Finals Prep
18.3: Age of the Universe
Oldest discrete objects w/ reliably determined ages: globular star clusters
Oldest ones though to be 12-13 billion yrs old
SO age of universe would be abt 14 billion yrs (bc universe must be at least as old as oldest objects in it)
ANOTHER technique–determine time elapsed since birth (Big Bang)
Hubble Time: Amount of time since big bang, assuming constant speed for any given galaxy
The quest for Hubble’s Constant
H0 = v/d (d=distance; v = recession speed)
Galaxy distances hard to determines, has led to large uncertainties in value of H0
Key Project of Hubble Space Telescope
Wanted to obtain distances to many important galaxies, but long wait time because of spherical aberration on the primary mirror; difficult to detect and reliably measure Cepheids
18.4 The Geometry and Fate of the Universe
Use Einstein’s general relativity theory to study expansion & overall geometry of universe
Cosmological principle: on the largest size scales, the universe is very uniform–same avg density everywhere at a given time and looks the same in all directions
Aka over large distances, universe is indeed uniform
No "cosmological constant”
Prior to hubble, ppl thought universe was static
Einstein postulated a lon-range repulsive force that would’ve kept gravity from making the universe contract–known as cosmological constant
BUT with discovery of expanding universe, constant no longer needed
“Biggest blunder of my life” - Einstein
3 kinds of possible universes
“Friedmann universes” - general relativity allows for 3 possible universes
In each case, expansion decelerates with time
Ultimate fate determined by overall avg density of matter
ΩM>1 (avg density above critical density) galaxies separate more slowly and eventually turn around and approach each other, ending in a “big crunch”
Closed universe
ΩM=1 (avg = critical) galaxies separate more and more slowly with time, but as time approaches infinity, recession speed approaches zero. Thus, universe will expand forever, though just barely
Known as flat or critical universe
ΩM<1 (avg below critical) galaxies separate more and more slowly with time, as time approaches infinity, recession speed approaches a constant nonzero value. Universe easily expand forever
open universe
18.5 Measuring the Expected Deceleration
Most accurate distance measurements made w/ type Ia supernova
Catch them in photographs of distant galaxies, candidates determined to be type Ia w/ spectrum measurements
An accelerating universe!
Several dozen measure but results revealed universe not slowing down, but accelerating
Data agrees best with the open universe, but we know matter density is greater than zero because we exist and there are no known forms of negative matter, even antimatter is positive!
Conclusion: a cosmic “antigravity” does exist
We live in an accelerating universe
Expansion rate is INCREASING
Many now considered the problem of what is CAUSING this acceleration one of the hottest topics in all of physics
Not actually Einstein's biggest blunder, reasoning for coming up with was wrong, but the idea of it actually maybe his greatest triumph!
Dark Energy
Causes space to expand more and more quickly
Dark matter is gravitationally attractive and produces deceleration
Present throughout space but only on largest scales does its density exceed that of luminous and dark matter
The cosmic jerk
Research data has shown for roughly first 9 billion yrs of its existence, the universe WAS deceleration, and it was only around 5 billion yrs ago that the expansion rate began to accelerate
Rate of change of acceleration is called “jerk,” this occurred when the expansion of universe went from state of deceleration to acceleration
18.6 The Future of the Universe
Currently live in the stelliferous era
Lots of stars!
Universe will then enter degenerate era
Filled w/ cold brown dewars, old WDS and neutron stars
Then, black hole era
Only discrete objects will be black holes
Finally, dark era
Only low-energy photons, neutrinos, and some elementary particle
Ch 19.1-19.4
Expansion of universe suggests, but does not demand, a big bang
Fatal blow to theory that universe has no beginning/end was discovery of the cosmic microwave background – a faint radio glow that uniformly pervades the universe
Produced before an age of abt 380,000 yrs
1990s: tiny ripples detected in cosmic microwave background radiation, corresponding to seeds from which superclusters and clusters of galaxies were formed
Total energy density of the universe:
Baryonic matter - 5%
Cold dark matter - 25%
Dark energy - 70%
When universe was very young: consisted of uniformly distributed gas and radiation at the same temperature
Up to an age of ~1 microsecond, universe consisted of quarks, gluons, photons, and number of other kinds of particles
Lightest elements formed in the first 10 minutes through primordial nucleosynthesis–formation of the nuclei of isotopes of hydrogen, helium, and lithium in first few minutes of the Universe
Relative amounts of diff isotopes, combined with other observations, tell us that much of the dark matter in the Universe must consist of exotic particles like WIMPS
Ch 20
Study of life elsewhere than on Earth: astrobiology, exobiology, or bioastronomy
One difficult in search for life– we don't have a clear definition of what LIFE is
Restrict ourselves to life as we know it
Based on organic compounds: those containing carbon
Life on Earth is based on chains of carbon called amino acids
Follow genetic code transmitted by DNA to link together and form proteins
Experiment where sparks were passed through certain mixtures of gases showed amino acids form easily
Can form in variety of environments including harsh ones (extremophiles)
Within our solar system, most likely candidates: Mars, Europa, & Enceladus bc we know liquid water exists within them
Extraterrestrial life outside our planetary system, especially intelligent life, most likely to have formed on moons or planets orbiting Sun-like main-sequence stars
Such stars have long life and relatively large habitable zone - range of distances in which conditions suitable for life might be found
Likely method of detections: searching for artificially produced electromagnetic signals, especially radio waves
Although we do not know how many communicating civilizations exist in the Milky Way, can estimate using the Drake Equation
Equation advanced by Frank Drake & popularized by Carl Sagan that attempts to estimate the number of communicating civilization by breaking the calculation down into series of steps that can be assessed individually, such as the rate of star formation, the fraction of stars with planets, and the avg lifetime of a communicating civilization
OFFICE HOURS 12/4
Qs for office hours
What to know abt Hubble’s constant/the search for it?
Know there is disagreement
How to manipulate the constant
Proportional
Inverting hubble’s constant gives you age of an (empty) universe
General story
Names to know:
Hubble
Lemaitre
Zwicky
Rubin
Leavitt
Cannon
Reorganize classification system to OBAFGKM
Payne
Key discovery tying in strength of hydrogen lines to temperature
Friedmann
Einstein
Herschel
Tying people to their concepts
Angular resolution formula
Go through the lecture where he ran through what the class will be
Equations to know
Kepler’s 3rd Law: p2[yrs] = a3[AU]
Newton’s revision: p2[yrs](M1+M2 [Mo]) = a3[AU]
Newton’s Gravity Fg = G(m1 + m2)/d2
Wave & Light EQs: v=λf →c=λf & E=hf=hc/λ
Wien’s Law: λpeak[nm] = 3 x 106 [nm x K]/T[K]
If double temp what would happen to wavelength peak (wavelength would be shorter (decrease by two)
Don't need to know the constant
Stefan Boltzmann Law: E/s/m2 = oT4
When temp increase by 2, energy increase by 16
Relationship between temp and energy
Doppler: v=c(λobs-λemit/λemit)
Object coming towards you
Telescopes: A= πD2/4 = πr2
For angular resolution, if you double the diameter, could see finer detail, angular resolution decreases
D=diameter
L=4πr2oT4 → b=4/4πd2 → d=1/p
d=distance
L=luminosity
b=apparent brightness
Leavitt’s Law: L ∝ P
Hubble-Lemaitre Law: v=H0d –. z= H0d/c
Z: redshift →v=z/c
Hubble time: Age= 1/H0
Concept of critical density: P critical → Ω = Pactual/ Pcritical
Don’t need to know General relativity formula, but do need to know concept
Formula differs from Newton's gravity
Can use Newton’s at long distances, but really strong forces of gravity need to take into account Einstein’s gravity
Cosmic Microwave Background
Penzias-Wilson: observed hum in radio bands throughout universe
In general observation methods favor larger planets closer to the stars they are orbiting
BIG THING: expect there to be more planets than stars in the universe
Histogram techniques on how many they’ve found
Most successful method: transit method, followed by radial velocity
have been ~1000 planets discovered with the radial velocity method (not ~10 or ~100),
Kepler mission discovered ~3000 planets (not ~3 or ~300,000).
Planets – know the broad features
Which ones have atmospheres
Which ones have moons
Most amt of moons: saturn
CERN & FermiLab
Watch subatomic particle debris & hope to piece together how they were put together
If big bang happened → universe should’ve been VERY dense
Ωall matter=0.31
Ωbaryon= 0.05
+ ΩDark matter= 0.26
+ ΩDark Energy= 0.69
Ω= 1.00
Rubin first to observe that Galaxy rotation plateaued instead of a predicted decrease, implied dark matter’s existence
Observatory Questions:
Why so much struggle to get the observatory?
What college was initially supposed to have it?
Dark matter v Dark Energy
dark matter is mass/matter, and dark energy is energy/pressure.
Quarks are building blocks that create protons and neutrons
Most luminous star dependent on radius
Cosmological Distance Ladder
Why do we have to have different rungs? Distance limitations
Hubble-Lemaitre Law
Supernovae (Ia)
Cepheid Variables
Spectroscopic “parallax”
Parallax of stars
Distance to the sun
Distance to the moon
Radius of the Earth