserved from accumulated activity, which is being analyzed

By studying the CMB cosmologists can extract information regarding the history of the Universe, such as ordinary matter, dark matter, dark energy, and the curvature of the Universe

Status 2014

It has been possible to infer model parameters from analyzing observational data and comparing to alternative explanations

The best fit so far is a model called ΛCDM which is shorthand for “inflationary cosmology with cold dark matter and a cosmological constant”

Status 2014

The latest Planck results affirm the ΛCDM model excluding some scale invariant sub-models

The current Hubble parameter was measured to be 67.80±0.77 (km/s)/Mpc

Images courtesy ESA - Planck

The B-mode polarization data will be released in Q1 of 2015

The B-mode ratio factor has been estimated as constrained to 0.11 by dust and other effects

Particle Overview

Image courtesy LBNL

Quantum foamevent(s)

Status 2014

It is assumed that the primordial universe supports quantum wave phenomena as is generally the case

It appears quantum wave activity present in the inflationary scalar field was then propagated in following stages

This pattern was apparently transformed in scale from smaller than a proton to larger than a galaxy by inflation then expansion

Evidence?

Image courtesy CALTECHWe are viewing the universe through a local distribution of mass, energy and fields

Earth

Evidence?

Image courtesy ESA - PlanckMultifrequency scan of Cosmic Microwave Background (CMB)

Evidence?

Image courtesy ESA - Planck

Dust MapDust Map

We are viewing the universe through a dust cloud

Evidence?

Image courtesy Coldcreation

The CMB is coming through a maze of intermediate factors

Evidence?

Images courtesy NASA- Hubble

The CMB is viewed through a field of galaxies and particles

Evidence?

When radiation was still trapped by matter, the tightly coupled system of photons, electrons and protons behaved as a single gas or fluid, with photons scattering off electrons like ricocheting bulletsAs in the air, a small disturbance in gas density would have propagated as compression wavesThe compressions heated the gas and the rarefactions cooled it, so any disturbance in the early universe resulted in a shifting pattern of temperature fluctuations.

Evidence?

Right after recombination you see an isotropic CMB (blue circle in centre, same in all directions) since only photons from the region right around you have had time to reach you.As time progresses, you see photons from more distant regions. You first see a quadrupole variation around you (red and blue at 90 degree angles), then an octopole, etc. by the current epoch some 10 billion years later what you see is very fine angular scale structure in the CMB temperature.

Evidence?

Image courtesy ESA - PlanckFine angular structure of CMB temperature variations (~70 µK)

Evidence?

Image courtesy Wayne Hu – PhD Thesis – Berkeley 1995

The LSS has density structure which scatters photons, the density variations are responsible for subsequent gravitational evolution

Evidence?

Image courtesy ESA - PlanckThe angular power spectrum peak at about 1° corresponds to the density variations

ΛCDM prediction

l = 180/Ѳ

Evidence?

Image courtesy Bryan Christie DesignThe event horizon never allows thermal equilibrium to occur except during inflation

Evidence for Inflation

The cosmic background radiation is uniform in temperature to 1 part in 100,000.

It was released when the universe was about 370,000 years old.

In standard cosmology without inflation, a mechanism to establish this uniformity would need to transmit information at ~100 times the speed of light.

In inflationary models, the universe begins so small that uniformity is easily established - just like the air in this room spreading to fill it uniformly.

Then inflation stretches the region to be large enough to include the visible universe.

Inflation DetailsThe process of inflation consists of three steps:

Prior to the expansion period, an inflaton field was at a higher-energy state (false vacuum).

Random quantum fluctuations triggered a phase transition whereby a part of the inflaton field decayed releasing its potential energy, as it settled to a lower energy state (the rest just kept on expanding)

This action generated a repulsive force that drove a bubble to expand

Evidence for Inflation

According to general relativity, the flatness of the universe is related to its mass density:

Image courtesy NASA WMAP

Ω0=actualmassdensitycriticalmassdensity

closed

open

flat

Evidence for Inflation

A universe at the critical density is like a pencil balancing on its tip.

If Ω in the early universe was slightly below 1, it would rapidly fall to zero - and no galaxies would form

If Ω was slightly greater than 1, it would rapidly rise to infinity,the universe would recollapse, and no galaxies would form

To be as close to critical density as we measure today, at one second after the big bang, Ω must have been equal to one to 15 decimal places,

i.e. ΩM + Ω

R + Ω

Λ = 1

Evidence for Inflation

Since inflation makes gravity become repulsive, the evolution of Ω changes too.

Ω is driven towards one, extremely rapidly (it could begin at almost any value)

Since the mechanism by which inflation explains the flatness of the early universe almost always overshoots, it predicts that even today the universe should have near critical density

Evidence for Inflation

Until 1998, observation pointed to Ω ≈ 0.2–0.3.

Latest observations by the ESA Planck satellite (combined with other astronomical observations):

Ω = 1.0010 ± 0.0065 with dark energy ~68.3%

In 1998 it was discovered that the expansion of the universe has been accelerating for about the last 5 billion years

Dark energy is causing this to happen

Slow Roll Inflation

Scalar Field

Po

ten

tial E

ne

rgy

Matter Creation

Expansion

Quantum Fluctuations Slow Roll

Field within event horizon and at “thermal equilibrium”

End of InflationQuantum contributions to seeding CMB power spectrum

Pressure WaveThe dark matter only interacts with gravity and so stays near the centre Before decoupling (due to expansion cooling), the photons and baryons move togetherAfter decoupling photons are interacting less with baryonic matter so they diffuse awayThis relieves the pressure leaving baryonic matter at a location often referred to as the sound horizon

Pressure Wave

Start with a single density perturbation The plasma is totally uniform except for a small excess at the origin (at the end of inflation) High pressure drives the gas + photon fluid outward at ~0.5c leaving dark matter behind

Pressure Wave

Initially both the photons and the baryons move outward together

This wave continues for ~105 years at the same time covariantly as the universe is expanding

Pressure Wave

After ~3.7 x 105 years the universe has cooled enough for protons to capture electrons forming neutral HydrogenThis decouples the photons which stream away leaving the baryon peak stalled

Baryons Photons

Pressure Wave

The photons continue to stream away forming the CMB while the baryons, having lost their motive pressure, remain in place

Baryons Photons

Sound Horizon

Pressure Wave

Photons become more uniform, baryons remain over dense in a shell ~100 Mpc in radius at 370,000 years

100 Mpc

Baryons Photons

Pressure Wave The final configuration is a peak at the centre and

an “echo” in a spherical shell

Baryons Photons CMB spectrum

Scaffold for massive black holes e.g. at centre of galaxies

Pressure Wave

There are also several lower amplitude harmonics of the main wave similar to a flute

Power Spectrum

Image courtesy ESA - Planck

1st Fundamental Wave

2nd Harmonic Wave

3rd Harmonic Wave

Damping Tail

Multiverse Overview

Image courtesy National Geographic

Continuous inflation infers a multiverse

M Theory Steinhardt–Turok model

Collision of 2 out of many multidimensional spaces called branes (from string theory)

Predicts similar physical data to inflation but from a lower initial temperature

Lower prediction than inflation for gravity waves

and hence B-modes

Inflation Gravity Waves

Image courtesy CALTECH - BICEP2

What is B-Mode Polarization?

Image courtesy CALTECH - BICEP2

Tensor Mode (T)

Scalar Mode (S)

B-Mode Evidence

Image courtesy University of Chicago – Wayne Hu

As the tensor mode gravity waves (T) increase relative to the density waves (S) a B-polarization mode appears close to the main peakCross = temperature-polarization cross power spectra

B-Mode Evidence

Image courtesy CALTECH - BICEP2Patch of the CMB viewed from a south pole observatory

B-Mode Research

The tensor to scalar mode ratio factor (r) for B-Modes to E-Modes is approximated as

r = 16ϵ

where ϵ is a slow roll inflation parameter

the parameter ϵ ~ -Ḣ/H2

Thus r directly probes and constrains the value of the Hubble parameter (H) during inflation

Gaussian Research

Recent research is concentrating on the presence of subtle non Gaussian effects in the CMB spectra

Open Discussion