26. cosmology significance of a dark night sky the universe is expanding the big bang initiated the...
TRANSCRIPT
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
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
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
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 areidentical
• 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/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
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
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
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
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
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 meter
Extremely 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 meter
Extremely high energy per hydrogen atom
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
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
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
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
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 ! ! !
• 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