26. Cosmology

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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

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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 - PowerPoint PPT Presentation

Transcript of 26. Cosmology

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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• Dark matter & dark energy determine the future

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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

L– Introduced to produce the preconceived static Universe

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The Dark Sky Anomaly

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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

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The Expanding Balloon Analogy

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The Hubble Law & Expansion

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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 lookidentical

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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/H0 Age of the Universe⇒

– The Universe began 13.8 Bya if 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

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The Observable Universe• Expected size of the Universe

– 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 13.8 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

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The Changing Observable UniverseEntire

Universe

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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

– This radiation is about 1% the strength of analog TV noise

– Earth’s atmosphere is largely opaque at 1.1 mm l• Satellites are a better option

– Cosmic Background Explorer (CoBE)

1989

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Tri-Color Noise

Noise

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Bell Labs Horn Antenna

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Microwave Background Spectrum

Blackbody curveT = 2.725 K

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Microwave Background Temperature

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Our Motion Through the Universe

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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, 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– Prevailed since the Universe was ~ 2,500 years old

• This may prevail for all time in the future

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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

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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

todayrrad = 4.6 . 10–31 kg . m–3

Equivalent to 550,000,000 photons per cubic meterExtremely low energy per severely redshifted photon

– Universe’s average density of mattertoday

rm = 2 to 11 . 10–27 kg . m–3

Equivalent to 1 to 6 hydrogen atoms per cubic meterExtremely high energy per hydrogen atom

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Evolution of Density

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Evolution of Radiation Temperature

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Before & After RecombinationFree protons Bound protons& electrons & electrons

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Microwave Temperature Variations

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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

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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

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The Geometry of the Universe

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Microwave Background & Curved Space

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A Universe Filled With Dark Energy• The observational evidence

– Temperature variations A nearly flat Universe⇒– Galaxy clusters Matter density ⇒ Wm = 0.2 to 0.4

• This clearly suggests a non-flat Universe

• The tentative conclusion– There is substantial dark energy in our Universe

– Necessarily WL = 0.6 to 0.8• Presumed dark energy must be 60 – 80% of the Universe

Dark Energy

Dark Matter

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Proportions of Mass & Energy

http://upload.wikimedia.org/wikipedia/commons/4/4b/DarkMatterPie.jpg

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Universe’s Variable Expansion Rate

http://www.nasa.gov/images/content/56200main_dark_expansion-lg.jpg

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Universe’s Matter & Energy Distribution

http://upload.wikimedia.org/wikipedia/commons/b/b9/Cosmological_composition.jpg

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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 constantmay be correct after all ! ! !

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Varying Rates of Cosmic Expansion

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Distant Supernovae & Hubble Diagram

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Limits on the Nature of the Universe

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The Evolution of Density Revisited

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The Growth of the Universe

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• 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

Chapter 28: Important Concepts