26. Cosmology. Significance of a dark night sky. The Universe Is Expanding

Transcription

1 26. Cosmology Significance of a dark night sky The Universe is expanding The Big Bang initiated the expanding Universe Microwave radiation evidence of the Big Bang The Universe was initially hot & opaque The importance of the shape of the Universe The Universe seems filled with dark energy The Universe s expansion rate is accelerating Matter & dark energy determine the future Significance of a Dark Night Sky Physical cosmology Science of the structure & evolution of the Universe One fundamental expectation Universe is large enough to fill the sky with stars Night sky should be filled with light Night sky is not filled with light Olber 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 all observers Einstein s general theory of relativity Space is warped by massive objects Einstein introduced the cosmological constant Λ Introduced to produce the preconceived static Universe The Dark Sky Anomaly The Universe Is Expanding The Hubble law 1920 Galaxy clusters are moving away from each other Pervasive galaxy cluster redshift is compelling evidence The Hubble law is a linear relationship Tenfold distance increase Tenfold recessional speed increase Expansion of space, not 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

2 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 imagine but cannot 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 Big Bang & the Expanding Universe Observational evidence Inverse of the Hubble law In the distant past, the Universe was very small Far enough back, all matter & energy were in one point 1/H 0 Age of the Universe The Universe began 13.8 Bya if H 0 = 75 km. sec 1. Mpc This assumes that H 0 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 Almost certainly far larger than what we can see The Changing Observable Universe Entire Universe 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 13.8 billion years old We can only see objects < 13.8 Bly away in all directions This is our cosmic particle horizon As the Universe ages, we see back even farther 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 λ Cosmic background microwave radiation Penzias & Wilson at Bell Labs Early 1960s Observed microwave background radiation from space This radiation is about 1% the strength of analog TV noise Earth s atmosphere is largely opaque at 1.1 mm λ Satellites are a better option Cosmic Background Explorer (CoBE) 1989 Tri-Color [Red, Green, Blue] Noise Noise

3 Bell Labs Horn Antenna Microwave Background Spectrum Blackbody curve T = K Microwave Background Temperature Our Motion Through the Universe Density Dominance in the Universe Radiation - dominated Universe Prevailed in the earliest stages of the Universe This corresponds to a cosmic redshift of z = 25,000 Since then, λ s have been stretched by a factor of 25,000 This occurred when the Universe was ~ 2,500 years old EMR had λ = 40 nm in the UV part of the spectrum Matter - dominated Universe 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 Misnomer because they were never previously combined

4 Radiation & Matter In the Universe Two basic possibilities in the Universe Something is either energy or matter Special relativity: They are two forms of the same entity Two important concepts Universe s average mass density of radiation today ρ rad = kg. m 3 Equivalent to 550,000,000 photons per cubic meter Extremely low energy per severely redshifted photon Universe s average density of matter today ρ m = 2 to 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 Universe s Shape 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 The Universe will continue expanding forever

5 Measuring the Universe s Shape 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 A nearly flat Universe Galaxy clusters Matter density Ω m = 0.2 to 0.4 This clearly suggests a non-flat Universe The tentative conclusion There is substantial dark energy in our Universe Necessarily Ω Λ = 0.6 to 0.8 Presumed dark energy must be 60 80% of the Universe Proportions of Mass & Energy Universe s Variable Expansion Rate

6 Universe s Matter & Energy Distribution Universe s Expansion is Accelerating The observational evidence Type Ia supernovae in distant galaxies A highly reliable standard candle 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

7 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