Cosmology: The Study of the Universe as a Whole

Physics 360 Geol 360

Astronomy

John Swez

Ancient Cosmology:Why A Finite Age Universe

Even simple observations can lead to thoughtful questions about the Universe

• Olbers’ Paradox – Why is the night dark?

• Newton’s Law of Gravity – Why hasn’t everything fallen together?

Why A Finite Age Universe: Olbers’ Paradox (1823)

• Assume Universe is infinitely old• Then every line of sight will

cross a star• The Night Sky would not be

black• The black night sky implies the

Universe is finite

Why A Finite Age Universe: Newton’s Law of Gravity

• Every object attracts every other object in the Universe

• Therefore, every star attracts every other star

• After infinite time, all stars should clump together into one big blob

• This could imply a finite aged Universe, or (from Newton) Divine Intervention

Modern Cosmology ObservationsThings Any Modern Cosmology Must Explain

• Expansion of the Universe

• Cosmic Microwave Background Radiation

• Evolution of the Universe

• Cosmic Element Abundances

Interpreting It All:The Underlying Principles of

Cosmology• Universality

– The same laws of physics apply everywhere

• Isotropy– At large scales, from any point the Universe

looks the same in every direction

• Isotropy with time would also be aesthetically pleasing, but doesn’t hold

Expansion of the Universe

Edwin Hubble:The redshift of the wavelength of spectral lines increases with distance

This is equated with expansion

Balloon Model of Expansion

Each button on the surface of a balloon expands away from all other buttons

The Universe is a 3-dimensional version of this

Expansion of the Universe

• Redshift of galaxies proportional to distance

• Implication: The Universe is Expanding

• Implication: In the past, the Universe was more dense than now

Redshift Interpretation

The redshift is due to space expanding, not a velocity effect

Space stretches light as it expands

Longer light is redder light

What Expansion Is Not

• Redshift is due to space expanding between galaxies

• Expansion is not ‘into’ anything

• The Universe doesn’t need an ‘edge’

• The Universe and expansion can be everywhere

Cosmic Background Radiation

• Space is filled with 2.7K blackbody radiation

• This peaks in the microwaves

• It has small variations, but is basically very uniform

Evolution of the Universe

• Gas clouds collapse into stars, which die

• Galaxy shapes vary with distance/time

• Number of radio galaxies increases with distance/time

• Older stars have less heavy elements like carbon and oxygen

• Implication: The Universe is changing

Galactic EvolutionHubble Deep Field

• A view of the most distant galaxies observed

• Many shapes besides common (spiral, elliptical) types

Cosmic Element Abundances

• The Universe appears to be– ~73% Hydrogen– ~25% Helium– ~2% everything else

• This appears to be true for most every star, in most every galaxy

• Deuterium abundance can’t come from stars• Isotope ratios for Helium and Lithium are also the

same everywhere

Modern Cosmology Observations (recap)

• Expansion of the Universe

• Cosmic Microwave Background Radiation

• Evolution of the Universe

• Cosmic Element Abundances

Big Bang Theory

• In the past, the Universe was very hot and dense• The Universe is expanding• Therefore, the Universe is cooling Modern Universe• Most credible theory of the early Universe• Supports all the modern observations• Does not address the origin of the Universe

Early Universe

Time Temperature Activities Implications

10-43 s Matter-antimatter <-> Energy

conversion Matter and Energy Indistinguishable

10-6 s 1013 K Pair-production Matter and energy change

forms

1 s 1010 K Pair-production stops Neutrino’s decouple

Matter / photon ratio set

3 minutes 107 K p+p-> d

d+p-> He Cosmic H, He abundance set

106 K

Radiation Era

Matter knocked around by radiation

~105 years 104 K

Radiation Decouples from

Matter Atoms form

109 years 103 K

Large Scale Structures Form Galaxies form

~1010 yrs Now

3 K

People Explore the Universe People engage in lifelong

learning

E=mc2 <-> e+ + e-

Fate of UniverseGeneral Theory of Relativity

• Energy/Mass shapes space and slows the expansion

• Cosmological Constant (‘dark force’) increases expansion

• Open: Universe expands forever

• Flat: Universe barely expands forever

• Closed: Universe eventually contracts

Geometry of the Universe

• Spherical– Surface of a sphere– Finite size

• Flat / Euclidean– ‘Normal’ geometry– Infinite size

• Hyperbolic– Surface of a saddle– Infinite size

Not Observed!

Observed!

Dark ForceCosmological Constant

• ‘Negative Pressure’ or ‘Dark Force’• Strong only over very long distances

(between galaxies)• A static universe requires it to fight gravity

(Einstein invented it for this)• An expanding universe does not require it

(until recently, it was discounted)• It may now dominate Universal expansion

Expansion Age of the Universe

• Running expansion backwards -> start

• Depends on current and past expansion rates

• Depends on relative strengths of:– gravity– Dark force, which may vary over time

Fate of Universe

Geometry Type Density Required

Cosmological Constant

Fate

Spherical Closed / Finite

> ~10-19 g/cm3 Negligible Collapse

Flat Open / Infinite

= ~10-19 g/cm3 Negligible Barely Expands Forever

Hyperbolic Open / Infinite

< ~10-19 g/cm3 Negligible Expands Forever

Flat Open / Infinite

Any Greater effect than gravity

Expansion Accelerates

Latest Results

• The expansion rate is measured by knowing the distance to very distant objects

• Type I Supernova are exploding stars (very bright!) and good distance indicators

• Recent observations support a universe where the expansion is now dominated by the Dark Force

Gravity Vs Cosmological Constant

Redshift

Fraction of mass to close

Universe

Relative strength of

Cosmological Constant

Dis

tanc

e

‘Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant’, Riess et al, 1998 Astronomical Journal v116 1009

Gravity Vs Cosmological

Constant

‘Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant’, Riess et al, 1998 Astronomical Journal v116 1009

Expansion Age of the Universe

Reiss et al Conclusions

• Dark Force / Cosmological Constant positive (99.7% likely)

• Expansion is accelerating (99.5% likely)

• Universe is flat

• Expansion Age of the Universe

14.2 ±1.5 Gyr

• These are reinforced by further observations

Ages of Objects in the Universe

Object Age (Years) Recorded History 6 Thousand Oceans (Salt Deposits) 3.5 Billion Earth (Radioactive Dating) 4.5 Billion Heavy Elements 10-12 Billion Oldest Star Clusters 12-14 Billion Expansion Age 14-15 Billion

These are all very consistent – a good sign!

Cosmology Summary

• The Universe was hot, dense and expanded– We observe galaxy redshifts– Assuming we are not at center implies

expansion– Expansion Age is consistent with other ages– Cosmic microwave background, element

abundances and evolution are integrated into Big Bang

• Universe will probably expand forever

Before the Big Bang - Inflation

Why is the Universe so uniform?

It stretched very early on

Origin of the UniverseBefore the Big Bang

• Idea: Universe Started as a Random Quantum Event– Universe appeared from nothing – net

energy is zero– ‘Early’ Universe did not have time as a

dimension– There is no ‘before’

• Idea: Universe properties initially unspecified, but the Universe selects for properties that lead to life (“Life of the Cosmos” by Smolin)

There is not a well established theory for before Inflation / Big Bang