26. Cosmology The significance of a dark night sky The Universe is expanding The Big Bang initiated the expanding Universe Microwave radiation as evidence of the Big Bang The Universe was initially hot & opaque The importance of the shape of the Universe The Universe seems to be filled with dark energy The Universe’s expansion rate is accelerating Matter & dark energy determine the future

The Significance of a Dark Night Sky Cosmology Science of the structure & evolution of the Universe One fundamental expectation The Universe is large enough to fill the sky with stars The night sky should be filled with light The night sky is not filled with light This is Olbers’s paradox The model of an infinite static Universe cannot be correct One possible solution Einstein’s special theory of relativity Distance, mass & time are all relative to different observers Einstein’s general theory of relativity Space is warped by massive objects The cosmological constant L Introduced to produce the preconceived static Universe

The Dark Sky Anomaly

The Universe Is Expanding The Hubble law 1920 Clusters of galaxies are moving away from each other Observed redshift is conclusive evidence The relationship is linear A tenfold distance increase yields a tenfold recessional speed increase An expansion of space, not of velocity through space Cosmological redshift rather than Doppler redshift Space does not expand in objects tightly bound by gravity Model of solid objects on an inflating balloon One fundamental conclusion The center of expansion cannot be identified Everything is moving away from everything else Every viewing location observes the same expansion

The Expanding Balloon Analogy

The Hubble Law & Expansion

Fundamental Assumptions Assumptions in scientific method Any reasonable assumptions can be made Assumptions are presumed unproveable for some reason Future observations may prove some assumptions invalid Assumptions in cosmology Observations can verify only one Universe Cosmologists can imagine but not produce other universes The cosmological principle The Universe is assumed to be homogeneous At the largest scale, all regions of space are identical The Universe is assumed to be isotropic At the largest scale, all directions in space look identical

The Big Bang & the Expanding Universe Observational evidence Reverse the Hubble law In the distant past, all things in the Universe were close together Far enough back, all matter & energy were in one point Reciprocal of the Hubble constant H0 gives age of the Universe The Universe began 13 Bya presuming H0 = 75 km . sec–1 . Mpc This assumes that H0 is now & has always been constant One major problem Stars cannot be older than the Universe Observational evidence suggests this may not be the case Recent evidence may resolve this issue The Big Bang “Cosmic singularity” may be a better term Remarkably similar to the singularity in a black hole

The Observable Universe Expected size of the Universe This is almost certainly far larger than what we can see Observational limits We only see as far back as the Universe is old This results from the finite speed of EMR in vacuum Presume the Universe is 13 billion years old We can only see objects < 13 Bly away This is our cosmic particle horizon As the Universe ages, we can see back even farther

The Changing Observable Universe Entire Universe

Microwave Radiation As Evidence The overabundance of He Theoretically there is too much He in the Universe Can be accounted if the early Universe was extremely hot If so, there should be evidence This would be severely redshifted to ~ 1.1 mm l Cosmic background microwave radiation Penzias & Wilson at Bell Labs Early 1960s Observed microwave background radiation from space About 1% of TV noise is this radiation Earth’s atmosphere is largely opaque at the 1.1 mm l Satellites are a better option Cosmic Background Explorer (CoBE) 1989

Bell Labs Horn Antenna

The Microwave Background Spectrum Blackbody curve T = 2.725 K

Temperature of Microwave Background

Our Motion Through the Universe

Density Dominance in the Universe Radiation-dominated Universe This prevailed in the earliest stages of the Universe This corresponds to a cosmic redshift of z = 25,000 Since then, l’s have been stretched by a factor of 25,000 This occurred when the Universe was ~ 2,500 years old EMR had l = 40 nm in the UV part of the spectrum Matter-dominated Universe This prevailed since the Universe was ~ 2,500 years old This may prevail for all time in the future

The Universe Began Hot & Opaque Basic physical processes High temperatures tend to ionize atoms & molecules Hydrogen has only 1 proton in its nucleus The bond with the 1 electron is therefore relatively weak Hydrogen ionizes at temperatures > 3,000 K This was true for ~ 300,000 years after the Big Bang An ionized gas is called a plasma Plasma interacts very strongly with EMR This makes a plasma opaque Cosmic redshift reduces energy content of EMR Energy is inversely proportional to wavelength Recombination Protons & electrons bind to form neutral hydrogen atoms Misnomer because they were never combined previously

Radiation & Matter In the Universe Two basic possibilities in the Universe Something is either energy or matter Special relativity says they are two forms of the same entity Important concepts The Universe’s average mass density of radiation today rrad = 4.6 . 10–31 kg . m–3 Equivalent to 550,000,000 photons per cubic meter Extremely low energy per severely redshifted photon The Universe’s average density of matter today rm = 2 to 11 . 10–27 kg . m–3 Equivalent to 1 to 6 hydrogen atoms per cubic meter Extremely high energy per hydrogen atom

Evolution of Density

Evolution of Radiation Temperature

Before & After Recombination Free protons Bound protons & electrons & electrons

Microwave Temperature Variations

Importance of the Shape of the Universe Three basic possibilities Positive curvature of space Spherical Two parallel beams of EMR will converge Three-dimensional analog of a sphere The Universe’s expansion rate will decrease The Universe will eventually collapse upon itself Zero curvature of space Flat Two parallel beams of EMR will remain parallel Three-dimensional analog of a plane The Universe’s expansion rate will remain constant The Universe will continue expanding forever Negative curvature of space Hyperbolic Two parallel beams of EMR will diverge Three-dimensional analog of a saddle The Universe’s expansion rate will increase

Measuring the Shape of the Universe Hypothetical Draw a huge triangle Measure the three angles Practical Count the number of extremely distant galaxies Spherical Concentration is highest nearby Flat Concentration is uniform Hyperbolic Concentration is highest far away

The Geometry of the Universe

Microwave Background & Curved Space

A Universe Filled With Dark Energy The observational evidence Temperature variations suggest a nearly flat Universe Galaxy clusters suggest matter density Wm = 0.2 to 0.4 This clearly suggests a non-flat Universe The tentative conclusion There must be substantial dark energy in our Universe Necessarily WL = 0.6 to 0.8 Presumed dark energy must be 60% to 80% of the Universe

Proportions of Mass & Energy http://upload.wikimedia.org/wikipedia/commons/4/4b/DarkMatterPie.jpg

Universe’s Variable Expansion Rate http://www.nasa.gov/images/content/56200main_dark_expansion-lg.jpg

Universe’s Matter & Energy Distribution http://upload.wikimedia.org/wikipedia/commons/b/b9/Cosmological_composition.jpg

The Universe’s Expansion is Accelerating The observational evidence Type Ia supernovae in distant galaxies The tentative conclusion The expansion rate of the Universe is increasing Einstein’s cosmological constant may be correct after all ! ! !

Varying Rates of Cosmic Expansion

Distant Supernovae & Hubble Diagram

Limits on the Nature of the Universe

The Evolution of Density Revisited

The Growth of the Universe

Chapter 28: Important Concepts The expanding Universe How can the sky be so dark? Stars should be literally everywhere The Hubble Law (1920) Space itself is expanding Concept of the Big Bang Matter & energy in a very small space Explosion created space & time Space expanded faster than light Expansion of space has slowed down Evidence for the Big Bang The cosmological red shift The cosmic background radiation Remnants of heat from the Big Bang Big Bang produced excess helium Required extremely high temperatures Milky Way moves amidst CBR The Great Attractor Characteristics of the Big Bang Extremely hot & opaque plasma The primordial fireball Hydrogen atoms eventually formed Temperature < 3,000 K Universe became transparent The shape of the Universe Three major possibilities Zero curvature = Flat *** Positive curvature = Closed Negative curvature = Open Two remarkable things ~90% of all matter is “dark” ~80% of all energy is “dark” The strangest thing yet Universe’s expansion is accelerating